Role of defects in reversible surface restructuring and activity of Co₃O₄ oxygen evolution electrocatalysts

Overcoming the slow kinetics of the oxygen evolution reaction at the anode is a key challenge for the sustainable production of hydrogen via electrolysis. This reaction operates at very positive potentials where the electrocatalyst is exposed to highly oxidative conditions and prone to potential-dependent surface structural transformations. While substantial evidence for such surface restructuring exists, its extent and relevance for the catalysts’ activity is unclear. We address this topic for the case of Co3O4, one of the best known electrocatalysts exhibiting surface transformations, by studies of epitaxial (111)-ordered electrodeposited films with combined operando surface X-ray diffraction, electrochemical impedance spectroscopy, and electrochemical measurements on rotating disk electrodes. Comparison of the as-prepared and the annealed state of the same samples, which both are stable even under long-term oxygen evolution conditions, provides clear insight into the role of surface defects. Our results show that defect-free annealed Co3O4(111) surfaces are structurally stable over a wide potential range and hydroxylate via adsorption at surface oxygen and Co sites. Potential-induced surface restructuring of the Co3O4 lattice only occurs in the presence of surface defects, leading to the formation of the well-known nanometer-thick oxyhydroxide skin layer. The presence of this skin layer promotes oxygen evolution at low overpotentials, but results in higher Tafel slopes. As a result, highly ordered Co3O4(111) surfaces are more active at high current densities than defective Co3O4 surfaces that undergo surface transformation. These results highlight that strategies for catalyst surface defect engineering need to be application-oriented.