Overcoming a bottleneck in carbon dioxide conversion

Date:
January 11, 2022
Source:
Massachusetts Institute of Technology
Summary:
A new study reveals why some attempts to convert carbon dioxide
into fuel have failed, and offers possible solutions.



FULL STORY ==========================================================================
If researchers could find a way to chemically convert carbon dioxide
into fuels or other products, they might make a major dent in greenhouse
gas emissions.

But many such processes that have seemed promising in the lab haven't
performed as expected in scaled-up formats that would be suitable for
use with a power plant or other emissions sources.


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Now, researchers at MIT have identified, quantified, and modeled a major
reason for poor performance in such conversion systems. The culprit
turns out to be a local depletion of the carbon dioxide gas right next
to the electrodes being used to catalyze the conversion. The problem
can be alleviated, the team found, by simply pulsing the current off
and on at specific intervals, allowing time for the gas to build back
up to the needed levels next to the electrode.

The findings, which could spur progress on developing a variety of
materials and designs for electrochemical carbon dioxide conversion
systems, were published today in the journal Langmuir, in a paper by MIT postdoc A'lvaro Moreno Soto, graduate student Jack Lake, and professor
of mechanical engineering Kripa Varanasi.

"Carbon dioxide mitigation is, I think, one of the important challenges
of our time," Varanasi says. While much of the research in the area has
focused on carbon capture and sequestration, in which the gas is pumped
into some kind of deep underground reservoir or converted to an inert
solid such as limestone, another promising avenue has been converting
the gas into other carbon compounds such as methane or ethanol, to be
used as fuel, or ethylene, which serves as a precursor to useful polymers.

There are several ways to do such conversions, including electrochemical, thermocatalytic, photothermal, or photochemical processes. "Each of these
has problems or challenges," Varanasi says. The thermal processes require
very high temperature, and they don't produce very high-value chemical products, which is a challenge with the light-activated processes
as well, he says. "Efficiency is always at play, always an issue."
The team has focused on the electrochemical approaches, with a goal of
getting "higher-C products" -- compounds that contain more carbon atoms
and tend to be higher-value fuels because of their energy per weight
or volume. In these reactions, the biggest challenge has been curbing
competing reactions that can take place at the same time, especially
the splitting of water molecules into oxygen and hydrogen.



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The reactions take place as a stream of liquid electrolyte with
the carbon dioxide dissolved in it passes over a metal catalytic
surface that is electrically charged. But as the carbon dioxide gets
converted, it leaves behind a region in the electrolyte stream where it
has essentially been used up, and so the reaction within this depleted
zone turns toward water splitting instead. This unwanted reaction uses
up energy and greatly reduces the overall efficiency of the conversion
process, the researchers found.

"There's a number of groups working on this, and a number of catalysts
that are out there," Varanasi says. "In all of these, I think the
hydrogen co-evolution becomes a bottleneck." One way of counteracting
this depletion, they found, can be achieved by a pulsed system --
a cycle of simply turning off the voltage, stopping the reaction and
giving the carbon dioxide time to spread back into the depleted zone
and reach usable levels again, and then resuming the reaction.

Often, the researchers say, groups have found promising catalyst
materials but haven't run their lab tests long enough to observe these depletion effects, and thus have been frustrated in trying to scale up
their systems. Furthermore, the concentration of carbon dioxide next
to the catalyst dictates the products that are made. Hence, depletion
can also change the mix of products that are produced and can make
the process unreliable. "If you want to be able to make a system that
works at industrial scale, you need to be able to run things over a long
period of time," Varanasi says, "and you need to not have these kinds
of effects that reduce the efficiency or reliability of the process."
The team studied three different catalyst materials, including copper,
and "we really focused on making sure that we understood and can quantify
the depletion effects," Lake says. In the process they were able to
develop a simple and reliable way of monitoring the efficiency of the conversion process as it happens, by measuring the changing pH levels,
a measure of acidity, in the system's electrolyte.



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In their tests, they used more sophisticated analytical tools to
characterize reaction products, including gas chromatography for analysis
of the gaseous products, and nuclear magnetic resonance characterization
for the system's liquid products. But their analysis showed that the
simple pH measurement of the electrolyte next to the electrode during
operation could provide a sufficient measure of the efficiency of the
reaction as it progressed.

This ability to easily monitor the reaction in real-time could ultimately
lead to a system optimized by machine-learning methods, controlling the production rate of the desired compounds through continuous feedback,
Moreno Soto says.

Now that the process is understood and quantified, other approaches
to mitigating the carbon dioxide depletion might be developed, the
researchers say, and could easily be tested using their methods.

This work shows, Lake says, that "no matter what your catalyst material
is" in such an electrocatalytic system, "you'll be affected by this
problem." And now, by using the model they developed, it's possible to determine exactly what kind of time window needs to be evaluated to get
an accurate sense of the material's overall efficiency and what kind of
system operations could maximize its effectiveness.

The research was supported by Shell, through the MIT Energy Initiative.

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by David
L. Chandler. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. A'lvaro Moreno Soto, Jack R. Lake, Kripa K. Varanasi. Transient
Effects
Caused by Gas Depletion during Carbon Dioxide
Electroreduction. Langmuir, 2022; DOI: 10.1021/acs.langmuir.1c02540 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220111111959.htm

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