Design ionic liquid/copper interface for electrocatalytic CO2 conversion to ethylene

The roles of ionic liquids (ILs) on modifying copper electrocatalyst for CO₂ reduction reaction (CO₂RR) pathways were systematically studied for selective conversion of CO₂ to ethylene in common aqueous alkaline electrolyte. The IL/copper interface was formed by adsorbing hydrophobic ILs on copper nanoparticle catalysts on carbon paper. ILs with phosphonium/ imidazolium cations and PF₆^- anion were selected for this study due to their stability and desirable functional groups. The amount and coverage of the ILs on the copper surface as well as the hydrophilicity of IL/copper interface, were characterized through the contact angle measurements. Electrochemical characterizations revealed that both phosphonium- and imidazolium- modified copper nanoparticle catalysts produce higher yield of C2+ products in CO₂RR compared to the bare copper nanoparticles. The ILs with different ligand group, i.e. chloro‑, bromo‑ and benzotriazole- phosphonium cations were further investigated to study their steric and coordination effects on copper electrode. Among the IL studied, the highest ethylene Faradaic efficiency (FE), up to 30 %, is achieved with the 0.15 μmol/ cm^-2 modification of bromo‑substituted phosphonium cation, which is 1.5 times of that on the bare Cu catalyst. Electrochemical results indicated that bromo‑coordinated Cu(I) is particularly effective to CO₂RR. In-situ surface enhanced Raman spectroscopy confirmed that IL steric effect facilitates the stabilization of the CO₂RR intermediate. This study presents a novel approach by exploring the synergy between the physicochemical properties of ILs and copper nanoparticle for catalytic CO₂RR in alkaline electrolytes that shows the enhancement of the C2+ product formation. It is expected the general scope of this approach could be applied to other electrocatalytic systems.