Operando Analysis of the Pre-OER Activation of Metal-Doped Co₃O₄ Nanoparticle Catalysts

Doped Co3O4 nanoparticles are investigated via spectro-electrochemistry in the (pre-) oxygen evolution reaction (OER) regime by tracing the absorption signal of the Co3+ d–d transition under applied bias for getting insight into the catalysts activation and the formation of catalytically active phases. In the low potential regime up to 1.37 VRHE, a rise in the optical absorption signal of the [Co3+]oct d–d transition is observed and attributed to a structural change from [Co2+]tet to [Co3+]oct due to an electrochemically induced surface restructuring with water. For applied potentials higher than 1.37 VRHE an overall offset of the absorption spectra in the UV–vis range, equivalent to a darkening of the materials is detected. This is attributed to the formation of a CoOx(OH)y skin layer as supported by high-energy X-ray diffraction (HE-XRD) measurements. We found that the kinetics of the Co3+ states are heavily influenced by the type of dopant with V-doped Co3O4 exhibiting stable Co3+ states (>20 min) while the Mn-doped Co3O4 Co3+ states reduce within 36 s under reductive bias. We conclude that doping Co3O4 with transition metals affects the formation and potential-dependent thickness of the CoOx(OH)y skin layer as the catalytically active phase and the formation of long-time stable surface Co3+ states after activation in the first OER cycle.