TAILIEUCHUNG - Electrochemical Capacitors: Ionic Liquid Electrolytes

Double-layer carbon supercapacitors (electrochemical double-layer capacitor (EDLCs)) based on two carbon electrodes of high surface area separated by an electrolyte are the most popular electrochemical supercapacitors. The charge process is electrostatic with charge separation at the two electrode–electrolyte interfaces. The EDLCs can thus be modeled with two capacitances in series with what is called equivalent series resistance (ESR). | Electrochemical Capacitors Ionic Liquid Electrolytes M Mastragostino and F Soavi University of Bologna Bologna Italy 2009 Elsevier . All rights reserved. Introduction Double-layer carbon supercapacitors electrochemical double-layer capacitor EDLCs based on two carbon electrodes of high surface area separated by an electrolyte are the most popular electrochemical supercapacitors. The charge process is electrostatic with charge separation at the two electrode-electrolyte interfaces. The EDLCs can thus be modeled with two capacitances in series with what is called equivalent series resistance ESR . Given that the electrode capacitance depends on the reciprocal of the double-layer thickness which is on the order of 10 10 m and that it is directly related to the carbon surface area typically of several hundred square meters per gram of carbon the capacitance of EDLCs is significantly higher than that of the dielectric and electrolytic capacitors. The stored energy of EDLCs is also higher than that of the dielectric and electrolytic capacitors but it is lower than that of batteries. Indeed the faradic charge processes in batteries involve all the bulk electrode materials and not just the surface. The maximum energy Emax of EDLC is directly related to its capacitance Csc and to the square of the maximum cell voltage Vmax as in the following equation Emax - CscVL 1 o for a discharge between Vmax and 1 2Vmax. Commercial EDLCs featuring the highest specific energy are based on organic electrolytes such as tetra alkylammonium salts tetra alkylammonium tetra fluoroborate Et4NBF4 in acetonitrile ACN or propylene carbonate PC which allow cell voltages of up to V and electrode-specific capacitances 100 F g 1 at room temperature RT . Although higher specific capacitances 150Fg 1 are feasible in aqueous electrolytes the narrow electrochemical stability window ESW of aqueous electrolytes means that EDLCs operate at Vmax and hence Emax is lower. High cell voltage is .

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