Residual Selenium–Assisted Oxidation Pathway Modulation in Cobalt Selenide for Enhanced Oxygen Evolution Reaction

Cobalt selenides (CoSe_x) are promising oxygen evolution reaction (OER) catalysts due to their tunable electronic structure and reconstruction behavior. However, the role of selenium (Se) in this process remains incompletely understood. In this study, the electrochemical transformation of CoSe is investigated during alkaline OER, and explore how residual Se influences catalytic activity. The results show that while surface selenium is largely leached away, selenium retention is observed in the subsurface and bulk regions, with a uniform distribution even after prolonged electrolysis. This residual selenium appears to aid in the formation and stabilization of highly oxidized cobalt species, such as Co³⁺ and Co⁴⁺, by influencing the local electronic structure during potential-driven reconstruction. In particular, it modulates the oxidation pathway toward CoO₂ via β-CoOOH. These structural and compositional changes correlate with enhanced OER activity compared to Se-free Co samples. The findings suggest that residual Se acts as a modulator, which provides valuable insights for designing high-performance transition metal chalcogenide electrocatalysts.