Introduction Imidazolium cations have been shown to enhance the performance of electrode catalysts for CO2 reduction in aqueous and also in non-aqueous media- for more details see Chapter 1. However, their underlying role and the reason for their enhanced performance over other electrolyte cations remain unclear. In this chapter, the objective is to develop a comprehensive understanding of the mechanisms underlying the effectiveness of imidazolium cations in facilitating CO2 reduction. To accomplish this, Au electrode and anhydrous acetonitrile were chosen as a model system for conducting a systematic investigation into how the molecular structure of imidazolium compounds influences their performance in electrochemical CO2 reduction. Regarding improvements in kinetics in non-aqueous media through the use of imidazolium cations, previous studies have reported an onset potential of approximately~ -0.4 V vs SHE for CO formation, albeit with the presence of 10 to 500 mM H2O66. Lau et al. reported onset potentials around -2.0 V vs. Fc/Fc+ when utilizing a C2-methylated imidazolium cation in acetonitrile8. Here it is found that the systematic chemical modification of the imidazolium cations under well-controlled anhydrous media leads to an onset potential of around -0.8 V vs. Ag/Ag+ (~ -0.258 V vs. SHE) which shows significant improvements over previous studies. Furthermore, the performance of the imidazolium cation is proved to be in strong correlation with the acidity of the C2-H bond, demonstrating the kinetic relevance of proton donation through isotopic labeling experiments. Finally, complementary DFT calculations are conducted to support the findings, confirming that a concerted coupled electron-proton transfer mechanism with partial proton transfer is the most likely operative mechanism. The demonstrated mechanism provides guidelines for improvement in the energy efficiency of non-aqueous electrochemical CO2 reduction by a tailored design of electrolyte cations.
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