Supercapacitor stores electricity on silicon chips
It is the first supercapacitor that is made out of silicon so it can be built into a silicon chip along with the microelectronic circuitry that it powers.
It should be possible to construct these power cells out of the excess silicon that exists in the current generation of solar cells, sensors, mobile phones and a variety of other electromechanical devices, providing a considerable cost savings, said the researchers.
"If you ask experts about making a supercapacitor out of silicon, they will tell you it is a crazy idea," said Cary Pint, the assistant professor of mechanical engineering who headed the development. "But we've found an easy way to do it."
Instead of storing energy in chemical reactions the way batteries do, "supercaps" 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.
Pint and his research team decided to take a radically different approach: using porous silicon, a material with a controllable and well-defined nanostructure made by electrochemically etching the surface of a silicon wafer. This allowed them to 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.
With experience in growing carbon nanostructures, the researchers decided to try to coat the porous silicon surface with carbon.
"We had no idea what would happen," said Pint. "Typically, researchers grow graphene from silicon-carbide materials at temperatures in excess of 1400°C. But at lower temperatures – 600 to 700°C – we certainly didn't expect graphene-like material growth."
When the researchers pulled the porous silicon out of the furnace, they found that it had turned from orange to purple or black. When they inspected it under a powerful scanning electron microscope they found that it looked nearly identical to the original material but it was coated by a layer of graphene a few nanometers thick.
When the researchers tested the coated material they found that it had chemically stabilised 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.
Pint's 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.