Double-layer capacitors electrolytic conductivity

In the case of ion conductive polymers, gel polymer electrolytes which consist of a polymer matrix, organic solvents and supporting electrolyte, were introduced as novel nonaqueous electrolyte systems in electrochemical applications, such as rechargeable batteries and electric double layer capacitors [3-5], Recently, considerable attention has been devoted to the application of gel poly-... 

To apply ionic liquids as an electrolyte for double-layer capacitors, they should have wide operational temperature range and high safety. Although ionic liquids have favorable properties such as nonvolatility and nonflammability from the viewpoint of thermal stability and safety, they have the disadvantage that because of their higher viscosity, they rapidly lose their electrolytic conductivity as the temperature decreases. 

Figure 17.7 shows the change of the capacitances of the double-layer capacitors using various electrolytes, when operating temperature was varied from 25 to —25°C. The capacitance of all ionic liquids decreased rapidly compared with the 1 M Et3MeNBFVPC due to the increase of internal resistance reflecting the temperature dependence of electrolytic conductivity in Figure 17.4. We have learned that this behavior is a fatal disadvantage of ionic liquids, and that EMIF -2.3HF is the one exception that affords enough capacitance even at —25°C, as reflects its high electrolytic conductivity.

Figure 17.10 Temperature dependences of electrolytic conductivity of a new ionic liquid (a) and capacitance of a double-layer capacitor using the ionic liquid (b).

Chiba, K., T. Ueda, and H. Yamamoto. 2007. Highly conductive electrolytic solution for electric double-layer capacitor using dimethylcarbonate and spiro-type quaternary ammonium salt. Electrochemistry 75 668—671. 

FIG U RE 4.18 (a) C V and conventional determination of stability limits of a popular nonaque-ous electrolyte for a double-layer capacitor 1.0 M EtjMeNPFg in EC/DMC (1 1 by weight) on nonporous WE (GC). (b) CV of the same electrolyte when porous composite based on M30 AC (95% with 5% PVdF) is used as WE. Scan rate 5 mV s Li as reference and Pt as CEs. Successive scans were conducted with an interval of 0.25 V between their cutoff potential limits (only sixth and tenth scans are shown). (Reprinted from Electrochimica Acta, 46, Xu, K., M. S. Ding, and T. R. Jow, A better quantification of electrochemical stability limits for electrolytes in double layer capacitors, 1823-1827, Copyright 2001, with permission from Elsevier.)... 

There has been growing interest in the field of supercapacitors due to their possible applications in medical devices, electrical vehicles, memory protection of computer electronics, and cellular communication devices. Their specific energies are generally greater than those of electrolytic capacitors and their specific power levels are higher than those of batteries. Supercapacitors can be divided into redox supercapacitors and electrical double layer capacitors (EDLCs). The former uses electroactive materials such as insertion-type compounds or conducting polymers as the electrode, while the latter uses carbon or other similar materials as the blocking electrode. 

Electrolytes for Electrochemical Double Layer Capacitors, Table 1 Specific conductivities electrolytes of capacitor... 

On the other hand, AC is used in order to avoid changes of electrolyte resistance due to changes in concentration of the electrolyte and buildup of electrolysis products at the electrode surfaces [5], Subsequently, R, is measured at potential frequency (j) by adjusting the resistance so that the current and potential are in-phase, which require a variable external conductance (C ). Also included in the Wheatstone bridge is the double-layer capacitor (C). Let s use two different balance conditions. 

As salts the most natural application of ILs is of course as electrolytes. In fact ILs are known for their high conductivity (lO" to lO S cm-i), high electrochemical stability (4-5.7 V) and thermostability (up to 300 °C). This set of properties together with the fact that most of ILs are nonvolatile and nonflammable, has driven their application as electrolytes for different electrochemical devices, such as dye synthesized solar cells, double layer capacitors, fuel cells, electrochemical windows and of course lithium secondary batteries. (Byrne et al., 2005 Fernicola et al., 2006 Galinski et al., 2006 Lu et al., 2002 Stephan, 2006)... 

In a double-layer capacitor, no faradic reaction takes place between the electrode and electrolyte. In a pseudocapadtor, most of the charge is transferred at the interface or by the material near the surface of the electrode, which does involve a faradic reaction. The most interesting case is the conducting polymer, in which charging/discharging is possible in the material itself, raising the possibility of increasing the capacitance to a new level. 

In the drive towards expanding the range of useful ionic liquids (ILs), with suitably low viscosities, sufficient stability, and conductivity for electrochemical uses, phosphonium-based ionic liquids (PILs) are exceptionally promising compounds. Due to the high thermal and electrochemical stabilities of PILs compared to ammonium-based ILs (allq l-ammonium, imidazolium, pyridinium, pyrrolidinium, and piperidinium compounds), phosphonium salts have been especially considered for applications as electrolytes. Their properties (both thermal and electrochemical stabilities) are in fact crucial to improve safety, durability, power and energy densities of electrochemical devices such as electric double layer capacitors. 

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