Flexible supercapacitors

As mentioned before, the supercapacitor is a high power density energy storage device with a particularly high reversibility and long life cycle [1-4]. With a great potential in the consumer electronics and high power applications, such as in communications devices, hybrid electrical vehicles (start-stop application), aviation and power industries, the development of supercapacitor devices has gained significant interest in recent years, due to their capability to produce large power and energy density, as compared with the conventional capacitors [5, 6]. Based on the type of electrode materials and working principles, supercapacitors can be categorized into three main classes, including Electrochemical Double-Layer Capacitors (EDLCs), pseudocapacitors and hybrid capacitors, as demonstrated in Fig. 6.1. In the EDLCs, the energy is stored via reversible adsorption and desorption of ions at the electrode/electrolyte interfaces, while there is no net charge transfer across the interfaces. Due to the difference in electrochemical potential between the electrodes, the open circuit potential of the cell is varied, which responds to the passage of the charges in the external circuit. The energy stored in this process is purely electrical, in which there is no contribution from the electrode chemical reactions or faradaic reactions. Carbon based supercapacitors are typical EDLCs [2, 7-12]. In comparison, pseudocapacitors are based on Faradaic reactions, i.e., oxidation-reduction reactions [2, 13, 14], with only a slight contribution of EDLCs by < 5%. Almost all the transition metal oxides, nitrides, carbides and conducting polymers can be used as electrode materials of pseudocapacitors. The energy storage mechanism of hybrid capacitors is the combination of the Faradaic and non-Faradaic reaction, such as the supercapacitors based on porous carbons and metal oxides.