Hysteresis Effects in the Potential-Dependent Double Layer Capacitance of Room Temperature Ionic Liquids at a Polycrystalline Platinum Interface

Abstract

We have measured the frequency- and potential-dependent differential capacitance of the room temperature ionic liquids [EMIm][N(Tf)2] and [BMP][N(Tf)2] at a polycrystalline platinum interface by means of broad-band electrochemical impedance spectroscopy. In a frequency range from 1 MHz to 10 Hz, we observe a transition from the bulk capacitance to a nonideal double layer capacitance. Below 10 Hz, the differential capacitance increases strongly with decreasing frequency, and the capacitance exceeds 0.5 mF/cm2. This low-frequency behavior points to the existence of slow pseudocapacitive processes which are most likely related to ion adsorption. We have fitted the capacitance spectra by means of an equivalent circuit containing constant-phase elements for the nonideal double layer capacitance and for the pseudocapacitance, respectively. When we plot the double layer capacitance estimated from the CPE parameters versus the dc potential of the working electrode, we find hysteresis effects, i.e., the double layer capacitance depends on the scan direction of the dc potential. The hysteresis is caused by slow processes at the ionic liquid/Pt interface taking place on the time scales of minutes to hours. We suggest that these are the same processes causing the pseudocapacitive behavior at frequencies below 10 Hz.