Acid-humidified CO ₂ gas input for stable electrochemical CO ₂ reduction reaction

(Bi)carbonate salt formation has been widely recognized as a primary factor in poor operational stability of the electrochemical carbon dioxide reduction reaction (CO₂RR). We demonstrate that flowing CO₂ gas into an acid bubbler-which carries trace amounts of acid vapor into a gas diffusion electrode for silver-catalyzed CO₂RR to carbon monoxide (CO)-can prevent salt accumulation. In a 100-square-centimeter, scaled-up CO₂RR membrane electrode assembly electrolyzer with single serpentine flow channels, the acid humidification method achieved the 4500 hours of stability milestone at 100 mA cm⁻² without compromising the CO faradaic efficiency, whereas a conventional water-humidified CO₂ feed only operated stably for ~80 hours. The acid-humidified CO₂ approach was extended to bismuth, copper, and zinc catalysts.