Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 7-Day Trial for You or Your Team.

Learn More →

Hydrogen evolution by a metal-free electrocatalyst

Hydrogen evolution by a metal-free electrocatalyst ARTICLE Received 12 Nov 2013 | Accepted 2 Apr 2014 | Published 28 Apr 2014 DOI: 10.1038/ncomms4783 1,2, 1, 3 4 3 4 5 Yao Zheng *, Yan Jiao *, Yihan Zhu , Lu Hua Li , Yu Han , Ying Chen , Aijun Du , 6 1 Mietek Jaroniec & Shi Zhang Qiao Electrocatalytic reduction of water to molecular hydrogen via the hydrogen evolution reaction may provide a sustainable energy supply for the future, but its commercial application is hampered by the use of precious platinum catalysts. All alternatives to platinum thus far are based on nonprecious metals, and, to our knowledge, there is no report about a catalyst for electrocatalytic hydrogen evolution beyond metals. Here we couple graphitic-carbon nitride with nitrogen-doped graphene to produce a metal-free hybrid catalyst, which shows an unexpected hydrogen evolution reaction activity with comparable overpotential and Tafel slope to some of well-developed metallic catalysts. Experimental observations in combination with density functional theory calculations reveal that its unusual electrocatalytic properties originate from an intrinsic chemical and electronic coupling that synergistically promotes the proton adsorption and reduction kinetics. 1 2 School of Chemical Engineering, University of Adelaide, Adelaide, South Australia 5005, Australia. Australian Institute for Bioengineering and Nanotechnology, University of Queensland, Brisbane, Queensland 4072, Australia. Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia. Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3216, Australia. School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia. Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, USA. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to S.Z.Q. (email: [email protected]). NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 ydrogen evolution reaction (HER), as a fundamental step sheets with complex Moire´ patterns, suggesting its multilayered of electrochemical water splitting and a cornerstone to structure formed by stacking multilayered g-C N on N-graphene 3 4 Hexplore the mechanism of other multi-electron transfer sheets (Fig. 1a). It is observed that the g-C N phase is less stable 3 4 processes in electrocatalysis, always requires a favourable catalyst than N-graphene under electron beam due to the rapid 1–3 25 to achieve fast kinetics for practical applications . Among a decomposition of its polymeric melon units . After selective wide variety of available catalysts, Pt supported on carbon shows removal of g-C N layers from the surface of N-graphene by 3 4 unbeatable electrocatalytic HER properties with extremely high transmission electron microscopy irradiation, a residual few- exchange current density and small Tafel slope . However, to gain layered N-graphene substrate with graphite-like atomic structure a sustainable hydrogen production, cost-effective alternatives to was resolved in the identical area of the initial g-C N -covered 3 4 precious and low abundant Pt catalyst should be developed sample (Fig. 1b). 5–8 with high electrocatalytic activities and stabilities . Despite tremendous efforts in this direction, all efficient HER electro- Electron microscopy characterization. Scanning transmission catalysts heretofore are based on transition metals, such as Co, Ni, electron microscopy combined with electron energy loss spec- 7–19 Fe, Mo and their molecular derivatives , which suffer inherent troscopy (EELS) was used to investigate the C N @NG hybrid. 3 4 corrosion susceptibility to the acidic proton exchange membrane High-angle-annular-dark-field–scanning transmission electron electrolysis. A robust catalytic system beyond metals has not been microscopy revealed that C N @NG consists of ultra-thin 3,8 3 4 found yet , largely narrowing the selection range of competitive nanosheets with a fairly uniform thickness (Fig. 1e). The spatial candidates as Pt’s replacements. distributions of different species in the nanosheet were visualized Various carbon-based materials feature unique advantages for by EELS mapping based on the intensity variation of their energy designated catalysis due to their tunable molecular structures, loss peaks. Note that unlikely, as in the case of carbon species that abundance and strong tolerance to acid/alkaline environments. are distributed over the entire sheet (Fig. 1f), nitrogen species Very recent advances in low-dimensional carbon materials, as concentrate in the specific regions (Fig. 1g). These results indicate metal-free catalysts, have shown their promising future in energy- that N-graphene having a low concentration of N (Supplementary related electrocatalytic oxygen reduction and evolution reac- Fig. 2) is partially covered by g-C N nanodomains on its surface. 20–23 3 4 tions . However, this innovative concept has not been Figure 1c shows two typical EELS spectra that were collected from explored yet for one of the most important electrolysis g-C N -containing and g-C N -free regions (site 1 and site 2 in 3 4 3 4 processes, hydrogen evolution, due to the poorly known HER Fig. 1e), respectively. The fine structures of carbon K-edges are mechanism on such materials. So far, all experimental and magnified in Fig. 1d, where the two p* edges at 283.4 eV (peak 1) theoretical studies on the electrocatalytic HER are exclusively and 285.5 eV (peak 2) and one s* edge at 290.2 eV (peak 3) are focused on the surface properties of metallic catalysts due to the attributed to the defective, graphitic and sp carbon species, 3,8 importance of metal–H bonds in this process ; whether a metal- 26,27 respectively . The EELS mapping with these energy loss peaks free material can exhibit similar catalytic behaviour and be more reveals that graphitic-sp carbon is present over the entire sheet active than metal-based electrocatalysts is still unknown. (Fig. 1i), whereas defective carbon species are only present in the In this work, we report the synthesis of a metal-free catalyst, g-C N -containing regions but absent in the g-C N -free regions 3 4 3 4 consisting of carbon and nitrogen only, by coupling graphitic- (Fig. 1h) or in a pure g-C N sample (Supplementary Fig. 3). 3 4 carbon nitride (g-C N ) with nitrogen-doped graphene 3 4 These defective species that can be assigned to low-coordinated (N-graphene; NG), and demonstrate that the resulting carbon atoms (26) are probably associated with the breakage of C N @NG hybrid possesses unique molecular structure and 3 4 the N–3C bridging bonds at the edge of g-C N moieties, due to 3 4 electronic properties for electrocatalytic HER application. This the strong interaction between g-C N and N-graphene substrate 3 4 metal-free hybrid shows comparable electrocatalytic HER activity (Supplementary Fig. 4). In addition, an enhanced intensity of s* with the existing well-developed metallic catalysts, such as excitation (peak 3) is observed in the g-C N -containing region 3 4 nanostructured MoS materials, although its activity is not as 2 3 (Fig. 1j), which is indicative of new sp carbon species formed high as that of the state-of-the-art Pt catalyst. Electrochemical through the growth of g-C N nanodomains on the N-graphene 3 4 measurements in combination with thermodynamic calculations surface. These observations suggest the generation of chemical reveal that its unusual electrocatalytic properties originate from a bonds between these two materials during coupling process of synergistic effect of this hybrid nanostructure, in which g-C N 3 4 g-C N and N-graphene. 3 4 provides highly active hydrogen adsorption sites, while N-graphene facilitates the electron-transfer process for the proton reduction. The findings provide clear evidence that, similar to NEXAFS and XPS studies. We used Synchrotron-based near- precious metals, the well-designed metal-free counterparts also edge X-ray absorption fine structure (NEXAFS) and X-ray pho- have great potential for highly efficient electrocatalytic HER, thus toelectron spectroscopy (XPS) to further probe this interfacial opening a new avenue towards replacing noble metals by broader interaction by exploring the lateral structure of C N @NG mul- 3 4 alternatives in a wide variety of applications. tilayered nanosheet. In the carbon K-edge NEXAFS spectra (Fig. 2a), the hybrid shows characteristic resonances of both g-C N and N-graphene individuals including structural defects 3 4 Results at B284 eV, p* at 285.5 eV and p* at 288.5 eV. In C¼ C C–N–C Catalyst synthesis and atomic structure. g-C N was grown nitrogen K-edge region (Fig. 2b), C N @NG shows two typical p* 3 4 3 4 directly on the surface of chemically exfoliated graphene oxide resonances at 399.7 and 402.6 eV corresponding to aromatic (GO) films to produce a strong chemical adhesion with the C–N–C coordination in one tri-s-triazine heteroring (N1, see the resultant N-graphene. GO was thermally reduced with simulta- inset of Fig. 2b) and N–3C bridging among three tri-s-triazine 24,25 neous nitrogen heteroatom incorporation into framework moieties (N2, see the inset of Fig. 2b) . As compared with pure driven by gaseous NH released during polycondensation of g-C N , a decreased intensity of N2 resonance in the hybrid 3 3 4 g-C N ’s precursor (dicyandiamide) to melem units , forming reveals a partial breaking of N–3C bridgings, which is in 3 4 aC N @NG hybrid (Supplementary Fig. 1). The aberration- agreement with the EELS results. More importantly, as can be 3 4 corrected high-resolution transmission electron microscopy seen from the carbon K-edge polarization-dependent NEXAFS image of the