A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

Yan-Jie Wang; Baizeng Fang; Dan Zhang; Aijun Li; David P. Wilkinson; Anna Ignaszak; Lei Zhang; Jiujun Zhang

doi:10.1007/s41918-018-0002-3

A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

Wang, Yan-Jie;Fang, Baizeng;Zhang, Dan;Li, Aijun;Wilkinson, David P.;Ignaszak, Anna;Zhang, Lei;Zhang, Jiujun 2018-03-01 00:00:00 Metal–air batteries (MABs), particularly rechargeable MABs, have gained renewed interests as a potential energy stor- age/conversion solution due to their high specific energy, low cost, and safety. The development of MABs has, however, been considerably hampered by its relatively low rate capability and its lack of efficient and stable air catalysts in which the former stems mainly from the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and the latter stems from the corrosion/oxidation of carbon materials in the presence of oxygen and high electrode potentials. In this review, various carbon-composited bifunctional electrocatalysts are reviewed to summarize progresses in the enhancement of ORR/OER and durability induced by the synergistic effects between carbon and other component(s). Catalyst mechanisms of the reaction processes and associated performance enhancements as well as technical challenges hindering commercialization are also analyzed. To facilitate further research and development, several research directions for overcoming these challenges are also proposed. Keywords Metal–air batteries · Oxygen reduction reaction · Oxygen evolution reaction · Carbon · Bifunctional electrocatalysts · Synergistic effect PACS 88.80.ff Batteries · 88.80.F− Energy storage technologies · 82.47.Aa Lithium-ion batteries 1 Introduction Metal–air batteries (MABs), in particular rechargeable MABs, possessing high specific energy, low cost, and safety [1, 2], have gained great attention in recent years due to their * Anna Ignaszak feasibility as electrochemical energy storage/conversion [email protected] solutions. A MAB system (see Fig. 1) is an electrochemical * Jiujun Zhang system consisting of a pure metal or metal alloy electrode [email protected] for metal oxidation reactions (discharge process) and metal compound reduction reactions (charge process), as well as a School of Sciences, Institute for Sustainable Energy, Shanghai University, Shanghai 200444, China second electrocatalyst coated air-electrode for oxygen reduc- tion reactions (ORR, discharge process) and oxygen evolu- School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, Guangdong, tion reactions (OER, charge process). Between these two China electrodes is an electrolyte serving as both a separator and Department of Chemical and Biological Engineering, an ion conductor [3]. Currently, the metal or alloy electrodes University of British Columbia, Vancouver, BC V6T 1Z3, that have been developed in the fabrication of RMABs con- Canada sist of Li, Zn, Al, Fe, Na, Ca, Mg, K, Sn, Si, Ge and/or their Department of Chemistry, University of New Brunswick, alloys [1, 4, 5] and typical catalysts of air electrodes include Fredericton, NB E3B 5A3, Canada materials based on Pt, Ir, Ru and their alloys as well as vari- Energy, Mining and Environment, National Research Council ous non-noble metals [6–10]. Canada, Vancouver, BC V6T 1W5, Canada Vol.:(0123456789) 1 3 2 Electrochemical Energy Reviews (2018) 1:1–34 Fig. 1 A basic metal–air battery (MAB) configuration with a simplified solid–liquid–gas three-phase zone Although MABs with oxygen in air possess drastically air electrodes, this review examines the most recent pro- higher theoretical energy densities than traditional aqueous gresses and research trends in both experimental and char- and lithium-ion batteries, the development and commerciali- acterization strategies, providing up-to-date knowledge and zation of MABs faces one major challenge: a lack of efficient information on MAB bifunctional catalysts. The reaction and robust bifunctional air-electrode catalysts which signifi- mechanisms of the catalytic processes, the catalyst compo- cantly limits battery performance in terms of both rate capa- nent interactions in the presence of carbon, the synergetic bility and long-term stability [4, 6]. To overcome this chal- effects induced by the addition of carbon into catalyst mate- lenge, global research of bifunctional air electrodes for MABs rials, as well as the relationship between physicochemical and in particular electrocatalysts for both ORR and OER has structure and catalytic performance are reviewed and ana- rapidly progressed in recent years. With respect to this, carbon lyzed. Current achievements and challenges in synthesizing materials have been employed as supports and components for catalysts and fabricating air electrodes are also summarized. catalysts to improve catalytic activity and stability. Future research directions are also proposed in this review to Normally, the air-electrode catalyst layer for both ORR accelerate research and development in this area to overcome and OER is a matrix structure containing primarily of cata- challenges. lyst particles and/or carbon particle-supported catalyst particles and ionomers [4, 5, 11, 12]. To synthesize these catalysts and fabricate these catalyst layers, different nano- 2 Composites of Different Carbons technologies, associated characterization techniques as well as performance validation methods have been carried out Combinations of two different carbon materials have been to significantly optimize the morphology and surface area previously reviewed and analyzed as modified carbon sup- of these advanced nanostructured catalysts to obtain high port materials for high-performance Pt-based fuel cell cata- catalytic activity and stability. As recognized, the interac- lysts [13]. Here, based on the advantages of different carbon tions and synergetic effects between carbon and other com- materials, formed carbon–carbon composite supports show ponents play an important role in performance optimization. enhanced ORR activities for Pt-based catalysts in polymer Tailored designs and geometries of such carbon-composited electrolyte membrane fuel cells (PEMFCs). Similar combi- bifunctional catalysts are also recognized as major factors in nations of two or more different carbon materials have also promoting catalytic activity and chemical/electrochemical been explored as metal-free bifunctional composite electro- stability. In addition, bi- or multi-carbon component cata- catalysts for RMABs. lysts can constitute new composites which not only possess In regard to zero-dimensional carbon nanostructures with original carbon characteristics but also present new proper- atomic sizes below 100 nm, graphene quantum dots (GQDs) ties for improving catalytic activity and stability. exhibit satisfactory ORR performance due to strong quan- With a focus on advanced carbon-composited materi- tum confinement, edge effects, and oxygen-rich functional als as bifunctional electrocatalysts for rechargeable MAB groups on the material surface [14, 15]. By encapsulating 1 3 Electrochemical Energy Reviews (2018) 1:1–34 3 GQDs in a graphene hydrogel structure, Wang et al. [16] the combined results in typical SEM and TEM images reported a carbon–carbon composite electrocatalyst in (Fig. 2b–e) not only demonstrate the porous structure of the which the GQDs provided more active sites and the gra- graphene hydrogel, but also indicate the existence of GQDs phene hydrogel served as a conductive substrate to fix and with different sizes ranging from 2 to 20 nm in the porous sputter GQDs to prevent agglomeration and facilitate elec- structure. For GQDs in particular, HRTEM images (Fig. 2f) tron transfer. According to the proposed synthesis mecha- proved the graphitization of carbon dots with no peaks being nism as shown in Fig. 2a, graphene quantum dot/graphene found at ~ 270 nm in the tested UV–Vis absorption spectra, hydrogel (GH–GQD) composite samples are prepared and confirming the presence of rich edges in the GQDs [15, 17]. labeled as GH, GH-GQD-45, GH-GQD-90, GH-GQD-180, At room temperature, the measured RDE results revealed corresponding to GQD amounts of 0, 45, 90, and 180 mg. In that the GH–GQD composite samples exhibited better ORR the structural characterization of these GH-GQD samples, activities than individual GH and graphene oxide because of GQDs. Reprinted with permission from Ref. [16]. Copyright 2016 Fig. 2 a Illustration of the synthesis of GQDs, b SEM image of Royal Society of Chemistry GQD-GH-90 with a scale bar of 500 nm, and TEM images of c GH, d GQD-GH-90, and e GQD, and f HRTEM image (scale bar = 5 nm) 1 3 4 Electrochemical Energy Reviews (2018) 1:1–34 of the synergistic effects between GH and GQDs in which mesoCs core–shell composite material. In their structural GQDs provided more active sites and GH served not only characterizations, the collected Raman spectra revealed that as a conductive substrate for enhancing the charge transfer pristine CNFs possessed a lower peak ratio of D band to of electrodes, but also as a good support material condu- G band than CNF@mesoCs, indicating that the surface of cive to the high dispersion of GQDs and their active sites. CNF@mesoCs possessed a more graphitized structure than The GH-GQD-90 sample demonstrated the highest ORR pristine CNFs. The CNF@mesoCs also displayed a better −1 onset potential of − 0.13 V among all GH–GQD compos- electrical conductivity of ~ 4.638 S cm and a larger surface 2 −1 −1 ite samples, indicating that the optimized GQD amount area of 2194 m g than pristine CNFs (~ 3.0759 S cm and 2 −1 is 90 mg. Linear sweep voltammetry (LSV) curves run in 708 m g ), demonstrating the effects of mesoporous carbon O -saturated 0.1 M KOH were used to evaluate the ORR onto CNFs in this CNF@mesoCs composite. The electro- mechanism, and it clearly demonstrated that the enhanced chemical performance of the CNF@mesoCs composite was electrocatalytic activity resulting from the rich edge defects also examined under an oxygen atmosphere in a discharging of the GH-GQD-90 composite sample was related to O test using a fabricated LAB with non-aqueous electrolyte. reactions through both two-electron and four-electron reac- Here, it was found that as the cathode, CNF@mesoCs dem- −1 tion pathways [18] as indicated by tested peroxide percent- onstrated a higher discharge capacity (4000 mA h g ) than −1 ages and electronic transfer numbers. Interestingly, further pristine CNFs (2750 mA h g ) because of its better electri- investigations of GH-GQD-90 as a cathode electrocatalyst cal conductivity and larger mesopore size and volume. And in a primary Zinc–air battery (ZAB) showed that the tested although battery performances need to be further improved, galvanodynamical discharging of the battery can increase to this research provides a significant new design route to cre- −2 100 mA cm , demonstrating that GH-GQD-90 is an effec- ate advanced core–shell catalysts for LAB cathodes. tive ORR catalyst that can provide sufficiently high battery performances. However, GH-GQD-90 only possessed com- parable discharge properties to commercial 20 wt% Pt/C at 3 Composites of Carbon and Non‑metals −2 higher current densities (~ 20 mA cm ) and no OER was discussed in the study. To further demonstrate the advan- 3.1 Composites of Carbon and Single Elements tages of hybrid phases in the development of advanced car- bon–carbon composite electrocatalysts in metal–air batteries In this subsection, carbon materials composited with a (MABs) applications, Luo et al. [19] prepared composite non-metal heteroatom such as nitrogen (N), boron (B), or cathode catalysts using Ketjan Black (EC-600JD)–carbon phosphorus (P) are reviewed. These non-metal heteroatoms paper (KB/CP) in which the ultra-large specific discharge are usually introduced into pure carbon to form RMAB air- capacity of the KB/CP cathode in a lithium–air battery (LIB) electrode catalysts through doping strategies. As identified, was found to be much greater than that of individual KB and dopants can act as a secondary phase; providing adsorption CP cathodes. This demonstrates that synergetic effects are structures for charge transfer and improving electrocata- present between the two different carbon components even if lytic activities of ORR and/or OER [21, 22]. Recently, vari- the two components have different structural morphologies ous carbon materials (e.g., porous carbon materials, CNT, and physical properties. graphenes, carbon fibers, carbon xerogel, carbon aerogel, Aside from hybrid phases, core–shell structured compos- nanocage carbons, carbon nano-onions) have been explored ite catalysts have also been explored as MAB cathodes in as heteroatom-doped carbon catalysts for BMABs and the which one carbon material acts as the core and the other results show that these composited carbon materials exhibit carbon acts as the shell. To improve the electrochemical desirable characteristics such as desirable pore size/volume, reactions of lithium–air batteries (LABs), Song et al. [20] large surface area, and high stability. designed and synthesized a carbon nanofiber@mesoporous carbon (CNF@mesoCs) core–shell composite catalyst to 3.1.1 N‑Doped Carbons optimize the advantages and characteristics of two individ- ual carbon materials. By coating mesoporous carbon onto To promote electrochemical properties, the doping of het- carbon nanofibers (CNFs), the resulting structure not only eroatoms is often used to modify the nature and chemical enlarged surface areas but also provided a highly conduc- properties of pure carbon materials and obtain advanced tive graphitized surface to increase electrical conductivity. metal-free carbon catalysts for RMABs. As a typical heter- In their synthesis, CNFs were first produced as a core that oatom, nitrogen (N) has become the most popular dopant to entangled with one another to form a self-standing three- facilitate catalytic reactions of carbon-based composite cath- dimensional cross-linked web structure using electrospin- odes because N has a larger electronegativity than carbon, ning techniques. Nanocasting was then carried out to coat a comparable atomic size to carbon, and five valence elec- mesoporous carbon on to the CNFs to form the final CNF@ trons for bonding with carbon atoms [23, 24]. Specifically, 1 3 Electrochemical Energy Reviews (2018) 1:1–34 5 N-doped carbon materials possess structural defects and can pyrrolic (399.4 eV), and graphitic (401.0 eV) nitrogen, result withdraw electrons from carbon atoms to enhance the con- from increasing carbonizing temperatures, demonstrating ductivity of the carbon material and thus improve battery increasing ratios of graphitic nitrogen. This correlates with performance [25–29]. XRD results in which meso-NdC-1000 gives two typical To develop favorable mesoporous structures and highly peaks at (002) and (101) that are associated with a higher active carbon-based advanced catalysts for LAB cathodes, degree of graphitization as compared with meso-NdC-900. Sakaushi et al. [30] investigated ionic liquid (IL)-based Elemental analysis showed that meso-NdC-1000 possessed mesoporous nitrogen-doped carbons (meso-NdCs) with a higher N content of ca. 12 wt% than meso-NdC-900 (~ 6 a designed mesoporous structure. In their silica template wt%). Based on a combination of Ex-situ XRD measure- assisted (Ludox HS40) carbonization method, ionic liquid ments and charge–discharge tests in an assembled LAB (N-butyl-3-methylpyridinium dicyanamide) was used as cell (the cell use 0.5 M LITFSI in TEGDME (Aldrich) as the source of both N and C, and carbonization temperatures the electrolyte and was measured in the potential range of + −1 of 900 and 1000 °C were performed to prepare two sam- 2.0–4.0 V versus Li/Li at 100 and 200 mA g ), meso- ples: meso-NdC-900 and meso-NdC-1000 [29, 31]. In the NdC-1000 was found to possess a lower discharge over- structural characterizations, the measured broad XRD pat- potential of 0.3 V as compared with metal-based catalysts terns and TEM images revealed typical characteristics of in LAB and the lowest charge overpotential of 0.45 V −1 highly porous materials with an average pore diameter of ca. (Table 1). At 100 and 200 mA g , meso-NdC-1000 also 8 nm as induced by defects at the carbon walls or irregular possessed specific discharge capacities of ca. 1750 and 1280 −1 fragments [32]. Importantly, as shown in N1s XPS spec-mAh g , indicating acceptable rate capabilities in LABs. tra (Fig. 3), three N species, such as pyridinic (398.3 eV), These results suggest that meso-NdC-1000 catalysts with Fig. 3 a Schematic illustration of different chemical structures of 398.3, 399.4, and 401.0 eV corresponding to pyridinic, pyrrolic, and doped nitrogen in a graphene framework. b, c X-ray photoemission graphitic nitrogen respectively. Reprinted with permission from Ref. spectra of meso-NdC-900 and meso-NdC-1000, with the peaks at [30]. Copyright 2015 John Wiley and Sons 1 3 6 Electrochemical Energy Reviews (2018) 1:1–34 Table 1 Compared data on the a −1 −1 Catalyst η (V) Q (mAh g ) I (mA g ) Electrolyte OER disk electrochemical properties of several air electrodes. Reprinted Noble carbon 0.45 1750 100 1 M LiTFSI in TEGDME with permission from Ref. [30]. Porous Au 0.6 325 5000.1 M LiClO in DMSO Copyright 2015 John Wiley and Co O /rGO 0.6 14,000 1401 M LiPF in TEGDME 3 4 6 Sons TiC 0.8 350 2500.5 M LiClO in DMSO Co O /carbon 0.9 2000 701 M LiPF in PC 3 4 6 α-MnO /carbon 1.0 2500 701 M LiPF in PC 2 6 Carbon (Super-S) 1.8 850 701 M LiPF in PC Comparison of average overpotentials@recharging (η ) for several catalysts to fully recharge a LAB. OER Here, overpotential (V) = E − 2.96. E (V vs. Li/Li ) is the average reaction potential of a LAB at OER OER recharging, that is, an OER reaction. The value 2.96 (V vs. Li/Li ) is the electromotive force of the follow- + − ing reaction: O + 2Li + 2e ↔ Li O . The η data are selected from reports showing full discharge- 2 2 2 OER recharge measurements b + LiFePO /carbon is substituted for Li metal anode for this measurement to supply Li . Other measurements use Li metal-based anodes Q is calculated based on the mass of carbon disk increased ratios of graphitic nitrogen at higher temperatures curves from − 0.8 to 0.05 V versus Hg/HgO in O -saturated possess favorable structures induced by doped nitrogen in 0.1 M KOH solution at an electrode rotating rate of 1600 r −1 mesoporous carbon. min showed that HMC provided higher ORR kinetic-lim- −2 In addition to the N-doped mesoporous carbon materi- iting current densities (~ − 4.95 mA cm ) than benchmark −2 als discussed above, other N-doped mesoporous carbon Pt/C catalysts (~ − 4.39 mA cm ) and more positive ORR materials have also been obtained for MAB cathode cata- onset potentials (ca. − 0.055 V) than Pt/C (ca. 0.001 V). In lysts [33–38]. For instance, a novel hierarchical N-doped a ZAB with 6 M KOH assembled using a Zn electrode, a carbon ORR catalyst (labeled by N:C-MgNTA) with a gra- separator, and an air cathode, the HMC catalyst also pro- phitic shell was prepared by Eisenberg et al. [38]. These vided a charge–discharge potential gap lower than that of 40 researchers employed an in situ templating method cou- wt% Pt/C (~ 60%). These results highlight the merits of the pled with etching and pyrolysis to synthesize microporous, structure-designed HMC material with its small charge–dis- mesoporous, and macroporous structured catalysts. The charge voltage polarization and high stability over repeated coexistence of microporous, mesoporous and macroporous cycling and provide new avenues to develop cost-effective, structures was demonstrated to provide short electron and high-performance metal-free electrocatalysts for MABs. ion transport paths and increased active surfaces, benefiting Interestingly, some groups [41, 42] have used biomass to catalytic activities [39, 40]. Based on an investigation of create porous carbon cathode materials for MABs and have electrocatalytic ORR in O - and N -saturated 0.1 M KOH produced decent electrocatalytic activity and stability, as 2 2 solutions, the researchers found that the synthesized N:C- well as satisfactory battery performance. MgNTA exhibited more effective ORR activities with a Among N-doped graphene materials [43–45], a special 4e transfer mechanism and was more stable than commer- N-doped three-dimensional graphene (N-3DG) cathode cial 20 wt% Pt/C or the undoped carbon reference sample. catalyst was designed and fabricated for LABs by He et al. A spherical N-doped hollow mesoporous carbon (HMC) [45]. In their study, melamine was selected as the nitrogen material was also obtained as an efficient ZAB cathode by source due to its high nitrogen content and strong interac- Hadidi et al. [34], in which hollow and mesoporous car- tions with graphene oxide (GO) to avoid severe stacking in bon structures were combined with N-doping to obtain a the structure of the graphene nanosheets (GNS) in 3DG. novel carbon bifunctional electrocatalyst through polym- A combination of TEM and EDX mapping images of the erization and carbonization of dopamine on a sacrificial N-3DG, coupled with SEM images, revealed a highly porous spherical silica (SiO ) template. Here, both polydopamine two-dimensional structure for the nitrogen-doped graphene (PDA) and F127 acted as excellent carbon sources and PDA nanosheets with a homogeneous distribution of carbon, afforded the doping of N. A combination of SEM, TEM and nitrogen and oxygen elements. Further comparisons of the high-resolution TEM (HR-TEM) images revealed a hollow N-3DG and 3DG samples in N adsorption–desorption anal- mesoporous sphere after the silica was removed in hydro- ysis indicated that due to the relatively severe stacking of fluoric acid solution. Carbon shells (ca. 21 nm ± 28%) were GNSs in the 3DG, more mesopores and micropores favoring also formed after carbonization, revealing graphitic and fast O diffusion and electrolyte infiltration were formed in amorphous domains. Linear sweep voltammetry (LSV) the N-3DG than the 3DG. In XPS, the revealed pyridinic 1 3 Electrochemical Energy Reviews (2018) 1:1–34 7 N (~ 54%) was regarded as an effective N type to improve mechanical and cycling stabilities with low overpotentials (a ORR in the graphene plane or edge because it can donate high discharge and low charge voltage of ~ 1.00 and 1.78 V −2 more available lone electron pairs for effective oxygen acti- at 2 mA cm , respectively) and long cycle life (6 h, and vation [46]. Furthermore, according to first-principle com- can be recharged by the mechanical replacement of the Zn putations [47], carbon sheets with pyridine N were found to anode) even under repeated bending conditions. The synthe- be more thermodynamically favorable in attracting Li and sized N-dope CP cathodes were tested in an assembled LAB lithiated pyridinic N can provide excellent active sites for O with a non-aqueous electrolyte (1 M lithium perchlorate in adsorption and activation in the discharge process of LABs. dimethyl sulfoxide) and produced a cyclability of more than −2 During testing of the N-3DG and 3DG-based cathodes in a 30 cycles at a constant current density of 0.1 mA cm . The −1 LAB, N-3DG demonstrated a higher ORR onset potential first discharge capacity reached 8040 mAh g with a cell −2 and a higher cathodic peak current, implying higher ORR voltage of ~ 2.81 V at 0.1 mA cm , and the coulombic −1 −2 activities than 3DG. At 50, 100, and 200 mA g , the tested efficiency was 81% on the first cycle at 0.2 mA cm . These discharge capacities of N-3DG were 7300, 5110, and 3900 results indicate that N-doped CP materials are promising −1 mAh g , respectively, corresponding to average operating in the development of low-cost, versatile paper-based O voltages at 2.67, 2.64, and 2.56 V. These were all higher electrodes for LABs. than those of 3DG. In cycling performance measurements −1 −1 with a cutoff capacity of 500 mAh g at 100 mA g , the 3.1.2 Other Heteroatom‑Doped Carbons N-3DG cathode exhibited better cycling performances over 21 cycles with a more stable reversible capacity and a better Like nitrogen, other heteroatoms such as boron (B), phos- initial round-trip efficiency of 65.9%, whereas 3DG only phorus (P), sulfur (S), and fluorine (F) have also been selec- performed for about 8 cycles with an initial cycle round-trip tively used as doped carbon materials for MABs to produce efficiency of 60.7%. different physicochemical properties. In contrast to N, B Similar to porous carbon and graphene, CNTs [27, possesses lower electronegativity (~ 2.04) than C (~ 2.55) 48, 49], CNFs [50–52], and carbon papers [53] have also [54], and if doped to carbon, B becomes an electron accep- attracted much research interest in the development of tor, inducing a positive charge and resulting in charged sites N-doped carbon cathodes for MABs. Mi et al. [27] used (B ) that favor O adsorption and thus increased ORR activ- a floating catalyst chemical vapor deposition (FCCVD) ity [55, 56]. To study the important effects of B-doping and method to synthesize a N-CNTs composite material in to create sufficient active sites, researchers [ 54, 57, 58] have which ethylene and melamine were used as the carbon and focused their attention on combining mesoporous carbon −1 nitrogen sources. In a discharge capacity test at 100 mA g structures with B-doping to explore advanced metal-free using both carbonate-based (propylene, carbonate/ethylene, carbon catalysts. For example, Shu et al. [57] prepared carbonate, PC/EC) and ether-based (1,3-dioxolane/ethylene novel mesoporous boron-doped onion-like carbon (B-OLC) glycol dimethyl ether) electrolytes, N-CNTs produced bet- microspheres using nanodiamond and boric acid as C and ter discharge capacities in both electrolytes compared with B sources for rechargeable sodium–oxygen (Na–O ) bat- CNTs. This is because of the better dispersion of N-doping teries. In their structural characterizations, a comparison and more available sites for O adsorption and reduction, of HRTEM images and selected area electron diffraction as well as a better electrical conductivity facilitating the (SAED) patterns of their B-OLC and OLC samples revealed reduction of kinetics during discharge. The enhancement the existence of multilayered sp fullerene shells forming a of ORR activity after N-doping was attributed to three quasi-spherical onion-like nanoparticle with particle sizes main reasons: improved conductivity, more nucleation sites of 5–8 nm. The appearance of the graphite ring (002) in around nitrogen and less aggregation of discharge products. the SAED patterns also confirmed the effective conversion CNFs and CPs were subsequently used as N-doped carbon of nanodiamonds into sp carbon phases with an identified cathode catalysts in MABs [50–53] and the nitrogen-doped interlayer spacing (~ 0.34 nm) that did not change after CNFs tested as the air cathode in a liquid ZAB operating B-doping [59, 60]. Based on the B 1s spectrum in XPS, six in ambient air produced a maximum power density of 185 deconvoluted peaks were obtained at 185.5, 188.8, 190.2, −2 mW cm and a maximum energy density of ~ 776 Wh 191.5, 192.9 and 194.8 eV, corresponding to BC, BC , 4 3 −1 kg with a high open-circuit voltage (1.48 V) [50]. The BCO, BCO and B O respectively [61, 62]. The six spe- 2 2 2 3 corresponding rechargeable liquid ZAB showed a small cies of B were also found in the XPS spectra of the refer- −2 charge–discharge voltage gap (0.73 V at 10 mA cm ), high enced B-doped super P (B-Super P), confirming the suc- reversibility (initial round-trip efficiency of 62%) and sta- cessful doping of boron, in which B-super P possessed a bility (voltage gap increased to ~ 0.13 V after 500 cycles). lower content (~ 0.83 at%) than B-OLC (~ 5.47 at%). At −2 Moreover, other flexible all-solid-state rechargeable ZABs 0.15 mA cm , the obtained discharge/charge curves in an using N-doped CNF films as a cathode displayed excellent assembled sodium–air battery showed that compared with 1 3 8 Electrochemical Energy Reviews (2018) 1:1–34 OLC and B-Super P cathodes, the B-OLC cathode delivered that the ORR activity increased significantly in the order: −1 a higher discharge capacity (~ 10,200 mAh g ) and a lower pure carbon < P-C-1 < P-C-2 < P-C-5 < P-C-4 < P-C-3. overpotential, demonstrating higher bifunctional activities Moreover, P-C-3 possessed a shift of ~ 70 mV in the half- toward ORR and OER. In the investigation of rate capabili- wave potential as compared with 20 wt% Pt/C. Tested LSVs −2 −1 ties, it was found that at 0.6 mA cm , B-OLC presented in O -saturated 0.1 M KOH with a scan rate of 10 mV s −1 a discharge capacity of ~ 7455 mAh g . This was nearly at different electrode rotating rates demonstrated that the −1 40 times greater than that of Super P (~ 160 mAh g ) and P-C-3 catalyst provided an ORR electron transfer number −1 about twice that of OLC (~ 3558 mAh g ). This enhanced of ~ 3.81, suggesting a four-electron reaction pathway for rate capability was attributed to the synergistic effects P-C-3 in ORR similar to 20 wt% Pt/C. According to DFT between B doping, mesoporous structures and particle calculations, the amount of P-doping was critical in the sizes of OLC. Interestingly, B-Super P produced similar improvement of ORR in P-doped carbon xerogels. Based battery performances compared to Super P. This is possibly on ORR behaviors in which ORR activity increased with because a low B doping content (~ 0.83 at%) produces no increasing P content from 0.78 at% (P-C-1) to 1.64 at% effects on super P. In a further examination of reversibility, (P-C-3) and then decreased with increasing P content from −2 cycling performance tests were conducted at 0.3 mA cm , 2.77 at% (P-C-4) to 3.56 at% (P-C-5), the optimal P content and B-OLC was found to possessed better cyclability (up should be 1.64 at%, providing maximum ORR activity. to 125 cycles) than OLC (~ 56 cycles) and Super P (~ 6 To obtain optimal bifunctional catalysts, an important cycles), demonstrating the combined effects of B-doping and comparison of the different types of heteroatom-doping mesoporous structures. was conducted by Su et al. [58], in which N, B, and P were P is an electron donor even though the electronegativity used to dope ordered mesoporous carbons (OMCs). The of P (~ 2.19) is lower than C (~ 2.55), and the incorpora- researchers found that improvements to charge transfer tion of P into carbon by P-doping is easier than that of N kinetics is strongly depended on the nature of the doped or B due to a much larger covalent radium [(107 ± 3) pm] heteroatom. At a doping level below 1.0 at%, the ORR than N [(71 ± 1) pm] and B [(84 ± 3) pm]. Similar to the activity tested in alkaline solutions increased in the order combination of B-doping (or N-doping) and porous struc- of: N-OMCs < P-OMCs < B-OMCs. No data was reported tures, P-doping is used in combination with porous struc- for stability in their study, however. Another study was con- tures in the development of carbon-containing bifunc- ducted by Zhang et al. [69] on heteroatom(s)-doped graph- tional composite catalysts for MABs [63–65]. Using a diyne, in which the dopants were N, S, B, and F. However, sol–gel polymerization method followed by pyrolysis and this research only focused on the comparison between single P-doping, Wu et al. [63] developed a low-cost and highly heteroatom and dual atom-doped graphdiyne cathode mate- active P-doped carbon xerogel electrocatalyst to examine