Cobalt Molybdenum Oxide Derived High-Performance Electrocatalyst

Apr 26, 2018 - The design and synthesis of high-performance hydrogen evolution reaction (HER) catalysts requires an overall consideration of intrinsic...
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Cobalt Molybdenum Oxide-Derived High-Performance Electrocatalyst for Hydrogen Evolution Reaction Mingjie Zang, Ning Xu, Guoxuan Cao, Zhengjun Chen, Jie Cui, Li-Yong Gan, Hongbin Dai, Xianfeng Yang, and Ping Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00949 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Cobalt Molybdenum Oxide-Derived High-Performance Electrocatalyst for Hydrogen Evolution Reaction Mingjie Zang,[a] Ning Xu,[a] Guoxuan Cao,[a] Zhengjun Chen,[a] Jie Cui,[b] Liyong Gan,*[a] Hongbin Dai,[a] Xianfeng Yang,[b] and Ping Wang*[a] a

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, P. R. China b Analytical and Testing Centre, South China University of Technology, Guangzhou 510641, P. R. China E-mail: [email protected] (L. Y. Gan), [email protected] (P. Wang) ABSTRACT: Design and synthesis of high-performance hydrogen evolution reaction (HER) catalysts requires an overall consideration of intrinsic activity, number of active sites as well as electric conductivity. We herein report a facile synthesis of a cost-effective catalyst that can simultaneously address these key issuesA cobalt molybdenum oxide hydrate (CoMoO4⋅nH2O) with a 3D hierarchical nanostructure can be readily grown on nickel foam using hydrothermal method. Calcination treatment of this precursor material under reductive atmosphere resulted in the formation of Co nanoparticles on Co2Mo3O8 surface, which worked in concert to act as active sites for the HER. Besides, the resulting Co2Mo3O8 from the dehydration and reduction reactions of CoMoO4⋅nH2O showed remarkable increases on both active surface area and electric conductivity. As a consequence of these favorable attributes, the catalyst exhibited comparable electrocatalytic performance to the commercial Pt/C catalyst for the HER in alkaline solution, which is promising for the practical water-splitting applications. KEYWORDS: electrocatalysis, hydrogen evolution reaction, electroconductibility, intrinsic activity, nanostructure INTRODUCTION

H2 is a clean and flexible energy carrier and is expected to play a key role in the future sustainable energy system. Electrochemical water-splitting provides a sustainable way for producing H2 and accordingly a promising energy conversion technology that convert electricity from renewable energy resources into chemical energy in the form of H2. This is of profound importance for the efficient and widespread utilization of renewable energy resources that are characterized by their variable and intermittent output.1−5 Crucial to enabling an energy-efficient splitting of water is the development of active and durable electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction

(OER).6−8 Platinum is well recognized as the benchmark catalyst for the HER, requiring negligible overpotential to achieve high reaction rate. However, due to its scarcity and high cost, Pt is severely limited in the widespread technological use.9,10 In the past decades, a variety of non-precious transition metal compounds, such as sulfides,11−16 phosphides,17,18 phosphosulfides,19−22 carbides,23,24 nitrides,25,26 selenides,27,28 oxides,29−31 borides32,33 and non-metallic materials34,35 have been investigated as potential alternatives to replace Pt. In a general view, non-precious metal catalysts show satisfactory stability under alkaline conditions, which is compatible to the industrial operation of 1 / 15

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water-alkali electrolyzer. But in terms of activity, the performances of most catalysts in alkaline conditions are usually remarkably inferior to those in acidic conditions.36,37 This phenomenon might be associated with the difference between the HER pathways in alkaline and acid solutions. More exactly, the sluggish HER kinetics in alkaline solutions might stem from the discharge of H intermediate from water instead of from hydronium ions (H3O+). According to Markovic et al., this problem can be circumvented by combination of metal hydroxide and metal to create concerted catalysts, wherein the hydroxide promotes the dissociation of water and the nearby metal atom facilitates the adsorption and association of hydrogen intermediates into molecular H2.38 Since the transition metal hydroxides like Ni(OH)2 and Co(OH)2 are stable under alkaline environments even in the HER potential region, this finding revealed a feasible approach to improve the intrinsic activities of non-precious metal catalysts for the HER in alkaline conditions. In practice, this approach can be used in conjunction with alloying strategy to pursue notable activity improvements. Beside the intrinsic activity, increasing the number of accessible active sites via nanostructuring the electrocatalyst is also an extensively used strategy to enhance the HER performance. In addition, owing to the intrinsic requirement of fast electron-transferring between the electrode and reactants, electrical conductivity is also a key factor influencing the catalytic performance of electrocatalysts.39,40 Herein, we report the synthesis of a cobalt molybdenum oxide (Co-Mo-O)-derived electrocatalyst using hydrothermal method following by calcination treatment. In the simple preparation process of this cost-effective catalyst,

