Redox capacitors

In the second type of supercapacitor, sometimes termed pseudocapacitors, redox capacitors or electrochemical capacitors, the non-Faradaic doublelayer charging process is accompanied by charge transfer. This Faradaic process must be characterized by extremely fast kinetics in order to allow device operation with high current density discharge pulses. 

In this section, we review recent developments of redox capacitors with conducting polymer electrodes and EDLC with SPE or gel electrolyte systems. 

Many electrochemically dopant-induced structure-property changes in conducting polymers have been described for use in, for example, electrochromic windows and displays, electrochemically controlled chemical separation and delivery systems, redox capacitors, electromechanical actuators, etc.,32. Some of these are apparently close to commercial application. Polypyrrole capacitors are already in conunercial production,27. 

The following asymmetrical PsC capacitor was studied as a typical example of a redox capacitor. 1,2-dihydroxybenzene (DHB also referred to as o-benzoquinone and catechol) obtained by modification of a carbon electrode was used in it as a positive electrode. The negative electrode used was an anthraquinone (AQ)-modified carbon electrode (Algharaibeh and Pickup, 2011). 

Komaba, S., T. Tsuchikawa, M. Tomita, N. Yabnnchi, and A. Ogata. 2013. Efficient electrolyte additives of phosphate, carbonate, and horate to improve redox capacitor performance of manganese oxide electrodes. Journal of the Electrochemical Society 160 A1952-A1961. 

The terms supercapacitor and ultracapacitor are used to describe any double layer or redox capacitor with specific energy and specific power intermediate to batteries and traditional capacitors. Typically, ultracapacitor refers to a device comprised of two carbonaceous electrodes whereas supercapacitor refers to a similar device in which the two carbonaceous electrodes are catalyzed with metal oxides such as Ru02. This chapter will use the term supercapacitor to describe EAP-based capacitors, since that seems to be the most commonly used term for such materials. Another charge storage configuration uses an EAP electrode and a battery-type carbonaceous electrode in what is known as a hybrid device (however, outside of the EAP-based supercapacitor field, hybrid may refer to the combination of a battery electrode such as nickel hydroxide with a carbon electrode) [1]. 

Clemente, A., et al. 1996. Solid-state, polymer-based, redox capacitors. Solid State Ionics 85 273. 

Panero, S., E. Spila, and B. Scrosati. 1995. On the use of ionically conducting membranes for the fabrication of laminated polymer-based redox capacitors. / Electroanal Chem 396 385. 

As charge is inserted or removed at low rates, the polymer film behaves as a (redox) capacitor. The redox capacity of electroactive films is a quantity of fundamental importance, and it may be determined by several complementary techniques including cyclic voltammetry, electrochemical impedance and coulometric titration. Cyclic voltammetry of a redox polymer film at a slow... 

Clemente, A., et al. 1996. Solid-state, polymer-based, redox capacitors. Solid State Ionics 85 273. Borjas, R., and D.A. Buttry. 1991. EQCM studies of film growth, redox cycling, and charge trapping of n-doped and p-doped poly(thiophene). Chem Mater 3 872. 

Morita M, Ohsimii N, Yoshimoto N, Egashira M (2007) Proton-conducting non-aqueous gel electrolyte for a redox capacitor system. Electrochemistry 75 641-644... 

The number of studies which utilize ionic liquid electrol54e in redox capacitor system is still small, probably due to the difficulty to reproduce the pseudo-capacitive reaction in ionic liquid media. While the principle of pseudo-capacitance of conductive polymer electrodes permits to utilize ionic liquid electrolytes, high viscosity and rather inactive ions of ionic liquid may make their pseudo-capacitive reaction slow. The combination of nanostmctured conductive polymer electrode and ionic liquid electrolyte is expected to be effective [27]. It is far difficult that ionic liquids are utilized in transition metal-based redox capacitors where proton frequently participates in the reaction mechanisms. Some anions such as thiocyanate have been reported to provide pseudo-capacitance of manganese oxide [28]. The pseudo-capacitance of hydrous ruthenium oxide is based on the adsorption of proton on the electrode surface and thus requires proton in electrolyte. Therefore ionic liquids having proton have been attempted to be utilized with ruthenium oxide electrode [29]. Recent report that 1,3-substituted imidazolium cations such as EMI promote pseudo-capacitive reaction of mthenium oxide is interesting on the viewpoint of the establishment of the pseudo-capacitive system based on chemical nature of ionic liquids [30]. 

Since 1995, amorphous hydrated ruthenium dioxide, Ru02 H20, has been extensively studied as the electrode materials for the redox capacitor [1-3]. Ru02 H20 composite electrode... 

The energy capacity of ECs arises from either double-layer capacitance for electric doublelayer capacitors (EDLCs) or pseudocapacitance for redox capacitors [2, 3]. The energy storage mechanism of EDLCs is based on non-faradic phenomena in electric double layer formed at an electrode/electrolyte interface. In regard to electrode active materials for EDLCs, carbon materials such as activated carbons have been most widely used [4] because of their reasonable cost, good electrical conductivity, and high specific surface area. However, there is a limitation in their specific capacitance the gravimetric capacitance of most carbon materials does not linearly increase with an increase in the specific surface area above 1,200 m g [5]. 

Redox Capacitor, Scheme 1 Organic materials for ECs include polypyrrole (a), polyaniline (b), polythiophene, its... 

Redox Capacitor, Table 1 Summary of the various classes of transition metal oxides... 

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