rials. These will be discussed in the next section. the effects of P-doping and porous structures on ORR. In their synthesis, resorcinol and formaldehyde were used as 3.2 Composites of Carbon and Dual Elements the carbon sources and H PO was used as the phospho- 3 4 rus source. The as-prepared samples were labeled as P-C-1, Compared with single heteroatom-doped carbon materials P-C-2, P-C-3, P-C-4 and P-C-5, corresponding to weight with ORR activities that are inferior to conventional Pt/C ratios of 1:10, 2:10, 3:10, 4:10, and 5:10 phosphoric acid catalysts, especially in acidic media and neutral solutions, to carbonized sample (Co–C). In their structure characteri- doped carbon materials with two or more selected heter- zations, a combination of XRD and Raman spectroscopy oatoms have been predicted to further improve ORR activ- confirmed increased defects and disordered carbon after ity due to the synergetic effects arising from the co-doping P-doping corresponding with increasing P contents from of two or more heteroatoms in carbon [70]. Based on the 0.78 to 3.56 at%. The P content measured by inductively fact that co-doping N-doped carbon with a second heter- coupled plasma-atomic emission spectroscopy (ICP-MS) oatom (e.g., B, S or P) may improve ORR activity [71–73], were 0.78, 1.41, 1.64, 2.77, and 3.56 at% for P-C-1, P-C-2, Zhang et al. [74] conducted a scalable fabrication of three- P-C-3, P-C-4, and P-C-5 samples, respectively. In the P 2p dimensional N and P co-doped mesoporous nanocarbons spectra of XPS, two deconvolved contributions at 132.5 and (NPMC foams) and investigated the OER of these co-doped 134.5 eV can be assigned to P–C bonding and P–O bonding carbon materials. In a template-free method, three N, P co- [66, 67], respectively. The presence of P–O bonding con- doped mesoporous carbon (NPMC) foams (i.e., NPMC-900, firms the formation of P–O–C in the forms of CPO, C PO , NPMC-1000, and NPMC-1100) were prepared at anneal- 3 2 2 and CPO , indicating the successful doping of P [68]. In ing temperatures of 900, 1000, and 1100 °C, respectively. the examination of ORR at room temperature using rotating TEM images coupled with elemental mapping for the typi- ring-disk electrode (RRDE) technique, the comparison of cal NPMC-1000 sample revealed a uniform distribution of onset potential and diffusion limiting current density showed C, N and P. A combination of XRD, Raman spectra, and 1 3 Electrochemical Energy Reviews (2018) 1:1–34 9 TEM indicated that pyrolysis can resulted in a majority of were used as the N and B sources. In their investigation of the thermally stable graphic carbon domains being occupied co-doping, a combination of TEM, SEM, electron energy by the co-dopants of N and P from the PANI and phytic loss spectroscopy (EELS), and elemental mapping confirmed acid. Moreover, many edge-like graphitic structures were the uniform distribution of B and N in a preserved graphene found in the HRTEM images of the examined NPMC-1000 nanosheet morphology, and FTIR revealed the existence of sample which oversaw active sites and thus catalytic activ- B and N coupled to C, indicating that by-products such as ity. It was found that these NPMC samples possessed three- h-BN did not exist in the synthesized B, N-graphene sample. dimensional mesoporous structures with large surface areas, This was also confirmed by XPS in which a one-step syn - high pore volume and suitable pore sizes for electrocata- thesized h-BN/graphene reference sample clearly showed lytic activity. Both the existence of the four N species (i.e., the h-BN phase. Cyclic voltammetry curves measured in pyridinic, pyrrolic, graphitic, and oxidized pyridinic N) and an O -saturated 0.1 M KOH showed that B, N-graphene the two P-related bonds (i.e., P–C and P–O) [75] observed possessed a higher cathodic current density, indicating bet- in XPS confirmed the successful doping of N and P het- ter ORR activity than h-BN/graphene. A series of LSVs in eroatoms into the carbon network through thermal pyroly- O -saturated 0.1 M KOH solution were collected using a −1 sis. Based on further LSV curves obtained at 1600 r min RDE technique, and it was observed that the onset potential in O -saturated 0.1 M KOH solution with a scan rate of of B, N-graphene was closer to commercial Pt/C and higher −1 5 mV s , NPMC-1000 was found to provide the best ORR than the one-step synthesized single heteroatom-doped gra- activity with comparable onset potentials (~ 0.94 V) and phenes (such as N-graphene and B-graphene). These results half-wave potentials (~ 0.85 V) to Pt/C. Although NPMC- suggest that the co-doping of B and N can result in higher 900 possessed a higher N and P content than NPMC-1000, synergistic activities than single heteroatom-doped graphene the relatively low pyrolysis temperature led to a possible and that N-doping in N-graphene is more effective on the higher charge transfer resistance and thus relatively poorer improvement of activity than B-doping in B-graphene. RDE electrocatalytic activity. As for NPMC-1100, the removal experiments at different rotation rates ranging from 500 to −1 of doped heteroatoms at higher temperatures (~ 1100 °C) 2500 r min revealed an electron transfer number (~ 3.97) resulted in a decrease in active sites and therefore a decrease for B, N-graphene similar to commercial Pt/C (~ 3.98) and a in overall electrocatalytic activity. For OER, NPMC-1000 RRDE technique was further used to confirm the ORR path- and NPMC-1100 samples displayed lower onset potentials way. Importantly, their DFT calculations revealed significant and higher currents, demonstrating better OER activities synergistic effects resulting from the coupling interactions than Pt/C. Compared with RuO nanoparticles, NPMC-1000 between pyridinic N (one of N species) and B. electrodes exhibited lower onset potentials but less current Similar to N and P, oxygen (O) also possesses higher densities at greater potentials. Interestingly, as a cathode electronegativity than carbon. Based on the effects of co- material in primary and rechargeable ZABs, NPMC-1000 doping, Li et al. [23] developed a novel 3D O, N co-doped primary batteries demonstrated an open-circuit potential of carbon nanoweb (ON-CNW) material as a metal-free cata- −1 1.48 V with a specific capacity of 735 mAh g (correspond- lyst for hybrid LABs. In the deconvolution of N1s and O1s Zn −1 ing to an energy density of 835 Wh kg ), a peak power den- spectra of XPS, the content of pyridone/pyrrolic on ON- Zn −2 −2 sity of 55 mW cm (at a current density of ~ 70 mA cm ), CNW was about three times that of a referenced N-doped and stable operations for 240 h after mechanical recharging. carbon nanoweb (N-CNW) material. The pyridone nitrogen For NPMC-1000 rechargeable ZABs, the battery cycled sta- was reported to stabilize singlet dioxygen by forming a sta- bly for 180 discharge/charge cycles over a period of 30 h at ble adduct, weakening and breaking the bond in the oxygen −2 2 mA cm . According to DFT calculations, these tested bat- molecule [79, 80] so that the activation process using KOH tery performances are induced by the synergistic effects of can further improve ORR activity. CV measurements using N, P co-doping along with graphene edges for the improve- N-doped carbon nanosphere (N-CNS) catalysts were run in ment of bifunctional electrocatalytic activity toward OER O -saturated 0.1 M KOH, and only a single ORR peak poten- and ORR [76]. tial (~ 0.4 V) was observed. This peak was weaker than that Compared with N, P co-doping, N, B co-doping into car- of N-CNW(~ 0.29 V), indicating a difference in morphology bon materials is more difficult because of the formation of and structure for the two PPY-derived carbon materials. The by-products (e.g., hexagonal boron nitride, h-BN) during N-CNS was also found to possess greater particle resistance doping that can act as inert de-activating agents that reduce and reduced mass transport in the packed carbon spheres, the electrochemical activity of catalysts [77]. To incorporate whereas the 3D interconnected N-doped carbon fibers in heteroatoms at select sites of graphene frameworks and pre- the N-CNW increased electron transport, oxygen transport vent the formation of inactive by-products, Zheng et al. [78] and electrolyte diffusion. In a further comparison of ORR prepared a B, N co-doped-graphene (B, N-graphene) catalyst peak potentials, ON-CNW presented a positive shift pos- using a two-step doping strategy in which NH and H BO sibly due to a higher pyridone/pyrrolic content (~ 20.14%). 3 3 3 1 3 10 Electrochemical Energy Reviews (2018) 1:1–34 The N-CNW and ON-CNW samples were subsequently sites. To obtain simultaneous optimization of both porous evaluated as cathode catalysts in an assembled hybrid LAB structures and surface functionalities of N and S co-doped and the discharge voltage profiles of the cell revealed an carbon, Wu et al. [81] designed and fabricated a polyquater- apparent reduction in the difference between ON-CNW nium-derived heteroatom (N and S) co-doped hierarchically and Pt/C samples, demonstrating that ON-CNW possessed porous carbon (N-S-HPC) catalyst for ORR using a simple, performances close to that of Pt/C catalysts. At 0.5, 1.0, large-scale and green synthetic route with the assistance −2 1.5, and 2.0 mA cm (Fig. 4a). the discharge plateau of of a silica template. In their electrochemical evaluation of ON-CNW is lower than that of Pt/C but higher than those ORR, the LSV curves in O -saturated 0.1 M KOH at 1600 r −1 −1 of acetylene black and N-CNW, suggesting that ON-CNW min with a scan rate of 5 mV s and a catalyst loading of −2 possesses higher ORR activities than N-CNW and carbon 500 μg cm were recorded and compared. Compared with black. A compared cycling test was carried out in a hybrid the two referenced samples [N-doped hierarchically porous −2 LAB with a constant current density of 0.5 mA cm and carbon (N-HPC) and N, S-doped porous carbon (N-S-PC)], the round-trip overpotential for N-CNW (Fig. 4b) increased N-S-HPC presented a higher onset potential (~ 0.99 V) and a from 1.00 to 1.43 V, whereas ON-CNW (Fig. 4c) increased higher half-wave potential (~ 0.86 V), demonstrating higher from 0.92 to 1.02 V, demonstrating the improved effects of ORR activity. In particular, even at a low catalyst loading −2 O, N co-doping on activity and stability, and also confirming of 100 μg cm , N-S-HPC still exhibited a better half-wave that ON-CNW is a promising ORR catalyst in hybrid LABs. potential (~ 0.83 V) and a greater diffusion-limited current −2 The co-doping of N and S [81–83] has also been dem- density (~ 7.5 mA cm ) than those of commercial 20 wt% −2 onstrated to provide benefits in the redistribution of spin Pt/C (~ 0.82 V, ~ 5.5 mA cm ), suggesting that N-S-HPC and charge densities and the formation of more ORR active possesses comparable ORR performance to Pt/C in alkaline Fig. 4 a Discharge voltage profiles of hybrid LAB with Pt/C, N-CNW or c ON-CNW as the ORR catalyst. Reprinted with permis- N-CNW, ON-CNW, or carbon black as the ORR catalyst at differ - sion from Ref. [23]. Copyright 2014 John Wiley and Sons ent current densities. Cycling performance of hybrid LABs with b 1 3 Electrochemical Energy Reviews (2018) 1:1–34 11 solutions. Interestingly, instead of 0.1 M KOH solution, an and H PtCl were used as Au and Pt sources to prepare 40 2 6 acidic medium (i.e., 0.5 M H SO solution) was also used wt% PtAu/C and TEM images revealed a uniform distribu- 2 4 in the evaluation of the ORR performance of N-S-HPC. tion of PtAu nanoparticles [(~ 6.8 ± 1.4) nm] on carbon The obtained CV curves in O -saturated 0.5 M H SO solu- with XRD data (Fig. 5a), indicating a solid-solution com- 2 2 4 tion showed that N-S-HPC produced a slightly lower ORR posed of Pt and Au corresponding to Pt Au . From the 0.5 0.5 activity than Pt/C as evidenced by the half-wave poten- CV curves (Fig. 5b) of the 40 wt% PtAu/C, the calculated tial (~ 0.73 V) as compared with the half-wave potential electrochemical surface area (ESA) of the Pt and Au was 2 −1 (~ 0.78 V) of Pt/C. To demonstrate the potential of N-S-HPC (38 ± 4) m g and correlated with the dispersion of PtAu PtAu catalysts in MABs, a ZAB with 6 M KOH was assembled for observed in the TEM. In their investigation of ORR/OER further electrochemical measurements using N-S-HPC as the using an assembled LAB, a comparison of first discharge air–cathode catalyst. The peak power density obtained using and charge voltages (Fig. 5c) showed that the discharge volt- −2 N-S-HPC was ~ 536 mW cm and is drastically superior to age of 40 wt% PtAu/C is comparable to that of 40 wt% Au/C, −2 Pt/C (~ 145 mW cm ). Therefore, the co-doping of N, S can and the charge voltage of 40 wt% PtAu/C is comparable to result in an optimized combination of porous structures and that of 40 wt% Pt/C. Moreover, 40 wt% PtAu/C as a cath- surface functionalities for N and S and thus the enhancement ode catalyst provided higher round-trip ec ffi iencies than pure of electrocatalytic activity. carbon even though both possessed similar specific capaci- −1 Zhang et al. [69] carried out experiments in synthesizing ties in the first cycle (~ 1200 mAh g ). This suggests carbon and comparing three different co-doped carbon catalysts: N, that the incorporation of Au into Pt surfaces can result in S co-doped graphdiyne (NSGD), N, B co-doped graphdiyne improved ORR and OER kinetics of 40 wt% PtAu/C. How- (NBGD), and N, F co-doped graphdiyne (NFGD). After a ever, at increasing charging cycles, the charge voltage of series of electrochemical measurements in an assembled 40 wt% PtAu/C was lower than that of 40 wt% Pt/C and if primary ZABs, the ORR activity of NFGD was found to be current densities were decreased, discharge and charge volt- comparable to that of commercial Pt/C (20 wt% Pt on Vulcan ages reduced considerably (Fig. 5d). This research crucially XC-72) and superior to those of the other two dual-doped demonstrates that PtAu/C is responsible for both ORR and GDs. A first-principle study to theoretically demonstrate the OER after the incorporation of Au into Pt atoms on the nan- synergistically enhanced catalytic effects of co-doping in B, oparticle surface, providing a reasonable strategy to develop P co-doped graphene was also carried out to compare with bifunctional catalysts for RMABs. To further understand the B-doped graphene and P-doped graphene [22]. effects of Pt and Au on ORR and OER, a Pt–Au alloy nano- particle catalyst supported on carbon black (Vulcan XC-72) [87] was found to played an important role in the charge/ 4 Composites of Carbon and Metals discharge performance of rechargeable LABs owing to their nanoscale alloying and phase properties inducing synergistic 4.1 Composites of Carbon and Noble Metals effects between the AuPt alloy and C. or Noble Metal‑Alloys Similar to Au, Co has also been used to form Pt–Co alloy nanoparticles supported on Vulcan XC-72 carbon in the 4.1.1 Composites of Carbon and Pt or Pt‑Alloys application of LABs. To fabricate Pt Co /Vulcan XC-72 x y catalysts with a loading of 20 wt%, Su et al. [89] conducted The composites of carbon and Pt have commonly been uti- a chemical reduction method to deposit a series of Pt Co x y lized as carbon-supported Pt fuel cell electrocatalysts for (x:y = 4, 2, 1, and 0.5) alloy nanoparticles onto Vulcan ORR in which different carbon materials act as high sur - XC-72 using H PtCl ·6H O and CoCl ·6H O as Pt and Co 2 6 2 2 2 face area substrates for the structuring and proper disper- sources. XRD confirmed the complete incorporation of Co sion of Pt nanoparticles to form carbon-supported Pt-based into the fcc crystal structure of Pt with an average crystallite catalysts. However, it has been well documented that Pt/C size for all catalyst samples being in the range of 5–8 nm. alone is not an efficient bifunctional electrocatalyst because This was also in agreement with TEM and HRTEM analysis. of its insufficient OER performances [84– 86]. To address A combination of SEM and TEM showed that all Pt Co /C x y the primary challenges of OER, metals such as Au [8, 87, samples possessed carbon nanoparticles with similar mor- 88], Co [89], Zn [90], Ir [7, 91], Pd [91], and Ru [91] have phologies in the size range of 50–100 nm and presented been used to form carbon-supported Pt-alloy catalysts in uniform distribution of Pt–Co alloy nanoparticles. In electro- various metal–air batteries. Using the active effects of Au chemical performance tests in LABs, the discharge capacity on the surface modification of Pt, Lu et al. [ 8] combined Au of the PtCo /C cathode was found to have decreased from −1 and Pt into individual PtAu nanoparticles on the surface of 2578 to 2074 mAh g after 5 cycles, whereas the referenced −1 carbon and evaluated the ORR and OER activities of the Vulcan XC-72 cathode degraded from 965 to 376 mAh g , resulting 40 wt% PtAu/C in LABs. In their study, HAuCl demonstrating that PtCo /C possessed much better capacity 4 2 1 3 12 Electrochemical Energy Reviews (2018) 1:1–34 −1 Fig. 5 a TEM image (top right) and XRD data of 40 wt% PtAu/C, charge profile of carbon at 85 mA g and of 40 wt% Au/C, 40 carbon −1 b cyclic voltammograms of 40 wt% PtAu/C collected in Ar-saturated wt% Pt/C, and 40 wt% PtAu/C at 100 mA g , d LAB discharge/ carbon 0.5 M H SO between 0.05 and 1.7 V versus RHE (room temperature charge profiles (first cycle) of 40 wt% PtAu/C at 50, 100, and 250 mA 2 4 −1 and 50 mV/s). insets: (left) HRTEM image of 40 wt% PtAu/C and g . Reprinted with permission from Ref. [8]. Copyright 2010 carbon (right) schematic representation of Pt–Au, with arrows indicating the American Chemical Society CV signatures for Pt (gray) and Au (yellow), c LAB first discharge/ retention. Among all Pt Co /C samples, PtCo /C achieved increased, leading to enhancements in both OER and ORR x y 2 −1 a maximum capacity of 3040 mAh g . This capacity was activities. Cycling tests in LABs also confirmed better cycla- −1 higher than that of the PtAu/C (1200 mAh g ) developed bility performances with Pt Co /C as compared with bare x y by Lu et al. [8]. This indicates that compared with Au, Co Vulcan XC-72. To further understand and obtain high ORR should be more suitable in Pt-alloys for the improvement of and OER activities from Pt-alloys with non-noble metals, both ORR and OER. In further analyses of discharge and Zhang et al. [90] synthesized PtZn/carbon aerogel alloy charge curves in the second cycle, Pt Co /C samples pos- catalysts and measured the electrochemical performance x y sessed much lower charge voltages (~ 3.94 V) and over- of these catalysts (PtCo/carbon aerogel, Pt/carbon aerogel potentials (~ 1.21 V) than Vulcan XC-72 with specific and Pt/carbon black) in magnesium–air batteries. In their capacities increasing positively with increasing Co con- study, the PtZn/carbon aerogel demonstrated a higher spe- −1 tent, indicating improved catalytic activities toward ORR cific discharge capacity (1349.5 mAh g ) than the PtCo/ −1 and OER in LABs. Along with increasing Co content near carbon aerogel (~ 1283.38 mAh g ), Pt/carbon aerogel −1 Pt atoms, surface Pt electron density and segregation also (~ 1113.53 mAh g ), and Pt/carbon black (~ 997.01 mAh 1 3 Electrochemical Energy Reviews (2018) 1:1–34 13 −1 g ) catalysts, suggesting that PtZn/carbon aerogel pos- DHG. Two large oxidation peaks at 3.25 and 4.37 V of Ir@ sessed better ORR and OER activities. DHG also indicated much higher anodic currents. These As a noble metal, iridium also was used to alloy with Pt results illustrated that Ir@DHG possesses superior OER to prepare carbon-supported Pt–Ir catalysts for rechargeable activity compared with DHG. Cycling performances at a −1 LABs by Ke et al. [92], in which H PtCl ·H O and IrCl current density of 2000 mA g with a limited capacity of 2 6 2 3 −1 were used as Pt and Ir sources. In a comparison of OER, 1000 mAh g revealed that Ir@DHG can run 150 cycles current–potential curves tested in a Ar-saturated 0.1 M KOH with terminal voltages greater than 2.5 V, demonstrating solution revealed that Pt/C provided a higher onset potential that Ir@DHG also possessed better reversibility and cycling (~ 0.66 V) than Pt–Ir/C (~ 0.53 V) and Ir/C (~ 0.50 V) cata- performance than DHG. lysts, indicating that Pt–Ir/C possessed better OER. For a Ruthenium (Ru) [96, 97] and palladium (Pd) [98–100] comparison of r fi st cycle discharge–charge potential prol fi es have also been used to functionalize carbon to reduce over- in discharge–charge measurements run in a LAB, Pt–Ir/C potentials and capacity decays during round-trip cycling in demonstrated the least discharge overpotential (~ 0.15 V) rechargeable LABs. Using a type of carbonized bacterial during the discharge process whereas at the end of the charg- cellulose (CBC) as a carbon support, Tong et al. [96] pre- ing process, Pt–Ir/C possessed the lowest potential differ - pared a 5 wt% Ru/CBC composite as the binder-free cath- ence (~ 0.727 V) between the discharge and charge poten- ode in non-aqueous LABs in which Ru nanoparticles can tials. Therefore, Pt–Ir/C clearly exhibits enhanced ORR and provide active sites for both ORR and OER, and CBC can OER. provide transport pathways for both electrons and oxygen. Both BET and TEM measurements showed a high surface 2 −1 4.1.2 Composites of Carbon and Other Noble Metals area of 397.6 m g and a porous network structure for or Alloys the 5 wt% Ru/CBC catalyst, favoring the distribution of active sites and mass transport. The obtained galvanostatic −1 Aside from Pt and Pt-alloys, other noble metals (Ir, Ru, discharge-recharge curves at a current density of 200 mA g Pd, Au and Ag) and alloys have also been used to prepare displayed a narrower discharge–charge potential gap than noble metal-based electrocatalysts supported on carbon that of CBC, indicating that 5wt% Ru/CBC possessed better materials for MABs. Using Ir as a catalytic noble metal ORR and OER activities. Corresponding SEM images of to functionalize catalytic surfaces of carbon, Zhou et al. 5wt% Ru/CBC after charging showed uniform deposition of [93] introduced nano-sized iridium (Ir) catalysts into a discharge products (i.e., Li O ) on the CBC surface, demon- 2 2 three-dimensional porous graphene for LABs using a non- strating the strong interaction between Li O and CBC and 2 2 aqueous electrolyte in which iridium functionalized deoxy- the improvement of stability of the tested LAB with 5 wt% genated hierarchical graphene (Ir@DHG) was synthesized Ru/CBC. Similar to Ru functionalized CBC catalysts, Pd- using a vacuum-promoted exfoliation method followed catalyzed carbon nanofibers (Pd/CNF) catalysts were also by a deoxygenation process. The Ir@DHG was tested to prepared for alkaline MABs by Alegre et al. [98] in which 2 −1 possess a high BET surface area (~ 372.5 m g ) and its a commercial Vulcan carbon was employed as a reference nitrogen adsorption–desorption curves showed pore sizes support material to support Pd nanoparticles. Characteriza- ranging from 2 to 200 nm, indicating the presence of a fer- tion by XRD, TEM, and SEM showed that the Pd/CNF and tile mesoporous and macroporous architecture favoring the Pd/Vulcan catalysts possessed typical fcc structures of Pd transport of oxygen, electrolytes and electrons to and from and the calculated crystallite sizes of the Pd/CNF and Pd/ catalytic sites during discharge and charge. According to Vulcan catalysts were 6.1 and 6.5 nm, respectively. In the XPS results, the Ir@DHG catalyst displayed smaller peaks examination of ORR and OER in a half-cell configuration, for C1s. This is possibly because Ir can closely cover gra- two full polarization curves obtained from 1.2 to 0.3 V under phene and decrease the exposure of carbon surfaces. Impor- ORR and from 1.2 to 2.0 V under OER showed that Pd/CNF tantly, Ir@DHG possessed a slightly lower intensity for O1s possessed slightly smaller overpotentials than Pd/Vulcan and than DHG, suggesting that the graphene in Ir@DHG can was therefore slightly more active. This was possibly attrib- maintain a highly deoxygenated surface and enhance the uted to the higher electrical conductivity and/or smaller Pd stability of the electrode/electrolyte interface [94, 95]. The size of Pd/CNF, favoring better charge transfers and thus high-resolution XPS spectrum of Ir 4f presented dominat- improved ORR and OER. Further charge/discharge cycle ing Ir(0) 4f and Ir(0) 4f with factional Ir(IV) 4f and tests showed that Pd/CNF could run more cycles than Pd/ 5/2 7/2 5/2 Ir(IV) 4f . This suggests a certain degree of surface oxida- Vulcan and was therefore more stable as well. Au [101] and 7/2 tion. TEM showed the uniform distribution of Ir nanopar- Ag [102] were also used to functionalize carbon materials ticles (~ 2.08 nm) onto the entire graphene plane and CV in the application of MABs. A recent study [97] of Ru- and −1 tests in LABs at 1 mV s showed that Ir@DHG possessed Pd-catalyzed carbon nanotube fabrics showed that Ru- and higher current peaks, indicating better ORR activity than Pd-CNT catalyzed cathodes for LABs do not show visible 1 3 14 Electrochemical Energy Reviews (2018) 1:1–34 improvements. This is possibly because the presence of favoring cycling performances. The incorporation of transi- noble metal catalysts impairs the reversibility of cells, caus- tional non-noble metals such as Co and Ni can also improve ing a decrease in O recovery efficiency (the ratio between the cycle life of MABs, as evidenced in Table 2. And the amount of O evolved during charge and the amount con- although noble metal-based bifunctional catalysts incorpo- sumed in the preceding discharge) coupled with an increase rated by other noble metal component(s) can play an impor- in CO evolution during charging. tant role in improving the bifunctional activity and stability Because carbon-supported Pt-based binary metal cata- of ORR/OER, their high price and scarcity present barriers lysts exhibit active ORR and OER activities for enhanced for scale-up deployment in MAB commercialization. LAB performance, carbon-supported non-platinum binary metal catalysts such as Pd Pb/C [103] and PdIr/C [91] have 4.2 Composites of Carbon and Non‑noble Metals also been studied for carbon-composited bifunctional cata- lysts in MABs. To replace expensive Pt-based alloy cata- Non-noble metals have been incorporated into carbon mate- lysts, Cui et al. [103] synthesized a Pd Pb/C catalyst using rials to enhance both ORR and OER, reduce carbon cor- a modified impregnation-reduction method in which they rosion, and improve reversibility and rate capabilities of carefully controlled the experimental conditions (e.g., tem- carbon-composited bifunctional catalysts for MABs. These perature, time, reduction agent, metal precursor) to obtain materials are inexpensive, easily fabricated, and possess a structurally ordered intermetallic phase that provided good interactions with carbon. For example, tantalum (Ta) uniform active sites on the same surface plane. In char- was found to reduce battery polarization and overpotential acterizations, XRD confirmed the ordered Pd Pb/C struc- by Yu et al. [105] in which they developed vertically aligned ture and TEM images showed that Pd Pb/C possessed a CNTs (VACNTs) with ultra-lengths on permeable Ta foils uniform particle distribution with an average particle size (VACNTs-Ta) as air cathodes for non-aqueous secondary of (7.2 ± 0.5) nm. The particle sizes of the Pd/C and Pt/C LABs using a thermal chemical vapor deposition (TCVD) catalysts were (6.3 ± 0.4) and (5.1 ± 0.3) nm, respectively. method. In their study, CNT powder (CNT-P, 20–40 μm Polarization curves in O -saturated 0.1 M KOH solution at in length and 15 nm in outer diameter) and commercial −1 1600 r min revealed that Pd Pb/C was more active than VACNT on a stainless steel (SS) mesh substrate (VACNTs- both Pd/C and Pt/C catalysts, evidenced by a more positive SS) were used as the reference samples. Compared with the onset potential and a greater half-wave potential (~ 0.92 V) VACNTs-SS and CNT-P samples, VACNTs-Ta presented than Pd/C (~ 0.88 V) and Pt/C (~ 0.88 V) catalysts. Impor- a lower ratio of D band and G band as observed by Raman tantly, after the incorporation of Pd, Pd Pb/C gained better spectroscopy, demonstrating that VACNTs-Ta possessed less catalytic activities toward ORR and OER and possessed bet- surface defects and lower reactivity with O reduction spe- ter durability than Pt/C. This was confirmed in the meas- cies. The combination of BET and BJH revealed the coex- ured discharge and charge voltage profiles of an assembled istence of micropores, mesopores, and macropores in the ZAB using Pd Pb/C as the cathode catalyst, in which initial VACNTs-Ta in which mesopores and macropores were the round-trip overpotentials increased from 0.72 V at the first majority, favoring the transport of mass and thus improving cycle to 0.86 V at the 135th cycle. Besides Pd Pb/C cata- catalytically properties. In the comparison of electrochemi- lysts, PdIr/C was also studied by Ko et al. [91]. Based on cal properties, the first discharge and charge behaviors of an initial charge–discharge behaviors, a comparison of ORR assembled LAB showed that as a cathode, VACNTs-Ta can and OER showed that PtRu/C possessed lower overpoten- deliver a larger gravimetric specific capacity (~ 4300 mAh −1 −1 −1 tials for charging and discharging than PtPd/C and PtIr/C, in g ) at 200 mA g than VACNTs-SS (~ 3200 mAh g ) −1 which the capacity followed an order: PtRu/C (~ 346 mAh and CNT-P (~ 700 mAh g ). These results confirmed that −1 −1 −1 g ) > PtPd/C (~ 153 mAh g ) > PdIr/C (~ 135 mAh g ). VACNTs-Ta possessed greater ORR and OER activities than Table 2 shows the electrochemical performances for typi- the other two samples. In further tests of cycling and rate cal noble metal–carbon composite bifunctional catalysts as capability, VACNTs-Ta also exhibited better cycling per- −1 cathodes in MABs [86, 87, 89, 91, 92, 96, 100, 104]. formances (65 cycles at 200 mA g as well as a curtailed −1 To improve the ORR and OER (especially OER) of specific capacity of 1000 mAh g ) and rate capabilities −1 −1 Pt/C catalysts, the incorporation of other metals can play (10,000 mAh g at 50 mA g ), suggesting that VACNT-Ta an important role in improving battery performance in possessed more a favorable architecture and higher stabil- rechargeable MABs due to the synergistic interactions ity than those of the VACNT-SS and CNT-P samples. The between different components. For example, composite researchers concluded that the use of Ta not only favored the catalysts such as PtCo /C [89] and Pt/CNTs/Ni [104] can fabrication of VACNT but also produced beneficial interac - −1 deliver high discharge capacities of 3040 and 4050 mAh g tions between Ta and VACNT, benefiting co-transportation/ for MABs, respectively. Compared to pure Pt catalysts, other reaction and thus the reduction of discharge product (e.g., noble metals (e.g., Ir or Ru) are also able to increase OER Li O ) aggregation. To further develop high-performance 2 2 1 3 Electrochemical Energy Reviews (2018) 1:1–34 15 Table 2 Electrochemical performance for typical noble metal-based bifunctional catalysts as air-electrode materials in the tested MABs No. Bifunctional Maximum Cycle Potential Potential differ - Coulombic Electrolyte type MAB type References catalyst as capacity/mAh number range for ence for ORR/ efficiency −1 cathode g (current (current cycle test- OER (initial (%, after a −1 materials