Double Layer at [BuMeIm][Tf2N] Ionic Liquid-Pt or -C Material Interfaces

The interface [BuMeIm][Tf2N]/electrode, where [BuMeIm][Tf2N] stands for the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was characterized by electrochemical impedance spectroscopy at different temperatures and for different electrode materials: platinum (Pt, metallic), glassy carbon (GC, high conductivity), carbon nitride (a-CNx, mean conductivity), and boron-doped diamond (BDD, semiconducting with a quasimetallic character). For Pt, GC, and a-CNx, the behavior of the interface could be described by the same equivalent electrical circuit. In the case of BDD, a parallel combination of R-sc and C-sc was introduced into the circuit to take into account the potential drop due to the development of a space charge region within the material. The Mott-Schottky plots have confirmed the polycrystalline semiconductor character of the BDD material, and the boron concentration estimated is fully consistent with the B amount introduced for the synthesis. The variations of the double-layer capacitance as a function of potential were found to be camel shaped for all electrode materials at the highest studied temperature. This is consistent with the prediction of Kornyshev's theory as low values of the packing parameter gamma were estimated by simulation (lower than 0.33). An increase of the double-layer capacitance is found with the temperature similarly to most of the results obtained for molten salts.