In this paper, the capacitive electrochemical behavior of the 1-butyl-3-methylimidazolium tetrafluoroborate (Bmim BF4)-polycrystalline gold electrode interface is reported over the potential range from -0.37 to 0.53 V vs Fc/Fc(+) (Fc = ferrocene). Experimental results are generated by analysis of data (RC model) obtained from large amplitude Fourier-transformed alternating current voltammetry (FT-ACV) over the frequency range of 10 Hz to 1 kHz. Results suggest a parabolic, U-shaped capacitance versus potential relationship, in stark contrast to present ionic liquid (IL) electrochemical double-layer (EDL) theory. The potential range analyzed was carefully selected to be free of Faradaic current and displays minimal hysteresis with respect to the potential scan direction. Over the selected potential window spanning 0.9 V, the capacitance versus potential curve at 9 Hz exhibits a U-shape, with a capacitance minimum of 19.9 +/- 1.3 mu F cm(-2) at 0.13 +/- 0.04 V, flanked by maximum values of 21.2 +/- 1.3 and 20.8 +/- 1.4 mu F cm(-2) at -0.37 and 0.53 V vs Fc/Fc(+), respectively. This capacitance versus potential profile is consistent with traditional Gouy-Chapman-Stern theory for dilute aqueous electrolyte solutions and high-temperature molten salts but distinctly misaligned with bell- or camel-shaped relationships that have recently been proposed in IL model systems. The minimum capacitance exhibits a small level of frequency dispersion, which increases linearly versus the logarithm of the applied frequency. The potential at which the minimum capacitance is located is also slightly dependent on frequency. This work demonstrates that large-amplitude FT-ACV provides a sensitive probe of the EDL from a single experiment and advances the convergence between theoretical predictions and experimental observations of IL-electrode EDL systems.