density/ mA g density/ ing (V) ORR/OER cycle b −2 −1 c or mA cm ) mA g or overpotentials , number) −2 + mA cm , V vs. Li/Li ) upper-limit capacity/ −1 mAh g ) a a 1 10 wt% Pt/1200 (70 )20 (100 , 2.0–4.8 0.97 (2.88/3.85) –1 M LiPF in LAB [86] GNS 720) EC:DMC (1:1 v/v) b b 2 20 wt% 1329 (0.12 ) 1 (0.12 , 2.0–4.5 1.30 (2.70/4.00) –1 M LiPF in LOB [87] Pt Au /C 240) EC:DMC (1:1 v/v) 51 49 a a 3 20 wt% 3040 (100 )5 (100 , –) 2.0–4.6 1.21 (2.73/3.94) –1 M LiClO in LAB [89] PtCo /C PC:DME (1:2 v/v) b b 4 44 wt% 700 (0.2 ) 40 (0.2 , 2.3–4.5 1.20 (2.66/3.86) –1 M LiPF in TEG- LAB [91] Pt Ru /C 700) DME 50 50 5 36.5 wt% –150 (2000 , 1.5–4.5 1.07 (2.74/3.81) –0.1 M LiClO in rLOB [93] Ir@DHG 1000) TEGDME:DMSO (1:2 v/v) 6 5 wt% Ru/ –25 (200 , 2.0–4.5 1.26 (2.71/3.97) – LiCF SO :TEGDME LOB [96] 3 3 CBC 500) (4:1 mol/mol) a a 7 40.12 wt% 616 (70 )10 (70 , –) 2.0–4.3 0.83 (2.72/3.55) –1 M LiPF in PC rLAB [100] Pt/C a a 40.03 wt% 855 (70 )10 (70 , –) 0.75 (2.65/3.40) – Pd/C a a 40.15 wt% 577 (70 )10 (70 , –) 0.84 (2.76/3.70) – Ru/C a a 8 Pt/CNTs/Ni4050 (20 )80 (400 , 2.0–4.2 1.10 (2.60/3.70) – 1 M LITFSI in TEG- LOB [104] 1500) DME a −1 mA g b −2 mA cm Initial overpotentials are obtained from the cyclic tests Dimethyl carbonate 1,2-Dimethoxyethane non-noble metal-functionalized carbon cathode materials, nanoparticles supported on graphene (CoCu/graphene) as Su et al. [106] studied the composites of carbon with Ni, a composite cathode catalyst for LABs. XRD confirmed the Co, and Cu quantum dots (QDs) and compared them with coexistence of Co and Cu along with graphene, and TEM a MnO/carbon composite. Based on capacitance and rate images revealed highly distributed CoCu nanoparticles performance results, the researchers demonstrated that Ni, with an average diameter of 10–20 nm on graphene sheets Co, and Cu QDs composited with carbon is an important without aggregation, meaning that more active sites were path in developing advanced carbon-based cathode materi- formed. The electrocatalytic activity measurements in the als in LABs. first discharge–charge profiles of their LAB with a voltage −1 Although single non-noble metals have been explored range of 2.0–4.3 V at 200 mA g showed larger discharge −1 for novel carbon-based composites, research has also been capacities (14,821 mAh g ) and coulombic efficiencies carried out using two non-noble metals (e.g., transition met- (92%) than compared with Co/graphene and Cu/graphene als such as Co, Ni, Fe, Cu, and Mn) in carbon-composited cathodes, indicating that CoCu/graphene possessed better catalysts for MABs in which stronger synergistic effects electrocatalytic activity. Moreover, Co/graphene presented between the two metals and carbon are expected. To hin- much higher coulombic efficiencies than Cu/graphene, der the restacking of graphene layers and increase the space demonstrating that Co can efficiently accelerate OER and between layers favorable to the transport of oxygen, lithium that Cu can have a positive influence on ORR. In a com- ions and electrolyte, Chen et al. [107] designed and synthe- parison of CV curves, CoCu/graphene exhibited the most sized cost-effective cobalt-copper bimetallic yolk-shelled positive ORR onset potential and the greatest ORR/OER 1 3 16 Electrochemical Energy Reviews (2018) 1:1–34 peak current density, confirming the higher catalytic activ - As listed in Table 3 [105, 107, 110, 111, 113–119], ity of CoCu/graphene as compared with Co/graphene and instead of high-priced noble metals, the use of non-noble Cu/graphene cathodes. In cycling tests performed in a LAB metals in bifunctional composite catalysts can generally −1 with a current density of 200 mA g and a cutoff capacity improve MAB performances because of the formation of −1 of 1000 mAh g , the discharge voltages of Co/graphene various decent structures and the synergistic effects between −1 and Cu/graphene cathodes degraded to less than 2.5 V after different components. For example, at 200 mA g , CoCu/ only 71 and 37 cycles, respectively, whereas CoCu/graphene grapheme [107] can deliver high discharge capacities of over −1 remained above 2.5 V for 122 cycles, once again demon- 100,000 mAh g as well as acceptable cycle performances strating the superior ORR stability of CoCu/graphene. This (i.e., cycle number above 70). Interestingly, based on non- cycling test was subsequently extended to a higher current noble metal and carbon, ternary and quaternary compos- −1 density of 500 mAh g with a cutoff capacity of 1000 mAh ite catalysts, such as Fe–Fe C/CNFs [113], FeNi @GR@ 3 3 −1 g , and the CoCu/graphene run stably for 204 cycles with Fe–NiOOH [114], Co O /Ni/C [119], and Co/C/NiFe LDH/ 3 4 a discharge terminal voltage of 2.0 V. This was better than AB [113] have demonstrated even better activities toward Co/graphene (~ 144 cycles) or Cu/graphene (~ 101 cycles). ORR and OER, resulting in enhanced MAB performances in The cycling performance of CoCu/graphene was shown to terms of capacity, cycling stability and/or energy efficiency. be comparable to those of the best cycling performances in In addition, by comparing non-noble metal-based with noble LABs [108, 109]. A similar study was conducted by Kwak metal-based bifunctional catalysts as shown in Table 3, it et al. [110], in which CNT was composited with Fe and can be seen that in terms of tested cycle numbers, non-noble Co as a cathode catalyst for LABs. In their characterization metal-based catalysts can achieve almost better cycling per- using SEM, TEM, XRD, EDS, and SAED patterns, bime- formances in MABs than noble metal-based catalysts, as tallic Fe and Co coupled with a small amount of oxidized shown in Table 2. Non-noble metals can also provide accept- states, were confirmed in the FeCo–CNTs catalyst (Fig. 6). able bifunctionality resulting from its formed nanostructures In an assembled LAB, the FeCo–CNT cathode demonstrated as well as its strong interactions with supporting materials −1 greater capacity (~ 3600 vs. ~ 1276 mAh g ) and better (like carbons). round-trip efficiency (72.15 vs. 62.5%) than pristine CNTs, In the search for next-generation bifunctional catalysts for indicating that FeCo–CNTs possessed superior ORR and scale-up deployment in MAB commercialization, non-noble OER activities. metal-based catalysts are more promising than noble metals. Besides the CoCu/graphene and FeCo–CNTs compos- Not only are non-noble metals cheaper and more abundant ite catalysts discussed above, electrospun graphitic carbon than noble metals, non-noble metals can easily be fabricated nanofibers with in situ encapsulated Co–Ni nanoparticles with supporting materials (e.g., carbon) in the formation of (Co–Ni/CNFs) were also developed by Huang et al. [111], in efficient nanostructures to improve bifunctional activities which the encapsulation of metal catalysts into nanocarbon and stability by building important interactions with the can suppress aggregation, presenting more active sites for supporting material. both ORR and OER. In their synthesis, cobalt(II) acetate ter- ahydrate (Co(Ac) ·4H O) and nickel(II) acetate tetrahydrate 2 2 (Ni(Ac) ·4H O) were used as Co and Ni sources. In their 5 Composites of Carbon and Oxides 2 2 examination of electrocatalytic activity using an assembled non-aqueous LAB without any binders or additives, a cyclic In recent years, metal oxides have been reported to exhibit −1 test with an upper-limit capacity of 1000 mAh g at 200 promising activities for ORR and OER. However, the self- −1 mA g showed that the Co–Ni/CNFs cathode can run 60 passivity and low electrical conductivity of metal oxides can cycles with initial overpotentials of only 0.22 and 0.70 V decrease active sites and hinder charge transport, leading to for ORR and OER, respectively. Importantly, in the first low catalytic performances [120, 121]. Therefore, composit- −1 discharge/charge curves obtained at 200 mA g , Co–Ni/ ing conductive carbon and oxides have become an effective CNFs displayed an initial discharge capacity of 8635 mAh strategy to enhance catalytic performances because these −1 g . This was better than that of CNFs, suggesting that the composites can overcome these low conductivity and cor- in situ formation of Co–Ni improved electrochemical perfor- rosion drawbacks [122, 123]. mances and enhanced ORR and OER. Their results showed that the inherently interconnected, conductive network of 5.1 Composites of Carbon and Perovskite Oxides Co–Ni/CNFs is sufficient for LABs without any binders and additives. Ren et al. [112] found that CuFe catalyzed carbon Perovskite oxides, with a general formula of AB O [124] black (Ketjenblack) materials can drastically increase the (Fig. 7, the A-site is a rare alkaline earth metal cation and density of catalytic sites, resulting in improvements of LAB the B-site is a 3d transition metal cation), have attracted ORR kinetics. increasing attention because of their defective structures, 1 3 Electrochemical Energy Reviews (2018) 1:1–34 17 Fig. 6 Morphology and structure of the synthesized FeCo–CNTs terns of FeCo–CNTs and pristine CNTs. e SAED patterns of FeCo– composite. SEM images of a FeCo–CNTs and b pristine CNTs; c CNTs. Reprinted with permission from Ref. [110]. Copyright 2016 TEM image of FeCo–CNTs with EDS mapping images; d XRD pat- Royal Society of Chemistry low costs, excellent oxygen mobility, and outstanding improve ORR and OER activities in alkaline environments activities toward ORR and OER [125, 126]. To improve owing to three aspects: the active oxide, the conductive overall energy efficiency and retain stability, perovskite carbon support, and the synergistic effects between them. oxides have been proposed to composite with carbon to 1 3 18 Electrochemical Energy Reviews (2018) 1:1–34 Table 3 Electrochemical performance for typical non-noble metal-based bifunctional catalysts as air-electrode materials in the tested MABs No. Bifunctional Maximum Cycle number Potential range Potential differ - Energy Electrolyte MAB type References catalyst as capacity/mAh (current den- for cycle test- ence for ORR/ efficiency type −1 −1 cathode mate- g (current sity/mA g ing (V) OER (initial (%) a −2 rials density/ mA or mA cm , ORR/OER −1 b c g or mA upper-limit overpotentials , −2 + cm ) capacity/mAh V vs. Li/Li ) −1 g ) a a 1 Ta/CNTs4300 (200 )65 (200 , 2.0–4.5 1.70 (2.60/4.30) – 1 M LiTFSI/ rLOB [105] 1000) TEGDME a a 2 CoCu/gra-14,821 (200 )122 (200 , 2.5–4.5 1.05 (2.75/3.80) – 1 M LiTFSI/ rLOB [107] phene 1000) TEGDME a a 3 FeCo/CNTs3600 (250 )50 (100 , 2.4–4.5 1.14 (2.75/3.89) – 1 M LiTFSI/ rLOB [110] 1000) TEGDME a a 4 CoNi/CNFs8635 (200 )60 (200 , 2.0–4.5 0.92 (–/–) – 0.5 M LiTFSI/ rLOB [111] 1000) TEGDME a a 5 Fe–Fe C/6250 (200 )41 (300 , 600) 2.0–4.3 1.05 (2.70/3.75) – 1 M LiTFSI/ rLOB [113] CNFs TEGDME 6 FeNi @GR@ –100 (1 , –) 0.9–2.1 0.90 (1.05/1.95) – 6 M KOH rZAB [114] Fe–NiOOH 7 Co/C/NiFe –300 (40 , –) 1.05–2.05 0.75 (1.20/1.95) 51.2% 6 M KOH rZAB [115] LDH/AB 8 Co–N/C –500 (2 , –) 0.4–1.4 0.94 (1.21/2.15) – 6 M KOH ZAB [116] 9 Fe–N/C731 (100 ) – – – – 6 M KOH ZAB [117] 10 Fe@N–C –100 (10 , –) 1.0–2.0 0.70 (1.25/1.95) – 6 M KOH ZAB [118] a a 11 Co O /Ni/C14,830 (400 )48 (100 , 2.0–4.3 0.55 (2.69/3.24) 75.1%0.1 M LiClO / LOB [119] 3 4 4 2000) DME a −1 mA g b −2 mA cm Initial overpotentials are obtained from the cyclic tests To enhance the electrical conduction of perovskite oxides and investigate the effects of perovskite oxides on ORR and OER, Xu et al. [127] studied BaMnO –carbon composites and their bifunctional electrocatalytic capa- bilities for ORR and OER. Their carbon-coated BaMnO nanorod (BaMnO @5%C) samples were synthesized through a coating method in which BaCl and MnO 2 2 were used as Ba and Mn sources with a ratio of Ba to Mn being 1:1. SEM images of the product revealed BaMnO nanorods 100–200 nm in diameter and 1–4 μm in length and TEM revealed that these nanorods were compactly and uniformly coated with a 10-nm-thick carbon. The compari- son of BaMnO with BaMnO @5%C samples by XRD evi- 3 3 denced the presence of perovskite BaMnO structures with a small amount of MnO resulting from residual raw reac- tants. In the investigation for ORR and OER, CV curves in O -saturated 0.1 M KOH solution showed that the peak −2 current density (~ 2.5 mA cm ) of BaMnO @5%C was −2 much greater than that of BaMnO (~ 0.94 mA cm ), demonstrating superior electrocatalytic activity. This result was further confirmed by tested LSV curves in Fig. 7 A unit cell in perovskite showing the relative positions of dif- ferent ions. Reprinted with permission from Ref. [124]. Copyright which BaMnO @5%C possessed a larger positive half- 2016 John Wiley and Sons wave potential as compared with BaMnO . Moreover, 1 3 Electrochemical Energy Reviews (2018) 1:1–34 19 the diffusion limiting current density of BaMnO @5%C of the carbon and perovskite oxides, as well as the syner- was comparable to commercial Pt/C (20wt% Pt on Vul- gistic effects between them. can XC-72) catalysts. Interestingly, BaMnO @5%C pos- In addition to the partial substitution of A-sites, the par- sessed an ORR electron transfer number of ~ 3.8. This tial substitution of B sites was also successfully conducted was higher than that of BaMnO (3.4 – 3.7) and close to to form new perovskite oxides for MAB carbon-composited that of Pt/C (~ 3.9). BaMnO @5%C also demonstrated catalysts. Yuasa et al. [131] prepared carbon-supported better OER activity because it exhibited greater current LaMn Fe O (C-LaMn Fe O ) electrocatalysts using 0.6 0.4 3 0.6 0.4 3 densities and more negative onset potentials than BaMnO a reverse homogeneous precipitation (RHP) method and and Pt/C as revealed by linear scanning voltammograms investigated the effects of LaMn Fe O on discharge 0.6 0.4 3 in N -saturated 0.1 M KOH solution at a rotation speed and charge properties in LABs. XRD revealed that the −1 of 1600 r min . In terms of stability, durability tests obtained C-LaMn Fe O was in a perovskite-phase of 0.6 0.4 3 were run for ORR and OER using a chronoamperometric an orthorhombic form without any other impurity phases method for 12 h in O - and N -saturated 0.1 M KOH at with a calculated crystalline size of 17.4 nm. TEM images 2 2 −1 1600 r min . Here, BaMnO @5%C provided better stabil- showed the uniform distribution of LaMn Fe O on car- 3 0.6 0.4 3 ity toward ORR and OER compared with both Pt/C and bon. The size range of LaMn Fe O was 15–20 nm, and 0.6 0.4 3 BaMnO samples. Not all carbon materials enhance ORR this was consistent with XRD results. Used in an LAB, −2 and OER after compositing with perovskite oxides, how- the obtained four charge/discharge curves at 0.5 mA cm ever. For example, LaMnO -C exhibited poor OER activity showed that as compared with C-LaMn Fe O , carbon 3 0.6 0.4 3 after the incorporation of carbon [128]. presented an unstable charge voltage due to its oxidation For perovskite oxides with the general formula of corrosion by anodic polarization [132]. With the addi- ABO , the substitution of the A-site and/or B-site metal tion of LaMn Fe O , C-LaMn Fe O exhibited sta- 3 0.6 0.4 3 0.6 0.4 3 cations to generate oxygen deficiency/vacancy can have ble discharge/charge curves because LaMn Fe O , with 0.6 0.4 3 large characteristic effects on their electronic structure a higher OER, effectively prevents oxidation corrosion and coordination chemistry, leading to enhanced ORR from anodic polarization. The discharge voltages of the and OER activities [129]. Two typical representatives C-LaMn Fe O were also greater than that of carbon, sug- 0.6 0.4 3 of perovskite oxides with a partial A-site substitution, gesting that LaMn Fe O can also act as an ORR catalyst 0.6 0.4 3 La Ca CoO (LCC) and Sr Sm CoO (SSC) have in non-aqueous electrolytes as well as in alkaline aqueous 0.6 0.4 3 0.5 0.5 3-δ 3+ 4+ been combined with carbon black to form two different solutions because Mn or Mn ions can act as active sites carbon-supported perovskite oxides (C-LCC and C-SSC) for ORR [133]. Researchers [134, 135] have also studied for rechargeable MABs [11, 130]. In a comparison of carbon-composited perovskite oxides derived from the sub- their bifunctionality using graphitized Vulcan XC-72R stitution of both A and B sites as oxygen electrodes and have as a reference, obtained cathodic polarization curves in a also found improved electrochemical properties. three-electrode cell with an 8.5 M KOH solution revealed that the C-SSC composite cathode provided better ORR 5.2 Composites of Carbon and Spinel Oxides activities that the C-LCC cathode and both cathodes exhibited better ORR activities than the graphitized Vul- As substitutes for expensive noble metal catalysts, spinel can XC-72R cathode. No significant difference in OER oxides [denoted as A B O (A, B=Co, Zn, Ni, Fe, Cu, x 3−x 4 activity was found between the C-SSC and C-LCC cath- Mn, etc.)] have gained much attention because of their low odes, however, but both still provided better OER activities costs, considerable activities, high abundance (stability), than the graphitized Vulcan XC-72R cathode. In cycling and environmental friendliness [136–138]. To achieve bet- −2 performance tests under 53 mA cm , C-SSC ran for 106 ter catalytic activities toward ORR and OER, however, net cycles at − 0.3 V, whereas C-LCC only ran for ~ 68 cycles spinel oxides are usually attached onto conducting carbon at − 0.3 V. The graphitized Vulcan XC-72R exhibited substrates with the aim of assuring fast electron transport unsatisfactory cycling performances because of its poor and good interaction between oxides and carbon [136]. For OER capabilities. Later, Velraj and Zhu [11] also prepared example, Li et al. [137] performed an in situ growth of spi- untreated Vulcan XC-72R supported LCC and SSC and nel CoFe O nanoparticles on rod-like ordered mesoporous 2 4 their results showed that because of the better corrosion carbon (CFO/RC) through a hydrothermal treatment process resistance of graphitized carbon, graphitized Vulcan-based with an annealing procedure in which Co(NO ) ·6H O and 3 2 2 electrodes can provide more than twice the cycle life of Fe(NO )·9H O were used as Co and Fe sources. According 3 2 untreated carbon-based electrodes. These results confirm to different annealing temperatures of 300, 400, 500, and that for carbon-supported perovskite oxide composites, the 600 °C, the final composites were labeled as CFO/RC-300, enhancement of catalytic activity and durability toward CFO/RC-400, CFO/RC-500, and CFO/RC-600, respectively. ORR and OER are related to the structure and properties In the examination of surface chemical composition and 1 3 20 Electrochemical Energy Reviews (2018) 1:1–34 cation oxidation states using XPS, no shifts were found for of defected CFO nanoparticles providing more active sites. the Co 2p peaks of the pure CFO and CFO/RC-400 catalysts, For Tafel plots in the low overpotential region, the Tafel indicating that the Co cations in both catalysts possessed the slope (99 mV per dec) of the CFO/RC-400 composite was same chemical surrounding. For the Fe 2p and Fe 2p lower than those of CFO (129 mV per dec) and RC (125 mV 3/2 1/2 of the Fe 2p spectra, however, CFO/RC-400 showed two per dec). Both results indicate that CFO/RC-400 possessed small shifts to higher binding energies as compared with enhanced ORR kinetics after the incorporation of CFO into CFO, revealing strong coupling between CFO and RC. In RC. Among all CFO/RC composites, CFO/RC-400 pro- −2 further analyses, the O1s peak of CFO/RC-400 was seen to duced the highest limiting current density (4.86 mA cm ) shift to higher binding energies in comparison with CFO, and the highest onset potential (~ − 0.10 V), indicating that confirming the strong coupling between CFO and RC from CFO/RC-400 possessed the optimal ORR activity among the lattice oxygen in the Co/Fe–O framework during the all CFO/RC composites. This is not only because anneal- hydrothermal treatment. In the assessment of electrocata- ing temperatures below 400 °C can result in the incomplete lytic properties, LSV curves (Fig. 8a) in O -saturated 0.1 M CTAB decomposition covering some active sites, but also −1 KOH solution at a rotation rate of 1600 r min showed because annealing temperatures above 400 °C can cause that all CFO/RC composites exhibited superior onset poten- the reduction of defects and the increase of particle sizes, tials (− 0.10 to − 0.13 V) compared with CFO (− 0.23 V) decreasing electrocatalytic activity. For OER, anodic LSV and RC (− 0.29 V). This was attributed to the synergistic curves (Fig. 8b) were recorded in N -saturated 0.1 M KOH −1 effects of the two components as well as the large amount solution at a rotation speed of 1600 r m in . Here, both the onset potential and Tafel slope (in the low overpoten- tial region) of CFO/RC-400 (~ 0.41 V, 92 mV per decade) were less than those of CFO (~ 0.43 V, 112 mV per dec- ade), RC (~ 0.75 V, 308 mV per decade), and 20 wt% Pt/C (~ 0.54 V, 105 mV per decade). At 1.0 V, CFO/RC-400 dis- −2 played a much higher current density of 39.6 mA cm than −2 −2 CFO (~ 34.5 mA cm ), RC (~ 3.37 mA cm ), and Pt/C −2 (~ 27.8 mA cm ), demonstrating the much higher OER activity of CFO/RC-400 as compared with CFO, RC, and Pt/C samples. This can possibly be attributed mainly to the hierarchical mesoporous structure of the RC matrix and the strong coupling and synergistic effects between CFO and RC. Based on the obtained ORR and OER activities, the CFO/RC-400 composite was also found to outperform other carbon-spinel oxide composite catalysts such as NiCo O / 2 4 grapheme [139], CoFe O /grapheme [140], and CoF e O / 2 4 2 4 biocarbon [141], suggesting that ordered mesoporous carbon rods are more suitable for spinel oxide nanoparticle loading to improve ORR and OER than other carbon matrixes such as CNTs, graphene, and biocarbon materials. Similar to CoFe O and NiCo O ternary spinel oxides, 2 4 2 4 Co O ; a binary spinel oxide, has also been investigated in 3 4 the development of carbon-spinel oxide bifunctional com- posite catalysts. Specifically, a facile hydrothermal route was used by Liu et al. [142] to form a cubic Co O and 3 4 multi-walled carbon nanotube (cCo O /MWCNT) compos- 3 4 ite in which MWCNTs were acid-functionalized as struc- ture directing/oxidizing agents and Co(CH COO) ·4H O 3 2 2 was used as the single Co source. For comparison, pure cCo O , acid-treated MWCNTs and a physical mixture of 3 4 cCo O + MWCNTs were chosen as reference samples. Fig. 8 a Linear sweep voltammetry (LSV) of RC, CFO, CFO/RC 3 4 composite and commercial Pt/C in O -saturated 0.1 M KOH solution 2 The comparison of XRD patterns of cCo O and cCo O / 3 4 3 4 −1 at a rotation rate of 1600 r min , and b anodic LSV of RC, CFO, MWCNTs revealed that after their spinel structures were CFO/RC-400 composite and commercial Pt/C in N -saturated 0.1 M −1 confirmed, the peaks of cCo O were shifted slightly to a 3 4 KOH solution at a rotation rate of 1600 r min . Reprinted with per- larger 2θ angle after the addition of MWCNTs, suggesting a mission from Ref. 137 Copyright 2016 Royal Society of Chemistry 1 3 Electrochemical Energy Reviews (2018) 1:1–34 21 2+ 2+ slight lattice contraction possibly due to variations in crystal Co in CoO/C, while two spin orbit doubles of Co and 3+ sizes [143, 144] or interactions between cCo O and MWC- Co has been assigned in their CoO reference sample. In 3 4 NTs. SEM and TEM images confirmed the cubic morphol- addition, analysis of O 1S showed the shift of O4 signal, ogy of cCo O and showed the attachment of MWCNTs resulting from CoO reduction (O-to-Co atomic ratio was 3 4 to the cCo O surface without free cubic particles. These less than calculated 1.39) in CoO/C sample. This indicated 3 4 results indicated the effective tethering between MWCNTs on existence of oxygen deficiencies in the CoO/C sample and cCo O as the acid-functionalized MWCNTs served after the addition of C species. In their evaluation of battery 3 4 as an oxidizing/structure directing agent to oxidize cobalt performance and electrocatalyst activities for both ORR and ions into spinel cobalt oxides and regulate the formation of OER, the CoO/C catalyst showed larger capacities (~ 7011 −1 −1 cubic cobalt oxide. According to TGA, the cCo O /MWC- and ~ 4074 mAh g at 100 and 400 mA g , respectively) 3 4 −1 NTs composite contained 54% cCo O similar to the physi- than CoO (~ 5189, ~ 2059 mAh g ), and cyclability tests in 3 4, −1 cal mixture of cCo O + MWCNTs. For ORR, LSV curves a rechargeable LAB with a current density of ~ 200 mA g 3 4 in O -saturated 0.1 M KOH solution showed that among and a high voltage cutoff of 4.5 V demonstrated that CoO/C all tested catalysts, cCo O /MWCNTs possessed the larg- can achieve higher cycle numbers (50) than CoO. These 3 4 est onset potential (~ − 0.15 V) and best current density results indicate that the addition of carbon can significantly −2 (~ − 2.91 mA cm at − 0.4 V), indicating that cCo O / improve electrocatalytic activities for both ORR and OER. 3 4 MWCNTs possessed superior ORR activity to cCo O , A further investigation of morphology and phase composi- 3 4 MWCNTs, and cCo O + MWCNTs. For OER, LSV curves tion under different charge/discharge states using SEM and 3 4 in N -saturated 0.1 KOH solution showed that cCo O / XRD demonstrated that the main discharge product, Li O 2 3 4 2 2, MWCNTs possessed the best OER activity as evidenced by can be completely decomposed by CoO/C but not by CoO. −2 a higher current density (~ 16.0 mA cm at 0.7 V) com- This quality attributed to the lower charge–discharge over- pared with other catalysts. A further cycling test with 500 potentials and higher cycling properties of CoO/C. Overall, continuous CV cycles showed that the ORR current density these enhanced electrochemical performances can be attrib- of cCo O /MWCNTs was about 4, 34, and 3 times better uted to the composition between carbon and CoO and can 3 4 than the MWCNTs, cCo O , and cCo O -MWCNTs cata- possibly be associated with the integration of carbon dotting 3 4 3 4 lysts, respectively, and that the final OER current density of and oxygen vacancies into CoO as well as the synergetic cCo O /MWCNTs was 49% higher than cCo O -MWCNTs effects of the two components. This promising strategy of 3 4 3 4 and greater than the individual components. These results compositing carbon with oxides to create positive effects suggested that cCo O /MWCNTs can produce stronger inter- on MAB ORR and OER has inspired more research into 3 4 actions between cCo O and MWCNTs, resulting in possible exploring and using different oxides, such as La O [151], 3 4 2 3 coupling effects and thus higher ORR and OER activities. zirconium doped ceria [152], and cobalt-manganese mixed oxide (Co Mn O) [153]. x 1-x 5.3 Composites of Carbon and Other Oxides Apart from perovskite oxides and spinel oxides, other oxides, 6 Composites of Carbon and Nitrides such as CoO [145, 146], MnO [147, 148], and RuO [149, 2 2 150], have also been used to prepare effective carbon-com- Combining carbon and nitrides (e.g., TiN or CN) to cre- posited bifunctional catalysts to tackle the sluggish kinetics ate novel catalysts for MAB ORR and OER has become of ORR and OER in MABs. Based on a novel strategy to an effective strategy to improve rechargeability and round- improve the catalytic performance of CoO through the inte- trip efficiency. Titanium nitride (TiN), a typical transition gration of dotted carbon species and oxygen vacancies, Gao metal nitride with high electronic conductivity and good et al. [145] designed and synthesized a carbon-dotted CoO electrochemical activity, is widely applied in electrochem- with oxygen vacancies (CoO/C) for LAB cathodes using a istry studies [154]. Using a template method (Fig. 9a), Li simple calcination of a formed pink precipitate of ethanol- et al. [155] prepared nano-sized TiN supported on Vulcan mediated Co(Ac) ·4H O. In their structure characteriza- XC-72 (n-TiN/VC) and used this as a bifunctional catalyst 2 2 tions, XRD spectra were used for the low composition of for non-aqueous LABs. Commercial micrometer scale TiN CoO from Rietveld refinement. Furthermore, the Raman was used by the researchers to prepare m-TiN/VC samples 0.89 spectra was used for analysis of the shift of Co–O vibration as reference. XRD patterns revealed that TiN in n-TiN/ peaks for both CoO and CoO/C, and revealed a negative VC possessed a crystallite size of 4.3 nm according to the shift of Co–O signal for CoO/C as compared to CoO due Scherrer equation and was consistent with TEM results. to suggested oxygen vacancies. This was further confirmed BET measurements showed that the surface areas of the n- by thorough XPS analysis of both Co 2p and O 1s signals. TiN/VC, m-TiN/VC, and VC samples were 172, 144, and 2 −1 A Co 2p spectra was only fitted with one peak related to 233 m g respectively. In electrochemical measurements, 1 3 22 Electrochemical Energy Reviews (2018) 1:1–34 −1 For OER curves (Fig. 9c) at 50 mA g , n-TiN/VC pro- carbon vided the lowest recharge voltage and accordingly the lowest voltage gap (~ 1.05 V), showing promoted OER activity. Moreover, the initial section of the recharge in the inset of Fig. 9c shows a lower onset potential (~ 2.9 V) for n-TiN/VC than those of m-TiN/VC (~ 3.1 V) and VC (~ 3.1 V) cath- odes, matching the strong oxidation peak of LiO produced by n-TiN/VC as shown in Fig. 9b, demonstrating better OER activities due to the interaction between n-TiN and VC. In five discharge–recharge cycles, n-TiN/VC exhibited higher recharge voltages at the 5th discharge–charge cycle than both m-TiN/VC and VC cathodes. This can be attributed to the strong interaction between the remaining Li O deposits 2 2 and the TiN nanoparticles on the carbon surface, resulting in high stability. However, because TiN can be oxidized to TiO based on the results of XRD and FTIR, LABs with n- TiN/VC cathodes possess a limited potential of 4.3 V despite its ability to enhance ORR and OER. Park et al. [158] stud- ied TiN/C composites and found that the porous structure and synergistic effects between TiN and carbon play a pro- moting role in the enhancement of catalytic activity for both ORR and OER in LABs. Graphitic carbon nitride (g-CN) is also an attractive metal-free material because of its abundance and negligi- ble metal pollution [159]. However, the ORR activity of g-CN itself is not satisfactory because of its poor electri- cal conductivity [160, 161] despite nitrogen atoms in g-CN being able to increase the electropositivity of adjacent car- bon atoms. Based on this, Fu et al. [162] combined car- bon with porous graphitic g-CN through a template-free synthesis route to obtain g-CN/C composite catalysts for Fig. 9 a Preparation process of n-TiN/VC. b CV curves of LAB with ORR in MABs. In their experiment, based on the amount of VC, m-TiN/VC, and n-TiN/VC as air-electrode catalysts under an O d -glucose (i.e., 15.84, 39.60, and 79.20 mmol) used, three −1 atmosphere from 2.0 to 4.0 V at 0.05 mV s . c Discharge-recharge g-CN/C samples (g-CN/C-1, g-CN/C-2, and g-CN/C-3) curves of VC, m-TiN/VC, and n-TiN/VC as cathode catalysts for −1 were obtained. For comparison, a physically mixed g-CN LABs with an enlarged section highlighted (inset) at 50 mA g . carbon Reprinted with permission from Ref. [155]. Copyright 2013 Royal and carbon, labeled as g-CN + C, was also prepared. The Society of Chemistry addition of carbon increased not only the BET surface area but also porosity. Table 4 presents the ratio of CN in the g-CN in terms of the atomic ratio of carbon and nitrogen CVs (Fig. 9b) under O from 2.0 to 4.0 V at a scan rate of as well as the relative composition ratios (%) of the four −1 0.05 mV s in a LAB revealed that for the cathodic scan, samples derived from XPS spectra. A comparison of ORR n-TiN/VC showed higher ORR currents than m-TiN/VC and activities for the three different composites (i.e., g-CN/C-1, VC cathodes. For the anodic scan, n-TiN/VC presented three g-CN/C-2, and g-CN/C-3) revealed that g-CN/C-2 with oxidation peaks rather than two as seen in the CVs of m-TiN/ 33.75 wt% N content delivered a more positive potential VC and VC cathodes. This suggests three varied OER path- and a larger limiting current than g-CN/C-1 (~ 50.35 wt% N) ways [i.e., I: Eq. (1), II: Eq. (2), and III: Eq. (3)] according and g-CN/C-1 (~ 26.09 wt% N) samples, demonstrating to the decomposition mechanisms [156, 157]: that g-CN/C-2 possessed the best ORR activity. This result suggests that high N content benefits the tradeoff between + − LiO → Li + O + e (1) 2 2 available ORR active sites and electron conduction. LSV + − curves of g-CN/C-2 obtained in O -saturated 0.1 M KOH Li O → 2Li + O + 2e (2) 2 2 2 −1 solution at 1600 r min exhibited significantly more posi- tive onset potentials (~ 0.90 V) and larger disk currents 2LiO → 4Li + O + 4e (3) 2 2 −2 (limiting current density, 4.10 mA cm ) than those of the 1 3 Electrochemical Energy Reviews (2018) 1:1–34 23 Table 4 Ratios of CN in g-CN based on the atomic ratio of carbon and nitrogen and relative composition ratio (%) of four carbon components derived from the deconvoluted XPS spectra. Reprinted with permission from Ref. [162]. Copyright 2016 Royal Society of Chemistry a b c Samples C1s (atm.