Enhanced Catalytic Activity in Nitrogen-Anion ... - ACS Publications

Research Article pubs.acs.org/acscatalysis

Enhanced Catalytic Activity in Nitrogen-Anion Modified Metallic Cobalt Disulfide Porous Nanowire Arrays for Hydrogen Evolution Pengzuo Chen,†,+ Tianpei Zhou,†,+ Minglong Chen,‡ Yun Tong,† Nan Zhang,§ Xu Peng,† Wangsheng Chu,§ Xiaojun Wu,‡ Changzheng Wu,*,† and Yi Xie† †

Hefei National Laboratory for Physical Science at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), and CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei, Anhui 230026 P. R. China ‡ CAS Key Laboratory of Materials for Energy Conversion and Department of Material Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China § National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P.R. China S Supporting Information *

ABSTRACT: Rational design of cost-effective and non-precious-metal electrocatalysts for the hydrogen evolution reaction (HER) still remains a great challenge for future applications in sustainable energy storage and conversion systems. In this work, we developed a simple nitrogen-anion decoration strategy to realize the synergistic regulation of the catalytically active sites, electronic structure, and reaction dynamics in metallic CoS2 porous nanowire (NW) arrays. Specifically, the introduction of nitrogenanion in the CoS2 system, revealed by the XPS and XANES spectra, not only modified the morphology, offering additional active sites, but also enhanced the electrical conductivity to promote rapid charge transfer for the HER process. Synergistically, density functional theory confirms that the N incorporation results in more optimal free energy of hydrogen adsorption for either S or Co active sites, benefiting the HER kinetics. As expected, the N−CoS2 NW/carbon cloth (CC) electrode showed significantly enhanced HER performance with a lower overpotential and a larger exchange current density than the pristine one. This work provides a promising idea to the rational design of advanced and highly efficient electrocatalysts for hydrogen production. KEYWORDS: nitrogen-anion, cobalt disulfide, metallic, porous nanowire arrays, hydrogen evolution reaction

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INTRODUCTION The sustained over consumption of fossil fuels brings the prominent energy crisis and environmental contamination, forcing considerable attentions to develop renewable and highly efficient energy sources. Molecular hydrogen (H2), which possesses the highest gravimetric energy density and no pollution byproduct, has been regarded as an attractive and promising energy carrier for the future energy supply.1,2 Electrochemical water splitting to generate H2 is one of the most economical and effective route for large-scale hydrogen production, in which the high-active electrocatalyst is required to guarantee the sustainable energy efficiency.3−6 Up to now, platinum (Pt) or Pt-based noble metals are still the most active catalysts for the hydrogen evolution reaction (HER); however, their scarcity and high cost substantially hinder the large-scale applications. Thus, the development of cost-effective and nonprecious-metal electrocatalysts is an urgently requirement for practical application.7,8 Until now, many research efforts have been devoted to develop various classes of earth-abundant transition metal catalysts, such as metal sulfides,9−13 phosphides,14−16 nitrides,17,18 and carbides,19,20 as promising candidates to substitute traditional noble metal Pt. However, most of the catalysts show wanting efficiency, suffering from © XXXX American Chemical Society

low active sites exposure, while many involve complicated and multistep preparation process to increase costs.21 The pyrite-type transition metal dichalcogenides with ideal atomic arrangement and high electrical transport have emerged as an interesting class of potential catalysts for the HER process,22 although a lack of thermodynamic driving force and basic active sites hindered the intrinsic catalytic activities.23 The significant breakthrough has been made by the DFT calculations, which suggests the chalcogenide atoms at the surface of pyrite electrocatalyst can act as HER active sites,24 providing the theoretical foundation to modulate the catalytic active sites of pyrite catalysts. Inspired by this, many efforts have been focused on improving their catalytic activity by optimizing the conductivity and the density of exposed active sites.25,26 Among them, adopting heterometal atom doping in pyrite-type catalyst has been identified as a direct and effective method benefiting from the simultaneously enhanced conductivity and induced additional active sites. However, the drawback is that the subsequent complication of metal-active Received: July 6, 2017 Revised: September 9, 2017 Published: September 15, 2017 7405

