Durable and Efficient Hollow Porous Oxide Spinel Microspheres for Oxygen Reduction

Transition metal oxide catalysts with high oxygen reduction activity and durability are highly desirable for use in fuel cells and metal-air batteries. Herein we report, for the first time, the oxygen reduction activity of hollow porous spinel AB2O4 microspheres, where A = Zn2+ and B = Mn3+ and/or Co3+ (i.e., ZnMnxCo2−xO4). Among them, ZnMnCoO4 (x = 1) microspheres exhibit the best oxygen reduction activity with a half-wave-potential only 50 mV lower than that of the Pt/C counterpart and an excellent durability in the alkaline solution. Importantly, the electronic transition of Co3+ ions from low-spin state in commercial Co3O4 catalyst to a mixed high-spin and low-spin state in ZnMnCoO4 catalyst was found to weaken the Co3+-OH bond and facilitate the O2−/OH− displacement. The density functional theory calculation substantiated that ZnMnCoO4 displayed a more favorable binding energy with O2 and oxygenated species, thereby enabling the fast reaction kinetics in the oxygen reduction reaction process.