Deciphering the role of aromatic cations in electrochemical CO2 reduction: interfacial ion assembly governs reaction pathways

The accumulation of ions at electrochemical interfaces governs the local chemical environment, which in turn determines the reaction pathways and rates of electrocatalytic processes, including electrochemical CO2 reduction. Imidazolium cations have been shown to promote CO2 reduction in nonaqueous electrolytes, where multiple mechanisms have been proposed for how imidazolium facilitates CO2 reduction. However, many puzzles persist surrounding how imidazolium cations modify local chemical environments at electrochemical interfaces during CO2 reduction. Dialkylimidazolium cations are multifunctional species that interact with adsorbed CO2 center dot- while also donating protons and forming carbene-mediated coordination complexes. In this work, we exploit the combination of independent proton donor [Et3NH]Cl and aprotic imidazolium cations, namely 1-ethyl-2,3-dimethylimidazolium ([EMMIm](+)) and 1-ethyl-2,3,4,5-tetramethylimidazolium ([EM(4)Im](+)) to further illuminate how imidazolium cations promote selective CO2 electrochemical reduction. Our data indicates that the presence of an aromatic, planar delocalized charge region on imidazolium rings plays an essential role in stabilizing CO2 center dot- to promote electrocatalytic reduction. Kinetic and steady-state electrochemical analysis demonstrates that ring substituents of [EMMIm](+) additionally tune local chemical environments to impact the rate and product distribution of CO2 reduction by limiting the transport of proton donors. Further, we leverage surface-enhanced Raman scattering in the presence of a molecular probe of local electric fields to illustrate that the unique interface-tuning properties of [EMMIm](+) stem from potential-driven assembly at cathodes. Our study highlights how imidazolium substituents can be tuned to regulate interfacial electrochemical environments and illustrates the importance of balancing CO2 center dot- stabilization and proton transport in sustaining steady-state electrochemical CO2 reduction with high rate and selectivity. More broadly, our results suggest that aromatic cations promote electrochemical CO2 reduction via a distinct "pi(+)-anion" interaction that appears to be the electrostatic analog of the more commonly investigated "cation-pi" interaction, which drives self-assembly in proteins and many other biological systems.