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Silicon supercapacitor touted as cheaper power storage option

24 Oct 2013

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Vanderbilt University material scientists have created a silicon supercapacitor that could one day be used to extend mobile phones' battery life from days to weeks or enable solar cells to produce electricity even without direct sunlight. In a study published in the journal Scientific Reports, mechanical engineering assistant professor Carl Pint and colleagues coated porous silicon with carbon to stabilise its surface and prevent adverse chemical reactions. The design deviates from the use of carbon-based nanomaterials, which are seen as a key to improving supercapacitors' energy density.

The novel supercapacitor is the first such device that is made out of silicon so it can be built into a silicon chip along with the microelectronic circuitry that it powers. In fact, 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. Pint's group is working 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.

Silicon supercapacitor touted as cheaper power storage option

Figure 1: Silicon chip with porous surface next to the special furnace where it was coated with graphene to create a supercapacitor electrode. Credit: Joe Howell / Vanderbilt.

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. Supercapacitors are currently used to store 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. While they are still unable to beat the electric energy storage capability of lithium-ion batteries, supercapacitors are gaining ground in the market for portable power storage.

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. Carbon coating turned the silicon from orange to purple or black and manifested a "graphene-like-material growth" a few nanometres 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.

"Despite the excellent device performance we achieved, our goal wasn't to create devices with record performance. It was to develop a road map for integrated energy storage," Pint said.

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