Distinguishing interfacial double layer and oxide-based capacitance on gold and pre-oxidized Fe-Cr in 1-ethyl-3-methylimidazolium methanesulfonate room temperature ionic liquid aqueous mixture

Abstract

The frequency-dependent impedance of a metal/oxide/room temperature ionic liquid (RTIL) interface was separated into oxide layer (Cox) and double layer (Cdl) capacitances using electrochemical impedance spectroscopy (EIS). A pre-formed oxide of known initial thickness on Fe-20 at.% Cr was investigated in 1-ethyl-3-methylimidazolium methanesulfonate ([Emim][MeSO3]) RTIL + aqueous mixtures. The oxide thickness was calculated from Cox using the complex capacitance representation and the power-law model. The result was additionally verified by cross-sectional analysis using transmission electron microscopy. The Cdl calculated from the constant-phase element (CPE) was distinguishable from Cox for Fe-20Cr, which enables monitoring of the interfaces of structural metals with oxide layers in RTILs containing water.