Oxygen-Resistant CO₂ Reduction Enabled by Electrolysis of Liquid Feedstocks

Electrolytic CO₂ reduction fails in the presence of O₂. This failure occurs because the reduction of O₂ is thermodynamically favored over the reduction of CO₂. Consequently, O₂ must be removed from the CO₂ feed prior to entering an electrolyzer, which is expensive. Here, we show that the use of liquid bicarbonate feedstocks (e.g., aqueous 3.0 M KHCO₃), rather than gaseous CO₂ feedstocks, enables efficient and selective CO₂ reduction without additional procedures for removing O₂. This effect is made possible because liquid bicarbonate solutions, which serve as a liquid CO₂ carrier, deliver high concentrations of captured CO₂ to the cathode, while the low solubility of O₂ in aqueous media maintains a low O₂ concentration at the same cathode surface. Consequently, electrolyzers fed with liquid bicarbonate feedstocks create an environment at the cathode that favors the reduction of CO₂ over O₂. We validate this claim by electrochemically converting CO₂ into CO with reaction selectivities of 65% at 100 mA cm^-2 using a 3.0 M KHCO₃ solution bubbled with 100% CO₂ or 100% O₂. Similar experiments performed with a gaseous CO₂ feedstock showed that merely 0.5% of O₂ in the feedstock reduced CO selectivity by >90% after 1 h of electrolysis. Our findings demonstrate that a liquid bicarbonate feedstock enables efficient CO₂ reduction without the need for expensive O₂ removal steps.