Berkeley Lab scientists have developed a digital model to accelerate the optimization of membrane-electrode assemblies to convert CO₂ to fuel and other products. A paper on the work appears in the journal Nature Chemical Engineering.

Carbon dioxide can be transformed into valuable feedstocks such as carbon monoxide and ethylene, which manufacturers use to make products including chemicals and packaging. One way to do this is with membrane-electrode assemblies, which are devices that consist of two electrodes separated by a membrane.

Also used in fuel cells that turn inputs such as hydrogen into electricity, membrane-electrode assemblies hold promise for being able to use surplus renewable power to run reaction sequences that catalyze carbon dioxide into other chemicals. However, these devices have problems with efficiency, and their workings are not yet fully understood.

Membrane-electrode assemblies are complicated systems with multiple layers. Each layer holds different chemical species, additives, and particles. Often, we don’t really know why experiments with membrane-electrode assemblies produce certain products, or why they fail to convert a larger percentage of a given amount of carbon dioxide.

—Adam Weber, a senior scientist at Berkeley Lab and corresponding author of the study

Computer modeling can help predict which device parameters will produce the best results, but they tend to be less accurate at anticipating issues such as crossover, which is when carbon dioxide moves across the membrane instead of reacting. To improve model accuracy, the researchers turned to Marcus–Hush–Chidsey kinetics, a theory that previously had not been integrated into membrane-electrode assembly modeling and is shown to be critical for understanding the reaction mechanism.

The researchers validated their model against experimental data, finding that it did a better job of predicting real-world outcomes than previous models. Among other advantages, the use of Marcus–Hush–Chidsey kinetics made it possible to account for the role of water orientation.

The team ran virtual experiments with its model to explore how different membrane-electrode assembly designs performed in terms of carbon-dioxide utilization and selectivity for desired products.

Some of the variables the team tested virtually included catalyst-layer thickness and catalyst-specific surface area. They also uncovered design rules around the importance of coupled ion and water transport, as well as tradeoffs between transport phenomena and reaction and buffer kinetics. All of these change the overall energy efficiency, products obtained, and amount of carbon dioxide converted.

Having a digital twin of a system allows you to probe a much larger parameter space much more rapidly than in experiments, which are typically complex and require special equipment. We can’t see where every molecule is in an experiment. But in a model, we can.

—Adam Weber

Weber said the next step in the research is to increase the model’s complexity to be able to look at performance over a membrane-electrode assembly’s lifetime, among other variables.

This research was supported in part by the Department of Energy’s Bioenergy Technologies Office and Berkeley Lab’s Laboratory Directed Research and Development Grant.

Resources

• Lees, E.W., Bui, J.C., Romiluyi, O. et al. Exploring CO₂ reduction and crossover in membrane electrode assemblies. Nat Chem Eng 1, 340–353 (2024). doi: 10.1038/s44286-024-00062-0