Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulfide via hydrazine-induced phase transformation for water splitting

Haoyi Li; Shuangming Chen; Xiaofan Jia; Biao Xu; Haifeng Lin; Haozhou Yang; Li Song; Xun Wang

doi:10.1038/ncomms15377

Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulfide via hydrazine-induced phase transformation for water splitting

Li, Haoyi; Chen, Shuangming; Jia, Xiaofan; Xu, Biao; Lin, Haifeng; Yang, Haozhou; Song, Li; Wang, Xun 2017-05-09 00:00:00 ARTICLE Received 5 Nov 2016 | Accepted 17 Mar 2017 | Published 9 May 2017 DOI: 10.1038/ncomms15377 OPEN Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulﬁde via hydrazine- induced phase transformation for water splitting 1 2 3 4 1 1 2 1 Haoyi Li , Shuangming Chen , Xiaofan Jia , Biao Xu , Haifeng Lin , Haozhou Yang , Li Song & Xun Wang Highly active and robust eletcrocatalysts based on earth-abundant elements are desirable to generate hydrogen and oxygen as fuels from water sustainably to replace noble metal materials. Here we report an approach to synthesize porous hybrid nanostructures combining amorphous nickel-cobalt complexes with 1T phase molybdenum disulﬁde (MoS ) via hydrazine-induced phase transformation for water splitting. The hybrid nanostructures exhibit overpotentials of 70 mV for hydrogen evolution and 235 mV for oxygen evolution at 10 mA cm with long-term stability, which have superior kinetics for hydrogen- and oxygen-evolution with Tafel slope values of 38.1 and 45.7 mVdec . Moreover, we achieve 10 mA cm at a low voltage of 1.44 V for 48 h in basic media for overall water splitting. We propose that such performance is likely due to the complete transformation of MoS to metallic 1T phase, high porosity and stabilization effect of nickel-cobalt complexes on 1T phase MoS . 1 2 Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China. National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230029, China. 3 4 Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, USA. Department of Chemical and Biological Engineering, Iowa State University, Ames, Iowa 50010, USA. Correspondence and requests for materials should be addressed to X.W. (email: [email protected]). NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 1 N N H H 2 2 4 4 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ydrogen (H ) has attracted extensive attention for decades compounds and the phase of MoS (Fig. 1, see Methods 2 2 1–4 as an environmentally friendly energy source . for synthetic details). With the increasing amount of HZH, HElectrochemical or photo-electrochemical water splitting Ni-Co-based compounds are changed to amorphous complexes is a convenient method to generate hydrogen, converting electricity from a partial metallic state and MoS is completely converted to 5,6 to chemical fuels for energy storage and transport . However, the metallic 1T phase. We propose that the phase transformation of dynamically unfavourable nature of water splitting is regarded as MoS is attributed to the enrichment of amorphous Ni-Co the bottleneck, in which both the hydrogen- and oxygen-evolution complexes with electron-donor ability of hydrazine because of the reactions (HER and OER) need high overpotentials for activation. stabilization effects of the complexes on 1T phase MoS . Metallic Currently, Pt alloys and Ir/Ru oxides are regarded as the state-of- 1T phase MoS can facilitate the electrode kinetics, increase the-art electrocatalysts for HER and OER respectively, but cost and the electric conductivity of the electrocatalysts and proliferate 30,31 scarcity are the barriers for the scale-up utilization of these noble- the catalytic active sites . Concurrently, porous nanostructures 7–9 metal catalysts in industrial deployment . The development of can create more catalytic active sites and improve the mass catalysts for HER and OER with non-noble materials has achieved transport and gas permeability effectively in the process of water 32,33 great success. Molybdenum disulﬁde (MoS ) is one of the most splitting . Moreover, hydrazine involved Ni-Co complexes promising candidates for HER electrocatalysts representing have the amine residues in the second coordination sphere where transition metal dichalcogenides, which has the possibility intramolecular proton transfer takes place preferentially, which is 10–15 to replace Pt-based electrocatalysts for practical applications . beneﬁcial to lower the overpotential of electrocatalytic H 34,35 For OER, low overpotentials and moderate durability have evolution . The large quantity of Ni-Co complexes in been exhibited by transition-metal (especially nickel and cobalt) the PHNCMs is helpful to promote the catalytic activities of 16,17 18,19 20,21 22 sulﬁdes ,selenides , oxides , phosphides and layered HER and OER simultaneously. Therefore, we fabricate the double hydroxides . However, it is quite difﬁcult to obtain high PHNCMs to integrate the advantages of every component in concentrations of H and OH simultaneously to motivate HER electrocatalysis, resulting in the enhancement of performance for þ 14 and OER because they follow the rule of [H ] [OH ] ¼ 10 . overall water splitting. Meanwhile, if both catalysts were employed for electrolysis, the cost would increase because of the complicated process of manufacturing electrodes. Thus, it is quite challenging to develop Results bifunctional electrocatalysts for HER and OER in one electrolyte. Synthesis and characterization of PHNCMs. We ﬁrst synthe- 24–29 Although some progress has been made in this ﬁeld ,more sized the Ni-Co hydroxides ultrathin nanosheets (NCUNs) to efforts should be devoted to designing the catalysts and enhancing provide the precursors and templates for the following synthesis their performance to control the industrial cost and lower the of PHNCMs. The pristine NCUNs showed B4 nm uniform energy consumption. A recent work illustrated that MoS /Ni S thickness and 30–150 nm diameters with circular shape according 2 3 2 heterostructures designed by interface engineering show excellent to atomic force microscopy (AFM) and transmission electron performance for overall water splitting, which could synergistically microscopy (TEM) images (Fig. 1a–c). The crystalline structure of chemisorb hydrogen and oxygen-containing intermediates .This NCUNs was demonstrated through X-ray diffraction pattern material provides the possibility for fabricating new and efﬁcient (Supplementary Fig. 1a). The X-ray photoelectron spectroscopy electrocatalysts by hybridizing nickel-cobalt-based (Ni-Co-based) (XPS) spectrum of O 1s orbital in NCUNs (Supplementary compounds with MoS . Fig. 1b) further conﬁrmed the hydroxide feature of NCUNs Herein, we present a facile strategy to synthesize porous hybrid owing to the peak position at 531.5 eV (ref. 36). nanostructures combining amorphous Ni-Co complexes with 1T After NCUNs reacted with ammonium tetrathiomolybdate phase MoS (denoted as PHNCMs) through hydrazine-inducing, ((NH ) MoS ) in N,N-dimethylformamide (DMF) solvent with 2 4 2 4 which have highly active and ultra-stable electrocatalytic different amount of HZH, the PHNCMs were formed, which performances towards HER and OER. Notably, the PHNCMs exhibited irregular nanosheet-like structures with uneven surface achieve overpotentials of 70 mV for HER and 235 mV for OER at and increasing porosity by TEM and high-resolution TEM 10 mA cm and fast kinetics shown by low Tafel slope values of (HRTEM) images (Fig. 2d–g and Supplementary Fig. 2, PHNCMs 38.1 and 45.7 mV dec for HER and OER respectively. Mean- with no HZH, 0.05 ml of HZH, 1 ml of HZH and 2.5 ml of HZH while, this material holds an overvoltage of 1.44 V to reach a denoted as 0H-PHNCMs, 0.05H-PHNCMs, 1H-PHNCMs and current density of 10 mA cm for 48 h operation without 2.5H-PHNCMs). The hybrid nanostructures maintained the degradation for overall water splitting. nanosheet morphology, as compared to the products synthesized In this work, we introduce hydrazine hydrate (HZH) into the from corresponding metal acetates directly in the same reaction reaction system to regulate the crystallization of Ni-Co-based system as 0H-PHNCMs (Supplementary Fig. 3). It suggested that Nickel hydroxides Cobalt hydroxides (NH ) MoS 4 2 4 Ni metal and tiny Co metal DMF 200 C 2H-MoS 1T-MoS Amorphous Ni-Co complexes Figure 1 | Schematic representation of the formation of PHNCMs. Upper route illustrates the synthesis of Ni metal and tiny Co metal hybridized with MoS in the blended phase of 2H and 1T without HZH using NCUNs and (NH ) MoS as the precursors by a solvothermal method. Lower route 2 4 2 4 demonstrates the synthesis of the hybrid nanostructures of amorphous Ni-Co complexes and 1T phase MoS with large amount of HZH using the same precursors as upper route. 2 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications no N H 2 4 NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE NCUNs were excellent templates for building hybrid nanosheet- correlated to the quantity of HZH based on the TEM images like structures. HZH could decompose into nitrogen (N ) and (Fig. 2d–g) and the corresponding pore size distribution curves ammonia (NH ) at high temperatures, after which N and NH (Fig. 2h–k). When the quantity of HZH reached 2.5 ml, the 3 2 3 bubbles acted as the gas templates resulting in the formation of number of pores increased markedly and pores with larger pores. We inferred that the number and size of pores positively diameters (B10 nm) appeared. Meanwhile, speciﬁc surface areas 4.096 nm ab c 4.092 nm 1 4.092 nm 4.083 nm –2 0 50 100 4.082 nm 4.086 nm 4.094 nm –2 0 60 120 4.083 nm 4.094 nm 4.092 nm –2 0 40 80 Distance (nm) d e f 1T MoS g 2H+1T MoS 2 2H+1T MoS 1T MoS 2 2 2.5 ml HZH 1 ml HZH No HZH 0.05 ml HZH 0.476 nm 0.267 nm MoS (004) 0.476 nm 2 new MoS (101) MoS (004) 2 new 0.476 nm MoS (004) 2 new 0.203 nm 0.476 nm MoS (004) 0.476 nm Ni (111) 2 new 0.307 nm 0.307 nm MoS (004) 0.476 nm 2 new MoS (004) MoS (004) 2 2 MoS (004) 0.307 nm 2 new MoS (004) h j 0.010 i 0.020 k 0.010 0.020 0.008 0.016 0.008 0.015 0.006 0.012 0.006 0.010 0.004 0.008 0.004 0.005 0.002 0.004 0.002 0.000 0.000 0.000 0.000 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Pore size (nm) Pore size (nm) Pore size (nm) Pore size (nm) 0H-PHNCMs N O Ni, JCPDS No.04-0850 MoS , JCPDS No.37-1492 0.05H-PHNCMs (002) new 0.952 nm 1H-PHNCMs (004) new 2.5H-PHNCMs Ni Co Mo 0.476 nm MoS , JCPDS No.37-1492 10 20 30 40 50 60 70 80 90 2 Theta (degree) NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 3 3 –1 –1 dV/dD (cm g nm ) Intensity (a.u.) 3 –1 –1 dV/dD (cm g nm ) 3 –1 –1 dV/dD (cm g nm ) Height (nm) 3 –1 –1 dV/dD (cm g nm ) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 were obtained by the N adsorption–desorption isotherms structure (EXAFS). Structural parameters of the bond lengths, the (Supplementary Fig. 4). 2.5H-PHNCMs had the largest speciﬁc coordination numbers, the Debye-Waller factors and the 2 1 surface area of 90.68 m g among the PHNCMs, leading comparison between experimental data and ﬁtting curves for to more active catalytic sites to enhance the performance of Ni, Co and Mo K-edge were summarized in Supplementary electrocatalysis. Furthermore, when the amount of HZH Table 1 and 2 and Supplementary Figs 10–12. The normalized increased continuously to 3 and 5 ml (denoted as 3H-PHNCMs X-ray absorption near edge structure (XANES) spectra of Ni and 5H-PHNCMs), no big differences were observed in and Co K-edge of the PHNCMs (Supplementary Fig. 13a morphoogies, speciﬁc surface area and pore size distribution and 13b) exhibited the content sequences of Ni and Co metal. curves (Supplementary Fig. 5) as compared with that of According to the intensity of white line and pre-edge peaks, 2.5H-PHNCMs. as compared with the standard samples, they were 0H-PHNCMs Integrating the results of HRTEM images (lower images in 44 1H-PHNCMs 4 0.05H-PHNCMs 4 2.5H-PHNCMs and Fig. 2d–g and Supplementary Fig. 6) and the corresponding X-ray 0H-PHNCMs 4 1H-PHNCMs 4 2.5H-PHNCMs 4 0.05H- diffraction patterns (Fig. 2l and Supplementary Fig. 7), Ni was PHNCMs, respectively for Ni and Co. Fig. 3a,b presented changed to amorphous state from metallic state, new (002) k -weighted Fourier transform (FT) proﬁles of XANES at Ni and and (004) planes of MoS with the lattice distance of 0.952 and Co K-edge of PHNCMs and standard Ni foil, NiO, Co foil, CoO 0.476 nm were observed and pristine (004) planes of MoS as contrastive samples. For 0H-PHNCMs, it was clearly seen that with the lattice distance of 0.307 nm disappeared gradually when Ni-Ni bond with 2.48 Å length were the main existing forms in the amount of HZH increased. The enlargement of interlayer the real space. The peaks at B1.72 Å for Ni in the ﬁrst shell of spacing of MoS indirectly suggested the existence of metallic the three PHNCMs with HZH were attributed to the major 1T phase of MoS in PHNCMs . We further conﬁrmed the contribution of Ni-O or Ni-N bonds. Meanwhile, the peaks for state of Ni and Co in 0H-PHNCMs. The X-ray diffraction Co in the ﬁrst shell of all the PHNCMs upshifted slightly as pattern (Supplementary Fig. 8) of Ni-only hybrid nanostructures compared to the ﬁrst peak position of standard CoO, which synthesized in the same way as 0H-PHNCMs could match with revealed that Co–O or Co-N bonds predominated but there was the standard patterns of MoS and Ni metal and that of Co-only still a small quantity of Co-Co bond. Surface features of Ni and hybrid nanostructures was only in accordance with the standard Co were probed by XPS measurement (Supplementary Fig. 14). pattern of MoS . It is well known that HZH is a strong reductant For 0H-PHNCMs, the peaks at B853.09 and B870.3 eV stood 38–40 as well as a strong coordination ligand . Therefore, the for Ni metal in Ni-Co alloy and the small peak at B856.3 eV introduction of HZH to the reaction system led to the preferential indicated that a little bit of Ni in oxidation state located on the formation of hydrazine coordinated Ni-Co complexes. According surface, while the peaks at B778.5 and B780.7 eV were indexed 41,42 to the X-ray diffraction matching consequences, the crystalline to Co metal and Co in oxidation state . Along with the structure in PHNCMs with HZH just agreed with MoS , which increasing amount of HZH, peaks for both of metallic Ni and Co indicated that Ni and Co existed in the amorphous states. The were attenuated and peaks of oxidation state at higher binding scanning TEM (STEM) images and energy-dispersive X-ray energies became stronger gradually. As Supplementary Fig. 15 (EDX) elemental mapping spectra of PHNCMs (Fig. 2m and shown, Co (III) and Ni (III) predominated and Co (II) and Ni (II) Supplementary Fig. 9) exhibited the elemental composition and also existed on the surface of 2.5H-PHNCMs. However, there uniform distribution of the six elements in PHNCMs. The were still a small quantity of Co and Ni metal, the small peaks of quantity of nitrogen increased signiﬁcantly in PHNCMs with which could be observed in the XPS spectra. This result HZH as compared with that in 0H-PHNCMs that was proven by corresponded well to the FT proﬁles of XANES at Ni and Co elemental proportion analyses of nitrogen (Table 1). The K-edge. During the synthesis of 2.5H-PHNCMs, hydrazine was increasing proportions of nitrogen revealed that it was likely for added dropwise slowly with stirring at the room temperature Ni and Co to form hydrazine coordinated amorphous complexes. before heating. In this case, Co and Ni ions preferentially Therefore, it was quite possible that PHNCMs with large amount coordinated with hydrazine to form complexes rather than being of HZH were composed of amorphous Ni-Co complexes and reduced. At the same time, a small amount of potassium 1T phase MoS , which was further revealed in the following hydroxide (KOH) was fed into the system to make NH form 2 3 characterization. and escape easily at a high temperature. The NH would be a gas template to make sure the formation of porous nanostructures. So the quantity of KOH was not enough to cause the preformed 39,40 Analyses for atomic structure of the PHNCMs. In-depth Ni-Co-hydrazine complexes to be reduced to metallic states . analyses to determine the atomic structure of PHNCMs were When Co and Ni coordinated with hydrazine to form complexes, made by several characterization methods. First, we investigated it is easy for Co (II) and Ni (II) complexes to be oxidized to Co the bonding situation of Ni and Co in PHNCMs through the (III) and Ni (III) ones. Because the redox potential of hydrazine- y 3 þ 2 þ y 3 þ 2 þ synchrotron-radiation-based extended X-ray absorption ﬁne coordinated complexes, c (Co /Co ) and c (Ni /Ni ), Figure 2 | Characterizations of the NCUNs and PHNCMs. (a) TEM image (b) AFM image and (c) the corresponding line-scan proﬁles of NCUNs, showing NCUNs with B4 nm thickness and 30–150 nm diameters. (d,e) TEM (upper) and HRTEM (lower) images of 0H-PHNCMs and 0.05H-PHNCMs showing porous nanosheet-like structures and uneven surface. The lattice distance of 0.476 nm is indexed to new (004) lattice plane of MoS as compared to pristine (004) planes with 0.307 nm of distance. The enlargement of lattice distance suggests that 1T phase MoS exists in the 0H-PHNCMs and 0.05H-PHNCMs. With the increasing amount of HZH to (f) 1 ml and (g) 2.5 ml, TEM and HRTEM images exhibit the disappearance of pristine (004) planes, suggesting the complete transformation of MoS to metallic 1T phase. The distances of 0.203 and 0.267 nm are consistent with the standard spacing of (111) planes in Ni metal and (101) planes in MoS , respectively. (h–k) Pore size distribution curves of 0H-, 0.05H-, 1H- and 2.5H-PHNCMs in accordance with TEM images in d–g.(l) X-ray diffraction patterns of PHNCMs. Lower patterns display the shifts of (002) and (004) peaks (corresponding to the lattice distances of 0.952 and 0.476 nm) and gradual disappearance of pristine (004) peaks at B29 of 2 Theta in the short dash circle, illustrating the increasing interlayer space of MoS .(m) STEM and EDX elemental mapping spectra of 2.5H-PHNCMs showing the porous nanostructures and uniform distribution of N (blue), O (red), S (orange), Ni (yellow), Co (pink) and Mo (cyan). Scale bars, 100 nm (a,b); upper, 20 nm (d–g, upper); 2 nm (lower); 50 nm (m). 