The search for commercially viable catalysts for the oxygen evolution reaction (OER), which is typically the bottleneck for electrochemical water splitting, is a key challenge in the worldwide transition to renewables-based electricity generation. Transition-metal oxides in general and cobalt (hydro-)oxides in particular are of great interest in this regard, as they show promising catalytic properties, are stable in alkaline and neutral electrolytes under ambient conditions, and can be easily prepared from earth-abundant materials [1]. A wide variety of different catalysts, based on oxides of Co, Fe, and Ni, have been synthesized and studied, exhibiting rather diverse nanoscale morphologies and usually an unknown or poorly defined surface structure. This makes it difficult to compare their reactivity and correlate it with ab initio theoretical studies, which hinders the development of clear structure-reactivity relationships and unambiguous determin­ation of the OER reaction mechanism.

We here present studies of structurally well-defined Co₃O₄ films of 10-30 nm thickness, prepared by electrodeposition or molecular beam epitaxy (MBE) on Au(111), Au(100), and Ir(100) substrates. In all cases a well ordered epitaxial Co₃O₄(111) arrangement results, but the film morphology differs substantially, as shown by AFM and X-ray diffraction measurements. The oxide structure under reaction conditions was studied by detailed operando surface X-ray diffraction [2,3] and in situ X-ray absorption measurements. These techniques allow characterization of the surface and bulk oxide structure over a wide potential range, including the OER regime, and can be performed simultaneously with electro­chemical measurements, enabling direct correlations of structure and reactivity.

We find that the films prepared by molecular beam epitaxy are perfectly stable whereas all electrodeposited Co₃O₄ films exhibit reversible changes in the oxide structure and oxidation state, in agreement with a previous study of polycrystalline Co₃O₄ catalysts [3]. Specifically, we observe the formation of an ultrathin X-ray-amorphous CoO_x(OH)_y skin layer above 1 V vs. RHE and the gradual buildup of tensile lattice strain with increasing potential. These structural changes occur for all electrodeposited samples, albeit to a different extent, depending on the film morphology. They were found at pH values between 7 and 13 and in the presence of different cations. Potential step experiments show that the structural transformation proceeds within seconds and is highly reversible. Despite the similar structure, differences in the OER overpotential of up to 150 mV are observed for these samples, with the structurally stable MBE-prepared films having a larger overpotential than the electrodeposited films. This indicates a beneficial effect of the CoO_x(OH)_y skin layer on the OER activity of Co oxide catalysts.

[1] C. C. L. McCrory et al., Journal of the American Chemical Society 2015, 137, 4347.

[2] F. Reikowski, et al., ACS Catalysis, 2019, 9, 3811.

[3] T. Wiegmann, et al., ACS Catalysis, 2022, 12, 256.

[4] A. Bergmann, et al., Nature Communications, 2015, 6, 8625.