Energy Storage On Silicon Chips — New Supercapacitor Creates Interesting Possibilities For Solar Cells

The first supercapacitor composed of silicon was recently created by researchers at Vanderbilt University — the novel supercapacitor opens up a number of very interesting possibilities with regard to solar cell technology and mobile electronics. In particular, the researchers note the possibility of developing solar cells that can provide electricity for a full 24 hours of the day, and of developing mobile phones that can recharge in seconds and work for weeks between charges.

The great strength of the new supercapacitor is that, since its created out of silicon, it can simply be built into a silicon chip along with and at the same time as the same microelectronic circuitry that it powers. The researchers even mention the possibility of constructing 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.”

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

“If you ask experts about making a supercapacitor out of silicon, they will tell you it is a crazy idea,” stated Cary Pint, the assistant professor of mechanical engineering who headed the development. “But we’ve found an easy way to do it.”

Most research to date to improve the energy density of supercapacitors has focused on the utilization of carbon-based nanomaterials like graphene and nanotubes, but because of the great difficulty in “constructing high-performance, functional devices out of nanoscale building blocks with any level of control,” improvements have been slow. So, the researchers decided to try something radically new — utilizing porous silicon, a material with a controllable and well-defined nanostructure made by electrochemically etching the surface of a silicon wafer.

Vanderbilt University provides details:

This allowed the researchers 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 the chemicals in the electrolytes that provide the ions that store the electrical charge.

With experience in growing carbon nanostructures, Pint’s group decided to try to coat the porous silicon surface with carbon. 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.

“We had no idea what would happen,” Pint explained. “Typically, researchers grow graphene from silicon-carbide materials at temperatures in excess of 1400 degrees Celsius. But at lower temperatures — 600 to 700 degrees Celsius — we certainly didn’t expect graphene-like material growth.”

After testing the coated material, the researchers found that it had chemically stabilized the silicon surface — and that, when it was used to create supercapacitors, 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.”

The researchers think that this approach very likely isn’t specific to graphene. “The ability to engineer surfaces with atomically thin layers of materials combined with the control achieved in designing porous materials opens opportunities for a number of different applications beyond energy storage,” Pint argued.

“Despite the excellent device performance we achieved, our goal wasn’t to create devices with record performance,” Pint continued. “It was to develop a road map for integrated energy storage. Silicon is an ideal material to focus on because it is the basis of so much of our modern technology and applications. In addition, most of the silicon in existing devices remains unused since it is very expensive and wasteful to produce thin silicon wafers.”

The researchers are now pursuing this line of thought — looking to develop energy storage that can be built into the excess materials and/or unused back-sides of solar cells.

The new research was detailed in a paper published in the journal Scientific Reports.