Phase Control of Cobalt Selenide: Unraveling the Relationship Between Phase Property and Hydrogen Evolution Catalysis

Abstract The development of phase engineering still confronts challenges including controlled synthesis of desired phase structures and unraveling of inner mechanism for phase‐dependent applications. Herein, taking CoSe₂ as a prototype, a facile wet‐chemical method is developed to synthesize metastable orthorhombic phase CoSe₂ (o‐CoSe₂). In the meantime, stable cubic phase CoSe₂ (c‐CoSe₂) can be prepared by phase transition from o‐CoSe₂ using a general annealing approach. It is demonstrated that CoSe₂ exhibits distinct phase‐dependent hydrogen evolution performance in photo‐ and electrocatalysis. In electrocatalytic reaction, o‐CoSe₂ shows an overpotential of 300 mV better than c‐CoSe₂ (365 mV) at –10 mA cm^‐2 in acid solution. Unlike electrocatalysis, as cocatalyst in photocatalytic hydrogen evolution, c‐CoSe₂/TiO₂ exhibits a roughly 4.5‐folds higher hydrogen evolution rate (12.001 µmol h^‐1) than o‐CoSe₂/TiO₂ (2.601 µmol h^‐1). Combined with comprehensive theoretical and experimental analysis, it has been proposed that appropriate hydrogen adsorption ability of o‐CoSe₂ contributes to efficient electrocatalysis, whilst fast interfacial charge migration between TiO₂ photo‐harvester and c‐CoSe₂ cocatalyst is the dominating factor for photocatalysis. This article demonstrates the feasibility of improving catalytic activity of CoSe₂ by phase engineering, and also, provides guidance into unmasking the relationship between phase structure and hydrogen evolution catalysis.