The waste fibres from hemp crops can be
transformed into high-performance energy storage devices, scientists say.
They "cooked" cannabis bark
into carbon nanosheets and built supercapacitors "on a par with or better
than graphene" - the industry gold standard.
Graphene is the name given to a novel
substance composed of a single layer of carbon atoms, extracted from graphite,
with astonishing properties: the stuff is stronger than diamond, more
conductive than copper and more flexible than rubber.
They presented their work at the American Chemical Society meeting in San Francisco.
"People ask me: why hemp? I say,
why not?" said Dr David Mitlin of Clarkson University, New York, who
describes his device in the journal ACS Nano.
"We're making graphene-like
materials for a thousandth of the price - and we're doing it with waste.
"The hemp we use is perfectly legal
to grow. It has no THC in it at all - so there's no overlap with any
recreational activities."
Secret sauce
In countries including China, Canada and
the UK, hemp can be grown industrially for clothing and building materials.
But the leftover bast fibre - the inner
bark - typically ends up as landfill.
Dr Mitlin's team took these fibres and
recycled them into supercapacitors - energy storage devices which are
transforming the way electronics are powered.
Conventional batteries store large
reservoirs of energy and drip-feed it slowly, whereas supercapacitors can
rapidly discharge their entire load.
They are ideal in machines that rely on
sharp bursts of power. In electric cars, for example, supercapacitors are used for
regenerative braking.
Releasing this torrent requires
electrodes with high surface area - one of graphene's many
phenomenal
properties.
Stronger than diamond, more conductive
than copper and more flexible than rubber, the "miracle material" was the target of a
£50m investment by UK Chancellor George Osborne.
But while this carbon monolayer is the
state-of-the-art material for commercial supercapacitors, it is prohibitively
expensive to produce.
Finding cheap, sustainable alternatives
is the speciality of Dr Mitlin's former research group at the University of Alberta.
They have experimented with all flavours
of biowaste - from peat moss to eggs. Most recently, they turned banana peel into batteries.
"You can do really interesting
things with bio-waste. We've pretty much figured out the secret sauce of
it," said Dr Mitlin.
The trick is to tailor the right plant
fibre to the right electrical device - according to their organic structure.
"With banana peels, you can turn
them into a dense block of carbon - we call it pseudo-graphite - and that's
great for sodium ion batteries," he explained.
UK Chancellor George Osborne |
"But if you look at hemp fibre its
structure is the opposite - it makes sheets with high surface area - and that's
very conducive to supercapacitors."
The first step, he explained, "is
to cook it - almost like a pressure cooker. It's called hydrothermal synthesis.
"Once you dissolve the lignin and
the semicellulose, it leaves these carbon nanosheets - a pseudo-graphene
structure."
By fabricating these sheets into
electrodes and adding an ionic liquid as the electrolyte, his team made
supercapacitors which operate at a broad range of temperatures and a high
energy density.
Direct comparisons with rival devices
are complicated by the variety of measures for performance.
But Mitlin's peer-reviewed journal paper ranks the device "on par with
or better than commercial graphene-based devices".
"They work down to 0C and display
some of the best power-energy combinations reported in the literature for any
carbon.
"For example, at a very high power
density of 20 kW/kg (kilowatt per kilo) and temperatures of 20, 60, and 100C,
the energy densities are 19, 34, and 40 Wh/kg (watt-hours per kilo)
respectively."
Fully assembled, their energy density is
12 Wh/kg, which can be achieved at a charge time less than six seconds.
Growth
industry
"Obviously hemp can't do all the
things graphene can," Dr Mitlin concedes.
"But for energy storage, it works
just as well. And it costs a fraction of the price
-$500-1,000 a tonne."
Having established a proof of principle, his start-up company Alta Supercaps is hoping to begin small-scale manufacturing.
It plans to market devices to the oil
and gas industries - where high-temperature operation is a valuable asset.
His move to the US coincides with a
change in regulatory attitudes - with signs that hemp could be making a comeback.
In China the crop is widely cultivated,
and in Canada, the industry for textiles is growing.
"Fifty miles down the road from my
house in Alberta there was an agricultural hemp processing facility. And all
that bast fibre - it just sits in a high bay, and they don't know what to do
with it," Dr Mitlin told BBC News.
"It's a waste product looking for a
value-added application. People are almost paying you to take it away."
And if the technology really takes off -
it could help economies, he argues.
"It's a robust plant - you can even
grow it in Alberta, Manitoba.
"A lot of farmers would be thrilled to grow hemp." Culled from BBC
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