1. Introduction Hydrogen has long been considered as a future clean energy carrier. One of the most promising methods for producing hydrogen is electrolysis of water.1-4 However, efficient electrocatalysts must be required to accelerate the up-hill, kinetically sluggish water splitting reaction. Noble metal Pt-based materials are now the most active electrocatalyst for the hydrogen evolution reaction (HER)―an important half-reaction of the water splitting. But it is impossible to use such noble metal electrocatalysts over a wide range of applications owing to their high price and low earth-abundance. Thus, developing alternative, noble metal-free, lower-cost and more abundant materials is highly desirable for the water splitting technology. 5-8 Recently, many efforts have been gaining a lot of attention to tackle these issues, and some of them have led to a number of nonprecious metal materials that can electrocatalyze HER efficiently. Notable progress has been made in hydrogen-evolving electrocatalysts based on earth-abundant transition metal elements, mainly including molybdenum,9,10 tungsten,11,12 nickel13,14 and cobalt.15-16 By comparison, copper-based hydrogen-evolution electrocatalysts are much less explored, although some properties of copper are quite attractive, e.g., high earth-abundance, bio-relevance, many accessible oxidation states (Cu0, CuI and CuI) and rich coordination chemistry17-21. A moderate H adsorption strength at its catalytic site is crucially required for a good hydrogen-evolution electrocatalyst, according to the Sabatier principle. 22,23 As for metallic copper, the adsorbed H on the metal surface is so weak that the slow discharge

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step limits the turnover rate. 24,25 Hence, the central issue in the development of Cu-based hydrogen-evolving electrocatalysts lies in how to optimize the H adsorption properties of copper catalytic sites. Following this thought, a few attempts have been made by researchers over the past two years, and a number of successful examples show that meticulously tuning the coordination environment of copper catalytic sites surrounded by different anions (such as, N, P and O) is an effective method to modify the H adsorption property of Cu, thereby improving the electrocatalytic activity. 26-33 In addition, the drift of electron density from Cu atoms to anions can lead to the positively-charged Cu atoms and the negatively-charged anions; and correspondingly, the Cu atoms and the anions might function as the hydride-acceptor and proton-acceptor centers, respectively, facilitating the HER.34-38 For instance, two molecular Cu-based electrocatalysts, in which copper are coordinated with N-containing ligands, have been synthesized, and they, as homogeneous materials, have been shown to be electrocatalytically active for HER.34,35 In addition, several solid-state compounds, in which copper are coordinated with P, O or N atoms, have also been reported to be efficient for electrocatalytic HER (e.g., Cu3P, Cu2O and Cu-C3N4).36-38 Despite the above achievements made in Cu-based hydrogen-evolving electrocatalysts, the electrocatalytic activity and/or stability of many of them are often unsatisfied, and a Cu-based electrocatalyst, in which Cu atoms are coordinated with S atoms, has not been explored yet. Herein, we report, for the first time, a copper(I) sulfide material that can electrocatalyze HER efficiently. The material (dubbed γ-Cu2S/CF) comprises

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single-crystalline γ-Cu2S nanoplates in situ grown on copper foam (CF). The synthesis of this material is achieved in a hydrothermal system, with the assistance of a small amount of cobalt(II) acetate. Furthermore, we show that the γ-Cu2S/CF material can serve as an efficient, stable, integrated 3D electrode toward HER at neutral pH.

2. Experimental section

2.1 Chemicals and Reagents Cobalt(II) acetate tetrahydrate, cobalt(II) chloride hexahydrate and cobalt(II) nitrate hexahydrate were purchased from Xilong Chemicals Co., Ltd. Thiourea and nickel(II) acetate tetrahydrate were purchased from Tianjin Fuchen Chemical Reagents Factory. Copper foam (CF) (thickness: 1.0 mm, bulk density: 0.26 g/cm3) was purchased from Suzhou Jiashide foam metal Co., Ltd. Dipotassium hydrogen phosphate (K2HPO4) and potassium dihydrogen phosphate (KH2PO4) were purchased from Beijing Chemical Factory. 20 wt% Pt/C catalyst was purchased from Sigma-Aldrich. Highly purified water (> 18 M cm resistivity) was obtained from a PALL PURELAB Plus system and used throughout the experiments.

