Tetraethylammonium Cations Enable C2+ Product Formation in Acidic CO2 Reduction

Cation effect is essential to obtain selectivity towards carbon dioxide (CO2) reduction reaction (CO2RR) over the hydrogen evolution reaction (HER) in acidic media. Selectivity towards CO2RR has been largely attributed to the coordination between alkali cations and CO2-derived intermediates. Here, we provide a different perspective by demonstrating that tetraethylammonium (Et4N+), a weakly coordinating cation, can also effectively suppress the HER in acidic media, favoring CO2RR on both Ag and Cu electrodes. The generation of both C1 and C2+ products on Cu indicates that the direct participation of alkali cations as co-catalyst is not a prerequisite for CO2 activation or C-C coupling. We find that CO2RR is independent of the bulk pH in the presence of 1 M Et4N+, as evidenced by similar selectivity across acidic, neutral, and alkaline media. In addition, rigorous impurities control reveals that millimolar-levels of triethylammonium chloride (Et3NHCl) substantially promote HER on Cu catalyst, hence calling for rigorous impurity control. Overall, this work demonstrates that weakly coordinating cations can promote CO2RR across a broad range of pH, indicating that a catalytic role of alkali metal cations is not necessary for CO2RR. Graphical Abstract CO2 reduction with weakly coordinating cations: The weakly coordinating organic cation tetraethylammonium (Et4N+) efficiently suppresses the hydrogen evolution reaction, favoring CO2RR on Ag and Cu electrodes. This demonstrates that the direct participation or catalytic role of alkali metal cations is not a prerequisite for CO2 activation or C-C coupling.