Sulfur-Doped CoSe₂ Porous Nanosheets as Efficient Electrocatalysts for the Hydrogen Evolution Reaction

The electrochemical hydrogen evolution reaction (HER), as a promising route for hydrogen production, demands efficient and robust noble-metal-free catalysts. Doping foreign atoms into an efficient catalyst such as CoSe₂ could further enhance its activity toward the HER. Herein, we developed a solvothermal ion exchange approach to doping S into CoSe₂ nanosheets (NSs). We provide a combined experimental and theoretical investigation to establish the obtained S-doped CoSe₂ (S-CoSe₂) nanoporous NSs as highly efficient and Earth-abundant catalysts for the HER. The optimal S-CoSe₂ catalyst delivers a catalytic current density of 10 mA·cm^-2 for the HER at an overpotential of only 88 mV, demonstrating that S-CoSe₂ is one of the most efficient CoSe- and CoS-based catalysts for the HER. We performed density functional theory (DFT) calculations to determine the stable structural configurations of S-CoSe₂, and on the basis of which, we calculated the hydrogen adsorption Gibbs free energy (ΔG_H) on CoSe₂, CoS₂, and the S-CoSe₂ and the barrier energies of the rate-determining step of the HER on S-CoSe₂. DFT calculations reveal that S-doping not only decreases the absolute value of ΔG_H (move toward zero) but also significantly lowers the kinetic barrier energy of the rate-determining step of the HER on S-CoSe₂, leading to a greatly improved HER performance.