Commentary on ‘The interface between Au(1 1 1) and an ionic liquid’

Abstract

Recently, Gnahm et al. [Electrochim. Acta 55 (2010) 6212] analyzed and discussed capacitive processes at the interface between the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate and a single-crystalline Au(1 1 1) electrode. In order to derive interfacial capacitance spectra, they subtracted the bulk resistance R S from the experimental impedance spectra Z ˆ ( v ) and used the following expression C ˆ s u b ( ν ) = ( i 2 π ν × ( Z ˆ ( ν ) − R s ) ) − 1 . The subtraction procedure was based on the assumption that lim ν → ∞ Z ′ ( ν ) = R s . In this commentary we argue that this assumption neglects the high-frequency capacitance C s acting in parallel to R S . In a three-electrode setup, the high-frequency capacitance is often governed by the input capacitance of the reference electrode. The parallel action of C S and R S results in lim ν → ∞ Z ′ ( ν ) = 0 . Consequently, the subtraction of R S leads, in general, to negative − C ″ s u b ( ν ) values at high frequencies which have no physical meaning. Moreover, the subtraction procedure causes an artificial distortion of the double layer semicircle in the complex capacitance plane.