Accelerated Hydrogen Evolution Reaction in CoS2 by Transition-Metal

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Accelerated Hydrogen Evolution Reaction in CoS2 by Transition-Metal Doping Jingyan Zhang, Yuchan Liu, Changqi Sun, Pinxian Xi, Shanglong Peng, Daqiang Gao, and Desheng Xue ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00066 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018

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ACS Energy Letters

Accelerated Hydrogen Evolution Reaction in CoS2 by Transition-Metal Doping Jingyan Zhang, Yuchan Liu, Changqi Sun, Pinxian Xi, Shanglong Peng, Daqiang Gao*, Desheng Xue*. Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of Special Function Materials and Structure Design of MOE, Lanzhou University, Lanzhou 730000, People’s Republic of China Email: [email protected], [email protected]

ABSTRACT Cobalt pyrite (CoS2) is one of the promising candidate catalysts for electrocatalytic hydrogen evolution because of its efficient catalytic activity sites and inherent metallicity. Herein, we report the greatly improved electrocatalytic activity of CoS2 by Mn doping. Firstly, we give the theoretical prediction that Mn is the most excellent dopant to activate the electrocatalytic activity of CoS2 with the smallest Gibbs free energy (|∆GH*|) and the remain metal property. Secondly, to give the experiment evidence, Mn doped CoS2 nanowires are prepared by hydrothermal and post sulfuration. Where the optimized sample shows a low overpotential of 43 mV at 10 mA/cm2, a Tafel slope of only 34 mV/dec as well as a long time stability for hydrogen evolution reaction. This work opens the new way to arouse the electrocatalytic activity of other pristine candidate catalysts.

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Hydrogen, as the most clean energy carrier for replacing traditional fossil fuels, has attracted much attention.1,2 Particularly, hydrogen production from electrolysis of water is considered to be an attractive candidate in future hydrogen economy, where the key step in this technology is to develop efficient electro-catalysts.3-5 At present, Pt-based catalysts are still the most efficient catalysts for hydrogen evolution reaction (HER), but their widespread applications are limited by their low abundance and high cost.6,7 Therefore, exploring cheap, durable and efficient HER electrocatalysts are scientists' ultimate goal. Generally, an efficient HER electrocatalyst should possess the properties of both having amounts of active sites and high conductivity.8-10 As the representative HER catalysts, transition metal dichalcogenides (MoS2, WS2, Co-Mo-S),11-13 selenides (MoSe2, NiSe2, CoP2xSe2(1.x)),14-16 nitrides (MoN, Ni3N),17,18 phosphide (Ni2P, FeP, CoP),19-21 carbide (Mo2C, WC)22,23 and boride (Ni2B, Co-Ni-B)24,25 have attracted significant research interest and exhibit some noticeable properties. Nevertheless, most of these catalysts' HER properties still can't catch up with and surpass the state-of-the-art Pt-based catalysts. Analyzing the causes, poor conductivity and inert sites are still their fatal drawbacks, even methods used by incorporating carbon-based 2


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conductive material with them26-28 or activating the catalysts' active sites by doping or recombination with other catalysts.29-31 Thus, efficient methods and strategies are still needed to design and develop earth-abundant alternatives that are suitable for water splitting. Recently, metallic cobalt pyrite (CoS2), as a representative of pyrite-type transition metal dichalcogenides has emerged as low-cost materials with high HER activity based on both computational and experimental studies.32,33 It is reported that the HER performance of CoS2 can be enhanced by nano-structuring,34,35 incorporating on the surface of MoS2/RGO to form three-tiered cake-style material,36 as well as, doping of nonmetal element N and P.37,38 Besides, our previous results demonstrated that Cu dopants can optimization Co sites and the inert S sites.39 These results provide an alternative pathway to adjust the electronic density and active sites of CoS2 for activating HER by doping. In addition, in view of the classic “volcano” theory of metal atoms doped MoS2,40 near zero free energy of H adsorption on MoS2 with optimized energy level and more activated inert sites leads to maximum activity toward HER. With these key concepts of active sites and energy level modulation in mind, chemically doping with transition-metal will be a useful model system to accelerate the kinetics of HER in CoS2. To study the dependence of transition-metal doping on the activating of catalytic activity in CoS2, we first use density function theory (DFT) calculations to investigate the HER activity of transition metal atoms doped CoS2 system. Generally, the Gibbs free energy of adsorbed H* on the active site (|∆GH*|) can be used to evaluate the 3



