Ionic liquids (ILs) in electrocatalysis have attracted a lot of attention from the seminal work of Rosen et al. [1] where they used a mixture with 18 mol% of 1-Ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) in water as a solvent supporting electrolyte system and achieved a relevant overpotential decrease for CO2 reduction on a silver electrode by forming adducts between CO2 and reduced imidazolium cations. In fact, it has been demonstrated more recently that imidazolium cations are reduced on the electrode surface as a first step and then form adducts with CO2, [2-3] which lowers the energy barrier of the unfavorable first one-electron reduction to CO2. Nevertheless, most of those studies using ILs have been carried out mainly using heterogeneous electrocatalysts. In contrast, much less work has been done exploring the role of ILs in molecular electrocatalysis for CO2 electroreduction. The role of different ILs as simultaneous supporting electrolyte and co-catalyst in acetonitrile for CO2 electroreduction in the presence of a model molecular catalyst [Re(bpy)(CO)3Cl] is presented here. [4] Thus, the structure-activity relationship of ILs on electrocatalytic CO2 reduction is evaluated by cyclic voltammetry and controlled potential electrolysis. In particular, the nature of the cation, anion and cation alkyl chain is varied by a choice of 5 different ILs, including imidazolium and pyrrolidinium cations and their results are compared to conventional benchmark supporting electrolyte. Under catalytic conditions an overpotential diminution of 0.33 V for CO2 to CO conversion is achieved in the presence of ILs. Finally, a mechanistic explanation is provided to justify this behavior and the partial suppression of the IL co-catalytic effect when CO2 electroreduction takes place in the presence of a proton source. References [1] Rosen, B.A.; Salehi-Khojin, A.; Thorson, M.R.; Zhu, W.; Whipple, D.T.; Kenis, P.J.A.; Masel, R.I. Science 2011, 334 (6056), 643–644. [2] Hanc-Scherer, F.A.; Montiel, M.A.; Montiel, V.; Herrero, E.; Sánchez-Sánchez, C.M. Phys. Chem. Chem. Phys. 2015, 17, 23909–23916. [3] Wang, Y.; Hayashi, T.; He, D.; Li, Y.; Jin, F.; Nakamura, R. Appl. Catal. B Environ. 2020, 264, 118495. [4] Vichou, E.; Xu-Li, Y.; Gomez-Mingot, M.; Fontecave, M.; Sánchez-Sánchez, C.M. ACS Catal, submitted.