The electrical double layer at the [BMIM][PF6] ionic liquid/electrode interface: effect of temperature on the differential capacitance

Impedance spectra (50 kHz-1 Hz) were acquired and used to obtain the differential capacitance at the interfaces between 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6] ionic liquid and three different electrode materials (Hg, Pt, and glassy carbon (GC)) as a function of the applied potential. The electrocapillary curve for the Hg/[BMIM][PF6] interface was obtained from drop time measurements, from which the potential of zero charge was calculated to be -0.39 V (Ag wire). The potential of zero charge is 0.30 V less negative than the potential of differential capacitance minimum. This disagreement suggests that the differential capacitance minimum is not related to a classical diffuse layer minimum. Additional support to this conclusion was obtained from positive temperature coefficient for the differential capacitance in contrast to the negative temperature predicted by the classic Gouy-Chapman model. The results do not support the recent model predictions of bell shaped capacitance curves for room temperature ionic liquids, RTILs.