The results, based on a single electrochemical process, could help reduce emissions from the hardest-to-decarbonize industries, such as steel and cement.
In the effort to curb global greenhouse gas emissions around the world, scientists have wood MY focuses on carbon capture technologies to decarbonize the most challenging industrial emitters.
Industries such as steel, cement and chemical manufacturing are particularly difficult to decarbonise due to the inherent use of carbon and fossil fuels in their processes. If technologies can be developed to capture CO2 emissions and reuse them in the production process, this could lead to a significant reduction in emissions from these “hard to reduce” sectors.
But the current experimental technologies that capture and convert carbon dioxide do so as two separate processes that themselves require a huge amount of energy to run. The MIT team seeks to combine the two processes into one integrated and far more energy-efficient system that could potentially run on renewable energy to both capture and convert carbon dioxide from concentrated, industrial sources.
Recent findings on carbon capture and conversion
In a study published Sept. 5 in the journal ACS catalysis, the researchers reveal the hidden function of how carbon dioxide can be both captured and converted through a single electrochemical process. The process involves using an electrode to attract carbon dioxide released from a sorbent and to convert it into a reduced, reusable form.
Others have reported similar demonstrations, but the mechanisms driving the electrochemical reaction have remained unclear. The MIT team performed extensive experiments to determine that driver, and found that it ultimately came down to the partial pressure of carbon dioxide. In other words, the more pure carbon dioxide that comes into contact with the electrode, the more efficiently the electrode can capture and convert the molecule.
Knowledge of this main driver, or “active species“, can help researchers tune and optimize similar electrochemical systems to efficiently capture and convert carbon dioxide in an integrated process.
The results of the study suggest that although these electrochemical systems would probably not work in very dilute environments (for example, to capture and convert carbon emissions directly from the air), they would be suitable for the highly concentrated emissions generated by industrial processes, especially those that that has no obvious sustainable alternative.
“We can and should switch to renewable energy for electricity production. But deeply decarbonizing industries like cement or steel production are challenging and will take longer,” says study author Betar Gallant, Class of 1922 Career Development Associate Professor at MIT. “Even if we get rid of all our power plants, we need some solutions to deal with the emissions from other industries in the shorter term before we can fully decarbonize them. That’s where we see a sweet spot where things like this system could fit in.”
The study’s MIT co-authors are lead author and postdoc Graham Leverick and graduate student Elizabeth Bernhardt, along with Aisyah Illyani Ismail, Jun Hui Law, Arif Arifutzzaman and Mohamed Kheireddine Aroua from Sunway University in Malaysia.
Understanding the carbon sequestration process
Carbon capture technologies are designed to capture emissions, or “flue gas,” from the smokestacks of power plants and manufacturing facilities. This is done primarily by using large retrofits to direct emissions into chambers filled with a “capture” solution – a mixture of amines or ammonia-based compounds that chemically bind with carbon dioxide, producing a stable form that can be separated from the rest of the the flue gas.
High temperatures are then applied, typically in the form of fossil fuel-generated steam, to release the captured carbon dioxide from its amine bond. In its pure form, the gas can then be pumped into storage tanks or underground, mineralized or further converted into chemicals or fuels.
“Carbon capture is a mature technology in that the chemistry has been known for about 100 years, but it requires really large installations and is quite expensive and energy intensive to run,” notes Gallant. “What we want are technologies that are more modular and flexible and can be adapted to more diverse sources of carbon dioxide. Electrochemical systems can help solve that.”
Her group at MIT is developing an electrochemical system that both recovers the captured carbon dioxide and converts it into a reduced, usable product. Such an integrated system, rather than a decoupled one, she says, could be entirely powered by renewable electricity rather than fossil fuel-derived steam.
Their concept centers around an electrode that fits into existing chambers with carbon capture solutions. When a voltage is applied to the electrode, electrons flow into the reactive form of carbon dioxide and convert it into a product with the help of protons supplied from water. This makes the sorbent available to bind more carbon dioxide instead of using steam to do the same.
Gallant previously demonstrated that this electrochemical process could work to capture and convert carbon dioxide into a solid carbonate form.
“We showed that this electrochemical process was feasible in very early concepts,” she says. “Since then, there have been other studies focused on using this process to try to produce useful chemicals and fuels. But there have been inconsistent explanations of how these reactions work, under the hood.”
The role of ‘Solo CO2’
In the new study, the MIT team took a magnifying glass under the hood to read the specific reactions that drive the electrochemical process. In the lab, they generated amine solutions similar to the industrial capture solutions used to extract carbon dioxide from flue gas. They methodically altered various properties of each solution, such as pH, concentration and type of amine, and then ran each solution past an electrode made of silver—a metal widely used in electrolysis studies and known to efficiently convert carbon dioxide to carbon monoxide. They then measured the concentration of carbon monoxide converted at the end of the reaction and compared that number to that of every other solution they tested to see which parameter had the most influence on how much carbon monoxide was converted produced.
In the end, they found that what mattered most was not the type of amine originally used to trap carbon dioxide, as many have suspected. Instead, it was the concentration of solo, free-floating carbon dioxide molecules that avoided binding with amines but were still present in the solution. This “solo-CO2” determined the concentration of carbon monoxide that was ultimately produced.
“We found that it is easier to react this ‘solo’ CO2 compared to CO2 that has been trapped by the amine,” says Leverick. “This tells future researchers that this process could be feasible for industrial streams where high concentrations of carbon dioxide could be efficiently captured and converted into useful chemicals and fuels.”
“This is not a removal technology, and it’s important to say that,” Gallant emphasizes. “The value it provides is that it allows us to recycle carbon dioxide a number of times, while maintaining existing industrial processes, for fewer associated emissions. Ultimately, my dream is that electrochemical systems can be used to facilitate mineralization and permanent storage of CO2—a true removal technology. That’s a long-term vision. And much of the science we’re beginning to understand is a first step toward designing those processes.”
Reference: “Uncovering the active species in amine-mediated CO2 Reduction to CO on Ag” by Graham Leverick, Elizabeth M. Bernhardt, Aisyah Ilyani Ismail, Jun Hui Law, A. Arifutzzaman, Mohamed Kheireddine Aroua and Betar M. Gallant*, 5 September 2023, ACS catalysis.
This research is supported by Sunway University in Malaysia.