Electrochemical double layer capacitor (EDLC) complements batteries in addressing the intermittency problems in renewable energy sources. Here, the charge is stored in ionic double layer at the surface of the electrode. The storage process is passive, and does not involve any moving part. These devices offer energy density, which is nearly one tenth of the lowest density batteries. At the same time they offer power density, nearly ten times that of the batteries. Their high charge utilization, close to that of batteries, and a fast response, similar to that of capacitor find potential use in high power sources, particularly where the intermittency is an issue. Other uses are in back-up memory for electronic devices e.g., video or compact disc players, computers, regenerative breaking devices, and high power lasers.
High internal surface area is an important requirement for storing charges in double layer through physical adsorption and electrostatic interaction. Porous carbon offers a large surface area with electrical conductivity, over which the charge can be stored. In addition to the capacitance from the double layer, the resistance encountered by the ions in reaching the end of the pore, or transport through the pore network, while charging or discharging determines the effectiveness of storage. The resistance arises from bulk electrolyte, and from the electrolyte inside the pore.
The porous carbon cannot be represented by a simple capacitance, or even a resistance in series with the capacitance. Instead, the charge is stored over a distributed network of resistive and capacitive elements. In a packaged capacitor, the resistance arises in many forms, e.g., electrolyte resistance, and the interfacial resistances in the inter particle regions, and at the external contacts. Additionally in porous carbon, the size of ions, and the hierarchy of pore structure decide whether the ions can penetrate into the pore completely, and access all the internal area of the pore. That is, the size and the hierarchy through which the ions access the pore determine the additional resistance in the charging and discharging of EDLC.
In this presentation, the impedance spectroscopy data is analyzed to model the transport of ion he resistance inside the pore. The equivalent circuits were fitted here to explain the complex impedance curve. The resistive and capacitive elements and their arrangement were studied for different thicknesses of the activated carbon overlay. The effect of binder on the internal surface area of activated carbon powder was evaluated through BET analysis and scanning electron microscopy. The BET analysis was performed using Autosorb 1 from Quantachrome Instruments, U.S.A. and the scanning electron microscopy was performed using ZEISS EVO 60 from Carl ZEISS SMT Germany. The carbon powder was obtained from activated charcoal, as purchased. For comparison, the carbon powder was synthesized by conducting pyrolysis of a dried precursor gel in a vacuum furnace. The porous network may get affected by the drying of gel precursor. In view of this, three methods of drying are evaluated. These are atmospheric, vacuum and freeze drying. Two electrolytes are used for comparison. Aqueous solution of potassium chloride as an aqueous electrolyte, and tetra ethyl ammonium tetra fluoroborate in acetonitrile as an organic electrolyte are used in this study.
The cyclic voltammetry and the impedance spectroscopy were performed using a potentiostat-galvanostat with frequency response analyzer from Princeton Applied Research, U.S.A. An electrochemical cell was constructed using plexiglass blocks. Two electrodes were assembled with a separator in between, and the chamber was filled with aqueous solution of potassium chloride. The self-discharge characteristics of the electrodes were studied prior to the experiments of cyclic voltammetry and impedance spectroscopy.
The scan rate for cyclic voltammetry varied within the range of 2 mV/s to 1V/s. The capacitances, thus obtained were compared with the values in literature. The impedance spectroscopy is performed within the frequency range of 0.1 Hz to 0.1 MHz. The equivalent series resistance is derived from the complex impedance plots for different overlay thicknesses, current collectors, type of electrolytes, and source of carbon.
The time constants in the complex impedance were analyzed through different equivalent circuits. The equivalent circuits take into account the branching of pores, as well as the parallel pores. The non-ideal behavior of charge transfer is accounted through Q type capacitance. The hierarchy of pores from macro to meso and down to micro scale is considered here. The access to the pores from higher to lower levels in the hierarchy, as the ions travel through the pore network during the charging and the discharging cycles is analyzed. The effect of various parameters of electrode and electrolyte are reviewed based on this analysis.
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