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the intrinsic activity, number of active sites and electrical conductivity issues can be simultaneously addressed. As a consequence, the non-precious Co-Mo-O-derived electrocatalyst exhibited exceptionally high activity and excellent stability towards the HER in alkaline solutions, which is comparable to the benchmark Pt/C catalyst. RESULT AND DISCUSSION The Co-Mo-O-derived electrocatalyst was synthesized by a two-step procedure, as schematically shown in Figure 1. First, a cobalt molybdenum oxide hydrate with a 3D nanostructure was grown on a piece of nickel foam (NF, 1×4 cm2) via a hydrothermal reaction at 150 °C for 6 h in 30 mL of deionized water containing Co(NO3)2⋅6H2O (0.02 M) and Na2MoO4⋅2H2O (0.01 M). The as-synthesized material was then calcined at 450 °C under a flowing H2/Ar (v/v, 1/10) atmosphere for 2 h to prepare the targeted catalyst. X-ray diffraction (XRD, Figure 2a) result showed that the sample obtained from hydrothermal process can be well indexed to crystalline CoMoO4⋅nH2O (JCPDS card 10-1016). Morphology observation by field-emission scanning electron microscopy (FE-SEM, Figure 3a and 3b) indicated that the NF surface was entirely covered by large numbers of nanosheets, which self-assembled into 3D flower-like structure. According to the high resolution transmission electron microscopy (HRTEM) observation and selective area electron diffraction (SAED) analysis (Figure 3e), these nanosheets were identified as a monocrystalline CoMoO4⋅nH2O phase, which was consistent with the XRD result.

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Figure 1. Schematic illustration of the formation of Co/Co2Mo3O8 electrocatalyst on NF. Calcination of the CoMoO4⋅nH2O/NF sample at elevated temperatures under reductive atmosphere resulted in notable changes of phase structure. As seen in Figure 2a, the diffraction peaks of CoMoO4⋅nH2O phase completely disappeared in the calcined sample at 450 °C, and the newly appeared sets of peaks could be assigned to the crystalline Co2Mo3O8 and metallic fcc Co. Consistently, the SAED pattern taken from a typical calcined nanosheet (Figure 3f and 3g) showed two sets of diffraction spots with six-fold symmetry, which corresponded to Co and Co2Mo3O8 long their zone axes of [111] and [001], respectively. Notably, these two types of monocrystalline phases adopted a specific crystallographic orientation relationship, that is, _ the (112) lattice plane of Co and (220) lattice plane of Co2Mo3O8 paralleled to each other. In the HRTEM image (Figure 3h), the lattice fringes with an interplanar spacing of 0.29 nm intersected with an interplanar angle of 60o could be safely assigned to {110} planes of Co2Mo3O8 and the lattice fringes with a distance of 0.25 nm and an interplanar angle of 60o to {110} planes of Co. Figure 3i-3m presented the two-dimensional elemental mapping results that were acquired in

high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode in combination with energy dispersive X-ray spectroscopy (EDS) analysis. It was clearly observed that calcination treatment resulted in the precipitation of Co nanoparticles with sizes ranging from several to several tens of nanometers. In sharp contrast, Mo element showed no appreciable aggregation in the post-calcined sample. A comparison of the SEM and TEM morphologies (Figure 3a-d) of the samples before and after the calcination treatment found that the nanosheet structure was well retained in the calcination process. But in sharp contrast to the CoMoO4⋅nH2O/NF sample, the post-calcined sample showed the dispersion of large numbers of nanopores throughout the nanosheet (Figure 3f and Figure S1, Supporting Information). In line with this observation, the N2 adsorption/desorption isotherm of the post-calcined sample showed a type IV isotherm with a type H3 hysteresis loop, which was characteristic for mesoporous materials (Figure 2b). The average pore size (Figure 2c) was determined to be around 8 nm, which was in good agreement with the HRTEM observation 3 / 15

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(Figure 3f and Figure S1, Supporting Information). Clearly, the formation of nanopores should stem from the dehydration reaction of CoMoO4⋅nH2O at elevated temperatures. As a

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consequence of the formation of nanoporous structure, the specific surface area of the catalyst sample was increased from 15.2 to 77.1 m2 g–1 after the calcination treatment.

