Consequences of Overtones in Raman Spectra for Structural Assignment Elucidated for NiOOH during Alkaline Oxygen Evolution

NiOOH serves as a catalyst for the oxygen evolution reaction (OER) that is both abundant and cost-effective. While the catalytic activity is predominantly influenced by the surface, the precise surface structure remains a subject of debate. Raman spectroscopy has frequently been utilized to probe surfaces, however, the precise assignment of bands through theoretical calculation remains to be elucidated. In this study, the surface of NiOOH is examined through the application of in situ surface-enhanced Raman spectroscopy (SERS), in conjunction with D and 18-O isotope labeling, enabling the probing of the surface for potential species. Potential structures of the compounds were calculated using Density Functional Theory (DFT) to assign structures to the detected bands in the experiment. From this comprehensive set of results we conclude, that the surface structure of NiOOH lacks protons and resembles Ni(IV)O2. Furthermore, we propose that bands previously assigned to superoxide species may in fact be connected to overtone and combination modes. In addition, the present study explores the impact of electrolyte conductivity and applied potential on the activation of Raman-inactive modes resulting from symmetry breaking.