Benchmarking Metal Oxide Electrocatalysts for Oxygen Reduction

Rationally designing highly active electrocatalysts for the oxygen reduction reaction (ORR) without relying on precious metals is critical for promoting the application of metal–air batteries and fuel cells. As a promising alternative to traditional Pt-group-based catalysts, transition-metal oxides (TMOs) with high corrosion resistivity and diverse structure configuration have exhibited great catalytic potential. However, the current favorable ORR performance of TMOs has often been achieved through their coupling with engineered carbons such as graphene and carbon nanotubes, which can obscure the intrinsic catalytic potential of TMOs. To promote TMO-based catalysts for ORR applications, comprehensively analyzing their catalytic properties and screening promising TMO-based parent materials is crucial. Herein to eliminate the research gap, 22 noble-metal-free TMOs (e.g., Co3O4, MnO, and Fe2O3) were benchmarked for ORR catalysis, with Pt/C and RuO2 as comparisons. Nb2O5 demonstrated the greatest potential as an ORR electrocatalyst among all of the studied TMOs, with higher specific activity surpassing that of Pt/C. In addition, it could achieve a larger peak power density than Pt/C in a more practical Zn–air battery setup, showing promise for applications. Regarding stability, most of the studied TMOs exhibited larger losses in limiting current densities than the degradation of half-wave potentials (from 0.65% to 7.54) after the 10,000 cycle accelerated degradation tests, pointing out the direction to improve their ORR stabilities.