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.