DOI: 10.1021/acscatal.7b02218 ACS Catal. 2017, 7, 7405−7411

Research Article

ACS Catalysis sites prejudice the in-depth study of intrinsic catalytic activity and the inset of metal ion with large atomic size is unfavorable for maintaining the stable crystal structure of pyrite catalysts.27 In this regard, the nonmetal dopants, especially nitrogen (N)anion dopants that have been extensively applied to regulate the active sites and electronic structure of carbon materials, will be highly promising candidates to modify the catalytic activity of pyrite-type catalysts; additional benefits are synergistic regulation of the active sites, electronic structure, and reaction dynamic.28,29 Metallic cobalt sulfide (CoS2), as a typical pyrite compounds, has been widely exploited as adequate electrocatalyst toward the HER.30 However, the further improvement of catalytic activity for CoS2 material has still been largely limited due to the low surface active sites and inferior reaction dynamic.31 Although there has been some progress in improving the catalytic activity of CoS2 by heteroatom metal doping,32−36 the heteroatom anion decoration strategy has rarely been reported up to now. Herein, taking the metallic pyrite CoS2 as a proofof-concept study, we rationally develop nitrogen-anion decorated CoS2 porous nanowire arrays for the first time, representing the high-active electrocatalyst for HER. The incorporation of nitrogen-anion atoms in CoS2 porous nanowire successfully brings more surface catalytic active sites, faster electron transport capacity, and optimal adsorption free energy of hydrogen, giving rise to greatly improved HER catalytic activity. Remarkably, the as-prepared N−CoS2 NW/ CC 3D electrode exhibits excellent HER performance and high stability under acidic condition. The original concept of our work will broaden the horizons in the rational design of advanced electrocatalysts for hydrogen evolution.

Figure 1. (a) XRD pattern of N−CoS2 NW/CC. (b) Low- and (c) high-magnification SEM images of N−CoS2 NW/CC. (d) TEM and (e) HRTEM images of N−CoS2 nanowire. (f) The HAADF-STEM image and corresponding elemental mapping images for N−CoS2 nanowire.

(at 25° and 44°) and without any additional diffraction peaks of other impurities.37 As a contrast, the pristine CoS2 NW/CC can also be obtained by using sulfur powder as reactants (Figure S3). The microstructure of the product was further investigated by scanning electron microscopy (SEM) and transition electron microscopy (TEM). Figure S1 shows that the entire surface of carbon cloths were uniformly coated with Co(OH)F nanowire arrays. After sulfuration reactions, the integral 1D morphology of N−CoS2 products were still intact, except for many holes emerging at the outmost surface (Figure 1b,c), which can be further verified by high-magnification TEM images (Figure 1d). Further insights into the structural and composition information on N−CoS2 were obtained from highresolution transmission electron microscopy (HRTEM). As shown in Figure 1e, the HRTEM image of N−CoS2 showed a distinct lattice fringe of 0.277 nm, which can be ascribed to the (200) lattice plane of the CoS2 sample.38 Furthermore, the corresponding energy dispersive X-ray (EDX) mapping results demonstrated the uniform spatial distributions of Co, S, and N elements in the as-obtained N−CoS2 porous nanowire. The composition and chemical state information on N− CoS2 product was further investigated by X-ray photoelectron spectroscopy (XPS). As shown in Figure 2a, for Co 2p, the two main peaks located at 779.0 and 794.1 eV correspond to the characteristic peaks of Co 2p3/2 and Co 2p1/2, respectively.39 These two main peaks can be divided into four peaks at 778.9, 780.2, 793.9, and 795.7 eV, which can be ascribed to the coexistence of Co3+ (778.9 and 793.9 eV) and Co2+ (780.2 and 795.7 eV) after N-anion modifications.40 Moreover, the peaks located at the binding energy of 784.4 and 803 eV can be ascribed to the satellite peaks of Co 2p3/2 and Co 2p1/2, respectively.41 The S 2p region exhibits binding energy located at 163.0, 164.0, and 168.9 eV, of which the first two can be

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RESULTS AND DISCUSSION In this study, the N-anion decorated CoS2 porous nanowires arrays grown on carbon cloth (N−CoS2 NW/CC) was prepared from Co(OH)F/CC precursor by a simple solid reaction, whereby the thiourea powder was utilized as both nitrogen and sulfur sources (Scheme 1). The basic structural information on the as-prepared N−CoS2 NW/CC 3D electrode was first evaluated by X-ray diffraction (XRD). As shown in Figure 1a, all the diffraction peaks match well with the pure CoS2 phase (JCPDS 89-1492) and carbon cloth substrate Scheme 1. Representation of Synthetic Process for N−CoS2 NW/CC Electrode Material

