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 broadband 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/cm 2 . 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.

The Differential Capacitance of Ionic Liquid / Metal Electrode Interfaces – A Critical Comparison of Experimental Results with Theoretical Predictions

“…As expected from the sizes FAP − > TFSI − > PF 6 − , there is a clear trend that the capacitance in the anodic regime decreases in the order PF 6 − > TFSI − > FAP − . For the imidazolium-based ILs, the differential capacitance in the cathodic range decreases in the order [ [53,54]. The FAP − ILs were dried for several hours under vacuum (10 −3 mbar) at 100 • C by the Endres group in Clausthal and sent to Marburg in sealed sample containers.…”

The Electrode/Ionic Liquid Interface: Electric Double Layer and Metal Electrodeposition

“…A similar effect is also observed in N(Tf) 2 À anion-based ionic liquids at polycrystalline Pt electrodes. [103] However, the use of polycrystalline electrodes in the abovementioned studies adds complexities for precise analysis and theoretical modeling. Further comprehensive understanding of the structure and properties of the electric double layer in ionic liquids relies on the employment of well-defined singlecrystal surfaces.…”

Static Capacitance at the Electrochemical Liquid-liquid Interface Between Ionic Liquids and Eutectic Ga-In Alloy Measured Using the Pendant Drop Method

“…In contrast to the progress of the understanding of the EDL of ILs from the theoretical side, experimental confirmation of the predicted IL-peculiar behavior is not straightforward. It has been found, from studies using electrochemical impedance spectroscopy (EIS), that C for the EDL of ILs has strong frequency dependence [11][12][13][14][15][16][17][18] and shows potential hysteresis, 11,[19][20][21] both of which seem to result from the structural ordering of IL interface [22][23][24][25][26][27][28][29] and the ultraslow dynamics [29][30][31][32][33][34][35][36][37][38] of such ordered structure. These tendencies have hampered rigorous evaluation of C experimentally.…”