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HER activity of catalysts, where thermo-neutral (|∆GH*|=0) reveals the outstanding HER performance of the catalysts.41 Thus, we calculated the ∆GH* on the stable (001) surface of CoS2 with different dopants. The hydrogen evolution activity was calculate via a 2×2×2 supercell, where we presumed one Mn atom instead of one Co atom in the case of Mn doped CoS2 (the calculation details are shown in supporting information). As shown in the volcano plots (Figure 1a), the doped metal atoms replace the Co atoms of CoS2, which can obviously decrease the ∆GH* of Co sites. Particular, some dopants (V, Mn, Fe, and Ni) could activate the adjacent Co atoms with the lower |∆GH*| (< 0.1 eV), revealing their potential excellent electrocatalytic activity. Besides, some dopants themselves (Mn, Fe, Ni and Cu) could act as the new active sites in CoS2 matrix, which can further increase the catalysts' catalytic activity in HER (supporting information Figure S1). Noting that whether these dopants could be easily and stable doped into the CoS2 matrix or not, is determined by the formation energy.42,43 Here, the case of the formation energies of substitution Co dopants by other transition metal atoms in CoS2 matrix are also calculated, where the zero formation energy generally suggests that the dopants can be easily doped into CoS2 matrix and can be stable in the system. As shown in Figure 1b, besides Ti, V and Zn dopants, other metal atoms could be stable doped into the CoS2 matrix with the relative low formation energies. Coincidently, the systems of Mn, Fe and Ni doped CoS2 have the both lower ∆GH* and formation energies, foreboding their high HER activity. Among them, Mn is the best candidate dopant to well tune the electrocatalytic activity of CoS2. 4


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Figure 1. The relation between currents (log(i0)) and the Gibbs free energy of adsorbed H* (∆GH*) presents a volcano curve. (a) ∆GH* on the (001) surface of CoS2 with different metal dopants. (b) Summarized formation energies of different metal dopants substituted Co in CoS2.

Moreover, the electronic structures of these doped systems are also studied by using the 2×2×2 supercell with one Co atom replaced by one transtion metal atom (Figure S2). As shown in Figure 2a, the pristine CoS2 is metallic. After Mn doping, the system still keeps metallic nature. Importantly, there are three new bands contributed by the hybridization of the S-p, Co-d and Mn-d orbitals appear near the Fermi level in Mn doped system (Figure 2b, Figure S3, Figure S4). What's more, the electron occupied states at the Feimi level for Mn doped CoS2 are more than that of pure CoS2 (Figure S5), which can accelerate the electron transfer in catalysis in HER.44 Further, to evaluate the effect of Mn dopant on activating the reaction dynamic, the calculated free energy change (∆GH*) of hydrogen adsorption on different sites are presented in Figure 2c. Compared to the pristine CoS2 catalyst, the value of ∆GH* for Mn doped CoS2 catalyst significantly decreased both on adjacent 5



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Co sites (∆GH*= -0.12 eV at Co2 site, 0.03 eV at Co1 site) and Mn dopant (∆GH* = 0.03 eV), indicating H adsorption on the surface of CoS2 catalyst will be optimized after incorporation of Mn atoms. The optimized atomic structures with H adsorption at Co sites and Mn site presented in Figure 2d. Thus, the strategy of introducing Mn dopant to optimize the HER activity of CoS2 is grounded in both engineering the electronic structure and activating the catalytic activity of Co atoms.

Figure 2. The calculated electronic bands and density of states (DOS) results of the (a) CoS2 and (b) Mn doped CoS2, respectively. (c) Free-energy diagram for H adsorption on the CoS2 and Mn doped CoS2 with the (001) surfaces for Co sites and Mn site. (d) The optimized atomic structures of Mn doped CoS2 with H adsorption at Co sites (Co1 and Co2 sites) and Mn site.