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DOI: 10.1021/acscatal.7b02218 ACS Catal. 2017, 7, 7405−7411

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ACS Catalysis

electrical conductivity of the CoS2 material system. The S Ledge spectra of the N−CoS2 product is similar to that of pristine CoS2 (Figure 2f), which indicates that the additional charge transfer is happening between Co and N, rather than between Co and S. All above results clearly indicate that the Nanion atoms have been successfully incorporated into the lattice and formed Co−N bonds in the N−CoS2 product, which can greatly modify the electronic structure of the N−CoS2 material and contribute to the HER catalytic activity. To investigate the HER performance of as-obtained N−CoS2 NW/CC electrode material, electrochemical measurements have been performed. The polarization curve of the blank CC exhibits negligible catalytic activity (Figure 3a), which in turn

Figure 2. XPS spectra of (a) Co 2p, (b) S 2p, and (c) N 1s for N− CoS2 NW/CC. (d) FT-EXAFS curves for N−CoS2 and CoS2 products. (e) Co L-edge and (f) S L-edge XANES spectra of N− CoS2 and CoS2 products.

attributed to S 2p3/2 and S 2p1/2 peaks of Co−S bonds.42 The peak located at 168.9 eV might be ascribed to S−O bonding (Figure 2b).43 Moreover, in the region of N spectrum (Figure 2c), an obvious peak appearing at approximately 397.6 eV can be observed, which matches well with the previously reported Co−N bonds,44 indicated successful nitrogen atom incorporation into the framework of the CoS2 product. To further study the local atomic and electronic structures of the N−CoS2 product, the synchrotron X-ray absorption spectroscopy has been performed. As shown in Figure 2d, Fourier transform curves of the k3-weighted EXAFS of CoS2 and N−CoS2 samples present a similar radial distribution function (RDF), with a typical Co−S peak at about 1.95 Å.45 Notably, after the introduction of N atoms, the position of the Co−S peak for N− CoS2 sample is slightly shifted to a lower value of R, associating with a distinctly reducing peak intensity because of the shorter Co−N bond length and the weaker photoelectron scattering ability of N atoms.46 Furthermore, X-ray absorption near edge structure (XANES) of Co L-edge and S L-edge further provided the subtle change of electronic state for N−CoS2 product. The Co L-edge spectra are contributed by the Co 2p to 3d transition and split into the two parts (L3-edge and L2edge) because of spin−orbital coupling.47 Meanwhile, the L3edge spectrum, located at lower photon energy, is strongly related to the multiple electronic structure dominated by the hybridization of Co 3d orbitals with ligand S 2p orbitals and the Co 3d−3d interaction. As shown in Figure 2e, the peaks intensity of both Co L3-edge and L2-edge for N−CoS2 product is obviously lower than that of pristine CoS2 sample, which suggests more electrons occupy Co 3d orbitals in the N−CoS2 product. This result indicates that the incorporation of N atoms brings remarkable electron injection and greatly improves the

Figure 3. (a) IR-corrected HER polarization curves and (b) corresponding Tafel plots of blank CC, Pt/C−CC, N−CoS2 NW/ CC, and CoS2 NW/CC electrodes. (c) Comparison of potentials required to reach j = 50 mA cm−2 and Tafel slopes for N−CoS2 NW/ CC and CoS2 NW/CC products. (d) Nyquist plots of electrochemical impedance spectra of different catalysts for HER process. (e) Longterm stability test for N−CoS2 NW/CC and CoS2 NW/CC at a current density of 50 mA cm−2. (f) The Cdl linear fitting and calculation of N−CoS2 NW/CC and CoS2 NW/CC products.

verifies the intrinsic superior catalytic activity of metallic porous N−CoS2 nanowire arrays. As shown in Figure 3a,c, the N− CoS2 NW/CC electrode only needs an low overpotential of 152 mV to achieve the current density of 50 mA cm−2, which is much smaller than that of pristine CoS2 NW/CC (245 mV). In addition, at the potential of −0.25 V vs RHE, the current density of N−CoS2 NW/CC is about 5.9 times that of pristine CoS2 NW/CC electrode, suggesting the greatly improved HER catalytic activity after the N-anion decoration. Notably, the asprepared N−CoS2 NW/CC electrode exhibits much higher current density than that of Pt/C−CC catalysts, indicating the superior HER catalytic activity of N−CoS2 NW/CC electrocatalysts. Generally, the kinetic mechanism for HER in acidic medium can be proposed by the obtained Tafel slope value. 7407

