Spectroelectrochemical Analysis of the Water Oxidation Mechanism on Doped Nickel Oxides

Metal oxides and oxyhydroxides exhibit state-of-the-artactivity for the oxygen evolution reaction (OER); however, their reactionmechanism, particularly the relationship between charging of the oxide andOER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-,and Zn-doped nickel oxides usingoperandoUV-vis spectroscopy coupledwith time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+redox peak potential is found to shift anodically from Mn- < Co- < Fe-< Zn-doped samples, suggesting a decrease in oxygen binding energeticsfrom Mn- to Zn-doped samples. At OER-relevant potentials, using opticalabsorption spectroscopy, we quantitatively detect the subsequent oxidationof these redox centers. The OER kinetics was found to have a second-orderdependence on the density of these oxidized species, suggesting a chemicalrate-determining step involving coupling of two oxo species. The intrinsicturnover frequency per oxidized species exhibits a volcano trend with thebinding energy of oxygen on the Ni site, having a maximum activity of similar to 0.05 s-1at 300 mV overpotential for the Fe-doped sample.Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited byO-O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limitedby the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energyand intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OERkinetics.