Type I supercapacitor

In a Type I supercapadtor, J x(anode) = ).ed(cathode), meaning that Type I supercapacitor voltages are limited by the overoxidation of the polymer, which usually occurs around 0.5-0.75 V [4j. Type I supercapacitors have a charged state in which one polymer layer is fully oxidized and the other layer is completely neutral. In the discharged state, both layers are 50% oxidized—Whence at most only 50% of the total polymer s p-doping capacity is used [7,143,144]. 

Figure 9.5 represents the charging cycle of a Type I supercapacitor as the anode oxidizes (going from half-oxidized to fully oxidized), the cathode neutralizes (going from half-oxidized to neutral). 

In Type III and Type IV supercapadtors, ox(anode)- red(cathode) typically ranges from 0.8 to 3 V [ 147]. The overvoltage is approximately the same as that of a Type I supercapacitor (perhaps less due to the sensitivity of n-doped polymers), meaning that Type in and Type IV supercapacitors typically have... 

Several other polymers have also been evaluated for use in Type I supercapacitors. While polythiophene exhibits an excellent specific capacitance for energy storage (250 F/g) [160], polythiophene supercapacitors are uncommon, possibly due to cycle life issues. However, derivatives of polythiophene have been used in the preparation of Type II, Type III, and Type IV supercapacitors (see Section 9.3.6.2 and Section 9.3.6.3). 

One class of polythiophene derivative has been studied for use in Type I supercapacitors research into poly(3,4-ethylenedioxythiophene) (PEDOT, Figure 9.4J)-based supercapacitors has been driven by PEDOT s superior chemical and electrochemical stability [148] as well as its fast switching times [161]. Carlberg and Inganas [148] demonstrated energy density of 1 Wh/kg at power densities of... 

Figure 9 shows the discharge curves of a Type I polypyrrole-based, a Type II polypyrrole/poly(3-methylthiophene)-based and a Type III poly(dithieno[3,4-6 3, 4 -d]thiophene-based supercapacitor at 4 mA cm discharge current. Types I and II can be assembled using such conventional heterocyclic polymers as polypyrrole, polyaniline and polythiophene, which are efficiently p-dopable polymers and can easily be chemically or electrochemically synthesized from inexpensive... 

Figure 9. Discharge curves at 4 mA cm of the three types of supercapacitors a) polypyrrole/LiC104 -propylene carbonate (PC)/polypyrrole b) polypyrrole/ LiC104-PC/poly(3-methylthiophene) c) poly(dithieno[3,4-6 3, 4 -rf ]thiophene)/ (C2Hs)4NBF4-PC/poly(dithieno[3,4-b . i, A -d]thiophene), potentiostatically charged at 1.1 V, 1.15 V, and 3.0 V, respectively.

Performance Implications of Supercapacitor Device Types 9.3.4.1 Type I... 

FIGURE 9.9 Comparison of voltage decay characteristics for Type I, Type n, and Type III and IV supercapacitors. 

The bulk of EAP-based supercapacitor work to date has focused on Type I devices. Polypyrrole (PPy, Figure 9.4C) has been studied [147,151-153] for this application, with specific capacitance values ranging from 40 to 200 F/g. Garcia-Belmonte and Bisquert [151] electrochemically deposited PPy devices that exhibit specific capacitances of 100-200 F/cm with no apparent dependence on film thickness or porosity extensive modeling of impedance characteristics was used. Hashmi et aL [153] prepared PPy-based devices using proton and lithium-ion conducting polymer electrolytes. As is often observed, electrochemical performance suffered somewhat in polymeric electrolytes single electrode specific capacitances of 40-84 F/g were observed with stability of 1000 cycles over a 1 V window. 

Relatively little work has been published on Type II supercapacitors. Arbizzani et al. [147] have prepared PPy/poly(3-methylthiophene) devices performance was similar to their PPy-based Type I device and to carbon supercapacitors. Clemente et al. [163] prepared PPy/PANI devices with specific capacitance values as high as 25 F/g, depending on electrolyte composition. Stenger-Smith et al. [37] prepared poly(3,4-ethylenedioxythiophene) (PEDOT, Figure 9.4J)/poly(3,4-propylenedioxythiophene) (PProDOT, Figure 9.4L) Type II supercapacitors. Switching speed and cycle life were found to depend heavily on electrolyte composition, with only 2% loss in capacity over 50,000 full cycles when 1-ethyl-3- methyl-IH-imidazolium bis(trifluoromethanesulfonyl)imide (EMI-BTI) was used as the electrolyte. 

In this chapter we examine the effect of column chromatography purification of ethyl methyl imidazolium bis(trifluoromethanesulfonylimide), (EMIBTI) (synthesized by our laboratory on the 100 gram scale) on the performance of a Type I electroactive polymer supercapacitor based upon poly(propylene dioxythiophene) (PProDOT). 

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