DOI: 10.1021/acscatal.7b02218 ACS Catal. 2017, 7, 7405−7411

Research Article

ACS Catalysis The calculated Tafel slopes of about 118, 39, and 28 mV dec−1 under standard conditions correspond to the Volmer, Heyrovsky, and Tafel reactions, respectively. As shown in Figure 3b, the Tafel slope value of the N−CoS2 NW/CC (58 mV dec−1) is obviously smaller than that of pristine CoS2 NW/ CC (90 mV dec−1), further indicating its faster kinetic activity for HER (Figure 3b, c). This result indicates that the asprepared N−CoS2 NW/CC electrocatalyst is termed to follow the Tafel−Heyrovsky mechanism, and the rate-limiting step is the electrochemical desorption step. Moreover, the exchange current density parameters for the HER were also compared in Figure S6 and Table S1. It can be seen that the N−CoS2 NW/ CC electrode has a much bigger exchange current density of 0.52 mA cm−2 than that of the pristine CoS2 NW/CC product (0.2 mA cm−2), which demonstrates the significantly improved electrocatalytic kinetics of N−CoS2 NW/CC. Moreover, the Nyquist plot of N−CoS2 NW/CC exhibits much small charge transfer resistance (9.2 Ω) than that of the CoS2 NW/CC (16.2 Ω), indicating that N-anion modification can effectively improve the HER kinetic activity of CoS2 catalyst (Figure 3d). Meanwhile, the stability of N−CoS2 NW/CC is further evaluated by the long-term CV cycling and chronoamperometric response tests. As shown in Figure S7 and Figure 3e, the N−CoS2 NW/CC electrode shows negligible degradation of current density after 3000 CV cycling tests. Moreover, after continuous testing at the same current density of 50 mA cm−2, both the overpotentials of N−CoS2 NW/CC and CoS2 NW/ CC remained stable for a long period of 12 h, indicating the excellent stabilities under acid conditions. The unique N-anion decorated metallic CoS2 porous nanowires combined with strong 3D electrode configuration, endowing N−CoS2 NW/CC electrodes with greatly enhanced HER catalytic activity than that of pristine CoS2 NW/CC materials. First, the synergetic incorporation of nitrogen atoms into CoS2 benefits from the use of thiourea as both nitrogen and sulfur sources, resulting in large modification of the morphology, including surface roughness and porous structure, which can offer more catalytic active sites for HER. In order to evaluate the effective active surfaces, the electrochemical double-layer capacitance (Cdl) and BET surface area were measured. As shown in Figure 3f, the N−CoS2 NW/CC electrode shows an extremely high Cdl of ca. 565.5 mF cm−2, which is about 1.9-times that of the pristine CoS2 NW/CC (292.3 mF cm−2). This result is consistent with the result from the BET surface areas (Figure S11), indicating the high exposure of effective electrochemical active sites. Since Cdl is expected to be linearly proportional to the active surface area of electrocatalysts,48−51 these results confirm that the incorporation of N-anion atoms into the framework of CoS2 favors enhancing the catalytic active sites in comparison to the pristine CoS2 material, thus greatly contributing to the enhanced HER performance. Second, the surface anion decoration strategy has been regard as an effective method to regulate the electronic structure of the catalyst without influencing the crystal structure. As shown in Figure 4a, DFT calculation reveals that the density of states (DOS) of N−CoS2 is continuous across the Fermi level, maintaining the intrinsic metallic character of N−CoS2 after the incorporation of N atoms. Meanwhile, the typical metallic behavior of N−CoS2 and pristine CoS2 can further be confirmed by temperaturedependent resistivity curves (Figure 4b). Notably, the resistivity of the N−CoS2 product is obviously lower than that of pristine CoS2 in the whole temperature range, indicating the greatly

Figure 4. (a) Calculated density of states for N−CoS2 material. The Fermi level is set at 0 eV. (b) Temperature-dependent resistivity of N−CoS2 and pristine CoS2 products. The calculated HER free-energy changes of the N−CoS2 catalyst on (c) Co site and (d) S site under acid condition.

