Structure and dynamics of the interfacial layer between ionic liquids and electrode materials

In this overview paper we present a combined in situ STM, AFM and EIS study on the structure and dynamics of the interfacial layers between Au(111) and two extremely pure ionic liquids, namely [Py1,4]FAP and [EMIM]FAP. The combination of these methods provides valuable information for both neutral and electrified interfaces. In situ STM and AFM results reveal that a multilayered ion morphology is present at the IL-Au(111) interface, with stronger near surface layering detected at higher electrode potentials. The in situ STM measurements show that the structure of the interfacial layers is dependent on the applied electrode potential, the number of subsequent STM scans and the scan rate. Furthermore, in the case of [Py1,4]FAP, the Au(111) surface undergoes herringbone reconstruction, Au(111)(22×3), in the cathodic potential regime, and the ultra-slow formation of vacancies in the herringbone structure is probed with in situ STM. EIS measurements reveal the presence of two distinct capacitive processes at the interface taking place on different time scales. The time scale of the fast process is typically in the millisecond range and is governed by the bulk ion transport in the IL, which exhibits a Vogel-Fulcher-Tammann-type temperature dependence. The slow process takes place on a time scale of seconds and is Arrhenius activated. The contribution of this process to the overall interfacial capacitance is particularly large in the potential regime where the herringbone structure is probed. Furthermore, we analyze the temperature dependence of the interfacial capacitance.