Sulfur-Doped CoSe₂Porous 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 (δGH) 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 δGH (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.