Burke, Andrew and Mati Arulepp (2001) Recent Developments in Carbon-Based Electrochemical Capacitators: Status of the Technology and Future Prospects. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-01-20
Electrochemical capacitors (ultracapacitors) are being developed as an alternative to pulse power batteries. In this paper, the status of the technology for ultracapacitors is summarized with a strong emphasis on carbon-based technologies and large devices. The performance of these devices has been measured for a wide range of current and power density and the characteristics of devices manufactured by Ness, Panasonic, Montena, and Maxwell compared. All the devices considered use an organic electrolyte with a cell voltage of 2.3–2.7V. It was found that the useable energy density of commercially available devices have improved by about 60% increasing from 2.5 to near 4.0 Wh/kg. The power density of the devices for a 95% efficient discharge is 1000–1800 W/kg with the matched impedance power density being greater than 7 kW/kg. Prototype devices using advanced carbons with specific capacitance of 110–120 F/gm with an organic electrolytes are being developed. These devices have a useable energy density of up to 5 Wh/kg at 2.3V and a peak power capability of 7 kW/kg for a 95% discharge. Future development of these devices using carbons with a specific capacitance of 125–135 F/gm and rated voltages near 3V will result in energy densities of 10–12 Wh/kg and peak power density greater than 10 kW/kg. Test data are presented for a 18-cell module of 2.5V, 2500F ultracapacitor devices that indicate that the energy density and peak power of a module or pack of ultracapacitors is close to that which would be expected based on the performance of the single cells. Cell-to-cell voltage variability of the module seemed to stabilize at relatively small values permitting cycling of the module for 36 hours. Self-discharge of the module followed closely from the self-discharge characteristics of the single cells used to assemble the module. High cost remains a critical obstacle to the mass marketing of ultracapacitors, but the cost of devices has been reduced markedly in recent years. It is projected that costs will continue to be reduced over the next several years, but whether the costs can be reduced sufficiently to be attractive to the mass markets is uncertain. This would require ultracapacitors that can be sold for $2–5/ Wh.