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 Rsc and Csc 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 γ 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.

EIS, CV and XPS experiments were carried out to address the evolution of H‐terminated BDD electrochemical properties under intensive use and air aging. A drastic drop of apparent electron transfer rate k0′ was thus recorded using Fe(CN6)3–/4– as the redox mediator by identifying components of the Randles equivalent circuit. Excellent agreement was observed between theoretical and experimental curves. The feasibility of recovering and stabilizing high reactivity and reversible behaviour by applying a suitable electrochemical post treatment was then demonstrated. Such a treatment was developed empirically and seemed to significantly improve the electrode performances. On the basis of our results, we claim that the electrochemical evolutions observed are strongly linked to chemical termination modifications and partial inactivation of the electrode surface. Further research on the mechanisms that govern electron mediation at the diamond surface would be of high interest for better control of diamond electrode stability and reactivity towards their applications for bio‐electronic devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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