Achieving Increased Electrochemical Accessibility and Lowered Oxygen Evolution Reaction Activation Energy for Co²⁺ Sites with a Simple Anion Preoxidation

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.