Motivated by above promising prediction, series Mn doped CoS2 nanowires were fabricated and we further established them as a cost-efficient and highly active electrocatalyst for HER. A two-step strategies were developed to synthesize Mn doped CoS2 nanowires on the glass substrate. Firstly, we fabricated the pure or Mn doped nanowires precursors, and then sulfuration them at 500°C in argon atmosphere. 6


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The Mn doping concentration is determined by the XPS measurement, which is about 0 at.%, 0.97 at.% and 4.36 at.% for the fabricated three samples, named of CoS2, Mn-1% and Mn-5% for convenience. The XRD result indicates that the precursor of CoS2 corresponds to Co(CO3)0.5OH and it shows the smooth nanowires morphology (Figure S6). The details of the preparation process are shown in support information, and the schematic is presented in Figure 3a. The crystallinity and phase information of Mn doped CoS2 nanowires are first confirmed by X-ray diffraction (XRD). As shown in Figure 3b, all the diffraction peaks are consistent with the standard pattern of CoS2 (JCPDS 41-1471) with lattice constants a=b=c=5.538 Å and a space group of Pa-3. It is worth mentioning that there are no obvious shifts in the peaks positions of XRD after incorporating of Mn, indicating that Mn ion is very compatible substitution into the phase structure of CoS2 matrix without forming MnCo alloys or manganese compounds. The homogeneity of substituting Mn2+ ion might attribute to its similar ionic radius and electronegativity as Co2+ ion. The Raman spectra of Mn doped CoS2 nanowires are shown in Figure S7, where the strong peak observed at 392 cm-1 and the weak peak appeared at 289 cm-1 correspond to the Ag and Eg in the vibrational modes of CoS2, respectively.45 Scanning electron microscopy (SEM) images reveal that the Mn doped samples have the similar morphology as pure CoS2 (Figure S8). As the representative, the SEM images of sample Mn-5% under different magnification are shown in Figure 3c and 3d. Obviously, it's made up of nanowires with the diameter about 100 nm, pointing in different directions. There are lots of pores in each nanowire, which can lead to the larger specific surface area of the sample and may 7



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create more active sites for further electrocatalytic hydrogen evolution reaction. Transmission electron microscopy (TEM) image of Mn-5% catalyst shown in Figure 3e further confirms the porous nanowire morphology. The high-resolution TEM result reveals visible lattice (Figure 3f), where two sets of noticeable lattice spacing corresponding to the (220) and (211) crystal plane are marked, respectively. The energy dispersive X-ray (EDX) mappings of Mn-5% catalyst reveal that the Co, S and Mn elements are homogeneous distributed in the whole nanowire, indicating that Mn element is uniformly incorporated into the CoS2 matrix.

Figure 3. (a) The schematic of sample preparation progress. (b) The X-ray diffraction (XRD) patterns of Mn doped CoS2 catalysts. (c) The low and (d) high magnified SEM images of sample Mn-5%. (e) The microscopy image obtained by TEM. (f) The high-resolution TEM image. (g) Element distribution mappings of Co, S and Mn in sample Mn-5%.

X-ray photoelectron spectroscopy (XPS) measurements were obtained to analyse

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elemental composition and valence states of Mn doped CoS2 catalysts. Figure S9 presents the wide XPS results of pure CoS2 and Mn-5% with the detected elements of C, O, S, Co, and Mn for pure and Mn doped CoS2. The high resolution XPS spectra of Co 2p, S 2p, Mn 2p for sample Mn-5% and pure CoS2 are shown in Figure 4a-c, respectively. The Co 2p spectra of both samples could be fitted into one group of spin–orbit peaks at 778.3 eV and 793.0 eV corresponding to Co 2p3/2 and Co 2p1/2 accompanied by two broad satellite peaks, revealing that Mn doping has no affection on the valence state of Co. Two peaks are at 641.7 eV and 653.5 eV for the Mn 2p spectra (Figure 4c), assigning to the Mn4+, the other two peaks at 643.3 eV and 654.7 eV relating to the Mn3+.46 As shown in Figure 4b, the high resolution spectra of S 2p has two peaks at 162.6eV and 163.9 eV matched to S 2p3/2 and S 2p1/2.47 Noting that the small peak of 161.5 eV in S 2p appears in the Mn doped CoS2 compared to pure CoS2, revealing the decreased valence state of S. It is caused by the Mn doping, where Mn dopant can provide more electrons to S than that of Co, corresponding to our bader charge calculation results (supporting information, Table S1). In addition, the electrical behaviors of as prepared pure and Mn doped CoS2 samples are measured by using four-electrode method. It can be seen that the resistivity of the samples decrease with the decreasing of the measured temperature, indicating their intrinsic metal properties. The important is that the Mn doped sample shows the lower resistivity than that of pure CoS2, which is consistent with our above calculation results. Thus, all the aforementioned characterizations confirm the successful synthesis of Mn doped CoS2 nanowires. 9



