A schematic of a two-layer coating of films on a copper surface. The negatively charged ionomer raises the pH. The positively charged ionomer attracts CO2. The effects of a pulsed voltage combined with a CO2 conversion result in increased rates of carbon-rich products. Berkeley Lab.
Carbon dioxide, a product of burning fossil fuels and the most prevalent greenhouse gas, has the potential to be converted back into useful fuels. The process of reducing CO2 emissions into a fuel is known as electrochemical reduction. To be commercially viable, the process needs to be improved, and a higher amount of desirable carbon-rich products need to be produced.
The researchers at Lawrence Berkeley National Laboratory have improved the process's selectivity by modifying the surface of the copper catalysts used to assist the reaction.
"Although we know copper is the best catalyst for this reaction, it doesn't give high selectivity to the desired products," said Alexis Bell, a faculty senior scientist in Berkeley Lab's Chemical Sciences Division and professor of chemical engineering at UC Berkeley. The group has found that there are tricks you can use to give that selectivity.
In previous studies, the researchers established the best conditions for creating carbon-rich products. Those conditions are different from those that occur in a typical fuel cell, which uses a water-based material.
Bell and his team were part of the Department of Energy's Liquid Sunlight Alliance Energy Innovation Hub project, and they wanted to find a design that could be used in the water environment of fuel cells. They are uniquely suited to have a strong influence on the microenvironment because of their highlyselective chemistry.
The lead author of the paper was a researcher in Bell's group who proposed to coat the copper catalyst with two ionomers. The team believed that the environment should be changed in a way that would steer the reaction to generate carbon-rich products that can be readily converted to useful chemicals and liquid fuels.
The researchers applied a thin layer of each ionomer, as well as a bilayer of both ionomers, to copper films supported by a polymer material, forming a membranes that they could insert near one end of a hand-sized electrochemical cell. They measured the total current flowing through the cell while feeding CO2 and applying a voltage. They measured the gasses and liquids that were collected during the reaction. The two-layer case found that carbon-rich products accounted for 80% of the energy consumed by the reaction.
"This sandwich coating gives the best of both worlds: high product selectivity and high activity," said Bell. The bi-layer surface favored carbon-rich products, but at the same time generated a strong electrical current, indicating increased activity.
The improved reaction was a result of the high CO2 concentration that built up in the coating layer immediately on top of the copper. A low local acidity was created by negatively-charged molecule that piled up in the region between the two ionomers. Concentration trade-off occurs when the ionomer films are not present.
To increase the reaction efficiency even further, the researchers turned to a technique that had been demonstrated before, without ionomer films, as another way to increase CO2 and pH. The researchers achieved a 250% increase in carbon-rich products by using a pulsed voltage with the bilayer ionomer coating.
Catalyst discovery doesn't consider the operating conditions, even though some researchers have focused their work on developing new catalysts. The environment at the catalyst's surface is a new approach.
Adam Weber, a senior staff scientist in Berkeley Lab's Energy Technologies, said that they took what they know about the reaction's speed and used it to guide their thinking about how to change the environment at the catalyst site.
The next step is to make more catalysts. The Berkeley Lab team's initial experiments involved small, flat model systems, which are far simpler to work with than the high-area, porous structures necessary for commercial applications. It's easy to coat a flat surface. Bell said a commercial approach might involve coating tiny copper spheres. Adding a second coating can be difficult. If you mix and deposit the two coating together in a solvent, you can hope that they separate as the solvent goes away. And if they don't? "We'll have to get more clever," Bell said.
Chanyeon Kim and her team have developed catalyst microenvironments for CO2 reduction to multicarbon products on copper using bilayer ionomer coating. There is a DOI titled " 10.1038/s41560-021-00920-8".
Nature Energy journal information.
A new technique improves the conversion of carbon dioxide into liquid fuels.
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