That banana peel, turned into
graphene, can help facilitate a massive reduction of the environmental impact
of concrete and other building materials. While you’re at it, toss in those
plastic empties.
A new process introduced by the Rice
University lab of chemist James Tour can turn bulk quantities of just about any
carbon source into valuable graphene flakes. The process is quick and cheap;
Tour said the “flash graphene” technique can convert a ton of coal, food waste
or plastic into graphene for a fraction of the cost used by other bulk
graphene-producing methods.
“This is a big deal,” Tour said.
“The world throws out 30% to 40% of all food, because it goes bad, and plastic
waste is of worldwide concern. We’ve already proven that any solid carbon-based
matter, including mixed plastic waste and rubber tires, can be turned into
graphene.”
As reported in Nature, flash
graphene is made in 10 milliseconds by heating carbon-containing materials to
3,000 Kelvin (about 5,000 degrees Fahrenheit). The source material can be
nearly anything with carbon content. Food waste, plastic waste, petroleum coke,
coal, wood clippings and biochar are prime candidates, Tour said. “With the
present commercial price of graphene being $67,000 to $200,000 per ton, the
prospects for this process look superb,” he said.
Tour said a concentration of as
little as 0.1% of flash graphene in the cement used to bind concrete could
lessen its massive environmental impact by a third. Production of cement
reportedly emits as much as 8% of human-made carbon dioxide every year.
“By strengthening concrete with
graphene, we could use less concrete for building, and it would cost less to
manufacture and less to transport,” he said. “Essentially, we’re trapping
greenhouse gases like carbon dioxide and methane that waste food would have
emitted in landfills. We are converting those carbons into graphene and adding
that graphene to concrete, thereby lowering the amount of carbon dioxide
generated in concrete manufacture. It’s a win-win environmental scenario using
graphene.”
“Turning trash to treasure is key to
the circular economy,” said co-corresponding author Rouzbeh Shahsavari, an
adjunct assistant professor of civil and environmental engineering and of
materials science and nanoengineering at Rice and president of C-Crete
Technologies. “Here, graphene acts both as a 2D template and a reinforcing
agent that controls cement hydration and subsequent strength development.”
In the past, Tour said, “graphene
has been too expensive to use in these applications. The flash process will
greatly lessen the price while it helps us better manage waste.”
“With our method, that carbon
becomes fixed,” he said. “It will not enter the air again.”
The process aligns nicely with
Rice’s recently announced Carbon Hub initiative to create a zero-emissions
future that repurposes hydrocarbons from oil and gas to generate hydrogen gas
and solid carbon with zero emission of carbon dioxide. The flash graphene
process can convert that solid carbon into graphene for concrete, asphalt,
buildings, cars, clothing and more, Tour said.
Flash Joule heating for bulk
graphene, developed in the Tour lab by Rice graduate student and lead author
Duy Luong, improves upon techniques like exfoliation from graphite and chemical
vapor deposition on a metal foil that require much more effort and cost to
produce just a little graphene.
Even better, the process produces
“turbostratic” graphene, with misaligned layers that are easy to separate. “A-B
stacked graphene from other processes, like exfoliation of graphite, is very
hard to pull apart,” Tour said. “The layers adhere strongly together.
But turbostratic graphene is much
easier to work with because the adhesion between layers is much lower. They
just come apart in solution or upon blending in composites.
“That’s important, because now we
can get each of these single-atomic layers to interact with a host composite,”
he said.
The lab noted that used coffee
grounds transformed into pristine single-layer sheets of graphene.
Bulk composites of graphene with
plastic, metals, plywood, concrete and other building materials would be a
major market for flash graphene, according to the researchers, who are already
testing graphene-enhanced concrete and plastic.
The flash process happens in a
custom-designed reactor that heats material quickly and emits all noncarbon
elements as gas. “When this process is industrialized, elements like oxygen and
nitrogen that exit the flash reactor can all be trapped as small molecules
because they have value,” Tour said.
He said the flash process produces
very little excess heat, channeling almost all of its energy into the target.
“You can put your finger right on the container a few seconds afterwards,” Tour
said. “And keep in mind this is almost three times hotter than the chemical
vapor deposition furnaces we formerly used to make graphene, but in the flash
process the heat is concentrated in the carbon material and none in a
surrounding reactor.
“All the excess energy comes out as
light, in a very bright flash, and because there aren’t any solvents, it’s a
super clean process,” he said.
Luong did not expect to find
graphene when he fired up the first small-scale device to find new phases of
material, beginning with a sample of carbon black. “This started when I took a
look at a Science paper talking about flash Joule heating to make
phase-changing nanoparticles of metals,” he said. But Luong quickly realized
the process produced nothing but high-quality graphene.
Atom-level simulations by Rice
researcher and co-author Ksenia Bets confirmed that temperature is key to the
material’s rapid formation. “We essentially speed up the slow geological
process by which carbon evolves into its ground state, graphite,” she said.
“Greatly accelerated by a heat spike, it is also stopped at the right instant,
at the graphene stage.
“It is amazing how state-of-the-art
computer simulations, notoriously slow for observing such kinetics, reveal the
details of high temperature-modulated atomic movements and transformation,”
Bets said.
Tour hopes to produce a kilogram
(2.2 pounds) a day of flash graphene within two years, starting with a project
recently funded by the Department of Energy to convert U.S.-sourced coal. “This
could provide an outlet for coal in large scale by converting it inexpensively
into a much-higher-value building material,” he said.
Journal
article: https://www.nature.com/articles/s41586-020-1938-0
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