Measurements of differential capacitance in room temperature ionic liquid at mercury, glassy carbon and gold electrode interfaces

Differential capacitances were measured in l-propyl-3-methylimidazolium tetrafluoroborate (PMIBF4) ionic liquid at three different electrode substrates (Hg, GC (glassy carbon) and Au) as a function of potential. Essentially different capacitance-potential curves were obtained at different electrodes. From the parabolic electrocapillary curve measured at dropping Hg electrode in PMIBF4, the potential of zero charge (PZC) was found to be -0.31 V vs. Ag/AgCl (wire). However, the capacitance-potential curve at Hg electrode was found not to show any valley related to the PZC, whereas at GC and An electrodes a minimum was observed at 0.29 and -0.51 V, respectively. The results are in disagreement with the recent theoretical study which implies that the capacitance-potential curve should be of bell shape with the maximum value of capacitance at PZC. The parabolic capacitance-potential curve similar to those obtained in inorganic molten salts was also observed for the first time at GC electrode. Probable causes of the difference in their capacitance-potential curves were also discussed. (c) 2007 Elsevier B.V. All rights reserved.