Researchers at MIT have identified, quantified, and modeled a major reason for the poor performance of electroreduction processes to convert CO₂ to fuel or other useful chemicals. The findings could spur progress on developing a variety of materials and designs for electrochemical carbon dioxide conversion systems. A paper on their work is published in the ACS journal Langmuir.

The novel use of carbon dioxide (CO₂) electroreduction to generate carbon-based products which do not contribute to the greenhouse effect has promoted the vision of carbon dioxide as a renewable feedstock for future clean fuel production. Depending on the material choice for the electrocatalysis, a certain variety of products is expected from the carbon dioxide reduction reaction (CO₂RR). However, as the CO₂ concentration in areas close to the working electrode (relative to the diffusive boundary layer) decreases as it is being consumed and transformed into other products, the generation of H₂ is favored to the detriment of CO₂ electroreduction.

Therefore, the extent to which H₂ is produced can be used as a metric to evaluate the efficiency of CO₂RR. This article proposes a model that accounts for the modes in which aqueous gas depletion evolves over time and affects the long-term CO₂ electroreduction and the corresponding pH evolution near the electrode’s surface. For the latter, two main contributions are distinguished: gas depletion due to CO₂ consumption and ion generation in areas close to the electrocatalyst surface. pH is then suggested as an accurate and indirect means to measure CO₂ concentration in a liquid electrolyte.

We conclude that CO₂ depletion causes a strong decay in the electrochemical reaction efficiency. In the end, we discuss several methods which may delay the onset of the adverse effects caused by gas depletion, such as the utilization of pulsed electroreduction, cycling the applied current to electrodes on and off periodically.

—Soto et al.

MIT researchers have identified a problem that tends to limit chemical processes for turning carbon dioxide into fuel or other useful chemicals and ways of addressing that problem. Credits: Image: Courtesy of the Varanasi Lab

While much of the research in the area of carbon dioxide mitigation has focused on carbon capture and sequestration, 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 with their own problems or challenges. 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, says co-corresponding author Kripa Varanasi. “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.

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. In all of these, I think the hydrogen co-evolution becomes a bottleneck.

—Kripa Varanasi

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, and you need to not have these kinds of effects that reduce the efficiency or reliability of the process.

—Kripa Varanasi

The team studied three different catalyst materials, including copper, and focused on making sure that it understood and can quantify the depletion effects. In the process, the researchers 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.

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. 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, co-corresponding author 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, co-author Jack Lake says, that “no matter what your catalyst material is” in such an electrocatalytic system, “you’ll be affected by this problem.” By using the model they developed, it’s now 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.

Resources

• Álvaro Moreno Soto, Jack R. Lake, and Kripa K. Varanasi (2022) “Transient Effects Caused by Gas Depletion during Carbon Dioxide Electroreduction” Langmuir doi: 10.1021/acs.langmuir.1c02540