2.2 Synthesis of γ-Cu2S/CF In order to prepare the γ-Cu2S/CF material, a piece of CF (0.5 cm × 5 cm) was cleaned sequentially with acetone and HCl solution (3 mol/L) under ultrasonic condition, and then washed with water for several times. The cleaned CF was submerged into an aqueous solution (30 mL) containing of thiourea (2 mmol/L) and of cobalt (II) acetate tetrahydrate (1 mmol/L) in a 50 mL Teflon-lined stainless autoclave.

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The autoclave was heated in an oven and maintained at 180 °C for 3 h. After the autoclave cooled down to room temperature, a CF covered with uniform black solids was obtained, giving the γ-Cu2S/CF material. In order to examine the roles of cobalt(II) acetate in the synthesis of the material, several control experiments were carried out. (i) An attempted experiment was carried out in the absence of cobalt(II) acetate, but other synthesis parameters kept unchanged. This particular experiment gave a Cu2O thin film supported on CF. (ii) Cobalt(II) chloride or cobalt(II) nitrate was also tried to replace cobalt(II) acetate in the synthesis, a Cu2O thin film supported on CF was also obtained. (iii) Nickel(II) acetate, in lieu of cobalt(II) acetate, was tried to use in the synthesis. This particular reaction yielded a β-Cu2S thin film supported on CF. (iv) In order to investigate the influence of the amount of cobalt (II) acetate in synthetic system on the morphology of γ-Cu2S/CF, we carried out some attempted experiments for the synthesis of the material with different amount of cobalt(II) acetate (e.g., 0.5, 1, 1.5 and 2 mmol), while keeping other reaction parameters unchanged during the hydrothermal process.

2.3 General Characterizations Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250 X-ray photoelectron spectrometer with a monochromatic X-ray source (Al Kα hυ = 1486.6 eV). Scanning electron microscopy (SEM) images were obtained with a JEOL JSM 6700F electron scanning electron microscope, and transmission

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electron microscopy (TEM) images were obtained with a Philips-FEI Tecnai G2S-Twin microscope equipped with a field emission gun operating at 200 kV. Elemental analysis was conducted on a Perkin-Elmer Optima 3300 DV inductively coupled plasma atomic emission spectrometer (ICP-OES). The evolved hydrogen during HER were detected by using a gas chromatograph (Shimadzu, GC-2014C). All the electrochemical properties of the materials were studied with an electrochemical workstation CHI 660E.

2.4 Electrochemical Measurements The electrocatalytic activity of the material was studied in a typical three-electrode system. 1 M phosphate buffer (pH 7) was used as the electrolyte. A saturated calomel electrode (SCE) was used as the reference electrode and a carbon rod was used as the counter electrode. The material (Cu2O/CF, β-Cu2S/CF or γ-Cu2S/CF) was used directly as the working electrode with a working surface area (0.3 cm × 0.3 cm) that was achieved by sealing the rest with a modified acrylate adhesive. According to the equation: Evs. RHE = Evs. SCE + 0.261 + 0.059pH, the potential, measured against the SCE reference electrode, was converted into the ones versus the reversible hydrogen electrode (RHE). A very slow scan rate of 0.033 mV/s was used for linear sweep voltammetry (LSV) measurements. It was found that the self-supported electrode produced a substantial background current, which is primarily capacitive in origin. 15,39,40,

Thus, correction was executed in this work to obtain a true analysis of catalytic

activity. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 100 kHz-1 Hz. To compare the relative electrochemical active area

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of the materials, cyclic voltammetry curves were measured in the potential range of 0.10-0.30 V vs. RHE. Based on this result, the double layer capacity and electrochemical active area of the materials were compared by using a previously reported method.41,42 A working electrode containing powered Pt/C was prepared according to the procedures as follows: (i) Pt/C (5.2 mg) was dispersed in isopropanol (200 μL) to form a homogeneous mixture; (ii) this mixture (10 μL) was drop-casted onto a Cu foam with an area of 0.09 cm2, resulting in a loading amount of Pt/C of 2.84 mg/cm2; and (iii) in order to protect the Pt/C film, 0.3 % Nafion solution in isopropanol (6 μL) was drop-casted onto the surface of the electrode. To measure the Faradic efficiency of HER, we collected the evolved H2 gas (H2exp) by water drainage method, and then calculated the moles of H2 (H2cal) generated from the reaction with a hypothetical 100% current efficiency. The Faradic efficiency of the material in HER is defined as the ratio of H2exp to H2cal.

