A promising way to transform CO2 emissions in a fuel feedstock are the process of electrochemical reduction (eg, previous post). However, to be commercially viable, the process must be improved to produce a greater amount of desirable carbon rich products.
Now, as the newspaper reports Natural energy, researchers from the Lawrence Berkeley National Laboratory (Berkeley Lab) have improved the selectivity of the process by developing a new approach to modify the surface area of copper catalysts used to assist the reaction.
Although we know that copper is the best catalyst for this reaction, it does not give high selectivity to the desired products. Our group has found that you can do various tricks with the local environment of the catalyst to provide this selectivity.
—Alexis Bell, Principal Investigator in the Chemical Sciences Division at the Berkeley Lab and Professor of Chemical Engineering at UC Berkeley
In previous studies, researchers had established the precise conditions that gave the best electrical and chemical environment to create commercially interesting carbon-rich products. But these conditions are contrary to those that occur naturally in a typical fuel cell, which uses a water-based conductive material.
To identify a design that could be used in the aqueous environment of fuel cells, Bell and his team, as part of the Department of Energy Liquid Sunlight Alliance Energy Innovation Hub (LiSA) project, focused on thin layers of ionomers, polymers which allow certain charged molecules (ions) to pass while excluding others. Due to their highly selective chemistry, they are particularly well suited to have a strong influence on the microenvironment.
Schematic of a two-layer coating of films, called ionomers, on a copper surface. The negatively charged ionomer, Nafion, increases the pH near the surface. The positively charged ionomer, Sustainion, attracts CO more strongly2. These effects combined with a pulsed voltage result in significantly improved CO levels.2 conversion to carbon-rich products. (Credit: Berkeley Lab)
Chanyeon Kim, postdoctoral researcher in Bell’s group and lead author of the paper, proposed to coat the surface of the copper catalyst with two common ionomers, Nafion and Sustainion. In doing so, the team hypothesized that it would have to alter the environment, including the pH and the amounts of water and CO.2– in the immediate vicinity of the catalyst in a manner which would orient the reaction to generate carbon-rich products which can be easily converted into useful chemicals and liquid fuels.
The researchers applied a thin layer of each ionomer, as well as a bilayer of the two ionomers, to films of copper supported by a polymeric material, forming membranes that they could insert near one end of an electrochemical cell. the size of a hand. During CO feed2 in the cell and by applying a voltage, they measured the total current flowing through the cell. Next, they measured the gases and liquids that accumulated in adjacent tanks during the reaction. For the two-layer case, they found that the carbon-rich products accounted for 80% of the energy consumed by the reaction, compared to 60% in the uncoated situation.
This sandwich coating offers the best of both worlds: high product selectivity and high activity.
The bilayer surface not only favored carbon-rich products, but at the same time generated a strong electric current, indicating increased activity.
The researchers concluded that the improved reaction was a consequence of the high CO content2 concentration that has accumulated in the coating layer immediately above the copper. In addition, negatively charged molecules that accumulated in the region between the two ionomers created weak local acidity. This combination countered the concentration compromise which tends to occur in the absence of the ionomer films.
To further increase the efficiency of the reaction, the researchers turned to a technique that had previously been demonstrated without ionomer films as another way to increase CO2 and pH: voltage pulsation. Using pulsed voltage with the ionomer bilayer coating, researchers achieved a 250% increase in carbon-rich products over uncoated copper and at static voltage.
While some researchers have focused their work on developing new catalysts, the discovery of catalysts does not take operating conditions into account. Controlling the environment on the catalyst surface is a new and different approach.
Rather than coming up with a brand new catalyst, we took what we know about the kinetics of a reaction and used that knowledge to guide our thinking on how to change the environment at the catalyst site.
—Adam Weber, senior energy technology scientist at the Berkeley Lab and co-author
The next step is to increase the production of the coated catalyst. The early experiments of the Berkeley Lab team involved small, flat model systems, which are much easier to work with compared to the large-area porous structures required for commercial applications.
While coating a flat surface is not difficult, a commercial approach could involve coating tiny copper spheres, Bell noted. Adding a second coating becomes difficult. One possibility is to mix and deposit the two coatings together in a solvent and expect them to separate as the solvent evaporates.
This work was supported by the DOE Office of Science.
Kim, C., Bui, JC, Luo, X. et al. (2021) “Tailor-made catalytic microenvironments for CO2 electro-reduction in multicarbon products on copper using ionomer bilayer coatings. Nat Energy 6, 1026-1034 doi: 10.1038 / s41560-021-00920-8