Interpretation of Raman Spectra of NiOOH Using DFT and Its Application to Monitoring the Surface Change during OER at Different pH

In electrolyzers, the electrocatalytic splitting of aqueous electrolytes is typically impeded by the sluggish kinetics of the Oxygen Evolution Reaction (OER) at the anode. In established alkaline electrolysis technologies, NiOOH anodes are commonly employed and have been the subject of extensive studies using various analysis methods, such as Raman spectroscopy. [1-4] In this study, we emphasize the origin of the vibrational modes of a NiOOH Raman spectrum, with the objective of attaining a more profound understanding of the surface structure of NiOOH before and during the OER. Additionally, we illustrate the impact of varying pH values on the OER activity and the surface structures of the NiOOH electrodes, with the aim of providing a comprehensive understanding of potential-dependent structural changes. The potential dependent surface structural changes of NiOOH are investigated using in situ Surface Enhanced Raman Spectroscopy (SERS) in electrolytes of varying neutral and alkaline pH. Isotope labeling experiments, employing D 2 O and H 2 18 O, were conducted to gain additional insights in the measured Raman modes, from possible peak shifts induced by the isotopes. Furthermore, Density Functional Theory (DFT) calculations on various Ni x O y (OH) z were performed to assign the peaks in the Raman spectrum. Utilizing this comprehensive set of experimental and theoretical data, we demonstrate that the surface structure of NiOOH is predominantly deprotonated under both alkaline and neutral pH conditions. Additionally, we discuss the prevailing view on the formation and detection of superoxides using Raman spectroscopy on NiOOH. [3,4] Moreover, while performing the OER on NiOOH under weak alkaline pH values, SERS reveals a band at 1040 cm-1, which has not yet been reported. Further Cyclic Voltammetry (CV) studies at various pH levels also show significant changes in OER region. Complementary Electrochemical Quartz Crystal Microbalance (EQCM) measurements reveal pH-dependent changes in the electrode mass at high potentials, which correlate with the variations observed in the CVs and SERS spectra. The variations in the CVs are rationalized in terms of local pH changes at the electrode surface. Combining the results from the various techniques we discuss the impact of pH and potential dependent surface changes on OER kinetics and its underlying mechanism. (1) Klaus, S.; Cai, Y.; Louie, M. W.; Trotochaud, L.; Bell, A. T. J. Phys. Chem. C 2015 , 119 (13), 7243–7254. (2) Merrill, M.; Worsley, M.; Wittstock, A.; Biener, J.; Stadermann, M. Journal of Electroanalytical Chemistry 2014 , 717–718 , 177–188. (3) Lee, S.; Chu, Y.-C.; Bai, L.; Chen, H. M.; Hu, X. Chem Catalysis 2023 , 3 (1), 100475. (4) Diaz-Morales, O.; Ferrus-Suspedra, D.; Koper, M. T. M. Chem. Sci. 2016 , 7 (4), 2639–2645. Figure 1