Carbon properties and their role in supercapacitors

Supercapacitors (also known as ‘ultracapacitors’) offer a promising alternative approach to meeting the increasing power demands of energy-storage systems in general, and of portable (digital) electronic devices in particular. Supercapacitors are able to store and deliver energy at relatively high rates (beyond those accessible with batteries) because the mechanism of energy storage is simple charge-separation (as in conventional capacitors). The vast increases in capacitance achieved by supercapacitors are due to the combination of: (i) an extremely small distance that separates the opposite charges, as defined by the electric double-layer; (ii) highly porous electrodes that embody very high surface-area. A variety of porous forms of carbon are currently preferred as the electrode materials because they have exceptionally high surface areas, relatively high electronic conductivity, and acceptable cost. The power and energy-storage capabilities of these devices are closely linked to the physical and chemical characteristics of the carbon electrodes. For example, increases in specific surface-area, obtained through activation of the carbon, generally lead to increased capacitance. Since only the electrolyte-wetted surface-area contributes to capacitance, the carbon processing is required to generate predominantly ‘open’ pores that are connected to the bulk pore network. While the supercapacitors available today perform well, it is generally agreed that there is considerable scope for improvement (e.g., improved performance at higher frequencies). Thus it is likely that carbon will continue to play a principal role in supercapacitor technology, mainly through further optimization of porosity, surface treatments to promote wettability, and reduced inter-particle contact resistance.