Engineers have constructed the first supercapacitor made out of silicon. These power cells could allow mobile devices that recharge in seconds and stay charged for weeks.
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.
“If you ask experts about making a supercapacitor out of silicon, they will tell you it is a crazy idea,” says Cary Pint, an assistant professor of mechanical engineering at Vanderbilt University 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.
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.
Radically different supercap
“The big challenge for this approach is assembling the materials,” says 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 decided to take a radically different approach: they used 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,” says Pint. “Typically, researchers grow graphene from silicon-carbide materials at temperatures in excess of 1,400 degrees Celsius. But at lower temperatures, 600 to 700 degrees Celsius, 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. Inspection under a powerful scanning electron microscope showed it looked nearly identical to the original material but it was coated by a layer of graphene a few nanometers thick.
They tested the coated material and realized it had chemically stabilized the silicon surface. And 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 novel supercapacitor design is described in a paper published in the journal Scientific Reports.
24/7 solar cells
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 says.
“Despite the excellent device performance we achieved, our goal wasn’t to create devices with record performance,” says 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 backsides of solar cells and sensors. The supercapacitors would store excess electricity that the cells generate at midday and release it when the demand peaks in the afternoon.
“All the things that define us in a modern environment require electricity,” says Pint. “The more that we can integrate power storage into existing materials and devices, the more compact and efficient they will become.”
The National Science Foundation and the Army Research Office funded the study.
Source: Vanderbilt University