Hemp nanosheets could be better than graphene for making the ideal supercapacitor
June 16, 2015
As hemp makes a comeback in the U.S. after a decades-long ban on its cultivation, scientists are reporting that fibers from the plant can pack as much energy and power as graphene, long-touted as the model material for supercapacitors, according to David Mitlin, Ph.D.
Supercapacitors are energy storage devices that have huge potential to transform the way future electronics are powered. Unlike today’s rechargeable batteries, which sip up energy over several hours, supercapacitors can charge and discharge within seconds. But they normally can’t store nearly as much energy as batteries, an important property known as energy density.
One approach researchers are taking to boost supercapacitors’ energy density is to design better electrodes. Mitlin’s team has figured out how to make them from certain hemp fibers — and they can hold as much energy as the current top contender: graphene.
“The key advantage is that our electrodes are made from biowaste using a simple process, and therefore, are much cheaper than graphene.”
His process uses hemp bast fibers. The fibers come from the inner bark of the plant and often are discarded from Canada’s fast-growing industries that use hemp for clothing, construction materials and other products. The US could soon become another supplier of bast.
His team found that if they heated the fibers for 24 hours at a little over 350 degrees Fahrenheit, and then blasted the resulting material with more intense heat, it would exfoliate into carbon nanosheets.
Mitlin’s team built their supercapacitors using the hemp-derived carbons as electrodes and an ionic liquid as the electrolyte. Fully assembled, the devices performed far better than commercial supercapacitors in both energy density and the range of temperatures over which they can work. The hemp-based devices yielded energy densities as high as 12 Watt-hours per kilogram, two to three times higher than commercial counterparts. They also operate over an impressive temperature range, from freezing to more than 200 degrees Fahrenheit.
They’re presenting their research, which a Canadian start-up company is working on scaling up, at the 248th National Meeting & Exposition of the American Chemical Society (ACS), the world’s largest scientific society.
Mitlin, who conducted the research while at the University of Alberta, received funding from Alberta Innovates Technology Futures, National Institute for Nanotechnology (Canada) and Alberta Livestock and Meat Agency.