Carbon Nanotube Supercapacitors

Supercapacitors (aka, electrochemical capacitors or ultracapacitors) are electrochemical energy storage devices that combine the high energy-storage-capability of conventional batteries with the high power-delivery-capability of conventional capacitors (Burke, 2000; Conway, 1999). Able to achieve higher power and longer cycle life than batteries, supercapacitors have been developed to provide power pulses for a wide range of applications including electric transportation technology (e.g., hybrid electric vehicles (HEVs) and plug-in HEVs), electric utility industry (e.g., emergency backup power and grid system stability improvement), consumer electronics (e.g., laptops, cell phones, pagers, and video cameras), medical electronics (e.g., portable defibrillators, drug delivery units, and neurological stimulators), and military / defense devices (e.g., communication devices, unmanned aerial vehicles, spacecraft probes, and missile systems) (Miller & Burke, 2008; Miller & Simon, 2008). In advanced electric transportation technologies, for example, supercapacitors are being developed as power assists for HEVs and plug-in HEVs, where the supercapacitor is operated to provide peak power during acceleration and hill-climbing, and it can be recharged during regenerative braking (Ehsani et al., 2005). This is extremely useful in achieving better fuel economy, decreasing harmful emissions, and reducing the reliance on petroleum sources. A recent report prepared by the US Department of Energy indicated that supercapacitors have been paid equal attention as to batteries as the future energy storage technologies (US DoE, 2007). The world market for supercapacitors has been growing steadily and rapidly. Nevertheless, in order to improve the performance of the state-of-the-art supercapacitors to satisfy the rapidly increasing performance demands for the applications mentioned above, new electrode materials having superior properties over those of the currently used activated carbon electrodes are needed. This chapter summarizes the recent research and technology in developing carbon nanotube (CNT)-based materials as a new type of electrode materials for supercapacitors. 29