Electrochemical Carbon Monoxide Reduction on Polycrystalline Copper: Effects of Potential, Pressure, and pH on Selectivity toward Multicarbon and Oxygenated Products

Understandingthe surface reactivity of CO, which is a key intermediateduring electrochemical CO₂ reduction, is crucial for thedevelopment of catalysts that selectively target desired productsfor the conversion of CO₂ to fuels and chemicals. In thisstudy, a custom-designed electrochemical cell is utilized to investigateplanar polycrystalline copper as an electrocatalyst for CO reductionunder alkaline conditions. Seven major CO reduction products havebeen observed including various hydrocarbons and oxygenates whichare also common CO₂ reduction products, strongly indicatingthat CO is a key reaction intermediate for these further-reduced products.A comparison of CO and CO₂ reduction demonstrates thatthere is a large decrease in the overpotential for C–C coupledproducts under CO reduction conditions. The effects of CO partialpressure and electrolyte pH are investigated; we conclude that theaforementioned large potential shift is primarily a pH effect. Thus,alkaline conditions can be used to increase the energy efficiencyof CO and CO₂ reduction to C–C coupled products,when these cathode reactions are coupled to the oxygen evolution reactionat the anode. Further analysis of the reaction products reveals commontrends in selectivity that indicate both the production of oxygenatesand C–C coupled products are favored at lower overpotentials.These selectivity trends are generalized by comparing the resultson planar Cu to current state-of-the-art high-surface-area Cu catalysts,which are able to achieve high oxygenate selectivity by operatingat the same geometric current density at lower overpotentials. Combined,these findings outline key principles for designing CO and CO₂ electrolyzers that are able to produce valuable C–Ccoupled products with high energy efficiency.