Organic ligands and CeO2-induced generic valence modulation strategies to design Fe active sites for promoted oxygen involved reactions in rechargeable zinc–air batteries

Abstract

The limited catalytic selectivity and stability of most metal oxides during reactions are primarily due to their irreversible valence transitions. To address this challenge, this study focuses on the development of CeO₂-induced valence-tunable Fe_xO nanoparticles, which were supported on a nitrogen-doped carbon substrate to enhance the catalytic performance of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). In situ Raman spectroscopy confirmed that the key intermediates involved in the ORR on the CeO₂–Fe_xO/NC catalyst surface are *O₂⁻ and *OOH, which remain stable even at low potentials. Ultraviolet photoelectron spectroscopy (UPS) revealed that the CeO₂–Fe_xO/NC catalyst exhibits a lower work function, which facilitates electron escape and promotes the activation of oxygen molecules. X-ray photoelectron spectroscopy (XPS) further showed that Fe in CeO₂–Fe_xO/NC has a reduced binding energy, with Fe_xO acting as a broad valence electron reservoir (0–3 e⁻), which enhances proton-coupled electron transfer and accelerates reaction kinetics during the catalytic process. As a result, the CeO₂–Fe_xO/NC sample demonstrates outstanding bifunctional oxygen electrocatalytic performance, with a potential gap of just 0.62 V, surpassing that of the Pt/C + RuO₂ catalyst (0.65 V). Moreover, zinc–air batteries utilizing the CeO₂–Fe_xO/NC catalyst exhibit a high specific capacity of 743.0 mA h g⁻¹.