Combination of Theory and Experiment Achieving a Rational Design

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Combination of Theory and Experiment Achieving a Rational Design of Electrocatalysts for Hydrogen Evolution on the Hierarchically Mesoporous CoS2 Microsphere Anqi Wang,†,∥ Man Zhang,†,∥ Haobo Li,‡ Fan Wu,§ Kai Yan,*,† and Jianping Xiao*,∥ †

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-sen University, 135 Xingang Xi Road, Guangzhou 510275, China ‡ Institute of Natural Sciences, Westlake Institute for Advanced Study, Westlake University, Hangzhou 310024, China § School of Materials Science and Engineering, Harvard University, Boston 02138, United States

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S Supporting Information *

ABSTRACT: We report that the combination of theoretical and experimental results has demonstrated that superior electrocatalytic performance can be achieved with the hierarchically mesoporous CoS2 microsphere terminated with CoS2 sheet arrays and highly exposed sulfur surfaces with an optimal hydrogen adsorption energy. The as-prepared mesoporous CoS2 microsphere shows a rather low overpotential of 90 mV at 10 mA cm−2 and a small Tafel slope of 67.0 mV dec−1, which is comparable to the precious 20 wt % Pt/C catalyst. Besides, two times higher electrochemical active surface area and excellent stability are obtained. The overall performance is superior to most of the previously reported catalysts. More importantly, we performed density functional theory calculations and have documented that a number of sulfur sites on the CoS2 surface exhibit high intrinsic hydrogen evolution activity and resistance to hydroxyl poisoning, leading to the superior catalytic performance and durability.

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variations,12 resulting in some quantitative deviations between the results under different experimental conditions, a clear activity trend of volcano-shaped behavior is visible. Based on the previous theoretical and experimental investigations, Pt-based catalysts are by far the most active ones and thus used as a benchmark for hydrogen evolution, but the high cost and scarcity of Pt have greatly hindered their widespread applications. After DFT calculations successfully predicted that MoS2 is a promising hydrogen evolution electrocatalyst,13 there have been remarkable advances in the exploitation of earth-abundant transition metal dichalcogenides, MoS2, MoSe2, etc., as electrocatalysts for hydrogen evolution because of their low cost and high abundance.14−20 MoS2 edge sites have been confirmed with high hydrogen evolution catalytic activity.14,20−24 Recently, catalysts based on Co doped MoS2 exhibit even higher hydrogen evolution activity because of the activated in-plane S sites neighboring to Co atoms.24−27 However, there is little attention to expose the sulfur surface efficient for hydrogen evolution. Our recent works have shown that the MoS2 sphere on Ni foam28 and the NiS microsphere10 could expose more active sites and enhance the activity, however, the morphology of the microsphere is less regular and controllable, the overpotential is still high

INTRODUCTION Hydrogen is an excellent medium for energy storage, several key industrial processes (e.g., hydrogen fuel cell), and playing a critical role as an energy carrier in future hydrogen economy.1−3 Electrocatalytic hydrogen production is expected to play a major role in the sustainable development of energy conversion processes in reducing our dependence on fossil fuels. Over the years a number of studies have been performed to investigate characteristics of hydrogen-related processes.4−7 Density functional theory (DFT) calculations and microkinetic modeling for hydrogen evolution on transition metal surfaces8−10 and Pd pseudomorphs11 have been previously developed. The hydrogen evolution activity trends are often described by the free energy of H adsorption (ΔGH). Experimental hydrogen evolution activities of various transition metals represent a volcano plot relationship with ΔGH.7,8 This yields a volcano shaped activity trend with the peak corresponding to ΔGH = 0 eV. From the reported volcano plot, Pt, Pd, and Ru in the form of a bulk crystal and Pd/PtRu(111) overlayer stand close to the top of the volcano plot. At ΔGH = 0 eV, their binding energy to hydrogen is neither too strong nor too weak, therefore yielding good catalytic performance. In comparison, metals and pseudomorphic surfaces on the left leg of the volcano curve bind hydrogen too strongly, whereas those catalysts on the right leg bind hydrogen too weakly and thus are not adequate catalysts. Although there are some factors, for instance, coverage and reaction mechanism © XXXX American Chemical Society