3. Results and Discussion

3.1 Synthesis and characterization of γ-Cu2S/CF The γ-Cu2S/CF material is synthesized by direct sulfidization of CF with thiourea in a hydrothermal system in the presence of a small amount of cobalt(II) acetate (see Experimental Section for details). During the hydrothermal reaction, the CF serves as a copper source and a support material, allowing the in situ growth of γ-Cu2S nanoplates on it. Figure 1 (A and B) show photographs of CF before and after sulfidization, exhibiting the obvious colour change from reddish

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brown for CF to black for γ-Cu2S/CF. In addition, the porosity of the CF can be maintained after the sulfidization reaction (Figure S1 in SI). Furthermore, during the sulfidization of CF, the color of the reaction solution maintained pink, indicating the presence of some Co2+ ions in the solution. The presence of Co2+ ions in the solution is further confirmed by ICP-OES analysis. These results imply that the sulfidization of CF, rather than cobalt ions, is the main reaction in the synthesis system. Figure 1C shows the X-ray diffraction (XRD) pattern of γ-Cu2S/CF. The result shows that the γ-Cu2S/CF contains a monoclinic phase of Cu2S (i.e., γ-Cu2S) besides metallic copper. In addition, no XRD peaks related to cobalt sulfides are observed. Figures 1D and S2 in SI present the scanning electron microscopy (SEM) images of γ-Cu2S/CF, showing that the entire surface of the CF is covered by bushy γ-Cu2S nanoplates. A close view (Figure 2E) reveals that these nanoplates, with a thickness of 100-200 nm and a smooth surface, appear to be vertically grown over the CF. In the HRTEM image (Figure 1F), it is seen that the nanoplate is highly crystalline with two sets of continuous lattice fringes of 0.62 and 0.60 nm. They are associated with the (111) and (210) crystallographic planes of the monoclinic phase of Cu2S, respectively, and the angle between (111) and (210) is 70.9o, which is consistent with the theoretical value. The selected area electron diffraction (SAED) pattern in Figure 1G shows a set of sharp spots, and some of them can be indexed as (111), (222) and (210) planes of γ-Cu2S.

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This result is indicative of the high crystallinity and single-crystalline nature of γ-Cu2S nanoplates. To obtain the composition information and the chemical states of the elements in the material, X-ray photoelectron spectroscopy (XPS) is performed. From the survey XPS spectrum of γ-Cu2S/CF (Figure 2A), four elements, including Cu, S, C and O, are observed. The presence of C1s XPS signal at 284.8 eV is due to adventitious elemental carbon from the XPS instrument. The presence of O1s XPS signal at 531.0 eV is originated from the adsorbed water molecules or the partial surface oxidation of the material.43 It is worth noting here that no Co signal is observed in the survey spectrum and high resolution Co 2P spectrum of γ-Cu2S/CF (Figure 2A and B). This result indicates that Co2+ has not been incorporated into the crystal lattice of γ-Cu2S in γ-Cu2S/CF, and no cobalt sulfides as the secondary phase are present in γ-Cu2S/CF. In addition, ICP-OES analysis further reveals that there was barely any cobalt species in γ-Cu2S/CF. Figure 2C shows the high-resolution Cu 2p XPS spectrum. The spectrum gives the Cu 2p3/2 and Cu 2p1/2 binding energies ranging from 928.0-937.7 eV and from 949.1-957.1 eV, respectively. The peaks can be fitting to the 2p 3/2 and 2p1/2 core levels of Cu+ and the 2p3/2 and 2p1/2 core levels of Cu2+, respectively.44-46 The shake-up satellite peaks of Cu species are also observed at 943.8 eV and 962.6 eV. The quantitative analysis of the XPS data reveals that the Cu2+/(Cu2+ + Cu+) atomic ratio is about 20.8%, indicating the partial surface oxidation of the material. Figure 2D shows the high-resolution S 2p XPS spectrum. The peaks

