Operando Analysis of the Pre-OER Activation of Metal-Doped Co ₃ O ₄ Nanoparticle Catalysts

Doped Co 3 O 4 nanoparticles are investigated via spectro-electrochemistry in the (pre-) oxygen evolution reaction (OER) regime by tracing the absorption signal of the Co 3+ 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 V RHE, a rise in the optical absorption signal of the [Co 3+ ] oct d–d transition is observed and attributed to a structural change from [Co 2+ ] tet to [Co 3+ ] oct due to an electrochemically induced surface restructuring with water. For applied potentials higher than 1.37 V RHE 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 CoO x (OH) y skin layer as supported by high-energy X-ray diffraction (HE-XRD) measurements. We found that the kinetics of the Co 3+ states are heavily influenced by the type of dopant with V-doped Co 3 O 4 exhibiting stable Co 3+ states (>20 min) while the Mn-doped Co 3 O 4 Co 3+ states reduce within 36 s under reductive bias. We conclude that doping Co 3 O 4 with transition metals affects the formation and potential-dependent thickness of the CoO x (OH) y skin layer as the catalytically active phase and the formation of long-time stable surface Co 3+ states after activation in the first OER cycle.