Structure of interfacial water at gold electrodes during hydrogen evolution in alkaline medium: a spectroscopic study through isotopic dilution

Abstract

Driven by the persisting poor understanding of the electrocatalytic hydrogen evolution reaction (HER) in alkaline medium, we studied the interfacial water structure at a polycrystalline Au surface using a combination of operando surface enhanced infrared and Raman spectroscopies. Employing an isotopic dilution strategy, we investigated the fundamental O-H and O-D vibrations of HOD molecules, where the study of the normal vibrational modes of water is simplified due to symmetry reduction. From the water structure analyses at the electrode–electrolyte interface, we unravelled the complementarities of infrared and Raman spectroscopies in probing an electrochemical interface. Taking advantage of the preferential accumulation of H2O over D2O at the Au surface, we analysed the orientational changes in the water molecules up to a few layers over the electrified Au surface. The major conclusions from our study are the following: (i) Interfacial water orients in a H-down manner immediately negative of the potential of zero charge (pzc); (ii) There is no strongly hydrogen-bonded ‘ice-like water’ or poorly hydrogen-bonded ‘free water’ at the Au electrode surface at any potentials. (iii) Interfacial water forms a stable backbone of water roughly parallel to the electrode surface which survives orientation with one H down at potentials negative to pzc [Chem Sci, 2024, 15, 17469–17480], and a very high negative surface charge density is required for further reorientation. Although experiments with H2O suggest that the maximum degree of orientation, and therefore dielectric saturation, is reached around -0.3 V vs. RHE in 1 M KOH, the analysis of the potential dependence of the O-H and O-D stretching modes of HOD reveals that in fact, further orientation of the water dipoles continues at least down to -0.9 V.