Silicon chip with porous surface next to the special furnace where it was coated withgraphene to create a supercapacitor electrode. (Joe Howell / Vanderbilt)
Researchers at the Vanderbilt University in Nashville propose a novel silicon supercapacitor design. Such supercapacitor can be, theoretically, integrated into a silicon chip, opening some interesting options for energy storage.
Batteries effectively store energy but do not deliver power efficiently because the charged carriers move slowly through the solid battery material. Capacitors, which store energy at the surface of a material, generally have low storage capabilities. Supercapacitors bridge the gap between conventional capacitors and rechargeable batteries. Supercapacitors currently cannot store as much energy as batteries, but are able to be charged and discharged much quicker. They also are distinguished from batteries by a much longer lifetime.
“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.”
According to the abstract of a paper published in Scientific Reports (see footnote), silicon materials remain unused for supercapacitors due to extreme reactivity of silicon with electrolytes. However, doped silicon materials boast a low mass density, excellent conductivity, a controllably etched nanoporous structure, and combined earth abundance and technological presence appealing to diverse energy storage frameworks.
Silicon supercapacitor research group (left to right): Landon Oakes, Shahana Chatterji, Andrew Westover and Cary Pint. (Credit: Joe Howell / Vanderbilt)
A Vanderbilt University article says that research to improve the energy density of supercapacitors has focused on graphene-based supercapacitors or some other carbon-based nanomaterials. Because these devices store electrical charge on the surface of their electrodes, the way to increase their energy density is to increase the electrodes’ surface area, which means making surfaces filled with nanoscale ridges and pores. An opposite approach would be to use an already structured material like silicon.
Research News @ Vanderbilt website provides more info:
“The big challenge for this approach is assembling the materials,” said Pint. “Constructing high-performance, functional devices out of nanoscale building blocks with any level of control has proven to be quite challenging, and when it is achieved it is difficult to repeat.”
So Pint and his research team—raduate students Landon Oakes, Andrew Westover and post-doctoral fellow Shahana Chatterjee—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, Pint’s group 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 degrees Celsius. But at lower temperatures—600 to 700 degrees Celsius—we certainly didn’t expect graphene-like material growth.”
Graph displays the power density (watts per kilogram) and energy density (watt-hours per kilogram) of capacitors made from porous silicon (P-Si), graphene-coated porous silicon and carbon-based commercial capacitors. (Credit: Cary Pint / Vanderbilt)
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 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.
The graphene layer acts as an atomically thin protective coating. Pint and his group argue that this approach isn’t limited 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,” he said.
“Despite the excellent device performance we achieved, our goal wasn’t to create devices with record performance,” said Pint. “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.”
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.
Oakes L, Westover A, Mares JW, Chatterjee S, Erwin WR, Bardhan R, Weiss SM, & Pint CL (2013). Surface engineered porous silicon for stable, high performance electrochemical supercapacitors. Scientific Reports, 3 PMID: 24145684
