Researchers at MIT’s Laboratory for Electromagnetic and Electronic Systems (LEES) are investigating the use of nanotube structures to improve the energy storage density of ultracapacitors to a level comparable with that of NiMH batteries, while maintaining or even improving the ultracapacitors’ high performance.
Capacitors store energy in an electric field, making them more efficient than standard batteries, which get their energy from chemical reactions. Although ultracapacitors can provide quick, massive bursts of energy—ideal for acceleration with an electric drive—they need to be much larger than batteries to hold an equivalent amount of energy.
Physical constraints on electrode surface area and spacing have limited ultracapacitors to an energy storage capacity around 20 times less than a similarly sized lithium-ion battery. Commercial ultracapacitors achieve an energy density of around 6 Wh/kg; NiMH batteries of around 60 Wh/kg; lithium-ion batteries of around 120 Wh/kg.
The LEES ultracapacitor overcomes this limitation by using a matrix of vertically aligned, single-wall carbon nanotubes (CNT) as an electrode.
Storage capacity in an ultracapacitor is proportional to the surface area of the electrodes. Today’s ultracapacitors use electrodes made of activated carbon, which is extremely porous and therefore has a very large surface area. However, the pores in the carbon are irregular in size and shape; the irregularities reduce efficiency.
The vertically-aligned nanotubes in the LEES ultracapacitor have a regular shape, and a size that is only several atomic diameters in width. The result is a significantly more effective surface area, which equates to significantly increased storage capacity.
The MIT analysis shows that the CNT ultracapacitor could have an energy density higher than 60 Wh/kg, a power density greater than 100 kW/kg (three orders of magnitude higher than batteries), and a lifetime longer than 300,000 cycles. At 60 Wh/kg, the CNT ultracapacitors would have comparable density to NiMH batteries.
The new nanotube-enhanced ultracapacitors could be made in any of the sizes currently available and be produced using conventional technology.
This configuration has the potential to maintain and even improve the high performance characteristics of ultracapacitors while providing energy storage densities comparable to batteries. Nanotube-enhanced ultracapacitors would combine the long life and high power characteristics of a commercial ultracapacitor with the higher energy storage density normally available only from a chemical battery.—Joel E. Schindall, associate director of LEES
The work has been funded in part by the MIT/Industry Consortium on Advanced Automotive Electrical/Electronic Components and Systems and in part by a grant from the Ford-MIT Alliance.