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

Driven by the poor understanding of the electrocatalytic hydrogen evolution reaction (HER) in alkaline medium, we studied the interfacial water structure at a polycrystalline Au surface using in situ 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. The major conclusions from our study are as follows: (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 [P. Gunasekaran et al., 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.