Consequences of Overtones in Raman Spectra for Structural Assignment of Nickel Anodes during Alkaline Electrolysis

Surface-enhanced Raman spectroscopy (SERS) is a valuable technique, particularly for in situ and operando studies. However, anharmonic features, such as overtones, while rare and often overlooked, may complicate structural interpretations. This aspect is illustrated for Nickel oxyhydroxide (NiOOH), a vividly discussed oxygen evolution reaction (OER) catalyst for application as anode material in alkaline electrolysers. Based on in situ SERS measurements in common electrolytes, including isotope-labeled water and Density Functional Theory (DFT) calculated Raman spectra, we question the presence of NiOOH upon electrochemical oxidation of Ni(OH) ₂ and instead suggest that the catalytically active surface consists of Nickel dioxide (NiO ₂ ) instead. Additional bands in the experimental spectrum, which were previously assigned to superoxide species, are attributed to overtones and combination bands. Possible reasons for the appearance of overtones are discussed. Previously observed shifts of these bands at varying pH and more positive OER potentials are rationalized by a vibrational Stark effect rather than changes in the material or adsorbate structure.