Electrochemical supercapacitors: Energy storage beyond batteries

Recently, a new class of reversible electrochemical energy storage systems have been developed that use: (a) the capacitance associated with charging and discharging of the electrical doublelayer at the electrode–electrolyte interface and are hence called electrical double-layer capacitors (EDLCs), and (b) the pseudocapacitance with electrosorption or surface redox reactions which are referred as pseudocapacitors. While EDLCs with capacities of many tens of farads per gram of the electrode material have been achieved employing high surface-area carbon powders, fibres, or felts, much higher capacitance values are accomplished with pseudocapacitors employing certain high surface-area oxides or conducting polymers. These electrochemical capacitors are being envisaged for several applications to complement the storage batteries. This article provides a brief introduction to scientific fundamentals and technological applications of electrochemical supercapacitors. It is also stressed that there is a substantial scope for technology development in this newly emerging area, where materials science and polymer technology will have a pivotal role in conjunction with electrochemistry.