The structure effect on the role of different ionic liquids (ILs) as simultaneous supporting electrolyte and co-catalyst for CO2 electroreduction in the presence of a model molecular catalyst [Re(bpy)(CO)3Cl] has not been addressed yet. In particular, we varied the nature of the cation, anion and cation alkyl chain by a choice of 5 different ILs, including imidazolium and pyrrolidinium cations and we compared their results to benchmark supporting electrolyte. We report an overpotential diminution of 0.33 V for CO2 to CO conversion in the presence of ILs under CO2 catalytic conditions. We prove the IL cation-dependent overpotential diminution due to the electrostatic stabilization of the negatively-charged active form of the catalyst, being the π-π stacking interaction provided by imidazolium cations responsible for their outstanding performance. Finally, a mechanistic explanation is provided to justify a weaker IL co-catalytic effect when CO2 electroreduction takes place in the presence of a proton source.