Consequences of overtones in Raman spectra for assigning nickel anode surface structures as NiO$_2$ during alkaline electrolysis

Ni-based anodes are among the most active and cost-effective oxygen evolution reaction catalysts for alkaline electrolysis. The current mechanistic understanding assumes that the anode surface is terminated by nickel oxohydroxide (NiOOH) under operating conditions. Here, by combining isotope-labelled in situ surface-enhanced Raman spectroscopy measurements with density functional theory-calculated Raman spectra, we elucidate that the catalytically active surface is exclusively terminated by nickel dioxide (NiO$_2$) instead. Furthermore, bands previously assigned to superoxides are attributed to overtones and combination bands. On the basis of additional measurements on Co- and Mn-based oxides, we suggest that these features predominantly appear in layered materials. Moreover, the observed shifts at varying pH, potentials and electrolyte composition are rationalized by a vibrational Stark effect rather than by changes in the material or adsorbate structure. Finally, the implications for the mechanistic interpretation of the oxygen evolution reaction on Ni-based anodes are discussed on the basis of the revised structural assignment.