Electrocatalytic Conversion of CO2 to Formate at Low Overpotential by Electrolyte Engineering in Model Molecular Catalysis

International audienceWe report electrolyte engineering strategy for CO 2 reduction into formate with a model molecular catalyst, i.e. [Rh(bpy)(Cp*)Cl]Cl, by modifying the solvent (organic or aqueous), the proton source (H 2 O or acetic acid) and the electrode/solution interface with imidazolium- and pyrrolidinium-based ionic liquids (ILs). Our experimental and theoretical DFT investigations suggest that π + -π interactions between the imidazolium-based IL cation and the reduced bipyridine ligand of the catalyst improve the efficiency of the CO 2 reduction reaction (CO 2 RR) by lowering the overpotential, while granting partial suppression of hydrogen evolution reaction (HER). This allows tuning the selectivity towards formate, reaching for this catalyst an unprecedented faradaic efficiency (FE HCOO -) ≥ 90 % and energy efficiency = 66 % in acetonitrile solution. For the first time, relevant CO 2 conversion to formic acid/formate is reached at low overpotential (0.28 V) using a homogenous catalyst in acidic aqueous solution (pH = 3.8). These results open up a new strategy based on electrolyte engineering for enhancing carbon balance in CO 2 RR