Understanding the Anion Effect of Basic Cobalt Salts for the Electrocatalytic Oxygen Evolution Reaction

Basic cobalt salts with distinct acidic anion Co(OH)x(A)y are efficient electrocatalysts toward the oxygen evolution reaction (OER). However, plenty of present studies are still in the try-and-wrong stage, while the underlying anion effect remains unclear. Herein, a series of Co(OH)x(A)y (A = F–, Cl–, and CO32–) are synthesized via a hydrothermal strategy, and the order of the OER activity is determined to be Co(OH)(CO3)0.5 > Co2(OH)3Cl > Co(OH)F > Co(OH)2. X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy studies reveal that these Co(OH)x(A)y materials undergo surface oxidation during the OER process, and the highly active Co(IV) content is the dominant factor in deciding the OER performance. Furthermore, quantitative analysis of anion concentrations in the electrolyte solution reveals that the anion leaching ability in Co(OH)x(A)y directly relates to the Co(IV) content and finally the OER catalytic activity, which is the essence of the anion effect and can be summarized as an "anion leaching─metal oxidation" model. Our work not only provides deep understanding of the anion effect of metal basic salt-based OER catalysts but also profound insights for the activation process of the OER pre-catalysts.