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 efficientelectrocatalyststoward the oxygen evolution reaction (OER). However, plenty of presentstudies are still in the try-and-wrong stage, while the underlyinganion effect remains unclear. Herein, a series of Co(OH)( x )(A)( y ) (A = F-, Cl-, and CO3 (2-))are synthesized via a hydrothermal strategy, and the order of theOER activity is determined to be Co(OH)(CO3)(0.5) > Co-2(OH)(3)Cl > Co(OH)F > Co(OH)(2). X-ray photoelectron spectroscopy and soft X-ray absorptionspectroscopystudies reveal that these Co(OH)( x )(A)( y ) materials undergo surface oxidation duringthe OER process, and the highly active Co(IV) content is the dominantfactor in deciding the OER performance. Furthermore, quantitativeanalysis of anion concentrations in the electrolyte solution revealsthat the anion leaching ability in Co(OH)( x )(A)( y ) directly relates to the Co(IV) contentand finally the OER catalytic activity, which is the essence of theanion effect and can be summarized as an "anion leaching metaloxidation" model. Our work not only provides deep understandingof the anion effect of metal basic salt-based OER catalysts but alsoprofound insights for the activation process of the OER pre-catalysts.