A full Volmer–Heyrovsky reaction catalyst based on sulfur vacancy structure optimization

Despite the introduction of vacancies being a notorious practice for improving electrocatalytic performance, mechanistic elucidation and rational control of such vacancies in electrocatalytic reactions remain elusive. Furthermore, prior catalysts tend to not consider the full hydrogen evolution reaction (HER) holistically, leading to suboptimal designs. Herein, we present a vacancy structure optimization strategy for catalyzing the full Volmer–Heyrovsky reaction. The proposed method has been demonstrated using FeS nanobranches with sulfur vacancies strategically substituted by oxygens (OSV-FeS NBs). Specifically, the as-prepared sample has achieved a significantly lower overpotential of 273 mV at 200 mA cm⁻² and a smaller Tafel slope of 82 mV dec⁻¹ compared to samples with non-oxygen-substituted sulfur vacancies (SV-FeS, 357 mV/114 mV dec⁻¹) and their pristine (p-FeS, 640 mV/160 mV dec⁻¹) counterparts. Computational and experimental results combined reveal, for the first time, that the strategic occupation of sulfur vacancies in OSV-FeS simultaneously reduces H₂O adsorption on the Fe sites, increases O–H dissociation, and weakens H* adsorption on the Fe-Fe sites, thus achieving the optimization of both the Volmer and Heyrovsky steps during the HER. This work opens a new avenue for designing high-performance electrocatalysts with enhancement of the entire Volmer-Heyrovsky reaction holistically. © The Royal Society of Chemistry 2024.