Ionomers Modify the Selectivity of Cu-Catalyzed Electrochemical CO2 Reduction

Electrochemical reduction of carbon dioxide (CO₂RR) to produce energy-rich fuels using copper (Cu)-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. Here we employ electrochemical and operando Raman spectroscopic measurements to study the effects of three archetypical ionomers (Nafion, Sustainion-type XA-9, and poly(terphenyl piperidinium)) on Cu-catalyzed CO₂ reduction at high current densities (up to 200 mA/cm²). We find Nafion has little influence, XA-9 promotes the formation of CO over multicarbon products, and PTP favors hydrogen and formate production. We identify charge and hydrophobicity/hydrophilicity as important parameters of the ionomers. To explain the observed effects, we propose the charge transfer between Cu and XA-9 weakens CO adsorption energy whereas the hydrophilicity of PTP reduces M-H energy. Our study reveals the structure-sensitive nature of the ionomer-catalyst interaction in CO₂RR.