Ionomer Side Chains Modulate Interfacial Microenvironments for Selective CO2 Electrolysis

Abstract

This study investigates how the molecular structure of imidazolium ionomers with linear alkyl side chains (CnH2n+1 where n = 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 n-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.