Review on supercapacitors: Technologies and performance evaluation

The development of electrochemical capacitors (i.e. supercapacitors) have attracted a lot of attention in recent years because of the increasing demand for efficient, high-power energy storage. Electrochemical capacitors (ECs) are particularly attractive for transportation and renewable energy generation applications, taking advantage of their superior power capability and outstanding cycle life. Over the past decade, various advanced electrode materials and cell design are being studied to improve the energy density of ECs. Hybrid Li-ion capacitors and pseudo-capacitors that utilize fast surface redox reactions of metal oxide and doped polymers are the prime candidates being considered. This paper is concerned with the metrics being used to describe the performance of ECs and how the metrics are evaluated by testing devices and how the data from the testing are best interpreted. Emphasize is on relating testing of advanced ECs using materials more complex than activated carbons to testing electric double-layer capacitors (EDLCs) using carbon in both electrodes. A second focus of the paper is projecting the potential of the advanced materials and ionic liquid electrolytes for the development of complete EC cells having an energy density more than a factor of ten greater the energy density of the EDLC devices currently on the market. This potential was evaluated by calculating the performance (energy and power) of a series of ECs that utilize the advanced materials that have been studied by electrochemists over the past 10–15 years. The capacitance and resistance of the advanced ECs were calculated utilizing specific capacitance (F/g or F/cm3) and porosity data for the electrode materials and ionic conductivity of the electrolytes. It was concluded that hybrid ECs can be developed with energy densities of at least 50 Wh/kg, 70 Wh/L with efficient power greater than 3 kW/kg. Continued research on micro-porous carbons with specific capacitance of 200F/g and greater is needed.to achieve these EC performance goals.