The carbon dioxide reduction reaction (CO2RR) presents the opportunity to consume CO2 and produce desirable products. However, the alkaline conditions required for productive CO2RR result in the bulk of input CO2 being lost to bicarbonate and carbonate. This loss imposes a 25% limit on the conversion of CO2 to multicarbon (C2+) products for systems that use anions as the charge carrier—and overcoming this limit is a challenge of singular importance to the field. Recently, a research group from Canada published a paper in ACS Energy Letters (ACS Energy Lett. 2021, 6, 2952−2959; https://doi.org/10.1021/acsenergylett.1c01122) where they developed a strategy to address the issue and improve the CO2 conversion rate.
First, the researchers applied a cation exchange membrane (CEM) at the place of a Sustainion anion exchange membrane (AEM) in the electrolyzer to block CO2 loss due to the formation of bicarbonate and carbonate. Then, the research group developed a permeable CO2 regeneration layer (PCRL) on the cathode surface by applying an anion exchange polymer coating on top of the Cu catalyst layer. The thickness of the PCRL is less than 10 µm. The functional groups of the anion exchange polymer create a positive space charge, enabling the transport of anions and impeding the transport of cations through the PCRL. The polymer coating on the cathode allows for CO2 transport to the catalyst via diffusion through the water-filled hydrated ionic domains in the polymer matrix. The thin layer also minimizes the obstruction of water and CO2 from the membrane junction to the catalyst surface. With the PCRL, the local alkaline environment is maintained, and CO2 lost to bicarbonate and carbonate is regenerated locally. The electrolyzer with the PCRL coupled with a CEM (Nafion® 117) in an electrolyzer achieved high CO2 conversion efficiency of 85% when combined with proper CO2 flow rate. It was also proved that the high CO2 conversion achieved with the PCRL approach does not come at the cost of other performance metrics. The cell voltage and faradaic efficiency with the PCTL are similar to those achieved with the conventional AEM cell tested with the same electrodes. It is believed that the approach provides a solution to the basic problem limiting the field of electrocatalytic conversion of CO2 to multicarbon products.