CO2-derived fuels, synthesized using renewable energy, present an attractive route towards fuel formation from sustainable energy. Impressive progress has been achieved in the production of CO and formic acid from CO2. However, despite considerable efforts in the field, the selective electrosynthesis of higher order products such as methane or ethylene remains elusive. A likely explanation lies in insufficient understanding of the complex reaction network involved in transferring multiple protons and electrons to surface-bound intermediates. To shed light into this fascinating maze, we investigate the electrochemical reduction of CO at Cu electrodes in nonaqueous electrolytes at low temperatures. Thereby, we gain 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 higher concentrations of CO serve to suppress methane production, in addition to hydrogen. In contrast, ethylene production remains unaffected by CO concentration. Together with electrochemical activation data, we exploit our findings to gain unprecedented insight into the mechanism of electrochemical CO and CO2 reduction. Among others, we are able to show how 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. Our results draw a comprehensive picture of the catalytic pathways governing CO reduction and provide an excellent example highlighting the interplay between electrochemical and thermally activated reactions. Based on these findings, 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.

Understanding the surface dynamics of hydrogen and CO in the electrochemical reduction of CO2

2019 Fall National Meeting

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