enhanced electrical conductivity of N−CoS2 catalyst after Nanion decoration, which can facilitate electron transport during HER. Third, the introduction of nitrogen-anion optimizes the adsorption free energy of hydrogen to greatly promote HER performance. To evaluate the effect of N-anions on activating the reaction dynamic, the calculation of the free energy change (ΔG*H) of hydrogen adsorption has been performed. As shown in Figure 4c,d, compared to the pristine CoS2 catalyst, the value of ΔGH* for N−CoS2 catalyst significantly decreased and is quite close to 0 eV in both the Co-sites and S-sites, indicating H adsorption on the surface of N−CoS2 catalyst will be optimized after the incorporation of N atoms.52,53 This result suggests that the N-anion doping in the CoS2 can further weaken the H−S bond between adsorbed H and S atoms, successfully activating the HER reaction dynamic for higher catalytic activity.

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CONCLUSIONS In conclusion, we highlighted that nitrogen-anion engineering is an effective strategy to improve the catalytic activity for HER. As an example, the nitrogen-anion decorated metallic CoS2 porous nanowire arrays are first successfully prepared by a simple solid reaction and represent a highly active HER electrocatalyst under acid conditions. The introduction of N atoms enables synergistic regulation of the active sites, electronic structure, and reaction dynamic of the CoS2 material. Benefiting from the enhanced electrical conductivity to expedite electron transfer, the modified porous nanowire arrays demonstrate more active site exposure and optimized free energy of hydrogen adsorption for better HER kinetics; the asobtained N−CoS2 NW/CC electrode exhibits excellent HER catalytic activity and stability. This work highlights that N-anion engineering favors the design of highly efficient HER electrocatalysts.

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EXPERIMENTAL SECTION The Synthesis of Co(OH)F NW/CC Precursor. The chemicals used in the synthesis process are all analytical grade and added without further treatment. In a typical procedure, carbon cloths were first washed by deionized water, acetone, and ethanol several times. Then, dilute HNO3 was used to 7408

DOI: 10.1021/acscatal.7b02218 ACS Catal. 2017, 7, 7405−7411

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activated using CV test at a scan rate of 50 mV s−1 several times, and then linear sweep voltammetry (LSV) tests were carried out. Linear sweep voltammetry (LSV) was executed at a scan rate of 10 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements of the catalysts were performed at the overpotential of −150 mV vs RHE by using an AC voltage with 5 mV amplitude in a frequency range from 100 kHz to 10 mHz under 0.5 M H2SO4 solution. Calculation Method. First-principle calculations were performed by the density functional theory (DFT) using the Vienna Ab-initio Simulation Package (VASP) package.54 The generalized gradient approximation (GGA) with the Perdew− Burke−Ernzerhof (PBE) functional were used to describe the electronic exchange and correlation effects.55 Spin-polarization was included for their magnetic properties. The calculated lattice constants are a = b = c = 5.52 Å after full relaxation. The surface was simulated with a single-layer-thick p(2 × 1) 001 plane of CoS2 unit slab with ∼15 Å vacuum. The K points meshing for Brillioun zone was set up as a 5 × 11 × 1 grid centered at the gamma point regarding Monkhorst Pack Scheme for geometric optimization of the slab surfaces. The simulation was run with a cutoff energy of 450 eV throughout the computations. Spin-polarized density functional theory (DFT) calculations were carried out with the Vienna Ab-initio Simulation Package (VASP),56 within the projector-augmented wave (PAW) method.57 The exchange-correlation interaction functional is described by generalized gradient approximation (GGA), in Perdew, Burke, and Ernzerhof (PBE) functional. The strongly correlated interaction of Co 3d electrons is considered with GGA+U (U = 4.0 eV) method.58 The cutoff energy of 500 eV was adopted for the plane-wave basis set. The structures were relaxed by conjugate gradient algorithm implemented in the VASP code until the forces on the atoms were less than 0.01 eV/Å. The criterion for the total energy is set as 1 × 10−5 eV.

