Electrolytes Used in EDLC

Shortage of electrolyte in dilute solutions can strongly affect the charging-discharge behavior of EDLC. The effect of resistance of electrolyte on the process of charge accumulation in EDLC carbon electrodes with solutions of tetraethyl ammonium tetrafluoroborate (TEABF4) in PC was studied. The charge 

At present, hundreds of ILs have been obtained, but most of them are inapplicable in EDLC because of inappropriate physical properties low conductivity, high viscosity, high melting point (Arbizzani et al. (2008)). 

The materials most widely used in ECSC are imidazolium and pyrrolidinium salts that are characterized by the highest conductivity, while the electrochemical stability window (ESW) reaches even 5V (these data, however, are obtained for smooth electrodes) then again, high cyclability has not been obtained as yet. Table 27.2 presents the values of conductivity, ESW, melting points with ILs in the limit, formula weight and density of A-butyl-A-methyl pyrrolidinium 

TABLE 27.2. Values of Conductivity (c), ESW, Melting Points with ILs in the Limit, Formula Weight and Density of V-Butyl-iV-Methyl Pyrrolidinium (PYRj4-l-) and V-Methoxyethyl-V-Methylpyrrolidinium (PYRj 2 y-l-)-Based ILs with Bis (trifluoromethanesulfonyl)imide (TFSI—) and Trifluoromethane Sulfonate (Tf—) Anions 

Studies of capacitance of negatively charged electrodes based on AC and aero/cryo/xerogel carbon in two ILs with the same anion and other cations with an almost similar size, that is, l-ethyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide (EMITFSI) and Af-butyl-A-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) ILs were made (Fig. 27.10). 

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The meso/macroporous carbons have attracted much attention in their application as electrode materials in EDLCs, since the meso/macropores promote the formation of an effective doublelayer or the transfer of ions into the pores, resulting in the increases in the electrolyte wettability and the rate capability.67,68 In this regard, there has been considerable research targeted towards developing the synthetic methods of novel meso/macroporous carbons.17,36"55,69 72 Various types of such inorganic templates as silica materials and zeolites are widely used for the synthesis of the meso/macroporous carbons, since it was revealed17,36"55 that these inorganic templates contribute to the formation of the meso/macropores with various pore structures and broad PSD. 

Rs (Figure 1.22a). The double layer capacitance is represented by the capacitance C, and Rs is the series resistance of the EDLC, also named the equivalent series resistance (ESR). This series resistance shows the nonideal behavior of the system. This resistance is the sum of various ohmic contributions that can be found in the system, such as the electrolyte resistance (ionic contribution), the contact resistance (between the carbon particles, at the current collector/carbon film interface), and the intrinsic resistance of the components (current collectors and carbon). Since the resistivity of the current collectors is low when A1 foils or grids are used, it is generally admitted that the main important contribution to the ESR is the electrolyte resistance (in the bulk and in the porosity of the electrode) and to a smaller extent the current collector/active film contact impedance [25,26], The Nyquist plot related to this simple RC circuit presented in Figure 1.22b shows a vertical line parallel to the imaginary axis. 

Carbon powder mixed with polymeric binder (PVdF, PTFE) has been widely used as anode material for lithium ion batteries and as the electrode material for EDLC with liquid electrolyte solutions. When such composite electrodes composed of carbon powder and polymer binder were used in all-solid-state EDLC, the performance was not good enough because of poor electrical contact between the electrode s active mass and the electrolyte. By having the electrolyte inside the composite electrode, the contact between the active mass in the electrode and the electrolyte can be considerably improved and hence the capacitance can... 

EDLCs store energy within the variation of potential at the electrode/electrolyte interface. This variation of potential at a surface (or interface) is known as the electric double layer or, more traditionally, the Helmholtz layer. The thickness of the double layer depends on the size of the ions and the concentration of the electrolyte. For concentrated electrolytes, the thickness is on the order of 10 A, while the double layer is 1000 A for dilute electrolytes (5). In essence, this double layer is a nanoscale model of a traditional capacitor where ions of opposite charges are stored by electrostatic attraction between charged ions and the electrode surface. EDLCs use high surface area materials as the electrode and therefore can store much more charge (higher capacitance) compared to traditional capacitors. 

Various t5pes of porous carbon have been widely studied for use as electrode materials for EDLCs. Their unusual structural and electronic properties make the carbon nanostructures applicable in the electrode materials of EDLCs and batteries. The principle of electrochemical capacitors, physical adsorption/desorption of electrolyte ions in solution, was applied for water purification by using different carbon materials [108-113]. 

A particular feature of operation of EDLC with AC electrodes is the formation of a certain amount of CO2 in the course of operation at the potentials above 1V (NHE). Analysis of the gas collected in the case of EDLC with electrolyte of 38% H2SO4 showed that it consisted predominantly of N2 and O2 with traces of CO2 and H2O at potentials below 1V. Eor charged EDLC, the gas consists mostly of CO2 with small amounts of Nj, O2, and HjO vapors. Presence of water vapors shows that the performance of the capacitor can decrease with time because of loss of electrolyte, which results in a decrease in capacitance of the capacitor. This, however, can occur only in unsealed EDLCs. Carbon dioxide is a product of oxidation of carbon electrodes. Its amount strongly depends on the voltage applied. To prevent CO2 accumulation in the course of EDLC operation, a release valve is used in some schemes. In most schemes, however, no valves are used. 

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