ENGINEERING OF NONSTOICHIOMETRIC HIGH-ENTROPY SPINEL OXIDE CATALYSTS FOR ZINC-AIR BATTERIES

Date

2026-4-20

Author

Geyikçi, Uygar

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The escalating global energy demand necessitates reliable, grid-scale storage solutions to support intermittent renewable energy sources. Rechargeable zinc-air batteries (ZABs) have emerged as highly compelling candidates due to their economic viability, safety, and exceptional theoretical energy density. However, the commercialization of ZABs is severely hindered by the sluggish kinetics of the cathodic oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), and by the reliance on expensive, degradation-prone noble-metal electrocatalysts. While transition-metal-based high-entropy oxides (HEOs) offer a robust, customizable alternative, the specific electrocatalytic impacts of non-stoichiometry remain largely unexplored, with current literature overly reliant on the surface oxygen vacancy paradigm. To bridge this knowledge gap, this thesis synthesizes a compositional gradient of non-stoichiometric spinel HEOs via a modified sol-gel Pechini method. Structural and electronic characterizations reveal that deviating from stoichiometry to a Co-rich/Fe-poor configuration induces a cooperative mixed-valence environment. Even while maintaining a rigidly constant oxygen vacancy defect density, this synergistic Co-Fe interaction physically contracts the metal-oxygen (M-O) bonds. It substantially lowers the work function of the catalyst surface. Among the synthesized variants, the Co-rich (Co0.3) catalyst demonstrated superior bifunctional performance, achieving a minimal OER overpotential of 428 mV at 10 mA cm-2 and a highly efficient 4-electron ORR pathway, yielding a narrow bifunctional index of 0.95 V. Crucially, the deliberate modulation of the work function and M-O covalency theoretically activates the Lattice Oxygen Mechanism (LOM), allowing the catalyst to bypass traditional thermodynamic scaling limitations. When integrated into a practical zinc-air battery, the Co0.3 cathode delivered a peak power density of 63 mW cm-2 and sustained exceptional cyclic durability for over 600 hours without structural lattice degradation. Ultimately, this research establishes a definitive, non-linear correlation between non-stoichiometric cation coordination, work function, and electrocatalytic activity, providing a robust design framework for next-generation ZAB electrocatalysts.

Subject Keywords

Zinc-air battery, Oxygen Evolution Reaction, Oxygen Reduction Reaction, High Entropy Oxide

URI

https://hdl.handle.net/11511/119414

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Graduate School of Natural and Applied Sciences, Thesis