Resolving Electric Fields and Ionic Motion at Electrochemical Interfaces with Vibrational Spectroscopy

Understanding ionic structure and dynamics at electrified interfaces is broadly relevant to a large range of phenomena, including electrocatalysis, electrochemical energy storage, and sensing. The complexities of interfacial environments, including the interfacial electric fields and the electric double layer structure often impede systematic understanding of the interface. Two spectroscopic approaches for understanding the ionic structure and dynamics will be presented. The first is based on surface-tethered benzonitriles as vibrational probes, with distinct frequency shifts in response to a variety of interfacial properties, including electric field, hydrogen bonding, and coordination with Lewis acidic ions. Using this probe, local electric fields generated by a small concentration of surfactants is measured and estimated to be in the order of ~1 V/nm. Furthermore, the ion-binding property of this probe is used to measure the slow desolvation of doubly charged ions such as Zn²⁺ from a common battery electrolyte and its adsorption on the surface. Surprisingly slow desolvation rates, on the order of several minutes, are measured. The second spectroscopic approach is a marriage between the electrochemical impedance spectroscopy (EIS) and interfacial IR spectroscopy. While EIS is a commonly used tool, its ability to resolve the molecular components of the overall electric response of the interface is extremely limited. Our combined Vibrationally Resolved Electrochemical Impedance Spectroscopy (VibREIS) approach allows to dissect the net electrochemical response of the interface to its various ionic contributions. We apply this technique to an aqueous solution of an organic surfactant, and an inorganic azide salt. We observe manifestly different dynamics between the larger and more sluggish organic ions and the smaller and more agile inorganic azides. Furthermore, we show that solvent motion is anticorrelated with the motion of the surfactants. Critical evaluation of the potentials and limitations of the two spectroscopic approaches will be presented.