Harnessing dispersed metal ions in salt-solution quenched ceria nanorods for promoting electrocatalytic activity towards alkaline oxygen reduction reaction

Abstract

Defect engineering, particularly through the creation of oxygen vacancies, combined with transition-metal incorporation, offers an effective route to tailor the catalytic properties of metal oxides. In this study, we report, for the first time, the simultaneous modulation of defects and the dispersion of transition-metal ions on ceria nanorods via quenching in ice-cold salt solutions to improve Oxygen Reduction Reaction (ORR) electrocatalysis in alkaline media. Hydrothermally synthesized ceria nanorods with high surface area and mesoporous architecture were quenched in Co2+, Ni2+, Cu2+, and Zn2+ salt solutions and comprehensively characterized using structural, morphological, and surface-sensitive techniques. Positron annihilation lifetime spectroscopy revealed a nearly balanced distribution of oxygen vacancies and vacancy clusters in the bulk and surface regions of the Zn2+-quenched catalyst (Zn-Q). Furthermore, Extended X-ray Absorption Fine Structure (EXAFS) and wavelet-transformed EXAFS analyses identified a distinct neighboring Zn-Ce interaction in Zn-Q, indicating a unique mode of Zn incorporation compared with the other quenched catalysts. The synergistic effects of defect modulation and transition-metal dispersion were subsequently correlated with ORR activity. Salt-solution quenching enhanced the half-wave potential and shifted the ORR pathway from a two-electron to a four-electron pathway. Among the investigated catalysts, Zn-Q exhibited the most favorable combination of thermodynamic and kinetic ORR descriptors, delivering the highest four-electron selectivity with minimal peroxide generation. These findings demonstrate a simple, cost-effective strategy for engineering high-performance ceria-based ORR electrocatalysts through concurrent defect modulation and transition-metal dispersion, without the addition of noble-metal components.