%) N1s (atm.%) Relative composition ratio of carbon (%) C/N in g-CN P1 P2 P3 P4 g-CN 50.74 43.11 42.72 0 0 57.28 0.67 g-CN/C-1 74.60 15.61 37.84 20.75 30.22 11.18 0.53 g-CN/C-2 74.70 19.88 29.03 27.51 25.03 18.43 0.69 g-CN/C-3 76.40 16.57 39.34 12.09 29.05 19.52 0.90 Atomic ratios of carbon and nitrogen in composites are obtained from XPS results Different carbon percentages deconvoluted from the C1s XPS spectra of composites The ratio of C/N in g-CN is calculated based on the carbon present in C–N bonds divided by nitrogen according to the equation C/N = P4 * C1s/N1s g-CN and g-CN + C samples, confirming the higher ORR activity of g-CN/C-2. Current-time chronoamperometric responses in O -saturated 0.1 M KOH solution showed that after 20,000 s, g-CN/C-2 decayed less (~ 20%), displaying superior stability compared with g-CN + C. All these results demonstrate the high ORR activity and good stability of the g-CN/C-2 catalyst. Recently, doping strategies (e.g., P-dop- ing) have also been used to fabricate P-doped g-CN for MAB applications [163] in which in situ growth of P-g-C N on 3 4 carbon-fiber paper (PCN-CFP) is carried out for ZAB flex- ible oxygen electrodes. These resultant PCN-CFP catalysts exhibit outstanding ORR and OER activity, stability, and reversibility in tested ZABs. 7 Composites of Carbon and Carbides Because carbides possess desirable ORR activities [164], researchers have recently attempted to utilize carbides such as tungsten carbide (WC) [165] and boron carbide (B C) [166] to create carbon-composited catalysts to improve capacity, rechargeability, and round-trip efficiency in MABs. For example, Koo et al. [165] coated a uniform WC layer onto a carbon (Ketjenblack EC600-JD) cathode using physical vapor deposition (PVD) (Fig. 10a) with TEM images revealing a 20-nm-thick uniform WC-coating layer and electrochemical measurements being performed in a non-aqueous LAB (Fig. 10b). The obtained discharge curves (down to 2.0 V) show that the WC-coated cath- Fig. 10 a Schematic diagram of PVD method for WC coating. b −1 Design and configuration of LAB. Reprinted with permission from ode can deliver a capacity of ~ 7000 mAh g . This carbon Ref. [165]. Copyright 2015 IOP publishing is twofold higher than that of the carbon cathode. Addi- tionally, it was found that in the comparison of the 1st discharge–charge curve and the 10th discharge–charge enhanced catalytic property and electrical conductivity −1 curve at 100 mA g , the WC-coated cathode produced of the carbon-WC composite. In further tests, at a cur- carbon −1 lower overpotential gaps of 700 and 1200 mV as compared rent density of ~ 1000 mA g and a voltage range of carbon with those of the carbon cathode. These results suggest 2.0–4.2 V, the WC-coated cathode was found to be able to that WC coating can enhance both ORR and OER and be efficiently operated beyond the 36th cycle, whereas the therefore limit discharge–charge overpotentials due to the carbon cathode stopped operating at the 12th cycle. This 1 3 24 Electrochemical Energy Reviews (2018) 1:1–34 increased cycle stability and round-trip efficiency clearly components are found to enhance electrocatalytic ORR and demonstrates the improvements in reaction rates and the OER performances in MABs. reduction in discharge and charge voltage gaps. Moreo- ver, it was shown that during the charge and discharge 8.1 Other Carbon‑Based Binary Composites cycling process, the addition of WC resulted in the rapid creation and decomposition of reaction products (i.e., To develop high-performance bifunctional catalysts and Li O ) because of the enhanced catalytic effects. And as solve carbon corrosion issues in MABs, Lyu et al. compos- 2 2 −1 current densities increased from 100 to 200 mA g , the ited cobalt sulfide, acting as an active ORR material, with carbon WC-coated cathode also showed decreased overpotentials carbon to form carbon-based binary composites for cathode compared with the carbon cathode, further confirming the materials in MABs [169]. In their experiment, cobalt acetate benefits of WC coating. (Co(Ac) ) and thioacetamide were used as Co and sulfur Similar to the WC-C composite above, B C nanowires- sources to obtain a novel CoS nanoparticles-reduced gra- 4 2 carbon nanotube (BC) composite cathodes, induced by the phene oxide (CoS /rGO) composite for aprotic LABs. TGA synergistic effect of B C and carbon, were also found to revealed 78 wt% CoS and XRD showed that the patterns 4 2 exhibit improved ORR/OER activity, rechargeability, and of cobalt sulfide on the graphene sheet were of a typical round-trip efficiency for LABs by Luo et al. [166]. In their cubic CoS phase (JCPDS 00-41-1471). In measurements typical experiment, Raman spectra, field emission scan- in an aprotic LAB using LiClO -DMSO as an electrolyte, ning electron microscopy (FESEM), STEM, and EDS were the CV curves showed that the CoS /rGO cathode delivered used for characterizations to confirm the formation of car - a higher ORR onset potential and a notably higher OER bon-B4C composites. Their Raman spectra showed bands current peak (at ~ 3.75 V), demonstrating its higher cata- −1 below 1200 cm , which matched the characteristics of B C lytic activities for both ORR and OER than those of rGO [167, 168], and FESEM, STEM and EDS confirmed a large and Vulcan XC-72. A combination of XRD, XPS, and SEM amount of B C nanowires growing from CNT aggregations revealed the significant role of CoS in lowering discharge/ 4 2 that possessed diameters in the range of 40–100 nm, and charge overpotentials by positively affecting the formation lengths above 2 μm. In RDE measurements in O -saturated and decomposition of Li O . Here, Li CO is still formed 2 2 2 2 3 −1 0.1 M KOH at a scan rate of 10 mV s , LSV curves at 900 r as a side product during discharging/charging, however. In −1 min showed that the B–C composite exhibited a more pos- measurements of rate capability and cyclability, it was found itive onset potential and a larger current density, correspond- that although rate capabilities increased with increasing rates −1 ing to a higher ORR activity in comparison with those of the from 50 to 500 mA g, CoS /rGO only ran 18 cycles in the −1 Pt/C and CN samples. Based on Koutecky–Levich plots, the cycling test at a rate of 200 mA g and a limited capacity −1 B–C composite was revealed to produce a nearly four-elec- of 500 mAh g . This indicates possible incomplete decom- tron ORR process comparable to Pt/C (n ~ 4.0) but different position of side products (e.g., Li CO ). Therefore, car- 2 3 from CN (n = 3.2–3.5). Subsequent comparisons of OER bon–cobalt sulfide composites need to be further improved curves revealed that the B–C composite provided a smaller in future studies to improve its potential in the development Tafel slope of ~ 70 mV per decade than Pt/C (~ 123 mV per of advanced bifunctional catalysts for LABs. decade), indicating that the B–C catalyst possessed superior NiCo S was also combined with rGO to synthesize 2 4 OER activity to 20 wt% Pt/C. Electrochemical properties NiCo S –rGO composites by Wu et al. [170] and their 2 4 were continuously tested in an assembled battery with B–C research found that the synergistic effects between NiCo S 2 4 and CN composite cathodes and the researchers found that and rGO can result in superior ORR performances as the B–C composite showed not only higher ORR potential compared with 20 wt% Pt/C, demonstrating potential as a and lower OER potential compared with CN, it also exhib- bifunctional catalyst in MABs. Aside from sulfides, other ited better cycling performances with a higher round-trip functional materials, such as FeOOH [171], soil [172], poly- efficiency. These results can possibly be attributed to B C imides [173], and Fe phthalocyanine (FePc) [174], have also and carbon providing efficient synergistic effects on electro - used in the development of carbon-based binary compos- catalytic reactions. ites for MAB bifunctional catalysts. In these studies, the incorporation of these functional materials yielded active synergistic effects with carbon, producing active sites for 8 Other Carbon‑Based Composites enhanced ORR and OER and reducing side reactions in elec- trocatalytic reactions to improve cycling performances and Other carbon-based composites can be classified into catalytic stability. carbon-based binary composites and ternary compos- ites in which strong synergistic effects between different 1 3 Electrochemical Energy Reviews (2018) 1:1–34 25 chemical/physical stability). For all components, the syner- 8.2 Other Carbon‑Based Ternary Composites gistic effects can be observed improving the catalytic activi- ties and cycling performances of MABs. Table 5 shows the Based on the synergistic effects of three separate components and their inter-correlation, various researchers have recently electrochemical performances of typical metal-free carbon- composited bifunctional catalysts for MAB cathodes [145, focused on developing carbon-based ternary composites as advanced cathodes in MABs. Among carbon-based ternary 146, 148–150, 152, 158, 165, 166, 171–174, 176, 178–183]. Metal-free catalysts composed of carbon and other com- composites for MABs, carbon; coupled with an oxide, is usually combined with a third component such as a poly- ponents (e.g., oxides, nitrides, carbides, sulfides, β-FeOOH, polyimide, Fe phthalocyanine, etc.) have been intensely electrolyte [175], a second oxide [176], or a metal [177] to exhibit enhanced catalytic activities toward ORR and/or studied as cathode materials for MABs and the results show advantages over pure carbon in terms of ORR/OER activ- OER and improved MAB performance. A positively charged polyelectrolyte, poly(diallyldimethylammonium chloride) ity and cycling stability/durability. For example, polyimide/ CNT catalysts [173] have been found to exhibit a high cycling (PDDA), was used by Zhai et al. [175] to modify carbon supports to provide available functional groups favorable stability over 130 cycles with a maximum discharge capacity −1 −1 of 11,000 mAh g at a high current density of 500 mA g to the formation of more active sites. In their study, PDDA functionalized CNTs were combined with spinel CoMn O under LAB operation conditions. As discussed previously, the 2 4 combination of carbon and oxide(s) is a sound strategy in the to form a ternary CoMn O /PDDA-CNTs composite ORR 2 4 catalyst. Their results showed that by increasing the loading development of abundant, low-cost, and efficient bifunctional catalysts for rechargeable MABs owing to the strong synergy of CoMn O from 36 to 83 wt%, the CoMn O /PDDA-CNTs 2 4 2 4 composite exhibited high ORR current densities in alkaline between carbon and oxides. For example, MnO /CP [148] and CoMn O /rGO [179] can achieve 500 and 200 cycles at 15 and and neutral conditions through a 4e reduction pathway 2 4 −2 that outperformed Pt/C due to the non-covalent coupling 20 mA cm , respectively, indicating a significant improve- ment of cycling performances due to strong resistances to effects between CoMn O and PDDA-CNTs. Moreover, 2 4 at an optimal content of 36 wt% CoMn O , the CoMn O / carbon corrosion. A SP/CaMnO catalyst [181] demonstrated 2 4 2 4 charge–discharge stability after over 80 cycles (with a high PDDA-CNTs potential difference between ORR and OER −1 was ~ 0.849 V, demonstrating potential bifunctionality. current density of 200 mA g and a limited capacity of 1000 −1 −1 mAh g ) and high discharge capacities of 7000 mAh g . To obtain a suitable third component, Ma and Wang [176] selected two different oxides to fabricate α-MnO –LaNiO / These results demonstrate the beneficial effects of combin- 2 3 ing different components with carbon material in terms of CNTs composites with the expectation that α-MnO and LaNiO can produce synergistic effects on bifunctional electrocatalytic activity, stability/durability, and associated MAB performances. However, further improvements and activity. According to the literature [4, 5, 126, 130], α-MnO acts as a good ORR catalyst and LaNiO as a good OER cat- studies are needed to enhance MAB performances to achieve commercialization. alyst. Their experiment showed that the α-MnO –LaNiO / 2 3 CNTs cathode provided excellent charge–discharge cycling Nevertheless, based on the collected data for several types of bifunctional catalysts in Tables 2, 3 and 5, the cycling per- performances within 75 charge–discharge cycles due to good bifunctional activity and durability. Wu et al. [177] studied formances of ORR and OER are not easily improved even with the use of different components compositing with carbon. This Ni-modified MnO /C (Ni-MnO /C) composites for ORR and x x their application in ZABs and they demonstrated that at an may be because the nanostructure and property of the formed carbon composites are closely associated with discharge prod- optimal Ni/Mn atomic ratio of 1:2, Ni-MnO /C cathodes in a tested ZAB can achieve a large power density of ~ 122 ucts during cycling. In a comparison of stability, Co/C/NiFe −2 −2 LDH/AB and Co–N/C composite catalysts in Table 3 show mW cm . This was better than MnO /C (~ 89 mW cm ) −2 and slightly higher than referenced cathodes of Pd/C (~ 121 300 cycle numbers at 40 mA cm and 500 cycle numbers −2 −2 −2 at 2 mA cm respectively, whereas the MnO /CP catalyst in mW cm ) and Pt/C (~ 120.5 mW cm ). −2 Other carbon-based ternary composites such as Fe/ Table 5 produces 500 cycle numbers at 15 mA cm . These results indicate a successful strategy without the use of noble Fe C–CNFs [113] and Fe-doped NiOOH grown on graphene-encapsulated FeNi nanodots (FeNi @GR@ metals in developing high-performance catalysts with high 3 3 bifunctional activities and stability. For the scale-up produc- Fe–NiOOH) [114] were also studied by Wang et al. for MAB ORR and OER. In these two carbon-based ternary compos- tion of next-generation bifunctional catalysts in commercial MABs, low costs, high bifunctional activities, and good sta- ites, carbon provided an excellent porous structure and high surface area, whereas the other two components provided bility are strongly desired. In terms of material cost and avail- ability, noble metals are being replaced by low-cost materials electrochemical properties associated with their catalytic performance (i.e., ORR and OER) and physiochemical prop- such as transitional metals, oxide, nitrides, carbides, sulfides, β-FeOOH, Fe phthalocyanine, and conducting polymers to erties (e.g., active sites, thermal ability, conductivity, and 1 3 26 Electrochemical Energy Reviews (2018) 1:1–34 1 3 Table 5 Electrochemical performance for typical metal-free carbon-composited bifunctional catalysts as air-electrode materials in the tested MABs No. Bifunctional catalyst as Maximum capacity/mAh Cycle number (current den- Potential range for Potential difference for Energy Electrolyte type MAB type References −1 a −1 −2 cathode materials g (current density/ mA sity/mA g or mA cm , cycle testing (V) ORR/OER (initial ORR/ efficiency −1 b −2 c g or mA cm ) upper-limit capacity/mAh OER overpotentials , V vs. (%) −1 + g ) Li/Li ) a a 1 CoO/SP5637 (200 )50 (200 , 1000) 2.0–4.5 1.32 (2.63/3.95) – 1 M LITFSI/TEGDME rLOB [145] b b 2 CoO/CNF 3882.5 (0.2 ) 50 (0.2 , 1000) 2.0–4.2 1.08 (2.72/3.80) – 1 M LiTFSI/TEGDME rLOB [146] b −1 3 MnO /CP –500 (15 , –) 1.0–2.4 0.75 (1.25/2.00) – 6 M KOH +20 g L ZnCl rZAB [148] x 2 b b 4 RuO /CNT 1150 (0.4 ) 50 (0.4 , 244) 2.0–4.4 1.38 (2.56/3.94) 65.4% 1 M LITFSI/TEGDME LOB [149] a a 5 RuO ·0.64H O/rGO5000 (500 )35 (200 , 2000) 2.0–4.3 1.40 (2.75/4.15) – LiCF SO /TEGDME LOB [150] 2 2 3 3 (1:4 mol/mol) b b 6 G/Zr–CeO 3254 (0.2 )15 (1 , 500) 2.0–4.5 1.60 (2.50/4.10) – 1 M LITFSI/TEGDME LOB [152] 7 TiN/C 7100 (100a)35 (200 , 1000) 2.0–4.5 1.15 (2.60/3.75) – LiCF SO /TEGDME (1:4, LOB [158] 3 3 mol/mol) a a 8 WC/C7000 (100 )36 (100 , 1000) 2.0–4.5 0.88 (2.75/3.63) – 1 M LITFSI/TEGDME LOB [165] b b 9 B C/CNT 16,000 (0.2 ) 120 (0.4 , 1000) 2.5–4.4 1.02 (2.73/3.75) – LiCF SO /TEGDME (1:4, LOB [166] 4 3 3 mol/mol) b b b b 10 β-FeOOH/C aerogels, 10,230 (0.1 ), 6050 (0.1 ) 60 (0.1 , 800), 42 (0.1 , 2.0–4.4 1.22 (2.69/3.91) – 1 M LITFSI/TEGDME rLOB [171] β-FeOOH/Super P 800) b b 11 VC/soil 7640 (0.2 ) 100 (0.2 , 1000) 2.3–4.2 1.59 (2.71/4.30) – 1 M LITFSI/TEGDME rLOB [172] a a 12 Polyimide/CNT11,000 (500 )137 (500 , 1500) 2.0–4.35 1.72 (2.63/4.35) – 1 M LITFSI/TEGDME LAB [173] b b b b 13 FePc/GNS, FePc/CNTs, 865.6 (0.5 ), 632.4 (0.5 ), 30 (0.5 , –), 30 (0.5 , –), 30 2.0–4.8 0.67 (3.00/3.67), 0.85 –1 M LiClO /ED/ LAB [174] b b FePc/AB 795.4 (0.5 ) (0.5 , –) (3.02/3.87), 1.05 DEC + 1 M LiNO /0.5 M (2.82/3.87) LiOH 14 MnO –LaNiO /CNT –75 (20 , –) 0.8–2.4 0.75 (1.20/1.95) – 6 M KOH + 0.4 M ZnO rZAB [176] 2 3 15 CNT/CoFe O 3670 (200a)35 (200 , 430) 2.0–4.3 1.55 (2.70/4.25) – 1 M LITFSI/TEGDME rLOB [178] 2 4 b b 16 CoMn O /rGO610 (20 )200 (20 , –), 0.8–2.4 0.95 (1.15/2.10) 6 M KOH ZAB [179] 2 4 17 VC/Co O –22, (20 , –), 0.5–3.0 1.25 (1.00/2.25) – 6 M KOH + 0.2 M Zinc rZAB [180] 3 4 acetate a a 18 SP/CaMnO7000 (200 )80 (200 , 1000) 2.0 -4.0 1.50 (2.25/3.75) –1 M NaSO CF /TEGDME NaOB [181] 3 3 3 m a a a 19 KB/RM-TIT , KB/RM-3250 (100 ), 2700 (100 ), 121 (400 , 1000), 105 2.0–4.8 0.74 (2.76/3.50), 1.25 – 1 M LITFSI/TEGDME rLOB [182] n a a a FIT , KB5950 (100 ) (400 , 1000), 24 (400 , (2.75/4.00), 1.70 1000) (2.68/4.38) a a 20 CNT@RuO4350 (100 )100 (500 , 300) 2.3–4.0 0.72 (2.70/3.42) – LITFSI/tri(ethylene) glycol rLOB [183] dimethyl ether (1:5 mol/ mol) a −1 mA g b −2 mA cm Initial overpotentials are obtained from the cyclic tests RuO tube in Mn O tube 2 2 3 RuO fiber in Mn O tube 2 2 3 Electrochemical Energy Reviews (2018) 1:1–34 27 composite with carbon to enhance bifunctional activities and interactions between different heteroatom dopants can stability. With the rapid development of nanotechnologies and also contribute significantly to electrocatalytic activities related sciences, processing techniques are becoming more for ORR and OER. Other material components such as facile in the fabrication of non-noble metal-based and metal- oxides and metals can have more effective influences on free bifunctional catalysts in scale-up productions. the electrocatalytic activity and cycling performance of carbon-composite catalyst MABs. In particular, perovskite oxides exhibit bifunctionality for catalytic ORR and OER 9 Summary, Challenges and Future in alkaline solutions. To solve issues of carbon corrosion Research Directions and oxidation in MABs, novel strategies such as carbon- composited ternary bifunctional catalysts have shown 9.1 Summary promising effects on the improvement of catalytic per - formances. Compared with individual components, these In this review, carbon-composited bifunctional catalysts ternary and/or trinary composite catalysts offer improved for MABs are comprehensively reviewed in terms of their catalytic performances and strong resistances to carbon material selection, synthesis method, structural characteri- corrosion in MABs. zation and electrochemical performance. Compared with single material catalysts, composited catalysts possess 9.2 Challenges synergistic effects on structural and electrical properties resulting from different components, leading to enhanced In recent years, considerable efforts have been concentrated ORR and OER performances in MAB air-electrode cata- on the development of carbon-composited bifunctional cata- lysts. In the design of advanced carbon-based composites lysts. Several major technological challenges are still pre- as bifunctional catalysts, the proper selection of carbon sent, however, in the development of commercial MABs: (1) materials is crucial because the properties of the carbon insuc ffi ient electrocatalytic activities for both ORR and OER material (e.g., porous structure, surface area and electronic at the air-electrode, leading to low energy/power density and conductivity) affect the transport of ionic/electronic/gas, efficiency of MABs; (2) insufficient stability/durability of electron conductivity and the distribution of catalytic bifunctional air-electrode catalysts due to low resistances to sites. In addition, the strong interaction between the car- electrochemical corrosion, resulting in degradation of MAB bon materials with other component(s) determines the performance; (3) insufficient strategies for catalyst design, bifunctional activities of ORR and OER and their stability/ starting material selection, scalable synthesis and catalyst durability. As compared with other porous structures, (i.e., performance optimization; (4) unoptimized MAB electrode/ macropore and microporous structures), it is known that cell design and fabrication; and (5) insufficient fundamental mesoporous structures coupled with high surface areas are understanding of catalyst interaction, synergy and bifunc- more beneficial to ionic and electronic transport because tionality mechanisms as well as MAB electrode/cell design of shorter conducting paths. Mesoporous structures also and fabrication. Overall, the immature capability for bifunc- possess the ability to retain electrocatalytic active sites tional catalyst optimization and scale-up production with with uniform distribution. cost-effective approaches for MAB air electrodes is a major As a special type of carbon material, graphene tends to hindrance and the understanding of fundamentals is highly hold active sites at the edge rather than the mesoporous useful for material selection and optimization of catalyst/ structure. However, the restacking of graphene layers is electrode designs. a vital issue that needs to be resolved in the development of advanced graphene-composited bifunctional catalysts 9.3 Proposed Future Research Directions for MABs. It is should also be emphasized that not all non-car- To overcome these technical challenges, several future bon material(s) are able to act as useful component(s) research directions can be proposed as follows for the fab- in carbon-composited bifunctional catalysts in terms of rication of next-generation carbon-composited bifunctional producing synergistic interactions and overcoming carbon catalysts for MAB air electrodes: corrosion/oxides in MABs. Heteroatom(s) can, however, be used as non-carbon component(s) for carbon com- 1. Improving the catalytic activity and stability/durability posite catalysts, and the resulting heteroatom(s)-doped of carbon-composited bifunctional catalysts for MAB carbon materials can exhibit improvements in ORR and air electrodes by developing novel methodologies for OER performance. 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He also serves as an associate editor for RSC tery: understanding the structure and morphology changes of Advances. graphene-supported CoMn O bifunctional catalysts under prac- 2 4 tical rechargeable conditions. ACS Appl. Mater. Interfaces 6, Dr. Dan Zhang is a Lecturer at the 16545–16555 (2014) Shanghai Institute of Mathe- 180. Lee, D.U., Park, M.G., Park, H.W., et al.: Highly active and matic and Mechanics at Shang- durable nanocrystal-decorated bifunctional electrocatalyst for hai University. Dr. Dan Zhang rechargeable zinc–air batteries. Chemsuschem 8, 3129–3138 received her Ph.D. in Manufac- (2015) turing Engineering of Aerospace 181. Hu, Y., Han, X., Zhao, Q., et al.: Porous perovskite calcium–man- Vehicle from the Northwestern ganese oxide microspheres as an efficient catalyst for recharge- Polytechnical University in able sodium–oxygen batteries. J. Mater. Chem. A 3, 3320–3324 2006. Dr. Zhang’s current (2015) research interests include the 182. Yoon, K.R., Lee, G.Y., Jung, J.W., et al.: One-dimensional R uO / hydrodynamic of microfluidic Mn O hollow architectures as efficient bifunctional catalysts for devices and the reaction mecha- 2 3 lithium–oxygen batteries. Nano Lett. 16, 2076–2083 (2016) nism simulations of surface elec- 183. Jian, Z., Liu, P., Li, F., et al.: Core–shell-structured CNT@RuO trocatalysis in the field of fuel composite as a high-performance cathode catalyst for recharge- cells. able Li–O batteries. Angew. Chem. Int. Ed. 53, 442–446 (2014) Dr. Aijun Li is currently a Profes- sor at the School of Materials Dr. Yan‑Jie Wang obtained his Science and Engineering at the Ph.D. in Materials Science and Shanghai University (SHU). He Engineering from Zhejiang Uni- received his Ph.D. in Materials versity, China, in 2005. Subse- Science from Northwestern Pol- quently, he conducted two post- ytechnical University, Xi’an doctoral research positions at the China in 2004. Dr. Li worked as Sungkyunkwan University, a senior scientist and then a Korea, and at the Pennsylvania group leader for carbon materials State University, USA, respec- at the Karlsruhe Institute of tively. In 2009, he worked as a Technology (KIT), Germany senior scientist at the University from 2010 to 2015, being of British Columbia, Canada, in involved with the research, cooperation with the National development and application of Research Council of Canada and composites. Dr. Li’s main Vancouver International Clean- research interests are in the complex interactions of multi-physical and Tech Research Institute Inc., chemical phenomena involved in chemically reacting flows; mainly respectively. In 2017, he became a full-Professor at the Dongguan Uni- focusing on modeling, simulation and synthesis of composites by versity of Technology, China. He is also an adjunct professor at the chemical vapor infiltration/deposition processes. Fuzhou University. His interests include energy storage and conversion, polymer science, biomass, and medical areas. 1 3 34 Electrochemical Energy Reviews (2018) 1:1–34 Dr. David P. Wilkinson is a profes- Lei Zhang is a Senior Research sor and Canada Research Chair Officer at National Research in the Department of Chemical Council Canada (NRC), a Fellow and Biological Engineering at of the Royal Society of Chemis- the University of British Colum- try (FRSC), an adjunct Professor bia (UBC). He previously held of various Universities, and a the positions of Executive direc- vice president of International tor of the UBC Clean Energy Academy of Electrochemical Research Center, Principal Energy Science (IAOEES). Lei’s Research Officer and Senior research interests include PEM Advisor with the National Fuel Cell electrocatalysis, super- Research Council of Canada capacitors, metal-air batteries, Institute for Fuel Cell Innova- batteries and hybrid batteries. tion, Director and Vice President She has co-authored more than of Research and development at 170 publications. She is the Ballard Power Systems, and member of the NSERC Indus- Group Leader at Moli Energy. His main research interests are in elec- trial R&D Fellowships College of Reviewers, the Editorial Board trochemical and photochemical devices, energy conversion and storage Member of Electrochemical Energy Reviews (EER) -Springer Nature. materials, and processes to create clean and sustainable energy and She is also an active member of the Royal Society of Chemistry (RSC), water. the Canadian Society for Chemistry (CSC), and the Canadian Society for Chemical Engineering (CSChE). Dr. Anna Ignaszak is an Assistant Professor at the University of Dr. Jiujun Zhang is a Professor in New Brunswick and an adjunct College of Sciences, Institute for assistant professor at the Frie- Sustainable Energy at Shanghai drich-Schiller University (Ger- University. He was a Principal many). She completed an Research Officer at the National appointment as a research asso- Research Council of Canada ciate at the Clean Energy (NRC) from 2004 to 2016. Dr. Research Center, at the Univer- Zhang received his B.S. and M.Sc. sity of British Columbia (Can- in electrochemistry from Peking ada), and as a research associate University in 1982 and 1985, at the National Research Coun- respectively, and his Ph.D. in elec- cil of Canada. She has a diverse trochemistry from Wuhan Univer- background in materials (car- sity in 1988. He then carried out bons, composites, metal clus- three terms of postdoctoral ters) for electrochemical energy research at the California Institute storage and conversion, electrochemical sensors, and heterogeneous of Technology, York University, catalysis. The research conducted in her laboratories in Canada and and the University of British Columbia. Dr. Zhang has over 30 years of Germany aims to synthesize morphology-controlled catalysts, under- scientific research experience, particularly in the area of electrochemical standing the structure–reactivity interplay for optimum redox energy storage and conversion. He is also the Adjunct Professor at the activity. University of British Columbia and the University of Waterloo. 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Electrochemical Energy Reviews Springer Journals http://www.deepdyve.com/lp/springer-journals/a-review-of-carbon-composited-materials-as-air-electrode-bifunctional-loSLOeMMt5

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Publisher: Springer Journals
Copyright: 2019 The Author(s)
ISSN: 2520-8489
eISSN: 2520-8136
DOI: 10.1007/s41918-018-0002-3
Publisher site: See Article on Publisher Site

Abstract

