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

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.