(674f) Enhanced CO2 Electroreduction to CH4 and C2H4 Via Selective Proton Transfer

CO₂-derived fuels, synthesized using renewable energy, present an attractive route towards fuel formation from sustainable energy. The role of proton transfer in dictating the efficiency and selectivity of the two-electron, two-proton reduction of CO₂ to CO has been well-studied. Indeed, impressive progress has been achieved in the production of CO from CO₂. However, the role of proton transfer to surface-bound CO intermediates in governing the selective electrosynthesis of higher order products beyond CO, such as methane or ethylene, remains poorly understood.

Here, we investigate the electrochemical reduction of CO at Cu electrodes in nonaqueous electrolytes at low temperatures. The choice of the electrolyte and reaction conditions allows fine-tuned control of the proton donor and the CO binding strength, enabling for the first time activation-controlled kinetic studies over an extended parameter range. Our studies lead to the surprising finding that increasing the concentration of CO serves to suppress methane production, in addition to hydrogen. In contrast, ethylene production remains unaffected by CO concentration and depends predominantly on the electrochemical potential.

These observations provide unprecedented insight into the mechanism of electrochemical CO and CO₂ reduction. Among others, we are able to show that the rate of methane and hydrogen formation is governed by the competition of CO and H for surface sites and that ethylene formation is taking place from sites which are saturated with CO, indicating that simply increasing the supply of CO may not be a viable strategy for improving CO and CO₂ reduction systems. Based on our results, we subsequently demonstrate how knowledge of the reaction mechanism can be exploited to selectively suppress hydrogen evolution relative to CO reduction by rational tuning of the proton donor.