4 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE Table 1 | Percentage proportions of nitrogen in different PHNCMs. Samples 0H-PHNCMs 0.05H-PHNCMs 1H-PHNCMs 2.5H-PHNCMs Ratios 3.37% 10.65% 13.92% 14.88% 1 0 was decreased dramatically to the 10 from 10 of magnitude, role of stabilization of 1T phase MoS , which could be further so that hydrazine-coordinated Co (III) and Ni (III) complexes demonstrated by the following results. could exist steadily. When the amount of HZH increased to 3 and 5 ml, Ni and Co in the oxidation states were observed quite similarly (Supplementary Fig. 14). The above analyses further Electrochemical evaluation of the PHNCMs. On the basis of the indicated the existence of hydrazine-coordinated Ni-Co successful synthesis and comprehensive characterization, we complexes in the PHNCMs with HZH. examined HER and OER electrocatalytic activities of the Subsequently, we investigated the local bond length of Mo in PHNCMs. The as-prepared PHNCMs mixing with pure carbon PHNCMs through XANES (Supplementary Fig. 13c) to demon- (Vulcan XC72) were evaluated using a typical three-electrode strate the phase transformation of MoS . According to the system in 1 M KOH media. A saturated calomel electrode (SCE) FT proﬁles and bond lengths at Mo K-edge (Fig. 3c and was used as the reference electrode. It was calibrated with Supplementary Table 2), the nearest Mo-Mo bonds of PHNCMs reversible hydrogen electrode (RHE) and the potentials were showed distinct decreasing lengths from 3.16 to 2.78 Å, which was reported versus RHE (Supplementary Fig. 19). MoS nanosheets in agreement with structural transformation from hexagonal to (with 0.05 ml of HZH), NCUNs, commercial Pt/C and IrO /C tetragonal phase . For 0H- and 0.05H-PHNCMs, the peaks at catalyst were also measured for comparison. 2.5H-PHNCMs 2.82 Å indicated that there was still some 2H phase MoS in the displayed excellent electrocatalytic activity for HER, which hybrid nanostructures. In contrast, MoS in 1H-PHNCMs and approached that of commercial Pt/C catalyst. As a consequence 2.5H-PHNCMs was completely transferred to 1T phase as the of polarization (Fig. 4a), 2.5H-PHNCMs only needed an over- evident disappearance of Mo-Mo peaks at 2.82 Å for 2H phase. potential of 70 mV to reach a current density of 10 mA cm that XPS spectrum of Mo (Fig. 3d and Supplementary Fig. 16a) was much better than that of 0H-PHNCMs (87 mV at displayed the obvious shifts of peaks to lower binding energies 10 mA cm ), not to mention 148 and 128 mV for 0.05H- with the increasing amount of HZH. For 1H-, 2.5H-, 3H- and PHNCMs and 1H-PHNCMs (Supplementary Fig. 20a). After 5H-PHNCMs, two characteristic peaks of Mo 3d and Mo 3d linear ﬁtting for the Tafel plot of 2.5H-PHNCMs, the slope was 5/2 3/2 orbitals were located at B228 and B231.0 eV, which were much calculated to be 38.1 mV dec (Supplementary Fig. 21a), which lower than that of the 2H phase counterparts (B229 and was lower than that of 0H-PHNCMs (40.3 mV dec ), MoS 30,43 1 1 232 eV) . Similar downshifts of binding energies for S 2p nanosheets (46.4 mV dec ) and NCUNs (89.4 mV dec ). Such orbitals were exhibited in Fig. 3e and Supplementary Fig. 16b. a low Tafel slope value illustrated the superior HER kinetics of These downward movements of Mo 3d and S 2p peak positions 2.5H-PHNCMs, further conﬁrming that they were indeed further proved the formation of metallic 1T phase MoS .In excellent HER electrocatalysts. Electrochemical impedance addition, we attempted to demonstrate the reason behind phase spectroscopy (EIS) Nyquist plots (Supplementary Fig. 21b) transformation of MoS . We synthesized MoS nanosheets manifested that 2.5H-PHNCMs had smaller reaction resistance as 2 2 with different amount of HZH in the same way as PHNCMs compared to other contrasting catalysts, suggesting that charge (MoS with no HZH, 0.05 ml of HZH, 1 ml of HZH and 2.5 ml transfer was facilitated in 2.5H-PHNCMs. To assess the stability of HZH denoted as 0H-MoS , 0.05H-MoS , 1H-MoS and of 2.5H-PHNCMs, Chronoamperometric response (Fig. 4c) was 2 2 2 2.5H-MoS ). TEM images (Supplementary Fig. 17a–d) showed recorded at a constant potential of 0.13 V versus RHE for 24 h. the distinct nanosheet morphology of MoS and the tendency to During the initial 3 h, the performance degraded; however, the assemble spherically. The two peaks below 20 in the X-ray current density increased for the remaining time. More efﬁcient diffraction pattern (Supplementary Fig. 17e) revealed that the HER active sites might exist in the interfaces between MoS and enlarged interlayer spacing emerged as compared to pristine Ni-Co complexes. The 3 h reduction allowed more electrolyte MoS , which was in good agreement with that of the PHNCMs. A to access the interfaces, obtaining the increasing HER current binding energy comparison of Mo was illustrated in the XPS density during the following time . To measure electrochemical spectrum (Fig. 3f) of the above a series of MoS nanosheets. active surface area (ECSA), we ﬁrst scanned cyclic voltammetry Inconspicuous downshifts (B0.25 eV) were observed for peaks (CV) cycles in the range of no Faradaic processes (Supplementary of Mo 3d and Mo 3d orbitals between 0H-MoS and Fig. 22), obtained double-layer capacitance (C ) (Fig. 4e) and 5/2 3/2 2 dl 2.5H-MoS , which were far less than that in PHNCMs (B1 eV). then converted it to ECSA. We found that 2.5H-PHNCMs had Analogously, the peaks of S 2p orbitals in XPS spectrum (Supple- the largest ECSA of 807.5 cm as compared to 0H-PHNCMs 2 2 2 mentary Fig. 18) showed the weak downshifts (B0.2 eV). The (502.5 cm ), MoS nanosheets (255 cm ) and NCUNs (45 cm ). above results indicated that 2.5H-MoS was not in pure 1T phase The number of catalytic active sites could be determined roughly and Ni-Co complexes were essential for the completely phase by ECSA. Therefore, 2.5H-PHNCMs possessed the maximum transformation of MoS . active sites for HER among all of the contrasts. Supplementary It has been reported that electron donor is quite necessary in Fig. 23a showed the comparison between the theoretical amount the phase transformation of MoS from 2H to 1T phase . In our of H calculated from a chronopotentiometric response and the 2 2 case, hydrazine could be regarded as the electron donor to induce evolved quantity of H experimentally measured from a gas the phase transformation. However, if NCUNs were not chromatography in the process of HER over 2.5H-PHNCMs for introduced to the reaction system of 2.5H-PHNCMs, pure 1T 120 min. The experimental values were observed to extremely phase MoS could not have been obtained. Compared to 2H approach to the theoretical values. This result indicated that phase MoS , 1T phase MoS has higher ground-state energy .It 2.5H-PHNCMs provided a Faradaic efﬁciency of E100% for the 2 2 is extremely possible that amorphous Ni-Co complexes play the HER. It is a convincing evidence of water splitting, which means NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 5 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ab 0H-PHNCMs Ni-Ni 0H-PHNCMs 0.05H-PHNCMs Co-Co 0.05H-PHNCMs 1H-PHNCMs 1H-PHNCMs 2.5H-PHNCMs 2.5H-PHNCMs Standard Ni foil Standard Co foil Standard NiO Standard CoO Ni-O/N Co-N/O 02 46 02 48 R(Å) R(Å) cd Mo-Mo 0H-PHNCMs Mo-S 0H-PHNCMs 0.05H-PHNCMs 0.05H-PHNCMs 1H-PHNCMs 1H-PHNCMs 2.5H-PHNCMs 2.5H-PHNCMs 2H phase MoS foil 2 Mo 3d 3/2 Mo 3d 5/2 S 2s Mo-O/N Mo-Mo(1T) Mo-Mo(2H) 224 226 228 230 232 234 236 238 024 68 R(Å) Binding Energy (eV) ef 0H-MoS 0H-PHNCMs 2 0.05H-PHNCMs 0.05H-MoS 1H-PHNCMs 1H-MoS Mo 3d S 2p 2.5H-PHNCMs 5/2 3/2 2.5H-MoS Mo 3d 3/2 S 2p 1/2 S 2s 158 160 162 164 166 168 170 172 224 226 228 230 232 234 236 238 Binding Energy (eV) Binding Energy (eV) Figure 3 | Characterization for atomic structure of the PHNCMs. (a–c) The k -weighted FTspectra of XANES from EXAFS at the Ni, Co and Mo K-edge of the PHNCMs and Ni foil, NiO, Co foil, CoO, 2H phase MoS foil as contrasting samples. A large quantity of Ni-Ni bond and tiny amount of Co-Co bond possess the major contribution in 0H-PHNCMs. Ni-O or Ni-N and Co-O or Co-N are the main bond for Ni and Co in PHNCMs with HZH. The third and fourth peaks at B2.40 Å and B2.85 Å of 0H-PHNCMs and 0.05H-PHNCMs for Mo-Mo bond indicate that 1T and 2H phase MoS coexist in the samples. The disappearance of the fourth peaks at B2.85 Å of 1H and 2.5H-PHNCMs illustrates the complete phase transformation of MoS to metallic 1T phase. The smoothing XPS spectra showing the binding energies of (d) Mo and (e) S in the PHNCMs. Obvious downshifts (B1 eV) of Mo 3d and S 2p peak positions demonstrates the phase transformation from 2H to 1Tphase. (f) The smoothing XPS spectrum of Mo 3d orbitals in MoS synthesized in the same way as PHNCMs. With the increasing quantity of HZH, the peaks of Mo 3d and Mo 3d orbitals in MoS have slight downshifts (B0.25 eV) to lower 3/2 5/2 2 binding energies, suggesting that amorphous Ni-Co complexes were the main cause in the phase conversion of MoS . 6 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 3 3 FT(k χ(k)) FT(k χ(k)) Intensity (a.u.) FT(k χ(k)) Intensity (a.u.) Intensity (a.u.) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE ab 0 2.5H-PHNCMs 2.5H-PHNCMs 0H-PHNCMs 0H-PHNCMs MoS nanosheets NCUNs MoS nanosheets –20 NCUNs Commercial Pt/C Commercial IrO /C –40 –60 –80 –100 1.3 1.4 1.5 1.6 1.7 1.8 1.9 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 Potential (V) vs RHE Potential (V) vs RHE cd –10 –20 40 –30 30 –40 20 –50 10 0 8 16 24 0 8 16 24 Time (h) Time (h) ef –2 2.5H-PHNCMs, C =32.3 mF cm –2 dl 2.5H-PHNCMs, C =108.1 mF cm dl –2 0H-PHNCMs, C =20.1 mF cm 1.2 dl –2 0H-PHNCMs, C =73.9 mF cm dl –2 MoS nanosheets, C =10.2 mF cm –2 2 dl NCUNs, C =42.8 mF cm dl –2 NCUNs, C =1.8 mF cm –2 dl 3 MoS nanosheets, C =2.0 mF cm 2 dl 1.8 1.4 0.0 0 102030 0 10 20 30 –1 –1 Scan rate (mV S ) Scan rate (mV S ) Figure 4 | Electrocatalytic hydrogen and oxygen evolution of different catalysts. (a,b) Polarization curves for HER and OER measured at a scan rate of 5mVs in 1 M KOH electrolyte; (c,d) chronoamperometric responses (i-t) recorded on 2.5H-PHNCMs for 24 h at a constant applied potential of 0.13 V versus RHE for HER and 1.53 V versus RHE for OER; (e,f) the ﬁtting plots showing C for HER and OER. dl that the cathodic currents over 2.5H-PHNCMs derived from the which outstandingly ranked as the best among reported OER 16–23 hydrogen evolution. catalysts and was much better than that of commercial In addition to the excellent performance for HER electro- IrO /C catalyst. A performance comparison of PHNCMs for catalysis, 2.5H-PHNCMs were also efﬁcient electrocatalysts for OER polarization (Supplementary Fig. 20b) was made and it was OER. During polarization process (Fig. 4b), 2.5H-PHNCMs found that 2.5H-PHNCMs and 0H-PHNCMs showed better could afford a current density of 10 mA cm with an activities and the current densities of them tended to be the overpotential of 235 mV and kept the activity for 24 h (Fig. 4d), same after 1.6 V. At the same time, the Tafel slope value of NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 7 –2 –2 –2 Current density (mA cm ) . Current density (mA cm ) Current density (mA cm ) –2 –2 Current density (mA cm ) –2 Current density (mA cm ) Current density (mA cm ) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 2.5H-PHNCMs (45.7 mV dec ) was the smallest among all of 2.5H-PHNCMs the tested catalysts (Supplementary Fig. 21c), implying that they 0H-PHNCMs had superior OER kinetics. Moreover, EIS Nyquist plots Commercial IrO /C-Pt/C couple (Supplementary Fig. 21d) showed that they made electrons 40 –2 3 mg cm loading transfer easier according to the o 30 ohm reaction resistance. Similarly, we measured the C to estimate the ECSA of dl each sample for OER (Fig. 4d and Supplementary Fig. 24). Because of the larger proportion of Ni and Co as compared to Mo (Supplementary Table 3), PHNCMs had more catalytic active sites for OER than that for HER. 2.5H-PHNCMs possessed 2,702.5 cm of ECSA, which was much larger than that of others 2 2 2 (1,847.5 cm of 0H-PHNCMs, 1,070 cm of NCUNs and 50 cm of MoS nanosheets). For the Faradaic efﬁciencies for OER, the contrast between the measured amount of O production and the theoretical values was shown in Supplementary Fig. 23b and Supplementary Table 4. It was found that the experimental values 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 were a little less than the theoretical values due to the complex Voltage (V) four-electron reaction process of OER, whose kinetics was quite unfavourable. The Faradaic efﬁciencies at the different periods were also calculated and the average Faradaic efﬁciency was –2 1 mg cm loading 91.23%. We also measured the electrocatalytic performances for HER and OER over 3H- and 5H-PHNCMs and made a comparison with 2.5H-PHNCMs. It was observed that the three PHNCMs exhibited quite similar activities for HER and OER electro- catalysis (Supplementary Fig. 25). Signiﬁcant evaluation indexes were summarized in Supplementary Table 5. 0 8 16 24 Furthermore, We made a comparison of XPS spectra –2 3 mg cm loading (Supplementary Fig. 26a,b) of Mo 3d orbitals in 2.5H-PHNCMs and 2.5H-MoS before and after 1,000 OER cycles. The positions of the two characteristic peaks of Mo 3d and Mo 3d orbitals 5/2 3/2 15 in 2.5H-PHNCMs did not change after 1,000 OER cycles. However, upshifts (B0.4 eV) were observed for the peaks of Mo 3d and Mo 3d orbitals in 2.5H-MoS after 1,000 OER cycles, 5/2 3/2 2 which revealed in situ electrochemical oxidation of 2.5H-MoS 016 32 48 during OER process. Besides, new (002) and (004) planes with the Time (h) enlargement lattice distance of 0.952 and 0.476 nm were observed in X-ray diffraction pattern (Supplementary Fig. 26c) and Figure 5 | Bifunctional electrocatalysts for overall water splitting. (a) Steady-state polarization curves of the catalysts on CFP with HRTEM image (Supplementary Fig. 26d) of 2.5H-PHNCMs after 1,000 OER cycles, which could further conﬁrm that MoS 1mgcm of mass loading for overall water splitting at a scan rate of 5mVs in 1 M KOH electrolyte with a two-electrode conﬁguration after in 2.5H-PHNCMs remained 1T phase after 1,000 OER cycles. a 20 min open circuit scan. The green dash line represents the polarization The above illustration demonstrated that amorphous Ni-Co curve of 2.5H-PHNCMs with 3 mg cm of mass loading, which holds just complexes played a role of stabilization of metallic 1T phase 1.44 V to reach a current density of 10 mA cm .(b) Chronoamperometric MoS . 2 2 curves of 2.5H-PHNCMs with 1 mg cm (upper) and 3 mg cm (lower) Such excellent electrocatalytic performance should be attrib- of mass loading for overall water splitting in a two-electrode conﬁguration uted to the contribution of both components in 2.5H-PHNCMs. at constant cell voltages of 1.49 and 1.44 V, respectively. On one hand, metallic 1T phase MoS can increase the electric 30,31 conductivity and catalytic active sites , which was proven by the low electrochemical reaction resistances and large ECSAs. On the other hand, large proportions of Ni and Co result in Evaluation of PHNCMs for overall water splitting. PHNCMs marvelous electrocatalytic OER activity. Here we propose a were also used as both cathodic and anodic materials for overall possible mechanism of intramolecular proton transfer in hydra- water splitting with a two-electrode conﬁguration in 1 M KOH zine coordinated Ni-Co complexes (Supplementary Fig. 27) that electrolyte. 2.5H-PHNCMs achieved a current density of 34,35 2 2 beneﬁcially lowers the overpotential of HER . Our Ni-Co 10 mA cm at a cell voltage of 1.48 V with 1 mg cm of mass complexes have amine residues in the second coordination loading (Fig. 5a). They had higher catalytic activity than sphere, which could be a hydrogen-exchanging site. As shown in 0H-PHNCMs (1.53 V at 10 mA cm ) and commercial IrO /C- Supplementary Fig. 15, Co (III) and Ni (III) predominated in Pt/C couple (1.63 V at 10 mA cm ). According to the polar- 2.5H-PHNCMs. The catalytic trivalent metal linking to hydrazine ization curves of the PHNCMs (Supplementary Fig. 28a), obtains two electrons to become the monovalent metal. Then, two 2.5H-PHNCMs and 0H-PHNCMs exhibited lower overpotential protons combine with the monovalent metal and nitrogen atom than that of 0.05H-PHNCMs and 1H-PHNCMs, in accordance of the amine residue in the second coordination sphere to form with the consequences of HER and OER electrocatalysis. A long- a metal hydride and a –NH group. Subsequently, the two term electrolysis process utilizing 2.5H-PHNCMs was operated at protons tend to combine with each other forming H . In this a constant potential of 1.49 V for 24 h (upper image in Fig. 5b), way, hydrazine coordinated Ni-Co complexes can facilitate the which showed remarkable durability with negligible degradation. electrocatalysis for HER, thereby enhancing the performance of However, the current density of commercial IrO /C-Pt/C couple overall water splitting. decreased continuously during a 7 h operation (Supplementary 8 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications –2 –2 Current density (mA cm ) Current density (mA cm ) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE Fig. 28b). After 24 h operation, many H and O bubbles Synthesis of MoS nanosheets. A total of 20 mg of (NH ) MoS was dispersed in 2 2 2 4 2 4 10 ml of DMF. The mixture was stirred at room temperature for 10 min until a remained on the carbon ﬁbre paper (CFP) daubed by homogeneous solution was achieved. After that, 0, 0.05 ml, 1 and 2.5 ml of HZH 2.5H-PHNCMs with 1 mg cm of mass loading (Supplementary were added to four of the above mixture respectively. The reaction solution Fig. 29). In addition, the activity of 2.5H-PHNCMs could be was further stirred for 10 min before transferred to 40 ml Teﬂon-lined autoclaves. further improved by increasing the mass loading of the active Then they were heated at 200 C for 10 h. After the system was cooling down, the suspension was centrifuged and washed with ethanol for three times. material on CFP to 3 mg cm (the green dash line in Fig. 5a). Then the precipitant was obtained and re-dispersed in ethanol for further The increasing mass loading decreased the overvoltage from 1.48 characterizations. to 1.44 V at a current density of 10 mA cm . It is noteworthy that this electrode was durable for 48 h without degradation Characterizations. The morphologies and structures of the samples were observed (lower image in Fig. 5b). Furthermore, we investigated the overall by TEM, which was conducted on a Hitachi H7700 at 100 kV, using the carbon- water splitting electrocatalytic performances of 3H- and coated copper grid. Details of morphologies and structures were obtained by 5H-PHNCMs with 3 mg cm of mass loading and made HRTEM that was carried out on a FEI G2 F20 S-Twin TEM at 200 kV equipped with high angle annular dark-ﬁeld STEM. EDX were taken with the same instru- a contrast with 2.5H-PHNCMs. The extremely close activities ment as HRTEM. AFM was conducted on a Dimension Icom, Bruker. Powder of the three PHNCMs were exhibited in Supplementary Fig. 30. X-ray diffraction characterization was performed on a Bruker D8 Advance X-ray Large volumes of H and O gases on the surface of electrodes 2 2 diffractometer using Cu-Ka radiation (l ¼ 1.5418 Å). XPS were recorded on a PHI over 2.5H-PHNCMs could be seen during the process of Quantera SXM spectrometer with monochromatic Al Ka X-ray sources (1,486.6 eV) at 2.0 kV and 20 mA. N adsorption/desorption measurements were a chronoamperometric test (Supplementary Movie 1, a video carried out on a Autosorb-iQ2, Quantachrome Instruments. The proportions of water electrolysis for around 2 min). The above results of N and Co, Ni, Mo were measured by Elemental Analyzer, Euro EA3000 and demonstrated that 2.5H-PHNCMs had a great potential to ICP-AES, VISTA-MPX, respectively. Ni, Co and Mo K-edge XAFS measurements serve effectively for the practical and long-term application of were made at the beamline 14W1 in 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). The X-ray was monochromatized by a double-crystal Si overall water splitting. (111) monochromator for BSRF. The energy was calibrated using a cobalt metal foil for the Co K-edge, a nickel metal foil for the Ni K-edge and a molybdenum Discussion metal foil for the Mo K-edge. The monochromator was detuned to reject higher harmonics. The acquired EXAFS data were processed according to the standard In summary, we have developed a facile approach to fabricate procedures using the WinXAS3.1 program . Theoretical amplitudes and amorphous Ni-Co complexes hybridized with 1T phase MoS as 2 48 phase-shift functions were calculated with the FEFF8.2 code using the crystal highly active bifunctional electrocatalysts for overall water splitting. structural parameters of the Co, CoO, Ni, NiO, MoO and MoS . 