In-Situ Seiras Observation of Co Electrodeposition in an Ionic Liquid: Correlation between the Reaction and Interfacial Restructuring

Abstract

Room temperature ionic liquids (RTILs), promising electrolyte materials for various electrochemical applications, requires molecular level description of their structures at the interface where electrochemical reactions occur. Recently, our studies using surface-enhanced infrared absorption spectroscopy (SEIRAS) that can selectively probe solid/liquid interfaces revealed that the first ionic layer is so rigid that a certain overpotential is required to change the adsorbed ions [1]. Therefore, the rigid first ionic layer possibly inhibits redox species from adsorption onto the electrode surface, which is the initial step of electrochemical reactions; however, no direct experimental evidence for such inhibition has been reported. For uncovering the correlation between electrochemical reactions and interfacial structures of RTILs, we measured electrochemical current and SEIRA spectra simultaneously during potential scans under reaction conditions of Co electrodeposition. SEIRA spectra recorded for a polycrystalline Au electrode in neat [C3mpy][TFSA] (Fig.1) during potential scans showed hysteretic potential dependence, indicating the requirement of a certain overpotential for anion-cation exchange in the first ionic layer in analogy with the other RTILs [1]. SEIRA spectra for a Au electrode in [C3mpy][TFSA] containing 0.1 M Co(TFSA)2 indicated that the anion-cation exchange starts at -0.8 V vs Pt during a negative-going scan. Simultaneously observed CV shows the onset potential of Co electrodeposition at the same potential as shown in Fig. 1. It suggested that the anionic first layer prevent Co2+ from adsorption onto the electrode and thus electrodeposition did not start, and partial mixing of the first ionic layer triggered the adsorption and reduction of Co2+. This is the first direct observation of the correlation between electrochemical reactions and potential-induced interfacial restructuring of RTILs. [1] K. Motobayashi et al., Electrochem. Commun. 100, 117 (2019).