A plant that is still banned in most of the U.S. offers amazing energy-storage potential that far exceeds anything currently on the market. Researchers presenting at the recent 248th National Meeting & Exposition of the American Chemical Society revealed that fibers from the hemp plant, a variety of the cannabis plant, can be turned into electrodes capable of better storing energy in supercapacitors.
The best material currently available for supercapacitor electrodes is graphene, a light-weight material that is both strong and able to be made thin enough to store high amounts of energy. Scientists have been developing novel ways to utilize graphene in things like batteries, touch-screen smartphones and devices, water filtration systems and solar panels, making these products more efficient and easier to recharge.
But graphene is expensive to produce, which makes it cost-prohibitive in many existing consumer applications. Its general lack of efficiency also makes graphene more of a budding technology with promising potential rather than an effective solution for today. Because of this, scientists have been investigating other solutions such as hemp, which is significantly cheaper to produce and far more effective.
According to Dr. David Mitlin, Ph.D., the bast fibers in hemp make much better electrodes than graphene because they have a much higher energy density, meaning they can store more power and last longer than traditional electrodes. Hemp also grows much more quickly than most other plants, making it an extremely viable source of the needed fibers.
After scraping the inner bark from the hemp plant, a material that is typically discarded, Dr. Mitlin and his team heated these fibers for 24 hours at just above 350 degrees Fahrenheit. Following this, they focused a more intense heat on the fibers, which exfoliated them into carbon nanosheets that can store energy.
Hemp supercapacitors work up to three times better than standard commercial varieties
These nanosheets would become the bread and butter of new-and-improved supercapacitors that Dr. Mitlin says perform better than existing commercial supercapacitors, both in terms of energy density and the range of temperatures in which they are capable of operating. Tests revealed that the hemp-based supercapacitors had energy densities topping 12 watt-hours per kilogram, which is up to three times higher than anything currently on the market.
“Our device’s electrochemical performance is on par with or better than graphene-based devices,” said Dr. Mitlin. “The key advantage is that our electrodes are made from biowaste using a simple process, and therefore, are much cheaper than graphene.”
As far as the conditions in which these hemp-based supercapacitors are capable of operating, Dr. Mitlin says his devices perform well in both freezing weather and at temperatures exceeding 200 degrees Fahrenheit. The temperature ranges of operation in a graphene context, on the other hand, are much more limited, making hemp the all-around better option.
“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,” said Dr. Mitlin. “We’re past the proof-of-principle stage for the fully functional supercapacitor. Now we’re gearing up for small-scale manufacturing.”
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