as-synthesized hybrid shows two-dimensional spectra (Fig. 2c), a weak shoulder at 287.4 eV (assigned to the p* 2 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 ARTICLE * C K 3 N K Site 1 Site 2 280 320 360 400 440 280 290 300 Energy loss (eV) Site1 Site2 Figure 1 | Electron microscopy characterization of C N @NG nanosheet. (a) Aberration-corrected and monochromated high-resolution transmission 3 4 electron microscopy image of freshly prepared C N @NG hybrid. (b) High-resolution transmission electron microscopy image of the same area as (a) after 3 4 removal of g-C N layer by electron beam irradiation (under a prolonged exposure ofB20 s). Scale bar, 2 nm (a,b). (c) EELS spectra of C N @NG collected 3 4 3 4 at two specific sites as indicated in e. Site 1 represents a g-C N -containing region with apparent nitrogen K-edge energy loss peak at B400 eV; site 2 3 4 represents a g-C N -free region. (d) Fine structure of the carbon K-edge EELS on C N @NG. (e–g), High-angle-annular-dark-field–scanning transmission 3 4 3 4 electron microscopy image of C N @NG nanosheet and EELS mappings of overall (f) carbon and (g) nitrogen species in the red-line areas. Scale bar, 2 mm 3 4 (e). (h–j), EELS mappings of various carbon species (peak 1–3, respectively) as indicated in d. A temperature colour code is used in mapping images where the intensity increases as the colour changes in the order of black-blue-green-red-yellow-white. resonance of C–N species) shows a different angular dependence result, the Fermi level crosses the conduction band of g-C N 3 4 to the in-plane C–N–C and C¼ C species, indicating the out-of- as demonstrated by the projected density of states shown in plane orientation of new species between g-C N and N-graphene Fig. 3c. This change in projected density of states between 3 4 28 3 basals . The XPS spectra also reveal extra sp C–N species pure g-C N and hybridized C N @NG indicates enhanced 3 4 3 4 at 287.2 eV involved in the in situ-formed C N @NG electron mobility in the latter, which is significant for the 3 4 hybrid , which do not exist in a physically mixed g-C N and electrocatalytic HER. 3 4 N-graphene (C N /NG) (Fig. 2d). Thus, a combination of 3 4 polarization-dependent NEXAFS and XPS results provides a Electrochemically-measured HER activity. The chemical cou- crucial evidence for the interfacial interaction model between pling and concomitant electron’s re-distribution between g-C N 3 4 g-C N and N-graphene, where the out-of-plane oriented C–N 3 4 and N-graphene can provide a resistance-less path for fast elec- bonds are clearly detected. These interlayer bonds can provide tron transfer through C N @NG’s interlayers and consequently 3 4 a high interconnectivity between g-C N and N-graphene 3 4 accelerate the electrocatalytic HER kinetics on its surface. The parallel layers for a rapid electron transfer to boost hybrid’s polarization curve (i–V) recorded on C N @NG shows an 3 4 electrocatalytic activity. overpotential of B240 mV to achieve a 10-mA cm HER current density and a Tafel slope of 51.5 mV dec (Fig. 4a,b, Electronic structure and electron-transfer properties.We Supplementary Fig. 7). The HER exchange current density (i ) for conducted density functional theory (DFT) calculations to show C N @NG, calculated from Tafel plot by extrapolation method, is 3 4 7  2 the interlayer electronic-coupling effect between g-C N and 3.5 10 Acm ; this value is comparable (within an order 3 4 N-graphene (Supplementary Figs 5,6). Contrary to the semi- of magnitude) to those of the well-developed nanostructured conductive pure g-C N ,C N @NG hybrid shows no band gap MoS -based metallic catalysts after normalizing them to the 3 4 3 4 2 due to the downshifting of the Dirac cone at the G point to same surface area and catalyst loading (Supplementary Fig. 8, 15,30–32 guarantee a fast electron transfer (Fig. 3a). More importantly, Supplementary Tables 1 and 2) . Importantly, the inter- after coupling g-C N with N-graphene, the charge density in facial covalent bonds between g-C N and N-graphene layers can 3 4 3 4 hybrid’s interlayer was redistributed in the form of an apparent also guarantee a strong molecular framework of the hybrid. electron transfer from conductive N-graphene to g-C N , leading Therefore, the C N @NG catalyst, formed solely from C and N 3 4 3 4 to an electron-rich region on g-C N layer and a hole-rich without any metallic elements, features a robust stability in 3 4 region on N-graphene layer (Fig. 3b). Such localized electron both acidic and alkaline solutions that is required for a accumulation (0.15 e per C N @NG unit cell) leads to the sustainable hydrogen production (Fig. 4c). Note that pure 3 4 downshifting of the valance and conduction bands of g-C N ;as a g-C N and N-graphene, used as control samples, show 3 4 3 4 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. Counts (e) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 1.2 2.0 * σ* Pure g-C N C–N–C C–C 3 4 C N @NG 3 4 C=C 1.5 * N1 0.8 C–N–C N1 * N2 C–N N2 Defects 1.0 0.4 0.5 C N @NG 3 4 Pure g-C N 3 4 N-graphene 0.0 0.0 280 285 290 295 395 400 405 410 Photon energy (eV) Photon energy (eV) 3.0 C1s X-ray 2.5 sp C–N 2.0 C N @NG 3 4 hybrid 1.5 Interlayered * C–N 1.0 =25° C N /NG 3 4 0.5 mixture =55° 0.0 285 290 295 292 290 288 286 284 282 Photon energy (eV) Binding energy (eV) Figure 2 | Chemical structure of C N @NG. (a,b) Carbon and nitrogen K-edge NEXAFS spectra of different catalysts. An inset illustrating two types 3 4 of nitrogen species in g-C N molecular network; blue refers nitrogen whereas grey refers carbon atoms. A weaker shoulder on the spectrum of C N @NG 3 4 3 4 at 398.3 eV is assigned to p* resonance of nitrogen heteroatom in N-graphene. (c) Polarization-dependent carbon K-edge NEXAFS spectra of C–N C N @NG with geometry of the experiment shown in inset. (d) High-resolution carbon 1s XPS spectra of C N @NG hybrid and C N /NG mixture. 3 4 3 4 3 4 Pure g-C N 3 4 Band gap C N @NG 3 4 g-C N 3 4 –2 N-graphene –4 KM  KM  –4 –2 0 2 4 Energy (eV) Figure 3 | DFT calculation studies of C N @NG. (a) Band structure of pure g-C N (left) and C N @NG hybrid (right). (b) Interfacial electron 3 4 3 4 3 4 transfer in C N @NG. Yellow and cyan iso-surface represents electron accumulation and electron depletion; the iso-surface value is 0.005 e Å . 3 4 (c) The projected density of states on pure g-C N (top) and C N @NG hybrid (down). 3 4 3 4 negligible HER activities due to their nonconductive or nonactive HER free-energy diagram. We carried out a series of DFT cal- nature, respectively (Fig. 4a). Strikingly, C N @NG hybrid shows culations to get a fundamental understanding of the synergistic 3 4 higher proton reduction current than C N /NG mixture effect leading to this unexpected high electrocatalytic activity of 3 4 34–36 (Supplementary Table 3), unambiguously confirming the critical C N @NG. In many cases , the overall HER pathway can be 3 4 role of the aforementioned chemical and electronic couplings in described by a three-state diagram comprising an initial state achieving an excellent HER activity of the former. Nevertheless, H þ e , an intermediate adsorbed H*, and a final product ½H the apparent lower Faradaic resistance in the electrochemical (Fig. 5a). The Gibbs free-energy of the intermediate state, |DG |, H* impendence spectrum (Fig. 4d) of in situ-formed hybrid, which is has been considered as a major descriptor of the HER activity for related to a charge-transfer process in HER , indicates that the a wide variety of metal catalysts. The optimum value of |DG | H* enhanced electrocatalytic performance of C N @NG originates should be zero ; for instance, this value for the well-known 3 4 Pt not simply from its increased electrical conductivity because of highly efficient Pt catalyst is near-zero as |DG |E0.09 eV H* incorporated conductive N-graphene into the hybrid, but from a (Fig. 5a) . Among three metal-free catalysts studied, C N @NG 3 4 complex interaction between g-C N and N-graphene to shows the smallest |DG | value of 0.19 eV (Fig. 5a, 3 4 H* synergistically promote the HER process. Supplementary Fig. 9 and Supplementary Table 4), which is a 4 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. Energy (eV) Normalized intensity (a.u.) Normalized intensity (a.u.) Normalized intensity (a.u.) Intensity (a.u.) PDOS =1 =0.5 NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 ARTICLE 0.6 0.5 –5 0.4 –10 0.3 g-C N 3 4 N-graphene 0.2 –15 C N /NG mixture 3 4 0.1 C N @NG 3 4 Pt/C –20 0.0 –1.0 –0.5 0.0 0.5 1.0 –0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 –2 E (V versus RHE) Log/(mA cm ) geometric –2 –4 –6 Initial in 0.5 M H SO 2 4 C N /NG mixture After 1,000 cycles 3 4 –8 C N @NG hybrid Initial in 0.1 M KOH 3 4 After 1,000 cycles –10 –0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 0 50 100 150 Z ′ (Ω cm ) E (V versus RHE) Figure 4 | Fundamental electrochemical relationships measured for HER on C N @NG. (a,b) The HER polarization curves and Tafel plots for four 3 4 metal-free electrocatalysts and 20% Pt/C (electrolyte: 0.5 M H SO , scan rate: 5 mVs ). The curve referring to C N @NG was recorded for the sample 2 4 3 4 with 33 wt% of g-C N in the hybrid (see Supplementary Fig. 7 for the performance of the samples with different g-C N concentrations). (c) Polarization 3 4 3 4 curves recorded for C N @NG hybrid before and after 1,000 potential sweeps (þ 0.2 to  0.6 V versus reversible hydrogen electrode) under 3 4 acidic and basic conditions. (d) Electrochemical impedance spectroscopy data for C N @NG hybrid and C N /NG mixture in H SO ; data were collected 3 4 3 4 2 4 for the electrodes under HER overpotential¼ 200 mV. 0.75 0.4 H* = 0.67 –2 0.50 N-graphene 0.3 Pt = 1 Pt(111) Pd 0.25 0.2 –4 Rh Ir + – H + e 1/2 H 2 0.1 0.00 MoS Pt Ni 0.0 + – W Co H + e 1/2 H –0.25 C N @NG Cu 3 4 –6 Ag(111) –0.1 Nb = 0.33 Mo C N @NG 3 4 –0.50 Au(111) –0.2 g-C N H* 3 4 Ag C N @NG 3 4 –8 –0.75 10 –0.3 –0.8 –0.4 0.0 0.4 0.8 Reaction coordinate Reaction coordinate G (eV) H* Figure 5 | DFT-calculated HER activities of C N @NG. (a) The calculated free-energy diagram of HER at the equilibrium potential for three metal-free 3 4 catalysts and Pt reference. (b) Volcano plots of i as a function of the DG for newly developed C N @NG (red triangle), common metal catalysts 0 H* 3 4 (open symbols, data collected from ref. 35) as well as a typical nanostructured MoS catalyst (closed symbol, data collected from ref. 14). (c) Free-energy diagram of HER on the surface of C N @NG under different H* coverage (1/3, 2/3 and 1 with the molecular configurations shown as insets) 3 4 conditions. clear indication of its best electrocatalytic activity from the Fig. 11) indicates very weak H* adsorption and easy product viewpoint of thermodynamics. Besides, pure g-C N and desorption, which both are unfavourable for electrocatalytic 3 4 N-graphene show reverse DG values (Fig. 5a) and H* HER . However, chemical coupling of g-C N and N-graphene H* 3 4 adsorption behaviours due to their different molecular into a uniform hybrid can result in a mediated adsorption– structures and electronic properties. A largely negative desorption behaviour (|DG |-0) to facilitate the overall HER H* DG ¼ 0.54 eV on g-C N (Supplementary Fig. 10) indicates kinetics, as shown in Fig. 3a. DFT calculations-derived adsorption H* 3 4 that chemical adsorption of H* on its surface is too strong, while a configuration analysis reveals that such characteristics on largely positive DG ¼ 0.57 on N-graphene (Supplementary C N @NG surface originates from the unique structure of H* 3 4 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. G (eV) H* –2 –2 I (mA cm ) I (mA cm ) geometric geometric –2 i (A cm ) –Z ″ (Ω cm ) Overpotential (V) G (eV) H* ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 1.2 U = 0 V RHE Volmer-Heyrovsky 0.8 H + – 2H + 2e 2H* + + H H 0.4 H + + – – – H H* + H + e H e e 2 0.0 – –0.4 1.2 H H U = 0 V U = –0.33 V RHE RHE – – e e Volmer-Tafel 0.8 H H + – H H 2H + 2e 2H* – H 0.4 e H* + H* H 0.0 –0.4 Reaction coordinate Figure 6 | HER mechanism. Reaction pathways of HER on C N @NG according to the Volmer–Heyrovsky route (a) and Volmer–Tafel route (b). Dashed 3 4 lines are activation barriers for each reaction step as reported elsewhere . g-C N molecule allowing one H* bonding with two pyridinic the case of nanostructured MoS (ref. 15), one can expect that the 3 4 2 nitrogens in one tri-s-triazine periodic unit to form a C N H design of C N @NG catalysts with largely exposed active sites for 2 3 3 4 heteroring, as illustrated in Supplementary Fig. 9. Simultaneously, favourable hydrogen adsorption would greatly enhance their a large number of electrons transferred from N-graphene to electrocatalytic HER performances. catalytically active g-C N layer can rapidly reduce these adsorbed 3 4 H* species to the final molecular hydrogen. Such atomic-level HER mechanism. Although kinetics of the Tafel step on the HER mechanism on the surface of C N @NG clearly 3 4 metal surfaces has been studied in great details, the mechanism of demonstrates the synergistic effect of chemical and electronic HER on the metal-free catalysts is largely unexplored. Therefore, couplings on the enhancement of the proton adsorption/ two generally accepted HER mechanisms, Volmer–Heyrovsky reduction kinetics and explains the origin of strikingly high and Volmer–Tafel reactions , were studied for the new hybrid electrocatalytic activity on C N @NG hybrid. 3 4 catalyst. Assuming that there is no extra energy barriers related to the whole process, only the solid lines in Fig. 6 can be taken into Volcano plot. The established HER free-energy diagram provides account. As regards the Volmer–Tafel mechanism (Fig. 6a), the a quantitative relationship between the measured electrochemical DFT calculation-derived HER pathway shows that there is a free- activity and theoretical free energy of hydrogen adsorption to energy difference of 0.33 eV for the second-step Tafel reaction further evaluate the electrocatalytic properties of the newly under equilibrium potential, which is higher than that of developed C N @NG in comparison with some typical metallic Heyrovsky reaction (0.19 eV, Fig. 6b). Such energy difference 3 4 catalysts. The normalized experimental value of i along with can be eliminated at higher overpotential, for example, 0.33 V theoretically calculated DG for C N @NG is marked on a under which the free energy of the second and third reaction H* 3 4 volcano-shaped plot shown in Fig. 5b. The catalyst’s performance steps is the same (the red line in panel of Fig. 6b). Therefore, the can be quantitatively evaluated by the position of its i and DG pathway selectivity on C N @NG is potential-dependent: at low 0 H* 3 4 values relative to the volcano peak (the closer the position of these overpotential the Volmer–Heyrovsky mechanism with a rate- values to the peak, the better is the catalyst) . As can be seen in limiting step of electrochemical desorption is the most probable, Fig. 5b, metal-free C N @NG catalyst perfectly follows the whereas at high overpotential it becomes the Volmer–Tafel 3 4 volcano trend along with a wide collection of metal catalysts, mechanism. Simultaneously, there might be extra barriers related which also validates the predictive capability of the DFT model to each reaction step as has been shown for Pt surface ; the employed beyond metals. More importantly, the HER activity of potential-dependent barrier values derived from previously the newly developed metal-free electrocatalyst, judged on the reported Pt surface are depicted as dashed lines in Fig. 6. basis of both electrocatalytic i and thermodynamic DG Under such assumption that C N @NG possesses the same 0 H* 3 4 properties, even surpasses those of the common nonprecious energy barriers for each reaction step as those on Pt surface, at metals and is comparable with that of the state-of-the-art low overpotentials the Volmer–Tafel mechanism is much faster 14,35 nanostructured MoS electrocatalyst . than the Volmer–Heyrovsky one, and they become equally fast around  1.0 V versus reversible hydrogen electrode. Hydrogen coverage. The DFT calculations further reveal the mechanistic HER behaviour difference on metal-free C N @NG Discussion 3 4 and traditional metallic catalysts. As shown in Fig. 5c, at low Our study suggests that the chemical coupling of two different overpotential, one C N @NG unit cell tends to allow for layered materials resulted in the formation of a simple, robust and 3 4 adsorption of only one H* due to its smallest |DG |, yielding a highly efficient metal-free hybrid catalyst, and the electrocatalytic H* low coverage of y¼ 1/3. This structure-oriented low-coverage HER performance of which is comparable or even better than adsorption of H* on C N @NG is very different from that on Pt, that of traditional metallic catalysts. Experimental observations in 3 4 which is prone to a very high coverage close to y¼ 1 (ref. 35). The combination with DFT calculations reveal that its unusual smaller number of active sites on a certain C N @NG unit cell is electrocatalytic properties originate from an intrinsic chemical 3 4 the inherent reason for its relatively low kinetic activity as and electronic coupling that synergistically promotes the proton compared with that of precious Pt-group catalysts. However, as in adsorption and reduction kinetics. This finding may shed light 6 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. G (eV) G (eV) H* H* NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 ARTICLE towards replacing noble metals by metal-free counterparts and 6. Le Goff, A. et al. From hydrogenases to noble metal-free catalytic pave the way for the performance-oriented molecular design of nanomaterials for H production and uptake. Science 326, 1384–1387 (2009). 7. Zhuo, J. et al. Salts of C (OH) electrodeposited onto a glassy carbon electrode: these innovative materials. 60 8 surprising catalytic performance in the hydrogen evolution reaction. Angew. Chem. Int. Ed. 52, 10867–10870 (2013). Methods 8. Cook, T. R. et al. Solar energy supply and storage for the legacy and nonlegacy Materials synthesis. C N @NG hybrid was synthesized by mixing a given worlds. Chem. Rev. 110, 6474–6502 (2010). 3 4 amount of dicyandiamide (DCDA) with 250 ml of chemically exfoliated GO 9. Artero, V., Chavarot-Kerlidou, M. & Fontecave, M. Splitting water with cobalt. 1 38 dispersion (B0.5 mg ml ); the mixture was concentrated by rotary evaporation Angew. Chem. Int. Ed. 50, 7238–7266 (2011). followed by lyophilization. DCDA was deposited on the GO surface by the 10. DuBois, M. R. & DuBois, D. L. The roles of the first and second coordination electrostatic interactions between negatively charged GO and positively charged spheres in the design of molecular catalysts for H production and oxidation. DCDA during drying process. The collected dark-brown mixture was then Chem. Soc. Rev. 38, 62–72 (2009). annealed under argon atmosphere at 600 C for 4 h at a heating rate of 4 Cmin 11. Cobo, S. et al. A Janus cobalt-based catalytic material for electro-splitting of (the sample was wrapped in aluminium foil to avoid melamine sublimation during water. Nat. Mater. 