Ether Coordination in Ionomers Switches Function with Length Scale in CO2 Electrolysis

Abstract CO2 electrolysis to CO relies on anion-exchange ionomers (AEIs) in two distinct roles, as the membrane and as the catalyst-layer binder, yet the design criteria for the two roles are rarely separated. Here, we demonstrate that side-chain architecture switches function with length scale by comparing AEIs bearing hexamethylene (HTMA) or ether-containing (EOTMA) trimethylammonium spacers in each role. As a membrane, EOTMA immobilizes most of its water, so 56% greater uptake produces no conductivity gain. The EOTMA membrane also admits K+, which accumulates at the cathode and lowers CO selectivity relative to HTMA. As a binder, the same ether groups improve CO2 accessibility and strengthen interfacial hydrogen bonding, promoting *COOH formation and weakening *CO binding, as shown by impedance, sorption, and spectroscopic analyses. Pairing the HTMA membrane with the EOTMA binder on commercial Ag delivers a CO partial current density of 550.3 mA cm–2 at 3.51 V and 60 °C. Role-optimized ionomer design therefore offers a general strategy for engineering electrochemical interfaces.