Metal–air batteries (MABs), particularly rechargeable MABs, have gained renewed interests as a potential energy stor- age/conversion solution due to their high specific energy, low cost, and safety. The development of MABs has, however, been considerably hampered by its relatively low rate capability and its lack of efficient and stable air catalysts in which the former stems mainly from the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and the latter stems from the corrosion/oxidation of carbon materials in the presence of oxygen and high electrode potentials. In this review, various carbon-composited bifunctional electrocatalysts are reviewed to summarize progresses in the enhancement of ORR/OER and durability induced by the synergistic effects between carbon and other component(s). Catalyst mechanisms of the reaction processes and associated performance enhancements as well as technical challenges hindering commercialization are also analyzed. To facilitate further research and development, several research directions for overcoming these challenges are also proposed. Keywords Metal–air batteries · Oxygen reduction reaction · Oxygen evolution reaction · Carbon · Bifunctional electrocatalysts · Synergistic effect PACS 88.80.ff Batteries · 88.80.F− Energy storage technologies · 82.47.Aa Lithium-ion batteries 1 Introduction Metal–air batteries (MABs), in particular rechargeable MABs, possessing high specific energy, low cost, and safety [1, 2], have gained great attention in recent years due to their * Anna Ignaszak feasibility as electrochemical energy storage/conversion [email protected] solutions. A MAB system (see Fig. 1) is an electrochemical * Jiujun Zhang system consisting of a pure metal or metal alloy electrode [email protected] for metal oxidation reactions (discharge process) and metal compound reduction reactions (charge process), as well as a School of Sciences, Institute for Sustainable Energy, Shanghai University, Shanghai 200444, China second electrocatalyst coated air-electrode for oxygen reduc- tion reactions (ORR, discharge process) and oxygen evolu- School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, Guangdong, tion reactions (OER, charge process). Between these two China electrodes is an electrolyte serving as both a separator and Department of Chemical and Biological Engineering, an ion conductor [3]. Currently, the metal or alloy electrodes University of British Columbia, Vancouver, BC V6T 1Z3, that have been developed in the fabrication of RMABs con- Canada sist of Li, Zn, Al, Fe, Na, Ca, Mg, K, Sn, Si, Ge and/or their Department of Chemistry, University of New Brunswick, alloys [1, 4, 5] and typical catalysts of air electrodes include Fredericton, NB E3B 5A3, Canada materials based on Pt, Ir, Ru and their alloys as well as vari- Energy, Mining and Environment, National Research Council ous non-noble metals [6–10]. Canada, Vancouver, BC V6T 1W5, Canada Vol.:(0123456789) 1 3 2 Electrochemical Energy Reviews (2018) 1:1–34 Fig. 1 A basic metal–air battery (MAB) configuration with a simplified solid–liquid–gas three-phase zone Although MABs with oxygen in air possess drastically air electrodes, this review examines the most recent pro- higher theoretical energy densities than traditional aqueous gresses and research trends in both experimental and char- and lithium-ion batteries, the development and commerciali- acterization strategies, providing up-to-date knowledge and zation of MABs faces one major challenge: a lack of efficient information on MAB bifunctional catalysts. The reaction and robust bifunctional air-electrode catalysts which signifi- mechanisms of the catalytic processes, the catalyst compo- cantly limits battery performance in terms of both rate capa- nent interactions in the presence of carbon, the synergetic bility and long-term stability [4, 6]. To overcome this chal- effects induced by the addition of carbon into catalyst mate- lenge, global research of bifunctional air electrodes for MABs rials, as well as the relationship between physicochemical and in particular electrocatalysts for both ORR and OER has structure and catalytic performance are reviewed and ana- rapidly progressed in recent years. With respect to this, carbon lyzed. Current achievements and challenges in synthesizing materials have been employed as supports and components for catalysts and fabricating air electrodes are also summarized. catalysts to improve catalytic activity and stability. Future research directions are also proposed in this review to Normally, the air-electrode catalyst layer for both ORR accelerate research and development in this area to overcome and OER is a matrix structure containing primarily of cata- challenges. lyst particles and/or carbon particle-supported catalyst particles and ionomers [4, 5, 11, 12]. To synthesize these catalysts and fabricate these catalyst layers, different nano- 2 Composites of Different Carbons technologies, associated characterization techniques as well as performance validation methods have been carried out Combinations of two different carbon materials have been to significantly optimize the morphology and surface area previously reviewed and analyzed as modified carbon sup- of these advanced nanostructured catalysts to obtain high port materials for high-performance Pt-based fuel cell cata- catalytic activity and stability. As recognized, the interac- lysts [13]. Here, based on the advantages of different carbon tions and synergetic effects between carbon and other com- materials, formed carbon–carbon composite supports show ponents play an important role in performance optimization. enhanced ORR activities for Pt-based catalysts in polymer Tailored designs and geometries of such carbon-composited electrolyte membrane fuel cells (PEMFCs). Similar combi- bifunctional catalysts are also recognized as major factors in nations of two or more different carbon materials have also promoting catalytic activity and chemical/electrochemical been explored as metal-free bifunctional composite electro- stability. In addition, bi- or multi-carbon component cata- catalysts for RMABs. lysts can constitute new composites which not only possess In regard to zero-dimensional carbon nanostructures with original carbon characteristics but also present new proper- atomic sizes below 100 nm, graphene quantum dots (GQDs) ties for improving catalytic activity and stability. exhibit satisfactory ORR performance due to strong quan- With a focus on advanced carbon-composited materi- tum confinement, edge effects, and oxygen-rich functional als as bifunctional electrocatalysts for rechargeable MAB groups on the material surface [14, 15]. By encapsulating 1 3 Electrochemical Energy Reviews (2018) 1:1–34 3 GQDs in a graphene hydrogel structure, Wang et al. [16] the combined results in typical SEM and TEM images reported a carbon–carbon composite electrocatalyst in (Fig. 2b–e) not only demonstrate the porous structure of the which the GQDs provided more active sites and the gra- graphene hydrogel, but also indicate the existence of GQDs phene hydrogel served as a conductive substrate to fix and with different sizes ranging from 2 to 20 nm in the porous sputter GQDs to prevent agglomeration and facilitate elec- structure. For GQDs in particular, HRTEM images (Fig. 2f) tron transfer. According to the proposed synthesis mecha- proved the graphitization of carbon dots with no peaks being nism as shown in Fig. 2a, graphene quantum dot/graphene found at ~ 270 nm in the tested UV–Vis absorption spectra, hydrogel (GH–GQD) composite samples are prepared and confirming the presence of rich edges in the GQDs [15, 17]. labeled as GH, GH-GQD-45, GH-GQD-90, GH-GQD-180, At room temperature, the measured RDE results revealed corresponding to GQD amounts of 0, 45, 90, and 180 mg. In that the GH–GQD composite samples exhibited better ORR the structural characterization of these GH-GQD samples, activities than individual GH and graphene oxide because of GQDs. Reprinted with permission from Ref. [16]. Copyright 2016 Fig. 2 a Illustration of the synthesis of GQDs, b SEM image of Royal Society of Chemistry GQD-GH-90 with a scale bar of 500 nm, and TEM images of c GH, d GQD-GH-90, and e GQD, and f HRTEM image (scale bar = 5 nm) 1 3 4 Electrochemical Energy Reviews (2018) 1:1–34 of the synergistic effects between GH and GQDs in which mesoCs core–shell composite material. In their structural GQDs provided more active sites and GH served not only characterizations, the collected Raman spectra revealed that as a conductive substrate for enhancing the charge transfer pristine CNFs possessed a lower peak ratio of D band to of electrodes, but also as a good support material condu- G band than CNF@mesoCs, indicating that the surface of cive to the high dispersion of GQDs and their active sites. CNF@mesoCs possessed a more graphitized structure than The GH-GQD-90 sample demonstrated the highest ORR pristine CNFs. The CNF@mesoCs also displayed a better −1 onset potential of − 0.13 V among all GH–GQD compos- electrical conductivity of ~ 4.638 S cm and a larger surface 2 −1 −1 ite samples, indicating that the optimized GQD amount area of 2194 m g than pristine CNFs (~ 3.0759 S cm and 2 −1 is 90 mg. Linear sweep voltammetry (LSV) curves run in 708 m g ), demonstrating the effects of mesoporous carbon O -saturated 0.1 M KOH were used to evaluate the ORR onto CNFs in this CNF@mesoCs composite. The electro- mechanism, and it clearly demonstrated that the enhanced chemical performance of the CNF@mesoCs composite was electrocatalytic activity resulting from the rich edge defects also examined under an oxygen atmosphere in a discharging of the GH-GQD-90 composite sample was related to O test using a fabricated LAB with non-aqueous electrolyte. reactions through both two-electron and four-electron reac- Here, it was found that as the cathode, CNF@mesoCs dem- −1 tion pathways [18] as indicated by tested peroxide percent- onstrated a higher discharge capacity (4000 mA h g ) than −1 ages and electronic transfer numbers. Interestingly, further pristine CNFs (2750 mA h g ) because of its better electri- investigations of GH-GQD-90 as a cathode electrocatalyst cal conductivity and larger mesopore size and volume. And in a primary Zinc–air battery (ZAB) showed that the tested although battery performances need to be further improved, galvanodynamical discharging of the battery can increase to this research provides a significant new design route to cre- −2 100 mA cm , demonstrating that GH-GQD-90 is an effec- ate advanced core–shell catalysts for LAB cathodes. tive ORR catalyst that can provide sufficiently high battery performances. However, GH-GQD-90 only possessed com- parable discharge properties to commercial 20 wt% Pt/C at 3 Composites of Carbon and Non‑metals −2 higher current densities (~ 20 mA cm ) and no OER was discussed in the study. To further demonstrate the advan- 3.1 Composites of Carbon and Single Elements tages of hybrid phases in the development of advanced car- bon–carbon composite electrocatalysts in metal–air batteries In this subsection, carbon materials composited with a (MABs) applications, Luo et al. [19] prepared composite non-metal heteroatom such as nitrogen (N), boron (B), or cathode catalysts using Ketjan Black (EC-600JD)–carbon phosphorus (P) are reviewed. These non-metal heteroatoms paper (KB/CP) in which the ultra-large specific discharge are usually introduced into pure carbon to form RMAB air- capacity of the KB/CP cathode in a lithium–air battery (LIB) electrode catalysts through doping strategies. As identified, was found to be much greater than that of individual KB and dopants can act as a secondary phase; providing adsorption CP cathodes. This demonstrates that synergetic effects are structures for charge transfer and improving electrocata- present between the two different carbon components even if lytic activities of ORR and/or OER [21, 22]. Recently, vari- the two components have different structural morphologies ous carbon materials (e.g., porous carbon materials, CNT, and physical properties. graphenes, carbon fibers, carbon xerogel, carbon aerogel, Aside from hybrid phases, core–shell structured compos- nanocage carbons, carbon nano-onions) have been explored ite catalysts have also been explored as MAB cathodes in as heteroatom-doped carbon catalysts for BMABs and the which one carbon material acts as the core and the other results show that these composited carbon materials exhibit carbon acts as the shell. To improve the electrochemical desirable characteristics such as desirable pore size/volume, reactions of lithium–air batteries (LABs), Song et al. [20] large surface area, and high stability. designed and synthesized a carbon nanofiber@mesoporous carbon (CNF@mesoCs) core–shell composite catalyst to 3.1.1 N‑Doped Carbons optimize the advantages and characteristics of two individ- ual carbon materials. By coating mesoporous carbon onto To promote electrochemical properties, the doping of het- carbon nanofibers (CNFs), the resulting structure not only eroatoms is often used to modify the nature and chemical enlarged surface areas but also provided a highly conduc- properties of pure carbon materials and obtain advanced tive graphitized surface to increase electrical conductivity. metal-free carbon catalysts for RMABs. As a typical heter- In their synthesis, CNFs were first produced as a core that oatom, nitrogen (N) has become the most popular dopant to entangled with one another to form a self-standing three- facilitate catalytic reactions of carbon-based composite cath- dimensional cross-linked web structure using electrospin- odes because N has a larger electronegativity than carbon, ning techniques. Nanocasting was then carried out to coat a comparable atomic size to carbon, and five valence elec- mesoporous carbon on to the CNFs to form the final CNF@ trons for bonding with carbon atoms [23, 24]. Specifically, 1 3 Electrochemical Energy Reviews (2018) 1:1–34 5 N-doped carbon materials possess structural defects and can pyrrolic (399.4 eV), and graphitic (401.0 eV) nitrogen, result withdraw electrons from carbon atoms to enhance the con- from increasing carbonizing temperatures, demonstrating ductivity of the carbon material and thus improve battery increasing ratios of graphitic nitrogen. This correlates with performance [25–29]. XRD results in which meso-NdC-1000 gives two typical To develop favorable mesoporous structures and highly peaks at (002) and (101) that are associated with a higher active carbon-based advanced catalysts for LAB cathodes, degree of graphitization as compared with meso-NdC-900. Sakaushi et al. [30] investigated ionic liquid (IL)-based Elemental analysis showed that meso-NdC-1000 possessed mesoporous nitrogen-doped carbons (meso-NdCs) with a higher N content of ca. 12 wt% than meso-NdC-900 (~ 6 a designed mesoporous structure. In their silica template wt%). Based on a combination of Ex-situ XRD measure- assisted (Ludox HS40) carbonization method, ionic liquid ments and charge–discharge tests in an assembled LAB (N-butyl-3-methylpyridinium dicyanamide) was used as cell (the cell use 0.5 M LITFSI in TEGDME (Aldrich) as the source of both N and C, and carbonization temperatures the electrolyte and was measured in the potential range of + −1 of 900 and 1000 °C were performed to prepare two sam- 2.0–4.0 V versus Li/Li at 100 and 200 mA g ), meso- ples: meso-NdC-900 and meso-NdC-1000 [29, 31]. In the NdC-1000 was found to possess a lower discharge over- structural characterizations, the measured broad XRD pat- potential of 0.3 V as compared with metal-based catalysts terns and TEM images revealed typical characteristics of in LAB and the lowest charge overpotential of 0.45 V −1 highly porous materials with an average pore diameter of ca. (Table 1). At 100 and 200 mA g , meso-NdC-1000 also 8 nm as induced by defects at the carbon walls or irregular possessed specific discharge capacities of ca. 1750 and 1280 −1 fragments [32]. Importantly, as shown in N1s XPS spec-mAh g , indicating acceptable rate capabilities in LABs. tra (Fig. 3), three N species, such as pyridinic (398.3 eV), These results suggest that meso-NdC-1000 catalysts with Fig. 3 a Schematic illustration of different chemical structures of 398.3, 399.4, and 401.0 eV corresponding to pyridinic, pyrrolic, and doped nitrogen in a graphene framework. b, c X-ray photoemission graphitic nitrogen respectively. Reprinted with permission from Ref. spectra of meso-NdC-900 and meso-NdC-1000, with the peaks at [30]. Copyright 2015 John Wiley and Sons 1 3 6 Electrochemical Energy Reviews (2018) 1:1–34 Table 1 Compared data on the a −1 −1 Catalyst η (V) Q (mAh g ) I (mA g ) Electrolyte OER disk electrochemical properties of several air electrodes. Reprinted Noble carbon 0.45 1750 100 1 M LiTFSI in TEGDME with permission from Ref. [30]. Porous Au 0.6 325 5000.1 M LiClO in DMSO Copyright 2015 John Wiley and Co O /rGO 0.6 14,000 1401 M LiPF in TEGDME 3 4 6 Sons TiC 0.8 350 2500.5 M LiClO in DMSO Co O /carbon 0.9 2000 701 M LiPF in PC 3 4 6 α-MnO /carbon 1.0 2500 701 M LiPF in PC 2 6 Carbon (Super-S) 1.8 850 701 M LiPF in PC Comparison of average overpotentials@recharging (η ) for several catalysts to fully recharge a LAB. OER Here, overpotential (V) = E − 2.96. E (V vs. Li/Li ) is the average reaction potential of a LAB at OER OER recharging, that is, an OER reaction. The value 2.96 (V vs. Li/Li ) is the electromotive force of the follow- + − ing reaction: O + 2Li + 2e ↔ Li O . The η data are selected from reports showing full discharge- 2 2 2 OER recharge measurements b + LiFePO /carbon is substituted for Li metal anode for this measurement to supply Li . Other measurements use Li metal-based anodes Q is calculated based on the mass of carbon disk increased ratios of graphitic nitrogen at higher temperatures curves from − 0.8 to 0.05 V versus Hg/HgO in O -saturated possess favorable structures induced by doped nitrogen in 0.1 M KOH solution at an electrode rotating rate of 1600 r −1 mesoporous carbon. min showed that HMC provided higher ORR kinetic-lim- −2 In addition to the N-doped mesoporous carbon materi- iting current densities (~ − 4.95 mA cm ) than benchmark −2 als discussed above, other N-doped mesoporous carbon Pt/C catalysts (~ − 4.39 mA cm ) and more positive ORR materials have also been obtained for MAB cathode cata- onset potentials (ca. − 0.055 V) than Pt/C (ca. 0.001 V). In lysts [33–38]. For instance, a novel hierarchical N-doped a ZAB with 6 M KOH assembled using a Zn electrode, a carbon ORR catalyst (labeled by N:C-MgNTA) with a gra- separator, and an air cathode, the HMC catalyst also pro- phitic shell was prepared by Eisenberg et al. [38]. These vided a charge–discharge potential gap lower than that of 40 researchers employed an in situ templating method cou- wt% Pt/C (~ 60%). These results highlight the merits of the pled with etching and pyrolysis to synthesize microporous, structure-designed HMC material with its small charge–dis- mesoporous, and macroporous structured catalysts. The charge voltage polarization and high stability over repeated coexistence of microporous, mesoporous and macroporous cycling and provide new avenues to develop cost-effective, structures was demonstrated to provide short electron and high-performance metal-free electrocatalysts for MABs. ion transport paths and increased active surfaces, benefiting Interestingly, some groups [41, 42] have used biomass to catalytic activities [39, 40]. Based on an investigation of create porous carbon cathode materials for MABs and have electrocatalytic ORR in O - and N -saturated 0.1 M KOH produced decent electrocatalytic activity and stability, as 2 2 solutions, the researchers found that the synthesized N:C- well as satisfactory battery performance. MgNTA exhibited more effective ORR activities with a Among N-doped graphene materials [43–45], a special 4e transfer mechanism and was more stable than commer- N-doped three-dimensional graphene (N-3DG) cathode cial 20 wt% Pt/C or the undoped carbon reference sample. catalyst was designed and fabricated for LABs by He et al. A spherical N-doped hollow mesoporous carbon (HMC) [45]. In their study, melamine was selected as the nitrogen material was also obtained as an efficient ZAB cathode by source due to its high nitrogen content and strong interac- Hadidi et al. [34], in which hollow and mesoporous car- tions with graphene oxide (GO) to avoid severe stacking in bon structures were combined with N-doping to obtain a the structure of the graphene nanosheets (GNS) in 3DG. novel carbon bifunctional electrocatalyst through polym- A combination of TEM and EDX mapping images of the erization and carbonization of dopamine on a sacrificial N-3DG, coupled with SEM images, revealed a highly porous spherical silica (SiO ) template. Here, both polydopamine two-dimensional structure for the nitrogen-doped graphene (PDA) and F127 acted as excellent carbon sources and PDA nanosheets with a homogeneous distribution of carbon, afforded the doping of N. A combination of SEM, TEM and nitrogen and oxygen elements. Further comparisons of the high-resolution TEM (HR-TEM) images revealed a hollow N-3DG and 3DG samples in N adsorption–desorption anal- mesoporous sphere after the silica was removed in hydro- ysis indicated that due to the relatively severe stacking of fluoric acid solution. Carbon shells (ca. 21 nm ± 28%) were GNSs in the 3DG, more mesopores and micropores favoring also formed after carbonization, revealing graphitic and fast O diffusion and electrolyte infiltration were formed in amorphous domains. Linear sweep voltammetry (LSV) the N-3DG than the 3DG. In XPS, the revealed pyridinic 1 3 Electrochemical Energy Reviews (2018) 1:1–34 7 N (~ 54%) was regarded as an effective N type to improve mechanical and cycling stabilities with low overpotentials (a ORR in the graphene plane or edge because it can donate high discharge and low charge voltage of ~ 1.00 and 1.78 V −2 more available lone electron pairs for effective oxygen acti- at 2 mA cm , respectively) and long cycle life (6 h, and vation [46]. Furthermore, according to first-principle com- can be recharged by the mechanical replacement of the Zn putations [47], carbon sheets with pyridine N were found to anode) even under repeated bending conditions. The synthe- be more thermodynamically favorable in attracting Li and sized N-dope CP cathodes were tested in an assembled LAB lithiated pyridinic N can provide excellent active sites for O with a non-aqueous electrolyte (1 M lithium perchlorate in adsorption and activation in the discharge process of LABs. dimethyl sulfoxide) and produced a cyclability of more than −2 During testing of the N-3DG and 3DG-based cathodes in a 30 cycles at a constant current density of 0.1 mA cm . The −1 LAB, N-3DG demonstrated a higher ORR onset potential first discharge capacity reached 8040 mAh g with a cell −2 and a higher cathodic peak current, implying higher ORR voltage of ~ 2.81 V at 0.1 mA cm , and the coulombic −1 −2 activities than 3DG. At 50, 100, and 200 mA g , the tested efficiency was 81% on the first cycle at 0.2 mA cm . These discharge capacities of N-3DG were 7300, 5110, and 3900 results indicate that N-doped CP materials are promising −1 mAh g , respectively, corresponding to average operating in the development of low-cost, versatile paper-based O voltages at 2.67, 2.64, and 2.56 V. These were all higher electrodes for LABs. than those of 3DG. In cycling performance measurements −1 −1 with a cutoff capacity of 500 mAh g at 100 mA g , the 3.1.2 Other Heteroatom‑Doped Carbons N-3DG cathode exhibited better cycling performances over 21 cycles with a more stable reversible capacity and a better Like nitrogen, other heteroatoms such as boron (B), phos- initial round-trip efficiency of 65.9%, whereas 3DG only phorus (P), sulfur (S), and fluorine (F) have also been selec- performed for about 8 cycles with an initial cycle round-trip tively used as doped carbon materials for MABs to produce efficiency of 60.7%. different physicochemical properties. In contrast to N, B Similar to porous carbon and graphene, CNTs [27, possesses lower electronegativity (~ 2.04) than C (~ 2.55) 48, 49], CNFs [50–52], and carbon papers [53] have also [54], and if doped to carbon, B becomes an electron accep- attracted much research interest in the development of tor, inducing a positive charge and resulting in charged sites N-doped carbon cathodes for MABs. Mi et al. [27] used (B ) that favor O adsorption and thus increased ORR activ- a floating catalyst chemical vapor deposition (FCCVD) ity [55, 56]. To study the important effects of B-doping and method to synthesize a N-CNTs composite material in to create sufficient active sites, researchers [ 54, 57, 58] have which ethylene and melamine were used as the carbon and focused their attention on combining mesoporous carbon −1 nitrogen sources. In a discharge capacity test at 100 mA g structures with B-doping to explore advanced metal-free using both carbonate-based (propylene, carbonate/ethylene, carbon catalysts. For example, Shu et al. [57] prepared carbonate, PC/EC) and ether-based (1,3-dioxolane/ethylene novel mesoporous boron-doped onion-like carbon (B-OLC) glycol dimethyl ether) electrolytes, N-CNTs produced bet- microspheres using nanodiamond and boric acid as C and ter discharge capacities in both electrolytes compared with B sources for rechargeable sodium–oxygen (Na–O ) bat- CNTs. This is because of the better dispersion of N-doping teries. In their structural characterizations, a comparison and more available sites for O adsorption and reduction, of HRTEM images and selected area electron diffraction as well as a better electrical conductivity facilitating the (SAED) patterns of their B-OLC and OLC samples revealed reduction of kinetics during discharge. The enhancement the existence of multilayered sp fullerene shells forming a of ORR activity after N-doping was attributed to three quasi-spherical onion-like nanoparticle with particle sizes main reasons: improved conductivity, more nucleation sites of 5–8 nm. The appearance of the graphite ring (002) in around nitrogen and less aggregation of discharge products. the SAED patterns also confirmed the effective conversion CNFs and CPs were subsequently used as N-doped carbon of nanodiamonds into sp carbon phases with an identified cathode catalysts in MABs [50–53] and the nitrogen-doped interlayer spacing (~ 0.34 nm) that did not change after CNFs tested as the air cathode in a liquid ZAB operating B-doping [59, 60]. Based on the B 1s spectrum in XPS, six in ambient air produced a maximum power density of 185 deconvoluted peaks were obtained at 185.5, 188.8, 190.2, −2 mW cm and a maximum energy density of ~ 776 Wh 191.5, 192.9 and 194.8 eV, corresponding to BC, BC , 4 3 −1 kg with a high open-circuit voltage (1.48 V) [50]. The BCO, BCO and B O respectively [61, 62]. The six spe- 2 2 2 3 corresponding rechargeable liquid ZAB showed a small cies of B were also found in the XPS spectra of the refer- −2 charge–discharge voltage gap (0.73 V at 10 mA cm ), high enced B-doped super P (B-Super P), confirming the suc- reversibility (initial round-trip efficiency of 62%) and sta- cessful doping of boron, in which B-super P possessed a bility (voltage gap increased to ~ 0.13 V after 500 cycles). lower content (~ 0.83 at%) than B-OLC (~ 5.47 at%). At −2 Moreover, other flexible all-solid-state rechargeable ZABs 0.15 mA cm , the obtained discharge/charge curves in an using N-doped CNF films as a cathode displayed excellent assembled sodium–air battery showed that compared with 1 3 8 Electrochemical Energy Reviews (2018) 1:1–34 OLC and B-Super P cathodes, the B-OLC cathode delivered that the ORR activity increased significantly in the order: −1 a higher discharge capacity (~ 10,200 mAh g ) and a lower pure carbon < P-C-1 < P-C-2 < P-C-5 < P-C-4 < P-C-3. overpotential, demonstrating higher bifunctional activities Moreover, P-C-3 possessed a shift of ~ 70 mV in the half- toward ORR and OER. In the investigation of rate capabili- wave potential as compared with 20 wt% Pt/C. Tested LSVs −2 −1 ties, it was found that at 0.6 mA cm , B-OLC presented in O -saturated 0.1 M KOH with a scan rate of 10 mV s −1 a discharge capacity of ~ 7455 mAh g . This was nearly at different electrode rotating rates demonstrated that the −1 40 times greater than that of Super P (~ 160 mAh g ) and P-C-3 catalyst provided an ORR electron transfer number −1 about twice that of OLC (~ 3558 mAh g ). This enhanced of ~ 3.81, suggesting a four-electron reaction pathway for rate capability was attributed to the synergistic effects P-C-3 in ORR similar to 20 wt% Pt/C. According to DFT between B doping, mesoporous structures and particle calculations, the amount of P-doping was critical in the sizes of OLC. Interestingly, B-Super P produced similar improvement of ORR in P-doped carbon xerogels. Based battery performances compared to Super P. This is possibly on ORR behaviors in which ORR activity increased with because a low B doping content (~ 0.83 at%) produces no increasing P content from 0.78 at% (P-C-1) to 1.64 at% effects on super P. In a further examination of reversibility, (P-C-3) and then decreased with increasing P content from −2 cycling performance tests were conducted at 0.3 mA cm , 2.77 at% (P-C-4) to 3.56 at% (P-C-5), the optimal P content and B-OLC was found to possessed better cyclability (up should be 1.64 at%, providing maximum ORR activity. to 125 cycles) than OLC (~ 56 cycles) and Super P (~ 6 To obtain optimal bifunctional catalysts, an important cycles), demonstrating the combined effects of B-doping and comparison of the different types of heteroatom-doping mesoporous structures. was conducted by Su et al. [58], in which N, B, and P were P is an electron donor even though the electronegativity used to dope ordered mesoporous carbons (OMCs). The of P (~ 2.19) is lower than C (~ 2.55), and the incorpora- researchers found that improvements to charge transfer tion of P into carbon by P-doping is easier than that of N kinetics is strongly depended on the nature of the doped or B due to a much larger covalent radium [(107 ± 3) pm] heteroatom. At a doping level below 1.0 at%, the ORR than N [(71 ± 1) pm] and B [(84 ± 3) pm]. Similar to the activity tested in alkaline solutions increased in the order combination of B-doping (or N-doping) and porous struc- of: N-OMCs < P-OMCs < B-OMCs. No data was reported tures, P-doping is used in combination with porous struc- for stability in their study, however. Another study was con- tures in the development of carbon-containing bifunc- ducted by Zhang et al. [69] on heteroatom(s)-doped graph- tional composite catalysts for MABs [63–65]. Using a diyne, in which the dopants were N, S, B, and F. However, sol–gel polymerization method followed by pyrolysis and this research only focused on the comparison between single P-doping, Wu et al. [63] developed a low-cost and highly heteroatom and dual atom-doped graphdiyne cathode mate- active P-doped carbon xerogel electrocatalyst to examine rials. These will be discussed in the next section. the effects of P-doping and porous structures on ORR. In their synthesis, resorcinol and formaldehyde were used as 3.2 Composites of Carbon and Dual Elements the carbon sources and H PO was used as the phospho- 3 4 rus source. The as-prepared samples were labeled as P-C-1, Compared with single heteroatom-doped carbon materials P-C-2, P-C-3, P-C-4 and P-C-5, corresponding to weight with ORR activities that are inferior to conventional Pt/C ratios of 1:10, 2:10, 3:10, 4:10, and 5:10 phosphoric acid catalysts, especially in acidic media and neutral solutions, to carbonized sample (Co–C). In their structure characteri- doped carbon materials with two or more selected heter- zations, a combination of XRD and Raman spectroscopy oatoms have been predicted to further improve ORR activ- confirmed increased defects and disordered carbon after ity due to the synergetic effects arising from the co-doping P-doping corresponding with increasing P contents from of two or more heteroatoms in carbon [70]. Based on the 