Supercapacitors
Supercapacitors are energy storage systems in which the energy is stored by pure electrosorption of ions on the surfaces of charged electrodes thus forming the double-layer (Figure 1). These devices occupy the area in the Ragone plot (the plot of power against energy density) between batteries and dielectric capacitors. Supercapacitors are designed for high power and long cycle life, but at the expense of energy density. Hence the research on supercapacitors is focused on improving their energy densities, i.e. the times of energy delivery. This can be achieved by using the electrolytes with wide potential window and/or by optimisation of the electrode structure.
Electrodes are the most important components of supercapacitors as the energy storage happens on their surfaces. Porous carbon has been the most studied electrode material for several good reasons: high surface area, good polarisability, chemical resistance, abundance, and low toxicity.
With regards to the commercial applications of supercapacitors, examples include memory back-up in toys, cameras, video recorders and mobile phones. More recently, supercapacitors have been employed in emergency doors on an Airbus A380 and as advanced energy storage for the Palmdale Water District’s Southern California water treatment plant. With a projected market of over $180 million in 2009, supercapacitors are early in commercialisation, though the technology is firmly in place.
