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.