Directing the Outcome of CO₂ Reduction at Bismuth Cathodes Using Varied Ionic Liquid Promoters

Ionic liquids (ILs) have been established as effective promoters for the electrocatalytic upconversion of CO₂ to various commodity chemicals. Imidazolium ([Im]⁺) cathode combinations have been reported to selectively catalyze the 2e^-/2H⁺ reduction of CO₂ to CO. Recently our laboratory has reported energy-efficient systems for CO production featuring inexpensive bismuth-based cathode materials and ILs comprised of 1,3-dialkylimidazolium cations. As part of our ongoing efforts to understand the factors that drive CO₂ reduction at electrode interfaces, we sought to evaluate the catalytic performance of alternative ILs in combination with previously described Bi cathodes. In this work, we demonstrate that protic ionic liquids (PILs) derived from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) effectively promote the electrochemical reduction of CO₂ to formate (HCOO^-) with high selectivity. The use of PILs comprised of the conjugate acid of DBU, [DBU-H]⁺, efficiently catalyzed the reduction of CO₂ to HCOO^- (FE_FA ≈ 80%) with significant suppression of CO production (FE_CO ≈ 20%) in either MeCN or MeCN/H₂O (95/5) solution. When they were used in combination with [DBU-H]⁺-based PILs, Bi-based cathodes achieved current densities for CO₂ reduction (j _tot ≈ 25-45 mA/cm²) that are comparable to or greater than those reported with imidazolium ILs such as [BMIM]PF₆. As we demonstrate herein, the selectivity of the 2e^- reduction of CO₂ toward HCOO^- or CO can be dictated through the choice of the IL promoter present in the electrolysis solution, even in cases in which the same electrocatalyst material is studied. These findings highlight the tunability of bismuth/IL systems for the electrochemical reduction of CO₂ with high efficiency and rapid kinetics.