A survey of supercapacitors, their applications, power design with supercapacitors, and future directions

Over the last 10 years supercapacitors have grown from infancy with sales of $US40M in 2002 to a strongly emerging technology with sales of US$450M last year. This represents an annual growth rate of 35%. Forecasts for the next 5 years show growth continuing at 25% 30% with sales of $1.5B in 2015. Supercapacitors are electrical double layer capacitors with no dielectric. Charge transport is by ions dissolved in an electrolyte. They are "super" because of the very high surface area of their porous carbon electrodes (2000 3000m2/gm), and the very small separation distance between the +ve and -ve charge, measured in Angstroms. The electrodes have a porous separator between them. The ions in the electrolyte are against the surface of the electrodes. Capacitance is proportional to the surface area and volume of the carbon electrodes. ESR depends on the active area, the conductivity of the electrolyte and carbon, the porosity, thickness and tortuosity of the separator. Since there is no dielectric, the rated voltage depends on the breakdown voltage of the electrolyte. There are two types of electrolyte: organic and aqueous. Aqueous electrolytes are stable to 0.9V, organic electrolytes to 2.7V Supercapacitor cells must be stacked in series to achieve a useful working voltage.