Probing the Impact of Interfacial Electric Fields and Hydrogen-Bonding Interactions on the Electroreduction of CO to Ethylene

The electrochemical reduction of carbon dioxide to hydrocarbons on Cu electrodes could potentially provide sustainable pathways to renewable fuels and valuable commodity chemicals, such as ethylene. However, the poor product selectivity of this process hinders the adoption of this promising technology. It is well-known that the electrolyte can profoundly impact the selectivity of electrocatalytic carbon dioxide reduction. However, the molecular-level origins of this dependence are still poorly understood. A better understanding of how surface intermediates interact with their liquid reaction environment is essential for improving the product selectivity of this process. Herein, we employed differential electrochemical mass spectrometry (DEMS) and surface-enhanced infrared absorption spectroscopy (SEIRAS) to dissect how the interfacial properties of the Cu/electrolyte contact impact the catalytic selectivity of the reduction of surface-adsorbed CO, a key intermediate in the reduction of carbon dioxide to ethylene. Specifically, we employed a series of four quaternary alkyl ammonium cations (methyl4N+, ethyl4N+, propyl4N+, and butyl4N+) to systematically tune the properties of the electrochemical double layer. Using DEMS, we found that ethylene is only produced in the presence of methyl4N+ and ethyl4N+. However, ethylene is not formed in the presence of propyl4N+ and butyl4N+. Using SEIRAS, we characterized the Cu/electrolyte interface in the presence of the different cations. We found that, irrespective of the cation of the electrolyte, approximately the same CO surface-coverage is reached, suggesting that the cations do not block CO-adsorption sites. Further, we extracted the electrochemical Stark tuning rates from the dependence of the CO stretch frequency on applied potential. Based on these rates, we have derived the interfacial electric field strengths in the presence of the four different cations. The electric fields are on the order of 0.1 V Å-1. A simple electrostatic model reveals that these electric fields make a negligible electrostatic contribution to the CO adsorption energy. Analysis of the O-H stretch spectrum of interfacial water reveals that the hydrogen bond between surface-adsorbed CO and interfacial water is disrupted in the presence of propyl4N+ and butyl4N+. Our observation suggests that this hydrogen bond is essential for the formation of ethylene during the reduction of CO on Cu. These insights are expected to guide the design of electrocatalytic interfaces with high selectivity for ethylene.