Lab's 'green' invention reduces
carbon dioxide into valuable fuels
September 3, 2019
Rice University
An electrocatalysis reactor
built at Rice University recycles carbon dioxide to produce pure liquid fuel
solutions using electricity. The scientists behind the invention hope it will
become an efficient and profitable way to reuse the greenhouse gas and keep it
out of the atmosphere.
A common greenhouse gas could
be repurposed in an efficient and environmentally friendly way with an
electrolyzer that uses renewable electricity to produce pure liquid fuels.
The catalytic reactor
developed by the Rice University lab of chemical and biomolecular engineer
Haotian Wang uses carbon dioxide as its feedstock and, in its latest prototype,
produces highly purified and high concentrations of formic acid.
Formic acid produced by
traditional carbon dioxide devices needs costly and energy-intensive
purification steps, Wang said. The direct production of pure formic acid
solutions will help to promote commercial carbon dioxide conversion technologies.
The method is detailed
in Nature Energy.
Wang, who joined Rice's Brown
School of Engineering in January, and his group pursue technologies that turn
greenhouse gases into useful products. In tests, the new electrocatalyst
reached an energy conversion efficiency of about 42%. That means nearly half of
the electrical energy can be stored in formic acid as liquid fuel.
"Formic acid is an energy
carrier," Wang said. "It's a fuel-cell fuel that can generate
electricity and emit carbon dioxide -- which you can grab and recycle again.
"It's also fundamental in
the chemical engineering industry as a feedstock for other chemicals, and a
storage material for hydrogen that can hold nearly 1,000 times the energy of
the same volume of hydrogen gas, which is difficult to compress," he said.
"That's currently a big challenge for hydrogen fuel-cell cars."
Two advances made the new
device possible, said lead author and Rice postdoctoral researcher Chuan Xia.
The first was his development of a robust, two-dimensional bismuth catalyst and
the second a solid-state electrolyte that eliminates the need for salt as part
of the reaction.
"Bismuth is a very heavy
atom, compared to transition metals like copper, iron or cobalt," Wang
said. "Its mobility is much lower, particularly under reaction conditions.
So that stabilizes the catalyst." He noted the reactor is structured to
keep water from contacting the catalyst, which also helps preserve it.
Xia can make the nanomaterials
in bulk. "Currently, people produce catalysts on the milligram or gram
scales," he said. "We developed a way to produce them at the kilogram
scale. That will make our process easier to scale up for industry."
The polymer-based solid
electrolyte is coated with sulfonic acid ligands to conduct positive charge or
amino functional groups to conduct negative ions. "Usually people reduce
carbon dioxide in a traditional liquid electrolyte like salty water," Wang
said. "You want the electricity to be conducted, but pure water
electrolyte is too resistant. You need to add salts like sodium chloride or
potassium bicarbonate so that ions can move freely in water.
"But when you generate
formic acid that way, it mixes with the salts," he said. "For a
majority of applications you have to remove the salts from the end product,
which takes a lot of energy and cost. So we employed solid electrolytes that
conduct protons and can be made of insoluble polymers or inorganic compounds,
eliminating the need for salts."
The rate at which water flows
through the product chamber determines the concentration of the solution. Slow
throughput with the current setup produces a solution that is nearly 30% formic
acid by weight, while faster flows allow the concentration to be customized.
The researchers expect to achieve higher concentrations from next-generation
reactors that accept gas flow to bring out pure formic acid vapors.
The Rice lab worked with
Brookhaven National Laboratory to view the process in progress. "X-ray
absorption spectroscopy, a powerful technique available at the Inner Shell
Spectroscopy (ISS) beamline at Brookhaven Lab's National Synchrotron Light
Source II, enables us to probe the electronic structure of electrocatalysts in
operando -- that is, during the actual chemical process," said co-author
Eli Stavitski, lead beamline scientist at ISS. "In this work, we followed
bismuth's oxidation states at different potentials and were able to identify
the catalyst's active state during carbon dioxide reduction."
With its current reactor, the
lab generated formic acid continuously for 100 hours with negligible
degradation of the reactor's components, including the nanoscale catalysts.
Wang suggested the reactor could be easily retooled to produce such
higher-value products as acetic acid, ethanol or propanol fuels.
"The big picture is that
carbon dioxide reduction is very important for its effect on global warming as
well as for green chemical synthesis," Wang said. "If the electricity
comes from renewable sources like the sun or wind, we can create a loop that turns
carbon dioxide into something important without emitting more of it."
Co-authors are Rice graduate
student Peng Zhu; graduate student Qiu Jiang and Husam Alshareef, a professor
of material science and engineering, at King Abdullah University of Science and
Technology, Saudi Arabia (KAUST); postdoctoral researcher Ying Pan of Harvard
University; and staff scientist Wentao Liang of Northeastern University. Wang
is the William Marsh Rice Trustee Assistant Professor of Chemical and
Biomolecular Engineering. Xia is a J. Evans Attwell-Welch Postdoctoral Fellow
at Rice.
Rice and the U.S. Department
of Energy Office of Science User Facilities supported the research.
Story Source:
Materials provided by Rice University. Note:
Content may be edited for style and length.
Journal Reference:
Chuan Xia, Peng Zhu, Qiu
Jiang, Ying Pan, Wentao Liang, Eli Stavitsk, Husam N. Alshareef, Haotian
Wang. Continuous production of pure liquid fuel solutions via
electrocatalytic CO2 reduction using solid-electrolyte devices. Nature
Energy, 2019; DOI: 10.1038/s41560-019-0451-x
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