11, 802–807 (2012). heating). In our tests, the amount of initial DCDA was selected as 400, 200, 100 and 12. Du, P. & Eisenberg, R. Catalysts made of earth-abundant elements (Co, Ni, Fe) 50 mg; correspondingly, the resultant C N @NG hybrids were numbered from #1 3 4 for water splitting: recent progress and future challenges. Energy Environ. Sci. 5, to #4 with colour changing from dark green to black. Among these four samples, 6012–6021 (2012). sample #3 with B33 wt% of g-C N showed the best electrocatalytic HER 3 4 13. Popczun, E. J. et al. Nanostructured nickel phosphide as an electrocatalyst performance (Supplementary Fig. 7); therefore, if no further notification is for the hydrogen evolution reaction. J. Am. Chem. Soc. 135, 9267–9270 provided, C N @NG notation in the paper refers to sample #3. 3 4 (2013). 14. Jaramillo, T. F. et al. Identification of active edge sites for electrochemical H evolution from MoS nanocatalysts. Science 317, 100–102 (2007). Imaging and spectroscopic characterization. Aberration-corrected high-reso- 15. Kibsgaard, J., Chen, Z., Reinecke, B. N. & Jaramillo, T. F. Engineering the lution transmission electron microscopy imaging was carried out on a FEI Titan G surface structure of MoS to preferentially expose active edge sites for electron microscope equipped with image-corrector operated at 60 kV. A 2 electrocatalysis. Nat. Mater. 11, 963–969 (2012). resolution limit of B1.2 Å was achieved under the imaging conditions used. The 16. Voiry, D. et al. Enhanced catalytic activity in strained chemically exfoliated scanning transmission electron microscopy imaging and EELS mapping were WS nanosheets for hydrogen evolution. Nat. Mater. 12, 850–855 (2013). carried out on a cubed FEI Titan G electron microscope equipped with both a 2 probe-corrector and a monochromator operated at 80 kV. The probe convergence 17. Najmaei, S. et al. Vapour phase growth and grain boundary structure of angle was 24.9 mrad and the inner detector angle was 76 mrad. A probe size of molybdenum disulphide atomic layers. Nat. Mater. 12, 754–759 (2013). 1.5 Å and a best energy resolution of 0.17 eV, as measured from the full-width-at- 18. Laursen, A. B., Kegnæs, S., Dahla, S. & Chorkendorff, I. Molybdenum sulfides– half-maximum of the zero-loss peak, were achieved. For simultaneous EELS efficient and viable materials for electro- and photoelectrocatalytic hydrogen mapping of K-edge structures for carbon and nitrogen, a 2-mm spectrometer evolution. Energy Environ. Sci. 5, 5577–5591 (2012). entrance aperture and 0.1 eV per channel dispersion were used (B0.4 eV energy 19. Merki, D. & Hu, X. Recent developments of molybdenum and tungsten resolution). sulfides as hydrogen evolution catalysts. Energy Environ. Sci. 4, 3878–3888 The synchrotron-based NEXAFS measurements were carried out on the soft (2011). X-ray spectroscopy beamline at the Australian Synchrotron, which is equipped 20. Gong, K., Du, F., Xia, Z., Durstock, M. & Dai, L. Nitrogen-doped carbon with a hemispherical electron analyser and a microchannel plate detector that nanotube arrays with high electrocatalytic activity for oxygen reduction. Science enables a simultaneous recording of the total electron yield and partial electron 323, 760–764 (2009). yield. The raw NEXAFS data were normalized to the photoelectron current of the 21. Mirzakulova, E. et al. Electrode-assisted catalytic water oxidation by a flavin photon beam, measured on an Au grid. The lab-based XPS measurements were derivative. Nat. Chem. 4, 794–801 (2012). performed on a Kratos Axis ULTRA X-ray Photoelectron Spectrometer 22. Zhao, Y., Nakamura, R., Kamiya, K., Nakanishi, S. & Hashimoto, K. Nitrogen- incorporating a 165-mm hemispherical electron energy analyser. The incident doped carbon nanomaterials as non-metal electrocatalysts for water oxidation. radiation was Monochromatic Al Ka X-rays (1,486.6 eV) at 225 W (15 kV, 15 mA). Nat. Commun. 4, 2390 (2013). Survey scans were taken at analyser pass energy of 160 eV and high-resolution 23. Zheng, Y. et al. Nanoporous graphitic-C N @Carbon metal-free 3 4 scans at 20 eV. electrocatalysts for highly efficient oxygen reduction. J. Am. Chem. Soc. 133, 20116–20119 (2011). 24. Wang, X. et al. A metal-free polymeric photocatalyst for hydrogen production Electrochemical characterization. Considering C N @NG electrode as an 3 4 from water under visible light. Nat. Mater. 8, 76–80 (2009). example, the catalyst was ultrasonically dispersed in pure water containing 0.1 wt% of Nafion. Next, 20 ml of catalyst aqueous dispersion (1.0 mg ml ) were trans- 25. Thomas, A. et al. Graphitic carbon nitride materials: variation of structure ferred onto the glassy carbon rotating disk electrode (0.196 cm ) serving as a and morphology and their use as metal-free catalysts. J. Mater. Chem. 18, working electrode. The reference electrode was an Ag/AgCl in 4 M AgCl–KCl 4893–4908 (2008). solution and the counter electrode was a graphite rod. All potentials were refer- 26. Suenaga, K. & Koshino, M. Atom-by-atom spectroscopy at graphene edge. enced to a reversible hydrogen electrode by adding a value of (0.205þ 0.059 pH) Nature 468, 1088–1090 (2010). 2 3 V and all polarization curves were corrected for the iR contribution within the cell. 27. Muller, D. A., Tzou, Y., Raj, R. & Silcox, J. Mapping sp and sp states of carbon A flow of N was maintained over the electrolyte during the experiment to elim- at sub-nanometre spatial resolution. Nature 366, 725–727 (1993). inate dissolved oxygen. The working electrode was rotated at 1,600 r.p.m. to 28. Rosenberg, R. A., Love, P. J. & Rehn, V. Polarization-dependent C(K) near-edge remove hydrogen gas bubbles formed at the catalyst surface. x-ray-absorption fine structure of graphite. Phys. Rev. B 33, 4034–4037 (1985). 29. Sheng, Z. et al. Catalyst-free synthesis of nitrogen-doped graphene via thermal annealing graphite oxide with melamine and its excellent electrocatalysis. ACS DFT calculations. The electronic structures of all catalysts were computed by Nano 5, 4350–4358 (2011). Vienna Ab-initio Simulation Package (VASP). An all-electron description, the 30. Chen, Z. et al. Core-shell MoO -MoS nanowires for hydrogen evolution: a 3 2 projector-augmented wave method was used to describe the electron–ion functional design for electrocatalytic materials. Nano Lett. 11, 4168–4175 (2011). interaction. All calculations were performed with PBE exchange-correlation 31. Chen, W. et al. Hydrogen-evolution catalysts based on non-noble metal nickel functional (the 1996 functional of Perdew, Burke and Ernzerhof) on periodically molybdenum nitride nanosheets. Angew. Chem. Int. Ed. 51, 6131–6135 repeated slabs. Molecular model-building details, free-energy calculations, HER (2012). mechanism and relevant references are given in the Supplementary Methods. 32. Merki, D., Vrubel, H., Rovelli, L., Fierro, S. & Hu, X. Fe, Co, and Ni ions promote the catalytic activity of amorphous molybdenum sulfide films for hydrogen evolution. Chem. Sci. 3, 2515–2525 (2012). References 33. Li, Y. et al. MoS nanoparticles grown on graphene: an advanced catalyst 1. Lewis, N. S. & Nocera, D. G. Powering the planet: chemical challenges in solar 2 energy utilization. Proc. Natl Acad. Sci. USA 103, 15729–15735 (2007). for the hydrogen evolution reaction. J. Am. Chem. Soc. 133, 7296–7299 2. Turner, J. A. Sustainable hydrogen production. Science 305, 972–974 (2004). (2011). 34. Nørskov, J. K., Bligaard, T., Rossmeisl, J. & Christensen, C. H. Towards the 3. Walter, M. G. et al. Solar water splitting cells. Chem. Rev. 110, 6446–6473 computational design of solid catalysts. Nat. Chem. 1, 37–46 (2009). (2010). 4. Conway, B. E. & Tilak, B. V. Interfacial processes involving electrocatalytic 35. Nørskov, J. K. et al. Trends in the exchange current for hydrogen evolution. evolution and oxidation of H , and the role of chemisorbed H. Electrochim. J. Electrochem. Soc. 152, J23–J26 (2005). Acta. 47, 3571–3594 (2002). 36. Greeley, J., Jaramillo, T. F., Bonde, J., Chorkendorff, I. & Nørskov, J. K. Computational high-throughput screening of electrocatalytic materials for 5. Subbaraman, R. et al. Enhancing hydrogen evolution activity in water splitting by tailoring Li -Ni(OH) -Pt interfaces. Science 334, 1256–1260 (2011). hydrogen evolution. Nat. Mater. 5, 909–913 (2006). NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications 7 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 calculations, assisted by A.D., Y.H.Z. and Y.H. carried out high-resolution transmission 37. Skulason, E. et al. Modeling the electrochemical hydrogen oxidation and electron microscopy imaging. L.H.L. and Y.C. made NEXAFS measurements. All authors evolution reactions on the basis of density functional theory calculations. discussed and analysed the data. S.Z.Q., Y.Z., Y.J. co-wrote the manuscript with input J. Phys. Chem. C 114, 18182–18197 (2010). from M.J. and Y.H. 38. Li, D., Muller, M. B., Gilje, S., Kaner, R. B. & Wallace, G. G. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotech. 3, 101–105 (2008). Additional information Acknowledgements Supplementary Information accompanies this paper at http://www.nature.com/ This research is financially supported by Australian Research Council (DP1095861, naturecommunications DP130104459). NEXAFS measurements were undertaken on the soft X-ray beamline at Australian Synchrotron. DFT calculations were undertaken on the NCI National Facility Competing financial interests: The authors declare no competing financial interests. systems through the National Computational Merit Allocation Scheme. Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/ Author contributions S.Z.Q. designed the research. Y.Z. synthesized catalysts and conducted chemical How to cite this article: Zheng, Y. et al. Hydrogen evolution by a metal-free characterizations and electrochemical measurements. Y.J. performed the DFT electrocatalyst. Nat. Commun. 5:3783 doi: 10.1038/ncomms4783 (2014). 8 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals

