Electrochemical water splitting provides sustainable means of generating hydrogen fuel, a carbon-free alternative for fossil fuels. The bottleneck in proceeding water splitting reaction is the sluggish kinetics of the oxygen evolution reaction (OER), resulted from its complex four-electron pathway. While precious metal oxide-based materials such as IrO₂ and RuO₂ deliver superior catalytic activity toward OER, their scarcity and high cost is not appropriate for the large-scale commercialization. In this respect, earth-abundant, first-row (3d) transition metal oxides have been of recent interest.¹ As an effective strategy to enhance the activity of 3d metal based catalysts, great efforts have been dedicated to multi-metal systems. Exploiting large diversity of metal compositions, various multi-metal oxides-based catalysts have been reported, including Ni-Fe, Co-Fe, Ni-Co, and ternary oxides/oxyhydroxides.² Among them, Fe-containing Co and Ni based oxide-based catalysts have known to be most active toward OER in alkaline electrolyte.³ Despite the substantial research efforts to unveil the origin of improved activity of Fe-containing Co and Ni oxide-based catalysts, one of the most important task that still remains unsolved is the accurate determination of catalytic active sites and their reaction mechanism at the atomistic scale.

In this work, we present a theoretical study on the electrocatalytic properties of pristine and transition metal (Fe, Ni, and Mn)-substituted Co oxides. Through the exhaustive consideration of oxygen sites in the vicinity of dopants, the prediction of the active sites and reaction mechanism is provided. The OER mechanisms estimated for the terminal sites of dopant and neighboring Co cations, and for the bridge site are introduced in detail. Notably, it is demonstrated that the bridge site which has been generally considered inactive for OER can be activated by Fe incorporation. We further investigate the structural properties of the relevant intermediates. With the structural analyses, we propose that the modulation of hydrogen bonding distance induced by the Jahn-teller active Fe(IV) cation leads to the activation of the bridge site. Finally, with the comprehensive view of estimated OER thermodynamics, we suggest an origin of improved OER performance of the Fe-containing Co oxide-based catalysts. We believe that our findings establish theoretical deign rule of Co based multi-metal oxide-based catalysts, and propose a viable strategy to resuscitate catalytic activity of inert sites by the structural modulation through cation doping.

Reference

1. P. et al. Energy Environ. Sci. 5, 6012 (2012).
2. N.T. et al. Chem. Soc. Rev. 46, 337 (2017).
3. C.L. et al. J. Am. Chem. Soc.137, 4347 (2015).