New Supercapacitor Made of Silicon Increases Chip Energy Storage

One of the biggest challenges of solar power is that the sun doesn’t shine 24 hours a day. On cloudy days or at night, solar panels cannot access the sun’s energy. A solution may be at hand. >Material science researchers at Vanderbilt University may have solved the problem with the creation of an innovative supercapacitor design fabricated from silicon. The device can be built into a silicon microchip side by side along with the electronic components that it powers.

According to the material scientists, the power cells  can be made out of “excess silicon” already  present in the  existing solar cells, mobile phones, sensors and other electronic devices, which  translate into lower costs.

Standard Supercapacitor Technology:  promising, but challenges remain.

Supercapacitors sometimes referred to as “ultracapacitors” are made out of activated carbon and have been around for decades. They charge faster than batteries and have a reputation for rapid-fire power-ups. Chemical reactions allow batteries to store energy. Supercapacitors amass ions on the on porous substrates-charging and discharging energy in minutes versus hours.

Since they do not require the amount charging and discharging that wear down batteries, supercapacitors last longer—millions of cycles compared to thousands for batteries

However, measured against lithium ion batteries of comparable weight and size, lithium ion batteries have perhaps 20 times the energy of supercapacitors. Supercapacitors are cost-prohibited because they would have to be much larger to hold the same amount of energy as batteries, which prevent their use in electronic devices.

Method of Energy Storage Is the Key

Cary Pint, assistant professor of mechanical engineering at Vanderbilt, heads a research team that has been working on Supercapacitor technology.  The teams efforts have centered on improving the energy density of carbon-based nanomaterials,  grapheme and nanotubes, because these materials store electrical charge on the  surface on their electrodes. Research has shown that the way to increase energy density is to increase the surface area of the electrodes.

Pint said that the primary challenge for researchers has to do with “assembling the materials.” Similar to other research involving the building of nanotube structures, the researchers struggled with controlling the process and building the necessary structure consistently.

In their efforts, Pint and the rest of the team — Shahana Chatterjee and graduate students Landon Oakes, Andrew Westover — decided to utilize porous silicon. The material, which is very controllable, consists of a precise nanostructure fabricated by etching the surface of a silicon wafer.

Silicon has always been used for supercapacitors because of the chemical reaction with certain chemicals present in the electrolytes, which supply the ions that store the electrical energy. The team coated the  surface with carbon and put it in the furnace at a  lower than normal temperature –600 to 700 degrees Celsius instead of  in excess of 1400 degrees Celsius.

The team did not anticipate the “graphene-like material growth” that resulted from the lower furnace temperature.

Subsequently, a microscopic examination revealed that the porous silicon appear similar to the original material but had developed a layer of graphene that was a couple of nanometers thick. Upon testing the material, they discovered that it had “chemically stabilized the surface.”

Using the material to make supercapacitors, the team discovered that coating the surface with graphene enhanced the energy density more than two times the magnitude of uncoated porous silicon. It also surpassed the energy density of supercapacitors available on the commercial market.

Pint emphasizes that the team did not have the mission of increasing the performance of the device, but to develop a system for ‘integrated energy storage.”  He points to the fact that today’s technology depends on silicon and that the very material is mostly wasted during the wafer fabrication process.

“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,” said Pint.

The paper is published in the October. 22 issue of the journal Scientific Reports.

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