Hydrogen evolution by a metal-free electrocatalyst

Download PDF
8 pages

Loading next page...
 
/lp/springer-journals/hydrogen-evolution-by-a-metal-free-electrocatalyst-GeUNOuWI0w

References (39)

Publisher
Springer Journals
Copyright
Copyright © 2014 by Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.
Subject
Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
eISSN
2041-1723
DOI
10.1038/ncomms4783
Publisher site
See Article on Publisher Site

Abstract

ARTICLE Received 12 Nov 2013 | Accepted 2 Apr 2014 | Published 28 Apr 2014 DOI: 10.1038/ncomms4783 1,2, 1, 3 4 3 4 5 Yao Zheng *, Yan Jiao *, Yihan Zhu , Lu Hua Li , Yu Han , Ying Chen , Aijun Du , 6 1 Mietek Jaroniec & Shi Zhang Qiao Electrocatalytic reduction of water to molecular hydrogen via the hydrogen evolution reaction may provide a sustainable energy supply for the future, but its commercial application is hampered by the use of precious platinum catalysts. All alternatives to platinum thus far are based on nonprecious metals, and, to our knowledge, there is no report about a catalyst for electrocatalytic hydrogen evolution beyond metals. Here we couple graphitic-carbon nitride with nitrogen-doped graphene to produce a metal-free hybrid catalyst, which shows an unexpected hydrogen evolution reaction activity with comparable overpotential and Tafel slope to some of well-developed metallic catalysts. Experimental observations in combination with density functional theory calculations reveal that its unusual electrocatalytic properties originate from an intrinsic chemical and electronic coupling that synergistically promotes the proton adsorption and reduction kinetics. 1 2 School of Chemical Engineering, University of Adelaide, Adelaide, South Australia 5005, Australia. Australian Institute for Bioengineering and Nanotechnology, University of Queensland, Brisbane, Queensland 4072, Australia. Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia. Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3216, Australia. School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia. Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, USA. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to S.Z.Q. (email: [email protected]). NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 ydrogen evolution reaction (HER), as a fundamental step sheets with complex Moire´ patterns, suggesting its multilayered of electrochemical water splitting and a cornerstone to structure formed by stacking multilayered g-C N on N-graphene 3 4 Hexplore the mechanism of other multi-electron transfer sheets (Fig. 1a). It is observed that the g-C N phase is less stable 3 4 processes in electrocatalysis, always requires a favourable catalyst than N-graphene under electron beam due to the rapid 1–3 25 to achieve fast kinetics for practical applications . Among a decomposition of its polymeric melon units . After selective wide variety of available catalysts, Pt supported on carbon shows removal of g-C N layers from the surface of N-graphene by 3 4 unbeatable electrocatalytic HER properties with extremely high transmission electron microscopy irradiation, a residual few- exchange current density and small Tafel slope . However, to gain layered N-graphene substrate with graphite-like atomic structure a sustainable hydrogen production, cost-effective alternatives to was resolved in the identical area of the initial g-C N -covered 3 4 precious and low abundant Pt catalyst should be developed sample (Fig. 1b). 5–8 with high electrocatalytic activities and stabilities . Despite tremendous efforts in this direction, all efficient HER electro- Electron microscopy characterization. Scanning transmission catalysts heretofore are based on transition metals, such as Co, Ni, electron microscopy combined with electron energy loss spec- 7–19 Fe, Mo and their molecular derivatives , which suffer inherent troscopy (EELS) was used to investigate the C N @NG hybrid. 3 4 corrosion susceptibility to the acidic proton exchange membrane High-angle-annular-dark-field–scanning transmission electron electrolysis. A robust catalytic system beyond metals has not been microscopy revealed that C N @NG consists of ultra-thin 3,8 3 4 found yet , largely narrowing the selection range of competitive nanosheets with a fairly uniform thickness (Fig. 1e). The spatial candidates as Pt’s replacements. distributions of different species in the nanosheet were visualized Various carbon-based materials feature unique advantages for by EELS mapping based on the intensity variation of their energy designated catalysis due to their tunable molecular structures, loss peaks. Note that unlikely, as in the case of carbon species that abundance and strong tolerance to acid/alkaline environments. are distributed over the entire sheet (Fig. 1f), nitrogen species Very recent advances in low-dimensional carbon materials, as concentrate in the specific regions (Fig. 1g). These results indicate metal-free catalysts, have shown their promising future in energy- that N-graphene having a low concentration of N (Supplementary related electrocatalytic oxygen reduction and evolution reac- Fig. 2) is partially covered by g-C N nanodomains on its surface. 20–23 3 4 tions . However, this innovative concept has not been Figure 1c shows two typical EELS spectra that were collected from explored yet for one of the most important electrolysis g-C N -containing and g-C N -free regions (site 1 and site 2 in 3 4 3 4 processes, hydrogen evolution, due to the poorly known HER Fig. 1e), respectively. The fine structures of carbon K-edges are mechanism on such materials. So far, all experimental and magnified in Fig. 1d, where the two p* edges at 283.4 eV (peak 1) theoretical studies on the electrocatalytic HER are exclusively and 285.5 eV (peak 2) and one s* edge at 290.2 eV (peak 3) are focused on the surface properties of metallic catalysts due to the attributed to the defective, graphitic and sp carbon species, 3,8 importance of metal–H bonds in this process ; whether a metal- 26,27 respectively . The EELS mapping with these energy loss peaks free material can exhibit similar catalytic behaviour and be more reveals that graphitic-sp carbon is present over the entire sheet active than metal-based electrocatalysts is still unknown. (Fig. 1i), whereas defective carbon species are only present in the In this work, we report the synthesis of a metal-free catalyst, g-C N -containing regions but absent in the g-C N -free regions 3 4 3 4 consisting of carbon and nitrogen only, by coupling graphitic- (Fig. 1h) or in a pure g-C N sample (Supplementary Fig. 3). 3 4 carbon nitride (g-C N ) with nitrogen-doped graphene 3 4 These defective species that can be assigned to low-coordinated (N-graphene; NG), and demonstrate that the resulting carbon atoms (26) are probably associated with the breakage of C N @NG hybrid possesses unique molecular structure and 3 4 the N–3C bridging bonds at the edge of g-C N moieties, due to 3 4 electronic properties for electrocatalytic HER application. This the strong interaction between g-C N and N-graphene substrate 3 4 metal-free hybrid shows comparable electrocatalytic HER activity (Supplementary Fig. 4). In addition, an enhanced intensity of s* with the existing well-developed metallic catalysts, such as excitation (peak 3) is observed in the g-C N -containing region 3 4 nanostructured MoS materials, although its activity is not as 2 3 (Fig. 1j), which is indicative of new sp carbon species formed high as that of the state-of-the-art Pt catalyst. Electrochemical through the growth of g-C N nanodomains on the N-graphene 3 4 measurements in combination with thermodynamic calculations surface. These observations suggest the generation of chemical reveal that its unusual electrocatalytic properties originate from a bonds between these two materials during coupling process of synergistic effect of this hybrid nanostructure, in which g-C N 3 4 g-C N and N-graphene. 3 4 provides highly active hydrogen adsorption sites, while N-graphene facilitates the electron-transfer process for the proton reduction. The findings provide clear evidence that, similar to NEXAFS and XPS studies. We used Synchrotron-based near- precious metals, the well-designed metal-free counterparts also edge X-ray absorption fine structure (NEXAFS) and X-ray pho- have great potential for highly efficient electrocatalytic HER, thus toelectron spectroscopy (XPS) to further probe this interfacial opening a new avenue towards replacing noble metals by broader interaction by exploring the lateral structure of C N @NG mul- 3 4 alternatives in a wide variety of applications. tilayered nanosheet. In the carbon K-edge NEXAFS spectra (Fig. 2a), the hybrid shows characteristic resonances of both g-C N and N-graphene individuals including structural defects 3 4 Results at B284 eV, p* at 285.5 eV and p* at 288.5 eV. In C¼ C C–N–C Catalyst synthesis and atomic structure. g-C N was grown nitrogen K-edge region (Fig. 2b), C N @NG shows two typical p* 3 4 3 4 directly on the surface of chemically exfoliated graphene oxide resonances at 399.7 and 402.6 eV corresponding to aromatic (GO) films to produce a strong chemical adhesion with the C–N–C coordination in one tri-s-triazine heteroring (N1, see the resultant N-graphene. GO was thermally reduced with simulta- inset of Fig. 2b) and N–3C bridging among three tri-s-triazine 24,25 neous nitrogen heteroatom incorporation into framework moieties (N2, see the inset of Fig. 2b) . As compared with pure driven by gaseous NH released during polycondensation of g-C N , a decreased intensity of N2 resonance in the hybrid 3 3 4 g-C N ’s precursor (dicyandiamide) to melem units , forming reveals a partial breaking of N–3C bridgings, which is in 3 4 aC N @NG hybrid (Supplementary Fig. 1). The aberration- agreement with the EELS results. More