3 2 The hybrid nanostructures exhibit an extremely low overpotential and long-term stability for both HER and OER, which can be Preparation of samples for HER and OER electrocatalysis. 5 mg of the active attributed to complete conversion of MoS to metallic 1T phase, 2 material and 1 mg of pure carbon (Vulcan XC72) were added into 0.95 ml of mixture solution of water and ethanol with the ratio of 3:1 and sonicated for at least increasing number of catalytic active sites and stabilization effect of 30 min. When they were dispersed uniformly, 0.05 ml of naﬁon D-521 dispersion amorphous Ni-Co complexes on 1T phase MoS . This catalyst only (5% w/w in water and 1-propanol) was added into the above mixture and then needs an overpotential of 1.44 V to afford an overall-water-splitting sonicated for at least 30 min. After that, 0.006 ml of the mixture solution was drop 2 2 current density of 10 mA cm with a mass loading of 3 mg cm casted on a rotating disk electrode (RDE) with glassy carbon, which has 0.196 cm and maintains its catalytic activity for 48 h operation without of effective area. When the sample solution dried naturally, it could be used as the working electrode for HER and OER electrocatalysis. degradation. The accessible and low-cost manufacturing as well as the excellent electrocatalytic performance may further inspire the Preparation of samples for overall water splitting. A total of 1 mg of development of non-noble-metal electrode materials for overall active material, 0.2 mg of acetylene black and 0.15 mg of polyvinylideneﬂuoride water splitting. were mixed using 0.04 ml of N-methyl-2-pyrrolidone as the solvent to yield a slurry. Then the slurry was daubed uniformly in 1 cm of area on a piece of 1 3 cm CFP and dried in vacuum at 100 C for 24 h. After that, it was Methods immersed in 1 M KOH solution for 12 h in vacuum for activation. Then it could be Reagents. All the reagents were purchased from Alfa Aesar in analytical grade and used as cathode and anode respectively for overall water splitting and the active used as received without further puriﬁcation. 2 2 area was 1 cm . The electrode with 3 mg cm of mass loading was prepared by changing the amount of active material, acetylene black and polyvinylidene Synthesis of NCUNs. In a typical synthesis of NCUNs, 0.8 ml of Nickel(II) acetate diﬂuoride to 3 mg, 0.6 and 0.45 mg, respectively and other procedures were the hydrate aqueous solution (0.2 M) and 0.8 ml of Cobalt(II) acetate tetrahydrate same as the above. aqueous solution (0.2 M) were added to 8 ml of anhydrous DMF and transferred into a 50 ml-ﬂask. An amount of 74 mg (2 mmol) of ammonium ﬂuoride (NH F) Electrochemical characterizations. HER and OER electrocatalysis were measured and 120 mg (2 mmol) of urea (CO(NH ) ) were dissolved into 0.3 ml of deionized 2 2 in a typical three-electrode conﬁguration with a Princeton PASTAT4000 instru- water, respectively. The NH F and CO(NH ) aqueous solution were dropwise 4 2 2 ment and a RDE system. Glassy carbon electrode with active sample was selected as added into the above mixture. Then the mixture was kept at 90 C with stirring for the working electrode, SCE as the reference electrode and a graphite rod as the 24 h. After the system was cooling down naturally, the suspension was centrifuged counter electrode. Overall water splitting was performed in a two-electrode system and washed with ethanol for three times. Then the precipitant was obtained and using the CFP with active sample as both the cathode and anode. For HER and re-dispersed in ethanol for further characterizations. Moreover, some precipitant OER, all of the polarization curves were measured in 1 M KOH solution using RDE was dried in a freezer dryer with vacuum for the next step reaction. For Ni-only or with 1,600 r.p.m. at a scan rate of 5 mV s . Chronoamperometric responses (i-t) Co-only hybrids, the precursors were synthesized in the same way above without were recorded on 2.5H-PHNCMs using RDE with 1,600 r.p.m. for 24 h at adding Cobalt(II) acetate tetrahydrate aqueous solution or Nickel(II) acetate a constant applied potential of 0.13 V versus RHE for HER and 1.53 V versus hydrate aqueous solution. RHE for OER. To measure electrochemical double-layer capacitance (C ), the dl potentials were swept for a cycle using RDE at 1,600 r.p.m. at a range of no faradic Synthesis of PHNCMs. First, 25 mg of NCUNs powders and 13 mg of processes six times at six different scan rates (5, 10, 15, 20, 25 and 30 mV s ). The measured capacitive current densities at the average potential in the selected range (NH ) MoS were dispersed in 10 ml of DMF and sonicated for 10 min to be 4 2 4 a homogeneous solution. After sonicating, 0, 0.05 ml, 1 ml, 2.5 ml of HZH was were plotted as a function of the scan rates and the slope of the linear ﬁt could be calculated as the C . The speciﬁc capacitance was generally found to be in the respectively added dropwise into above mixture with vigorously stirring and then dl 2 2 0.1 ml of potassium hydroxide (KOH) aqueous solution (including 20 mg of KOH) range of 20–60 mFcm and we used the average value of 40mFcm here. According to the following equation , was dropwise fed into the mixture with HZH, respectively. Then the mixture was separately transferred into 40 ml Teﬂon-lined autoclaves and heated dl ECSA ¼ cm ð1Þ at 200 C for 10 h. After the system was cooling down, the suspension was ECSA 40mF cm centrifuged and washed with ethanol for three times. Then the precipitant was obtained and re-dispersed in ethanol for further characterizations. Ni-only or we could obtain the ECSA roughly. EIS experiments were performed in the Co-only hybrids were synthesized by the same method above with Ni-only or frequency range of 10 kHz–100 mHz at a constant current density of 1 mA cm . Co-only precursors. All of the Tafel plots were measured in 1 M KOH solution using RDE with NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 9 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 1,600 r.p.m. at a scan rate of 1 mV s . As for the Faradaic efﬁciency 22. Popczun, E. J., Read, C. G., Roske, C. W., Lewis, N. S. & Schaak, R. E. Highly measurements, gas chromatography was used to determine the experimentally active electrocatalysis of the hydrogen evolution reaction by cobalt phosphide evolved amount of H and O . And we used the Faraday law to calculate the 2 2 nanoparticles. Angew Chem. Int. Ed. Engl. 126, 5531–5534 (2014). theoretical amount of H and O expected based on a chronopotentiometric 2 2 23. Song, F. & Hu, X. Exfoliation of layered double hydroxides for enhanced response at the constant current density of 20 mA cm for 120 min. For overall oxygen evolution catalysis. Nat. Commun. 5, 4477–4485 (2014). water splitting, all of the polarization curves were measured in 1 M KOH solution 24. Ledendecker, M. et al. The synthesis of nanostructured Ni P ﬁlms and their 5 4 at a scan rate of 5 mV s after an open circuit scan for 20 min. use as a non-noble bifunctional electrocatalyst for full water splitting. Angew Chronoamperometric curves of 2.5H-PHNCMs with 3 and 1 mg cm of mass Chem. Int. Ed. Engl. 127, 12538–12542 (2015). loading and commercial IrO /C-Pt/C couple were carried out at a constant cell 25. Wang, H. et al. Bifunctional non-noble metal oxide nanoparticle voltage of 1.44, 1.49 and 1.65 V. electrocatalysts through lithium-induced conversion for overall water splitting. Nat. Commun. 6, 7261–7268 (2015). 26. Stern, L. A., Feng, L., Song, F. & Hu, X. Ni P as a Janus catalyst for water RHE calibration. In all electrochemical characterizations, SCE was used as the splitting: the oxygen evolution activity of Ni P nanoparticles. Energy Environ. reference electrode and calibrated with RHE. The calibration was taken in the Sci. 8, 2347–2351 (2015). hydrogen-saturated electrolyte with the working electrode of a Pt wire. A single cycle 27. Jin, Y. et al. Porous MoO nanosheets as non-noble bifunctional electrocatalysts of cyclic voltammetry was measured at a scan rate of 1 mV s , and the average of for overall water splitting. Adv. Mater. 28, 3785–3790 (2016). the two potentials where the current crossed zero was regarded as the potential of the 28. Jaramillo, T. F. et al. Gold-supported cerium-doped NiO catalysts for water calibration value. In 1 M KOH electrolyte, E(RHE) ¼ E(SCE) þ 1.024 V. oxidation. Nat. Energy 1, 16053–16060 (2016). 29. Zhang, J. et al. Interface engineering of MoS /Ni S heterostructures for highly 2 3 2 enhanced electrochemical overall-water-splitting activity. Angew Chem. Int. Ed. Data availability. The data reported by this article are available from the corre- Engl. 128, 6814–6819 (2016). sponding author upon reasonable request. 30. Acerce, M., Voiry, D. & Chhowalla, M. Metallic 1T phase MoS nanosheets as supercapacitor electrode materials. Nat. Nanotechnol. 10, 313–318 (2015). 31. Voiry, D. et al. Conducting MoS nanosheets as catalysts for hydrogen References evolution reaction. Nano Lett. 13, 6222–6227 (2013). 1. Winsche, W. E., Hoffman, K. C. & Salzano, F. J. Hydrogen: its future role in the 32. Sasikumar, G., Ihm, J. W. & Ryu, H. Optimum Naﬁon content in PEM fuel cell nations energy economy. Science 180, 1325–1332 (1973). electrodes. Electrochim. Acta 50, 601–605 (2004). 2. Schlapbach, L. & Zu¨ttel, A. Hydrogen-storage materials for mobile applications. 33. Zhang, X. et al. Co/Co S @S,N-doped porous graphene sheets derived from S, 9 8 Nature 414, 353–358 (2001). N dual organic ligands assembled Co-MOFs as superior electrocatalysts for full 3. Bashyam, R. & Zelenay, P. A class of non-precious metal composite catalysts water splitting in alkaline media. Nano Energy 30, 93–102 (2016). for fuel cells. Nature 443, 63–66 (2006). 34. Wilson, A. D. et al. Hydrogen oxidation and production using nickel-based 4. Chu, P. & Majumdar, A. Opportunities and challenges for a sustainable energy molecular catalysts with positioned proton relays. J. Am. Chem. Soc. 128, future. Nature 488, 294–303 (2012). 358–366 (2006). 5. Maeda, K. et al. Photocatalyst releasing hydrogen from water. Nature 440, 35. Jacques, P. A. et al. Cobalt and nickel diimine-dioxime complexes as molecular 295–295 (2006). electrocatalysts for hydrogen evolution with low overvoltages. Proc. Natl Acad. 6. Mallouk, T. E. Water electrolysis: divide and conquer. Nat. Chem. 5, 362–363 Sci. USA 106, 20627–20632 (2009). (2013). 36. Tan, B. J., Klabunde, K. J. & Sherwood, P. M. A. X-ray photoelectron 7. Greeley, J., Jaramillo, T. F., Bonde, J., Chorkendorff, I. & Norskov, J. K. spectroscopy studies of solvated metal atom dispersed catalysts. Monometallic Computational high-throughput screening of electrocatalytic materials for iron and bimetallic iron-cobalt particles on alumina. Chem. Mater. 2, 186–191 hydrogen evolution. Nat. Mater. 5, 909–913 (2006). (1990). 8. Zeng, K. & Zhang, D. Recent progress in alkaline water electrolysis for 37. Liu, Q. et al. Gram-scale aqueous synthesis of stable few-layered 1T-MoS : hydrogen production and applications. Prog. Energy Combust. Sci. 36, 307–326 applications for visible-light driven photocatalytic hydrogen evolution. Small (2010). 11, 5556–5564 (2015). 9. Jiao, Y., Zheng, Y., Jaroniec, M. & Qiao, S. Z. Design of electrocatalysts for 38. Nicholls, D., Rowley, M. & Swindells, R. Hydrazine complexes of cobalt(II) oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. chloride. J. Chem. Soc. A 950–952 (1966). 44, 2060–2086 (2015). 39. Park, J. W. et al. Preparation of ﬁne Ni powders from nickel hydrazine 10. Jaramillo, T. F. et al. Identiﬁcation of active edge sites for electrochemical H 2 complex. Mater. Chem. Phys. 97, 371–378 (2006). evolution from MoS nanocatalysts. Science 317, 100–102 (2007). 40. Wu, Z. G., Munoz, M. & Montero, O. The synthesis of nickel nanoparticles by 11. Karunadasa, H. I. et al. A molecular MoS edge site mimic for catalytic 2 hydrazine reduction. Adv. Powder Technol. 21, 165–168 (2010). hydrogen generation. Science 335, 698–702 (2012). 41. Lian, K. K., Kirk, D. W. & Thorpe, S. J. Investigation of a ‘two-state’ Tafel 12. Kibsgaard, J., Chen, Z., Reinecke, B. N. & Jaramillo, T. F. Engineering the phenomenon for the oxygen evolution reaction on an amorphous Ni-Co alloy. surface structure of MoS to preferentially expose active edge sites for J. Electrochem. Soc. 142, 3704–3712 (1995). electrocatalysis. Nat. Mater. 11, 963–969 (2012). 42. Biesinger, M. C. et al. Resolving surface chemical states in XPS analysis of ﬁrst 13. Xie, J. et al. Controllable disorder engineering in oxygen-incorporated MoS 2 row transition metals, oxides and hydroxides: Cr, Mn, Fe, Ni and Co. Appl. ultrathin nanosheets for efﬁcient hydrogen evolution. J. Am. Chem. Soc. 135, Surf. Sci. 257, 2717–2730 (2011). 17881–17888 (2013). 43. Eda, G. et al. Photoluminescence from chemically exfoliated MoS . Nano Lett. 14. Xie, J. et al. Defect-rich MoS ultrathin nanosheets with additional active 2 11, 5111–5116 (2011). edge sites for enhanced electrocatalytic hydrogen evolution. Adv. Mater. 25, 44. Conley, H. J. et al. Bandgap engineering of strained monolayer and bilayer 5807–5813 (2013). MoS . Nano Lett. 13, 3626–3630 (2013). 15. Lu, Z. et al. Ultrahigh hydrogen evolution performance of under-water 45. Reed, E. J. et al. Structural phase transitions in two-dimentional Mo- and ‘superaerophobic’ MoS nanostructured electrodes. Adv. Mater. 26, 2 W-dichalcogenide monolayers. Nat. Commun. 5, 4214–4222 (2014). 2683–2687 (2014). 46. Gao, M. R. et al. An efﬁcient molybdenum disulﬁde/cobalt diselenide hybrid 16. Ganesan, P., Prabu, M., Sanetuntikul, J. & Shanmugam, S. Cobalt sulﬁde catalyst for electrochemical hydrogen generation. Nat. Commun. 6, 5982–5988 nanoparticles grown on nitrogen and sulfur codoped graphene oxide: an (2015). efﬁcient electrocatalyst for oxygen reduction and evolution reactions. ACS 47. Ressler, T. WinXAS: a program for X-ray absorption spectroscopy data analysis Catal. 5, 3625–3637 (2015). under MS-Windows. J. Synchrotron Radiat. 5, 118–122 (1998). 17. Feng, L.-L. et al. High-index faceted Ni S nanosheet arrays as highly active 3 2 48. Ankudinov, A. L., Ravel, B., Rehr, J. J. & Conradson, S. D. Real-space multiple- and ultrastable electrocatalysts for water splitting. J. Am. Chem. Soc. 137, scattering calculation and interpretation of X-ray-absorption near-edge 14023–14026 (2015). structure. Phys. Rev. B 58, 7565–7576 (1998). 18. Liu, Y. et al. Low overpotential in vacancy-rich ultrathin CoSe nanosheets for 2 49. Kibsgaard, J. & Jaramillo, T. F. Molybdenum phosphosulﬁde: an active, water oxidation. J. Am. Chem. Soc. 136, 15670–15675 (2014). acid-stable, earth-abundant catalyst for the hydrogen evolution reaction. Angew 19. Xu, K. et al. Solution-liquid-solid synthesis of hexagonal nickel selenide Chem. Int. Ed. Engl. 53, 14433–14437 (2014). nanowire arrays with a nonmetal catalyst. Angew Chem. In. Ed. Engl. 128, 1742–1745 (2016). 20. Li, Y., Hasin, P. & Wu, Y. Ni Co O nanowire arrays for electrocatalytic Acknowledgements x 3 x 4 oxygen evolution. Adv. Mater. 22, 1926–1929 (2010). This work was supported by NSFC (21431003, 21521091, 11605201, U1532112), China 21. Yeo, B. S. & Bell, A. T. Enhanced activity of gold-supported cobalt oxide for Ministry of Science and Technology under Contract of 2016YFA0202801 and the State the electrochemical evolution of oxygen. J. Am. Chem. Soc. 133, 5587–5593 Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011). (2014CB848900). 10 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE How to cite this article: Li, H. et al. Amorphous nickel-cobalt complexes hybridized with Author contributions 1T-phase molybdenum disulﬁde via hydrazine-induced phase transformation for water H. Li. and S.C. contributed equally to this work. X.W. led the research. H. Li. and X.W. splitting. Nat. Commun. 8, 15377 doi: 10.1038/ncomms15377 (2017). conceived the idea. H. Li. planned and performed the experiments, collected the data and analysed the data. S.C. and L.S. collected and analysed the EXAFS data. X.J. proposed and analysed the mechanism of intramolecular proton transfer. H. Li., X.J., B.X., H. Lin, H.Y. Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afﬁliations. and X.W. co-wrote the manuscript. All authors gave approval to the ﬁnal version of the manuscript. This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this Additional information article are included in the article’s Creative Commons license, unless indicated otherwise Supplementary Information accompanies this paper at http://www.nature.com/ in the credit line; if the material is not included under the Creative Commons license, naturecommunications users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ Competing interests: The authors declare no competing ﬁnancial interests. Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/ r The Author(s) 2017 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 11 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals http://www.deepdyve.com/lp/springer-journals/amorphous-nickel-cobalt-complexes-hybridized-with-1t-phase-molybdenum-Vs7a0fhfmx

