Intrinsic Electrocatalytic Activity of First-Row Transition Metal Antimonate Oxides for Oxygen Electrocatalysis

: Synthesis, Characterization, and Structure–Activity Relationships for Oxygen Reactions

Abstract

The development of earth-abundant oxygen electrocatalysts is essential for improving the efficiency and sustainability of energy-conversion technologies such as water electrolysers, fuel cells, and metal–air batteries. Trirutile-type transition-metal antimonates, MSb₂O₆ (M = Mn, Co, Ni, Fe), are promising candidates for these applications because their electronic properties can be systematically modified through compositional tuning while maintaining structurally robust oxide frameworks. This thesis investigates how the electrochemically accessible electronic structure of transition-metal centres governs oxygen-reduction (ORR) and oxygen-evolution (OER) activity. Emphasis is placed on intrinsic active-site behaviour, assessed by combining structural characterisation with cyclic-voltammetry-derived electrochemical descriptors, including the redox-active site density (Γₘ) and catalytic activity normalised per site.
The first part of the work examines phase-pure MSb₂O₆ (M = Mn, Co, Ni, Fe) oxides synthesised via a solid-state route. Electrochemical measurements reveal that MnSb₂O₆ exhibits the highest ORR activity, whereas CoSb₂O₆ displays superior OER performance. These trends are linked to differences in redox accessibility and the utilisation of transition-metal d states within the relevant potential windows, demonstrating that intrinsic catalytic activity depends strongly on how effectively metal centres participate in electrochemical redox processes, rather than on composition or crystal structure alone.
Building on these findings, Mn₁₋ₓCoₓSb₂O₆ solid solutions were investigated to explore the effects of controlled electronic tuning through cation substitution. Structural and electrochemical characterisation shows that intermediate Mn/Co ratios enhance both ORR and OER activity by reshaping the redox response and increasing the density of electrochemically accessible active sites. This approach is further extended to bimetallic antimonates of the form M₀.₅M′₀.₅Sb₂O₆ (M, M′ = Co, Ni, Fe), where mixed-metal environments lead to improved bifunctional activity and stability through more favourable redox behaviour and enhanced charge-transfer characteristics.
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Finally, lithium-modified LiₓCo₂SbO₆ phases were studied to assess the impact of lithium incorporation on structure and electrocatalytic performance. Lithium incorporation facilitates access to higher cobalt oxidation states, enhances intrinsic OER activity, and improves structural robustness under electrochemical operation.
Overall, this work demonstrates that oxygen electrocatalytic performance in transition-metal antimonate oxides is governed by both the number of electrochemically accessible redox sites and their intrinsic effectiveness. By relying exclusively on experimentally accessible electrochemical parameters, this thesis provides a robust, transferable framework for the rational design of earth-abundant, precious-metal-free electrocatalysts for ORR and OER.

Date of Award	12 May 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	David J Fermin (Supervisor) & David Cherns (Supervisor)