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located at 162.0 eV and 163.5 eV can be attributed to S 2p3/2 and S 2p1/2 binding energies, respectively.45 In order to examine the specific roles played by cobalt(II) acetate in the formation of Cu2S/CF, additional control experiments are carried out, and some results are shown as below (Figure 3). (i) When cobalt(II) acetate is not employed in the reaction, a thin film of Cu2O is formed over the CF in Figure S3 (A and B). (ii) When cobalt(II) acetate is replaced by cobalt(II) chloride or cobalt(II) nitrate in the reaction mixture, a thin film of Cu2O supported on the CF, similar to the one in (i), is obtained in Figure S3 (A, C and D). These control experiments (i-ii) demonstrate that the presence of acetate ions is critical for inhibiting the formation of Cu2O, or conversely, favouring the formation of γ-Cu2S. In addition, (iii) when nickel(II) acetate, in lieu of cobalt(II) acetate, is used in the reaction mixture, a thin film of β-Cu2S is formed over the CF (Figure S4). This result demonstrates the importance of Co2+ ions in directing the foramtion of γ-Cu2S. Furthermore, (iv) we study the importance of the amount of cobalt(II) acetate in the morphology of γ-Cu2S/CF. When the amount of cobalt(II) acetate is used in the wide range from 0.5 to 2 mmol, γ-Cu2S is always formed on the CF (Figure S5 in SI). In addition, the formation of well-defined γ-Cu2S nanoplates on CF is quite sensitive with the amount of cobalt(II) acetate, and cobaltous acetate of 1.0 mmol has been shown to be most suitable amount for the synthesis of γ-Cu2S nanoplates on CF (Figure 1 and Figure S5 in SI).

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The above results suggest that a small amount of cobalt(II) acetate in the synthesis system has three functions: (i) inhibiting the formation of Cu2O; (ii) tuning the crystal phase of Cu2S (γ-Cu2S versus β-Cu2S); and (iii) controlling the morphology of the as-formed γ-Cu2S. On the basis of the above results, a formation mechanism of γ-Cu2S on CF is proposed and briefly described as follows, although at current stage the full understanding of the cobalt(II) acetate-directed formation of γ-Cu2S is still a challenge. First, in the reaction system, oxygen is the only oxidant that can oxidize metallic copper into Cu + species, and thus, the CF should be oxidized by oxygen first (eqn (1)). This is why a thin film of Cu2O supported on the CF was obtained when no cobalt(II) acetate was involved in the reaction system. However, in the presence of cobalt(II) acetate, acetate ions might coordinate with copper ions, leading to the dissolution of Cu2O and the simultaneous formation of Cu2S (Eqn(2)-(3)). Because the solubility product constant of copper sulfides (c.a. 1.0 × 10-48) is much smaller than that of cobalt sulfides (c.a. 3.9 × 10-21), Cu2S is the only sulfide product in the reaction system, without the formation of coablt sulfides.46

