Cobalt chalcogenides are excellent oxygen evolution reaction (OER) precatalysts in alkaline medium as they readily form O2-evolving CoOOH entities in electrochemically accessible Co2+ sites when subjected to anodic potential. A key factor that determines the efficiency of OER in cobalt chalcogenides is the number of electrochemically accessible Co2+ sites. Here, an easy way of increasing the electrochemical accessibility of Co2+ sites in CoSe2 has been identified, which is the simple preoxidation of selenide to selenite. When screened for OER in alkali, it was found that the electrochemical accessibility of Co2+ after preoxidation of Se in CoSe2 was increased by 7.8 ± 2 times in the first cycle and 2–3 times after activation by potential sweeping and redox cycling. The corresponding OER activation energy lowered to ∼1/2 at overpotentials 450 mV or higher due to such preoxidation of Se. Irrespective of the lowering in the electrochemical accessibility of Co2+ sites from the 1st cycle to the 100th cycle, the overall OER activity was maintained to be the same. This is quite relatable as a major portion of Co2+ oxidized in the first cycle is shuttling between 3+ and 4+ states while evolving O2. Altogether, preoxidation of Se in CoSe2 benefitted the realization of increased electrochemical accessibility for Co2+ sites, improved ECSA, improved charge transfer at catalytic turnover conditions, and lowered OER activation energy.

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