Received: February 25, 2019 Revised: May 9, 2019

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DOI: 10.1021/acs.jpcc.9b01814 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (∼100 mV at 10 mA cm−2), and the durability still has large space to improve.10,28 Herein, we reported that the significantly enhanced hydrogen evolution activity can be achieved by the controllable fabrication of hierarchically mesoporous CoS2 microspheres with the exposure of the sulfur surface. The hierarchically mesoporous CoS2 microsphere terminated with vertical arrays of CoS2 sheets displays a rather low overpotential of 90 mV at 10 mA cm−2, a small Tafel slope of 67.0 mV dec−1, and excellent stability, which are comparable to the precious 20 wt % Pt/C catalyst and far superior to that of the commercial CoS2 catalyst. More importantly, theoretical and experimental results confirmed that hierarchically mesoporous CoS2 microspheres terminated with vertical arrays of CoS2 sheets have proved to be an outstanding hydrogen evolution electrocatalyst. The overall activities are far better than those of previously reported metal sulfide materials and superior to most of many traditional materials.

rotation symmetry by inversion through the center of CoS2 (Figure 1d), which belongs to the (Pa 3̅) symmetry and space group no. 205. The crystalline phase of CoS2 nanoparticles was further documented by the powder X-ray diffraction (XRD) (Figure 2a). XRD patterns show the strongest peak from (311) and other weak peaks from (200), (210), (211), (220), and (321). The XRD pattern of the mesoporous CoS2 was indexed to the cubic phase of CoS2 (JCPDS no. 89-1492), indicating the successful synthesis of the CoS2 material. Based on the Scherrer equation, the average crystallite size of mesoporous CoS2 microspheres was calculated to be 15−20 nm. No other peak was visible, indicating the good phase purity. Inductively coupled plasma was further used to confirm the molar ratio of S versus Co was 1.97. The N2 adsorption−desorption isotherm suggested an obvious hysteresis loop at a relative pressure in the range of 0.35−1.0, indicating the CoS2 samples display the type-IV adsorption isotherm with a H3 type hysteresis loop of the mesoporous structure (Figure 2b). A large BET surface area of 64.5 m2·g−1 and total pore volume of 0.33 cm3·g−1 were calculated on the mesoporous CoS2 material. The pore size of the as-made CoS2 was calculated based on the BJH method, showing a pore size of 4.7 nm. X-ray photoelectron spectroscopy (XPS) was further investigated for CoS2 as shown in Figure 2c,d. The broad scan spectrum (Figure 2c) indicated the presence of Co and S elements, together with C and O due to air exposure. The Co 2p spectra were fitted by the Gaussian fitting method and shown in Figure 2d. The Co 2p spectrum consists of two well-resolved peaks (Figure 2d) located at 794.6 and 779.7 eV, which are corresponding to Co 2p1/2 and Co 2p3/2, respectively. As shown in Figure S3a, the peaks at 163.7 and 162.6 eV are ascribed to 2p1/2 and 2p3/2 core levels of S2− in CoS2, respectively. The C 1s spectra of CoS2 (Figure S3c) indicates the existence of C−C (284.7 eV) and O-attached carbons because of the presence of CO as well as C−O peaks (Figure S3d). Electrocatalytic hydrogen evolution behavior of the synthesized CoS2 microsphere was evaluated using the threeelectrode facility with Ar-saturation in acidic media (0.5 M H2SO4, pH = 0.38). For reference, the commercial CoS2, 20 wt % Pt/C, and bare CFP were also used to investigate. As shown in Figure 3a, bare CFP shows negligible hydrogen evolution activity. In comparison, the mesoporous CoS2 on CFP (abbreviated as meso-CoS2), commercial CoS2 on CFP (abbreviated as com-CoS2), and 20 wt % Pt/C catalysts all manifested high electrocatalytic activities of hydrogen evolution. It was clear to note that a much lower overpotential from the mesoporous CoS2 microsphere. Besides, the electrochemical active surface area (ECSA) of the mesoporous CoS2 (0.546 mF cm−2) material was two-folder higher than that (0.269 mF cm−2) of the commercial CoS2 (Figure S4), further indicating the important role of the mesoporous structure of the CoS2 microsphere in enhancing the activities. To better understand the reaction kinetics of the hydrogen evolution process, Tafel plots were performed. In Figure 3b, the calculated value of the mesoporous CoS2 microsphere is ∼67.0 mV·dec−1 which is close to that of the precious 20 wt % Pt/C, suggesting a facile Volmer−Heyrovsky mechanism. Electrochemical impedance spectroscopy measurements (Figure S5) reveal that the radius of the semicircle is sharply reduced in comparison with commercial CoS2. The high durability of the mesoporous CoS2 microsphere material is confirmed by the chronoamperometric curve in Figure 3c. The

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RESULTS AND DISCUSSION To purposely build up the hierarchically porous CoS 2 microsphere, a soft template-assisted process has been developed. This new route involved the direct interactions of the P123 template with the cobalt precursor, atoms and ions, resulting in an easily controllable process. Scanning electron microscopy (SEM) images of the as-obtained CoS2 on carbon fiber paper (CFP) are shown in Figure 1a,b. The CoS2 particles

