• Efforts to convert carbon dioxide into fuels or other products have been largely unsuccessful.
  • Information technology's predicted that such processes could make a major dent in greenhouse gas emissions.
  • Now, researchers at MIT take identified, quantified and modelled a major reason for poor performance in such conversion systems.
  • And they offer solutions for how information technology could be successfully converted going forward.

If researchers could notice a way to chemically convert carbon dioxide into fuels or other products, they might make a major dent in greenhouse gas emissions. Only many such processes that have seemed promising in the lab haven't performed as expected in scaled-upward formats that would be suitable for use with a power found or other emissions sources.

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.

MIT researchers accept identified a problem that tends to limit chemic processes for turning carbon dioxide into fuel or other useful chemicals.

Image: Varanasi Lab

Now, researchers at MIT have identified, quantified and modelled a major reason for poor performance in such conversion systems. The culprit turns out to exist a local depletion of the carbon dioxide gas right adjacent to the electrodes existence used to catalyze the conversion. The problem can exist alleviated, the team found, by simply pulsing the current off and on at specific intervals, allowing time for the gas to build support 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 Álvaro Moreno Soto, graduate student Jack Lake, and professor of mechanical engineering science Kripa Varanasi.

CO2 conversion comes with challenges

"Carbon dioxide mitigation is, I think, one of the important challenges of our time," Varanasi says. While much of the research in the expanse 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 equally limestone, another promising artery has been converting the gas into other carbon compounds such as methane or ethanol, to exist used every bit fuel, or ethylene, which serves equally a forerunner 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 loftier-value chemical products, which is a challenge with the lite-activated processes as well, he says. "Efficiency is e'er at play, always an upshot."

The team has focused on the electrochemical approaches, with a goal of getting "higher-C products" - compounds that comprise more than carbon atoms and tend to be higher-value fuels considering of their energy per weight or book. In these reactions, the biggest challenge has been curbing competing reactions that tin take place at the same time, especially the splitting of water molecules into oxygen and hydrogen.

The reactions have identify as a stream of liquid electrolyte with the carbon dioxide dissolved in it passes over a metal catalytic surface that is electrically charged. Only every bit 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 upwards energy and greatly reduces the overall efficiency of the conversion process, the researchers constitute.

"There'due south a number of groups working on this, and a number of catalysts that are out at that place," Varanasi says. "In all of these, I think the hydrogen co-development becomes a bottleneck."

1 manner of counteracting this depletion, they institute, tin can be accomplished by a pulsed arrangement - a bicycle 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 and so resuming the reaction.

Often, the researchers say, groups accept found promising catalyst materials but haven't run their lab tests long enough to observe these depletion effects, and thus take been frustrated in trying to scale up their systems. Furthermore, the concentration of carbon dioxide side by side to the catalyst dictates the products that are made. Hence, depletion tin likewise change the mix of products that are produced and can brand the process unreliable. "If you want to exist able to make a system that works at industrial scale, you need to exist able to run things over a long period of time," Varanasi says, "and you demand to not have these kinds of effects that reduce the efficiency or reliability of the process."

Understanding carbon dioxide depletion

The squad studied three different catalyst materials, including copper, and "we really focused on making sure that we understood and tin can quantify the depletion effects," Lake says. In the process they were able to develop a uncomplicated 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 than sophisticated analytical tools to characterize reaction products, including gas chromatography for assay of the gaseous products, and nuclear magnetic resonance label for the organization's liquid products. But their analysis showed that the simple pH measurement of the electrolyte next to the electrode during functioning could provide a sufficient measure of the efficiency of the reaction as it progressed.

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

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

The inquiry was supported by Trounce, through the MIT Energy Initiative.