Carbon Nanotube Based Electrochemical Supercapacitors
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Several approaches have been used to develop carbon nanotube (CNT) based electrochemical supercapacitors. These approaches include the following: (a) stabilization and carbonization of ternary composites of polyacrylonitrile (PAN), poly (styrene co-acrylonitrile) (SAN) copolymer, and single wall carbon nanotubes (SWNTs); (b) SWNT membranes functionalized with aryl chloride, sodium sulfonate, aryl sulfonic acid, bis(3,5-di-tert-butylphenyl)5-aminobenzene-1,3-dioate, and 4,4 -methylenedianiline; and (c) pyrrole treated SWNTs. In addition nitric acid functionalized and heat-treated SWNT membranes have been studied. The electrochemical supercapacitor behavior of these membrane electrodes has been characterized by cyclic voltammetry, constant current charging-discharging, and impedance analysis in aqueous and ionic liquid electrolytes. Long term performance of selected electrodes has been evaluated. The surface area and pore size distribution was quantified by N2 gas adsorption/desorption and correlated with capacitance performance. The surface functional groups have been characterized by X-ray photoelectron spectroscopy. CNT electrode/electrolyte interaction has been characterized using contact angle measurements. Electrolyte absorption by the electrodes has also been characterized. Carbonized PAN/SAN/SWNT ternary composites exhibit double layer capacity of over 200 μF/cm2. By comparison, the double layer capacity of classical meso-porous carbons is in the range of 10-50 μF/cm2. The capacitance of functionalized SWNTs is up to 2 times that of the control bucky paper made from unfunctionalized SWNTs. Energy density of functionalized electrodes when evaluated in an ionic liquid is as high as 28 kJ/kg. High capacitance (up to 350 F/g) was obtained for pyrrole-treated functionalized SWNT membranes in 6 M KOH. This value is almost seven times that of the control bucky paper. Correlating the capacitance with surface area and pore size distribution, it was observed that macropores (pore width greater than 50 nm) play an important role for achieving high capacitance.