Tech Briefs Magazine - February 2023 - 39

peppered with nanoscale balls of metal,
1/2000th of a millimeter across, that use
those electrons and holes to help direct
the reaction.
A simple insulating layer atop the panel
keeps the temperature at a toasty
75 °C, or 167 °F, warm enough to help
encourage the reaction while also being
cool enough for the semiconductor catalyst
to perform well. The outdoor version
of the experiment, with less reliable
sunlight and temperature, achieved 6.1
percent efficiency at turning the energy
from the sun into hydrogen fuel. However,
indoors, the system achieved 9 percent
efficiency.
The next challenges the team intends
to tackle are to further improve the efficiency
and to achieve ultrahigh purity
hydrogen that can be directly fed into
fuel cells.
For more information, contact Kate
McAlpine at kmca@umich.edu; 734-6477087.
Scrubbing
Carbon Dioxide from Power Plant Smokestacks
An easily synthesized chemical filter could stop the greenhouse gas from reaching the atmosphere.
National Institute of Standards and Technology, Gaithersburg, MD
H
ow can we remove carbon dioxide, a
greenhouse gas, from fossil-fuel power
plant exhaust before it ever reaches
the atmosphere? New findings suggest a
promising answer lies in a simple, economical,
and potentially reusable material
analyzed at the National Institute
of Standards and Technology (NIST),
where scientists from several institutions
have determined why this material works
as well as it does.
The team's object of study is aluminum
formate, one of a class of substances
called metal-organic frameworks
(MOFs). As a group, MOFs have exhibited
great potential for filtering and
separating organic materials - often
the various hydrocarbons in fossil fuels
- from one another. Some MOFs have
shown promise at refining natural gas
or separating the octane components
of gasoline; others might contribute
to reducing the cost of plastics manufacturing
or cheaply converting one
substance to another. Their capacity to
perform such separations comes from
their inherently porous nature.
Aluminum formate (ALF) has a talent
for separating carbon dioxide from the
other gases that commonly fly out of the
smokestacks of coal-fired power plants.
It also lacks the shortcomings that other
proposed carbon filtration materials
have, said NIST's Hayden Evans, one of
the lead authors of the team's research
paper, published in the journal Science
Advances.
" What makes this work exciting is
adsorbents,
that ALF performs really well relative to
other high-performing CO2
but it rivals designer compounds in its
simplicity, overall stability and ease of
preparation, " said Evans, a Chemist at
the NIST Center for Neutron Research.
" It is made of two substances found easily
and abundantly, so creating enough ALF
Tech Briefs, February 2023
Exhaust from coal-fired power plants (left) contains large quantities of the greenhouse gas carbon
dioxide (purple tripartite molecules). Aluminum formate, a metal-organic framework whose structure
is highlighted at right, can selectively capture carbon dioxide from dried flue gas conditions,
potentially at a fraction of the cost of using other carbon filtration materials. (Image: B. Hayes/NIST)
to use widely should be possible at very
low cost. "
Scrubbing the CO2 from flue gas before
it reaches the atmosphere in the
first place is a logical approach, but it has
proved challenging to create an effective
scrubber. The mixture of gases that flows
up the smokestacks of coal-fired power
plants is typically hot, humid, and corrosive
- characteristics that have made it
difficult to find an economical material
that can do the job efficiently.
ALF is made from aluminum hydroxide
and formic acid, two chemicals that
are abundant and readily available on
the market. It would cost less than a dollar
per kilogram, Evans said, which is up
to 100 times less expensive than other
materials with similar performance. Low
cost is important because carbon capture
at a single plant could require up
to tens of thousands of tons of filtration
material. The amount needed for the
entire world would be enormous.
On a microscopic scale, ALF resembles
a three-dimensional wire cage with
innumerable small holes. These holes
are just large enough to allow CO2
molwww.techbriefs.com
ecules
to enter and get trapped, but just
small enough to exclude the slightly larger
nitrogen molecules that make up the
majority of flue gas. Neutron diffraction
work at the NCNR showed the team how
the individual cages in the material collect
and fill with CO2
, revealing that the
gas molecules fit inside certain cages within
ALF like a hand in a glove, Evans said.
Despite its potential, ALF is not ready
for immediate use. Engineers would need
to design a procedure to create ALF at
large scales. A coal-fired plant would also
need a compatible process to reduce the
humidity of the flue gas before scrubbing
it. Evans said that a great deal is already
understood about how to address these
issues, and that they would not make the
cost of using ALF prohibitive.
The research team includes scientists
from the National University of Singapore;
Singapore's Agency for Science,
Technology and Research; the University
of Delaware; and the University of California,
Santa Barbara.
For more information, contact Chad
Boutin at charles.boutin@nist.gov; 301975-4261.
39

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