Hemp is used in many countries as a cost-effective and greener building material, but it does produce some waste. Now, though, researchers believe that they could repurpose those waste fibers into next-generation energy storage devices.
The research, presented by Dr. David Mitlin at the 248th National Meeting and Exposition of the American Chemical Society (ACS) and published this month in the journal ACS Nano, details how the researchers took those hemp fibers and turned them into what are known as supercapacitors. Such supercapacitors carry a distinct advantage over conventional batteries and larger energy storage devices because they can be charged and discharged within a matter of seconds, which is handy for a number of technologies, including for electric cars that, rather than being drip-fed energy from regular batteries, require sharp bursts of power for things like regenerative braking.
However, super-capacitor technologies are currently restricted by issues like the cost of materials and the size limitations. In addition to this, they usually can’t store nearly as much energy as today’s conventional battery technology.
To get around this problem, scientists have been trying to create better electrodes that would allow larger energy density rates. At the moment, the best material for this process is the wonder-material graphene, which has become the star of the manufacturing and scientific world for its hundreds of uses. For the purpose of creating a supercapacitor though, graphene is incredibly costly to use.
That’s where hemp might offer a significant advantage.
“People ask me: why hemp? I say, why not?” Dr Mitlin told the BBC. “We’re making graphene-like materials for a thousandth of the price — and we’re doing it with waste.”
The process for making these graphene-like carbon fibers is actually relatively simple. From any cultivated crop of non-THC hemp (meaning that it’s perfectly legal and you can’t get high from the plant), there is inevitably a certain amount of bio-waste. This includes the hemp’s so-called inner bark or bast, and that usually ends up in landfills. Scientists have long been interested in bast fiber because of its engineering possibilities, but they’ve struggled to find the best way to process the material.
Mitlin and team, however, believe they’ve discovered what they call the “secret sauce.” They found that applying heat of over 350 degrees Fahrenheit for a 24 hour period, and then blasting the material with intense bursts of heat, caused the bast fiber to peel into what are known as nanosheets. The team then built their supercapacitors using the sheets as electrodes alongside an ionic liquid as the electrolyte. These supercapacitors yield 12 Watt-hours per kilogram, which is at least double most of the commercial counterparts and rivals even graphene, and costs as little as $500 a ton.
The researchers have managed to produce a proof-of-principle model which was successful, and so are moving on to a limited manufacturing stage that, they believe, will prove the viability of this technology. They hope that this research could eventually lead to a more environmentally friendly method of creating supercapacitors that, in turn, could spare us energy and waste products in other manufacturing endeavors.
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