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Figure 4. The XPS results for pure CoS2 and Mn-5%.(a) Co 2p, (b) S 2p and (c) Mn 2p spectra. (d) Temperature-dependent resistivity for sample CoS2 and Mn-5%.

Next, the electrochemical hydrogen evolution reaction (HER) performance of the prepared CoS2 and Mn doped CoS2 nanowires were evaluated by electrochemical testing in 0.5 M H2SO4 solution, which is measured via a typical three-electrode system, including reference electrode, counter electrode and working electrode. The electrocatalytic activities of commercial Pt/C (20 wt% platinum on Vulcan XC72) were also measured as reference under identical condition. During the testing, Vulcan XC72 was added in the catalysts to make the catalysts distribute uniform.48,49 All the electrochemical measurements have been iR-corrected, where i is the test current, and R is the compensation resistance. Figure 5a shows the polarization curves measured by linear sweep voltammetry (LSV) of all the samples. The Polarization curves of Vulcan XC72 and the catalysts without Vulcan XC72 were also tested (shown in Figure S10, Figure S11). Here, we obtained onset overpotentials of samples at the polarization curves by tangent method (Figure 5a, Figure S12). It can be seen that 10


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pure CoS2 nanowires exhibits the onset overpotential of 137 mV and an overpotential of 187 mV is demanded to reach the current density of 10 mV/cm2. Although its nanowires structure results in its better performance than other reported CoS2,50 it is still not satisfactory to hydrogen economy. After Mn doping, the sample Mn-5% exhibits a lowest onset overpotential of 15 mV. When the current density is 10 mA/cm2, an overpotential of only 43 mV is required. These results indicate that incorporation of Mn dopant into CoS2 can greatly improve the electrocatalytic performance. Tafel slope is an inherent parameter to evaluate the HER reaction kinetics of catalysts, which is common calculated by linear fitting the polarization curves according to the Tafel equation.51 As shown in Figure 5b, the Tafel slope of catalyst Mn-5% is only 34 mV/dec, which is comparable to commercial Pt/C (~30 mV/dec). The smallest Tafel slope of catalyst Mn-5% (87 mV/dec for pure CoS2 and 73 mV/dec for Mn-1% catalyst) reveals its fastest electron transfer progress in HER. The electrocatalytic hydrogen evolution properities of Mn-7% and Mn-9% with higher Mn contents were prepared and tested (Figure S13), but they were not as outstanding as Mn-5%. To further explore the effect of Mn doping on the samples’ electrochemically active surface area (EASA), the double-layer capacitances (Cdl) was measured by cyclic voltammetry at different scan rates according reported method.34,52,53 The details of test and calculation are described in the supporting information (Figure S14). A large double layer capacitance reveals more catalytic active sites, that is, a greater catalytic activity surface area. As shown in Figure 5c, the capacitance of Mn-5% 11



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electrode is 28 mF/cm2, larger twice than that of pure CoS2, revealing that the effective active sites increased with the increasing of Mn dopant. Further, to reveal the intrinsic activity of the Mn doped CoS2 catalysts, we estimated the turnover frequency (TOF) by the method described in supporting information (Figure S15). The results are in accordance with the double-layer measurements. The best catalyst Mn-5% shows the highest TOF at a certain overpotential. It can be seen from Figure 5d that when to reach 0.725 s-1 of TOF, overpotential of 169 mV, 117 mV and 60 mV need to be provided for sample CoS2, Mn-1% and Mn-5%, respectively. Moreover, to further illustrate the superior HER catalytic performance of CoS2 catalysts after Mn doping, electrochemical impedance spectroscopy (EIS) was measured, which is related to the catalytic kinetics. The testing is performed used alternating current (AC) amplitude at -0.28 V (vs. RHE) from 10 K Hz to 0.1 Hz in 0.5 M H2SO4. As shown in Figure S16, Rs is related to uncompensated solution resistance and Rct reflects charge transfer resistance. A smaller Rct is conducive to a faster reaction rate.54 The sample Mn-5% has the lowest Rct (~33 ohm), which is about one ninth of pure CoS2, revealing a fast electron-transfer rate between the interface of the catalyst and electrolyte, which corresponding contributes to excellent HER activity. By measuring the Cdl, TOF and EIS for the pure and Mn doped CoS2 catalysts, we draw a conclusion that the amount of active HER sites and the electron-transfer rate increase with the increasing of Mn dopant, which are well agree with our above calculation results, revealing that Mn dopant can indeed enhance the electrocatalytic activity of CoS2. An ideal electrocatalyst requires not only a low initial overpotential, a small 12


