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., ZnMn_xCo_2−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. Transition metal oxide catalysts with high oxygen reduction activity and durability are highly desirable as they promise new opportunities as alternatives to rare noble-metal catalysts for use in fuel cells and metal-air batteries. However, transition metal oxides often possess high overpotentials. In this paper, we report the crafting of a family of Co³⁺-based spinel oxides (AB₂O₄: A = Zn²⁺ and B = Mn³⁺ and/or Co³⁺) and the subsequent exploration of their use as catalysts in the oxygen reduction reaction (ORR) by scrutinizing the correlation between the electronic structure of Co³⁺ ions and the resulting ORR performance. Among them, ZnMnCoO₄ (x = 1) microspheres where Mn and Co ions are located at the B site of spinel AB₂O₄ were found to 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. 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 oxygen reduction reaction performance of the hollow porous oxide spinel microspheres was investigated. The ZnMnCoO₄ possessed a high onset potential of 1.00 V and an outstanding durability in the alkaline solution. The electronic transition of Co³⁺ ions was found to weaken the Co³⁺-OH bond and facilitate the O²⁻/OH⁻ displacement. Thus, it may offer promising potential for use as an effective catalyst with high oxygen reduction activity and durability in fuel cells and metal-air batteries, among other applications.