Competition between ionic adsorption and desorption on electrochemical double layer capacitor electrodes in acetonitrile solutions at different currents and temperatures
Graphical abstract
Introduction
The rapid supply of energy and long cycle life of electrochemical double layer capacitors (EDLCs) allow frequent and persistent cell cycling, allowing their use in electric vehicles, such as electric buses [1] and trams [2], where quick charge facilities are available in every station. In addition, the fuel efficiency of vehicles can be improved by using regenerative brakes [3], [4], [5], [6] and stop-start systems (or idle stop and go system) [7], [8], [9] by taking advantage of the rapid energy management of EDLCs. The operation of EDLCs at high currents causes the cell temperature to rise because of the cell resistance and reversible heat cycles [10]. In addition, in automobiles, the engine under the hood is another heat source that raises the cell temperature. Under these conditions, cells are constantly exposed to high and variable temperatures. While cells containing ionic liquids remain relatively stable at high temperatures [11], [12], [13], cells containing organic solvents suffer from increased vapor pressures at high temperatures [14]. These continual temperature changes have a negative effect on the cycle life of EDLCs because of the resultant electrochemical decomposition [15], [16], [17]. Nevertheless, if the temperature is well managed, the physical, chemical, and electrochemical damage can be limited; thus, it is necessary to find the optimum long-term operating conditions for a given device.
As the cell temperature increases, adsorbed ions on the EDLC electrode surface are desorbed, which reduces the coulombic efficiency of the cell. Even at room temperature, competition between ion adsorption and desorption exists. As soon as the charging process ends, the desorption of ions (self-discharge) becomes dominant. In this study, we investigated the change in behavior of energy-storage EDLCs with increasing temperature. The temperature dependence of the capacitance values at various currents can offer insights into the ion movement, adsorption, and desorption. Cell currents can change many factors for cells, for example, the operating time, ion population on the surface, and self-discharge time. Tetraethylammonium tetrafluoroborate (TEA BF4) and ethylmethylimidazolium tetrafluoroborate (EMI BF4) are the most widely used electrolyte salts for EDLCs. The individual properties of these two electrolytes [18] result in different cell behaviors. Understanding the adsorption and desorption behaviors, which are affected by many variables, can aid in the proper selection of cell operating conditions in terms of electrolytes, currents, and temperatures. In particular, we have studied the effect of temperature because long-term exposure to elevated temperatures can reduce the cycle life [19]. In addition, we focus on ionic adsorption and desorption behavior in cells where the electrolysis of the electrolytes over long cycles is not observed. The results from this study support further investigations into the severe electrolysis activities during long cycles.
Section snippets
Materials
The syntheses of EMI BF4 and TEA BF4 have been reported elsewhere [18], [20]. For the recrystallization of salts, all salts were dissolved in acetonitrile (AN, Sigma-Aldrich, 99.8%). n-Butanol was added to induce precipitation. The salts were filtered and dried thoroughly before use. The active material for the EDLC electrode was activated carbon (AC, CEP21KSN, Power Carbon Technology). The AC was mixed with an emulsion binder of styrene butyl rubber (SBR, Zeon, BM-400B), and
Results and discussion
Curves of capacitance vs. current density for the electrodes in 1.0 M TEA BF4 are shown in Fig. 1A; this figure shows the temperature dependence of the capacitance. The discharge capacitances at the lowest current density, 0.1 A g−1, are higher than those at any other current densities because the low current allows sufficient time for ions to enter the narrow pores in the electrodes. At 0.1 A g−1, the highest capacitance was obtained at the lowest temperature because the ion desorption rate is
Conclusions
The capacitances of EDLCs were not simply improved by increasing the temperature. At low currents, there is sufficient time for ions to occupy the maximum number of adsorption sites. When the ion population is high, increasing the temperature supplies energy to ions, allowing them to leave the electrode. However, improvements in the capacitance on increasing the temperature are observed when the surface ion population is not dense. The operation of EDLCs at a high-current density and low
Acknowledgements
This research was supported by the Ministry of Science, ICT and Future Planning of the Korean Government (NRF-2016-R1A2B1013121), Korean Ministry of Environment (Chemical Accident Prevention Technology Development Project) and a grant from the R&D Program of the Korea Railroad Research Institute, Republic of Korea.
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