A recirculation system for concentrating CO2 electrolyzer products

Abstract

Electrochemical carbon dioxide reduction represents a promising path to utilize CO₂ as a feedstock for generating valuable products such as fuels and chemicals. Faradaic efficiencies near 100% have been achieved for certain CO₂ reduction products such as CO, but the electrolyzer outlet streams usually contain large fractions of unreacted CO₂, dropping the product concentrations below 1% in many cases. The system disclosed here recycles the unreacted CO₂ together with the products and flows them back into the CO₂ reduction reactor, enabling much higher CO₂ conversion rates without dropping the gas flow rate. However, simple recirculation is shown to accumulate significant amounts of hydrogen, impeding effective CO₂ reduction. In this looped system, an electrochemical H₂ pump is placed in series with the CO₂ reactor, which effectively removes all the H₂ from the recycled gas stream, increasing the concentrations of carbon-containing products. The system was initially tested with a CO-generating catalyst and CO concentrations above 70% were achieved in the recycled gas stream, compared to a maximum CO concentration of 8% in single-pass configuration. Results with a CO₂ reactor targeting ethylene as the main product show that ethylene concentrations of at least 10% can be achieved, which is roughly 20 times higher compared to a single-pass system.