importantly, as can be 3 4 corrected high-resolution transmission electron microscopy seen from the carbon K-edge polarization-dependent NEXAFS image of the as-synthesized hybrid shows two-dimensional spectra (Fig. 2c), a weak shoulder at 287.4 eV (assigned to the p* 2 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 ARTICLE * C K 3 N K Site 1 Site 2 280 320 360 400 440 280 290 300 Energy loss (eV) Site1 Site2 Figure 1 | Electron microscopy characterization of C N @NG nanosheet. (a) Aberration-corrected and monochromated high-resolution transmission 3 4 electron microscopy image of freshly prepared C N @NG hybrid. (b) High-resolution transmission electron microscopy image of the same area as (a) after 3 4 removal of g-C N layer by electron beam irradiation (under a prolonged exposure ofB20 s). Scale bar, 2 nm (a,b). (c) EELS spectra of C N @NG collected 3 4 3 4 at two specific sites as indicated in e. Site 1 represents a g-C N -containing region with apparent nitrogen K-edge energy loss peak at B400 eV; site 2 3 4 represents a g-C N -free region. (d) Fine structure of the carbon K-edge EELS on C N @NG. (e–g), High-angle-annular-dark-field–scanning transmission 3 4 3 4 electron microscopy image of C N @NG nanosheet and EELS mappings of overall (f) carbon and (g) nitrogen species in the red-line areas. Scale bar, 2 mm 3 4 (e). (h–j), EELS mappings of various carbon species (peak 1–3, respectively) as indicated in d. A temperature colour code is used in mapping images where the intensity increases as the colour changes in the order of black-blue-green-red-yellow-white. resonance of C–N species) shows a different angular dependence result, the Fermi level crosses the conduction band of g-C N 3 4 to the in-plane C–N–C and C¼ C species, indicating the out-of- as demonstrated by the projected density of states shown in plane orientation of new species between g-C N and N-graphene Fig. 3c. This change in projected density of states between 3 4 28 3 basals . The XPS spectra also reveal extra sp C–N species pure g-C N and hybridized C N @NG indicates enhanced 3 4 3 4 at 287.2 eV involved in the in situ-formed C N @NG electron mobility in the latter, which is significant for the 3 4 hybrid , which do not exist in a physically mixed g-C N and electrocatalytic HER. 3 4 N-graphene (C N /NG) (Fig. 2d). Thus, a combination of 3 4 polarization-dependent NEXAFS and XPS results provides a Electrochemically-measured HER activity. The chemical cou- crucial evidence for the interfacial interaction model between pling and concomitant electron’s re-distribution between g-C N 3 4 g-C N and N-graphene, where the out-of-plane oriented C–N 3 4 and N-graphene can provide a resistance-less path for fast elec- bonds are clearly detected. These interlayer bonds can provide tron transfer through C N @NG’s interlayers and consequently 3 4 a high interconnectivity between g-C N and N-graphene 3 4 accelerate the electrocatalytic HER kinetics on its surface. The parallel layers for a rapid electron transfer to boost hybrid’s polarization curve (i–V) recorded on C N @NG shows an 3 4 electrocatalytic activity. overpotential of B240 mV to achieve a 10-mA cm HER current density and a Tafel slope of 51.5 mV dec (Fig. 4a,b, Electronic structure and electron-transfer properties.We Supplementary Fig. 7). The HER exchange current density (i ) for conducted density functional theory (DFT) calculations to show C N @NG, calculated from Tafel plot by extrapolation method, is 3 4 7  2 the interlayer electronic-coupling effect between g-C N and 3.5 10 Acm ; this value is comparable (within an order 3 4 N-graphene (Supplementary Figs 5,6). Contrary to the semi- of magnitude) to those of the well-developed nanostructured conductive pure g-C N ,C N @NG hybrid shows no band gap MoS -based metallic catalysts after normalizing them to the 3 4 3 4 2 due to the downshifting of the Dirac cone at the G point to same surface area and catalyst loading (Supplementary Fig. 8, 15,30–32 guarantee a fast electron transfer (Fig. 3a). More importantly, Supplementary Tables 1 and 2) . Importantly, the inter- after coupling g-C N with N-graphene, the charge density in facial covalent bonds between g-C N and N-graphene layers can 3 4 3 4 hybrid’s interlayer was redistributed in the form of an apparent also guarantee a strong molecular framework of the hybrid. electron transfer from conductive N-graphene to g-C N , leading Therefore, the C N @NG catalyst, formed solely from C and N 3 4 3 4 to an electron-rich region on g-C N layer and a hole-rich without any metallic elements, features a robust stability in 3 4 region on N-graphene layer (Fig. 3b). Such localized electron both acidic and alkaline solutions that is required for a accumulation (0.15 e per C N @NG unit cell) leads to the sustainable hydrogen production (Fig. 4c). Note that pure 3 4 downshifting of the valance and conduction bands of g-C N ;as a g-C N and N-graphene, used as control samples, show 3 4 3 4 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. Counts (e) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 1.2 2.0 * σ* Pure g-C N C–N–C C–C 3 4 C N @NG 3 4 C=C 1.5 * N1 0.8 C–N–C N1 * N2 C–N N2 Defects 1.0 0.4 0.5 C N @NG 3 4 Pure g-C N 3 4 N-graphene 0.0 0.0 280 285 290 295 395 400 405 410 Photon energy (eV) Photon energy (eV) 3.0 C1s X-ray 2.5 sp C–N 2.0 C N @NG 3 4 hybrid 1.5 Interlayered * C–N 1.0 =25° C N /NG 3 4 0.5 mixture =55° 0.0 285 290 295 292 290 288 286 284 282 Photon energy (eV) Binding energy (eV) Figure 2 | Chemical structure of C N @NG. (a,b) Carbon and nitrogen K-edge NEXAFS spectra of different catalysts. An inset illustrating two types 3 4 of nitrogen species in g-C N molecular network; blue refers nitrogen whereas grey refers carbon atoms. A weaker shoulder on the spectrum of C N @NG 3 4 3 4 at 398.3 eV is assigned to p* resonance of nitrogen heteroatom in N-graphene. (c) Polarization-dependent carbon K-edge NEXAFS spectra of C–N C N @NG with geometry of the experiment shown in inset. (d) High-resolution carbon 1s XPS spectra of C N @NG hybrid and C N /NG mixture. 3 4 3 4 3 4 Pure g-C N 3 4 Band gap C N @NG 3 4 g-C N 3 4 –2 N-graphene –4 KM  KM  –4 –2 0 2 4 Energy (eV) Figure 3 | DFT calculation studies of C N @NG. (a) Band structure of pure g-C N (left) and C N @NG hybrid (right). (b) Interfacial electron 3 4 3 4 3 4 transfer in C N @NG. Yellow and cyan iso-surface represents electron accumulation and electron depletion; the iso-surface value is 0.005 e Å . 3 4 (c) The projected density of states on pure g-C N (top) and C N @NG hybrid (down). 3 4 3 4 negligible HER activities due to their nonconductive or nonactive HER free-energy diagram. We carried out a series of DFT cal- nature, respectively (Fig. 4a). Strikingly, C N @NG hybrid shows culations to get a fundamental understanding of the synergistic 3 4 higher proton reduction current than C N /NG mixture effect leading to this unexpected high electrocatalytic activity of 3 4 34–36 (Supplementary Table 3), unambiguously confirming the critical C N @NG. In many cases , the overall HER pathway can be 3 4 role of the aforementioned chemical and electronic couplings in described by a three-state diagram comprising an initial state achieving an excellent HER activity of the former. Nevertheless, H þ e , an intermediate adsorbed H*, and a final product ½H the apparent lower Faradaic resistance in the electrochemical (Fig. 5a). The Gibbs free-energy of the intermediate state, |DG |, H* impendence spectrum (Fig. 4d) of in situ-formed hybrid, which is has been considered as a major descriptor of the HER activity for related to a charge-transfer process in HER , indicates that the a wide variety of metal catalysts. The optimum value of |DG | H* enhanced electrocatalytic performance of C N @NG originates should be zero ; for instance, this value for the well-known 3 4 Pt not simply from its increased electrical conductivity because of highly efficient Pt catalyst is near-zero as |DG |E0.09 eV H* incorporated conductive N-graphene into the hybrid, but from a (Fig. 5a) . Among three metal-free catalysts studied, C N @NG 3 4 complex interaction between g-C N and N-graphene to shows the smallest |DG | value of 0.19 eV (Fig. 5a, 3 4 H* synergistically promote the HER process. Supplementary Fig. 9 and Supplementary Table 4), which is a 4 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. Energy (eV) Normalized intensity (a.u.) Normalized intensity (a.u.) Normalized intensity (a.u.) Intensity (a.u.) PDOS =1 =0.5 NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 ARTICLE 0.6 0.5 –5 0.4 –10 0.3 g-C N 3 4 N-graphene 0.2 –15 C N /NG mixture 3 4 0.1 C N @NG 3 4 Pt/C –20 0.0 –1.0 –0.5 0.0 0.5 1.0 –0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 –2 E (V versus RHE) Log/(mA cm ) geometric –2 –4 –6 Initial in 0.5 M H SO 2 4 C N /NG mixture After 1,000 cycles 3 4 –8 C N @NG hybrid Initial in 0.1 M KOH 3 4 After 1,000 cycles –10 –0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 0 50 100 150 Z ′ (Ω cm ) E (V versus RHE) Figure 4 | Fundamental electrochemical relationships measured for HER on C N @NG. (a,b) The HER polarization curves and Tafel plots for four 3 4 metal-free electrocatalysts and 20% Pt/C (electrolyte: 0.5 M H SO , scan rate: 5 mVs ). The curve referring to C N @NG was recorded for the sample 2 4 3 4 with 33 wt% of g-C N in the hybrid (see Supplementary Fig. 7 for the performance of the samples with different g-C N concentrations). (c) Polarization 3 4 3 4 curves recorded for C N @NG hybrid before and after 1,000 potential sweeps (þ 0.2 to  0.6 V versus reversible hydrogen electrode) under 3 4 acidic and basic conditions. (d) Electrochemical impedance spectroscopy data for C N @NG hybrid and C N /NG mixture in H SO ; data were collected 3 4 3 4 2 4 for the electrodes under HER overpotential¼ 200 mV. 0.75 0.4 H* = 0.67 –2 0.50 