Figure 1. SEM images (a,b), TEM images (c), and the center (d) of the as-obtained CoS2 microsphere.

are subsphacroidal on the CFP substrate (Figure 1a). The porous morphology over the microspherical material with the hierarchically porous surface terminated with CoS2 sheet arrays (Figure 1b). In comparison, the SEM image of the commercial CoS2 was also performed for comparison (Figure S1). An irregular morphology of the bulk material was observed and no porosity existed in the commercial CoS2 material. Transmission electron microscopy (TEM) images further indicate the mesoporous internal structure of CoS2 spheres (Figure 1c). In Figure 1c, the lattice spacing of d = 0.30 nm is well matched with the exposed (311) planes of the CoS2 crystal. The selected area electron diffraction pattern (inset in Figure 1c) further manifests the polycrystalline CoS2. Figures S1a and S2 show the high-resolution (HR)-TEM image of a CoS2 particle that is on the ⟨111⟩ zone axis, with a schematic illustration of the CoS2 crystal lattice (Figure S1b). This confirms the 3-fold B

DOI: 10.1021/acs.jpcc.9b01814 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. XRD pattern (a), N2 adsorption−desorption isotherm (b, inset was pore size distribution), XPS survey (c), and Co 2p analysis (d) of the synthesized mesoporous CoS2 microsphere.

Figure 3. Electrochemical performance: (a) CV curves of CFP, commercial CoS2 on CFP (abbreviated as com-CoS2), as-obtained mesoporous CoS2 on CFP (abbreviated as meso-CoS2), and commercial 20 wt % Pt/C; (b) Tafel plots of com-CoS2, meso-CoS2, and commercial 20 wt % Pt/ C; (c) chronoamperometric test of meso-CoS2 catalyst; (d) present performance of meso-CoS2 compared with previously reported catalysts from literatures.

surface structures of CoS2 catalysts (Figure S6). The first step was to search for surface sites with high hydrogen evolution activity. The exchange current density [log(i0)] was chosen to represent the intrinsic activity of hydrogen evolution, which could be analytically obtained from the adsorption free energy of *H on the surface (ΔGH) through microkinetic modeling8 (Figure S7a). As shown in Figure S7b, we found that a number of various sites on CoS2 surfaces (e.g., (100), (210), and (311)) also show high hydrogen evolution activity comparable to that on the Pt(111) surface. Three CoS2 surfaces, that is (100), (210), and (311) (Figures S6 and S8), have been taken into account with different active structures. The Co sites on CoS2(100), one S site on CoS2(311) and four S sites on CoS2(210), were found with high hydrogen evolution activity

current density maintained over 10 h without an observable decrease under the continuous operation at an overpotential of 188 mV, confirming the high durability of the fabricated mesoporous CoS2 microsphere. To benchmark with previously reported catalysts, the mesoporous CoS2 microsphere in this work requires a very low overpotential of 90 mV at 10 mA· cm−2. This is, by far, one of the best chalcogenide catalysts for hydrogen evolution in acidic media (Figure 3d and Table S1). The overall activities are far better than those of many traditional materials.15,27,29−36 Besides, the facile synthesis made this methodology more attractive and practical. Using DFT calculations, we studied the hydrogen evolution activity of CoS2 catalysts. Theoretical models for CoS2(100), (210), and (311) surfaces have been built to simulate the C

DOI: 10.1021/acs.jpcc.9b01814 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. (a) The hydrogen evolution activity trend over a variety of CoS2 surfaces and Pt(111). The number 1−4 represent different adsorption sites. (b) *H adsorb on the Co site of the (100) and S site on (210) and (311) surfaces. Blue, yellow, and white balls represent Co, S, and H atoms, respectively. (c) Calculated binding energies (ΔEOH) of *OH on different sites of CoS2 surfaces, in comparison to that on the Pt(111) surface. A greater ΔEOH represents weaker OH binding.

active CoS2 electrode. The free energy change (ΔGH) of hydrogen adsorption has been calculated and the ΔGH for a CoS2 electrode was quite close to 0 eV on CoS2(100) (0.04 eV), CoS2(210) (0.05 eV), and CoS2(311) (0.06 eV). As the hydrogen adsorption and H2 desorption can be compromised and achieve the optimum kinetics simultaneously at ΔGH = 0, indicating that the strength of hydrogen adsorption on the surface of mesoporous CoS2 electrodes would be ideal for hydrogen evolution. The present experimental synthesis has successfully fabricated a CoS2 polycrystalline electrode with a high ratio of CoS2(311) exposure, validating that the hydrogen evolution activity of the hierarchically porous CoS2 microsphere was pretty high.

