The material could replace rare metals and lead to more economical production of carbon-neutral fuels.
By David L. Chandler, MIT News Office
February 24, 2022 -- An
electrochemical reaction that splits apart water molecules to produce oxygen is
at the heart of multiple approaches aiming to produce alternative fuels for
transportation. But this reaction has to be facilitated by a catalyst material,
and today’s versions require the use of rare and expensive elements such as iridium,
limiting the potential of such fuel production.
Now, researchers at MIT and elsewhere have developed an entirely new type
of catalyst material, called a metal hydroxide-organic framework (MHOF), which
is made of inexpensive and abundant components. The family of materials allows
engineers to precisely tune the catalyst’s structure and composition to the
needs of a particular chemical process, and it can then match or exceed the
performance of conventional, more expensive catalysts.
The findings are described today in the journal Nature Materials,
in a paper by MIT postdoc Shuai Yuan, graduate student Jiayu Peng, Professor
Yang Shao-Horn, Professor Yuriy Román-Leshkov, and nine others.
Oxygen evolution reactions are one of the reactions common to the
electrochemical production of fuels, chemicals, and materials. These processes
include the generation of hydrogen as a byproduct of the oxygen evolution,
which can be used directly as a fuel or undergo chemical reactions to produce
other transportation fuels; the manufacture of ammonia, for use as a fertilizer
or chemical feedstock; and carbon dioxide reduction in order to control
emissions.
But without help, “these reactions are sluggish,” Shao-Horn says. “For a
reaction with slow kinetics, you have to sacrifice voltage or energy to promote
the reaction rate.” Because of the extra energy input required, “the overall
efficiency is low. So that’s why people use catalysts,” she says, as these
materials naturally promote reactions by lowering energy input.
But until now, these catalysts “are all relying on expensive materials or
late transition metals that are very scarce, for example iridium oxide, and
there has been a big effort in the community to find alternatives based on
Earth-abundant materials that have the same performance in terms of activity
and stability,” Román-Leshkov says. The team says they have found materials
that provide exactly that combination of characteristics.
Other teams have explored the use of metal hydroxides, such as
nickel-iron hydroxides, Román-Leshkov says. But such materials have been difficult
to tailor to the requirements of specific applications. Now, though, “the
reason our work is quite exciting and quite relevant is that we’ve found a way
of tailoring the properties by nanostructuring these metal hydroxides in a
unique way.”
The team borrowed from research that has been done on a related class of
compounds known as metal-organic frameworks (MOFs), which are a kind of
crystalline structure made of metal oxide nodes linked together with organic
linker molecules. By replacing the metal oxide in such materials with certain
metal hydroxides, the team found, it became possible to create precisely
tunable materials that also had the necessary stability to be potentially
useful as catalysts.
“You put these chains of these organic linkers next to each other, and
they actually direct the formation of metal hydroxide sheets that are
interconnected with these organic linkers, which are then stacked, and have a
higher stability,” Román-Leshkov says. This has multiple benefits, he says, by
allowing a precise control over the nanostructured patterning, allowing precise
control of the electronic properties of the metal, and also providing greater
stability, enabling them to stand up to long periods of use.
In testing such materials, the researchers found the catalysts’
performance to be “surprising,” Shao-Horn says. “It is comparable to that of
the state-of-the-art oxide materials catalyzing for the oxygen evolution
reaction.”
Being composed largely of nickel and iron, these materials should be at
least 100 times cheaper than existing catalysts, they say, although the team
has not yet done a full economic analysis.
This family of materials “really offers a new space to tune the active
sites for catalyzing water splitting to produce hydrogen with reduced energy
input,” Shao-Horn says, to meet the exact needs of any given chemical process
where such catalysts are needed.
The materials can provide “five times greater tunability” than existing
nickel-based catalysts, Peng says, simply by substituting different metals in
place of nickel in the compound. “This would potentially offer many relevant
avenues for future discoveries.” The materials can also be produced in
extremely thin sheets, which could then be coated onto another material,
further reducing the material costs of such systems.
So far, the materials have been tested in small-scale laboratory test
devices, and the team is now addressing the issues of trying to scale up the
process to commercially relevant scales, which could still take a few years. But
the idea has great potential, Shao-Horn says, to help catalyze the production
of clean, emissions-free hydrogen fuel, so that “we can bring down the cost of
hydrogen from this process while not being constrained by the availability of
precious metals. This is important, because we need hydrogen production
technologies that can scale.”
The research team included others at MIT, Stockholm University in Sweden,
SLAC National Accelerator Laboratory, and Institute of Ion Beam Physics and
Materials Research in Dresden, Germany. The work was supported by the Toyota
Research Institute.
https://news.mit.edu/2022/metal-hydroxide-organic-framework-oxygen-0224
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