Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) are complicated materials with dynamic structures. They exhibit potential-induced phase transitions, potential-dependent electronic properties, variable oxidation and protonation states, and disordered local/surface phases. These properties make understanding the OER, and ultimately designing higher-efficiency catalysts, challenging. Measurements of intrinsic activity show that, by far, the most-active phases for OER under alkaline conditions are Fe-containing mixed-metal oxyhydroxides, but exactly how they function remains controversial. I will discuss our work to understand the key properties of these catalysts, including morphology, composition, and molecular/electronic structure, and how they evolve and are dynamic under active catalytic conditions. Specifically I will highlight new work where we have been able to systematically control the concentration of surface Fe species on NiOOH and CoOOH host structures. We discover that the per-Fe turn-over frequency (TOF<sub>Fe</sub>) for the OER increases nearly linearly with the Fe concentration, providing clear evidence for cooperative effects between Fe atoms and suggestive of a mechanism involving multiple Fe sites that is supported by density functional theory calculations. In sum, these concepts inform design strategies for higher-performance catalyst architectures and for their incorporation into practical electrolyzer devices to make clean hydrogen fuel from inexpensive renewable electricity for green long-distance transportation and long-duration energy storage.

Towards a molecular understanding of dynamic Fe-based oxygen evolution catalysts

ACS Spring 2022

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