Charge-Storage Mechanism

The main classes of organic materials are as follows polyacetylene, polypyrrole, polyaniline, polyphenylene, and polythiophene. These materials share the characteristic that, on Ionic doping, electronic carriers are introduced into a delocalized ir system, rendering them very good conductors (98). As in the case of the oxides, this is a double-injection process, with counterions entering the poiymer structure. Several of the polymers can be produced by in situ electrochemical processes. Their microstructure is typicaiiy very dependent on preparation conditions. 

Some of the poiymers, e.g., poiyacetylene, can be either p- or n-doped. For exampie, the empiricai equations describing potential vs. a lithium electrode, as a function of doping for this material are (99)  

Polyaniline can be prepared electrochemically, and its microstructure depends on the preparative conditions (102). The charge capacity is comparable to the inorganic oxides Gottesfeld et al. (103) report a capacity of 800 C/cm in an aqueous acidic electrolyte and Genies et al. (102) a value of 450 C/g in propylene carbonate/LiCIO,j. 

The use of aqueous electrolytes is certainly an advantage for high-rate applications, and polyaniline can be cycled in water over a limited potential range. Poiyacetylene, however, is very unstable, in both n- and p-doped forms, to traces of water and oxygen. At more positive potentials, polyaniline is unstable to oxidation and undergoes fragmentation and dissolution (104). Similarly, polypyrrole is fatally oxidized above 1 volt vs. SCE in aqueous solutions (105). 

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Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

The knowledge that conducting polymers can be charged, i.e. oxidized and reduced, raised early on the question of possible applications, such as the construction of a polymer battery. But basic research was long unable to explain the charge storage mechanism. 

Electrochemical measurements on polyaniline (PANI) produce a picture of the charge storage mechanism of conducting polymers which differs fundamentally from that obtained using PTh or PPy. In the cyclic voltammetric experiment one observes at least two reversible waves in the potential range between —0.2 and -)-1.23 V vs SCE. Above -1-1.0 V the charging current tends to zero. Capacitive currents and overoxidation effects, as with PPy and PTh, do not occur The striking... 

Closely coimected with the problems of the charge storage mechanism is the... 

Depending on the charge storage mechanism, one must distinguish between the electrical doublelayer capacitors (EDLC) and the pseudocapacitors. The principles and properties of both types of supercapacitors will be further described. 

Due to the confinement of ions in a situation very close to intercalation/insertion, the term double layer seems to be no longer relevant for explaining the charge storage mechanism in electrochemical capacitors based on nanoporous carbons. 

Supercapacitors are categorized into two classes based on their charge storage mechanisms.67,68 Electric charge may be stored in a capacitor through charge separation at the electrode-electrolyte... 

Zheng, J.P., Jow, T.R. 1995. A new charge storage mechanism for electrochemical capacitors. Journal of the Electrochemical Society 142, L6-L8. 

FIGURE 2.18 Dependence of capacitance on the salt concentration. (Reprinted from Journal of Power Sources, 196, Xu, C. J. et al., Charge storage mechanism of manganese dioxide for capacitor application Effect of the mild electrolytes containing alkaline and alkaline-earth metal cations, 7854-7859, Copyright 2011, with permission from Elsevier.)... 

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