Charge it

Researchers at the University of Arizona are developing capacitors built on nano-scale structures that quickly store and discharge large amounts of power.

Capacitors have been around for a long time. In their simplest form, they consist of two conducting plates separated by an insulating material. If they’re connected to a battery, a charge appears on their plates. But conventional capacitors aren’t suitable for use in hybrid vehicles because they would have to be enormous to store the required amount of energy.

Now, however, researchers at the University of Arizona are developing a technology based on DESDs (Digitated Energy Storage Devices) that could solve this problem. The DESD breakthrough was made by Professor Olgierd Palusinski, his former graduate student Ken Bartley, Research Engineer Jaeheon Lee, and others on their team in the electrical and computer engineering department.

DESDs have a very high capacitance-to-volume ratio that’s more than 10,000 times larger than a conventional parallel-plate capacitor of the same size. This makes for a device with large capacitance in a small package.

The UA researchers construct DESD capacitors by using commercially available porous membranes. The membranes have a pore diameter ranging from 15 nanometres to 1 micron and a hole density of 10 million to 100 trillion pores per square centimetre. To form the capacitors, the membrane pores are filled with copper to create a large copper surface area in a small space. This is important because the ability to store electric charge is proportional to the surface area of a capacitor’s plates.

The honeycomb of conductors formed in the nanometre-sized membrane pores has a much larger surface area and ability to store electricity than a conductor with just the surface area of the membrane alone.
In addition to making hybrid vehicles more efficient, DESDs also could make them more environmentally friendly because DESDs don’t wear out like batteries and would last for the life of the vehicle and beyond.

“The limiting factor right now is the low voltage (less than 5V) that can be imposed on the DESDs,” Palusinski said. The voltage limit is caused by the small space between conductors in the membrane. At higher voltages, electricity will spark between the conductors, causing loss of charge.

This voltage limitation can be bypassed by connecting the DESDs in series, with the voltage capacity increasing in direct proportion to their number. Unfortunately, connecting them in series lowers the overall capacitance of the array. “But this reduction in capacitance can be compensated by connecting several DESD arrays in parallel,” Palusinski explained. The capacitance of devices adds when they are connected in parallel.

“We are looking to both industry and NSF for additional funding to pursue this research,” Palusinski added. “We are getting close to the commercial development stage, but still need to do additional studies.”