Loading next page...

References (57)

S. Chu, A. Majumdar (2012)
Opportunities and challenges for a sustainable energy future
Nature, 488
J. Greeley, T. Jaramillo, J. Bonde, I. Chorkendorff, J. Nørskov (2006)
Computational high-throughput screening of electrocatalytic materials for hydrogen evolution
Nature Materials, 5
G. Swanson, M. Cattoni, William Hailer, Sholom Pearlman (1973)
Hydrogen: Its Future Role in the Nation's Energy Economy
Science, 180
Kun Xu, Hui Ding, Kaicheng Jia, Xiuli Lu, Pengzuo Chen, Tianpei Zhou, Han Cheng, Si Liu, Changzheng Wu, Yi Xie (2016)
Solution-Liquid-Solid Synthesis of Hexagonal Nickel Selenide Nanowire Arrays with a Nonmetal Catalyst.
Angewandte Chemie, 55 5
L. Schlapbach, A. Züttel (2001)
Hydrogen-storage materials for mobile applications
Nature, 414
A. Ankudinov, B. Ravel, J. Rehr, S. Conradson (1998)
REAL-SPACE MULTIPLE-SCATTERING CALCULATION AND INTERPRETATION OF X-RAY-ABSORPTION NEAR-EDGE STRUCTURE
Physical Review B, 58
Junfeng Xie, Hao Zhang, Shuang Li, Ruoxing Wang, Xu Sun, Min Zhou, Jingfang Zhou, X. Lou, Yi Xie (2013)
Defect‐Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution
Advanced Materials, 25
T. Ressler (1998)
WinXAS: a program for X-ray absorption spectroscopy data analysis under MS-Windows.
Journal of synchrotron radiation, 5 Pt 2
Jian Zhang, Tao Wang, D. Pohl, B. Rellinghaus, Renhao Dong, Shaohua Liu, Xiaodong Zhuang, Xinliang Feng (2016)
Interface Engineering of MoS2 /Ni3 S2 Heterostructures for Highly Enhanced Electrochemical Overall-Water-Splitting Activity.
Angewandte Chemie, 55 23
(2013)
Conducting MoS2 nanosheets as catalysts for hydrogen evolution reaction
, 13
G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, Mingwei Chen, M. Chhowalla (2011)
Photoluminescence from chemically exfoliated MoS2.
Nano letters, 11 12
B. Yeo, A. Bell (2011)
Enhanced activity of gold-supported cobalt oxide for the electrochemical evolution of oxygen.
Journal of the American Chemical Society, 133 14
Yanguang Li, Panitat Hasin, Yiying Wu (2010)
NixCo3−xO4 Nanowire Arrays for Electrocatalytic Oxygen Evolution
Advanced Materials, 22
Muharrem Acerce, D. Voiry, M. Chhowalla (2015)
Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials.
Nature nanotechnology, 10 4
T. Mallouk (2013)
Water electrolysis: Divide and conquer.
Nature chemistry, 5 5
Minrui Gao, Jin-Xia Liang, Ya-Rong Zheng, Yunfei Xu, Jun Jiang, Q. Gao, Jun Li, Shuhong Yu (2015)
An efficient molybdenum disulfide/cobalt diselenide hybrid catalyst for electrochemical hydrogen generation
Nature Communications, 6
M. Biesinger, L. Lau, A. Gerson, R. Smart (2010)
Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn
Applied Surface Science, 257
Junfeng Xie, Jiajia Zhang, Shuang Li, Fabian Grote, Xiaodong Zhang, Hao Zhang, Ruoxing Wang, Y. Lei, B. Pan, Yi Xie (2013)
Controllable disorder engineering in oxygen-incorporated MoS2 ultrathin nanosheets for efficient hydrogen evolution.
Journal of the American Chemical Society, 135 47
Y. Jiao, Yao Zheng, M. Jaroniec, S. Qiao (2015)
Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions.
Chemical Society reviews, 44 8
D Voiry (2013)
6222
Nano Lett., 13
Pandian Ganesan, M. Prabu, Jakkid Sanetuntikul, S. Shanmugam (2015)
Cobalt Sulfide Nanoparticles Grown on Nitrogen and Sulfur Codoped Graphene Oxide: An Efficient Electrocatalyst for Oxygen Reduction and Evolution Reactions
ACS Catalysis, 5
H. Karunadasa, E. Montalvo, Yujie Sun, M. Majda, J. Long, Christopher Chang (2012)
A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen Generation
Science, 335
E. Popczun, Carlos Read, Christopher Roske, N. Lewis, R. Schaak (2014)
Highly active electrocatalysis of the hydrogen evolution reaction by cobalt phosphide nanoparticles.
Angewandte Chemie, 53 21
M. Biesinger, B. Payne, A. Grosvenor, L. Lau, A. Gerson
Resolving surface chemical states in XPS analysis of ﬁrst row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni
T. Jaramillo, K. Jørgensen, J. Bonde, J. Nielsen, S. Horch, I. Chorkendorff (2007)
Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts
Science, 317
Qin Liu, Xiuling Li, Qun He, A. Khalil, Daobin Liu, Ting Xiang, Xiaojun Wu, L. Song (2015)
Gram-Scale Aqueous Synthesis of Stable Few-Layered 1T-MoS2 : Applications for Visible-Light-Driven Photocatalytic Hydrogen Evolution.
Small, 11 41
K. Zeng, Dongke Zhang (2010)
Recent progress in alkaline water electrolysis for hydrogen production and applications
Progress in Energy and Combustion Science, 36
B. Tan, K. Klabunde, P. Sherwood (1990)
X-ray photoelectron spectroscopy studies of solvated metal atom dispersed catalysts. Monometallic iron and bimetallic iron-cobalt particles on alumina
Chemistry of Materials, 2
Lucas-Alexandre Stern, Ligang Feng, Fang Song, Xile Hu (2015)
Janus catalyst for water splitting : the oxygen evolution activity of Ni 2 P nanoparticles
Fang Song, Xile Hu (2014)
Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis
Nature Communications, 5
Haotian Wang, Hyun‐Wook Lee, Yong Deng, Zhiyi Lu, Po-Chun Hsu, Yayuan Liu, Dingchang Lin, Yi Cui (2015)
Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting
Nature Communications, 6
K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen (2006)
Photocatalyst releasing hydrogen from water
Nature, 440
J. Ng, M. García‐Melchor, M. Bajdich, Pongkarn Chakthranont, Charlotte Kirk, A. Vojvodić, T. Jaramillo (2016)
Gold-supported cerium-doped NiOx catalysts for water oxidation
Nature Energy, 1
Amit Klein (2017)
Divide and Conquer
J Zhang (2016)
6814
Angew Chem. Int. Ed. Engl., 128
J. Kibsgaard, T. Jaramillo (2014)
Molybdenum phosphosulfide: an active, acid-stable, earth-abundant catalyst for the hydrogen evolution reaction.
Angewandte Chemie, 53 52
Z. Wu, M. Muñoz, O. Montero (2010)
The synthesis of nickel nanoparticles by hydrazine reduction
Advanced Powder Technology, 21
M Ledendecker (2015)
12538
Angew Chem. Int. Ed. Engl., 127
Zhiyi Lu, Wei Zhu, Xiaoyou Yu, Haichuan Zhang, Yingjie Li, Xiaoming Sun, Xinwei Wang, Hao Wang, Jingming Wang, Jun Luo, Xiaodong Lei, Lei Jiang (2014)
Ultrahigh Hydrogen Evolution Performance of Under‐Water “Superaerophobic” MoS2 Nanostructured Electrodes
Advanced Materials, 26
P. Jacques, V. Artero, J. Pécaut, M. Fontecave (2009)
Cobalt and nickel diimine-dioxime complexes as molecular electrocatalysts for hydrogen evolution with low overvoltages
Proceedings of the National Academy of Sciences, 106
K Xu (2016)
1742
Angew Chem. In. Ed. Engl., 128
G. Sasikumar, J. Ihm, H. Ryu (2004)
Optimum Nafion content in PEM fuel cell electrodes
Electrochimica Acta, 50
EJ Popczun (2014)
5531
Angew Chem. Int. Ed. Engl., 126
M. Ledendecker, Sandra Calderón, C. Papp, H. Steinrück, M. Antonietti, M. Shalom (2015)
The synthesis of nanostructured Ni5 P4 films and their use as a non-noble bifunctional electrocatalyst for full water splitting.
Angewandte Chemie, 54 42
D. Nicholls, M. Rowley, R. Swindells (1966)
Hydrazine complexes of cobalt(II) chloride
Journal of The Chemical Society A: Inorganic, Physical, Theoretical
Liang-Liang Feng, Guangtao Yu, Yuanyuan Wu, Guodong Li, Hui Li, Yuanhui Sun, Tewodros Asefa, Wei Chen, Xiaoxin Zou (2015)
High-index faceted Ni3S2 nanosheet arrays as highly active and ultrastable electrocatalysts for water splitting.
Journal of the American Chemical Society, 137 44
Youwen Liu, H. Cheng, Mengjie Lyu, Shaojuan Fan, Qinghua Liu, Wenshuai Zhang, Yuduo Zhi, Chengming Wang, Chong Xiao, Shiqiang Wei, B. Ye, Yi Xie (2014)
Low overpotential in vacancy-rich ultrathin CoSe2 nanosheets for water oxidation.
Journal of the American Chemical Society, 136 44
Rajesh Bashyam, P. Zelenay (2006)
A class of non-precious metal composite catalysts for fuel cells
Nature, 443
J. Kibsgaard, Zhebo Chen, Benjamin Reinecke, T. Jaramillo (2012)
Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis.
Nature materials, 11 11
Hiram Conley, Bin Wang, Jed Ziegler, R. Haglund, S. Pantelides, K. Bolotin (2013)
Bandgap engineering of strained monolayer and bilayer MoS2.
Nano letters, 13 8
K. Lian, D. Kirk, S. Thorpe (1995)
Investigation of a “Two‐State” Tafel Phenomenon for the Oxygen Evolution Reaction on an Amorphous Ni‐Co Alloy
Journal of The Electrochemical Society, 142
Karel-Alexander Duerloo, Yao Li, E. Reed (2014)
Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers
Nature Communications, 5
J. Park, Eun-Hyuk Chae, S. Kim, J. Lee, J. Kim, S. Yoon, Jae-Young Choi (2006)
Preparation of fine Ni powders from nickel hydrazine complex
Materials Chemistry and Physics, 97
Yanshuo Jin, Haotian Wang, Junjie Li, Xin Yue, Yujie Han, P. Shen, Yi Cui (2016)
Porous MoO2 Nanosheets as Non‐noble Bifunctional Electrocatalysts for Overall Water Splitting
Advanced Materials, 28
Xian Zhang, Shengwen Liu, Y. Zang, Rongrong Liu, Guoqiang Liu, Guozhong Wang, Yunxia Zhang, Haimin Zhang, Huijun Zhao (2016)
Co/Co9S8@S,N-doped porous graphene sheets derived from S, N dual organic ligands assembled Co-MOFs as superior electrocatalysts for full water splitting in alkaline media
Nano Energy, 30
Aaron Wilson, R. Newell, M. McNevin, J. Muckerman, M. DuBois, D. Dubois (2006)
Hydrogen oxidation and production using nickel-based molecular catalysts with positioned proton relays.
Journal of the American Chemical Society, 128 1
Lucas-Alexandre Stern, Ligang Feng, Fang Song, Xile Hu (2015)
Ni2P as a Janus catalyst for water splitting: the oxygen evolution activity of Ni2P nanoparticles
Energy and Environmental Science, 8

