Core–shell Fe$_3$O$_4$@CoFe$_2$O$_4$ nanoparticles as high-performance anode catalysts for enhanced oxygen evolution reaction

Water electrolysis is a promising and environmentally friendly means for renewable energy storage. Recent progress in the development of anion exchange membranes (AEMs) has provided new perspectives for high-performance anode catalysts based on transition metal oxides (TMOs) for the sluggish anodic oxygen evolution reaction (OER). Here, we report on core–shell nanoparticles (Fe$_3$O$_4$@CoFe$_2$O$_4$) which allow combining an electrocatalytic shell (CoFe$_2$O$_4$) with a conductive core (Fe$_3$O$_4$). Such an original approach significantly minimizes critical Co content in the catalyst and avoids addition of unstable conductive carbon black. The core–shell nanoparticles outperform Co$_{(1−x)}$Fe$_{(2+x)}$O$_4$ nanoparticles and show an exceptional OER activity per Co unit mass (2800 A g$_{cobalt}$$^{−1}$ at 1.65 V vs. RHE) suggesting synergistic interaction between the core and the shell. Along with the core–shell structure, the size of the Fe3O4 core is a critical parameter, with a large conductive Fe$_3$O$_4$ core being beneficial for OER enhancement.