Avoiding Pitfalls in Probing Interfacial Solvation Structures Using Surface-Enhanced Infrared Absorption Spectroscopy

The solvation properties of aqueous electrolytes have been shown to dramatically impact the outcomes of many electrocatalytic reactions. Understanding the structure and dynamics of interfacial water at the metal electrode/electrolyte contact is therefore of central importance in electrocatalysis. The electrode potential affects the interfacial water structure because it controls the concentration of ions in the electrochemical double layer (EDL), the preferential orientation of solvent dipoles, and the coverage of surface-adsorbed species. Surface-enhanced infrared absorption spectroscopy (SEIRAS) has been widely utilized for probing electrocatalytic interfaces, including potential-dependent solvent structure. However, the electrode/electrolyte interface can undergo time-dependent changes that are irreversible with electrode potential. Such subtle irreversible changes of the interface could lead to imperfect subtraction of infrared signals from the bulk or could otherwise alter interfacial solvation. If unrecognized, this phenomenon could lead to misinterpretations of the SEIRA spectra of water and other solvents at electrode/electrolyte interfaces. Yet, these effects have not been systematically investigated to date. Herein, we probed the O–H stretch band of interfacial water at Cu, Au, and Pt electrodes in the presence of different electrolytes. The SEIRA spectra of water at Cu and Au electrodes can be dominated by irreversible effects, which could inadvertently be misinterpreted as the intrinsic potential dependence of the interfacial water structure. In contrast, the SEIRA spectra of water at Pt electrodes typically exhibit a comparatively higher degree of reversibility with electrode potential. We established robust SEIRAS protocols for isolating changes in interfacial solvation that are reversible with electrode potential, that is, changes that reflect the intrinsic potential dependence of the EDL.