Figure 2. XRD patterns (a), N2 adsorption–desorption isotherms (b) and the corresponding pore size distribution (c) of the CoMoO4⋅nH2O and Co/Co2Mo3O8 powders peeled off from NF, respectively.

Figure 3. FE-SEM images of CoMoO4⋅nH2O/NF (a, b) and Co/Co2Mo3O8/NF (c, d) at different resolutions. HRTEM image of CoMoO4⋅nH2O (e) and the inset shows the corresponding SAED pattern. The corresponding SAED images and HRTEM images of Co/Co2Mo3O8/NF (f-h), inset in h was fast Fourier transform spectrum of corresponding HRTEM. HAADF-STEM images and corresponding EDS maps of the Co/Co2Mo3O8/NF for Co, Mo, O and combined image (i-m). The samples at different states were further

investigated by the surface-sensitive X-ray 4 / 15

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photoelectron spectroscopy (XPS) technique to determine the chemical state of the constituent elements. As seen in Figure 4a and 4b, the XPS spectra of the CoMoO4⋅nH2O/NF sample showed only the signals of Co2+ and Mo6+ species. In contrast, the Co 2p spectrum of the post-calcinated sample showed extra signals of metallic Co0. These results agreed well with those obtained from XRD (Figure 2a), SAED and HRTEM results (Figure 3g and 3h), showing clearly the partial reduction of CoMoO4 in the calcination process. Analysis of Mo 3d spectrum found that, besides the expected Mo4+ species, the post-calcinated sample also showed strong Mo5+ and Mo6+ signals. Since the parallel Raman spectroscopy analyses (Figure S2, Supporting Information) could not detect the characteristic Raman bands of substoichiometric MoO3−x species,41 the high oxidation states of Mo should be accommodated within the crystal structure of Co2Mo3O8. The resulting detective structure is expected to possess improved electric conductivity. This was validated by the measurement results of conductivity resistance (Figure 5b) as well as the theoretical calculation results of electronic structure (Figure 6a) as stated below.

According to the electrochemical property examination (Figure S3, Supporting Information), the CoMoO4⋅nH2O-derived catalyst showed considerable HER activity dependence on the calcination treatment temperature. The 450 °C-calcined catalyst exhibited an optimal HER activity and was thereby selected for detailed study. The HER activity of the Co/Co2Mo3O8/NF catalyst was evaluated in 1.0 M KOH by linear sweep voltammetry without iR compensation (Figure 5a) and with iR compensation (Figure S4, Supporting Information) in 1.0 M KOH. The as-prepared CoMoO4⋅nH2O/NF, neat NF and Pt/C catalysts were also measured under the identical condition for property comparison. It was found that the Co/Co2Mo3O8/NF catalyst showed much higher activity than the relevant samples (Figure 5a). The required overpotential to afford a current density of 10 mA cm−2 was only 25 mV. This catalytic performance is comparable to that of Pt/C and is among the top levels of the non-precious HER catalysts in alkaline conditions (Table S1, Supporting Information). To gain insight into the variation of HER activities of the series of electrocatalysts, we used

Figure 4. XPS spectra of the CoMoO4⋅nH2O/NF and Co/Co2Mo3O8/NF catalysts. 5 / 15

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electrochemical impedance spectroscopy (EIS) technique to measure the charge transfer resistance (Rct), and the electrical double-layer capacitance (EDLC) to determine the electrochemically active surface area (ECSA, Figure S5, Supporting Information). As seen in Figure 5b and 5c, calcination treatment of the CoMoO4⋅nH2O/NF sample resulted in a remarkably reduced charge-transfer resistance (from 23.8 to 2.45 Ω) and a ~25 folds increase of ECSA. The increase of ECSA was consistent with the specific surface area (SSA) results that were measured by Brunauer-Emmett-Teller (BET) N2-adsorption method. Evidently, it was the formation of nanoporous structure through the dehydration reaction of CoMoO4⋅nH2O that caused remarkable increases of ECSA and SSA of the catalyst sample. The improved electric conductivity should be attributed to the defective structure of Co2Mo3O8 as well as its close contact with metallic Co nanoparticles, which resulted from the in situ partial reduction of CoMoO4. Theoretical simulations suggested that the defect-free Co2Mo3O8 is a semiconductor with gaps of ~1.40 eV in both spin channels, as shown in Figure 6a. But upon incorporating Co vacancies or interstitial O defects into Co2Mo3O8 lattice (Figure S7, Supporting Information), shadow defect levels will be created near the valence band edge, which results in enhanced electric conductivity of the catalysts. In principle, the formation of both Co vacancies and interstitial O defects is consistent with the identification of Mo5+ and Mo6+ species in the Co2Mo3O8 lattice. But in consideration of the lower formation energy of Co vacancies in comparison with the interstitial O defect (0.54 vs. 0.97 eV), the observation of precipitation of metallic Co and in particular the reductive conditions in the calcination treatment, we speculated that the enhanced electric conductivity should be more likely attributed to

