The study explores cobalt sulfide electrocatalysts for alkaline water splitting, focusing on the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). We compare sulfur-rich and sulfur-poor amorphous cobalt-based materials, specifically examining how composition, morphology, and chemical speciation influence catalytic activity and stability under reaction conditions. A key finding is the different transformation behaviors of sulfur-poor and sulfur-rich materials in alkaline environments and their impact on HER and OER performance. The study highlights that the chosen electrodeposition method significantly impacts the composition, morphology, and most importantly, the dynamic surface transformation behavior of cobalt sulfide catalysts, which, in turn, dictates their performance and stability for either OER or HER in alkaline water splitting. The sulfur-rich materials deposited from nonaqueous solutions underwent a fast and complete transformation into oxyhydroxide species, which represent the OER-active centers. Sulfur-poor materials obtained from aqueous solutions, which exhibited relative stabilization of the sulfide functionality, are more effective for the HER. However, the active sulfide species present in these materials is unstable under prolonged polarization in alkaline HER conditions, and as the pristine sulfide transforms into a hydroxide, the catalytic activity declines significantly. • Aqueous vs. nonaqueous electrodeposition tailors the Co:S ratio in deposits. • Sulfur-rich films show high OER activity via rapid transformation to oxyhydroxide. • Sulfur-poor films are active for HER but show degradation upon electrolysis. • Raman spectroscopy reveals dynamic catalyst restructuring.
Kokin et al. (2026) studied this question.