Publisher: Springer Journals
Copyright: Copyright © 2017 by The Author(s)
Subject: Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
eISSN: 2041-1723
DOI: 10.1038/ncomms15377
Publisher site: See Article on Publisher Site

Abstract

ARTICLE Received 5 Nov 2016 | Accepted 17 Mar 2017 | Published 9 May 2017 DOI: 10.1038/ncomms15377 OPEN Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulﬁde via hydrazine- induced phase transformation for water splitting 1 2 3 4 1 1 2 1 Haoyi Li , Shuangming Chen , Xiaofan Jia , Biao Xu , Haifeng Lin , Haozhou Yang , Li Song & Xun Wang Highly active and robust eletcrocatalysts based on earth-abundant elements are desirable to generate hydrogen and oxygen as fuels from water sustainably to replace noble metal materials. Here we report an approach to synthesize porous hybrid nanostructures combining amorphous nickel-cobalt complexes with 1T phase molybdenum disulﬁde (MoS ) via hydrazine-induced phase transformation for water splitting. The hybrid nanostructures exhibit overpotentials of 70 mV for hydrogen evolution and 235 mV for oxygen evolution at 10 mA cm with long-term stability, which have superior kinetics for hydrogen- and oxygen-evolution with Tafel slope values of 38.1 and 45.7 mVdec . Moreover, we achieve 10 mA cm at a low voltage of 1.44 V for 48 h in basic media for overall water splitting. We propose that such performance is likely due to the complete transformation of MoS to metallic 1T phase, high porosity and stabilization effect of nickel-cobalt complexes on 1T phase MoS . 1 2 Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China. National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230029, China. 3 4 Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, USA. Department of Chemical and Biological Engineering, Iowa State University, Ames, Iowa 50010, USA. Correspondence and requests for materials should be addressed to X.W. (email: [email protected]). NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 1 N N H H 2 2 4 4 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ydrogen (H ) has attracted extensive attention for decades compounds and the phase of MoS (Fig. 1, see Methods 2 2 1–4 as an environmentally friendly energy source . for synthetic details). With the increasing amount of HZH, HElectrochemical or photo-electrochemical water splitting Ni-Co-based compounds are changed to amorphous complexes is a convenient method to generate hydrogen, converting electricity from a partial metallic state and MoS is completely converted to 5,6 to chemical fuels for energy storage and transport . However, the metallic 1T phase. We propose that the phase transformation of dynamically unfavourable nature of water splitting is regarded as MoS is attributed to the enrichment of amorphous Ni-Co the bottleneck, in which both the hydrogen- and oxygen-evolution complexes with electron-donor ability of hydrazine because of the reactions (HER and OER) need high overpotentials for activation. stabilization effects of the complexes on 1T phase MoS . Metallic Currently, Pt alloys and Ir/Ru oxides are regarded as the state-of- 1T phase MoS can facilitate the electrode kinetics, increase the-art electrocatalysts for HER and OER respectively, but cost and the electric conductivity of the electrocatalysts and proliferate 30,31 scarcity are the barriers for the scale-up utilization of these noble- the catalytic active sites . Concurrently, porous nanostructures 7–9 metal catalysts in industrial deployment . The development of can create more catalytic active sites and improve the mass catalysts for HER and OER with non-noble materials has achieved transport and gas permeability effectively in the process of water 32,33 great success. Molybdenum disulﬁde (MoS ) is one of the most splitting . Moreover, hydrazine involved Ni-Co complexes promising candidates for HER electrocatalysts representing have the amine residues in the second coordination sphere where transition metal dichalcogenides, which has the possibility intramolecular proton transfer takes place preferentially, which is 10–15 to replace Pt-based electrocatalysts for practical applications . beneﬁcial to lower the overpotential of electrocatalytic H 34,35 For OER, low overpotentials and moderate durability have evolution . The large quantity of Ni-Co complexes in been exhibited by transition-metal (especially nickel and cobalt) the PHNCMs is helpful to promote the catalytic activities of 16,17 18,19 20,21 22 sulﬁdes ,selenides , oxides , phosphides and layered HER and OER simultaneously. Therefore, we fabricate the double hydroxides . However, it is quite difﬁcult to obtain high PHNCMs to integrate the advantages of every component in concentrations of H and OH simultaneously to motivate HER electrocatalysis, resulting in the enhancement of performance for þ 14 and OER because they follow the rule of [H ] [OH ] ¼ 10 . overall water splitting. Meanwhile, if both catalysts were employed for electrolysis, the cost would increase because of the complicated process of manufacturing electrodes. Thus, it is quite challenging to develop Results bifunctional electrocatalysts for HER and OER in one electrolyte. Synthesis and characterization of PHNCMs. We ﬁrst synthe- 24–29 Although some progress has been made in this ﬁeld ,more sized the Ni-Co hydroxides ultrathin nanosheets (NCUNs) to efforts should be devoted to designing the catalysts and enhancing provide the precursors and templates for the following synthesis their performance to control the industrial cost and lower the of PHNCMs. The pristine NCUNs showed B4 nm uniform energy consumption. A recent work illustrated that MoS /Ni S thickness and 30–150 nm diameters with circular shape according 2 3 2 heterostructures designed by interface engineering show excellent to atomic force microscopy (AFM) and transmission electron performance for overall water splitting, which could synergistically microscopy (TEM) images (Fig. 1a–c). The crystalline structure of chemisorb hydrogen and oxygen-containing intermediates .This NCUNs was demonstrated through X-ray diffraction pattern material provides the possibility for fabricating new and efﬁcient (Supplementary Fig. 1a). The X-ray photoelectron spectroscopy electrocatalysts by hybridizing nickel-cobalt-based (Ni-Co-based) (XPS) spectrum of O 1s orbital in NCUNs (Supplementary compounds with MoS . Fig. 1b) further conﬁrmed the hydroxide feature of NCUNs Herein, we present a facile strategy to synthesize porous hybrid owing to the peak position at 531.5 eV (ref. 36). nanostructures combining amorphous Ni-Co complexes with 1T After NCUNs reacted with ammonium tetrathiomolybdate phase MoS (denoted as PHNCMs) through hydrazine-inducing, ((NH ) MoS ) in N,N-dimethylformamide (DMF) solvent with 2 4 2 4 which have highly active and ultra-stable electrocatalytic different amount of HZH, the PHNCMs were formed, which performances towards HER and OER. Notably, the PHNCMs exhibited irregular nanosheet-like structures with uneven surface achieve overpotentials of 70 mV for HER and 235 mV for OER at and increasing porosity by TEM and high-resolution TEM 10 mA cm and fast kinetics shown by low Tafel slope values of (HRTEM) images (Fig. 2d–g and Supplementary Fig. 2, PHNCMs 38.1 and 45.7 mV dec for HER and OER respectively. Mean- with no HZH, 0.05 ml of HZH, 1 ml of HZH and 2.5 ml of HZH while, this material holds an overvoltage of 1.44 V to reach a denoted as 0H-PHNCMs, 0.05H-PHNCMs, 1H-PHNCMs and current density of 10 mA cm for 48 h operation without 2.5H-PHNCMs). The hybrid nanostructures maintained the degradation for overall water splitting. nanosheet morphology, as compared to the products synthesized In this work, we introduce hydrazine hydrate (HZH) into the from corresponding metal acetates directly in the same reaction reaction system to regulate the crystallization of Ni-Co-based system as 0H-PHNCMs (Supplementary Fig. 3). It suggested that Nickel hydroxides Cobalt hydroxides (NH ) MoS 4 2 4 Ni metal and tiny Co metal DMF 200 C 2H-MoS 1T-MoS Amorphous Ni-Co complexes Figure 1 | Schematic representation of the formation of PHNCMs. Upper route illustrates the synthesis of Ni metal and tiny Co metal hybridized with MoS in the blended phase of 2H and 1T without HZH using NCUNs and (NH ) MoS as the precursors by a solvothermal method. Lower route 2 4 2 4 demonstrates the synthesis of the hybrid nanostructures of amorphous Ni-Co complexes and 1T phase MoS with large amount of HZH using the same precursors as upper route. 2 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications no N H 2 4 NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE NCUNs were excellent templates for building hybrid nanosheet- correlated to the quantity of HZH based on the TEM images like structures. HZH could decompose into nitrogen (N ) and (Fig. 2d–g) and the corresponding pore size distribution curves ammonia (NH ) at high temperatures, after which N and NH (Fig. 2h–k). When the quantity of HZH reached 2.5 ml, the 3 2 3 bubbles acted as the gas templates resulting in the formation of number of pores increased markedly and pores with larger pores. We inferred that the number and size of pores positively diameters (B10 nm) appeared. Meanwhile, speciﬁc surface areas 4.096 nm ab c 4.092 nm 1 4.092 nm 4.083 nm –2 0 50 100 4.082 nm 4.086 nm 4.094 nm –2 0 60 120 4.083 nm 4.094 nm 4.092 nm –2 0 40 80 Distance (nm) d e f 1T MoS g 2H+1T MoS 2 2H+1T MoS 1T MoS 2 2 2.5 ml HZH 1 ml HZH No HZH 0.05 ml HZH 0.476 nm 0.267 nm MoS (004) 0.476 nm 2 new MoS (101) MoS (004) 2 new 0.476 nm MoS (004) 2 new 0.203 nm 0.476 nm MoS (004) 0.476 nm Ni (111) 2 new 0.307 nm 0.307 nm MoS (004) 0.476 nm 2 new MoS (004) MoS (004) 2 2 MoS (004) 0.307 nm 2 new MoS (004) h j 0.010 i 0.020 k 0.010 0.020 0.008 0.016 0.008 0.015 0.006 0.012 0.006 0.010 0.004 0.008 0.004 0.005 0.002 0.004 0.002 0.000 0.000 0.000 0.000 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Pore size (nm) Pore size (nm) Pore size (nm) Pore size (nm) 0H-PHNCMs N O Ni, JCPDS No.04-0850 MoS , JCPDS No.37-1492 0.05H-PHNCMs (002) new 0.952 nm 1H-PHNCMs (004) new 2.5H-PHNCMs Ni Co Mo 0.476 nm MoS , JCPDS No.37-1492 10 20 30 40 50 60 70 80 90 2 Theta (degree) NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 3 3 –1 –1 dV/dD (cm g nm ) Intensity (a.u.) 3 –1 –1 dV/dD (cm g nm ) 3 –1 –1 dV/dD (cm g nm ) Height (nm) 3 –1 –1 dV/dD (cm g nm ) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 were obtained by the N adsorption–desorption isotherms structure (EXAFS). Structural parameters of the bond lengths, the (Supplementary Fig. 4). 2.5H-PHNCMs had the largest speciﬁc coordination numbers, the Debye-Waller factors and the 2 1 surface area of 90.68 m g among the PHNCMs, leading comparison between experimental data and ﬁtting curves for to more active catalytic sites to enhance the performance of Ni, Co and Mo K-edge were summarized in Supplementary electrocatalysis. Furthermore, when the amount of HZH Table 1 and 2 and Supplementary Figs 10–12. The normalized increased continuously to 3 and 5 ml (denoted as 3H-PHNCMs X-ray absorption near edge structure (XANES) spectra of Ni and 5H-PHNCMs), no big differences were observed in and Co K-edge of the PHNCMs (Supplementary Fig. 13a morphoogies, speciﬁc surface area and pore size distribution and 13b) exhibited the content sequences of Ni and Co metal. curves (Supplementary Fig. 5) as compared with that of According to the intensity of white line and pre-edge peaks, 2.5H-PHNCMs. as compared with the standard samples, they were 0H-PHNCMs Integrating the results of HRTEM images (lower images in 44 1H-PHNCMs 4 0.05H-PHNCMs 4 2.5H-PHNCMs and Fig. 2d–g and Supplementary Fig. 6) and the corresponding X-ray 0H-PHNCMs 4 1H-PHNCMs 4 2.5H-PHNCMs 4 0.05H- diffraction patterns (Fig. 2l and Supplementary Fig. 7), Ni was PHNCMs, respectively for Ni and Co. Fig. 3a,b presented changed to amorphous state from metallic state, new (002) k -weighted Fourier transform (FT) proﬁles of XANES at Ni and and (004) planes of MoS with the lattice distance of 0.952 and Co K-edge of PHNCMs and standard Ni foil, NiO, Co foil, CoO 0.476 nm were observed and pristine (004) planes of MoS as contrastive samples. For 0H-PHNCMs, it was clearly seen that with the lattice distance of 0.307 nm disappeared gradually when Ni-Ni bond with 2.48 Å length were the main existing forms in the amount of HZH increased. The enlargement of interlayer the real space. The peaks at B1.72 Å for Ni in the ﬁrst shell of spacing of MoS indirectly suggested the existence of metallic the three PHNCMs with HZH were attributed to the major 1T phase of MoS in PHNCMs . We further conﬁrmed the contribution of Ni-O or Ni-N bonds. Meanwhile, the peaks for state of Ni and Co in 0H-PHNCMs. The X-ray diffraction Co in the ﬁrst shell of all the PHNCMs upshifted slightly as pattern (Supplementary Fig. 8) of Ni-only hybrid nanostructures compared to the ﬁrst peak position of standard CoO, which synthesized in the same way as 0H-PHNCMs could match with revealed that Co–O or Co-N bonds predominated but there was the standard patterns of MoS and Ni metal and that of Co-only still a small quantity of Co-Co bond. Surface features of Ni and hybrid nanostructures was only in accordance with the standard Co were probed by XPS measurement (Supplementary Fig. 14). pattern of MoS . It is well known that HZH is a strong reductant For 0H-PHNCMs, the peaks at B853.09 and B870.3 eV stood 38–40 as well as a strong coordination ligand . Therefore, the for Ni metal in Ni-Co alloy and the small peak at B856.3 eV introduction of HZH to the reaction system led to the preferential indicated that a little bit of Ni in oxidation state located on the formation of hydrazine coordinated Ni-Co complexes. According surface, while the peaks at B778.5 and B780.7 eV were indexed 41,42 to the X-ray diffraction matching consequences, the crystalline to Co metal and Co in oxidation state . Along with the structure in PHNCMs with HZH just agreed with MoS , which increasing amount of HZH, peaks for both of metallic Ni and Co indicated that Ni and Co existed in the amorphous states. The were attenuated and peaks of oxidation state at