Achieving Increased Electrochemical Accessibility and Lowered Oxygen Evolution Reaction Activation Energy for Co<sup>2+</sup> Sites with a Simple Anion Preoxidation
Sengeni Anantharaj, Hisashi Sugime, Bo Zhi Chen, Natsuho Akagi, Suguru Noda
Abstract
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.