Designing Efficient Oxygen Evolution Reaction Electrocatalysts Based on Transition Metal Oxides

Due to our growing global population and escalating energy demands, concerns over future energy security are intensifying. The development of eco-friendly and sustainable energy production pathways is not only urgently needed, but also represents a significant scientific challenge. Moreover, the creation of low-cost and highly-efficient electrochemical energy storage and conversion systems is crucial to achieving the optimal utilization of renewable energy sources. Electrochemical water splitting has emerged as one of the most promising methods for scalable and sustainable hydrogen production. Electrocatalysts for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) are crucial in enabling rapid water splitting. Compared with the HER, the OER is a more complex process with sluggish kinetics, given that it encompasses not only four sequential proton-coupled electron transfer steps, but also a challenging spin state transition. Therefore, the rational design of high-performance OER electrocatalysts with enhanced catalytic efficiency is of the utmost importance. Our research has successfully employed electronic structural engineering strategies to modify the physicochemical properties of electrocatalysts, thereby improving their catalytic activity and accelerating the reaction kinetics. As a proof-of-concept, we have developed electrocatalyst design strategies rooted in electronic structural engineering, including doping engineering, heterostructure engineering, coordination modulation, and two-dimensional (2D) morphology engineering. These strategies have been effectively applied to tune the catalytic performance of layered Co2Mo3O8 for accelerated water splitting kinetics.

As discussed in Chapter 4, clear structure-property correlations have been established and the impact of coordination structure modulation on catalytic activity have been detailed. In this instance, a coupled heteroatom doping approach with 2D engineering was utilized to activate the dual coordination sites (both tetrahedral and octahedral sites) and enhance the oxygen electrocatalysis performance of FeCoMo3O8. The formation of the Fe-O-Co bond within the catalyst significantly enhances the adsorption of reaction intermediates. Additionally, the presence of high-spin cations is instrumental in establishing an unobstructed spin channel that facilitates electron transport. Furthermore, the structural design of this catalyst enabling specific transition metals to fully occupy a designated coordination site opens up the potential for the rational design of highly efficient catalysts. By selectively activating different sites, it becomes possible to create catalysts that are not only cost-effective, but also boast superior performance, thereby pushing the boundaries of what is achievable in the realm of catalyst design for energy conversion applications.

In another case, discussed in Chapter 5, a heterostructure engineering strategy was applied to develop a CoFe2O4@CoFeMo3O8 magnetic electrocatalyst with a tailored electronic structure. This novel heterostructured electrocatalyst, CoFe2O4@CoFeMo3O8 composite, with intrinsic electric and magnetic fields, was designed for highly efficient overall water splitting. The formation of the electric field governs the dynamics of electric charge, leading to optimized adsorption of reaction intermediates and a concomitant reduction in the energy barrier for electron transfer. Moreover, the heterostructure leverages a localized magnetic field to enhance the transfer of spin-polarized electrons, which significantly accelerates the OER process. Additionally, the pronounced sensitivity of these magnetic heterostructures to external alternating magnetic fields (AMF) suggests a potential avenue for further enhancing catalytic performance through external stimuli. Through the fine-tuning of the intrinsic properties of electrons, encompassing both charge and spin, the CoFe2O4@CoFeMo3O8 electrocatalyst has demonstrated remarkable intrinsic OER activity.

This study broadens the scope of design strategies for the development of efficient electrocatalysts based on transition metal oxides, and they are applicable to water electrolysis and other relevant electrocatalysis applications.