Electrocatalytic oxygen evolution activity of surfactant-free cobalt- and manganese-based spinel oxide nanoparticles in acid

Abstract

Acidic oxygen evolution electrocatalysts based on earth abundant elements are essential for the widespread application of proton-exchange membrane water electrolyzers. Spinel oxides containing cobalt and manganese are promising candidates. Here, the oxygen evolution activity and stability of surfactant-free cobalt and manganese spinel oxides with an average side length of 7 nm are evaluated in 0.1 M HClO₄. The oxygen evolution activity follows the trend Co₃O₄/CP > Co_1.27Mn_1.27Fe_0.46O₄/CP > Co₂MnO₄/CP > Mn₃O₄/CP, and only Co₃O₄/CP is stable towards the long-term OER in acid. The origin for increased activity of Co₃O₄/CP is also investigated, and the results indicate that the Co site is intrinsically more active towards the OER than the Mn site. The lower charge transfer resistance, lower apparent activation energy and close to optimum hydroxyl group adsorption energy corroborate the higher intrinsic OER activity of Co₃O₄/CP. For Co_1.27Mn_1.27Fe_0.46O₄ and Co₂MnO₄, electron interaction that alters the acidity of the metal center exists. The OER mechanism involves a decoupled proton–electron transfer process, as indicated by the non-zero reaction order of [H⁺] in the reversible hydrogen scale. Severe leaching of metal-containing species is detected during the long-term oxygen evolution reaction in acid.