@article{64182,
  abstract     = {{Overcoming the slow kinetics of the oxygen evolution reaction at the anode is a key challenge for the 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 transformation of the near-surface region. While substantial evidence for such surface restructuring exists, its extent and relevance for the catalyst’s activity are unclear. We address this topic for the case of Co3O4, one of the best-known electrocatalysts exhibiting surface restructuring, by studies of epitaxial (111)-ordered electrodeposited films with combined operando X-ray surface diffraction and absorption spectroscopy, electrochemical impedance spectroscopy, and electrochemical measurements on rotating disk electrodes. Comparison of the as-prepared and 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 occurs only 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 restructuring. These results highlight that strategies for catalyst surface defect engineering need to be application-oriented.}},
  author       = {{Scharf, Carl Hendric and Chandraraj, Alex and Dyk, Konrad and Stebner, Felix and Lepin, Sören and Tian, Jing and El Bergmi Byaz, Laila and Stettner, Jochim and Leppin, Christian and Kotova, Anastasiia and Reinke, Sebastian and Linnemann, Julia and Maroun, Fouad and Magnussen, Olaf M.}},
  issn         = {{2155-5435}},
  journal      = {{ACS Catalysis}},
  keywords     = {{electrocatalysis, oxygen evolution reaction, cobalt spinel, operando characterization}},
  publisher    = {{American Chemical Society (ACS)}},
  title        = {{{Role of Defects in Reversible Surface Restructuring and Activity of Co<sub>3</sub>O<sub>4</sub> Oxygen Evolution Electrocatalysts}}},
  doi          = {{10.1021/acscatal.5c08785}},
  year         = {{2026}},
}

@article{61982,
  abstract     = {{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.}},
  author       = {{Kampermann, L. and Klein, J. and Wagner, T. and Kotova, A. and Placke-Yan, C. and Yasar, A. and Jacobse, L. and Lasagna, S. and Leppin, Christian and Schulz, S. and Linnemann, Julia and Bergmann, A. and Roldan Cuenya, B. and Bacher, G.}},
  issn         = {{2155-5435}},
  journal      = {{ACS Catalysis}},
  keywords     = {{electrocatalysis, oxygen evolution reaction, cobalt spinel, operando characterization, spectroelectrochemistry}},
  number       = {{21}},
  pages        = {{18391--18403}},
  publisher    = {{American Chemical Society (ACS)}},
  title        = {{{Operando Analysis of the Pre-OER Activation of Metal-Doped Co<sub>3</sub>O<sub>4</sub> Nanoparticle Catalysts}}},
  doi          = {{10.1021/acscatal.5c03900}},
  volume       = {{15}},
  year         = {{2025}},
}