Porous calcium–manganese oxide microspheres for electrocatalytic oxygen reduction with high activity

Abstract

A series of calcium–manganese oxides (Ca–Mn–O) were prepared through thermal decomposition of carbonate solid–solution precursors and investigated as electrocatalysts for oxygen reduction reaction (ORR). The synthesized crystalline Ca–Mn–O compounds, including perovskite-type CaMnO₃, layered structured Ca₂Mn₃O₈, post-spinel CaMn₂O₄ and CaMn₃O₆, presented similar morphologies of porous microspheres with agglomerated nanoparticles. Electrochemical results, surface analysis, and computational studies revealed that the catalytic activities of Ca–Mn–O oxides, in terms of onset potential, reduction current, and transferred electron number, depended strongly on both the surface Mn oxidation state and the crystallographic structures. Remarkably, the as-synthesized CaMnO₃ and CaMn₃O₆ exhibited considerable activity and enabled an apparent quasi 4-electron oxygen reduction with low yield of peroxide species in alkaline solutions, suggesting their potential applications as cheap and abundant ORR catalysts.