Background, fundamental understanding and progress in electrochemical capacitors

Supercapacitors means electrochemical capacitors are being considered these days to be a good alternative for the conventional power sources (fuel cells and batteries) in many applications because of their high power density, long cycle life and less charging and discharging time. This review article presents an overview of different types of supercapacitors (electrical double-layer capacitors (EDLCs), pseudocapacitors and hybrid supercapacitors. The device configurations (symmetric, asymmetric and hybrid), the mechanism of charge storing at the surface (ion adsorption for EDLCs and fast surface redox reactions for pseudocapacitors) and the effect of electrode material (activated carbon, carbon aerogels, carbon fabrics, carbide-derived carbons, carbon nanotubes (CNTs), graphene, biomass, etc. for EDLCs and conducting polymers and insertion type compounds for pseudocapacitors) and electrolytes are crucial. Electrolytes used in the supercapacitors also play important role to determine its operating voltage range, energy density, power density, etc. Both the classes of electrolytes, liquid electrolytes (aqueous, organic, ionic liquids) and solid electrolytes (polymer-based electrolytes) are also discussed in the last section of this review. The voltage range, energy density and power density ultimately define their use for different applications namely heavy electric vehicles and portable electronic devices.