N-graphene 0.3 Pt = 1 Pt(111) Pd 0.25 0.2 –4 Rh Ir + – H + e 1/2 H 2 0.1 0.00 MoS Pt Ni 0.0 + – W Co H + e 1/2 H –0.25 C N @NG Cu 3 4 –6 Ag(111) –0.1 Nb = 0.33 Mo C N @NG 3 4 –0.50 Au(111) –0.2 g-C N H* 3 4 Ag C N @NG 3 4 –8 –0.75 10 –0.3 –0.8 –0.4 0.0 0.4 0.8 Reaction coordinate Reaction coordinate G (eV) H* Figure 5 | DFT-calculated HER activities of C N @NG. (a) The calculated free-energy diagram of HER at the equilibrium potential for three metal-free 3 4 catalysts and Pt reference. (b) Volcano plots of i as a function of the DG for newly developed C N @NG (red triangle), common metal catalysts 0 H* 3 4 (open symbols, data collected from ref. 35) as well as a typical nanostructured MoS catalyst (closed symbol, data collected from ref. 14). (c) Free-energy diagram of HER on the surface of C N @NG under different H* coverage (1/3, 2/3 and 1 with the molecular configurations shown as insets) 3 4 conditions. clear indication of its best electrocatalytic activity from the Fig. 11) indicates very weak H* adsorption and easy product viewpoint of thermodynamics. Besides, pure g-C N and desorption, which both are unfavourable for electrocatalytic 3 4 N-graphene show reverse DG values (Fig. 5a) and H* HER . However, chemical coupling of g-C N and N-graphene H* 3 4 adsorption behaviours due to their different molecular into a uniform hybrid can result in a mediated adsorption– structures and electronic properties. A largely negative desorption behaviour (|DG |-0) to facilitate the overall HER H* DG ¼ 0.54 eV on g-C N (Supplementary Fig. 10) indicates kinetics, as shown in Fig. 3a. DFT calculations-derived adsorption H* 3 4 that chemical adsorption of H* on its surface is too strong, while a configuration analysis reveals that such characteristics on largely positive DG ¼ 0.57 on N-graphene (Supplementary C N @NG surface originates from the unique structure of H* 3 4 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. G (eV) H* –2 –2 I (mA cm ) I (mA cm ) geometric geometric –2 i (A cm ) –Z ″ (Ω cm ) Overpotential (V) G (eV) H* ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 1.2 U = 0 V RHE Volmer-Heyrovsky 0.8 H + – 2H + 2e 2H* + + H H 0.4 H + + – – – H H* + H + e H e e 2 0.0 – –0.4 1.2 H H U = 0 V U = –0.33 V RHE RHE – – e e Volmer-Tafel 0.8 H H + – H H 2H + 2e 2H* – H 0.4 e H* + H* H 0.0 –0.4 Reaction coordinate Figure 6 | HER mechanism. Reaction pathways of HER on C N @NG according to the Volmer–Heyrovsky route (a) and Volmer–Tafel route (b). Dashed 3 4 lines are activation barriers for each reaction step as reported elsewhere . g-C N molecule allowing one H* bonding with two pyridinic the case of nanostructured MoS (ref. 15), one can expect that the 3 4 2 nitrogens in one tri-s-triazine periodic unit to form a C N H design of C N @NG catalysts with largely exposed active sites for 2 3 3 4 heteroring, as illustrated in Supplementary Fig. 9. Simultaneously, favourable hydrogen adsorption would greatly enhance their a large number of electrons transferred from N-graphene to electrocatalytic HER performances. catalytically active g-C N layer can rapidly reduce these adsorbed 3 4 H* species to the final molecular hydrogen. Such atomic-level HER mechanism. Although kinetics of the Tafel step on the HER mechanism on the surface of C N @NG clearly 3 4 metal surfaces has been studied in great details, the mechanism of demonstrates the synergistic effect of chemical and electronic HER on the metal-free catalysts is largely unexplored. Therefore, couplings on the enhancement of the proton adsorption/ two generally accepted HER mechanisms, Volmer–Heyrovsky reduction kinetics and explains the origin of strikingly high and Volmer–Tafel reactions , were studied for the new hybrid electrocatalytic activity on C N @NG hybrid. 3 4 catalyst. Assuming that there is no extra energy barriers related to the whole process, only the solid lines in Fig. 6 can be taken into Volcano plot. The established HER free-energy diagram provides account. As regards the Volmer–Tafel mechanism (Fig. 6a), the a quantitative relationship between the measured electrochemical DFT calculation-derived HER pathway shows that there is a free- activity and theoretical free energy of hydrogen adsorption to energy difference of 0.33 eV for the second-step Tafel reaction further evaluate the electrocatalytic properties of the newly under equilibrium potential, which is higher than that of developed C N @NG in comparison with some typical metallic Heyrovsky reaction (0.19 eV, Fig. 6b). Such energy difference 3 4 catalysts. The normalized experimental value of i along with can be eliminated at higher overpotential, for example, 0.33 V theoretically calculated DG for C N @NG is marked on a under which the free energy of the second and third reaction H* 3 4 volcano-shaped plot shown in Fig. 5b. The catalyst’s performance steps is the same (the red line in panel of Fig. 6b). Therefore, the can be quantitatively evaluated by the position of its i and DG pathway selectivity on C N @NG is potential-dependent: at low 0 H* 3 4 values relative to the volcano peak (the closer the position of these overpotential the Volmer–Heyrovsky mechanism with a rate- values to the peak, the better is the catalyst) . As can be seen in limiting step of electrochemical desorption is the most probable, Fig. 5b, metal-free C N @NG catalyst perfectly follows the whereas at high overpotential it becomes the Volmer–Tafel 3 4 volcano trend along with a wide collection of metal catalysts, mechanism. Simultaneously, there might be extra barriers related which also validates the predictive capability of the DFT model to each reaction step as has been shown for Pt surface ; the employed beyond metals. More importantly, the HER activity of potential-dependent barrier values derived from previously the newly developed metal-free electrocatalyst, judged on the reported Pt surface are depicted as dashed lines in Fig. 6. basis of both electrocatalytic i and thermodynamic DG Under such assumption that C N @NG possesses the same 0 H* 3 4 properties, even surpasses those of the common nonprecious energy barriers for each reaction step as those on Pt surface, at metals and is comparable with that of the state-of-the-art low overpotentials the Volmer–Tafel mechanism is much faster 14,35 nanostructured MoS electrocatalyst . than the Volmer–Heyrovsky one, and they become equally fast around  1.0 V versus reversible hydrogen electrode. Hydrogen coverage. The DFT calculations further reveal the mechanistic HER behaviour difference on metal-free C N @NG Discussion 3 4 and traditional metallic catalysts. As shown in Fig. 5c, at low Our study suggests that the chemical coupling of two different overpotential, one C N @NG unit cell tends to allow for layered materials resulted in the formation of a simple, robust and 3 4 adsorption of only one H* due to its smallest |DG |, yielding a highly efficient metal-free hybrid catalyst, and the electrocatalytic H* low coverage of y¼ 1/3. This structure-oriented low-coverage HER performance of which is comparable or even better than adsorption of H* on C N @NG is very different from that on Pt, that of traditional metallic catalysts. Experimental observations in 3 4 which is prone to a very high coverage close to y¼ 1 (ref. 35). The combination with DFT calculations reveal that its unusual smaller number of active sites on a certain C N @NG unit cell is electrocatalytic properties originate from an intrinsic chemical 3 4 the inherent reason for its relatively low kinetic activity as and electronic coupling that synergistically promotes the proton compared with that of precious Pt-group catalysts. However, as in adsorption and reduction kinetics. This finding may shed light 6 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. G (eV) G (eV) H* H* NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 ARTICLE towards replacing noble metals by metal-free counterparts and 6. Le Goff, A. et al. From hydrogenases to noble metal-free catalytic pave the way for the performance-oriented molecular design of nanomaterials for H production and uptake. Science 326, 1384–1387 (2009). 7. Zhuo, J. et al. Salts of C (OH) electrodeposited onto a glassy carbon electrode: these innovative materials. 60 8 surprising catalytic performance in the hydrogen evolution reaction. Angew. Chem. Int. Ed. 52, 10867–10870 (2013). Methods 8. Cook, T. R. et al. Solar energy supply and storage for the legacy and nonlegacy Materials synthesis. C N @NG hybrid was synthesized by mixing a given worlds. Chem. Rev. 110, 6474–6502 (2010). 3 4 amount of dicyandiamide (DCDA) with 250 ml of chemically exfoliated GO 9. Artero, V., Chavarot-Kerlidou, M. & Fontecave, M. Splitting water with cobalt. 1 38 dispersion (B0.5 mg ml ); the mixture was concentrated by rotary evaporation Angew. Chem. Int. Ed. 50, 7238–7266 (2011). followed by lyophilization. DCDA was deposited on the GO surface by the 10. DuBois, M. R. & DuBois, D. L. The roles of the first and second coordination electrostatic interactions between negatively charged GO and positively charged spheres in the design of molecular catalysts for H production and oxidation. DCDA during drying process. The collected dark-brown mixture was then Chem. Soc. Rev. 38, 62–72 (2009). annealed under argon atmosphere at 600 C for 4 h at a heating rate of 4 Cmin 11. Cobo, S. et al. A Janus cobalt-based catalytic material for electro-splitting of (the sample was wrapped in aluminium foil to avoid melamine sublimation during water. Nat. Mater. 