4 Cu + O2 →2 Cu2O Eqn. (1) H2O + 4 OAc- + Cu2O → 2Cu[OAc]2- + 2OH- Eqn. (2)

2 Cu[OAc]2- + S2- → Cu2S + 4 OAc- Eqn. (3)

3.2 Electrocatalytic properties of γ-Cu2S/CF

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The electrocatalytic performance of γ-Cu2S/CF was evaluated using a typical three-electrode system under neutral media (pH 7). For comparative purpose, the electrocatalytic activity of blank CF, Cu2O/CF (Figure 4B) and β-Cu2S/CF (Figure 5B) was also tested. In addition, 20 wt. % Pt/C was drop-casted on a blank CF to make the working electrode with the assistance of polymer binder (see Experimental Section for details), and its electrocatalytic activity was also measured. Figure 4A shows the polarization curves of the CF, Cu2O/CF, β-Cu2S/CF, γ-Cu2S/CF and Pt/C (20 wt. %). As expected, the blank CF has barely catalytic activity towards HER, and Pt/C shows remarkable electrocatalytic activity with a near-zero onset overpotential (the overpotential that the electrocatalyst needs to yield a current density of 1 mA/cm2). In addition, all the three materials (i.e., Cu2O/CF, β-Cu2S/CF, γ-Cu2S/CF) exhibit significant electrocatalytic activity for hydrogen evolution, with the materials’ catalytic activity increasing in the order of Cu2O/CF < β-Cu2S/CF < γ-Cu2S/CF. To be specific, the overpotentials needed to achieve a catalytic current density of 10 mA/cm2 are 255, 240 and 190 mV for Cu2O/CF, β-Cu2S/CF and γ-Cu2S/CF, respectively. Moreover, the results also show that the catalytic activity of γ-Cu2S/CF is superior to most of the noble metal-free Cu-based electrocatalysts for the HER (Table 1), and is among the efficient nonprecious HER electrocatalysts in neutral media (Table S1 in SI). Figure 4B shows the Tafel slopes of Cu2O/CF, γ-Cu2S/CF and β-Cu2S/CF during the HER. The slope values of them are 58.8, 98.9 and 125.5 mV/dec,

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respectively. Tafel slope is well known to be related to the rate limiting step in HER.49,50 According to the classic theory on the mechanism of hydrogen evolution, HER processes with three basic steps: Volmer (discharge), Tafel (recombination) and Heyrovsky (desorption) steps. And Tafel slope can be 29, 38, or 116 mV dec-1, depending on which step as the rate limiting step, respectively. Herein, β-Cu2S/CF has an experimental slope value quite close to the expected 120 mV dec-1. Hence, the rate limiting Volmer step is proposed in this case. However, the Tafel slopes for Cu2O/CF and γ-Cu2S/CF have a large deviation from the any expected values, indicating the complexity of the hydrogen evolution process in these two cases. The detailed catalytic mechanisms in these two cases still call for further in-depth investigation. Figure 4C displays a multi-step chronoamperometric curve for HER with γ-Cu2S/CF in neutral buffer solution with the potential being increased from 0 to 500 mV vs. RHE and an increment of 50 mV every 500 s. The result shows that the current remains very stable at each potential in the entire range, and the current can switch quite rapidly, indicating that the material has a good mass transport property. To evaluate the stability of γ-Cu2S/CF, an I-t curve for the HER in neutral buffer solution was measured for 10 h at a current density of 10 mA/cm2 (Figure 4D). The result shows that γ-Cu2S/CF retains its activity even after 10 h-long electrocatalytic HER, demonstrating the good catalytic stability of the material. In addition, γ-Cu2S/CF has a good structural stability because the crystal

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structure and chemical composition of γ-Cu2S remain intact after the electrocatalysis test, as judged by XRD and XPS measurements (Figure S6 in SI). Fig. 5 shows the comparative curves of experimentally detected H2 amount vs. the theoretical H2 amount during an 80 min of the electrochemical hydrogen evolution reaction. γ-Cu2S/CF gives a stable hydrogen evolution rate that is very close to the theoretically achievable value. This result indicates that γ-Cu2S/CF shows nearly 100% Faradaic yield, and the observed current during HER over γ-Cu2S/CF originates from the hydrogen evolution reaction (Note that possible non-catalytic current during HER might come from the reduction of the catalyst itself and the capacitive current.). To assess the electron transfer kinetic properties of the materials, electrochemical impedance spectroscopy (EIS) was carried out. Figure 6A shows the Nyquist plots for CF, Cu2O/CF, β-Cu2S/CF and γ-Cu2S/CF. The series resistance (Rs) and charge-transfer resistance (Rct) values for the four materials, which were obtained by fitting the experimental data to the corresponding equivalent circuit model (Figure 6A, inset), are summarized in Table 2. Generally, the Rs is related to the electrical transport property of a catalyst, and the Rct is associated to the catalytic kinetics at the catalyst/electrolyte interface. From the Table 1,

it is concluded that γ-Cu2S/CF has the smallest Rs and Rct among the three

materials. This result further suggests that γ-Cu2S/CF has an advantageous structure for both the electronic transport and electrocatalytic kinetics during HER. In order to compare the effective electrode surface areas of CF, Cu2O/CF, β-Cu2S/CF and γ-Cu2S/CF, the electrochemical double layer capacitance of the three materials at nonfaradaic overpotentials was determined first and the geometric double layer capacitance (Cdl) values were calculated to be 1.1, 22.8, 14

