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

Differential capacitances were measured in 1-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 Au 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.