Ionomers Modify the Selectivity of Cu-Catalyzed Electrochemical CO₂ Reduction

Electrochemical reduction of carbon dioxide (CO₂ RR) to produce energy-rich fuels using copper-based electrocatalysts is widely studied as a possible solution to CO₂ recycling. Ionomers are commonly used as binders to prepare catalyst-loaded electrodes, but their effects on the performance have not been fully investigated. In this study, electrochemical and operando Raman spectroscopic measurements are used to study the effects of three archetypical ionomers [Nafion, Sustainion-type XA-9, and poly(terphenyl piperidinium) (PTP)] on Cu-catalyzed CO₂ reduction at high current densities (up to 200 mA cm^-2 ). Nafion is found to have little influence, whereas XA-9 promotes the formation of CO over multicarbon products and PTP favors hydrogen and formate production. Charge and hydrophobicity/hydrophilicity are found to be important parameters of the ionomers. The observed effects are attributed to the charge transfer between Cu and XA-9 weakening the CO adsorption energy, whereas the hydrophilicity of PTP reduces M-H energy. This study reveals the structure-sensitive nature of the ionomer-catalyst interaction in CO₂ RR.