0.78 to 3.56 at%. The P content measured by inductively fact that co-doping N-doped carbon with a second heter- coupled plasma-atomic emission spectroscopy (ICP-MS) oatom (e.g., B, S or P) may improve ORR activity [71–73], were 0.78, 1.41, 1.64, 2.77, and 3.56 at% for P-C-1, P-C-2, Zhang et al. [74] conducted a scalable fabrication of three- P-C-3, P-C-4, and P-C-5 samples, respectively. In the P 2p dimensional N and P co-doped mesoporous nanocarbons spectra of XPS, two deconvolved contributions at 132.5 and (NPMC foams) and investigated the OER of these co-doped 134.5 eV can be assigned to P–C bonding and P–O bonding carbon materials. In a template-free method, three N, P co- [66, 67], respectively. The presence of P–O bonding con- doped mesoporous carbon (NPMC) foams (i.e., NPMC-900, firms the formation of P–O–C in the forms of CPO, C PO , NPMC-1000, and NPMC-1100) were prepared at anneal- 3 2 2 and CPO , indicating the successful doping of P [68]. In ing temperatures of 900, 1000, and 1100 °C, respectively. the examination of ORR at room temperature using rotating TEM images coupled with elemental mapping for the typi- ring-disk electrode (RRDE) technique, the comparison of cal NPMC-1000 sample revealed a uniform distribution of onset potential and diffusion limiting current density showed C, N and P. A combination of XRD, Raman spectra, and 1 3 Electrochemical Energy Reviews (2018) 1:1–34 9 TEM indicated that pyrolysis can resulted in a majority of were used as the N and B sources. In their investigation of the thermally stable graphic carbon domains being occupied co-doping, a combination of TEM, SEM, electron energy by the co-dopants of N and P from the PANI and phytic loss spectroscopy (EELS), and elemental mapping confirmed acid. Moreover, many edge-like graphitic structures were the uniform distribution of B and N in a preserved graphene found in the HRTEM images of the examined NPMC-1000 nanosheet morphology, and FTIR revealed the existence of sample which oversaw active sites and thus catalytic activ- B and N coupled to C, indicating that by-products such as ity. It was found that these NPMC samples possessed three- h-BN did not exist in the synthesized B, N-graphene sample. dimensional mesoporous structures with large surface areas, This was also confirmed by XPS in which a one-step syn - high pore volume and suitable pore sizes for electrocata- thesized h-BN/graphene reference sample clearly showed lytic activity. Both the existence of the four N species (i.e., the h-BN phase. Cyclic voltammetry curves measured in pyridinic, pyrrolic, graphitic, and oxidized pyridinic N) and an O -saturated 0.1 M KOH showed that B, N-graphene the two P-related bonds (i.e., P–C and P–O) [75] observed possessed a higher cathodic current density, indicating bet- in XPS confirmed the successful doping of N and P het- ter ORR activity than h-BN/graphene. A series of LSVs in eroatoms into the carbon network through thermal pyroly- O -saturated 0.1 M KOH solution were collected using a −1 sis. Based on further LSV curves obtained at 1600 r min RDE technique, and it was observed that the onset potential in O -saturated 0.1 M KOH solution with a scan rate of of B, N-graphene was closer to commercial Pt/C and higher −1 5 mV s , NPMC-1000 was found to provide the best ORR than the one-step synthesized single heteroatom-doped gra- activity with comparable onset potentials (~ 0.94 V) and phenes (such as N-graphene and B-graphene). These results half-wave potentials (~ 0.85 V) to Pt/C. Although NPMC- suggest that the co-doping of B and N can result in higher 900 possessed a higher N and P content than NPMC-1000, synergistic activities than single heteroatom-doped graphene the relatively low pyrolysis temperature led to a possible and that N-doping in N-graphene is more effective on the higher charge transfer resistance and thus relatively poorer improvement of activity than B-doping in B-graphene. RDE electrocatalytic activity. As for NPMC-1100, the removal experiments at different rotation rates ranging from 500 to −1 of doped heteroatoms at higher temperatures (~ 1100 °C) 2500 r min revealed an electron transfer number (~ 3.97) resulted in a decrease in active sites and therefore a decrease for B, N-graphene similar to commercial Pt/C (~ 3.98) and a in overall electrocatalytic activity. For OER, NPMC-1000 RRDE technique was further used to confirm the ORR path- and NPMC-1100 samples displayed lower onset potentials way. Importantly, their DFT calculations revealed significant and higher currents, demonstrating better OER activities synergistic effects resulting from the coupling interactions than Pt/C. Compared with RuO nanoparticles, NPMC-1000 between pyridinic N (one of N species) and B. electrodes exhibited lower onset potentials but less current Similar to N and P, oxygen (O) also possesses higher densities at greater potentials. Interestingly, as a cathode electronegativity than carbon. Based on the effects of co- material in primary and rechargeable ZABs, NPMC-1000 doping, Li et al. [23] developed a novel 3D O, N co-doped primary batteries demonstrated an open-circuit potential of carbon nanoweb (ON-CNW) material as a metal-free cata- −1 1.48 V with a specific capacity of 735 mAh g (correspond- lyst for hybrid LABs. In the deconvolution of N1s and O1s Zn −1 ing to an energy density of 835 Wh kg ), a peak power den- spectra of XPS, the content of pyridone/pyrrolic on ON- Zn −2 −2 sity of 55 mW cm (at a current density of ~ 70 mA cm ), CNW was about three times that of a referenced N-doped and stable operations for 240 h after mechanical recharging. carbon nanoweb (N-CNW) material. The pyridone nitrogen For NPMC-1000 rechargeable ZABs, the battery cycled sta- was reported to stabilize singlet dioxygen by forming a sta- bly for 180 discharge/charge cycles over a period of 30 h at ble adduct, weakening and breaking the bond in the oxygen −2 2 mA cm . According to DFT calculations, these tested bat- molecule [79, 80] so that the activation process using KOH tery performances are induced by the synergistic effects of can further improve ORR activity. CV measurements using N, P co-doping along with graphene edges for the improve- N-doped carbon nanosphere (N-CNS) catalysts were run in ment of bifunctional electrocatalytic activity toward OER O -saturated 0.1 M KOH, and only a single ORR peak poten- and ORR [76]. tial (~ 0.4 V) was observed. This peak was weaker than that Compared with N, P co-doping, N, B co-doping into car- of N-CNW(~ 0.29 V), indicating a difference in morphology bon materials is more difficult because of the formation of and structure for the two PPY-derived carbon materials. The by-products (e.g., hexagonal boron nitride, h-BN) during N-CNS was also found to possess greater particle resistance doping that can act as inert de-activating agents that reduce and reduced mass transport in the packed carbon spheres, the electrochemical activity of catalysts [77]. To incorporate whereas the 3D interconnected N-doped carbon fibers in heteroatoms at select sites of graphene frameworks and pre- the N-CNW increased electron transport, oxygen transport vent the formation of inactive by-products, Zheng et al. [78] and electrolyte diffusion. In a further comparison of ORR prepared a B, N co-doped-graphene (B, N-graphene) catalyst peak potentials, ON-CNW presented a positive shift pos- using a two-step doping strategy in which NH and H BO sibly due to a higher pyridone/pyrrolic content (~ 20.14%). 3 3 3 1 3 10 Electrochemical Energy Reviews (2018) 1:1–34 The N-CNW and ON-CNW samples were subsequently sites. To obtain simultaneous optimization of both porous evaluated as cathode catalysts in an assembled hybrid LAB structures and surface functionalities of N and S co-doped and the discharge voltage profiles of the cell revealed an carbon, Wu et al. [81] designed and fabricated a polyquater- apparent reduction in the difference between ON-CNW nium-derived heteroatom (N and S) co-doped hierarchically and Pt/C samples, demonstrating that ON-CNW possessed porous carbon (N-S-HPC) catalyst for ORR using a simple, performances close to that of Pt/C catalysts. At 0.5, 1.0, large-scale and green synthetic route with the assistance −2 1.5, and 2.0 mA cm (Fig. 4a). the discharge plateau of of a silica template. In their electrochemical evaluation of ON-CNW is lower than that of Pt/C but higher than those ORR, the LSV curves in O -saturated 0.1 M KOH at 1600 r −1 −1 of acetylene black and N-CNW, suggesting that ON-CNW min with a scan rate of 5 mV s and a catalyst loading of −2 possesses higher ORR activities than N-CNW and carbon 500 μg cm were recorded and compared. Compared with black. A compared cycling test was carried out in a hybrid the two referenced samples [N-doped hierarchically porous −2 LAB with a constant current density of 0.5 mA cm and carbon (N-HPC) and N, S-doped porous carbon (N-S-PC)], the round-trip overpotential for N-CNW (Fig. 4b) increased N-S-HPC presented a higher onset potential (~ 0.99 V) and a from 1.00 to 1.43 V, whereas ON-CNW (Fig. 4c) increased higher half-wave potential (~ 0.86 V), demonstrating higher from 0.92 to 1.02 V, demonstrating the improved effects of ORR activity. In particular, even at a low catalyst loading −2 O, N co-doping on activity and stability, and also confirming of 100 μg cm , N-S-HPC still exhibited a better half-wave that ON-CNW is a promising ORR catalyst in hybrid LABs. potential (~ 0.83 V) and a greater diffusion-limited current −2 The co-doping of N and S [81–83] has also been dem- density (~ 7.5 mA cm ) than those of commercial 20 wt% −2 onstrated to provide benefits in the redistribution of spin Pt/C (~ 0.82 V, ~ 5.5 mA cm ), suggesting that N-S-HPC and charge densities and the formation of more ORR active possesses comparable ORR performance to Pt/C in alkaline Fig. 4 a Discharge voltage profiles of hybrid LAB with Pt/C, N-CNW or c ON-CNW as the ORR catalyst. Reprinted with permis- N-CNW, ON-CNW, or carbon black as the ORR catalyst at differ - sion from Ref. [23]. Copyright 2014 John Wiley and Sons ent current densities. Cycling performance of hybrid LABs with b 1 3 Electrochemical Energy Reviews (2018) 1:1–34 11 solutions. Interestingly, instead of 0.1 M KOH solution, an and H PtCl were used as Au and Pt sources to prepare 40 2 6 acidic medium (i.e., 0.5 M H SO solution) was also used wt% PtAu/C and TEM images revealed a uniform distribu- 2 4 in the evaluation of the ORR performance of N-S-HPC. tion of PtAu nanoparticles [(~ 6.8 ± 1.4) nm] on carbon The obtained CV curves in O -saturated 0.5 M H SO solu- with XRD data (Fig. 5a), indicating a solid-solution com- 2 2 4 tion showed that N-S-HPC produced a slightly lower ORR posed of Pt and Au corresponding to Pt Au . From the 0.5 0.5 activity than Pt/C as evidenced by the half-wave poten- CV curves (Fig. 5b) of the 40 wt% PtAu/C, the calculated tial (~ 0.73 V) as compared with the half-wave potential electrochemical surface area (ESA) of the Pt and Au was 2 −1 (~ 0.78 V) of Pt/C. To demonstrate the potential of N-S-HPC (38 ± 4) m g and correlated with the dispersion of PtAu PtAu catalysts in MABs, a ZAB with 6 M KOH was assembled for observed in the TEM. In their investigation of ORR/OER further electrochemical measurements using N-S-HPC as the using an assembled LAB, a comparison of first discharge air–cathode catalyst. The peak power density obtained using and charge voltages (Fig. 5c) showed that the discharge volt- −2 N-S-HPC was ~ 536 mW cm and is drastically superior to age of 40 wt% PtAu/C is comparable to that of 40 wt% Au/C, −2 Pt/C (~ 145 mW cm ). Therefore, the co-doping of N, S can and the charge voltage of 40 wt% PtAu/C is comparable to result in an optimized combination of porous structures and that of 40 wt% Pt/C. Moreover, 40 wt% PtAu/C as a cath- surface functionalities for N and S and thus the enhancement ode catalyst provided higher round-trip ec ffi iencies than pure of electrocatalytic activity. carbon even though both possessed similar specific capaci- −1 Zhang et al. [69] carried out experiments in synthesizing ties in the first cycle (~ 1200 mAh g ). This suggests carbon and comparing three different co-doped carbon catalysts: N, that the incorporation of Au into Pt surfaces can result in S co-doped graphdiyne (NSGD), N, B co-doped graphdiyne improved ORR and OER kinetics of 40 wt% PtAu/C. How- (NBGD), and N, F co-doped graphdiyne (NFGD). After a ever, at increasing charging cycles, the charge voltage of series of electrochemical measurements in an assembled 40 wt% PtAu/C was lower than that of 40 wt% Pt/C and if primary ZABs, the ORR activity of NFGD was found to be current densities were decreased, discharge and charge volt- comparable to that of commercial Pt/C (20 wt% Pt on Vulcan ages reduced considerably (Fig. 5d). This research crucially XC-72) and superior to those of the other two dual-doped demonstrates that PtAu/C is responsible for both ORR and GDs. A first-principle study to theoretically demonstrate the OER after the incorporation of Au into Pt atoms on the nan- synergistically enhanced catalytic effects of co-doping in B, oparticle surface, providing a reasonable strategy to develop P co-doped graphene was also carried out to compare with bifunctional catalysts for RMABs. To further understand the B-doped graphene and P-doped graphene [22]. effects of Pt and Au on ORR and OER, a Pt–Au alloy nano- particle catalyst supported on carbon black (Vulcan XC-72) [87] was found to played an important role in the charge/ 4 Composites of Carbon and Metals discharge performance of rechargeable LABs owing to their nanoscale alloying and phase properties inducing synergistic 4.1 Composites of Carbon and Noble Metals effects between the AuPt alloy and C. or Noble Metal‑Alloys Similar to Au, Co has also been used to form Pt–Co alloy nanoparticles supported on Vulcan XC-72 carbon in the 4.1.1 Composites of Carbon and Pt or Pt‑Alloys application of LABs. To fabricate Pt Co /Vulcan XC-72 x y catalysts with a loading of 20 wt%, Su et al. [89] conducted The composites of carbon and Pt have commonly been uti- a chemical reduction method to deposit a series of Pt Co x y lized as carbon-supported Pt fuel cell electrocatalysts for (x:y = 4, 2, 1, and 0.5) alloy nanoparticles onto Vulcan ORR in which different carbon materials act as high sur - XC-72 using H PtCl ·6H O and CoCl ·6H O as Pt and Co 2 6 2 2 2 face area substrates for the structuring and proper disper- sources. XRD confirmed the complete incorporation of Co sion of Pt nanoparticles to form carbon-supported Pt-based into the fcc crystal structure of Pt with an average crystallite catalysts. However, it has been well documented that Pt/C size for all catalyst samples being in the range of 5–8 nm. alone is not an efficient bifunctional electrocatalyst because This was also in agreement with TEM and HRTEM analysis. of its insufficient OER performances [84– 86]. To address A combination of SEM and TEM showed that all Pt Co /C x y the primary challenges of OER, metals such as Au [8, 87, samples possessed carbon nanoparticles with similar mor- 88], Co [89], Zn [90], Ir [7, 91], Pd [91], and Ru [91] have phologies in the size range of 50–100 nm and presented been used to form carbon-supported Pt-alloy catalysts in uniform distribution of Pt–Co alloy nanoparticles. In electro- various metal–air batteries. Using the active effects of Au chemical performance tests in LABs, the discharge capacity on the surface modification of Pt, Lu et al. [ 8] combined Au of the PtCo /C cathode was found to have decreased from −1 and Pt into individual PtAu nanoparticles on the surface of 2578 to 2074 mAh g after 5 cycles, whereas the referenced −1 carbon and evaluated the ORR and OER activities of the Vulcan XC-72 cathode degraded from 965 to 376 mAh g , resulting 40 wt% PtAu/C in LABs. In their study, HAuCl demonstrating that PtCo /C possessed much better capacity 4 2 1 3 12 Electrochemical Energy Reviews (2018) 1:1–34 −1 Fig. 5 a TEM image (top right) and XRD data of 40 wt% PtAu/C, charge profile of carbon at 85 mA g and of 40 wt% Au/C, 40 carbon −1 b cyclic voltammograms of 40 wt% PtAu/C collected in Ar-saturated wt% Pt/C, and 40 wt% PtAu/C at 100 mA g , d LAB discharge/ carbon 0.5 M H SO between 0.05 and 1.7 V versus RHE (room temperature charge profiles (first cycle) of 40 wt% PtAu/C at 50, 100, and 250 mA 2 4 −1 and 50 mV/s). insets: (left) HRTEM image of 40 wt% PtAu/C and g . Reprinted with permission from Ref. [8]. Copyright 2010 carbon (right) schematic representation of Pt–Au, with arrows indicating the American Chemical Society CV signatures for Pt (gray) and Au (yellow), c LAB first discharge/ retention. Among all Pt Co /C samples, PtCo /C achieved increased, leading to enhancements in both OER and ORR x y 2 −1 a maximum capacity of 3040 mAh g . This capacity was activities. Cycling tests in LABs also confirmed better cycla- −1 higher than that of the PtAu/C (1200 mAh g ) developed bility performances with Pt Co /C as compared with bare x y by Lu et al. [8]. This indicates that compared with Au, Co Vulcan XC-72. To further understand and obtain high ORR should be more suitable in Pt-alloys for the improvement of and OER activities from Pt-alloys with non-noble metals, both ORR and OER. In further analyses of discharge and Zhang et al. [90] synthesized PtZn/carbon aerogel alloy charge curves in the second cycle, Pt Co /C samples pos- catalysts and measured the electrochemical performance x y sessed much lower charge voltages (~ 3.94 V) and over- of these catalysts (PtCo/carbon aerogel, Pt/carbon aerogel potentials (~ 1.21 V) than Vulcan XC-72 with specific and Pt/carbon black) in magnesium–air batteries. In their capacities increasing positively with increasing Co con- study, the PtZn/carbon aerogel demonstrated a higher spe- −1 tent, indicating improved catalytic activities toward ORR cific discharge capacity (1349.5 mAh g ) than the PtCo/ −1 and OER in LABs. Along with increasing Co content near carbon aerogel (~ 1283.38 mAh g ), Pt/carbon aerogel −1 Pt atoms, surface Pt electron density and segregation also (~ 1113.53 mAh g ), and Pt/carbon black (~ 997.01 mAh 1 3 Electrochemical Energy Reviews (2018) 1:1–34 13 −1 g ) catalysts, suggesting that PtZn/carbon aerogel pos- DHG. Two large oxidation peaks at 3.25 and 4.37 V of Ir@ sessed better ORR and OER activities. DHG also indicated much higher anodic currents. These As a noble metal, iridium also was used to alloy with Pt results illustrated that Ir@DHG possesses superior OER to prepare carbon-supported Pt–Ir catalysts for rechargeable activity compared with DHG. Cycling performances at a −1 LABs by Ke et al. [92], in which H PtCl ·H O and IrCl current density of 2000 mA g with a limited capacity of 2 6 2 3 −1 were used as Pt and Ir sources. In a comparison of OER, 1000 mAh g revealed that Ir@DHG can run 150 cycles current–potential curves tested in a Ar-saturated 0.1 M KOH with terminal voltages greater than 2.5 V, demonstrating solution revealed that Pt/C provided a higher onset potential that Ir@DHG also possessed better reversibility and cycling (~ 0.66 V) than Pt–Ir/C (~ 0.53 V) and Ir/C (~ 0.50 V) cata- performance than DHG. lysts, indicating that Pt–Ir/C possessed better OER. For a Ruthenium (Ru) [96, 97] and palladium (Pd) [98–100] comparison of r fi st cycle discharge–charge potential prol fi es have also been used to functionalize carbon to reduce over- in discharge–charge measurements run in a LAB, Pt–Ir/C potentials and capacity decays during round-trip cycling in demonstrated the least discharge overpotential (~ 0.15 V) rechargeable LABs. Using a type of carbonized bacterial during the discharge process whereas at the end of the charg- cellulose (CBC) as a carbon support, Tong et al. [96] pre- ing process, Pt–Ir/C possessed the lowest potential differ - pared a 5 wt% Ru/CBC composite as the binder-free cath- ence (~ 0.727 V) between the discharge and charge poten- ode in non-aqueous LABs in which Ru nanoparticles can tials. Therefore, Pt–Ir/C clearly exhibits enhanced ORR and provide active sites for both ORR and OER, and CBC can OER. provide transport pathways for both electrons and oxygen. Both BET and TEM measurements showed a high surface 2 −1 4.1.2 Composites of Carbon and Other Noble Metals area of 397.6 m g and a porous network structure for or Alloys the 5 wt% Ru/CBC catalyst, favoring the distribution of active sites and mass transport. The obtained galvanostatic −1 Aside from Pt and Pt-alloys, other noble metals (Ir, Ru, discharge-recharge curves at a current density of 200 mA g Pd, Au and Ag) and alloys have also been used to prepare displayed a narrower discharge–charge potential gap than noble metal-based electrocatalysts supported on carbon that of CBC, indicating that 5wt% Ru/CBC possessed better materials for MABs. Using Ir as a catalytic noble metal ORR and OER activities. Corresponding SEM images of to functionalize catalytic surfaces of carbon, Zhou et al. 5wt% Ru/CBC after charging showed uniform deposition of [93] introduced nano-sized iridium (Ir) catalysts into a discharge products (i.e., Li O ) on the CBC surface, demon- 2 2 three-dimensional porous graphene for LABs using a non- strating the strong interaction between Li O and CBC and 2 2 aqueous electrolyte in which iridium functionalized deoxy- the improvement of stability of the tested LAB with 5 wt% genated hierarchical graphene (Ir@DHG) was synthesized Ru/CBC. Similar to Ru functionalized CBC catalysts, Pd- using a vacuum-promoted exfoliation method followed catalyzed carbon nanofibers (Pd/CNF) catalysts were also by a deoxygenation process. The Ir@DHG was tested to prepared for alkaline MABs by Alegre et al. [98] in which 2 −1 possess a high BET surface area (~ 372.5 m g ) and its a commercial Vulcan carbon was employed as a reference nitrogen adsorption–desorption curves showed pore sizes support material to support Pd nanoparticles. Characteriza- ranging from 2 to 200 nm, indicating the presence of a fer- tion by XRD, TEM, and SEM showed that the Pd/CNF and tile mesoporous and macroporous architecture favoring the Pd/Vulcan catalysts possessed typical fcc structures of Pd transport of oxygen, electrolytes and electrons to and from and the calculated crystallite sizes of the Pd/CNF and Pd/ catalytic sites during discharge and charge. According to Vulcan catalysts were 6.1 and 6.5 nm, respectively. In the XPS results, the Ir@DHG catalyst displayed smaller peaks examination of ORR and OER in a half-cell configuration, for C1s. This is possibly because Ir can closely cover gra- two full polarization curves obtained from 1.2 to 0.3 V under phene and decrease the exposure of carbon surfaces. Impor- ORR and from 1.2 to 2.0 V under OER showed that Pd/CNF tantly, Ir@DHG possessed a slightly lower intensity for O1s possessed slightly smaller overpotentials than Pd/Vulcan and than DHG, suggesting that the graphene in Ir@DHG can was therefore slightly more active. This was possibly attrib- maintain a highly deoxygenated surface and enhance the uted to the higher electrical conductivity and/or smaller Pd stability of the electrode/electrolyte interface [94, 95]. The size of Pd/CNF, favoring better charge transfers and thus high-resolution XPS spectrum of Ir 4f presented dominat- improved ORR and OER. Further charge/discharge cycle ing Ir(0) 4f and Ir(0) 4f with factional Ir(IV) 4f and tests showed that Pd/CNF could run more cycles than Pd/ 5/2 7/2 5/2 Ir(IV) 4f . This suggests a certain degree of surface oxida- Vulcan and was therefore more stable as well. Au [101] and 7/2 tion. TEM showed the uniform distribution of Ir nanopar- Ag [102] were also used to functionalize carbon materials ticles (~ 2.08 nm) onto the entire graphene plane and CV in the application of MABs. A recent study [97] of Ru- and −1 tests in LABs at 1 mV s showed that Ir@DHG possessed Pd-catalyzed carbon nanotube fabrics showed that Ru- and higher current peaks, indicating better ORR activity than Pd-CNT catalyzed cathodes for LABs do not show visible 1 3 14 Electrochemical Energy Reviews (2018) 1:1–34 improvements. This is possibly because the presence of favoring cycling performances. The incorporation of transi- noble metal catalysts impairs the reversibility of cells, caus- tional non-noble metals such as Co and Ni can also improve ing a decrease in O recovery efficiency (the ratio between the cycle life of MABs, as evidenced in Table 2. And the amount of O evolved during charge and the amount con- although noble metal-based bifunctional catalysts incorpo- sumed in the preceding discharge) coupled with an increase rated by other noble metal component(s) can play an impor- in CO evolution during charging. tant role in improving the bifunctional activity and stability Because carbon-supported Pt-based binary metal cata- of ORR/OER, their high price and scarcity present barriers lysts exhibit active ORR and OER activities for enhanced for scale-up deployment in MAB commercialization. LAB performance, carbon-supported non-platinum binary metal catalysts such as Pd Pb/C [103] and PdIr/C [91] have 4.2 Composites of Carbon and Non‑noble Metals also been studied for carbon-composited bifunctional cata- lysts in MABs. To replace expensive Pt-based alloy cata- Non-noble metals have been incorporated into carbon mate- lysts, Cui et al. [103] synthesized a Pd Pb/C catalyst using rials to enhance both ORR and OER, reduce carbon cor- a modified impregnation-reduction method in which they rosion, and improve reversibility and rate capabilities of carefully controlled the experimental conditions (e.g., tem- carbon-composited bifunctional catalysts for MABs. These perature, time, reduction agent, metal precursor) to obtain materials are inexpensive, easily fabricated, and possess a structurally ordered intermetallic phase that provided good interactions with carbon. For example, tantalum (Ta) uniform active sites on the same surface plane. In char- was found to reduce battery polarization and overpotential acterizations, XRD confirmed the ordered Pd Pb/C struc- by Yu et al. [105] in which they developed vertically aligned ture and TEM images showed that Pd Pb/C possessed a CNTs (VACNTs) with ultra-lengths on permeable Ta foils uniform particle distribution with an average particle size (VACNTs-Ta) as air cathodes for non-aqueous secondary of (7.2 ± 0.5) nm. The particle sizes of the Pd/C and Pt/C LABs using a thermal chemical vapor deposition (TCVD) catalysts were (6.3 ± 0.4) and (5.1 ± 0.3) nm, respectively. method. In their study, CNT powder (CNT-P, 20–40 μm Polarization curves in O -saturated 0.1 M KOH solution at in length and 15 nm in outer diameter) and commercial −1 1600 r min revealed that Pd Pb/C was more active than VACNT on a stainless steel (SS) mesh substrate (VACNTs- both Pd/C and Pt/C catalysts, evidenced by a more positive SS) were used as the reference samples. Compared with the onset potential and a greater half-wave potential (~ 0.92 V) VACNTs-SS and CNT-P samples, VACNTs-Ta presented than Pd/C (~ 0.88 V) and Pt/C (~ 0.88 V) catalysts. Impor- a lower ratio of D band and G band as observed by Raman tantly, after the incorporation of Pd, Pd Pb/C gained better spectroscopy, demonstrating that VACNTs-Ta possessed less catalytic activities toward ORR and OER and possessed bet- surface defects and lower reactivity with O reduction spe- ter durability than Pt/C. This was confirmed in the meas- cies. The combination of BET and BJH revealed the coex- ured discharge and charge voltage profiles of an assembled istence of micropores, mesopores, and macropores in the ZAB using Pd Pb/C as the cathode catalyst, in which initial VACNTs-Ta in which mesopores and macropores were the round-trip overpotentials increased from 0.72 V at the first majority, favoring the transport of mass and thus improving cycle to 0.86 V at the 135th cycle. Besides Pd Pb/C cata- catalytically properties. In the comparison of electrochemi- lysts, PdIr/C was also studied by Ko et al. [91]. Based on cal properties, the first discharge and charge behaviors of an initial charge–discharge behaviors, a comparison of ORR assembled LAB showed that as a cathode, VACNTs-Ta can and OER showed that PtRu/C possessed lower overpoten- deliver a larger gravimetric specific capacity (~ 4300 mAh −1 −1 −1 tials for charging and discharging than PtPd/C and PtIr/C, in g ) at 200 mA g than VACNTs-SS (~ 3200 mAh g ) −1 which the capacity followed an order: PtRu/C (~ 346 mAh and CNT-P (~ 700 mAh g ). These results confirmed that −1 −1 −1 g ) > PtPd/C (~ 153 mAh g ) > PdIr/C (~ 135 mAh g ). VACNTs-Ta possessed greater ORR and OER activities than Table 2 shows the electrochemical performances for typi- the other two samples. In further tests of cycling and rate cal noble metal–carbon composite bifunctional catalysts as capability, VACNTs-Ta also exhibited better cycling per- −1 cathodes in MABs [86, 87, 89, 91, 92, 96, 100, 104]. formances (65 cycles at 200 mA g as well as a curtailed −1 To improve the ORR and OER (especially OER) of specific capacity of 1000 mAh g ) and rate capabilities −1 −1 Pt/C catalysts, the incorporation of other metals can play (10,000 mAh g at 50 mA g ), suggesting that VACNT-Ta an important role in improving battery performance in possessed more a favorable architecture and higher stabil- rechargeable MABs due to the synergistic interactions ity than those of the VACNT-SS and CNT-P samples. The between different components. For example, composite researchers concluded that the use of Ta not only favored the catalysts such as PtCo /C [89] and Pt/CNTs/Ni [104] can fabrication of VACNT but also produced beneficial interac - −1 deliver high discharge capacities of 3040 and 4050 mAh g tions between Ta and VACNT, benefiting co-transportation/ for MABs, respectively. Compared to pure Pt catalysts, other reaction and thus the reduction of discharge product (e.g., noble metals (e.g., Ir or Ru) are also able to increase OER Li O ) aggregation. To further develop high-performance 2 2 1 3 Electrochemical Energy Reviews (2018) 1:1–34 15 Table 2 Electrochemical performance for typical noble metal-based bifunctional catalysts as air-electrode materials in the tested MABs No. Bifunctional Maximum Cycle Potential Potential differ - Coulombic Electrolyte type MAB type References catalyst as capacity/mAh number range for ence for ORR/ efficiency −1 cathode g (current (current cycle test- OER (initial (%, after a −1 materials density/ mA g density/ ing (V) ORR/OER cycle b −2 −1 c or mA cm ) mA g or overpotentials , number) −2 + mA cm , V vs. Li/Li ) upper-limit capacity/ −1 mAh g ) a a 1 10 wt% Pt/1200 (70 )20 (100 , 2.0–4.8 0.97 (2.88/3.85) –1 M LiPF in LAB [86] GNS 720) EC:DMC (1:1 v/v) b b 2 20 wt% 1329 (0.12 ) 1 (0.12 , 2.0–4.5 1.30 (2.70/4.00) –1 M LiPF in LOB [87] Pt Au /C 240) EC:DMC (1:1 v/v) 51 49 a a 3 20 wt% 3040 (100 )5 (100 , –) 2.0–4.6 1.21 (2.73/3.94) –1 M LiClO in LAB [89] PtCo /C PC:DME (1:2 v/v) b b 4 44 wt% 700 (0.2 ) 40 (0.2 , 2.3–4.5 1.20 (2.66/3.86) –1 M LiPF in TEG- LAB [91] Pt Ru /C 700) DME 50 50 5 36.5 wt% –150 (2000 , 1.5–4.5 1.07 (2.74/3.81) –0.1 M LiClO in rLOB [93] Ir@DHG 1000) TEGDME:DMSO (1:2 v/v) 6 5 wt% Ru/ –25 (200 , 2.0–4.5 1.26 (2.71/3.97) – LiCF SO :TEGDME LOB [96] 3 3 CBC 500) (4:1 mol/mol) a a 7 40.12 wt% 616 (70 )10 (70 , –) 2.0–4.3 0.83 (2.72/3.55) –1 M LiPF in PC rLAB [100] Pt/C a a 40.03 wt% 855 (70 )10 (70 , –) 0.75 (2.65/3.40) – Pd/C a a 40.15 wt% 577 (70 )10 (70 , –) 0.84 (2.76/3.70) – Ru/C a a 8 Pt/CNTs/Ni4050 (20 )80 (400 , 