37.6, and 102.0 mF/cm2 from the linear relation of current density and the scan rate derived from cyclic voltammetry curves (Figure 6B). Since the Cdl values are proportional to the effective electrode surface areas of the materials, we can conclude that γ-Cu2S/CF has a much larger electrochemically active surface area than Cu2O/CF, β-Cu2S/CF. The high effective electrode surface areas of γ-Cu2S/CF among them is mainly attributed to its nanoplate array structure. Overall, the good performance of the γ-Cu2S/CF electrocatalyst can be attributed to the following aspects: (i) the direct integration of γ-Cu2S nanoplates onto CF enables a good mechanical adhesion and facilitates the electron transfer from CF (a support) to γ-Cu2S (catalytically active phase) during HER. (ii) The excellent intrinsic electrical conductivity of Cu substrate favors the fast electron transport. (iii) The open hierarchical structure of γ-Cu2S/CF renders an enhanced contact with the electrolyte and ensures the diffusion of the electrolyte. (iv) polymer binders, which must be used for powered electrocatalysts, are not required for γ-Cu2S/CF. This should be beneficial for the catalytic stability as well as the exposure of the catalytically active sites.

4. Conclusions In summary, we have presented a novel ion-induced method for the synthesis of CF-supported, single-crystalline γ-Cu2S nanoplates, with the assistance of a small amount of cobalt(II) acetate. The roles of cobalt(II) acetate in the synthesis of the material have been studied. In addition, the resulting material is shown to efficiently electrocatalyze the HER at neutral pH. These findings would open up

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new possibilities to develop highly efficient and robust HER catalysts based on low-cost, earth-abundant elements, such as Cu. Acknowledgements This work was supported by the NSFC (21371070, 21401066); the National Basic Research Program of China (2013CB632403); Science and Technology Research Program of Education Department of Jilin Province ([2016] No. 410); Jilin province science and technology development projects (20140101041JC, 20130204001GX, 20150520003JH); Graduate Innovation Fund of Jilin University (2014052). References [1] M. Balla, M. Wietschel,;1; The future of hydrogen-opportunities and challenges, Int. J. Hydrogen Energy 34 (2009) 615-627. [2] T. Bak, J. Nowotny, M. Rekas and C. C. Sorrell,;1; Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects, Int. J. Hydrogen Energy 27 (2002) 991-1022. [3] X. Zou and Y. Zhang,;1; Noble metal-free hydrogen evolution catalysts for water splitting, Chem. Soc. Rev. 44 (2015) 5148-5180. [4] M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori and N. S. Lewis,;1; Solar water splitting cells, Chem. Rev. 110 (2010) 644-6473. [5] D. V. Esposito, S. T. Hunt, Y. C. Kimmel and J. G. Chen,;1; A new class of electrocatalysts for hydrogen production from water electrolysis: metal monolayers supported on low-cost transition metal carbides, J. Am. Chem. Soc. 134 (2012) 3025-3033. [6] A. L. Goff, V. Artero, B. Jousselme, P. D. Tran, N. Guillet, R. Métayé, A. Fihri, S. Palacin and M. Fontecave,;1; From hydrogenases to noble metal–free catalytic nanomaterials for H2 production and uptake, Science 4 (2009) 1384-1387. [7] L. A. Stern, L. G. Feng, F. Song and X. L. Hu,;1; Ni2P as a Janus catalyst for water splitting: the oxygen evolution activity of Ni2P nanoparticles, Energy Environ. Sci. 8 (2015) 2347-2351. [8] C. G. M. Guio, L. A. Stern and X. L. Hu,;1; Nanostructured hydrotreating catalysts for electrochemical hydrogen evolution, Chem. Soc. Rev. 43 (2014) 6555-6569. [9] J. F. Xie, J. J. Zhang, S. Li, F. Grote, X. D. Zhang, H. Zhang, R. X. Wang, Y. Lei, B. Pan and Y. Xie,;1; Controllable disorder engineering in oxygen-incorporated MoS2 16

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Fig. captions:

Fig. 1 (A), (B) Digital images of CF and γ-Cu2S/CF, respectively. (C) XRD pattern, (D, E) SEM images, (F) HRTEM image and (G) the corresponding selected area electron diffraction (SAED) pattern of γ-Cu2S/CF. The standard XRD cards of the Joint Committee on Powder Diffraction Standards (JCPDS) for γ-Cu2S’s (No. 33-0490) and metallic copper’s (No. 03-1018) are shown in the Fig. 1C, for comparison.

