Ionomer Side Chains Modulate Interfacial Microenvironments for Selective CO₂ Electrolysis

This study investigates how the molecular structure of imidazolium ionomers with linear alkyl side chains (C_<i>n</i>H_2<i>n</i>+1 where <i>n</i> = 1, 4, 10, 16) modulates interfacial microenvironments in the Ag-catalyzed CO₂ reduction reaction (CO₂RR). Variations in side chain length and molecular weight establish structure–performance relationships that link hydrophobicity and ion transport to activity and selectivity. Longer side chains suppress hydrogen evolution and enhance the CO₂RR, with the <i>n</i>-hexadecyl ionomer achieving the highest Faradaic efficiency for the CO₂RR of 90.1% in a two-compartment cell. Incorporation of this ionomer in a cation-exchange membrane-based membrane electrode assembly achieves selective CO production with a partial current density exceeding 100 mA cm^–2, outperforming a commercial benchmark. Controlled studies under lean and acidic electrolytes reveal that the ionomer maintains local alkaline environments by restricting the interfacial water and proton transport. These findings provide molecular-level insights into ionomer function and design principles for selective CO₂RR in practical electrolyzers.