Modulating Electronic Interaction in Electrocatalysts for Boosting Oxygen Evolution Reaction
Authors: Sung Yung-Eun , Lee Seongbeom , Shim Jaehyuk 
Published: December 2024

The oxygen evolution reaction (OER) is a crucial half-reaction in numerous electrochemical processes including water electrolysis, CO2 reduction, and rechargeable metal-air batteries. While precious metal-based catalysts such as IrO2 and RuO2 demonstrate effectiveness in OER, their high cost and instability hinder widespread adoption. Consequently, there is an urgent demand to develop low-cost transition metal-based electrocatalysts, satisfying both low overpotentials as well as robust stability. Given this perspective, research has concentrated on employing multi-metal systems as a catalyst design strategy to engineer effective transition metal-based catalysts for the OER. This approach facilitates the fine-tuning of catalytic properties by harnessing the synergistic interactions inherent within the constituent elements. Achieving this goal necessitates a profound comprehension of electronic structural modulations based on composition, coupled with rigorous analysis of the intricate redox mechanisms underlying the catalytic activity of these materials. In the course of our investigation, we synthesized high-entropy oxide in a spinel phase with a nano-sheet morphology. In addition to the flexible structure of spinel oxide accommodating various coordination environments and oxidation states, the unique physicochemical properties of high-entropy materials arising from their multi-elemental configuration make it possible to tailor the catalytic properties. We utilized OER-active metals Fe, Co, and Ni, complemented by the strategic introduction of non-3d elements Cr and Mo in their high-valence states, to significantly enhance the intrinsic activity of the catalysts by adjusting the oxide electronic structure. The resulting high-entropy spinel oxide (CrFeCoNiMo)3O4 displayed promising OER activity and stability outperforming the IrO2 benchmark, demonstrating its feasibility for practical applications. In particular, the sequential incorporation of Cr and Mo in (FeCoNi)3O4 led to performance enhancements, showcasing the tunable nature of high-entropy oxides. The prepared catalysts were further extended to a lab-scale anion exchange membrane water electrolyzer (AEMWE) to test the reliability, achieving lower overpotential than required by the IrO2 benchmark. For elucidation of catalytic activity enhancement, careful X-ray analysis was conducted. The presence of highly electronegative species such as Cr 6+ and Mo 6+ is expected to modulate the electronic structure of the other metal sites by inducing partial electron transfer through the bridging oxygen. Upon the inclusion of high-valence elements, a decrease in the electron density of the existing elements is observed, indicating the electron-withdrawing effect of high-valence elements. This decrease in electron density correlates with an increase in the oxidation state of transition metals, which in turn has been associated with enhanced oxygen activity. Consequently, the metal-oxygen bond weakened increasing the electron density at the O site, potentially enabling the detachment of oxygen from the catalysts. In conclusion, our study unveils the potential of integrating high-valence elements into high-entropy spinel oxide as a novel catalyst design approach. This optimizes the electronic structure of the catalyst and fine-tunes the metal-oxygen bond properties, thereby activating the water oxidation reaction pathway towards oxygen redox chemistry. By shedding light on this methodology, our study contributes valuable insights to the design strategy of effective electrocatalysts, emphasizing the significance of considering interactions among the constituent elements. Moreover, our research presents a diverse array of analytical approaches for elucidating elemental interactions, enriching our understanding of catalyst behavior. Additionally, experimental tests confirm the feasibility of applying these catalysts in practical electrochemical devices. Overall, our findings underscore the versability of high-entropy spinel oxides as catalysts for advancing electrochemical applications.