activate carbon cloths under appropriate temperature. After oxidation treatment for a certain time, carbon cloths were sonicated in deionized water to remove the residual acid. Next, 1 mmol of Co(NO3)2·6H2O, 3 mmol of NH4F, and 5 mmol of urea were dissolved in 35 mL of deionized water under vigorous stirring for a while, and then a cleaned CC (1 cm × 2 cm) was immersed into the solution. The solution together with the CC was transferred to a 40 mL Teflon-lined stainlesssteel autoclave and held at 120 °C for 12 h. After the autoclave was naturally cooled down to room temperature, the Co(OH)F NW/CC was washed with water and ethanol several times and dried in a vacuum oven. The Synthesis of Pt/C−CC, N−CoS2 NW/CC, and CoS2 NW/CC 3D Electrode Material. To synthesize the N−CoS2 NW/CC sample, a piece of Co(OH)F NW/CC was placed next to the thiourea powder in a porcelain boat with thiourea powder at the upstream side of the tube furnace. Subsequently, the furnace was heated to 500 °C at 10 °C min−1 under a flowing Ar atmosphere and maintained for 2 h, and then program-cooled to ambient temperature. The synthesis process of the CoS2 NW/CC 3D electrode material was similar to that of the N−CoS2 NW/CC electrode except for replacing thiourea with S powder as reactant. Moreover, the Pt/C−CC electrode was also prepared by drop coating method, and the loading amount of Pt/C was about 4 mg/cm2. Materials Characterization. X-ray powder diffraction (XRD) was acquired using a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). Transmission electron microscopy (TEM) was performed on a JEM-2100F field-emission electron microscope operated at an acceleration voltage of 200 kV. Field-emission scanning electron microscopy (FE-SEM) images were obtained on a JEOL JSM-6700F SEM. X-ray photoelectron spectra (XPS) were performed on an ESCALAB MK II X-ray photoelectron spectrometer with Mg Kα as the excitation source. Highresolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and corresponding energy-dispersive spectroscopy (EDS) mapping analyses were executed on a JEOL JEM-ARF200F TEM/STEM with a spherical aberration corrector. Co L-edge XANES spectra were measured at the beamline U19 of National Synchrotron Radiation Laboratory (NSRL, Hefei) in total electron yield (TEY) mode by collecting the sample drain current under a vacuum greater than 10−7 Pa. The absorption spectra of Co K-edge were collected in transmission mode using a Si(111) double-crystal monochromator at the X-ray absorption fine structure (XAFS) station of the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF) and the beamline 14W1 of the Shanghai Synchrotron Radiation Laboratory (SSRL, Shanghai) at room temperature. The electrical transport property measurements were carried out on pressed pellets using a Keithley 4200-SCS semiconductor characterization system and the four-point probe method. Electrochemical Measurements. HER electrochemical performance was tested in a three-electrode system on an electrochemical workstation (CHI660B). Ag/AgCl (3.3 M KCl) electrode was used as the reference electrode, while the counter electrode was a graphite rod electrode. All of the potentials were calibrated to the reversible hydrogen electrode (RHE) according to Nernst equation in 0.5 M H2SO4. As a 3D electrode, the as-obtained N−CoS2 NW/CC was tailored into 0.5 cm × 0.5 cm and directly used as the working electrodes for the electrochemical tests. The working electrodes were first

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b02218. SEM and TEM images of Co-based precursor electrode and CoS2 NW/CC, XRD pattern of as-prepared CoS2 NW/CC product, XPS of as-prepared N-CoS2 and CoS2, exchange current densitites overpotential and Tafel slope of as-prepared catalysts, stability tests of the CoS2 NW/ CC in acid medium, double-layer capacitance of asobtained electrodes, photograph showing the evolution of H2 and O2 gases from the electrodes, chronoamperometric response of as-prepared N-CoS2 and CoS2, N2 adsorption/desorption isotherms, XRD and IR-corrected HER of N-CoS2 NW/CC synthesized at different temperatures, and comparison of HER performance with that of other cobalt sulfide materials (PDF)

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AUTHOR INFORMATION

Corresponding Author

*C. Z. Wu. E-mail: [email protected]. ORCID

Xiaojun Wu: 0000-0003-3606-1211 Changzheng Wu: 0000-0002-4416-6358 Yi Xie: 0000-0002-1416-5557 7409

DOI: 10.1021/acscatal.7b02218 ACS Catal. 2017, 7, 7405−7411

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ACS Catalysis Author Contributions

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P.C. and T.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2015CB932302), Natural Science Foundation of China (Nos. U1432133 21501164, 1162163, U1632154), National Program for support of Top-notch Young Professionals, the Anhui Provincial Natural Science Foundation (No. 1608085QA08), and the Fundamental Research Funds for the Central Universities (No. WK2060190080). We also appreciate the support from the Major/Innovative Program of Development Foundation of Hefei Center for physical Science and Technology.

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DOI: 10.1021/acscatal.7b02218 ACS Catal. 2017, 7, 7405−7411