Hemp could replace graphene in supercapacitors

As hemp makes a comeback in the US after a decades-long ban on its cultivation, scientists are reporting that fibres from the plant can pack as much energy and power as graphene, long-touted as the model material for supercapacitors.

David Mitlin, formerly of the University of Alberta, now at the Clarkson University in New York, says better supercapacitor electrodes can be made from certain hemp fibres, and they perform as well as the current top contender: graphene.

"Our device's electrochemical performance is on par with, or better than, graphene-based devices," Mitlin says. "The key advantage is that our electrodes are made from biowaste using a simple process, and therefore, are much cheaper than graphene."

The race toward the ideal supercapacitor has largely focused on graphene but the processes are expensive. Mitlin's group wanted to find out if they could make graphene-like carbons from hemp bast fibres.

These fibres come from the inner bark of the plant and are often discarded by manufacturers in Canada that use hemp for clothing, construction materials and other products. The US could become another supplier of bast, as it now allows limited cultivation. Hemp, unlike its close cousin, does not induce highs!

Hemp bast has long been a subject of interest for scientists, but finding the right way to process the material in such a way as to give it the 'amazing' properties associated with graphene, has hitherto alluded researchers. Mitlin believes they've now found a way.

His team discovered that if they heated the fibres for 24 hours at a little over 350°F, and then subjected the resulting material to 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 terms of the energy density and the range of temperatures over which they could 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°F.

"We're past the proof-of-principle stage for the fully functional supercapacitor," he says. "Now we're gearing up for small-scale manufacturing."

Mitlin, who conducted the research while at the University of Alberta, acknowledges funding from Alberta Innovates Technology Futures, National Institute for Nanotechnology (Canada) and Alberta Livestock and Meat Agency.

Mitlin's group presented their research (which a Canadian start-up company is currently working on scaling up) at the 248th National Meeting & Exposition of the American Chemical Society in San Fransisco last week.

Les Hunt
Editor