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Tafel slope, but also a long time stability. Compared the linear sweep voltammetry curves before and after 10000 cycles shown Figure 5e, they are almost coincident, indicating that the HER properties of the sample Mn-5% hardly change after 10000 cycles, and it is still efficient. The inset in Figure 5c is the current depend time relation (i-t curve), which shows the current change with time of the Mn-5% catalyst at a constant overpotential of 100 mV. After 30 hours, the current density decreased less than 1%, which indicates its excellent outstanding electrochemical stability. The chronopotentiometry is also used to further test the stability of catalyst Mn-5% at different current density. Showing in Figure 5f, the demanded voltage to maintain a set current density exhibits a sluggish change over a long-time testing, again demonstrating the excellent durability of catalyst. Some electro-catalytic parameters for catalysts of pure CoS2 and Mn doped CoS2 have been listed in Table 1. What’s more, the HER performance of sample Mn-5% has been also compared with various CoS2-based electrocatalysts (shown in Table S2).

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Figure 5. (a) The polarization curves for Mn doped CoS2 catalysts, pure CoS2 and Vulan XC72 (carbon black) in 0.5 M H2SO4 at 2 mV/s. (b) Tafel plots obtained from the polarization curves. (c) Dependence of current on the scan rate at different double layer capacitance. (d) Turn over frequency (TOF) curves of the Mn doped CoS2 and pure CoS2 catalysts. (e) The linear sweep voltammetry (LSV) of the Mn-5% catalyst for the before and after 10000 CV scans; the inset exhibits the i-t curve of Mn-5% catalyst tested at an overpotential of 100 mV for 30h. (f) The electrochemical stability of the Mn-5% catalyst measured by chronopotentiometry at different current densities.

Table 1. Electro-catalytic parameters for catalysts of pure CoS2 and Mn doped CoS2.

Catalyst

Onset overpotential (mV)

η at J =2100 mA/cm (mV)

Tafel slope (mV/dec)

Rct (ohm)

Cdl (mF/cm2)

Potential at the TOF -1of 0.725 s (mV)

CoS2

137

327

87

287

10

169

Mn-1%

96

255

73

134

16

117

14


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Mn-5%

15

103

34

33

28

60

As calculation results shown in the volcano plot for series metal-doped CoS2 (Figure 1), transition metal dopants such as Fe and Ni also are excellent candidates to enhance the electrocatalytic activity of CoS2 by replacing the Co site. Therefore, we fabricated Fe-doped CoS2 and Ni-doped CoS2 nanowires by following the same synthetic process, where the Fe and Ni concentration is also set to 5%. The polarization curves of Fe-doped CoS2 and Ni-doped CoS2 nanowires are shown in Figure S18. The onset overpotential of sample Fe-5% and Ni-5% are 28 mV and 39 mV (15 mV for Mn-5%), respectively. When the current density reaches 10 mV/cm2, an overpotential of 52 mV and 66 mV are required for sample Fe-5% and Ni-5% (43 mV for Mn-5%), respectively. These results indicate that the trendency of HER activity for various dopants in CoS2 is Mn>Fe>Ni, which is good consistent with the above predicted volcano plot, further demonstrated that Mn is the most excellent candidate dopant to activate the electrocatalyst activity of CoS2. In summary, our DFT calculation results demonstrate that the electrocatalytic activity of CoS2 can be efficiently triggered by varied transition metal dopants. The volcano plot reveals that element Mn is the best candidate dopant to tune the adsorption behavior of H atoms on adjacent Co atoms, dopant itself and consequently the HER activity. It is further confirmed by the experiments results that Mn doped CoS2 nanowires possess a significantly improved HER property and a superior catalytic stability compared to pure CoS2. Finally, the screened HER tests of Fe- and 15