higher binding scanning TEM (STEM) images and energy-dispersive X-ray energies became stronger gradually. As Supplementary Fig. 15 (EDX) elemental mapping spectra of PHNCMs (Fig. 2m and shown, Co (III) and Ni (III) predominated and Co (II) and Ni (II) Supplementary Fig. 9) exhibited the elemental composition and also existed on the surface of 2.5H-PHNCMs. However, there uniform distribution of the six elements in PHNCMs. The were still a small quantity of Co and Ni metal, the small peaks of quantity of nitrogen increased signiﬁcantly in PHNCMs with which could be observed in the XPS spectra. This result HZH as compared with that in 0H-PHNCMs that was proven by corresponded well to the FT proﬁles of XANES at Ni and Co elemental proportion analyses of nitrogen (Table 1). The K-edge. During the synthesis of 2.5H-PHNCMs, hydrazine was increasing proportions of nitrogen revealed that it was likely for added dropwise slowly with stirring at the room temperature Ni and Co to form hydrazine coordinated amorphous complexes. before heating. In this case, Co and Ni ions preferentially Therefore, it was quite possible that PHNCMs with large amount coordinated with hydrazine to form complexes rather than being of HZH were composed of amorphous Ni-Co complexes and reduced. At the same time, a small amount of potassium 1T phase MoS , which was further revealed in the following hydroxide (KOH) was fed into the system to make NH form 2 3 characterization. and escape easily at a high temperature. The NH would be a gas template to make sure the formation of porous nanostructures. So the quantity of KOH was not enough to cause the preformed 39,40 Analyses for atomic structure of the PHNCMs. In-depth Ni-Co-hydrazine complexes to be reduced to metallic states . analyses to determine the atomic structure of PHNCMs were When Co and Ni coordinated with hydrazine to form complexes, made by several characterization methods. First, we investigated it is easy for Co (II) and Ni (II) complexes to be oxidized to Co the bonding situation of Ni and Co in PHNCMs through the (III) and Ni (III) ones. Because the redox potential of hydrazine- y 3 þ 2 þ y 3 þ 2 þ synchrotron-radiation-based extended X-ray absorption ﬁne coordinated complexes, c (Co /Co ) and c (Ni /Ni ), Figure 2 | Characterizations of the NCUNs and PHNCMs. (a) TEM image (b) AFM image and (c) the corresponding line-scan proﬁles of NCUNs, showing NCUNs with B4 nm thickness and 30–150 nm diameters. (d,e) TEM (upper) and HRTEM (lower) images of 0H-PHNCMs and 0.05H-PHNCMs showing porous nanosheet-like structures and uneven surface. The lattice distance of 0.476 nm is indexed to new (004) lattice plane of MoS as compared to pristine (004) planes with 0.307 nm of distance. The enlargement of lattice distance suggests that 1T phase MoS exists in the 0H-PHNCMs and 0.05H-PHNCMs. With the increasing amount of HZH to (f) 1 ml and (g) 2.5 ml, TEM and HRTEM images exhibit the disappearance of pristine (004) planes, suggesting the complete transformation of MoS to metallic 1T phase. The distances of 0.203 and 0.267 nm are consistent with the standard spacing of (111) planes in Ni metal and (101) planes in MoS , respectively. (h–k) Pore size distribution curves of 0H-, 0.05H-, 1H- and 2.5H-PHNCMs in accordance with TEM images in d–g.(l) X-ray diffraction patterns of PHNCMs. Lower patterns display the shifts of (002) and (004) peaks (corresponding to the lattice distances of 0.952 and 0.476 nm) and gradual disappearance of pristine (004) peaks at B29 of 2 Theta in the short dash circle, illustrating the increasing interlayer space of MoS .(m) STEM and EDX elemental mapping spectra of 2.5H-PHNCMs showing the porous nanostructures and uniform distribution of N (blue), O (red), S (orange), Ni (yellow), Co (pink) and Mo (cyan). Scale bars, 100 nm (a,b); upper, 20 nm (d–g, upper); 2 nm (lower); 50 nm (m). 4 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE Table 1 | Percentage proportions of nitrogen in different PHNCMs. Samples 0H-PHNCMs 0.05H-PHNCMs 1H-PHNCMs 2.5H-PHNCMs Ratios 3.37% 10.65% 13.92% 14.88% 1 0 was decreased dramatically to the 10 from 10 of magnitude, role of stabilization of 1T phase MoS , which could be further so that hydrazine-coordinated Co (III) and Ni (III) complexes demonstrated by the following results. could exist steadily. When the amount of HZH increased to 3 and 5 ml, Ni and Co in the oxidation states were observed quite similarly (Supplementary Fig. 14). The above analyses further Electrochemical evaluation of the PHNCMs. On the basis of the indicated the existence of hydrazine-coordinated Ni-Co successful synthesis and comprehensive characterization, we complexes in the PHNCMs with HZH. examined HER and OER electrocatalytic activities of the Subsequently, we investigated the local bond length of Mo in PHNCMs. The as-prepared PHNCMs mixing with pure carbon PHNCMs through XANES (Supplementary Fig. 13c) to demon- (Vulcan XC72) were evaluated using a typical three-electrode strate the phase transformation of MoS . According to the system in 1 M KOH media. A saturated calomel electrode (SCE) FT proﬁles and bond lengths at Mo K-edge (Fig. 3c and was used as the reference electrode. It was calibrated with Supplementary Table 2), the nearest Mo-Mo bonds of PHNCMs reversible hydrogen electrode (RHE) and the potentials were showed distinct decreasing lengths from 3.16 to 2.78 Å, which was reported versus RHE (Supplementary Fig. 19). MoS nanosheets in agreement with structural transformation from hexagonal to (with 0.05 ml of HZH), NCUNs, commercial Pt/C and IrO /C tetragonal phase . For 0H- and 0.05H-PHNCMs, the peaks at catalyst were also measured for comparison. 2.5H-PHNCMs 2.82 Å indicated that there was still some 2H phase MoS in the displayed excellent electrocatalytic activity for HER, which hybrid nanostructures. In contrast, MoS in 1H-PHNCMs and approached that of commercial Pt/C catalyst. As a consequence 2.5H-PHNCMs was completely transferred to 1T phase as the of polarization (Fig. 4a), 2.5H-PHNCMs only needed an over- evident disappearance of Mo-Mo peaks at 2.82 Å for 2H phase. potential of 70 mV to reach a current density of 10 mA cm that XPS spectrum of Mo (Fig. 3d and Supplementary Fig. 16a) was much better than that of 0H-PHNCMs (87 mV at displayed the obvious shifts of peaks to lower binding energies 10 mA cm ), not to mention 148 and 128 mV for 0.05H- with the increasing amount of HZH. For 1H-, 2.5H-, 3H- and PHNCMs and 1H-PHNCMs (Supplementary Fig. 20a). After 5H-PHNCMs, two characteristic peaks of Mo 3d and Mo 3d linear ﬁtting for the Tafel plot of 2.5H-PHNCMs, the slope was 5/2 3/2 orbitals were located at B228 and B231.0 eV, which were much calculated to be 38.1 mV dec (Supplementary Fig. 21a), which lower than that of the 2H phase counterparts (B229 and was lower than that of 0H-PHNCMs (40.3 mV dec ), MoS 30,43 1 1 232 eV) . Similar downshifts of binding energies for S 2p nanosheets (46.4 mV dec ) and NCUNs (89.4 mV dec ). Such orbitals were exhibited in Fig. 3e and Supplementary Fig. 16b. a low Tafel slope value illustrated the superior HER kinetics of These downward movements of Mo 3d and S 2p peak positions 2.5H-PHNCMs, further conﬁrming that they were indeed further proved the formation of metallic 1T phase MoS .In excellent HER electrocatalysts. Electrochemical impedance addition, we attempted to demonstrate the reason behind phase spectroscopy (EIS) Nyquist plots (Supplementary Fig. 21b) transformation of MoS . We synthesized MoS nanosheets manifested that 2.5H-PHNCMs had smaller reaction resistance as 2 2 with different amount of HZH in the same way as PHNCMs compared to other contrasting catalysts, suggesting that charge (MoS with no HZH, 0.05 ml of HZH, 1 ml of HZH and 2.5 ml transfer was facilitated in 2.5H-PHNCMs. To assess the stability of HZH denoted as 0H-MoS , 0.05H-MoS , 1H-MoS and of 2.5H-PHNCMs, Chronoamperometric response (Fig. 4c) was 2 2 2 2.5H-MoS ). TEM images (Supplementary Fig. 17a–d) showed recorded at a constant potential of 0.13 V versus RHE for 24 h. the distinct nanosheet morphology of MoS and the tendency to During the initial 3 h, the performance degraded; however, the assemble spherically. The two peaks below 20 in the X-ray current density increased for the remaining time. More efﬁcient diffraction pattern (Supplementary Fig. 17e) revealed that the HER active sites might exist in the interfaces between MoS and enlarged interlayer spacing emerged as compared to pristine Ni-Co complexes. The 3 h reduction allowed more electrolyte MoS , which was in good agreement with that of the PHNCMs. A to access the interfaces, obtaining the increasing HER current binding energy comparison of Mo was illustrated in the XPS density during the following time . To measure electrochemical spectrum (Fig. 3f) of the above a series of MoS nanosheets. active surface area (ECSA), we ﬁrst scanned cyclic voltammetry Inconspicuous downshifts (B0.25 eV) were observed for peaks (CV) cycles in the range of no Faradaic processes (Supplementary of Mo 3d and Mo 3d orbitals between 0H-MoS and Fig. 22), obtained double-layer capacitance (C ) (Fig. 4e) and 5/2 3/2 2 dl 2.5H-MoS , which were far less than that in PHNCMs (B1 eV). then converted it to ECSA. We found that 2.5H-PHNCMs had Analogously, the peaks of S 2p orbitals in XPS spectrum (Supple- the largest ECSA of 807.5 cm as compared to 0H-PHNCMs 2 2 2 mentary Fig. 18) showed the weak downshifts (B0.2 eV). The (502.5 cm ), MoS nanosheets (255 cm ) and NCUNs (45 cm ). above results indicated that 2.5H-MoS was not in pure 1T phase The number of catalytic active sites could be determined roughly and Ni-Co complexes were essential for the completely phase by ECSA. Therefore, 2.5H-PHNCMs possessed the maximum transformation of MoS . active sites for HER among all of the contrasts. Supplementary It has been reported that electron donor is quite necessary in Fig. 23a showed the comparison between the theoretical amount the phase transformation of MoS from 2H to 1T phase . In our of H calculated from a chronopotentiometric response and the 2 2 case, hydrazine could be regarded as the electron donor to induce evolved quantity of H experimentally measured from a gas the phase transformation. However, if NCUNs were not chromatography in the process of HER over 2.5H-PHNCMs for introduced to the reaction system of 2.5H-PHNCMs, pure 1T 120 min. The experimental values were observed to extremely phase MoS could not have been obtained. Compared to 2H approach to the theoretical values. This result indicated that phase MoS , 1T phase MoS has higher ground-state energy .It 2.5H-PHNCMs provided a Faradaic efﬁciency of E100% for the 2 2 is extremely possible that amorphous Ni-Co complexes play the HER. It is a convincing evidence of water splitting, which means NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 5 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ab 0H-PHNCMs Ni-Ni 0H-PHNCMs 0.05H-PHNCMs Co-Co 0.05H-PHNCMs 1H-PHNCMs 1H-PHNCMs 2.5H-PHNCMs 2.5H-PHNCMs Standard Ni foil Standard Co foil Standard NiO Standard CoO Ni-O/N Co-N/O 02 46 02 48 R(Å) R(Å) cd Mo-Mo 0H-PHNCMs Mo-S 0H-PHNCMs 0.05H-PHNCMs 0.05H-PHNCMs 1H-PHNCMs 1H-PHNCMs 2.5H-PHNCMs 2.5H-PHNCMs 2H phase MoS foil 2 Mo 3d 3/2 Mo 3d 5/2 S 2s Mo-O/N Mo-Mo(1T) Mo-Mo(2H) 224 226 228 230 232 234 236 238 024 68 R(Å) Binding Energy (eV) ef 0H-MoS 0H-PHNCMs 2 0.05H-PHNCMs 0.05H-MoS 1H-PHNCMs 1H-MoS Mo 3d S 2p 2.5H-PHNCMs 5/2 3/2 2.5H-MoS Mo 3d 3/2 S 2p 1/2 S 2s 158 160 162 164 166 168 170 172 224 226 228 230 232 234 236 238 Binding Energy (eV) Binding Energy (eV) Figure 3 | Characterization for atomic structure of the PHNCMs. (a–c) The k -weighted FTspectra of XANES from EXAFS at the Ni, Co and Mo K-edge of the PHNCMs and Ni foil, NiO, Co foil, CoO, 2H phase MoS foil as contrasting samples. A large quantity of Ni-Ni bond and tiny amount of Co-Co bond possess the major contribution in 0H-PHNCMs. Ni-O or Ni-N and Co-O or Co-N are the main bond for Ni and Co in PHNCMs with HZH. The third and fourth peaks at B2.40 Å and B2.85 Å of 0H-PHNCMs and 0.05H-PHNCMs for Mo-Mo bond indicate that 1T and 2H phase MoS coexist in the samples. The disappearance of the fourth peaks at B2.85 Å of 1H and 2.5H-PHNCMs illustrates the complete phase transformation of MoS to metallic 1T phase. The smoothing XPS spectra showing the binding energies of (d) Mo and (e) S in the PHNCMs. Obvious downshifts (B1 eV) of Mo 3d and S 2p peak positions demonstrates the phase transformation from 2H to 1Tphase. (f) The smoothing XPS spectrum of Mo 3d orbitals in MoS synthesized in the same way as PHNCMs. With the increasing quantity of HZH, the peaks of Mo 3d and Mo 3d orbitals in MoS have slight downshifts (B0.25 eV) to lower 3/2 5/2 2 binding energies, suggesting that amorphous Ni-Co complexes were the main cause in the phase conversion of MoS . 6 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 3 3 FT(k χ(k)) FT(k χ(k)) Intensity (a.u.) FT(k χ(k)) Intensity (a.u.) Intensity (a.u.) