Could hemp nanosheets topple graphene for better supercapacitor electrodes?

No, you can't smoke the supercapacitor
August 14, 2014

Hemp field in Côtes-d’Armor, Brittany, France (credit: Barbetorte, Creative Commons)

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.

David Mitlin, Ph.D., explains that 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.

Move over, 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.

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

“We’ve pretty much figured out the secret sauce of it,” says Mitlin, who’s now with Clarkson University in New York. “The trick is to really understand the structure of a starter material and to tune how it’s processed to give you what would rightfully be called amazing properties.”

How to create carbon nanosheets from hemp

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.

“We’re past the proof-of-principle stage for the fully functional supercapacitor,” he says. “Now we’re gearing up for small-scale manufacturing.” A Canadian startup company is working on scaling up the technology.

Mitlin’s team is presenting their research at the 248th National Meeting & Exposition of the American Chemical Society (ACS) this week.

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.

* Hemp and marijuana both come from the same plant — Cannabis Sativa L. The term “hemp” commonly refers to the industrial/commercial use of the cannabis stalk and seed for textiles, foods, papers, body care products, detergents, plastics and building materials. Industrial hemp is technically from the same species of plant that psychoactive marijuana comes from. However, it is from a different variety, or subspecies that contains many important differences. Hemp fiber is the longest, strongest and most durable of all natural fibers. Hemp cultivation requires no chemicals, pesticides or herbicides. Grown in rotation with other crops such as corn and legumes, hemp farming is completely sustainable. — Hempethics