Hemp has been shown to store as much energy as graphene, potentially toppling the wonder material as the top choice for building future supercapacitors.
Supercapacitors differ from today's rechargeable batteries primarily by their ability to charge and discharge within seconds, but this comes at the price of greatly reduced energy density, which means they can't store nearly as much energy as batteries.
One approach to boosting supercapacitors' energy density is to design better electrodes. A team led by Dr David Mitlin from Clarkson University in New York has found that electrodes made of carbon nanosheets derived from certain hemp fibres can hold as much energy as the current top contender graphene, a one-atom-thick layer of carbon atoms with extraordinary conductivity.
"Our device's electrochemical performance is on par with or better than graphene-based devices," Dr Mitlin said. "The key advantage is that our electrodes are made from biowaste using a simple process and, therefore, are much cheaper than graphene."
The team are presenting their research, which a Canadian start-up company is working on scaling up commercially, at the 248th National Meeting & Exposition of the American Chemical Society (ACS) currently underway in San Francisco.
The race toward the ideal supercapacitor has largely focused on graphene, which when stacked can be made into electrodes, but the material is expensive.
Dr Mitlin's group decided to see if they could make graphene-like carbons from hemp bast fibres, which 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 following the repeal of a decades-long ban on its cultivation due to its similarity to cannabis – the plant’s psychoactive cousin. The US now allows limited cultivation of hemp, which unlike cannabis 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 of it," said Mitlin. "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."
His team found that if they heated the fibres for 24 hours at a little over 175°C and then blasted the resulting material with more intense heat, it would exfoliate into carbon nanosheets.
Dr 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 and also operate over an impressive temperature range, from freezing to more than 90°C.
"We're past the proof-of-principle stage for the fully functional supercapacitor," added Dr Mitlin. "Now we're gearing up for small-scale manufacturing."