Amid global efforts to curb greenhouse gas emissions, MIT scientists are focusing on carbon capture technology to decarbonize the most challenging industrial emissions. These findings, based on a single electrochemical process, could help reduce emissions from industries hardest to decarbonize, such as steel and cement.

Researchers have revealed how to capture and convert carbon dioxide through a single electrochemical process. In this process, electrodes (like the one covered with bubbles in the picture) are used to absorb the carbon dioxide released from the adsorbent and convert it into a carbon-neutral product. Photo credit: JohnFreidah/MITMechE

Industries such as steel, cement and chemical manufacturing are particularly difficult to decarbonize due to their inherent use of carbon and fossil fuels in their production processes. If technology could be developed to capture carbon emissions and reuse them in the production process, it would be possible to significantly reduce emissions from these "hard-to-abate" industries.

However, current experimental technologies for capturing and converting carbon dioxide are two separate processes that themselves require large amounts of energy to run. The MIT research team hopes to combine the two processes into an integrated, far more energy-efficient system that could potentially use renewable energy to capture and convert carbon dioxide from concentrated industrial resources.

Latest research results on carbon capture and conversion

In a study published September 5 in the journal ACSCatalysis, researchers reveal the hidden power of how to capture and convert carbon dioxide through a single electrochemical process. The process involves using electrodes to absorb carbon dioxide released from an adsorbent and convert it into a reduced, reusable form.

Others have reported similar demonstrations, but the mechanism driving the electrochemical reaction remains unclear. The MIT team conducted numerous experiments to determine this driver and found that, ultimately, it depends on the partial pressure of carbon dioxide. In other words, the purer the carbon dioxide in contact with the electrode, the more efficiently the electrode can capture and convert carbon dioxide molecules.

Understanding this primary driver, or "active species," could help scientists tune and optimize similar electrochemical systems to efficiently capture and convert carbon dioxide in an integrated process.

The results of this study suggest that while these electrochemical systems may not be suitable for very rarefied environments (e.g., capturing and converting carbon emissions directly from the air), they are well suited for the high concentrations of emissions produced by industrial processes, particularly those for which there are no obvious renewable alternatives.

"We can and should switch to renewable energy for electricity generation," said study author Betar Gallant, MIT Career Development Associate Professor 1922. "But deep decarbonization of industries like cement or steel production is challenging and will take longer. Even if we retire all power plants, we will need some solutions to address emissions in other industries in the short term before we can fully decarbonize those industries. That's where we see a sweet spot, and something like this system could fit into that sweet spot."

MIT co-authors of the study include lead author, postdoc Graham Leverick and graduate student Elizabeth Bernhardt, as well as Athea Iliani-Esse of Sunway University in Malaysia. Aisyah Illyani Ismail, Jun Hui Law, Arif Arifutzzaman and Mohamed Kheireddine Aroua.

Learn about the carbon capture process

Carbon capture technology is designed to capture emissions, or "flues," from the smokestacks of power plants and manufacturing facilities. Emissions are directed, primarily through large retrofits, into a chamber containing a "capture" solution (a mixture of amines or amino compounds that chemically combine with carbon dioxide to create a stable form that can be separated from the rest of the flue gas).

The captured carbon dioxide is then treated with high temperatures, often using steam generated from fossil fuels, to release the trapped carbon dioxide from its amine bonds. Pure carbon dioxide gas can be pumped into storage tanks or underground, mineralized or further converted into chemicals or fuels.

"Carbon capture is a mature technology and the chemistry is about 100 years old, but it requires really large installations and is quite expensive and energy-intensive to run," Gallant points out. "What we need is more modular and flexible technology that can adapt to more diverse sources of carbon dioxide. Electrochemical systems can help solve this problem."

Her research group at MIT is developing an electrochemical system that can both recycle captured carbon dioxide and convert it into reduced, usable products. Such an integrated system, rather than a separate one, could be powered entirely by renewable energy rather than steam generated from fossil fuels, she said.

Their concept centers on an electrode that can fit into the cavity of an existing carbon capture solution. When a voltage is applied to the electrode, electrons flow toward the active form of carbon dioxide and are converted into products using protons supplied from the water. This way, the adsorbent can absorb more carbon dioxide instead of using steam to absorb the carbon dioxide.

Gallant has previously shown that this electrochemical process can capture carbon dioxide and convert it into a solid carbonate form. "We showed in very early concepts that this electrochemical process was possible," she said. "Since then, there have been other studies focused on using this process to try to produce useful chemicals and fuels. But there has been inconsistent explanation of how these reactions work."

The role of "carbon dioxide alone"

In the new study, the MIT research team used a magnifying glass to observe the specific reactions that drive electrochemical processes. In the lab, they generated amine solutions similar to industrial capture solutions used to extract carbon dioxide from flue gases. They methodically varied various properties of each solution, such as pH, concentration and type of amine, and then ran each solution through a silver electrode, a metal widely used in electrolysis research and known to efficiently convert carbon dioxide into carbon monoxide. They then measured the concentration of carbon monoxide converted at the end of the reaction and compared that number with that of each of the other solutions they tested to determine which parameter had the greatest impact on the amount of carbon monoxide produced.

In the end, they found that what mattered most wasn't the type of amine used to capture the carbon dioxide in the first place, as many suspected. Rather, what is most important is the concentration of free-floating carbon dioxide molecules in the solution that avoids binding to amines. This "carbon dioxide alone" determines the final concentration of carbon monoxide produced.

"We found that this 'alone' CO2 reacts more readily than CO2 captured by amines," Leverick said. "This tells future researchers that this process is feasible in industrial streams and can efficiently capture high concentrations of CO2 and convert it into useful chemicals and fuels."

"This is not a removal technology, which is important," Gallant emphasizes. "The value it brings is that it allows us to recycle CO2 many times while maintaining existing industrial processes, thereby reducing associated emissions. Ultimately, my dream is to use electrochemical systems to promote the mineralization and permanent storage of CO2, which is a true removal technology. This is a longer-term vision. And a lot of the science we are starting to understand is the first step in designing these processes."