Carbon Nanotubes for Storage of Energy: Super Capacitors
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Carbon nanotubes (CNTs), because of their exceptional physicochemical properties, have been used for energy storage devices such as super capacitors. Especially, conducting and microtextural properties of CNTs are noteworthy for this electrochemical application. The presence of more or less continuous graphitic layers, cylindrically rolled, together with a tight entanglement of nanotubes supply a conducting and well accessible carbon network with a moderate surface area (ca. 100–400 m2/g). Because the performance of electrochemical capacitors is connected with the accumulation of charges in the electrode/electrolyte interface, such a material in the form of an easily available web is of great interest. However, for this application, apart from mesopores, which are perfect for ion transportation, accessible micropores are required for the accumulation of ions in the electrical double layer. Generally, for this target a highly developed specific surface area is demanded. Carbon nanotubes can be used as electrode material or as a perfect additive or support in composite electrodes. For their application as an electrode material, the values of capacitance will strictly depend on the microtexture and catalyst impurities. The more defected the outer walls of CNTs the higher the values of capacitance. The presence of a thin amorphous carbon layer on the CNT walls also supply better conditions for charge accumulation. Additionally, catalytic impurities of nanotubes can be a source of redox reactions giving pseudocapacitance effects.
Generally, from the literature data, a high discrepancy of capacitance (from 5 to 115 F/g) is found because the values strongly depend on CNTs' preparation method and purity and on the conditions for capacitance measurements. Some research has been devoted to elucidate the effect of microtexture on capacitance properties to improve capacitance values. Enhancement of CNTs' capacitance can be achieved by an increase of their surface area or introduction of pseudocapacitance effects through redox species or conducting polymers. The specific surface area of nanotubes has been increased by KOH activation at 800°C. A few fold increase of capacitance has been obtained due to the development of microporosity while preserving the mesoporous character. This interesting phenomenon can be explained by the insertion or intercalation of potassium in the walls of nanotubes at 800°C and the subsequent formation of micropores. Consequently, the opening of nanotube tips was also observed that is profitable for charge accumulation. Higher values of capacitance can also be obtained if CNTs are used as a conducting component and/or support in the composite electrode. In this case, conducting polymers or redox species are mainly responsible for the capacitance values (giving the so-called pseudocapacitance). Generally, a synergy effect is obtained between the components of the super capacitor electrode. A nanocomposite gives better results than its components alone.
Carbon nanotubes can also be used as a frame for carbon obtained from different polymeric precursors. In this case, the nanotube skeleton is very profitable to prevent shrinkage during the carbonization process and, consequently, an interesting CNT/carbon composite is obtained for super capacitor application.