Vanderbilt University researchers say they have come up with a way to store electricity on a silicon-based supercapacitor that would let mobile phones recharge in seconds and let them continue to operate for weeks without recharging.
The researchers said in a paper that instead of storing energy in chemical reactions the way batteries do, silicon supercapacitors store electricity by assembling ions on the surface of a porous material. As a result, they tend to charge and discharge in minutes, instead of hours, and operate for a few million cycles, instead of a few thousand cycles like batteries, the researchers stated.
"These properties have allowed commercial supercapacitors, which are made out of activated carbon, to capture a few niche markets, such as storing energy captured by regenerative braking systems on buses and electric vehicles and to provide the bursts of power required to adjust of the blades of giant wind turbines to changing wind conditions. Supercapacitors still lag behind the electrical energy storage capability of lithium-ion batteries, so they are too bulky to power most consumer devices. However, they have been catching up rapidly," the researchers said.
The Vanderbilt team said they used porous silicon -- a material with a controllable and well-defined nanostructure made by electrochemically etching the surface of a silicon wafer. This let them create surfaces with optimal nanostructures for supercapacitor electrodes, but it left them with a major problem: Silicon is generally considered unsuitable for use in supercapacitors because it reacts readily with some of chemicals in the electrolytes that provide the ions that store the electrical charge, the researchers said.
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To remedy the situation, the researchers said they coated the porous silicon surface with carbon and found that it had chemically stabilized the silicon surface. When they used it to make supercapacitors, they found that the graphene coating improved energy densities by over two orders of magnitude compared to those made from uncoated porous silicon and significantly better than commercial supercapacitors.
Cary Pint, the assistant professor of mechanical engineering who headed the development said the group is currently using this approach to develop energy storage that can be formed in the excess materials or on the unused back sides of solar cells and sensors. The supercapacitors would store excess the electricity that the cells generate at midday and release it when the demand peaks in the afternoon.
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