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The electrochemical CO2 reduction reaction (CO2RR) is an emerging carbon utilisation technology. The use of ionomers in fabricating catalytic electrodes was found to exert substantial effects on activity and product selectivity, particularly for Cu-catalyzed multicarbon (C2+) product formation. Here, we study the CO2RR performance of Cu catalysts coated with 8 different commercial ionomers to understand how ionomers impact C2+ product formation in zero-gap membrane electrode assembly (MEA) reactors. We find that the ionomer hydrophobicity plays the most decisive role in determining the faradaic efficiency (FE) and partial current density of C2+ products. Cu coated with the most hydrophobic ionomer converts CO2 into C2+ products at an FE of about 75% and 180 mA cm−2. This is due to the hydrophobicity-induced *CO adsorption stabilisation. The ionised side chains of ionomers may also impact the selectivity and activity of C2+ products by altering the electric double layer (EDL) structures. Ionised side chains with bulkier molecular structures and smaller hydration numbers presumably result in less compact EDLs, pertinent to enhanced C–C coupling activity. [ABSTRACT FROM AUTHOR]