Double-layer capacitors electrode

Care is not always sufficiently brought to this electrode. An insoluble redox system, much more capacitive than the working electrode is often a good choice, or possibly a large double layer capacitor electrode, which does not pollute the electrolyte. 

Segalini, J., B. Daffos, P. L. Tabema, Y. Gogotsi, and P. Simon. 2010. Qualitative electrochemical impedance spectroscopy study of ion transport into sub-nanometer carbon pores in electrochemical double layer capacitor electrodes. Electrochimica Acta 55 7489-7494. 

Ohta, T., I. T. Kim, M. Egashira, N. Yoshimoto, and M. Morita. 2012. Effects of electrolyte composition on the electrochemical activation of alkali-treated soft carbon as an electric double-layer capacitor electrode. Journal of Power Sources 198 408-415. 

Shah, R., X. F. Zhang, and S. Talapatra. 2009. Electrochemical double layer capacitor electrodes using aligned carbon nanotubes grown directly on metals. Nanotechnology 20 395202. 

Dsoke, S., X. Tian, C. Taubert, S. Schluter, and M. Wohlfahrt-Mehrens. 2013. Strategies to reduce the resistance sources on electrochemical double layer capacitor electrodes. Journal of Power Sources 238 422-429. 

FIGURE 4.34 Proposed electrochemical processes occurring on the MojC-derived carbon electrode at high positive polarizations in [EMIMJCBFJ electrolyte (a) oxidative dimerization of EMIM+ cations via N-N bond formation (b) possible complexation of in situ elec-trochemically formed BF3 with a free electron pair of a nitrogen atom in the EMIM cation and/or the substitution of an alkyl chain to the EMIM+ cation. (From Kruusma, J. et al. 2014. In situ XPS studies of electrochemically positively polarized molybdenum carbide derived carbon double layer capacitor electrode. Journal of the Electrochemical Society, 161 A1266-A1277. Reproduced by permission of The Electrochemical Society.)... 

Tonisoo, A., J. Kruusma, R. Parna et al. 2013. In situ XPS studies of electrochemicaUy negatively polarized molybdenum carbide derived carbon double layer capacitor electrode. Journal of the Electrochemical Society 160 A1084-A1093. 

Tanahashi, 1., Yoshida, A. and Nishino, A., Electrochemical characterization of activated carbon fiber cloth polarizable electrodes for electric double layer capacitors. J. Electrochem. Soc., 1990, 137(10), 3052 3056. 

Ishikawa, M., Morita, M., lhara, M. and Matsuda, Y., Electric double layer capacitor composed of activated carbon fiber cloth electrodes and solid polymer electrolytes containing alkylammonium salt, J. Electrochem. Soc., 1994, 141(7), 1730 1734. 

Ishikawa, M., Sakamoto, A., Morita, M., Matsuda, Y. and Ishida, K., Effect of treatment of activated carbon fiber cloth electrodes with cold plasma upon performance of electric double layer capacitors, J. Power Sources, 1996, 60(2), 233 238. 

Work in this area has been conducted in many laboratories since the early 1980s. The electrodes to be used in such a double-layer capacitor should be ideally polarizable (i.e., all charges supplied should be expended), exclusively for the change of charge density in the double layer [not for any electrochemical (faradaic) reactions]. Ideal polarizability can be found in certain metal electrodes in contact with elelctrolyte solutions free of substances that could become involved in electrochemical reactions, and extends over a certain interval of electrode potentials. Beyond these limits ideal polarizability is lost, owing to the onset of reactions involving the solvent or other solution components. 

In recent years, many types of double-layer capacitors have been built with porous or extremely rough carbon electrodes. Activated carbon or materials produced by carbonization and partial activation of textile cloth can be used for these purposes. At carbon materials, the specific capacity is on the order of 10 J,F/cm of trae surface area in the region of ideal polarizability. Activated carbons have specific surface areas attaining thousands of mVg. The double-layer capacity can thus attain several tens of farads per gram of electrode material at the surfaces of such carbons. 

The operation of a double-layer capacitor is tied to a displacement of electrolyte ions. In a fully charged capacitor, anions accumulate as counterions in the solution layer next to the positively charged electrode while the eoncentration of the cations decreases. At the negative electrode, the opposite situation is seen. During discharge, the ionic concentrations level out to the bulk solution values by migration and diffusion. 

The total capacity of a ruthenium oxide electrode [the usual double-layer capacity plus the pseudocapacity of reaction (21.4)] is rather high (i.e., several hundred F/g), even more than at the electrodes of carbon double-layer capacitors. The maximum working voltage of ruthenium oxide pseudocapacitors is about 1.4 V. 

High porosity carbons ranging from typically microporous solids of narrow pore size distribution to materials with over 30% of mesopore contribution were produced by the treatment of various polymeric-type (coal) and carbonaceous (mesophase, semi-cokes, commercial active carbon) precursors with an excess of KOH. The effects related to parent material nature, KOH/precursor ratio and reaction temperature and time on the porosity characteristics and surface chemistry is described. The results are discussed in terms of suitability of produced carbons as an electrode material in electric double-layer capacitors. 

Porous carbons are among the most attractive electrode materials for electric double layer capacitors (EDLC), where the charge accumulation occurs mainly by electrostatic attraction forces at the clcctrode/electrolyte interface [1-3]. Advantages of this class of materials include high surface... 

Fig. 7. A schematic diagram of an electric double-layer capacitor using active carbon electrodes.

Recently supercapacitors are attracting much attention as new power sources complementary to secondary batteries. The term supercapacitors is used for both electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs are based on the double-layer capacitance at carbon electrodes of high specific areas, while the pseudocapacitors are based on the pseudocapacitance of the films of redox oxides (Ru02, Ir02, etc.) or redox polymers (polypyrrole, polythiophene, etc.). 

The arrows above and the symbols below the interfaces indicate the transfer of the charge at each interface when the concentration of NaF in the sample is abruptly increased. It is possible to estimate the actual number of ions that are required to establish the potential difference at the interfaces. A typical value for the doublelayer capacitor is 10 5 F cm 2. If a potential difference of n = 100 mV is established at this interface, the double-layer capacitor must be charged by the charge Q = nCdi = 10 6 coulombs. From Faraday s law (6.3), we see that it corresponds to approximately 10 11 mol cm 2 or 1012 ions cm 2 of the electrode surface area. Thus, a finite amount of the potential determining ions is removed from the sample but this charge is replenished through the liquid junction, in order to maintain electroneutrality. 

As we have seen already, there are two kinds of selectivity thermodynamic selectivity and kinetic selectivity (Chapter 2). Let us first consider how we could use thermodynamic selectivity for amperometric sensors. The electrode and the solution form a double-layer capacitor. The minimum energy of this capacitor occurs at... 

Figure 17. Plots of the reduced current density against the applied potential obtained from the as-activated carbon (dotted line) and as-reactivated carbon (dashed line) electrode specimens. Solid line represents the ideal double layer capacitor where the time constant is zero. Reprinted from C.-H. Kim, S.-I. Pyun, and H.-C. Shin, J. Electrochem. Soc. 149 (2002) A93. Copyright 2001, with permission from The Electrochemical Society.

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