Fig. 2 (A) Survey and high-resolution of (B) Co2p, (C) Cu 2p, and (D) S2p XPS spectra of γ-Cu2S/CF.

Fig. 3 (A) Schematic illustration showing the growth of Cu2O thin film, γ-Cu2S thin film or β-Cu2S thin film on CF under different conditions. (B-D) Crystal structures of Cu2O, γ-Cu2S or β-Cu2S.

Fig. 4 Polarization curves during HER obtained in 1 M phosphate buffer solution over CF, Cu2O/CF, β-Cu2S/CF, γ-Cu2S/CF and Pt/C (20 wt%); (B) The corresponding Tafel slops for the HER; (C) Multi-step chronoamperometric curve obtained with γ-Cu2S/CF in 1 M phosphate buffer solution, measured at different overpotentials, starting at 50 mV and ending at 550 mV with an increment of 50 mV every 500 s. (C) Current density vs. time (I–t) curve obtained over γ-Cu2S/CF at an overpotential of 190 mV over 10 h-long electrocatalytic HER.

Fig. 5 Hydrogen production efficiency of γ-Cu2S/CF for HER with a current density of 10 mA/cm2 in neutral buffer solution. The exposed electrode area is about 0.5 cm × 0.5 cm. The calculated H2 lines represent the expected H2 amount assuming a 20

quantitative Faradaic yield (black line). The measured H2 lines represent the experimentally detected H2 (red circle).

Electrolyte

 at j = 1 mA/cm2 (mV)

 at j = 10 mA/cm2 (mV)

Refs

γ-Cu2S/CF

1 M KPi

105

190

This work

[(bztpen)Cu](BF4)2

0.1 M KPi

720

770

34

H2-Cucat

0.1 M KBi

558

628

32

Cu3P NW/CF

0.5 M H2SO4

80

143

36

Cu-C3N4

0.5 M H2SO4

30

390

38

Catalyst

Fig. 6 (A) Electrochemical impedance spectroscopy (EIS) Nyquist plots during HER of CF, Cu2O/CF, β-Cu2S/CF and γ-Cu2S/CF. Inset: the equivalent circuit model. (B) Plots used for determining the double-layer capacitance (Cdl) for CF, Cu2O/CF, β-Cu2S/CF and γ-Cu2S/CF. The Cdl, in turn, was used to estimate the electrochemical surface area (ECSA) of the materials. Table 1. Comparison of the electrocatalytic activity of γ-Cu2S/CF with some Cu-based HER electrocatalysts that have been recently reported. Table 2. The simulated series resistance (Rs) and charge transfer resistance (Rct) of CF, Cu2O/CF, β-Cu2S/CF and γ-Cu2S/CF based on Figure 6A.

21

Cu/Cu2O

0.1 M KPi

160

-

37

MoS2/N-CNT

0.5 M H2SO4

80

110

1

Ni3S2/NF

1 M KPi

45

170

14

Mo2C@NC

0.5 M H2SO4

60

124

10

Co-NRCNTs

1 M KPi

330

NiSe/NF

CF

1 M KOH Rs (Ω cm2 ) 2.9 Rct (Ω

cm2

)

77.53

10

4.0

540 β-Cu2S/CF 96 5.2

12.2

24.3

Cu2O/CF

2 γ-Cu2S/CF 3 2.9 4.9

Table 1. Comparison of the electrocatalytic activity of γ-Cu2S/CF with some Cu-based HER electrocatalysts that have been recently reported.

Table 2. The simulated series resistance (Rs) and charge transfer resistance (Rct) of CF, Cu2O/CF, β-Cu2S/CF and γ-Cu2S/CF based on Figure 6A.

TDENDOFDOCTD

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