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the formation of Co vacancies in the Co2Mo3O8 lattice. Tafel analysis of the polarization curves may provide valuable mechanistic insight into the HER. As seen in Figure 5d, the Tafel slopes of bare NF and the CoMoO4⋅nH2O/NF sample were determined to be 168 and 162 mV dec-1, respectively, suggesting that the rate-determining step is the initial electron-transfer through the discharge of water. In sharp contrast, the Co/Co2Mo3O8/NF catalyst showed a low and quite similar Tafel slope to the Pt/C catalyst (33 vs. 32 mV dec-1). According to the current understanding of the HER pathway, this result suggested that the HER over the Co/Co2Mo3O8/NF catalyst occurred through Volmer-Tafel or Volmer-Heyrovsky [9,42] mechanism. Electrocatalytic stability is another important criterion for evaluating the HER catalysts. In the present study, we tested the stability of the Co/Co2Mo3O8/NF catalyst using cyclic voltammetry (CV) and chronoamperometry methods. As presented in Figure 5e, the catalyst showed an overpotential increase of 6 mV in the first 2 h and a slight overpotential fluctuation of ±1 mV in the following 22 h of constant-current measurement at 10 mA cm-2. Interestingly, a comparison of the polarization curves found that the activity of the catalyst was even slightly improved in terms of activity after 24 h of constant-current measurement or 5000 CV cycles. SEM observations found that the morphological feature of the catalyst was well retained after long-term operation (Figure S6, Supporting Information) These results clearly indicated the outstanding stability of the catalyst derived from CoMoO4⋅nH2O/NF for the HER in alkaline condition. Currently, the mechanistic reason for the slight activity improvement arising upon long-term operation remains unclear. Presumably, it might originate from subtle phase evolution

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and/or surface rearrangement of the active sites in the HER potential region. In this regard, coupled experimental and theoretical studies are still required to gain better understanding of this interesting phenomenon. The Faradic efficiency (FE) was evaluated by comparing the

experimentally determined gas amount with theoretical value. The measured H2 amounts at different reaction durations matched well with the calculated values, indicating a nearly 100% FE for the HER (Figure 5f).

Figure 5. HER polarization curves without iR compensation (a). EIS Nyquist plots and fitting curves at onset potentials for various electrocatalysts (b). The capacitive current densities at OCP as a function of scan rate (c). The corresponding Tafel plots for the virous eletrocatalysts (d). A comparison of the polarization curves of the Co/Co2Mo3O8/NF before and after 5000 cycles in 1 M KOH (e). The inset shows the chronopotentiometric curves at 10 mA cm−2. Experimentally measured H2 production versus theoretically calculated quantities under constant current densities of 10 mA cm−2 (f). 7 / 15

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Figure 6. Density of states of Co2Mo3O8 without defects, with a Co vacancy and an interstitial oxygen atom (a). Calculated adsorption energies (∆Ead) of H2O (b) and Gibbs free energy (∆GH*) diagram (c) for HER on Co (fcc) and Co- (Co-T), Mo- (Mo-T) and O-terminated (O-T) Co2Mo3O8 surfaces. To gain insight into the HER mechanism and in particular the nature of active sites, we conducted first-principles calculations to investigate the elementary reaction steps involved in HER, i.e., dissociative adsorption of H2O molecule and the associative desorption of H2 molecule, on the surface of metallic Co and Co2Mo3O8 that were experimentally detected in the post-calcined catalyst sample. The feasibility of water dissociation strongly depends on its adsorption strength (∆Ead) on catalysts, while Gibbs free energy of hydrogen adsorption on catalysts (∆GH*) has been generally accepted as a catalytic descriptor to evaluate the associative desorption of H2 molecule.43 Figure 6b and 6c show the calculation results for the two processes on the four representative surfaces (Figure S8, Supporting Information). The calculated adsorption of H2O on the three modeled Co2Mo3O8 surfaces is significantly stronger than that on metallic Co surfaces, highlighting the pivotal role of Co2Mo3O8 in promoting the dissociative adsorption of water. In sharp contrast, the combination of two H intermediates from water dissociation is facile on metallic Co with calculated ∆GH* of only –0.13 eV, which is close

