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

An electrolyte engineering strategy was developed for CO2 reduction into formate with a model molecular catalyst, [Rh(bpy)(Cp*)Cl]Cl, by modifying the solvent (organic or aqueous), the proton source (H2O or acetic acid), and the electrode/solution interface with imidazolium‐ and pyrrolidinium‐based ionic liquids (ILs). Experimental and theoretical density functional theory investigations suggested that π+‐π interactions between the imidazolium‐based IL cation and the reduced bipyridine ligand of the catalyst improved the efficiency of the CO2 reduction reaction (CO2RR) by lowering the overpotential, while granting partial suppression of the hydrogen evolution reaction. This allowed tuning the selectivity towards formate, reaching for this catalyst an unprecedented faradaic efficiency (FEHCOO−) ≥90 % and energy efficiency of 66 % in acetonitrile solution. For the first time, relevant CO2 conversion to formic acid/formate was reached at low overpotential (0.28 V) using a homogeneous catalyst in acidic aqueous solution (pH=3.8). These results open up a new strategy based on electrolyte engineering for enhancing carbon balance in CO2RR. CO2 reduction: The presence of imidazolium‐based ionic liquids in solution significantly impacts on molecular electrocatalysis selectivity and activity by favoring the CO2 reduction reaction vs. hydrogen evolution reaction in both acetonitrile and aqueous solution. The combination of electrolytes [EMIM][BF4] and acetic acid amplifies their individual roles by enhancing CO2 conversion to formate at very low overpotential (0.28 V) in acidic aqueous solution.