Hemp nanosheets could trump graphene for supercapacitors

August 13, 2014 // By Jean-Pierre Joosting
After a decades-long ban on its cultivation in the USA, scientists are reporting that fibers from the hemp plant can pack as much energy and power as graphene, long-touted as the model material for supercapacitors.

It should be noted hemp and marijuana both come from the same species of plant — cannabis — but hemp is generally harvested from a subspecies of the Cannabis genus that has no psychoactive properties.

David Mitlin, Ph.D., explains that supercapacitors are energy storage devices but unlike today's rechargeable batteries, which sip up energy over several hours, supercapacitors can charge and discharge within seconds. However, they normally can't store nearly as much energy as batteries. 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.

"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 — a strong, light material made of atom-thick layers of carbon, which when stacked, can be made into electrodes. Scientists are investigating how they can take advantage of graphene's unique properties to build better solar cells, water filtration systems, touch-screen technology, as well as batteries and supercapacitors. The problem is it's expensive.

Mitlin's group decided to see if they could make graphene-like carbons from 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 U.S. could soon become another supplier of bast. It now allows limited cultivation of hemp, which unlike its close cousin, does not induce highs.

Scientists had long suspected there was more value to the hemp bast — it was just a matter of finding the right way to process the material.

"We've pretty much figured out the secret sauce