comparable with the Pt(111) surface (Figure 4a), with the corresponding adsorption configuration shown in Figure 4b. Note that these sites were also the most favorable sites for hydrogen adsorption on the surfaces (the most stable ΔGH in Figure 4a), thus hydrogen evolution processes were likely to be facile on CoS2 surfaces. The active sulfur atoms in Co-doped MoS2 were unsaturated 3-coordinated sites decorated by Co doping, while some unsaturated 2-coordinated sulfur sites were active for the stepped CoS2 surface (e.g., 210 and 311). In comparison, the saturated 3-coordinated sulfur sites in the pristine MoS2 or flat CoS2(100) surface were not active. Therefore, the unsaturated coordinate environment for S atoms (less than 3-coordinated), that is, S-rich surface, made the exposed sulfur sites more active for hydrogen evolution. In addition to the intrinsic activity on the catalyst surface, the competition of other possible coadsorbed species may also have an impact on the actual catalytic performance. Under hydrogen evolution reducing conditions, the water can dissociate, and the adsorbed *OH groups might occupy some of active sites, thus a good hydrogen evolution catalyst is preferable with poor *OH adsorption ability. Therefore, the corresponding binding energy of *OH (ΔEOH) on the active sites has been also examined. As shown in Figure 4c, compared with the Pt(111), the Co site on CoS2(100) is more easily poisoned by *OH, while the S sites on CoS2(210) and (311) show comparable or even better resistance to *OH poisoning. Furthermore, these CoS2 surfaces also show good stability relative to Pt(111), according to the lower surface energy (γ) shown in Table S2. In summary, CoS2 can be predicted to be a good hydrogen evolution catalyst with the S sites exhibiting both high hydrogen evolution activity, stability, and resistance to *OH poisoning, showing great potential as a nonprecious alternative to Pt. Certainly, the apparent activity of catalysts was highly correlated with the total number of active sulfur sites. Through controllable synthesis, exposing more surface S sites would be important for the large enhancement of the catalytic apparent activity. DFT calculations and microkinetic modeling analysis indicated that CoS2(100), CoS2(210), and CoS2(311) were quite active in comparison with the Pt electrode. Meanwhile, we found that CoS2(311) was quite stable in terms of surface energy (Table S2), showing the feasibility of preparing a highly

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CONCLUSIONS In summary, we have successfully built the hierarchically mesoporous CoS2 microsphere with a high ratio of CoS2(311) exposure by a soft template P123-assisted process. The measured hydrogen evolution activity over the hierarchical mesoporous CoS2 microsphere with the exposure of sulfur surface was indeed quite high. The hierarchically mesoporous CoS2 microsphere shows a rather low overpotential of 90 mV at 10 mA cm−2 and a small Tafel slope of 67.0 mV dec−1, which were comparable to the precious 20 wt % Pt/C catalyst. Besides, two times higher ECSA and excellent stability were obtained. The overall performance was superior to most of the previously reported catalysts. DFT calculations were further performed to elucidate the hydrogen evolution activity over a number of CoS2 facets and sites on these facets, indicating that the CoS2(311) facet was a promising active site for hydrogen evolution in terms of the most stable surface energy and high intrinsic activity. The combination of the controllable experimental synthesis, DFT calculations, and microkinetic analysis provides a useful guideline for the catalyst design of hydrogen evolution.

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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/acs.jpcc.9b01814. Details of catalyst characterizations, summarized Table compared with the previous literature, calculated surface D

DOI: 10.1021/acs.jpcc.9b01814 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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energy, SEM images, HR-TEM images, XPS analysis, ECSA calculation, and DFT calculation of the representation of the three studied CoS2 surface structures and adsorption configurations (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.Y.). *E-mail: [email protected] (J.X.). ORCID

Kai Yan: 0000-0001-9001-6337 Jianping Xiao: 0000-0003-1779-6140 Present Address ∥

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China.

Author Contributions ⊥

A.W. and M.Z. authors contribute equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Key R&D Program of China (2018YFD0800700), Ten Thousand Talent Plan, National Natural Science Foundation of China (21776324, 21802124 and 91845103), Science and Technology Planning Project of Guangdong Province, China (2014A050503032), and Hundred Talent Plan (201602) from Sun Yat-sen University, China.

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DOI: 10.1021/acs.jpcc.9b01814 J. Phys. Chem. C XXXX, XXX, XXX−XXX