2.0–4.2 1.10 (2.60/3.70) – 1 M LITFSI in TEG- LOB [104] 1500) DME a −1 mA g b −2 mA cm Initial overpotentials are obtained from the cyclic tests Dimethyl carbonate 1,2-Dimethoxyethane non-noble metal-functionalized carbon cathode materials, nanoparticles supported on graphene (CoCu/graphene) as Su et al. [106] studied the composites of carbon with Ni, a composite cathode catalyst for LABs. XRD confirmed the Co, and Cu quantum dots (QDs) and compared them with coexistence of Co and Cu along with graphene, and TEM a MnO/carbon composite. Based on capacitance and rate images revealed highly distributed CoCu nanoparticles performance results, the researchers demonstrated that Ni, with an average diameter of 10–20 nm on graphene sheets Co, and Cu QDs composited with carbon is an important without aggregation, meaning that more active sites were path in developing advanced carbon-based cathode materi- formed. The electrocatalytic activity measurements in the als in LABs. first discharge–charge profiles of their LAB with a voltage −1 Although single non-noble metals have been explored range of 2.0–4.3 V at 200 mA g showed larger discharge −1 for novel carbon-based composites, research has also been capacities (14,821 mAh g ) and coulombic efficiencies carried out using two non-noble metals (e.g., transition met- (92%) than compared with Co/graphene and Cu/graphene als such as Co, Ni, Fe, Cu, and Mn) in carbon-composited cathodes, indicating that CoCu/graphene possessed better catalysts for MABs in which stronger synergistic effects electrocatalytic activity. Moreover, Co/graphene presented between the two metals and carbon are expected. To hin- much higher coulombic efficiencies than Cu/graphene, der the restacking of graphene layers and increase the space demonstrating that Co can efficiently accelerate OER and between layers favorable to the transport of oxygen, lithium that Cu can have a positive influence on ORR. In a com- ions and electrolyte, Chen et al. [107] designed and synthe- parison of CV curves, CoCu/graphene exhibited the most sized cost-effective cobalt-copper bimetallic yolk-shelled positive ORR onset potential and the greatest ORR/OER 1 3 16 Electrochemical Energy Reviews (2018) 1:1–34 peak current density, confirming the higher catalytic activ - As listed in Table 3 [105, 107, 110, 111, 113–119], ity of CoCu/graphene as compared with Co/graphene and instead of high-priced noble metals, the use of non-noble Cu/graphene cathodes. In cycling tests performed in a LAB metals in bifunctional composite catalysts can generally −1 with a current density of 200 mA g and a cutoff capacity improve MAB performances because of the formation of −1 of 1000 mAh g , the discharge voltages of Co/graphene various decent structures and the synergistic effects between −1 and Cu/graphene cathodes degraded to less than 2.5 V after different components. For example, at 200 mA g , CoCu/ only 71 and 37 cycles, respectively, whereas CoCu/graphene grapheme [107] can deliver high discharge capacities of over −1 remained above 2.5 V for 122 cycles, once again demon- 100,000 mAh g as well as acceptable cycle performances strating the superior ORR stability of CoCu/graphene. This (i.e., cycle number above 70). Interestingly, based on non- cycling test was subsequently extended to a higher current noble metal and carbon, ternary and quaternary compos- −1 density of 500 mAh g with a cutoff capacity of 1000 mAh ite catalysts, such as Fe–Fe C/CNFs [113], FeNi @GR@ 3 3 −1 g , and the CoCu/graphene run stably for 204 cycles with Fe–NiOOH [114], Co O /Ni/C [119], and Co/C/NiFe LDH/ 3 4 a discharge terminal voltage of 2.0 V. This was better than AB [113] have demonstrated even better activities toward Co/graphene (~ 144 cycles) or Cu/graphene (~ 101 cycles). ORR and OER, resulting in enhanced MAB performances in The cycling performance of CoCu/graphene was shown to terms of capacity, cycling stability and/or energy efficiency. be comparable to those of the best cycling performances in In addition, by comparing non-noble metal-based with noble LABs [108, 109]. A similar study was conducted by Kwak metal-based bifunctional catalysts as shown in Table 3, it et al. [110], in which CNT was composited with Fe and can be seen that in terms of tested cycle numbers, non-noble Co as a cathode catalyst for LABs. In their characterization metal-based catalysts can achieve almost better cycling per- using SEM, TEM, XRD, EDS, and SAED patterns, bime- formances in MABs than noble metal-based catalysts, as tallic Fe and Co coupled with a small amount of oxidized shown in Table 2. Non-noble metals can also provide accept- states, were confirmed in the FeCo–CNTs catalyst (Fig. 6). able bifunctionality resulting from its formed nanostructures In an assembled LAB, the FeCo–CNT cathode demonstrated as well as its strong interactions with supporting materials −1 greater capacity (~ 3600 vs. ~ 1276 mAh g ) and better (like carbons). round-trip efficiency (72.15 vs. 62.5%) than pristine CNTs, In the search for next-generation bifunctional catalysts for indicating that FeCo–CNTs possessed superior ORR and scale-up deployment in MAB commercialization, non-noble OER activities. metal-based catalysts are more promising than noble metals. Besides the CoCu/graphene and FeCo–CNTs compos- Not only are non-noble metals cheaper and more abundant ite catalysts discussed above, electrospun graphitic carbon than noble metals, non-noble metals can easily be fabricated nanofibers with in situ encapsulated Co–Ni nanoparticles with supporting materials (e.g., carbon) in the formation of (Co–Ni/CNFs) were also developed by Huang et al. [111], in efficient nanostructures to improve bifunctional activities which the encapsulation of metal catalysts into nanocarbon and stability by building important interactions with the can suppress aggregation, presenting more active sites for supporting material. both ORR and OER. In their synthesis, cobalt(II) acetate ter- ahydrate (Co(Ac) ·4H O) and nickel(II) acetate tetrahydrate 2 2 (Ni(Ac) ·4H O) were used as Co and Ni sources. In their 5 Composites of Carbon and Oxides 2 2 examination of electrocatalytic activity using an assembled non-aqueous LAB without any binders or additives, a cyclic In recent years, metal oxides have been reported to exhibit −1 test with an upper-limit capacity of 1000 mAh g at 200 promising activities for ORR and OER. However, the self- −1 mA g showed that the Co–Ni/CNFs cathode can run 60 passivity and low electrical conductivity of metal oxides can cycles with initial overpotentials of only 0.22 and 0.70 V decrease active sites and hinder charge transport, leading to for ORR and OER, respectively. Importantly, in the first low catalytic performances [120, 121]. Therefore, composit- −1 discharge/charge curves obtained at 200 mA g , Co–Ni/ ing conductive carbon and oxides have become an effective CNFs displayed an initial discharge capacity of 8635 mAh strategy to enhance catalytic performances because these −1 g . This was better than that of CNFs, suggesting that the composites can overcome these low conductivity and cor- in situ formation of Co–Ni improved electrochemical perfor- rosion drawbacks [122, 123]. mances and enhanced ORR and OER. Their results showed that the inherently interconnected, conductive network of 5.1 Composites of Carbon and Perovskite Oxides Co–Ni/CNFs is sufficient for LABs without any binders and additives. Ren et al. [112] found that CuFe catalyzed carbon Perovskite oxides, with a general formula of AB O [124] black (Ketjenblack) materials can drastically increase the (Fig. 7, the A-site is a rare alkaline earth metal cation and density of catalytic sites, resulting in improvements of LAB the B-site is a 3d transition metal cation), have attracted ORR kinetics. increasing attention because of their defective structures, 1 3 Electrochemical Energy Reviews (2018) 1:1–34 17 Fig. 6 Morphology and structure of the synthesized FeCo–CNTs terns of FeCo–CNTs and pristine CNTs. e SAED patterns of FeCo– composite. SEM images of a FeCo–CNTs and b pristine CNTs; c CNTs. Reprinted with permission from Ref. [110]. Copyright 2016 TEM image of FeCo–CNTs with EDS mapping images; d XRD pat- Royal Society of Chemistry low costs, excellent oxygen mobility, and outstanding improve ORR and OER activities in alkaline environments activities toward ORR and OER [125, 126]. To improve owing to three aspects: the active oxide, the conductive overall energy efficiency and retain stability, perovskite carbon support, and the synergistic effects between them. oxides have been proposed to composite with carbon to 1 3 18 Electrochemical Energy Reviews (2018) 1:1–34 Table 3 Electrochemical performance for typical non-noble metal-based bifunctional catalysts as air-electrode materials in the tested MABs No. Bifunctional Maximum Cycle number Potential range Potential differ - Energy Electrolyte MAB type References catalyst as capacity/mAh (current den- for cycle test- ence for ORR/ efficiency type −1 −1 cathode mate- g (current sity/mA g ing (V) OER (initial (%) a −2 rials density/ mA or mA cm , ORR/OER −1 b c g or mA upper-limit overpotentials , −2 + cm ) capacity/mAh V vs. Li/Li ) −1 g ) a a 1 Ta/CNTs4300 (200 )65 (200 , 2.0–4.5 1.70 (2.60/4.30) – 1 M LiTFSI/ rLOB [105] 1000) TEGDME a a 2 CoCu/gra-14,821 (200 )122 (200 , 2.5–4.5 1.05 (2.75/3.80) – 1 M LiTFSI/ rLOB [107] phene 1000) TEGDME a a 3 FeCo/CNTs3600 (250 )50 (100 , 2.4–4.5 1.14 (2.75/3.89) – 1 M LiTFSI/ rLOB [110] 1000) TEGDME a a 4 CoNi/CNFs8635 (200 )60 (200 , 2.0–4.5 0.92 (–/–) – 0.5 M LiTFSI/ rLOB [111] 1000) TEGDME a a 5 Fe–Fe C/6250 (200 )41 (300 , 600) 2.0–4.3 1.05 (2.70/3.75) – 1 M LiTFSI/ rLOB [113] CNFs TEGDME 6 FeNi @GR@ –100 (1 , –) 0.9–2.1 0.90 (1.05/1.95) – 6 M KOH rZAB [114] Fe–NiOOH 7 Co/C/NiFe –300 (40 , –) 1.05–2.05 0.75 (1.20/1.95) 51.2% 6 M KOH rZAB [115] LDH/AB 8 Co–N/C –500 (2 , –) 0.4–1.4 0.94 (1.21/2.15) – 6 M KOH ZAB [116] 9 Fe–N/C731 (100 ) – – – – 6 M KOH ZAB [117] 10 Fe@N–C –100 (10 , –) 1.0–2.0 0.70 (1.25/1.95) – 6 M KOH ZAB [118] a a 11 Co O /Ni/C14,830 (400 )48 (100 , 2.0–4.3 0.55 (2.69/3.24) 75.1%0.1 M LiClO / LOB [119] 3 4 4 2000) DME a −1 mA g b −2 mA cm Initial overpotentials are obtained from the cyclic tests To enhance the electrical conduction of perovskite oxides and investigate the effects of perovskite oxides on ORR and OER, Xu et al. [127] studied BaMnO –carbon composites and their bifunctional electrocatalytic capa- bilities for ORR and OER. Their carbon-coated BaMnO nanorod (BaMnO @5%C) samples were synthesized through a coating method in which BaCl and MnO 2 2 were used as Ba and Mn sources with a ratio of Ba to Mn being 1:1. SEM images of the product revealed BaMnO nanorods 100–200 nm in diameter and 1–4 μm in length and TEM revealed that these nanorods were compactly and uniformly coated with a 10-nm-thick carbon. The compari- son of BaMnO with BaMnO @5%C samples by XRD evi- 3 3 denced the presence of perovskite BaMnO structures with a small amount of MnO resulting from residual raw reac- tants. In the investigation for ORR and OER, CV curves in O -saturated 0.1 M KOH solution showed that the peak −2 current density (~ 2.5 mA cm ) of BaMnO @5%C was −2 much greater than that of BaMnO (~ 0.94 mA cm ), demonstrating superior electrocatalytic activity. This result was further confirmed by tested LSV curves in Fig. 7 A unit cell in perovskite showing the relative positions of dif- ferent ions. Reprinted with permission from Ref. [124]. Copyright which BaMnO @5%C possessed a larger positive half- 2016 John Wiley and Sons wave potential as compared with BaMnO . Moreover, 1 3 Electrochemical Energy Reviews (2018) 1:1–34 19 the diffusion limiting current density of BaMnO @5%C of the carbon and perovskite oxides, as well as the syner- was comparable to commercial Pt/C (20wt% Pt on Vul- gistic effects between them. can XC-72) catalysts. Interestingly, BaMnO @5%C pos- In addition to the partial substitution of A-sites, the par- sessed an ORR electron transfer number of ~ 3.8. This tial substitution of B sites was also successfully conducted was higher than that of BaMnO (3.4 – 3.7) and close to to form new perovskite oxides for MAB carbon-composited that of Pt/C (~ 3.9). BaMnO @5%C also demonstrated catalysts. Yuasa et al. [131] prepared carbon-supported better OER activity because it exhibited greater current LaMn Fe O (C-LaMn Fe O ) electrocatalysts using 0.6 0.4 3 0.6 0.4 3 densities and more negative onset potentials than BaMnO a reverse homogeneous precipitation (RHP) method and and Pt/C as revealed by linear scanning voltammograms investigated the effects of LaMn Fe O on discharge 0.6 0.4 3 in N -saturated 0.1 M KOH solution at a rotation speed and charge properties in LABs. XRD revealed that the −1 of 1600 r min . In terms of stability, durability tests obtained C-LaMn Fe O was in a perovskite-phase of 0.6 0.4 3 were run for ORR and OER using a chronoamperometric an orthorhombic form without any other impurity phases method for 12 h in O - and N -saturated 0.1 M KOH at with a calculated crystalline size of 17.4 nm. TEM images 2 2 −1 1600 r min . Here, BaMnO @5%C provided better stabil- showed the uniform distribution of LaMn Fe O on car- 3 0.6 0.4 3 ity toward ORR and OER compared with both Pt/C and bon. The size range of LaMn Fe O was 15–20 nm, and 0.6 0.4 3 BaMnO samples. Not all carbon materials enhance ORR this was consistent with XRD results. Used in an LAB, −2 and OER after compositing with perovskite oxides, how- the obtained four charge/discharge curves at 0.5 mA cm ever. For example, LaMnO -C exhibited poor OER activity showed that as compared with C-LaMn Fe O , carbon 3 0.6 0.4 3 after the incorporation of carbon [128]. presented an unstable charge voltage due to its oxidation For perovskite oxides with the general formula of corrosion by anodic polarization [132]. With the addi- ABO , the substitution of the A-site and/or B-site metal tion of LaMn Fe O , C-LaMn Fe O exhibited sta- 3 0.6 0.4 3 0.6 0.4 3 cations to generate oxygen deficiency/vacancy can have ble discharge/charge curves because LaMn Fe O , with 0.6 0.4 3 large characteristic effects on their electronic structure a higher OER, effectively prevents oxidation corrosion and coordination chemistry, leading to enhanced ORR from anodic polarization. The discharge voltages of the and OER activities [129]. Two typical representatives C-LaMn Fe O were also greater than that of carbon, sug- 0.6 0.4 3 of perovskite oxides with a partial A-site substitution, gesting that LaMn Fe O can also act as an ORR catalyst 0.6 0.4 3 La Ca CoO (LCC) and Sr Sm CoO (SSC) have in non-aqueous electrolytes as well as in alkaline aqueous 0.6 0.4 3 0.5 0.5 3-δ 3+ 4+ been combined with carbon black to form two different solutions because Mn or Mn ions can act as active sites carbon-supported perovskite oxides (C-LCC and C-SSC) for ORR [133]. Researchers [134, 135] have also studied for rechargeable MABs [11, 130]. In a comparison of carbon-composited perovskite oxides derived from the sub- their bifunctionality using graphitized Vulcan XC-72R stitution of both A and B sites as oxygen electrodes and have as a reference, obtained cathodic polarization curves in a also found improved electrochemical properties. three-electrode cell with an 8.5 M KOH solution revealed that the C-SSC composite cathode provided better ORR 5.2 Composites of Carbon and Spinel Oxides activities that the C-LCC cathode and both cathodes exhibited better ORR activities than the graphitized Vul- As substitutes for expensive noble metal catalysts, spinel can XC-72R cathode. No significant difference in OER oxides [denoted as A B O (A, B=Co, Zn, Ni, Fe, Cu, x 3−x 4 activity was found between the C-SSC and C-LCC cath- Mn, etc.)] have gained much attention because of their low odes, however, but both still provided better OER activities costs, considerable activities, high abundance (stability), than the graphitized Vulcan XC-72R cathode. In cycling and environmental friendliness [136–138]. To achieve bet- −2 performance tests under 53 mA cm , C-SSC ran for 106 ter catalytic activities toward ORR and OER, however, net cycles at − 0.3 V, whereas C-LCC only ran for ~ 68 cycles spinel oxides are usually attached onto conducting carbon at − 0.3 V. The graphitized Vulcan XC-72R exhibited substrates with the aim of assuring fast electron transport unsatisfactory cycling performances because of its poor and good interaction between oxides and carbon [136]. For OER capabilities. Later, Velraj and Zhu [11] also prepared example, Li et al. [137] performed an in situ growth of spi- untreated Vulcan XC-72R supported LCC and SSC and nel CoFe O nanoparticles on rod-like ordered mesoporous 2 4 their results showed that because of the better corrosion carbon (CFO/RC) through a hydrothermal treatment process resistance of graphitized carbon, graphitized Vulcan-based with an annealing procedure in which Co(NO ) ·6H O and 3 2 2 electrodes can provide more than twice the cycle life of Fe(NO )·9H O were used as Co and Fe sources. According 3 2 untreated carbon-based electrodes. These results confirm to different annealing temperatures of 300, 400, 500, and that for carbon-supported perovskite oxide composites, the 600 °C, the final composites were labeled as CFO/RC-300, enhancement of catalytic activity and durability toward CFO/RC-400, CFO/RC-500, and CFO/RC-600, respectively. ORR and OER are related to the structure and properties In the examination of surface chemical composition and 1 3 20 Electrochemical Energy Reviews (2018) 1:1–34 cation oxidation states using XPS, no shifts were found for of defected CFO nanoparticles providing more active sites. the Co 2p peaks of the pure CFO and CFO/RC-400 catalysts, For Tafel plots in the low overpotential region, the Tafel indicating that the Co cations in both catalysts possessed the slope (99 mV per dec) of the CFO/RC-400 composite was same chemical surrounding. For the Fe 2p and Fe 2p lower than those of CFO (129 mV per dec) and RC (125 mV 3/2 1/2 of the Fe 2p spectra, however, CFO/RC-400 showed two per dec). Both results indicate that CFO/RC-400 possessed small shifts to higher binding energies as compared with enhanced ORR kinetics after the incorporation of CFO into CFO, revealing strong coupling between CFO and RC. In RC. Among all CFO/RC composites, CFO/RC-400 pro- −2 further analyses, the O1s peak of CFO/RC-400 was seen to duced the highest limiting current density (4.86 mA cm ) shift to higher binding energies in comparison with CFO, and the highest onset potential (~ − 0.10 V), indicating that confirming the strong coupling between CFO and RC from CFO/RC-400 possessed the optimal ORR activity among the lattice oxygen in the Co/Fe–O framework during the all CFO/RC composites. This is not only because anneal- hydrothermal treatment. In the assessment of electrocata- ing temperatures below 400 °C can result in the incomplete lytic properties, LSV curves (Fig. 8a) in O -saturated 0.1 M CTAB decomposition covering some active sites, but also −1 KOH solution at a rotation rate of 1600 r min showed because annealing temperatures above 400 °C can cause that all CFO/RC composites exhibited superior onset poten- the reduction of defects and the increase of particle sizes, tials (− 0.10 to − 0.13 V) compared with CFO (− 0.23 V) decreasing electrocatalytic activity. For OER, anodic LSV and RC (− 0.29 V). This was attributed to the synergistic curves (Fig. 8b) were recorded in N -saturated 0.1 M KOH −1 effects of the two components as well as the large amount solution at a rotation speed of 1600 r m in . Here, both the onset potential and Tafel slope (in the low overpoten- tial region) of CFO/RC-400 (~ 0.41 V, 92 mV per decade) were less than those of CFO (~ 0.43 V, 112 mV per dec- ade), RC (~ 0.75 V, 308 mV per decade), and 20 wt% Pt/C (~ 0.54 V, 105 mV per decade). At 1.0 V, CFO/RC-400 dis- −2 played a much higher current density of 39.6 mA cm than −2 −2 CFO (~ 34.5 mA cm ), RC (~ 3.37 mA cm ), and Pt/C −2 (~ 27.8 mA cm ), demonstrating the much higher OER activity of CFO/RC-400 as compared with CFO, RC, and Pt/C samples. This can possibly be attributed mainly to the hierarchical mesoporous structure of the RC matrix and the strong coupling and synergistic effects between CFO and RC. Based on the obtained ORR and OER activities, the CFO/RC-400 composite was also found to outperform other carbon-spinel oxide composite catalysts such as NiCo O / 2 4 grapheme [139], CoFe O /grapheme [140], and CoF e O / 2 4 2 4 biocarbon [141], suggesting that ordered mesoporous carbon rods are more suitable for spinel oxide nanoparticle loading to improve ORR and OER than other carbon matrixes such as CNTs, graphene, and biocarbon materials. Similar to CoFe O and NiCo O ternary spinel oxides, 2 4 2 4 Co O ; a binary spinel oxide, has also been investigated in 3 4 the development of carbon-spinel oxide bifunctional com- posite catalysts. Specifically, a facile hydrothermal route was used by Liu et al. [142] to form a cubic Co O and 3 4 multi-walled carbon nanotube (cCo O /MWCNT) compos- 3 4 ite in which MWCNTs were acid-functionalized as struc- ture directing/oxidizing agents and Co(CH COO) ·4H O 3 2 2 was used as the single Co source. For comparison, pure cCo O , acid-treated MWCNTs and a physical mixture of 3 4 cCo O + MWCNTs were chosen as reference samples. Fig. 8 a Linear sweep voltammetry (LSV) of RC, CFO, CFO/RC 3 4 composite and commercial Pt/C in O -saturated 0.1 M KOH solution 2 The comparison of XRD patterns of cCo O and cCo O / 3 4 3 4 −1 at a rotation rate of 1600 r min , and b anodic LSV of RC, CFO, MWCNTs revealed that after their spinel structures were CFO/RC-400 composite and commercial Pt/C in N -saturated 0.1 M −1 confirmed, the peaks of cCo O were shifted slightly to a 3 4 KOH solution at a rotation rate of 1600 r min . Reprinted with per- larger 2θ angle after the addition of MWCNTs, suggesting a mission from Ref. 137 Copyright 2016 Royal Society of Chemistry 1 3 Electrochemical Energy Reviews (2018) 1:1–34 21 2+ 2+ slight lattice contraction possibly due to variations in crystal Co in CoO/C, while two spin orbit doubles of Co and 3+ sizes [143, 144] or interactions between cCo O and MWC- Co has been assigned in their CoO reference sample. In 3 4 NTs. SEM and TEM images confirmed the cubic morphol- addition, analysis of O 1S showed the shift of O4 signal, ogy of cCo O and showed the attachment of MWCNTs resulting from CoO reduction (O-to-Co atomic ratio was 3 4 to the cCo O surface without free cubic particles. These less than calculated 1.39) in CoO/C sample. This indicated 3 4 results indicated the effective tethering between MWCNTs on existence of oxygen deficiencies in the CoO/C sample and cCo O as the acid-functionalized MWCNTs served after the addition of C species. In their evaluation of battery 3 4 as an oxidizing/structure directing agent to oxidize cobalt performance and electrocatalyst activities for both ORR and ions into spinel cobalt oxides and regulate the formation of OER, the CoO/C catalyst showed larger capacities (~ 7011 −1 −1 cubic cobalt oxide. According to TGA, the cCo O /MWC- and ~ 4074 mAh g at 100 and 400 mA g , respectively) 3 4 −1 NTs composite contained 54% cCo O similar to the physi- than CoO (~ 5189, ~ 2059 mAh g ), and cyclability tests in 3 4, −1 cal mixture of cCo O + MWCNTs. For ORR, LSV curves a rechargeable LAB with a current density of ~ 200 mA g 3 4 in O -saturated 0.1 M KOH solution showed that among and a high voltage cutoff of 4.5 V demonstrated that CoO/C all tested catalysts, cCo O /MWCNTs possessed the larg- can achieve higher cycle numbers (50) than CoO. These 3 4 est onset potential (~ − 0.15 V) and best current density results indicate that the addition of carbon can significantly −2 (~ − 2.91 mA cm at − 0.4 V), indicating that cCo O / improve electrocatalytic activities for both ORR and OER. 3 4 MWCNTs possessed superior ORR activity to cCo O , A further investigation of morphology and phase composi- 3 4 MWCNTs, and cCo O + MWCNTs. For OER, LSV curves tion under different charge/discharge states using SEM and 3 4 in N -saturated 0.1 KOH solution showed that cCo O / XRD demonstrated that the main discharge product, Li O 2 3 4 2 2, MWCNTs possessed the best OER activity as evidenced by can be completely decomposed by CoO/C but not by CoO. −2 a higher current density (~ 16.0 mA cm at 0.7 V) com- This quality attributed to the lower charge–discharge over- pared with other catalysts. A further cycling test with 500 potentials and higher cycling properties of CoO/C. Overall, continuous CV cycles showed that the ORR current density these enhanced electrochemical performances can be attrib- of cCo O /MWCNTs was about 4, 34, and 3 times better uted to the composition between carbon and CoO and can 3 4 than the MWCNTs, cCo O , and cCo O -MWCNTs cata- possibly be associated with the integration of carbon dotting 3 4 3 4 lysts, respectively, and that the final OER current density of and oxygen vacancies into CoO as well as the synergetic cCo O /MWCNTs was 49% higher than cCo O -MWCNTs effects of the two components. This promising strategy of 3 4 3 4 and greater than the individual components. These results compositing carbon with oxides to create positive effects suggested that cCo O /MWCNTs can produce stronger inter- on MAB ORR and OER has inspired more research into 3 4 actions between cCo O and MWCNTs, resulting in possible exploring and using different oxides, such as La O [151], 3 4 2 3 coupling effects and thus higher ORR and OER activities. zirconium doped ceria [152], and cobalt-manganese mixed oxide (Co Mn O) [153]. x 1-x 5.3 Composites of Carbon and Other Oxides Apart from perovskite oxides and spinel oxides, other oxides, 6 Composites of Carbon and Nitrides such as CoO [145, 146], MnO [147, 148], and RuO [149, 2 2 150], have also been used to prepare effective carbon-com- Combining carbon and nitrides (e.g., TiN or CN) to cre- posited bifunctional catalysts to tackle the sluggish kinetics ate novel catalysts for MAB ORR and OER has become of ORR and OER in MABs. Based on a novel strategy to an effective strategy to improve rechargeability and round- improve the catalytic performance of CoO through the inte- trip efficiency. Titanium nitride (TiN), a typical transition gration of dotted carbon species and oxygen vacancies, Gao metal nitride with high electronic conductivity and good et al. [145] designed and synthesized a carbon-dotted CoO electrochemical activity, is widely applied in electrochem- with oxygen vacancies (CoO/C) for LAB cathodes using a istry studies [154]. Using a template method (Fig. 9a), Li simple calcination of a formed pink precipitate of ethanol- et al. [155] prepared nano-sized TiN supported on Vulcan mediated Co(Ac) ·4H O. In their structure characteriza- XC-72 (n-TiN/VC) and used this as a bifunctional catalyst 2 2 tions, XRD spectra were used for the low composition of for non-aqueous LABs. Commercial micrometer scale TiN CoO from Rietveld refinement. Furthermore, the Raman was used by the researchers to prepare m-TiN/VC samples 0.89 spectra was used for analysis of the shift of Co–O vibration as reference. XRD patterns revealed that TiN in n-TiN/ peaks for both CoO and CoO/C, and revealed a negative VC possessed a crystallite size of 4.3 nm according to the shift of Co–O signal for CoO/C as compared to CoO due Scherrer equation and was consistent with TEM results. to suggested oxygen vacancies. This was further confirmed BET measurements showed that the surface areas of the n- by thorough XPS analysis of both Co 2p and O 1s signals. TiN/VC, m-TiN/VC, and VC samples were 172, 144, and 2 −1 A Co 2p spectra was only fitted with one peak related to 233 m g respectively. In electrochemical measurements, 1 3 22 Electrochemical Energy Reviews (2018) 1:1–34 −1 For OER curves (Fig. 9c) at 50 mA g , n-TiN/VC pro- carbon vided the lowest recharge voltage and accordingly the lowest voltage gap (~ 1.05 V), showing promoted OER activity. Moreover, the initial section of the recharge in the inset of Fig. 9c shows a lower onset potential (~ 2.9 V) for n-TiN/VC than those of m-TiN/VC (~ 3.1 V) and VC (~ 3.1 V) cath- odes, matching the strong oxidation peak of LiO produced by n-TiN/VC as shown in Fig. 9b, demonstrating better OER activities due to the interaction between n-TiN and VC. In five discharge–recharge cycles, n-TiN/VC exhibited higher recharge voltages at the 5th discharge–charge cycle than both m-TiN/VC and VC cathodes. This can be attributed to the strong interaction between the remaining Li O deposits 2 2 and the TiN nanoparticles on the carbon surface, resulting in high stability. However, because TiN can be oxidized to TiO based on the results of XRD and FTIR, LABs with n- TiN/VC cathodes possess a limited potential of 4.3 V despite its ability to enhance ORR and OER. Park et al. [158] stud- ied TiN/C composites and found that the porous structure and synergistic effects between TiN and carbon play a pro- moting role in the enhancement of catalytic activity for both ORR and OER in LABs. Graphitic carbon nitride (g-CN) is also an attractive metal-free material because of its abundance and negligi- ble metal pollution [159]. However, the ORR activity of g-CN itself is not satisfactory because of its poor electri- cal conductivity [160, 161] despite nitrogen atoms in g-CN being able to increase the electropositivity of adjacent car- bon atoms. Based on this, Fu et al. [162] combined car- bon with porous graphitic g-CN through a template-free synthesis route to obtain g-CN/C composite catalysts for Fig. 9 a Preparation process of n-TiN/VC. b CV curves of LAB with ORR in MABs. In their experiment, based on the amount of VC, m-TiN/VC, and n-TiN/VC as air-electrode catalysts under an O d -glucose (i.e., 15.84, 39.60, and 79.20 mmol) used, three −1 atmosphere from 2.0 to 4.0 V at 0.05 mV s . c Discharge-recharge g-CN/C samples (g-CN/C-1, g-CN/C-2, and g-CN/C-3) curves of VC, m-TiN/VC, and n-TiN/VC as cathode catalysts for −1 were obtained. For comparison, a physically mixed g-CN LABs with an enlarged section highlighted (inset) at 50 mA g . carbon Reprinted with permission from Ref. [155]. Copyright 2013 Royal and carbon, labeled as g-CN + C, was also prepared. The Society of Chemistry addition of carbon increased not only the BET surface area but also porosity. Table 4 presents the ratio of CN in the g-CN in terms of the atomic ratio of carbon and nitrogen CVs (Fig. 9b) under O from 2.0 to 4.0 V at a scan rate of as well as the relative composition ratios (%) of the four −1 0.05 mV s in a LAB revealed that for the cathodic scan, samples derived from XPS spectra. A comparison of ORR n-TiN/VC showed higher ORR currents than m-TiN/VC and activities for the three different composites (i.e., g-CN/C-1, VC cathodes. For the anodic scan, n-TiN/VC presented three g-CN/C-2, and g-CN/C-3) revealed that g-CN/C-2 with oxidation peaks rather than two as seen in the CVs of m-TiN/ 33.75 wt% N content delivered a more positive potential VC and VC cathodes. This suggests three varied OER path- and a larger limiting current than g-CN/C-1 (~ 50.35 wt% N) ways [i.e., I: Eq. (1), II: Eq. (2), and III: Eq. (3)] according and g-CN/C-1 (~ 26.09 wt% N) samples, demonstrating to the decomposition mechanisms [156, 157]: that g-CN/C-2 possessed the best ORR activity. This result suggests that high N content benefits the tradeoff between + − LiO → Li + O + e (1) 2 2 available ORR active sites and electron conduction. LSV + − curves of g-CN/C-2 obtained in O -saturated 0.1 M KOH Li O → 2Li + O + 2e (2) 2 2 2 −1 solution at 1600 r min exhibited significantly more posi- tive onset potentials (~ 0.90 V) and larger disk currents 2LiO → 4Li + O + 4e (3) 2 2 −2 (limiting current density, 4.10 mA cm ) than those of the 1 3 Electrochemical Energy Reviews (2018) 1:1–34 23 Table 4 Ratios of CN in g-CN based on the atomic ratio of carbon and nitrogen and relative composition ratio (%) of four carbon components derived from the deconvoluted XPS spectra. Reprinted with permission from Ref. [162]. Copyright 2016 Royal Society of Chemistry a b c Samples C1s (atm.