to the value on Pt (–0.06 eV). These calculation results clearly suggest that Co2Mo3O8 and metallic Co may work in concert to properly address different elementary steps of HER, and thereby synergistically providing highly active sites to catalyze HER. CONCLUSION In summary, CoMoO4⋅nH2O with a 3D hierarchical nanostructure can be readily synthesized and grown on nickel foam using a simple hydrothermal method. Calcination treatment of this precursor material under reductive atmosphere resulted in the in situ formation of Co nanoparticles on Co2Mo3O8 surface, which worked in concert to provide highly active sites for the HER. Meanwhile, due to the dehydration-induced formation of nanoporous structure and reduction-induced defective structure, the yielded Co2Mo3O8 from the conversion of CoMoO4⋅nH2O possesses simultaneously large electrochemical active surface area and good electric conductivity. As a consequence of these favorable attributes, the catalyst derived from CoMoO4⋅nH2O/NF

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exhibited extraordinarily high activity and excellent stability for the HER in alkaline solution, which are among the top levels of the non-precious metal HER catalysts. A favorable combination of cost-effectiveness, facile and scalable synthesis, outstanding catalytic performance makes cobalt molybdenum oxide-derived catalyst very promising alternative to Pt catalyst for emerging applications in energy conversion. EXPERIMENT SECTION Chemicals and materials. Cobaltous nitrate hexahydrate (Co(NO3)2·6H2O ， 99%), sodium molybdate dihydrate (Na2MoO4·2H2O ， 99%), hydrochloric acid (HCl, ca. 36~38% solution in water), potassium hydroxide(KOH，95%), Pt/C (20 wt% Pt on Vulcan XC-72R), Nafion solution (5 wt% in alcohol) and other reagents of analytical reagent grade were purchased from commercial sources and used without purification. NF (≥99%) was purchased from Incoatm, which has a thickness of 1.60 mm, an area density of ~650 g/m2 and a pore size ranging from 0.25 to 0.80 mm. Deionized (DI) water was used in preparation of all the aqueous solutions. Preparation of electrode materials. A NF-supported cobalt molybdenum oxide-derived electrocatalyst was prepared using a hydrothermal method followed by calcination treatment. The NF were consecutively cleaned in HCl solution (3 M), ethanol and DI water under sonication to remove nickel oxides, organics and other impurities on the surface. Co(NO3)2·6H2O (0.02 M) and Na2MoO4·2H2O (0.01 M) were dissolved in 30 mL DI water, and then the solution together with the cleaned NF were transferred into a 50 mL Teflon-lined stainless autoclave. The sealed autoclave was kept at 150 °C for 6 h, and then cooled naturally down to ambient temperature. The collected samples were

washed thoroughly with DI water and ethanol, and then dried in vacuum at ambient condition. Calcination of the catalyst samples was conducted at 350 ~ 500 °C in a flowing H2/Ar (1:10) atmosphere for 2 h. The ramping rate is 10 °C min−1. According to the weight change of the sample after hydrothermal and calcination processes, the loading amount of the catalyst on NF was determined to be around 2.5 mg cm−2. The Pt/C working electrode was prepared in a two-step process. First, Pt/C (5 mg) was dispersed in isopropanol (1 mL), followed by addition of Nafion solution (50 µL). After being sonicated for at least 30 min, 20 µL of the resulting catalyst ink was drop casted on a glass carbon electrode. Characterization. The phase structure, morphology and microstructure of the catalyst samples were characterized by XRD (RigakuRINT 2000, Cu Kα radiation), FE-SEM (ZEISS MERLIN) and HRTEM (JEOL-2100F). XPS analyses were performed in a Thermo Scientific K-ALPHA+ spectrometer with binding energies referenced to adventitious carbon at 284.8 eV. Raman spectra were collected using a Thermo Fisher Micro DXR microscope with a He-Ne laser (532 nm) excitation source at a resolution of 2 cm−1. Before the measurement session, the instrument calibration was performed using the alignment/calibration tool supplied by the manufacturer. The surface area, pore volume and pore size distribution of the catalyst samples were measured using BET-N2 adsorption method in a Micromeritics ASAP 2460 apparatus. Electrochemical measurements. Electrochemical measurements were carried out in an electrochemical workstation (CHI 660E) using a standard three-electrode setup in 1.0 M KOH. The as-prepared catalysts were used as working electrode, and graphite rod and Hg/HgO as counter electrode and reference electrode, respectively. All the measured potentials were 9 / 15

