Tuning Surface Transformations in Cobalt-Based Spinel Oxides for Enhanced Activity and Stability in the Oxygen Evolution Reaction

Achieving efficient hydrogen production via water electrolysis requires the development of low-cost, highly active, and stable oxygen evolution reaction (OER) electrocatalysts. However, the OER electrocatalyst surface is not static; instead, it reconstructs and transforms dynamically and continuously during the OER. Multiple elementary processes occur simultaneously, and the OER electrocatalyst surface changes substantially as the OER proceeds. Thus, understanding these elementary processes and how they affect electrocatalytic performance is essential for OER electrocatalyst design. In this study, we demonstrate that vanadium substitution markedly enhances the OER performance of cobalt spinel oxide. Co2VO4 nanoparticles achieve a low overpotential of approximately 306 mV at 10 mA cm-2 and maintain their activity for over 80 h of chronopotentiometry measurements, significantly outperforming the lower-substituted Co2.75V0.25O4 and Co3O4. Using a multimodal characterization approach, we reveal that the enhanced activity and electrochemical stability of Co2VO4 originate from the in situ formation of hydrated and defective γ-type (Co0.75V0.25)OOH clusters that are possibly continuously self-healed through highly reversible CoII(OH)2 ↔ γ-type oxyhydroxide transformation. This reversible transformation is triggered by pronounced vanadium dissolution due to severe lattice distortion induced by tetrahedrally coordinated vanadium (+5) sites in Co2VO4. In contrast, Co2.75V0.25O4 forms an epitaxial, less hydrated β-type (Co0.92V0.08)OOH layer that degrades via vanadium leaching, causing poor stability. Overall, our findings reveal that a pronounced vanadium (+5) content in Co-based spinel facilitates the highly reversible CoII(OH) 2 ↔ γ-type oxyhydroxide transformation, which is an effective strategy to promote OER activity and stability of cobalt-based spinel oxide catalysts toward OER.