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Ni-doped CoS2 in experiments further verified the calculated volcano plot, where the trendency of HER activity for various dopants in CoS2 is Mn>Fe>Ni. This finding provides a rational strategy to trigger the HER activity of CoS2 by transition element doping, which simultaneous point out the new directions to activate the electrocatalytic performance of other catalysts.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. The Supporting information includes the following: Experimental Section, Calculation details, extra SEM pictures, XRD and XPS results, Raman, CV curves, Polarization curves and Tafel plot.

AUTHOR INFORMATION Corresponding Authors * Email: [email protected] (D. Q. Gao), [email protected] (D. S. Xue) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 11474137, 11674143), Program for Changjiang Scholars and Innovative Research Team in University (IRT 16R35). We thank Prof. Jun Ding from National University 16


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of Singapore for the calculations and useful discussion.

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electrolyte. Sci. Adv. 2015, 1, e1500259-e1500259.

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Accelerated Hydrogen Evolution Reaction in CoS2 by Transition-Metal Doping Jingyan Zhang, Yuchan Liu, Changqi Sun, Pinxian Xi, Shanglong Peng, Daqiang Gao*, Desheng Xue*. Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of Special Function Materials and Structure Design of MOE, Lanzhou University, Lanzhou 730000, People’s Republic of China Email: [email protected], [email protected]

Figure 1. The relation between currents (log(i0)) and the Gibbs free energy of adsorbed H* (ΔGH*) 1



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presents a volcano curve. (a) ΔGH* on the (001) surface of CoS2 with different metal dopants. (b) Summarized formation energies of different metal dopants substituted Co in CoS2.

Figure 2. The calculated electronic bands and density of states (DOS) results of the (a) CoS2 and (b) Mn doped CoS2, respectively. (c) Free-energy diagram for H adsorption on the CoS2 and Mn doped CoS2 with the (001) surfaces for Co sites and Mn site. (d) The optimized atomic structures of Mn doped CoS2 with H adsorption at Co sites (Co1 and Co2 sites) and Mn site.

Figure 3. (a) The schematic of sample preparation progress. (b) The X-ray diffraction (XRD) 2


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patterns of Mn doped CoS2 catalysts. (c) The low and (d) high magnified SEM images of sample Mn-5%. (e) The microscopy image obtained by TEM. (f) The high-resolution TEM image. (g) Element distribution mappings of Co, S and Mn in sample Mn-5%.

Figure 4. The XPS results for pure CoS2 and Mn-5%.(a) Co 2p, (b) S 2p and (c) Mn 2p spectra. (d) Temperature-dependent resistivity for sample CoS2 and Mn-5%.

3



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Figure 5. (a) The polarization curves for Mn doped CoS2 catalysts, pure CoS2 and Vulan XC72 (carbon black) in 0.5 M H2SO4 at 2 mV/s. (b) Tafel plots obtained from the polarization curves. (c) Dependence of current on the scan rate at different double layer capacitance. (d) Turn over frequency (TOF) curves of the Mn doped CoS2 and pure CoS2 catalysts. (e) The linear sweep voltammetry (LSV) of the Mn-5% catalyst for the before and after 10000 CV scans; the inset exhibits the i-t curve of Mn-5% catalyst tested at an overpotential of 100 mV for 30h. (f) The electrochemical stability of the Mn-5% catalyst measured by chronopotentiometry at different current densities.

Table 1. Electro-catalytic parameters for catalysts of pure CoS2 and Mn doped CoS2. Rct (ohm)

Cdl (mF/cm2)

Potential at the TOF-1of 0.725 s (mV)

87

287

10

169

73

134

16

117

Catalyst

Onset overpotential (mV)

η at J =2100 mA/cm (mV)

Tafel slope (mV/dec)

CoS2

137

327

Mn-1%

96

255

4


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Mn-5%

15

103

34

33

28

60

5


Accelerated Hydrogen Evolution Reaction in CoS2 by Transition-Metal

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