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE ab 0 2.5H-PHNCMs 2.5H-PHNCMs 0H-PHNCMs 0H-PHNCMs MoS nanosheets NCUNs MoS nanosheets –20 NCUNs Commercial Pt/C Commercial IrO /C –40 –60 –80 –100 1.3 1.4 1.5 1.6 1.7 1.8 1.9 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 Potential (V) vs RHE Potential (V) vs RHE cd –10 –20 40 –30 30 –40 20 –50 10 0 8 16 24 0 8 16 24 Time (h) Time (h) ef –2 2.5H-PHNCMs, C =32.3 mF cm –2 dl 2.5H-PHNCMs, C =108.1 mF cm dl –2 0H-PHNCMs, C =20.1 mF cm 1.2 dl –2 0H-PHNCMs, C =73.9 mF cm dl –2 MoS nanosheets, C =10.2 mF cm –2 2 dl NCUNs, C =42.8 mF cm dl –2 NCUNs, C =1.8 mF cm –2 dl 3 MoS nanosheets, C =2.0 mF cm 2 dl 1.8 1.4 0.0 0 102030 0 10 20 30 –1 –1 Scan rate (mV S ) Scan rate (mV S ) Figure 4 | Electrocatalytic hydrogen and oxygen evolution of different catalysts. (a,b) Polarization curves for HER and OER measured at a scan rate of 5mVs in 1 M KOH electrolyte; (c,d) chronoamperometric responses (i-t) recorded on 2.5H-PHNCMs for 24 h at a constant applied potential of 0.13 V versus RHE for HER and 1.53 V versus RHE for OER; (e,f) the ﬁtting plots showing C for HER and OER. dl that the cathodic currents over 2.5H-PHNCMs derived from the which outstandingly ranked as the best among reported OER 16–23 hydrogen evolution. catalysts and was much better than that of commercial In addition to the excellent performance for HER electro- IrO /C catalyst. A performance comparison of PHNCMs for catalysis, 2.5H-PHNCMs were also efﬁcient electrocatalysts for OER polarization (Supplementary Fig. 20b) was made and it was OER. During polarization process (Fig. 4b), 2.5H-PHNCMs found that 2.5H-PHNCMs and 0H-PHNCMs showed better could afford a current density of 10 mA cm with an activities and the current densities of them tended to be the overpotential of 235 mV and kept the activity for 24 h (Fig. 4d), same after 1.6 V. At the same time, the Tafel slope value of NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 7 –2 –2 –2 Current density (mA cm ) . Current density (mA cm ) Current density (mA cm ) –2 –2 Current density (mA cm ) –2 Current density (mA cm ) Current density (mA cm ) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 2.5H-PHNCMs (45.7 mV dec ) was the smallest among all of 2.5H-PHNCMs the tested catalysts (Supplementary Fig. 21c), implying that they 0H-PHNCMs had superior OER kinetics. Moreover, EIS Nyquist plots Commercial IrO /C-Pt/C couple (Supplementary Fig. 21d) showed that they made electrons 40 –2 3 mg cm loading transfer easier according to the o 30 ohm reaction resistance. Similarly, we measured the C to estimate the ECSA of dl each sample for OER (Fig. 4d and Supplementary Fig. 24). Because of the larger proportion of Ni and Co as compared to Mo (Supplementary Table 3), PHNCMs had more catalytic active sites for OER than that for HER. 2.5H-PHNCMs possessed 2,702.5 cm of ECSA, which was much larger than that of others 2 2 2 (1,847.5 cm of 0H-PHNCMs, 1,070 cm of NCUNs and 50 cm of MoS nanosheets). For the Faradaic efﬁciencies for OER, the contrast between the measured amount of O production and the theoretical values was shown in Supplementary Fig. 23b and Supplementary Table 4. It was found that the experimental values 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 were a little less than the theoretical values due to the complex Voltage (V) four-electron reaction process of OER, whose kinetics was quite unfavourable. The Faradaic efﬁciencies at the different periods were also calculated and the average Faradaic efﬁciency was –2 1 mg cm loading 91.23%. We also measured the electrocatalytic performances for HER and OER over 3H- and 5H-PHNCMs and made a comparison with 2.5H-PHNCMs. It was observed that the three PHNCMs exhibited quite similar activities for HER and OER electro- catalysis (Supplementary Fig. 25). Signiﬁcant evaluation indexes were summarized in Supplementary Table 5. 0 8 16 24 Furthermore, We made a comparison of XPS spectra –2 3 mg cm loading (Supplementary Fig. 26a,b) of Mo 3d orbitals in 2.5H-PHNCMs and 2.5H-MoS before and after 1,000 OER cycles. The positions of the two characteristic peaks of Mo 3d and Mo 3d orbitals 5/2 3/2 15 in 2.5H-PHNCMs did not change after 1,000 OER cycles. However, upshifts (B0.4 eV) were observed for the peaks of Mo 3d and Mo 3d orbitals in 2.5H-MoS after 1,000 OER cycles, 5/2 3/2 2 which revealed in situ electrochemical oxidation of 2.5H-MoS 016 32 48 during OER process. Besides, new (002) and (004) planes with the Time (h) enlargement lattice distance of 0.952 and 0.476 nm were observed in X-ray diffraction pattern (Supplementary Fig. 26c) and Figure 5 | Bifunctional electrocatalysts for overall water splitting. (a) Steady-state polarization curves of the catalysts on CFP with HRTEM image (Supplementary Fig. 26d) of 2.5H-PHNCMs after 1,000 OER cycles, which could further conﬁrm that MoS 1mgcm of mass loading for overall water splitting at a scan rate of 5mVs in 1 M KOH electrolyte with a two-electrode conﬁguration after in 2.5H-PHNCMs remained 1T phase after 1,000 OER cycles. a 20 min open circuit scan. The green dash line represents the polarization The above illustration demonstrated that amorphous Ni-Co curve of 2.5H-PHNCMs with 3 mg cm of mass loading, which holds just complexes played a role of stabilization of metallic 1T phase 1.44 V to reach a current density of 10 mA cm .(b) Chronoamperometric MoS . 2 2 curves of 2.5H-PHNCMs with 1 mg cm (upper) and 3 mg cm (lower) Such excellent electrocatalytic performance should be attrib- of mass loading for overall water splitting in a two-electrode conﬁguration uted to the contribution of both components in 2.5H-PHNCMs. at constant cell voltages of 1.49 and 1.44 V, respectively. On one hand, metallic 1T phase MoS can increase the electric 30,31 conductivity and catalytic active sites , which was proven by the low electrochemical reaction resistances and large ECSAs. On the other hand, large proportions of Ni and Co result in Evaluation of PHNCMs for overall water splitting. PHNCMs marvelous electrocatalytic OER activity. Here we propose a were also used as both cathodic and anodic materials for overall possible mechanism of intramolecular proton transfer in hydra- water splitting with a two-electrode conﬁguration in 1 M KOH zine coordinated Ni-Co complexes (Supplementary Fig. 27) that electrolyte. 2.5H-PHNCMs achieved a current density of 34,35 2 2 beneﬁcially lowers the overpotential of HER . Our Ni-Co 10 mA cm at a cell voltage of 1.48 V with 1 mg cm of mass complexes have amine residues in the second coordination loading (Fig. 5a). They had higher catalytic activity than sphere, which could be a hydrogen-exchanging site. As shown in 0H-PHNCMs (1.53 V at 10 mA cm ) and commercial IrO /C- Supplementary Fig. 15, Co (III) and Ni (III) predominated in Pt/C couple (1.63 V at 10 mA cm ). According to the polar- 2.5H-PHNCMs. The catalytic trivalent metal linking to hydrazine ization curves of the PHNCMs (Supplementary Fig. 28a), obtains two electrons to become the monovalent metal. Then, two 2.5H-PHNCMs and 0H-PHNCMs exhibited lower overpotential protons combine with the monovalent metal and nitrogen atom than that of 0.05H-PHNCMs and 1H-PHNCMs, in accordance of the amine residue in the second coordination sphere to form with the consequences of HER and OER electrocatalysis. A long- a metal hydride and a –NH group. Subsequently, the two term electrolysis process utilizing 2.5H-PHNCMs was operated at protons tend to combine with each other forming H . In this a constant potential of 1.49 V for 24 h (upper image in Fig. 5b), way, hydrazine coordinated Ni-Co complexes can facilitate the which showed remarkable durability with negligible degradation. electrocatalysis for HER, thereby enhancing the performance of However, the current density of commercial IrO /C-Pt/C couple overall water splitting. decreased continuously during a 7 h operation (Supplementary 8 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications –2 –2 Current density (mA cm ) Current density (mA cm ) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE Fig. 28b). After 24 h operation, many H and O bubbles Synthesis of MoS nanosheets. A total of 20 mg of (NH ) MoS was dispersed in 2 2 2 4 2 4 10 ml of DMF. The mixture was stirred at room temperature for 10 min until a remained on the carbon ﬁbre paper (CFP) daubed by homogeneous solution was achieved. After that, 0, 0.05 ml, 1 and 2.5 ml of HZH 2.5H-PHNCMs with 1 mg cm of mass loading (Supplementary were added to four of the above mixture respectively. The reaction solution Fig. 29). In addition, the activity of 2.5H-PHNCMs could be was further stirred for 10 min before transferred to 40 ml Teﬂon-lined autoclaves. further improved by increasing the mass loading of the active Then they were heated at 200 C for 10 h. After the system was cooling down, the suspension was centrifuged and washed with ethanol for three times. material on CFP to 3 mg cm (the green dash line in Fig. 5a). Then the precipitant was obtained and re-dispersed in ethanol for further The increasing mass loading decreased the overvoltage from 1.48 characterizations. to 1.44 V at a current density of 10 mA cm . It is noteworthy that this electrode was durable for 48 h without degradation Characterizations. The morphologies and structures of the samples were observed (lower image in Fig. 5b). Furthermore, we investigated the overall by TEM, which was conducted on a Hitachi H7700 at 100 kV, using the carbon- water splitting electrocatalytic performances of 3H- and coated copper grid. Details of morphologies and structures were obtained by 5H-PHNCMs with 3 mg cm of mass loading and made HRTEM that was carried out on a FEI G2 F20 S-Twin TEM at 200 kV equipped with high angle annular dark-ﬁeld STEM. EDX were taken with the same instru- a contrast with 2.5H-PHNCMs. The extremely close activities ment as HRTEM. AFM was conducted on a Dimension Icom, Bruker. Powder of the three PHNCMs were exhibited in Supplementary Fig. 30. X-ray diffraction characterization was performed on a Bruker D8 Advance X-ray Large volumes of H and O gases on the surface of electrodes 2 2 diffractometer using Cu-Ka radiation (l ¼ 1.5418 Å). XPS were recorded on a PHI over 2.5H-PHNCMs could be seen during the process of Quantera SXM spectrometer with monochromatic Al Ka X-ray sources (1,486.6 eV) at 2.0 kV and 20 mA. N adsorption/desorption measurements were a chronoamperometric test (Supplementary Movie 1, a video carried out on a Autosorb-iQ2, Quantachrome Instruments. The proportions of water electrolysis for around 2 min). The above results of N and Co, Ni, Mo were measured by Elemental Analyzer, Euro EA3000 and demonstrated that 2.5H-PHNCMs had a great potential to ICP-AES, VISTA-MPX, respectively. Ni, Co and Mo K-edge XAFS measurements serve effectively for the practical and long-term application of were made at the beamline 14W1 in 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). The X-ray was monochromatized by a double-crystal Si overall water splitting. (111) monochromator for BSRF. The energy was calibrated using a cobalt metal foil for the Co K-edge, a nickel metal foil for the Ni K-edge and a molybdenum Discussion metal foil for the Mo K-edge. The monochromator was detuned to reject higher harmonics. The acquired EXAFS data were processed according to the standard In summary, we have developed a facile approach to fabricate procedures using the WinXAS3.1 program . Theoretical amplitudes and amorphous Ni-Co complexes hybridized with 1T phase MoS as 2 48 phase-shift functions were calculated with the FEFF8.2 code using the crystal highly active bifunctional electrocatalysts for overall water splitting. structural parameters of the Co, CoO, Ni, NiO, MoO and MoS . 3 2 The hybrid nanostructures exhibit an extremely low overpotential and long-term stability for both HER and OER, which can be Preparation of samples for HER and OER electrocatalysis. 5 mg of the active attributed to complete conversion of MoS to metallic 1T phase, 2 material and 1 mg of pure carbon (Vulcan XC72) were added into 0.95 ml of mixture solution of water and ethanol with the ratio of 3:1 and sonicated for at least increasing number of catalytic active sites and stabilization effect of 30 min. When they were dispersed uniformly, 0.05 ml of naﬁon D-521 dispersion amorphous Ni-Co complexes on 1T phase MoS . This catalyst only (5% w/w in water and 1-propanol) was added into the above mixture and then needs an overpotential of 1.44 V to afford an overall-water-splitting sonicated for at least 30 min. After that, 0.006 ml of the mixture solution was drop 2 2 current density of 10 mA cm with a mass loading of 3 mg cm casted on a rotating disk electrode (RDE) with glassy carbon, which has 0.196 cm and maintains its catalytic activity for 48 h operation without of effective area. When the sample solution dried naturally, it could be used as the working electrode for HER and OER electrocatalysis. degradation. The accessible and low-cost manufacturing as well as the excellent electrocatalytic performance may further inspire the Preparation of samples for overall water splitting. A total of 1 mg of development of non-noble-metal electrode materials for overall active material, 0.2 mg of acetylene black and 0.15 mg of polyvinylideneﬂuoride water splitting. were mixed using 0.04 ml of N-methyl-2-pyrrolidone as the solvent to yield a slurry. Then the slurry was daubed uniformly in 1 cm of area on a piece of 1 3 cm CFP and dried in vacuum at 100 C for 24 h. After that, it was Methods immersed in 1 M KOH solution for 12 h in vacuum for activation. Then it could be Reagents. All the reagents were purchased from Alfa Aesar in analytical grade and used as cathode and anode respectively for overall water splitting and the active used as received without further puriﬁcation. 