Structural Insights into Multi-Metal Spinel Oxide Nanoparticles for Boosting Oxygen Reduction Electrocatalysis.

Multi-metal oxide (MMO) materials have significant potential to facilitate various demanding reactions by providing additional degrees of freedom in catalyst design. However, fundamental understanding of the (electro)catalytic activity of MMOs is limited because of the intrinsic complexity of multi-element nature. Additional complexities arise when MMO catalysts have crystalline structures with two different metal site occupancies, such as spinel structure. Differences in quantum electronic structure and metal-oxygen bonds and the resulting differences in the tetrahedral and octahedral environments make it more challenging to investigate the origin of the (electro)catalytic activity of MMOs. Here, we synthesize uniform-sized multi-metal spinel oxide nanoparticles composed of Mn, Co, and Fe as model MMO electrocatalysts and systematically study the contributions of each element to the structural flexibility of the spinel oxides, which boosts the electrocatalytic oxygen reduction reaction (ORR) activity. Detailed crystal and electronic structure characterizations combined with electrochemical and computational studies reveal that the incorporation of Co not only increases the preferential octahedral site occupancy, but also modifies the electronic state of the ORR-active Mn site to enhance the intrinsic ORR activity. As a result, nanoparticles of the optimized catalyst, Co0.25 Mn0.75 Fe2.0 -MMO, exhibit a half-wave potential of 0.904 V (versus RHE) and mass activity of 46.9 A goxide -1 (at 0.9 V versus RHE) with promising stability. This article is protected by copyright. All rights reserved.