Energy storage, electrochemical capacitors

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. 

Carbonaceous materials play a key role in achieving the necessary performance parameters of electrochemical capacitors (EC). In fact, various forms of carbon constitute more than 95% of electrode composition [1], Double layer capacity and energy storage capacity of the capacitor is directly proportional to the accessible electrode surface, which is defined as surface that is wetted with electrolyte and participating in the electrochemical process. 

Lithium ion batteries (LIBs) and electrochemical capacitors (ECs) are two important energy storage devices that can complement each other. LIBs work slowly but provide high energy density whereas ECs offer high power density, but suffer from lower energy density [30],... 

Figure 2. Representation of (A, top) an electrochemical capacitor (supercapacitor), illustrating the energy storage in the electric double layers at the electrode—electrolyte interfaces, and (B, bottom) a fuel cell showing the continuous supply of reactants (hydrogen at the anode and oxygen at the cathode) and redox reactions in the cell.

Figure 3. Simplified Ragone plot of the energy storage domains for the various electrochemical energy conversion systems compared to an internal combustion engine and turbines and conventional capacitors.

Lewandowski, A. and Galmski, M., General Properties Of Ionic Liquids As Electrolytes For Carbon-based Double Layer Capacitors, New Carbon Based Materials for Electrochemical Energy Storage Systems, Barsukov et al. (Eds), Springer, The Netherlands, 2006, 73-83. 

Types of capacitors and mode of energy storage after Ref.206 Reprinted from B.E. Conway, Electrochemical supercapacitors. Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York (1999). Copyright 1999 with permission from Kluwer Academic Pubhshers. 

The topic of this book is focused on active masses containing carbon, either as an active mass (e.g., negative mass of lithium-ion battery or electrical double layer capacitors), as an electronically conducting additive, or as an electronically conductive support for catalysts. In some cases, carbon can also be used as a current collector (e.g., Leclanche cell). This chapter presents the basic electrochemical characterization methods, as applicable to carbon-based active materials used in energy storage and laboratory scale devices. 

D. J. Derwin, K. Kinoshita, T. D. Tran, and P. Zaleski, Symposium on Materials for Electrochemical Energy Storage and Conversion II-Batteries, Capacitors and Fuel Cells, at the 1997MRS Fall Meeting, Vol. 496, Materials Research Society, Boston, MA, 1998, pp. 575-580. 

In general, two modes of energy storage are combined in electrochemical capacitors (1) the electrostatic attraction between the surface charges and the ions of opposite charge (EDL) and (2) a pseudocapacitive contribution which is related with quick faradic charge transfer reactions between the electrolyte and the electrode [7,8], Whereas the redox process occurs at almost constant potential in an accumulator, the electrode potential varies proportionally to the charge utilized dr/ in a pseudocapacitor, which can be summarized by Equation 8.5 ... 

Zheng JP, Jow TR. The effect of salt concentration in electrolytes on the maximum energy storage for double layer capacitors. Journal of the Electrochemical Society 1997 144(7) 2417-2420. 

Goltser I, Butler S, Miller JR. Reliability assessment of electrochemical capacitors Method demonstration using 1-F commercial components, Proceedings of the 15th International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, FL, 2005, p. 215. 

The capacity per unit area that arises at a purely polarizable electrochemical interface (using the equation C = e/4nS), is about 20 fiF cm-1. The energy storage of such an electrochemical capacitor is proportional to this value. However, one may be able to involve the phenomenon of pseudo-capacitance to increase the capacity above that for a polarizable interface. Then one could envisage an electrochemical ultracapacitor with capacities of150-200/iF cm-2, an increase of up to 10 times that available from the capacity of polarizable interfaces. 

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