%) N1s (atm.%) Relative composition ratio of carbon (%) C/N in g-CN P1 P2 P3 P4 g-CN 50.74 43.11 42.72 0 0 57.28 0.67 g-CN/C-1 74.60 15.61 37.84 20.75 30.22 11.18 0.53 g-CN/C-2 74.70 19.88 29.03 27.51 25.03 18.43 0.69 g-CN/C-3 76.40 16.57 39.34 12.09 29.05 19.52 0.90 Atomic ratios of carbon and nitrogen in composites are obtained from XPS results Different carbon percentages deconvoluted from the C1s XPS spectra of composites The ratio of C/N in g-CN is calculated based on the carbon present in C–N bonds divided by nitrogen according to the equation C/N = P4 * C1s/N1s g-CN and g-CN + C samples, confirming the higher ORR activity of g-CN/C-2. Current-time chronoamperometric responses in O -saturated 0.1 M KOH solution showed that after 20,000 s, g-CN/C-2 decayed less (~ 20%), displaying superior stability compared with g-CN + C. All these results demonstrate the high ORR activity and good stability of the g-CN/C-2 catalyst. Recently, doping strategies (e.g., P-dop- ing) have also been used to fabricate P-doped g-CN for MAB applications [163] in which in situ growth of P-g-C N on 3 4 carbon-fiber paper (PCN-CFP) is carried out for ZAB flex- ible oxygen electrodes. These resultant PCN-CFP catalysts exhibit outstanding ORR and OER activity, stability, and reversibility in tested ZABs. 7 Composites of Carbon and Carbides Because carbides possess desirable ORR activities [164], researchers have recently attempted to utilize carbides such as tungsten carbide (WC) [165] and boron carbide (B C) [166] to create carbon-composited catalysts to improve capacity, rechargeability, and round-trip efficiency in MABs. For example, Koo et al. [165] coated a uniform WC layer onto a carbon (Ketjenblack EC600-JD) cathode using physical vapor deposition (PVD) (Fig. 10a) with TEM images revealing a 20-nm-thick uniform WC-coating layer and electrochemical measurements being performed in a non-aqueous LAB (Fig. 10b). The obtained discharge curves (down to 2.0 V) show that the WC-coated cath- Fig. 10 a Schematic diagram of PVD method for WC coating. b −1 Design and configuration of LAB. Reprinted with permission from ode can deliver a capacity of ~ 7000 mAh g . This carbon Ref. [165]. Copyright 2015 IOP publishing is twofold higher than that of the carbon cathode. Addi- tionally, it was found that in the comparison of the 1st discharge–charge curve and the 10th discharge–charge enhanced catalytic property and electrical conductivity −1 curve at 100 mA g , the WC-coated cathode produced of the carbon-WC composite. In further tests, at a cur- carbon −1 lower overpotential gaps of 700 and 1200 mV as compared rent density of ~ 1000 mA g and a voltage range of carbon with those of the carbon cathode. These results suggest 2.0–4.2 V, the WC-coated cathode was found to be able to that WC coating can enhance both ORR and OER and be efficiently operated beyond the 36th cycle, whereas the therefore limit discharge–charge overpotentials due to the carbon cathode stopped operating at the 12th cycle. This 1 3 24 Electrochemical Energy Reviews (2018) 1:1–34 increased cycle stability and round-trip efficiency clearly components are found to enhance electrocatalytic ORR and demonstrates the improvements in reaction rates and the OER performances in MABs. reduction in discharge and charge voltage gaps. Moreo- ver, it was shown that during the charge and discharge 8.1 Other Carbon‑Based Binary Composites cycling process, the addition of WC resulted in the rapid creation and decomposition of reaction products (i.e., To develop high-performance bifunctional catalysts and Li O ) because of the enhanced catalytic effects. And as solve carbon corrosion issues in MABs, Lyu et al. compos- 2 2 −1 current densities increased from 100 to 200 mA g , the ited cobalt sulfide, acting as an active ORR material, with carbon WC-coated cathode also showed decreased overpotentials carbon to form carbon-based binary composites for cathode compared with the carbon cathode, further confirming the materials in MABs [169]. In their experiment, cobalt acetate benefits of WC coating. (Co(Ac) ) and thioacetamide were used as Co and sulfur Similar to the WC-C composite above, B C nanowires- sources to obtain a novel CoS nanoparticles-reduced gra- 4 2 carbon nanotube (BC) composite cathodes, induced by the phene oxide (CoS /rGO) composite for aprotic LABs. TGA synergistic effect of B C and carbon, were also found to revealed 78 wt% CoS and XRD showed that the patterns 4 2 exhibit improved ORR/OER activity, rechargeability, and of cobalt sulfide on the graphene sheet were of a typical round-trip efficiency for LABs by Luo et al. [166]. In their cubic CoS phase (JCPDS 00-41-1471). In measurements typical experiment, Raman spectra, field emission scan- in an aprotic LAB using LiClO -DMSO as an electrolyte, ning electron microscopy (FESEM), STEM, and EDS were the CV curves showed that the CoS /rGO cathode delivered used for characterizations to confirm the formation of car - a higher ORR onset potential and a notably higher OER bon-B4C composites. Their Raman spectra showed bands current peak (at ~ 3.75 V), demonstrating its higher cata- −1 below 1200 cm , which matched the characteristics of B C lytic activities for both ORR and OER than those of rGO [167, 168], and FESEM, STEM and EDS confirmed a large and Vulcan XC-72. A combination of XRD, XPS, and SEM amount of B C nanowires growing from CNT aggregations revealed the significant role of CoS in lowering discharge/ 4 2 that possessed diameters in the range of 40–100 nm, and charge overpotentials by positively affecting the formation lengths above 2 μm. In RDE measurements in O -saturated and decomposition of Li O . Here, Li CO is still formed 2 2 2 2 3 −1 0.1 M KOH at a scan rate of 10 mV s , LSV curves at 900 r as a side product during discharging/charging, however. In −1 min showed that the B–C composite exhibited a more pos- measurements of rate capability and cyclability, it was found itive onset potential and a larger current density, correspond- that although rate capabilities increased with increasing rates −1 ing to a higher ORR activity in comparison with those of the from 50 to 500 mA g, CoS /rGO only ran 18 cycles in the −1 Pt/C and CN samples. Based on Koutecky–Levich plots, the cycling test at a rate of 200 mA g and a limited capacity −1 B–C composite was revealed to produce a nearly four-elec- of 500 mAh g . This indicates possible incomplete decom- tron ORR process comparable to Pt/C (n ~ 4.0) but different position of side products (e.g., Li CO ). Therefore, car- 2 3 from CN (n = 3.2–3.5). Subsequent comparisons of OER bon–cobalt sulfide composites need to be further improved curves revealed that the B–C composite provided a smaller in future studies to improve its potential in the development Tafel slope of ~ 70 mV per decade than Pt/C (~ 123 mV per of advanced bifunctional catalysts for LABs. decade), indicating that the B–C catalyst possessed superior NiCo S was also combined with rGO to synthesize 2 4 OER activity to 20 wt% Pt/C. Electrochemical properties NiCo S –rGO composites by Wu et al. [170] and their 2 4 were continuously tested in an assembled battery with B–C research found that the synergistic effects between NiCo S 2 4 and CN composite cathodes and the researchers found that and rGO can result in superior ORR performances as the B–C composite showed not only higher ORR potential compared with 20 wt% Pt/C, demonstrating potential as a and lower OER potential compared with CN, it also exhib- bifunctional catalyst in MABs. Aside from sulfides, other ited better cycling performances with a higher round-trip functional materials, such as FeOOH [171], soil [172], poly- efficiency. These results can possibly be attributed to B C imides [173], and Fe phthalocyanine (FePc) [174], have also and carbon providing efficient synergistic effects on electro - used in the development of carbon-based binary compos- catalytic reactions. ites for MAB bifunctional catalysts. In these studies, the incorporation of these functional materials yielded active synergistic effects with carbon, producing active sites for 8 Other Carbon‑Based Composites enhanced ORR and OER and reducing side reactions in elec- trocatalytic reactions to improve cycling performances and Other carbon-based composites can be classified into catalytic stability. carbon-based binary composites and ternary compos- ites in which strong synergistic effects between different 1 3 Electrochemical Energy Reviews (2018) 1:1–34 25 chemical/physical stability). For all components, the syner- 8.2 Other Carbon‑Based Ternary Composites gistic effects can be observed improving the catalytic activi- ties and cycling performances of MABs. Table 5 shows the Based on the synergistic effects of three separate components and their inter-correlation, various researchers have recently electrochemical performances of typical metal-free carbon- composited bifunctional catalysts for MAB cathodes [145, focused on developing carbon-based ternary composites as advanced cathodes in MABs. Among carbon-based ternary 146, 148–150, 152, 158, 165, 166, 171–174, 176, 178–183]. Metal-free catalysts composed of carbon and other com- composites for MABs, carbon; coupled with an oxide, is usually combined with a third component such as a poly- ponents (e.g., oxides, nitrides, carbides, sulfides, β-FeOOH, polyimide, Fe phthalocyanine, etc.) have been intensely electrolyte [175], a second oxide [176], or a metal [177] to exhibit enhanced catalytic activities toward ORR and/or studied as cathode materials for MABs and the results show advantages over pure carbon in terms of ORR/OER activ- OER and improved MAB performance. A positively charged polyelectrolyte, poly(diallyldimethylammonium chloride) ity and cycling stability/durability. For example, polyimide/ CNT catalysts [173] have been found to exhibit a high cycling (PDDA), was used by Zhai et al. [175] to modify carbon supports to provide available functional groups favorable stability over 130 cycles with a maximum discharge capacity −1 −1 of 11,000 mAh g at a high current density of 500 mA g to the formation of more active sites. In their study, PDDA functionalized CNTs were combined with spinel CoMn O under LAB operation conditions. As discussed previously, the 2 4 combination of carbon and oxide(s) is a sound strategy in the to form a ternary CoMn O /PDDA-CNTs composite ORR 2 4 catalyst. Their results showed that by increasing the loading development of abundant, low-cost, and efficient bifunctional catalysts for rechargeable MABs owing to the strong synergy of CoMn O from 36 to 83 wt%, the CoMn O /PDDA-CNTs 2 4 2 4 composite exhibited high ORR current densities in alkaline between carbon and oxides. For example, MnO /CP [148] and CoMn O /rGO [179] can achieve 500 and 200 cycles at 15 and and neutral conditions through a 4e reduction pathway 2 4 −2 that outperformed Pt/C due to the non-covalent coupling 20 mA cm , respectively, indicating a significant improve- ment of cycling performances due to strong resistances to effects between CoMn O and PDDA-CNTs. Moreover, 2 4 at an optimal content of 36 wt% CoMn O , the CoMn O / carbon corrosion. A SP/CaMnO catalyst [181] demonstrated 2 4 2 4 charge–discharge stability after over 80 cycles (with a high PDDA-CNTs potential difference between ORR and OER −1 was ~ 0.849 V, demonstrating potential bifunctionality. current density of 200 mA g and a limited capacity of 1000 −1 −1 mAh g ) and high discharge capacities of 7000 mAh g . To obtain a suitable third component, Ma and Wang [176] selected two different oxides to fabricate α-MnO –LaNiO / These results demonstrate the beneficial effects of combin- 2 3 ing different components with carbon material in terms of CNTs composites with the expectation that α-MnO and LaNiO can produce synergistic effects on bifunctional electrocatalytic activity, stability/durability, and associated MAB performances. However, further improvements and activity. According to the literature [4, 5, 126, 130], α-MnO acts as a good ORR catalyst and LaNiO as a good OER cat- studies are needed to enhance MAB performances to achieve commercialization. alyst. Their experiment showed that the α-MnO –LaNiO / 2 3 CNTs cathode provided excellent charge–discharge cycling Nevertheless, based on the collected data for several types of bifunctional catalysts in Tables 2, 3 and 5, the cycling per- performances within 75 charge–discharge cycles due to good bifunctional activity and durability. Wu et al. [177] studied formances of ORR and OER are not easily improved even with the use of different components compositing with carbon. This Ni-modified MnO /C (Ni-MnO /C) composites for ORR and x x their application in ZABs and they demonstrated that at an may be because the nanostructure and property of the formed carbon composites are closely associated with discharge prod- optimal Ni/Mn atomic ratio of 1:2, Ni-MnO /C cathodes in a tested ZAB can achieve a large power density of ~ 122 ucts during cycling. In a comparison of stability, Co/C/NiFe −2 −2 LDH/AB and Co–N/C composite catalysts in Table 3 show mW cm . This was better than MnO /C (~ 89 mW cm ) −2 and slightly higher than referenced cathodes of Pd/C (~ 121 300 cycle numbers at 40 mA cm and 500 cycle numbers −2 −2 −2 at 2 mA cm respectively, whereas the MnO /CP catalyst in mW cm ) and Pt/C (~ 120.5 mW cm ). −2 Other carbon-based ternary composites such as Fe/ Table 5 produces 500 cycle numbers at 15 mA cm . These results indicate a successful strategy without the use of noble Fe C–CNFs [113] and Fe-doped NiOOH grown on graphene-encapsulated FeNi nanodots (FeNi @GR@ metals in developing high-performance catalysts with high 3 3 bifunctional activities and stability. For the scale-up produc- Fe–NiOOH) [114] were also studied by Wang et al. for MAB ORR and OER. In these two carbon-based ternary compos- tion of next-generation bifunctional catalysts in commercial MABs, low costs, high bifunctional activities, and good sta- ites, carbon provided an excellent porous structure and high surface area, whereas the other two components provided bility are strongly desired. In terms of material cost and avail- ability, noble metals are being replaced by low-cost materials electrochemical properties associated with their catalytic performance (i.e., ORR and OER) and physiochemical prop- such as transitional metals, oxide, nitrides, carbides, sulfides, β-FeOOH, Fe phthalocyanine, and conducting polymers to erties (e.g., active sites, thermal ability, conductivity, and 1 3 26 Electrochemical Energy Reviews (2018) 1:1–34 1 3 Table 5 Electrochemical performance for typical metal-free carbon-composited bifunctional catalysts as air-electrode materials in the tested MABs No. Bifunctional catalyst as Maximum capacity/mAh Cycle number (current den- Potential range for Potential difference for Energy Electrolyte type MAB type References −1 a −1 −2 cathode materials g (current density/ mA sity/mA g or mA cm , cycle testing (V) ORR/OER (initial ORR/ efficiency −1 b −2 c g or mA cm ) upper-limit capacity/mAh OER overpotentials , V vs. (%) −1 + g ) Li/Li ) a a 1 CoO/SP5637 (200 )50 (200 , 1000) 2.0–4.5 1.32 (2.63/3.95) – 1 M LITFSI/TEGDME rLOB [145] b b 2 CoO/CNF 3882.5 (0.2 ) 50 (0.2 , 1000) 2.0–4.2 1.08 (2.72/3.80) – 1 M LiTFSI/TEGDME rLOB [146] b −1 3 MnO /CP –500 (15 , –) 1.0–2.4 0.75 (1.25/2.00) – 6 M KOH +20 g L ZnCl rZAB [148] x 2 b b 4 RuO /CNT 1150 (0.4 ) 50 (0.4 , 244) 2.0–4.4 1.38 (2.56/3.94) 65.4% 1 M LITFSI/TEGDME LOB [149] a a 5 RuO ·0.64H O/rGO5000 (500 )35 (200 , 2000) 2.0–4.3 1.40 (2.75/4.15) – LiCF SO /TEGDME LOB [150] 2 2 3 3 (1:4 mol/mol) b b 6 G/Zr–CeO 3254 (0.2 )15 (1 , 500) 2.0–4.5 1.60 (2.50/4.10) – 1 M LITFSI/TEGDME LOB [152] 7 TiN/C 7100 (100a)35 (200 , 1000) 2.0–4.5 1.15 (2.60/3.75) – LiCF SO /TEGDME (1:4, LOB [158] 3 3 mol/mol) a a 8 WC/C7000 (100 )36 (100 , 1000) 2.0–4.5 0.88 (2.75/3.63) – 1 M LITFSI/TEGDME LOB [165] b b 9 B C/CNT 16,000 (0.2 ) 120 (0.4 , 1000) 2.5–4.4 1.02 (2.73/3.75) – LiCF SO /TEGDME (1:4, LOB [166] 4 3 3 mol/mol) b b b b 10 β-FeOOH/C aerogels, 10,230 (0.1 ), 6050 (0.1 ) 60 (0.1 , 800), 42 (0.1 , 2.0–4.4 1.22 (2.69/3.91) – 1 M LITFSI/TEGDME rLOB [171] β-FeOOH/Super P 800) b b 11 VC/soil 7640 (0.2 ) 100 (0.2 , 1000) 2.3–4.2 1.59 (2.71/4.30) – 1 M LITFSI/TEGDME rLOB [172] a a 12 Polyimide/CNT11,000 (500 )137 (500 , 1500) 2.0–4.35 1.72 (2.63/4.35) – 1 M LITFSI/TEGDME LAB [173] b b b b 13 FePc/GNS, FePc/CNTs, 865.6 (0.5 ), 632.4 (0.5 ), 30 (0.5 , –), 30 (0.5 , –), 30 2.0–4.8 0.67 (3.00/3.67), 0.85 –1 M LiClO /ED/ LAB [174] b b FePc/AB 795.4 (0.5 ) (0.5 , –) (3.02/3.87), 1.05 DEC + 1 M LiNO /0.5 M (2.82/3.87) LiOH 14 MnO –LaNiO /CNT –75 (20 , –) 0.8–2.4 0.75 (1.20/1.95) – 6 M KOH + 0.4 M ZnO rZAB [176] 2 3 15 CNT/CoFe O 3670 (200a)35 (200 , 430) 2.0–4.3 1.55 (2.70/4.25) – 1 M LITFSI/TEGDME rLOB [178] 2 4 b b 16 CoMn O /rGO610 (20 )200 (20 , –), 0.8–2.4 0.95 (1.15/2.10) 6 M KOH ZAB [179] 2 4 17 VC/Co O –22, (20 , –), 0.5–3.0 1.25 (1.00/2.25) – 6 M KOH + 0.2 M Zinc rZAB [180] 3 4 acetate a a 18 SP/CaMnO7000 (200 )80 (200 , 1000) 2.0 -4.0 1.50 (2.25/3.75) –1 M NaSO CF /TEGDME NaOB [181] 3 3 3 m a a a 19 KB/RM-TIT , KB/RM-3250 (100 ), 2700 (100 ), 121 (400 , 1000), 105 2.0–4.8 0.74 (2.76/3.50), 1.25 – 1 M LITFSI/TEGDME rLOB [182] n a a a FIT , KB5950 (100 ) (400 , 1000), 24 (400 , (2.75/4.00), 1.70 1000) (2.68/4.38) a a 20 CNT@RuO4350 (100 )100 (500 , 300) 2.3–4.0 0.72 (2.70/3.42) – LITFSI/tri(ethylene) glycol rLOB [183] dimethyl ether (1:5 mol/ mol) a −1 mA g b −2 mA cm Initial overpotentials are obtained from the cyclic tests RuO tube in Mn O tube 2 2 3 RuO fiber in Mn O tube 2 2 3 Electrochemical Energy Reviews (2018) 1:1–34 27 composite with carbon to enhance bifunctional activities and interactions between different heteroatom dopants can stability. With the rapid development of nanotechnologies and also contribute significantly to electrocatalytic activities related sciences, processing techniques are becoming more for ORR and OER. Other material components such as facile in the fabrication of non-noble metal-based and metal- oxides and metals can have more effective influences on free bifunctional catalysts in scale-up productions. the electrocatalytic activity and cycling performance of carbon-composite catalyst MABs. In particular, perovskite oxides exhibit bifunctionality for catalytic ORR and OER 9 Summary, Challenges and Future in alkaline solutions. To solve issues of carbon corrosion Research Directions and oxidation in MABs, novel strategies such as carbon- composited ternary bifunctional catalysts have shown 9.1 Summary promising effects on the improvement of catalytic per - formances. Compared with individual components, these In this review, carbon-composited bifunctional catalysts ternary and/or trinary composite catalysts offer improved for MABs are comprehensively reviewed in terms of their catalytic performances and strong resistances to carbon material selection, synthesis method, structural characteri- corrosion in MABs. zation and electrochemical performance. Compared with single material catalysts, composited catalysts possess 9.2 Challenges synergistic effects on structural and electrical properties resulting from different components, leading to enhanced In recent years, considerable efforts have been concentrated ORR and OER performances in MAB air-electrode cata- on the development of carbon-composited bifunctional cata- lysts. In the design of advanced carbon-based composites lysts. Several major technological challenges are still pre- as bifunctional catalysts, the proper selection of carbon sent, however, in the development of commercial MABs: (1) materials is crucial because the properties of the carbon insuc ffi ient electrocatalytic activities for both ORR and OER material (e.g., porous structure, surface area and electronic at the air-electrode, leading to low energy/power density and conductivity) affect the transport of ionic/electronic/gas, efficiency of MABs; (2) insufficient stability/durability of electron conductivity and the distribution of catalytic bifunctional air-electrode catalysts due to low resistances to sites. In addition, the strong interaction between the car- electrochemical corrosion, resulting in degradation of MAB bon materials with other component(s) determines the performance; (3) insufficient strategies for catalyst design, bifunctional activities of ORR and OER and their stability/ starting material selection, scalable synthesis and catalyst durability. As compared with other porous structures, (i.e., performance optimization; (4) unoptimized MAB electrode/ macropore and microporous structures), it is known that cell design and fabrication; and (5) insufficient fundamental mesoporous structures coupled with high surface areas are understanding of catalyst interaction, synergy and bifunc- more beneficial to ionic and electronic transport because tionality mechanisms as well as MAB electrode/cell design of shorter conducting paths. Mesoporous structures also and fabrication. Overall, the immature capability for bifunc- possess the ability to retain electrocatalytic active sites tional catalyst optimization and scale-up production with with uniform distribution. cost-effective approaches for MAB air electrodes is a major As a special type of carbon material, graphene tends to hindrance and the understanding of fundamentals is highly hold active sites at the edge rather than the mesoporous useful for material selection and optimization of catalyst/ structure. However, the restacking of graphene layers is electrode designs. a vital issue that needs to be resolved in the development of advanced graphene-composited bifunctional catalysts 9.3 Proposed Future Research Directions for MABs. It is should also be emphasized that not all non-car- To overcome these technical challenges, several future bon material(s) are able to act as useful component(s) research directions can be proposed as follows for the fab- in carbon-composited bifunctional catalysts in terms of rication of next-generation carbon-composited bifunctional producing synergistic interactions and overcoming carbon catalysts for MAB air electrodes: corrosion/oxides in MABs. Heteroatom(s) can, however, be used as non-carbon component(s) for carbon com- 1. Improving the catalytic activity and stability/durability posite catalysts, and the resulting heteroatom(s)-doped of carbon-composited bifunctional catalysts for MAB carbon materials can exhibit improvements in ORR and air electrodes by developing novel methodologies for OER performance. 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Commun. 51, 5376–5381 (2012) 1210–1213 (2015) 1 3 Electrochemical Energy Reviews (2018) 1:1–34 33 174. Yoo, E., Zhou, H.: Fe phthalocyanine supported by graphene Dr. Baizeng Fang earned his nanosheet as catalyst in Li–air battery with the hybrid electrolyte. Ph.D. in Materials Science from J. Power Sources 244, 429–434 (2013) the University of Science and 175. Zhai, X., Yang, W., Li, M., et al.: Noncovalent hybrid of CoMn O Technology, Beijing in 1997. He 2 4 spinel nanocrystals and poly(diallyldimethylammonium chloride) then worked as a postdoc in Hol- functionalized carbon nanotubes as efficient electrocatalyst for land, a JSPS fellow in Japan, a oxygen reduction reaction. Carbon 65, 277–286 (2013) Lise Meitner scientist in Austria 176. Ma, H., Wang, B.: A bifunctional electrocatalyst α-MnO – and a research professor at Korea LaNiO /carbon nanotube composite for rechargeable zinc–air University. He is a senior scien- batteries. RSC Adv. 4, 46084–46092 (2014) tist at the University of British 177. Wu, Q., Jiang, L., Qi, L., et al.: Electrocatalytic performance Columbia, Canada. He has pub- of Ni modified MnO /C composites toward oxygen reduction lished over 100 peer-reviewed reaction and their application. Int. J. Hydrogen Energy 39, 3423– papers in high-profile journals 3432 (2014) including the Journal of the 178. Li, J., Zou, M., Wen, W., et al.: Spinel MFe O (M=Co, Ni) nan- American Chemical Society, 2 4 oparticles coated on multi-walled carbon nanotubes as electro- Accounts of Chemical Research catalysts for Li–O batteries. J. Mater. Chem. A 2, 10257–10262 and Chemical Reviews. His research interests include nanostructured (2014) materials for electrochemical energy storage/conversion, and artificial 179. Prabu, M., Ramakrishnan, P., Nara, H., et al.: Zinc–air bat- photosynthesis. He also serves as an associate editor for RSC tery: understanding the structure and morphology changes of Advances. graphene-supported CoMn O bifunctional catalysts under prac- 2 4 tical rechargeable conditions. ACS Appl. Mater. Interfaces 6, Dr. Dan Zhang is a Lecturer at the 16545–16555 (2014) Shanghai Institute of Mathe- 180. Lee, D.U., Park, M.G., Park, H.W., et al.: Highly active and matic and Mechanics at Shang- durable nanocrystal-decorated bifunctional electrocatalyst for hai University. Dr. Dan Zhang rechargeable zinc–air batteries. Chemsuschem 8, 3129–3138 received her Ph.D. in Manufac- (2015) turing Engineering of Aerospace 181. Hu, Y., Han, X., Zhao, Q., et al.: Porous perovskite calcium–man- Vehicle from the Northwestern ganese oxide microspheres as an efficient catalyst for recharge- Polytechnical University in able sodium–oxygen batteries. J. Mater. Chem. A 3, 3320–3324 2006. Dr. Zhang’s current (2015) research interests include the 182. Yoon, K.R., Lee, G.Y., Jung, J.W., et al.: One-dimensional R uO / hydrodynamic of microfluidic Mn O hollow architectures as efficient bifunctional catalysts for devices and the reaction mecha- 2 3 lithium–oxygen batteries. Nano Lett. 16, 2076–2083 (2016) nism simulations of surface elec- 183. Jian, Z., Liu, P., Li, F., et al.: Core–shell-structured CNT@RuO trocatalysis in the field of fuel composite as a high-performance cathode catalyst for recharge- cells. able Li–O batteries. Angew. Chem. Int. Ed. 53, 442–446 (2014) Dr. Aijun Li is currently a Profes- sor at the School of Materials Dr. Yan‑Jie Wang obtained his Science and Engineering at the Ph.D. in Materials Science and Shanghai University (SHU). He Engineering from Zhejiang Uni- received his Ph.D. in Materials versity, China, in 2005. Subse- Science from Northwestern Pol- quently, he conducted two post- ytechnical University, Xi’an doctoral research positions at the China in 2004. Dr. Li worked as Sungkyunkwan University, a senior scientist and then a Korea, and at the Pennsylvania group leader for carbon materials State University, USA, respec- at the Karlsruhe Institute of tively. In 2009, he worked as a Technology (KIT), Germany senior scientist at the University from 2010 to 2015, being of British Columbia, Canada, in involved with the research, cooperation with the National development and application of Research Council of Canada and composites. Dr. Li’s main Vancouver International Clean- research interests are in the complex interactions of multi-physical and Tech Research Institute Inc., chemical phenomena involved in chemically reacting flows; mainly respectively. In 2017, he became a full-Professor at the Dongguan Uni- focusing on modeling, simulation and synthesis of composites by versity of Technology, China. He is also an adjunct professor at the chemical vapor infiltration/deposition processes. Fuzhou University. His interests include energy storage and conversion, polymer science, biomass, and medical areas. 1 3 34 Electrochemical Energy Reviews (2018) 1:1–34 Dr. David P. Wilkinson is a profes- Lei Zhang is a Senior Research sor and Canada Research Chair Officer at National Research in the Department of Chemical Council Canada (NRC), a Fellow and Biological Engineering at of the Royal Society of Chemis- the University of British Colum- try (FRSC), an adjunct Professor bia (UBC). He previously held of various Universities, and a the positions of Executive direc- vice president of International tor of the UBC Clean Energy Academy of Electrochemical Research Center, Principal Energy Science (IAOEES). Lei’s Research Officer and Senior research interests include PEM Advisor with the National Fuel Cell electrocatalysis, super- Research Council of Canada capacitors, metal-air batteries, Institute for Fuel Cell Innova- batteries and hybrid batteries. tion, Director and Vice President She has co-authored more than of Research and development at 170 publications. She is the Ballard Power Systems, and member of the NSERC Indus- Group Leader at Moli Energy. His main research interests are in elec- trial R&D Fellowships College of Reviewers, the Editorial Board trochemical and photochemical devices, energy conversion and storage Member of Electrochemical Energy Reviews (EER) -Springer Nature. materials, and processes to create clean and sustainable energy and She is also an active member of the Royal Society of Chemistry (RSC), water. the Canadian Society for Chemistry (CSC), and the Canadian Society for Chemical Engineering (CSChE). Dr. Anna Ignaszak is an Assistant Professor at the University of Dr. Jiujun Zhang is a Professor in New Brunswick and an adjunct College of Sciences, Institute for assistant professor at the Frie- Sustainable Energy at Shanghai drich-Schiller University (Ger- University. He was a Principal many). She completed an Research Officer at the National appointment as a research asso- Research Council of Canada ciate at the Clean Energy (NRC) from 2004 to 2016. Dr. Research Center, at the Univer- Zhang received his B.S. and M.Sc. sity of British Columbia (Can- in electrochemistry from Peking ada), and as a research associate University in 1982 and 1985, at the National Research Coun- respectively, and his Ph.D. in elec- cil of Canada. She has a diverse trochemistry from Wuhan Univer- background in materials (car- sity in 1988. He then carried out bons, composites, metal clus- three terms of postdoctoral ters) for electrochemical energy research at the California Institute storage and conversion, electrochemical sensors, and heterogeneous of Technology, York University, catalysis. The research conducted in her laboratories in Canada and and the University of British Columbia. Dr. Zhang has over 30 years of Germany aims to synthesize morphology-controlled catalysts, under- scientific research experience, particularly in the area of electrochemical standing the structure–reactivity interplay for optimum redox energy storage and conversion. He is also the Adjunct Professor at the activity. University of British Columbia and the University of Waterloo. 1 3

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Electrochemical Energy Reviews – Springer Journals

Published: Mar 1, 2018

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A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

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A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

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