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* 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 ACS Energy Lett. 2018.3:779-786. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/23/19. For personal use only.
S Supporting Information *
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 resulting from Mn doping. First, 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 (|ΔGH*|) while remaining metallic. Second, to provide experimental evidence, Mndoped CoS2 nanowires are prepared by a hydrothermal and postsulfuration method. The optimized sample shows a low overpotential of 43 mV at 10 mA/cm2, a Tafel slope of only 34 mV/dec, and long-time stability for the hydrogen evolution reaction. This work reveals a new way to stimulate the electrocatalytic activity of other pristine candidate catalysts.
H
Recently, metallic cobalt pyrite (CoS2), as a representative of pyrite-type transition-metal dichalcogenides, has emerged as a low-cost material 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 nanostructuring,34,35 incorporating on the surface of MoS2/RGO to form a three-tiered cake-style material,36 as well as doping of nonmetal elements N and P.37,38 In addition, our previous results demonstrated that Cu dopants can optimize 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 atom-doped MoS2,40 near zero free energy of H adsorption on MoS2 with optimized energy level and more activated inert sites lead to maximum activity toward HER. With these key concepts of active sites and energy level modulation in mind, chemically doping with transition metals 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 atom-doped CoS2 systems. Generally, the Gibbs free energy of adsorbed H* on the active site
ydrogen, 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 a future hydrogen economy, where the key step in this technology is developing efficient electrocatalysts.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, realizing cheap, durable, and efficient HER electrocatalysts is scientists’ ultimate goal. Generally, an efficient HER electrocatalyst should have both active sites and high conductivity.8−10 As the representative HER catalysts, transition-metal dichalcogenides (MoS2, WS2, and Co−Mo−S), 11−13 selenides (MoSe 2 , NiSe 2, and CoP2xSe2(1−x)),14−16 nitrides (MoN and Ni3N),17,18 phosphide (Ni2P, FeP, and CoP),19−21 carbide (Mo2C and WC),22,23 and boride (Ni2B and Co−Ni−B)24,25 have attracted significant research interest and exhibit some noticeable properties. Nevertheless, most of these catalysts’ HER properties still cannot catch up with and surpass the state-of-the-art Pt-based catalysts. The causes of poor conductivity and inert sites are still their fatal drawbacks, even when using methods such as incorporating carbon-based 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. © 2018 American Chemical Society
Received: January 16, 2018 Accepted: March 5, 2018 Published: March 5, 2018 779
DOI: 10.1021/acsenergylett.8b00066 ACS Energy Lett. 2018, 3, 779−786
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Cite This: ACS Energy Lett. 2018, 3, 779−786
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lower |ΔGH*| ( Fe > Ni, which is consistent with the predicted volcano plot, further demonstrating 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, the dopant itself, and consequently the HER activity. This is further confirmed by the experiment results that Mn-doped CoS2 nanowires possess a significantly improved HER property and a superior catalytic stability compared to those of pure CoS2. Finally, the screened HER tests of Fe- and Ni-doped CoS2 in experiments further verified the calculated volcano plot, where the trend of HER activity for various dopants in CoS2 is Mn > Fe > Ni. This finding provides a rational strategy for triggering the HER activity of CoS2 by transition element doping, which simultaneously indicates new directions for activating the electrocatalytic performance of other catalysts.
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ACKNOWLEDGMENTS
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REFERENCES
This work is supported by the National Natural Science Foundation of China (Grant Nos. 11474137 and 11674143), Program for Changjiang Scholars and Innovative Research Team in University (IRT 16R35). We thank Prof. Jun Ding from National University of Singapore for the calculations and useful discussion.
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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/acsenergylett.8b00066. Experimental Section, calculation details, extra SEM pictures, XRD and XPS results, Raman results, CV curves, polarization curves, and Tafel plot (PDF)
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AUTHOR INFORMATION
Corresponding Authors
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Pinxian Xi: 0000-0001-5064-5622 Daqiang Gao: 0000-0002-8206-1258 Notes
The authors declare no competing financial interest. 784
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