Complex Capacitance Analysis on Leakage Current Appearing in Electric Double-layer Capacitor Carbon Electrode

Abstract

Complex capacitance analysis was done on the porous carbon electrode-electrolyte interface, where a minor leakage current is involved in addition to the dominant capacitor charging current. Based on the transmission line model, imaginary capacitance profiles ͑C im vs. log f͒ were theoretically derived for a cylindrical pore and multiple pore systems of nonuniform pore geometry. The parallel RC circuit was assumed for the interfacial impedance, where R is the charge-transfer resistance for leakage current and C the double-layer capacitance. The theoretical derivation illustrated that the resistive tail relevant to the leakage current appears in addition to the capacitive peak, which was in accordance with the experimental data taken on the porous carbon electrode. The electric double-layer capacitor ͑EDLC͒ parameters such as the extent of leakage current, total capacitance, and rate capability were visually estimated from the imaginary capacitance profiles. The more quantitative EDLC parameters were obtained by a nonlinear fitting to the experimental data. Electric double-layer capacitor ͑EDLC͒ utilizes the double layer formed at the electrode-electrolyte interface, where electric charges are stored on the electrode surface and ions of counter charge are arranged in the electrolyte side. The most demanding feature for EDLC electrodes is the ideally polarized behavior over a wide potential range. The practical EDLC electrodes, however, suffer from a self-discharge at the charged state that is caused by leakage currents. [1][2][3][4][5] This nonideally polarized behavior should thus be minimized to improve the charge-discharge efficiency and reliability of commercial cells.The leakage current appearing in EDLC electrodes can be analyzed using ac impedance spectroscopy. When impedance data are analyzed using the conventional Nyquist plot, the vertical line in the low-frequency region that is observed for ideally polarized electrodes becomes inclined with an increase in the leakage current. [6][7][8] The extent of leakage current can thus be estimated from the degree of inclination. The problem here, however, is that the inclination is also observed even in ideally polarized electrodes when they have a nonuniform pore geometry that is common in most practical porous EDLC electrodes. [9][10][11][12] In theory, the differentiation between the leakage current and nonuniform pore geometry for the cause of inclination is possible when the measurement is made at very low frequencies. In the extremely low-frequency region, ideally polarized electrodes with nonuniform pore geometry give a vertical line on the Nyquist plot, whereas the leakage current is visualized as a semicircle whose diameter reflects the charge-transfer resistance for leakage reaction. In practice, however, it is difficult to obtain data at such a low frequency due to an instrumental limitation and extremely long measuring time.In this work, the complex capacitance analysis is done on the leakage current involved in porous carbon electrode. From the imagi...

Electrostatic spray deposition of graphene nanoplatelets for high-power thin-film supercapacitor electrodes

“…e Specific capacitances obtained from galvanostatic charge/discharge measurements at various current densities extracted from impedance data according to Eqs. (1) and (2) [28,29].…”

Electrophoretic deposition (EPD) of hydrous ruthenium oxides with PTFE and their supercapacitor performances

“…(2)), and specific energy and power were calculated as a function of ac frequency (Eq. (3)), providing energy-power plot for supercapacitor electrode [27][28][29][30]:…”

Electrochemical impedance studies of IrO 2 catalysts for oxygen evolution

“…Hence, the most appropriate approach is to obtain the total capacitance from the manipulation of raw impedance data at the limit of very low frequencies. This has been attempted by using a complex capacitance analysis [38,39] but it is difficult to extrapolate the corresponding C im vs. C re arcs, while resorting to the more accurate procedure of integrating the C im vs. logf curves still does not alleviate the need for data at extremely low frequencies. An alternative, very elegant methodology was proposed by Pickup and co-workers for dispersed Pt/polymer electrolyte systems [40,41] and entails plotting 1/(ωZ im ) vs. Z re .…”