Rice University – September 3, 2019 -- 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.
No comments:
Post a Comment