Pyrite-type electrocatalysts for hydrogen evolution
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Introduction Hydrogen is a sustainable energy carrier that bridges the gap of time and geography between the production and application of electricity from clean energy.1 Water splitting driven by solar energy or electricity is an essential method for hydrogen production with high purity.2 Water splitting consists of two key steps of the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), where noble metal (e.g., Pt, Ir, Ru)-based materials are the most frequently used active catalysts due to their low overpotentials and large catalytic current densities. Unfortunately, their high cost and scarcity limit their scalable usage.3 Recently, earth-abundant materials with high performance have attracted significant attention as substitutes for noble metal-based catalysts. Generally, natural enzymes, including hydrogenases and nitrogenases, can efficiently split water at sulfur coordinated Mo and Ni/Fe metal centers.4 Pyrite-type transition-metal dichalcogenides (MX2, where M = Fe, Co, Ni, and X = S/Se) that are found widely in metamorphosed ores have been found to be attractive low-cost materials with significant potential for energy conversion.5 For example, the representative cobalt pyrite (CoS2) displays decent performance for HER and/or OER.6 This article summarizes recent progress in pyrite-type electrocatalytic nanomaterials. Topics covered include properties
of pyrite-type materials, S-based, Se-based, and ternary and other pyrite-type electrocataysts. Finally, the opportunities and challenges of pyrite-type catalysts are speculated.
Crystal and electronic structures of pyrites Pyrites crystallize in the cubic system similar to NaCl, leading to rigid and sometimes large cubic crystals (Figure 1a).7 One (M) metal atom and six X atoms form octahedral coordination, and one X atom and surrounding three metal atoms form tetrahedral coordination (Figure 1b).8 Typically, metal cations on the low-index surfaces of these dichalcogenides tend to present a reduced coordination number (Figure 1c).9 A structural analogy can be found for the structure of the active center of hydrogenase, which includes an Fe site with five permanent ligands in a distorted octahedral shell. This suggests that the square-pyramidal surface metal centers bridged by dichalcogenide dumbbells could be the active center for HER, where both the metal and chalcogenide atoms function as active sites.10 Thus, their activity and stability can be influenced by the electronic structures of metal centers and electron-donating character of the chalcogen ligands. The electronic structure of pyrites is strongly dependent on the d-electron count of the transition metals. The antibonding d-electron in the metal conduction band gradually increases from FeS2 to ZnS2, which leads to their markedly different
Wangyan Gou, Taiyuan University of Technology, China; [email protected] Mingkai Zhang, Xi’an Jiaotong University, China; [email protected] Jian Wu, Taiyuan University of Technology, China; [email protected] Qingchen Dong, Taiyuan
