Freestanding carbonaceous cathodes for flexible supercapacitors - A comprehensive prospect on materials' composition & structure, and device configuration

Among energy storage devices, flexible supercapacitors are the research hotspot and are used to power up electronic devices that are wearable and portable owing to their excellent specific capacitance, extraordinary power density, fast rate of charging-to-discharging, and excellent flexibility. The development of supercapacitors having high energy and high-power density is conditioned with the replacement of the conventional capacitive cathode by the battery-type electrode. Concerning this, various metal-based compounds (such as metal oxides, nitrides, sulfides, selenides, carbides, phosphides, and double hydroxides) are being used as active materials for the fabrication of efficient symmetric and asymmetric supercapacitor. In the construction of supercapacitors with high power and energy density, carbon is not only a promising primary active material, but it also has remarkable application as secondary material for the construction of flexible composite cathodes. Either as a conductive additive or as a structural scaffold, carbon is the top ingredient in the construction of flexible electrodes. In flexible supercapacitors, a prominent role is played by the freestanding electrodes. The flexible carbonaceous substrates are extensively used in this regard due to their high electronic conductivity, adjustable porosity and surface area, excellent mechanical properties, low density, and easy functionalization. It is also abundant and economical. This review thoroughly summarized the metal-based active material used to fabricate the cathode in flexible supercapacitors, various carbonaceous substrates in fabricating flexible electrodes, the modifying strategies to boost the electrochemical activity of active material, and also the substrate, and work done in designing and fabricating supercapacitors that are flexible with high power and energy density.