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referenced to the reversible hydrogen electrode (RHE) using the equation as follows: E(RHE) = E(Hg/HgO) + 0.924 V. Typically, a potential cycling in a range of +0.024 to −0.676 V vs. RHE with a sweep rate of 100 mV s-1 was applied before the measurement of polarization curves. The polarization curves were measured by linear sweep voltammetry (LSV) at a scan rate of 2 mV s-1 to evaluate the HER performance of working electrode. Unless specifically noted, all of the potentials were given without iR compensation. EIS measurements were carried out in a frequency range of 0.01 Hz to 100 kHz with an amplitude of 5mV at onset potential. The curve fitting was performed by Zview2 software. The Cdl was measured using a cyclic voltammetry (CV) method and on the basis of which ECSA of the catalysts was determined. The FE for H2 production was determined by comparing the measured quantity of gas with the theoretically calculated value. Theoretical calculations. Density functional theory calculations were conducted using the Vienna Ab-Initio Simulation Package with the frozen-core projector augmented method.44,45 The spin-polarized Perdew-Burke-Ernzerhof generalized gradient approximation including van der Waals corrections (optB88) was employed for the exchange correlation functional.46-48 A cutoff energy of 500 eV for the plane-wave expansion was used in this work.44 A 3 × 3 lateral supercell and a four-layer slab were used to model fcc Co (111) surfaces. The (001) surfaces of Co2Mo3O8 were performed using symmetric slabs to cancel out possible dipole moments. The Co-, Mo, and O-terminated surfaces were simulated by a size of 1×1 periodicity (6.246 Å × 6.246 Å) and slabs of total thirty-five, thirty-three and thirty-one layers, respectively, as shown in Figure S5, Supporting Information. Periodic images normal to the surface were separated by a vacuum thickness of

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over 20 Å. A Monkhorst-Pack k-mesh of 5×5×1 was employed to sample the Brillouin zone. To simulate the strong electronic correlations in the localized d orbitals of the Co and Mo ions in Co2Mo3O8, an on-site Coulomb interaction with respective effective parameters of U−J = 3.3 and 4.4 eV was included.49 The geometry was fully relaxed until all residual forces were less than 0.02 eV/Å. The adsorption energies of X (X = H and H2O) on the five surfaces were calculated as ∆Ead = EX/surface – Esurface –EX (Eq. 1) where EX/surface, Esurface and EX are the total energies of surfaces with and without X adsorbed and of X in gas phase, respectively. The associated change of Gibbs free energy of H is defined as ∆G =∆E + ∆ZPE - T∆S (Eq. 2) wherein ∆ZPE and ∆S are the zero-point energy and entropy differences between the adsorbed state and gas phase, respectively, and were calculated from the vibrational frequencies.50,51 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website including SEM and TEM images, Raman spectrum, Models of calculation and other related data. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Ping Wang: 0000-0002-7704-2538 Liyong Gan: 0000-0002-1879-1918 Notes The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS The financial supports from the National Natural Science Foundation of China (Grant Nos. 51671087 and 11504303); the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51621001); the Foundation for Research Groups of the Natural Science Foundation of Guangdong Province (Grant No. 2016A030312011) and the Special Support Plan for National 10000-talents Program. REFERENCES (1) Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332-337. (2) Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972-974. (3) Nørskov, J. K.; Christensen, C. H. Toward Efficient Hydrogen Production at Surfaces. Science 2006, 312, 1322-1323. (4) Mckone, J. R.; Marinescu, S. C.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Earth-Abundant Hydrogen Evolution Electrocatalysts. Chem. Sci. 2014, 5, 865-878. (5) Zou, X. X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (6) Wang, J.; Zhang, H.; Wang, X. Recent Methods for the Synthesis of Noble-Metal-Free Hydrogen-Evolution Electrocatalysts: from Nanoscale to Sub-nanoscale. Small Methods 2017, 1, 1700118-1700133. (7) Wang, J.; Ge, X. M.; Liu, Z. L.; Thia, L.; Yan, Y.; Xiao, W.; Wang, X. Heterogeneous Electrocatalyst with Molecular Cobalt Ions Serving as the Center of Active Sites. J. Am. Chem. Soc. 2017, 139, 1878-1884.

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