Maximizing single-pass conversion does not result in practical readiness for CO2 reduction electrolyzers

To integrate electrochemical CO2 reduction into the chemicals industry at scale, the outlet streams from CO2 electrolyzers must be product rich. Single-pass conversion is becoming increasingly common as a performance benchmark for CO2 electrolyzers because it suggests concentrated products and reduced separation energy for many catalytic processes. However, our analysis shows that CO2R reactor configurations that maximize single-pass conversion currently suffer from low product concentration in the outlet stream. This is because they restrict CO2 flow or operate in acidic cathode environments, which promote considerable hydrogen evolution as a side reaction. For any gas products besides syngas, high single-pass conversion does not signify that separation energy losses have been eliminated, or that product streams are directly suitable as feedstocks for downstream processes. We therefore recommend that researchers targeting CO2R scaleup report product concentrations rather than relying on single-pass conversion as an indicator of overall performance. Maximizing product concentration is a more meaningful target than maximizing single-pass conversion.