On Capacitive Processes at the Interface between 1-Ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate and Au(111)

Electrochemical impedance spectroscopy was used for characterizing the interface between the ultrapure room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate, [EMIm]FAP, and a Au(111) working electrode (WE). Plots of the potential-dependent spectroscopic data in the complex capacitance plane (CCP) reveal the existence of three distinct processes taking place on different time scales. At all WE potentials ranging from -0.5 to +1.0 V versus a Pt pseudo-reference electrode, a high-frequency semicircle was detected in the CCP, which was attributed to the formation of an electrochemical double layer (EDL). At intermediate frequencies, a second capacitive process was observed, which is most likely related to electrode de-/reconstruction in the cathodic regime and to a strong interaction between the Au(111) surface and FAP(-) anions in the anodic regime. When the WE potential becomes either more negative than -0.4 V or more positive than +0.8 V, a third ultraslow process was detected, which seems to become Faradaic in the low-frequency limit. To extract differential capacitance values for EDL formation and for the second capacitive process, the complex capacitance data were fitted to an empirical Cole Cole type equation. We find a significant hysteresis in the potential dependence of the differential double-layer capacitance (C-EDL). The capacitance relaxation strength of the second process is particular high at electrode potentials around +0.4 V and at potentials more negative than -0.4 V.