Cooperative Redox Transitions Drive Electrocatalysis of the Oxygen Evolution Reaction on Cobalt–Iron Core–Shell Nanoparticles

Transition metal oxides are promising materials for the development of cost-effective catalysts for the oxygen evolution reaction (OER) in alkaline media. Understanding the catalysts’ transformations occurring during the harsh oxidative conditions of the OER remains a bottleneck for the development of stable and active catalysts. Here, we studied redox transformations of core–shell Fe$_3$O$_4$@CoFe$_2$O$_4$ oxide nanoparticles over a wide range of potentials by using operando near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in total electron yield (TEY) detection mode. The analysis of the NEXAFS spectra reveals that the Fe$_3$O$_4$ core strongly affects the surface chemistry of the CoFe$_2$O$_4$ shell under the OER conditions. The spinel structure of the particles with Co (II) in the shell is preserved at potentials as high as 1.8 V vs RHE, at which Co (II) is expected to be oxidized into Co (III); whereas Fe (II) in the core is reversibly oxidized to Fe (III).