11, 802–807 (2012). heating). In our tests, the amount of initial DCDA was selected as 400, 200, 100 and 12. Du, P. & Eisenberg, R. Catalysts made of earth-abundant elements (Co, Ni, Fe) 50 mg; correspondingly, the resultant C N @NG hybrids were numbered from #1 3 4 for water splitting: recent progress and future challenges. Energy Environ. Sci. 5, to #4 with colour changing from dark green to black. Among these four samples, 6012–6021 (2012). sample #3 with B33 wt% of g-C N showed the best electrocatalytic HER 3 4 13. Popczun, E. J. et al. Nanostructured nickel phosphide as an electrocatalyst performance (Supplementary Fig. 7); therefore, if no further notification is for the hydrogen evolution reaction. J. Am. Chem. Soc. 135, 9267–9270 provided, C N @NG notation in the paper refers to sample #3. 3 4 (2013). 14. Jaramillo, T. F. et al. Identification of active edge sites for electrochemical H evolution from MoS nanocatalysts. Science 317, 100–102 (2007). Imaging and spectroscopic characterization. Aberration-corrected high-reso- 15. Kibsgaard, J., Chen, Z., Reinecke, B. N. & Jaramillo, T. F. Engineering the lution transmission electron microscopy imaging was carried out on a FEI Titan G surface structure of MoS to preferentially expose active edge sites for electron microscope equipped with image-corrector operated at 60 kV. A 2 electrocatalysis. Nat. Mater. 11, 963–969 (2012). resolution limit of B1.2 Å was achieved under the imaging conditions used. The 16. Voiry, D. et al. Enhanced catalytic activity in strained chemically exfoliated scanning transmission electron microscopy imaging and EELS mapping were WS nanosheets for hydrogen evolution. Nat. Mater. 12, 850–855 (2013). carried out on a cubed FEI Titan G electron microscope equipped with both a 2 probe-corrector and a monochromator operated at 80 kV. The probe convergence 17. Najmaei, S. et al. Vapour phase growth and grain boundary structure of angle was 24.9 mrad and the inner detector angle was 76 mrad. A probe size of molybdenum disulphide atomic layers. Nat. Mater. 12, 754–759 (2013). 1.5 Å and a best energy resolution of 0.17 eV, as measured from the full-width-at- 18. Laursen, A. B., Kegnæs, S., Dahla, S. & Chorkendorff, I. Molybdenum sulfides– half-maximum of the zero-loss peak, were achieved. For simultaneous EELS efficient and viable materials for electro- and photoelectrocatalytic hydrogen mapping of K-edge structures for carbon and nitrogen, a 2-mm spectrometer evolution. Energy Environ. Sci. 5, 5577–5591 (2012). entrance aperture and 0.1 eV per channel dispersion were used (B0.4 eV energy 19. Merki, D. & Hu, X. Recent developments of molybdenum and tungsten resolution). sulfides as hydrogen evolution catalysts. Energy Environ. Sci. 4, 3878–3888 The synchrotron-based NEXAFS measurements were carried out on the soft (2011). X-ray spectroscopy beamline at the Australian Synchrotron, which is equipped 20. Gong, K., Du, F., Xia, Z., Durstock, M. & Dai, L. Nitrogen-doped carbon with a hemispherical electron analyser and a microchannel plate detector that nanotube arrays with high electrocatalytic activity for oxygen reduction. Science enables a simultaneous recording of the total electron yield and partial electron 323, 760–764 (2009). yield. The raw NEXAFS data were normalized to the photoelectron current of the 21. Mirzakulova, E. et al. Electrode-assisted catalytic water oxidation by a flavin photon beam, measured on an Au grid. The lab-based XPS measurements were derivative. Nat. Chem. 4, 794–801 (2012). performed on a Kratos Axis ULTRA X-ray Photoelectron Spectrometer 22. Zhao, Y., Nakamura, R., Kamiya, K., Nakanishi, S. & Hashimoto, K. Nitrogen- incorporating a 165-mm hemispherical electron energy analyser. The incident doped carbon nanomaterials as non-metal electrocatalysts for water oxidation. radiation was Monochromatic Al Ka X-rays (1,486.6 eV) at 225 W (15 kV, 15 mA). Nat. Commun. 4, 2390 (2013). Survey scans were taken at analyser pass energy of 160 eV and high-resolution 23. Zheng, Y. et al. Nanoporous graphitic-C N @Carbon metal-free 3 4 scans at 20 eV. electrocatalysts for highly efficient oxygen reduction. J. Am. Chem. Soc. 133, 20116–20119 (2011). 24. Wang, X. et al. A metal-free polymeric photocatalyst for hydrogen production Electrochemical characterization. Considering C N @NG electrode as an 3 4 from water under visible light. Nat. Mater. 8, 76–80 (2009). example, the catalyst was ultrasonically dispersed in pure water containing 0.1 wt% of Nafion. Next, 20 ml of catalyst aqueous dispersion (1.0 mg ml ) were trans- 25. Thomas, A. et al. Graphitic carbon nitride materials: variation of structure ferred onto the glassy carbon rotating disk electrode (0.196 cm ) serving as a and morphology and their use as metal-free catalysts. J. Mater. Chem. 18, working electrode. The reference electrode was an Ag/AgCl in 4 M AgCl–KCl 4893–4908 (2008). solution and the counter electrode was a graphite rod. All potentials were refer- 26. Suenaga, K. & Koshino, M. Atom-by-atom spectroscopy at graphene edge. enced to a reversible hydrogen electrode by adding a value of (0.205þ 0.059 pH) Nature 468, 1088–1090 (2010). 2 3 V and all polarization curves were corrected for the iR contribution within the cell. 27. Muller, D. A., Tzou, Y., Raj, R. & Silcox, J. Mapping sp and sp states of carbon A flow of N was maintained over the electrolyte during the experiment to elim- at sub-nanometre spatial resolution. Nature 366, 725–727 (1993). inate dissolved oxygen. The working electrode was rotated at 1,600 r.p.m. to 28. Rosenberg, R. A., Love, P. J. & Rehn, V. Polarization-dependent C(K) near-edge remove hydrogen gas bubbles formed at the catalyst surface. x-ray-absorption fine structure of graphite. Phys. Rev. B 33, 4034–4037 (1985). 29. Sheng, Z. et al. Catalyst-free synthesis of nitrogen-doped graphene via thermal annealing graphite oxide with melamine and its excellent electrocatalysis. ACS DFT calculations. The electronic structures of all catalysts were computed by Nano 5, 4350–4358 (2011). Vienna Ab-initio Simulation Package (VASP). An all-electron description, the 30. Chen, Z. et al. Core-shell MoO -MoS nanowires for hydrogen evolution: a 3 2 projector-augmented wave method was used to describe the electron–ion functional design for electrocatalytic materials. Nano Lett. 11, 4168–4175 (2011). interaction. All calculations were performed with PBE exchange-correlation 31. Chen, W. et al. Hydrogen-evolution catalysts based on non-noble metal nickel functional (the 1996 functional of Perdew, Burke and Ernzerhof) on periodically molybdenum nitride nanosheets. Angew. Chem. Int. Ed. 51, 6131–6135 repeated slabs. Molecular model-building details, free-energy calculations, HER (2012). mechanism and relevant references are given in the Supplementary Methods. 32. Merki, D., Vrubel, H., Rovelli, L., Fierro, S. & Hu, X. Fe, Co, and Ni ions promote the catalytic activity of amorphous molybdenum sulfide films for hydrogen evolution. Chem. Sci. 3, 2515–2525 (2012). References 33. Li, Y. et al. MoS nanoparticles grown on graphene: an advanced catalyst 1. Lewis, N. S. & Nocera, D. G. Powering the planet: chemical challenges in solar 2 energy utilization. Proc. Natl Acad. Sci. USA 103, 15729–15735 (2007). for the hydrogen evolution reaction. J. Am. Chem. Soc. 133, 7296–7299 2. Turner, J. A. Sustainable hydrogen production. Science 305, 972–974 (2004). (2011). 34. Nørskov, J. K., Bligaard, T., Rossmeisl, J. & Christensen, C. H. Towards the 3. Walter, M. G. et al. Solar water splitting cells. Chem. Rev. 110, 6446–6473 computational design of solid catalysts. Nat. Chem. 1, 37–46 (2009). (2010). 4. Conway, B. E. & Tilak, B. V. Interfacial processes involving electrocatalytic 35. Nørskov, J. K. et al. Trends in the exchange current for hydrogen evolution. evolution and oxidation of H , and the role of chemisorbed H. Electrochim. J. Electrochem. Soc. 152, J23–J26 (2005). Acta. 47, 3571–3594 (2002). 36. Greeley, J., Jaramillo, T. F., Bonde, J., Chorkendorff, I. & Nørskov, J. K. Computational high-throughput screening of electrocatalytic materials for 5. Subbaraman, R. et al. Enhancing hydrogen evolution activity in water splitting by tailoring Li -Ni(OH) -Pt interfaces. Science 334, 1256–1260 (2011). hydrogen evolution. Nat. Mater. 5, 909–913 (2006). NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications 7 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4783 calculations, assisted by A.D., Y.H.Z. and Y.H. carried out high-resolution transmission 37. Skulason, E. et al. Modeling the electrochemical hydrogen oxidation and electron microscopy imaging. L.H.L. and Y.C. made NEXAFS measurements. All authors evolution reactions on the basis of density functional theory calculations. discussed and analysed the data. S.Z.Q., Y.Z., Y.J. co-wrote the manuscript with input J. Phys. Chem. C 114, 18182–18197 (2010). from M.J. and Y.H. 38. Li, D., Muller, M. B., Gilje, S., Kaner, R. B. & Wallace, G. G. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotech. 3, 101–105 (2008). Additional information Acknowledgements Supplementary Information accompanies this paper at http://www.nature.com/ This research is financially supported by Australian Research Council (DP1095861, naturecommunications DP130104459). NEXAFS measurements were undertaken on the soft X-ray beamline at Australian Synchrotron. DFT calculations were undertaken on the NCI National Facility Competing financial interests: The authors declare no competing financial interests. systems through the National Computational Merit Allocation Scheme. Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/ Author contributions S.Z.Q. designed the research. Y.Z. synthesized catalysts and conducted chemical How to cite this article: Zheng, Y. et al. Hydrogen evolution by a metal-free characterizations and electrochemical measurements. Y.J. performed the DFT electrocatalyst. Nat. Commun. 5:3783 doi: 10.1038/ncomms4783 (2014). 8 NATURE COMMUNICATIONS | 5:3783 | DOI: 10.1038/ncomms4783 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved.

Journal

Nature Communications – Springer Journals

Published: Apr 28, 2014

There are no references for this article.