2 2 area was 1 cm . The electrode with 3 mg cm of mass loading was prepared by changing the amount of active material, acetylene black and polyvinylidene Synthesis of NCUNs. In a typical synthesis of NCUNs, 0.8 ml of Nickel(II) acetate diﬂuoride to 3 mg, 0.6 and 0.45 mg, respectively and other procedures were the hydrate aqueous solution (0.2 M) and 0.8 ml of Cobalt(II) acetate tetrahydrate same as the above. aqueous solution (0.2 M) were added to 8 ml of anhydrous DMF and transferred into a 50 ml-ﬂask. An amount of 74 mg (2 mmol) of ammonium ﬂuoride (NH F) Electrochemical characterizations. HER and OER electrocatalysis were measured and 120 mg (2 mmol) of urea (CO(NH ) ) were dissolved into 0.3 ml of deionized 2 2 in a typical three-electrode conﬁguration with a Princeton PASTAT4000 instru- water, respectively. The NH F and CO(NH ) aqueous solution were dropwise 4 2 2 ment and a RDE system. Glassy carbon electrode with active sample was selected as added into the above mixture. Then the mixture was kept at 90 C with stirring for the working electrode, SCE as the reference electrode and a graphite rod as the 24 h. After the system was cooling down naturally, the suspension was centrifuged counter electrode. Overall water splitting was performed in a two-electrode system and washed with ethanol for three times. Then the precipitant was obtained and using the CFP with active sample as both the cathode and anode. For HER and re-dispersed in ethanol for further characterizations. Moreover, some precipitant OER, all of the polarization curves were measured in 1 M KOH solution using RDE was dried in a freezer dryer with vacuum for the next step reaction. For Ni-only or with 1,600 r.p.m. at a scan rate of 5 mV s . Chronoamperometric responses (i-t) Co-only hybrids, the precursors were synthesized in the same way above without were recorded on 2.5H-PHNCMs using RDE with 1,600 r.p.m. for 24 h at adding Cobalt(II) acetate tetrahydrate aqueous solution or Nickel(II) acetate a constant applied potential of 0.13 V versus RHE for HER and 1.53 V versus hydrate aqueous solution. RHE for OER. To measure electrochemical double-layer capacitance (C ), the dl potentials were swept for a cycle using RDE at 1,600 r.p.m. at a range of no faradic Synthesis of PHNCMs. First, 25 mg of NCUNs powders and 13 mg of processes six times at six different scan rates (5, 10, 15, 20, 25 and 30 mV s ). The measured capacitive current densities at the average potential in the selected range (NH ) MoS were dispersed in 10 ml of DMF and sonicated for 10 min to be 4 2 4 a homogeneous solution. After sonicating, 0, 0.05 ml, 1 ml, 2.5 ml of HZH was were plotted as a function of the scan rates and the slope of the linear ﬁt could be calculated as the C . The speciﬁc capacitance was generally found to be in the respectively added dropwise into above mixture with vigorously stirring and then dl 2 2 0.1 ml of potassium hydroxide (KOH) aqueous solution (including 20 mg of KOH) range of 20–60 mFcm and we used the average value of 40mFcm here. According to the following equation , was dropwise fed into the mixture with HZH, respectively. Then the mixture was separately transferred into 40 ml Teﬂon-lined autoclaves and heated dl ECSA ¼ cm ð1Þ at 200 C for 10 h. After the system was cooling down, the suspension was ECSA 40mF cm centrifuged and washed with ethanol for three times. Then the precipitant was obtained and re-dispersed in ethanol for further characterizations. Ni-only or we could obtain the ECSA roughly. EIS experiments were performed in the Co-only hybrids were synthesized by the same method above with Ni-only or frequency range of 10 kHz–100 mHz at a constant current density of 1 mA cm . Co-only precursors. All of the Tafel plots were measured in 1 M KOH solution using RDE with NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 9 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 1,600 r.p.m. at a scan rate of 1 mV s . As for the Faradaic efﬁciency 22. Popczun, E. J., Read, C. G., Roske, C. W., Lewis, N. S. & Schaak, R. E. Highly measurements, gas chromatography was used to determine the experimentally active electrocatalysis of the hydrogen evolution reaction by cobalt phosphide evolved amount of H and O . And we used the Faraday law to calculate the 2 2 nanoparticles. Angew Chem. Int. Ed. Engl. 126, 5531–5534 (2014). theoretical amount of H and O expected based on a chronopotentiometric 2 2 23. Song, F. & Hu, X. Exfoliation of layered double hydroxides for enhanced response at the constant current density of 20 mA cm for 120 min. For overall oxygen evolution catalysis. Nat. Commun. 5, 4477–4485 (2014). water splitting, all of the polarization curves were measured in 1 M KOH solution 24. Ledendecker, M. et al. The synthesis of nanostructured Ni P ﬁlms and their 5 4 at a scan rate of 5 mV s after an open circuit scan for 20 min. use as a non-noble bifunctional electrocatalyst for full water splitting. Angew Chronoamperometric curves of 2.5H-PHNCMs with 3 and 1 mg cm of mass Chem. Int. Ed. Engl. 127, 12538–12542 (2015). loading and commercial IrO /C-Pt/C couple were carried out at a constant cell 25. Wang, H. et al. Bifunctional non-noble metal oxide nanoparticle voltage of 1.44, 1.49 and 1.65 V. electrocatalysts through lithium-induced conversion for overall water splitting. Nat. Commun. 6, 7261–7268 (2015). 26. Stern, L. A., Feng, L., Song, F. & Hu, X. Ni P as a Janus catalyst for water RHE calibration. In all electrochemical characterizations, SCE was used as the splitting: the oxygen evolution activity of Ni P nanoparticles. Energy Environ. reference electrode and calibrated with RHE. The calibration was taken in the Sci. 8, 2347–2351 (2015). hydrogen-saturated electrolyte with the working electrode of a Pt wire. A single cycle 27. Jin, Y. et al. Porous MoO nanosheets as non-noble bifunctional electrocatalysts of cyclic voltammetry was measured at a scan rate of 1 mV s , and the average of for overall water splitting. Adv. Mater. 28, 3785–3790 (2016). the two potentials where the current crossed zero was regarded as the potential of the 28. Jaramillo, T. F. et al. Gold-supported cerium-doped NiO catalysts for water calibration value. In 1 M KOH electrolyte, E(RHE) ¼ E(SCE) þ 1.024 V. oxidation. Nat. Energy 1, 16053–16060 (2016). 29. Zhang, J. et al. Interface engineering of MoS /Ni S heterostructures for highly 2 3 2 enhanced electrochemical overall-water-splitting activity. Angew Chem. Int. Ed. Data availability. The data reported by this article are available from the corre- Engl. 128, 6814–6819 (2016). sponding author upon reasonable request. 30. Acerce, M., Voiry, D. & Chhowalla, M. Metallic 1T phase MoS nanosheets as supercapacitor electrode materials. Nat. Nanotechnol. 10, 313–318 (2015). 31. Voiry, D. et al. Conducting MoS nanosheets as catalysts for hydrogen References evolution reaction. Nano Lett. 13, 6222–6227 (2013). 1. Winsche, W. E., Hoffman, K. C. & Salzano, F. J. Hydrogen: its future role in the 32. Sasikumar, G., Ihm, J. W. & Ryu, H. Optimum Naﬁon content in PEM fuel cell nations energy economy. Science 180, 1325–1332 (1973). electrodes. Electrochim. Acta 50, 601–605 (2004). 2. Schlapbach, L. & Zu¨ttel, A. Hydrogen-storage materials for mobile applications. 33. Zhang, X. et al. Co/Co S @S,N-doped porous graphene sheets derived from S, 9 8 Nature 414, 353–358 (2001). N dual organic ligands assembled Co-MOFs as superior electrocatalysts for full 3. Bashyam, R. & Zelenay, P. A class of non-precious metal composite catalysts water splitting in alkaline media. Nano Energy 30, 93–102 (2016). for fuel cells. Nature 443, 63–66 (2006). 34. Wilson, A. D. et al. Hydrogen oxidation and production using nickel-based 4. Chu, P. & Majumdar, A. Opportunities and challenges for a sustainable energy molecular catalysts with positioned proton relays. J. Am. Chem. Soc. 128, future. Nature 488, 294–303 (2012). 358–366 (2006). 5. Maeda, K. et al. Photocatalyst releasing hydrogen from water. Nature 440, 35. Jacques, P. A. et al. Cobalt and nickel diimine-dioxime complexes as molecular 295–295 (2006). electrocatalysts for hydrogen evolution with low overvoltages. Proc. Natl Acad. 6. Mallouk, T. E. Water electrolysis: divide and conquer. Nat. Chem. 5, 362–363 Sci. USA 106, 20627–20632 (2009). (2013). 36. Tan, B. J., Klabunde, K. J. & Sherwood, P. M. A. X-ray photoelectron 7. Greeley, J., Jaramillo, T. F., Bonde, J., Chorkendorff, I. & Norskov, J. K. spectroscopy studies of solvated metal atom dispersed catalysts. Monometallic Computational high-throughput screening of electrocatalytic materials for iron and bimetallic iron-cobalt particles on alumina. Chem. Mater. 2, 186–191 hydrogen evolution. Nat. Mater. 5, 909–913 (2006). (1990). 8. Zeng, K. & Zhang, D. Recent progress in alkaline water electrolysis for 37. Liu, Q. et al. Gram-scale aqueous synthesis of stable few-layered 1T-MoS : hydrogen production and applications. Prog. Energy Combust. Sci. 36, 307–326 applications for visible-light driven photocatalytic hydrogen evolution. Small (2010). 11, 5556–5564 (2015). 9. Jiao, Y., Zheng, Y., Jaroniec, M. & Qiao, S. Z. Design of electrocatalysts for 38. Nicholls, D., Rowley, M. & Swindells, R. Hydrazine complexes of cobalt(II) oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. chloride. J. Chem. Soc. A 950–952 (1966). 44, 2060–2086 (2015). 39. Park, J. W. et al. Preparation of ﬁne Ni powders from nickel hydrazine 10. Jaramillo, T. F. et al. Identiﬁcation of active edge sites for electrochemical H 2 complex. Mater. Chem. Phys. 97, 371–378 (2006). evolution from MoS nanocatalysts. Science 317, 100–102 (2007). 40. Wu, Z. G., Munoz, M. & Montero, O. The synthesis of nickel nanoparticles by 11. Karunadasa, H. I. et al. A molecular MoS edge site mimic for catalytic 2 hydrazine reduction. Adv. Powder Technol. 21, 165–168 (2010). hydrogen generation. Science 335, 698–702 (2012). 41. Lian, K. K., Kirk, D. W. & Thorpe, S. J. Investigation of a ‘two-state’ Tafel 12. Kibsgaard, J., Chen, Z., Reinecke, B. N. & Jaramillo, T. F. Engineering the phenomenon for the oxygen evolution reaction on an amorphous Ni-Co alloy. surface structure of MoS to preferentially expose active edge sites for J. Electrochem. Soc. 142, 3704–3712 (1995). electrocatalysis. Nat. Mater. 11, 963–969 (2012). 42. Biesinger, M. C. et al. Resolving surface chemical states in XPS analysis of ﬁrst 13. Xie, J. et al. Controllable disorder engineering in oxygen-incorporated MoS 2 row transition metals, oxides and hydroxides: Cr, Mn, Fe, Ni and Co. Appl. ultrathin nanosheets for efﬁcient hydrogen evolution. J. Am. Chem. Soc. 135, Surf. Sci. 257, 2717–2730 (2011). 17881–17888 (2013). 43. Eda, G. et al. Photoluminescence from chemically exfoliated MoS . Nano Lett. 14. Xie, J. et al. Defect-rich MoS ultrathin nanosheets with additional active 2 11, 5111–5116 (2011). edge sites for enhanced electrocatalytic hydrogen evolution. Adv. Mater. 25, 44. Conley, H. J. et al. Bandgap engineering of strained monolayer and bilayer 5807–5813 (2013). MoS . Nano Lett. 13, 3626–3630 (2013). 15. Lu, Z. et al. Ultrahigh hydrogen evolution performance of under-water 45. Reed, E. J. et al. Structural phase transitions in two-dimentional Mo- and ‘superaerophobic’ MoS nanostructured electrodes. Adv. Mater. 26, 2 W-dichalcogenide monolayers. Nat. Commun. 5, 4214–4222 (2014). 2683–2687 (2014). 46. Gao, M. R. et al. An efﬁcient molybdenum disulﬁde/cobalt diselenide hybrid 16. Ganesan, P., Prabu, M., Sanetuntikul, J. & Shanmugam, S. Cobalt sulﬁde catalyst for electrochemical hydrogen generation. Nat. Commun. 6, 5982–5988 nanoparticles grown on nitrogen and sulfur codoped graphene oxide: an (2015). efﬁcient electrocatalyst for oxygen reduction and evolution reactions. ACS 47. Ressler, T. WinXAS: a program for X-ray absorption spectroscopy data analysis Catal. 5, 3625–3637 (2015). under MS-Windows. J. Synchrotron Radiat. 5, 118–122 (1998). 17. Feng, L.-L. et al. High-index faceted Ni S nanosheet arrays as highly active 3 2 48. Ankudinov, A. L., Ravel, B., Rehr, J. J. & Conradson, S. D. Real-space multiple- and ultrastable electrocatalysts for water splitting. J. Am. Chem. Soc. 137, scattering calculation and interpretation of X-ray-absorption near-edge 14023–14026 (2015). structure. Phys. Rev. B 58, 7565–7576 (1998). 18. Liu, Y. et al. Low overpotential in vacancy-rich ultrathin CoSe nanosheets for 2 49. Kibsgaard, J. & Jaramillo, T. F. Molybdenum phosphosulﬁde: an active, water oxidation. J. Am. Chem. Soc. 136, 15670–15675 (2014). acid-stable, earth-abundant catalyst for the hydrogen evolution reaction. Angew 19. Xu, K. et al. Solution-liquid-solid synthesis of hexagonal nickel selenide Chem. Int. Ed. Engl. 53, 14433–14437 (2014). nanowire arrays with a nonmetal catalyst. Angew Chem. In. Ed. Engl. 128, 1742–1745 (2016). 20. Li, Y., Hasin, P. & Wu, Y. Ni Co O nanowire arrays for electrocatalytic Acknowledgements x 3 x 4 oxygen evolution. Adv. Mater. 22, 1926–1929 (2010). This work was supported by NSFC (21431003, 21521091, 11605201, U1532112), China 21. Yeo, B. S. & Bell, A. T. Enhanced activity of gold-supported cobalt oxide for Ministry of Science and Technology under Contract of 2016YFA0202801 and the State the electrochemical evolution of oxygen. J. Am. Chem. Soc. 133, 5587–5593 Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011). (2014CB848900). 10 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15377 ARTICLE How to cite this article: Li, H. et al. Amorphous nickel-cobalt complexes hybridized with Author contributions 1T-phase molybdenum disulﬁde via hydrazine-induced phase transformation for water H. Li. and S.C. contributed equally to this work. X.W. led the research. H. Li. and X.W. splitting. Nat. Commun. 8, 15377 doi: 10.1038/ncomms15377 (2017). conceived the idea. H. Li. planned and performed the experiments, collected the data and analysed the data. S.C. and L.S. collected and analysed the EXAFS data. X.J. proposed and analysed the mechanism of intramolecular proton transfer. H. Li., X.J., B.X., H. Lin, H.Y. Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afﬁliations. and X.W. co-wrote the manuscript. All authors gave approval to the ﬁnal version of the manuscript. This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this Additional information article are included in the article’s Creative Commons license, unless indicated otherwise Supplementary Information accompanies this paper at http://www.nature.com/ in the credit line; if the material is not included under the Creative Commons license, naturecommunications users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ Competing interests: The authors declare no competing ﬁnancial interests. Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/ r The Author(s) 2017 NATURE COMMUNICATIONS | 8:15377 | DOI: 10.1038/ncomms15377 | www.nature.com/naturecommunications 11

Journal

Nature Communications – Springer Journals

Published: May 9, 2017

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

Learn More →

Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulfide via hydrazine-induced phase transformation for water splitting

Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulfide via hydrazine-induced phase transformation for water splitting

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

Learn More →

Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulfide via hydrazine-induced phase transformation for water splitting

Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulfide via hydrazine-induced phase transformation for water splitting

References (57)

Abstract

Journal

Recommended Articles

There are no references for this article.

Our policy towards the use of cookies