From Membranes to Catalyst Layers: Molecular and Interfacial Design of Anion-Exchange Polyelectrolytes

Conspectus Sustainable electrochemical energy conversion relies on functional materials that enable efficient transport while withstanding demanding operating environments. Anion-exchange membrane fuel cells (AEMFCs) and anion-exchange membrane water electrolyzers (AEMWEs) are attractive platforms because their alkaline environments enable the use of nonprecious-metal catalysts and hydrocarbon-based polymer electrolytes. Anion-exchange polyelectrolytes (APEs) are central to these devices, serving as both free-standing anion-exchange membranes (AEMs) and catalyst-layer anion-exchange ionomers (AEIs). Although AEMs and AEIs share related polymer chemistries, their distinct operating environments and functional roles impose different requirements on molecular design. Our work on AEMs has focused on the coupled requirements of hydroxide transport, chemical stability, dimensional control, and mechanical durability. We first examined how polymer architecture and chain packing affect the formation of ion-conducting domains and confined transport environments. As conductivity improved, chemical degradation and mechanically induced failure became increasingly relevant, leading us to investigate cation stability, local electronic environments, dimensional constraints, and molecular-level stress dissipation. These studies showed that AEMs cannot be optimized on the basis of a single performance metric but instead require the coordinated regulation of ion transport, molecular stability, and mechanical response. We also extended this work to kilogram-scale polymer synthesis and continuous roll-to-roll membrane fabrication, where synthetic reproducibility and processing compatibility become additional constraints on molecular design. The requirements change when APEs are used as catalyst-layer ionomers. AEIs form nanometer-scale films around catalyst particles and must provide local hydroxide conduction while allowing water and gas transport, maintaining catalyst accessibility, and contributing to interfacial integrity. Our studies on AEIs have therefore examined multiphase transport, catalyst-ionomer interactions, interfacial adhesion, ionomer distribution, and catalyst-layer structure. Fluorinated and porous ionomers were used to regulate oxygen transport and gas-bubble removal, while conformational and interfacial design was used to reduce unfavorable catalyst-ionomer interactions. Further control of adhesion, ionomer-film organization, and catalyst immobilization showed that catalyst-layer performance also depends strongly on how the ionomer is distributed and organized during electrode formation. The studies discussed in this Account suggest that APE molecular design should be matched to the specific operating environment and functional role of the polymer within the membrane-electrode assembly. For AEMs, this includes balancing transport, durability, and manufacturability; for AEIs, it additionally requires control of local transport, catalyst interactions, and catalyst-layer organization.