Ionomer Structure Engineering for Optimal Triple-Phase Boundaries in Anion Exchange Membrane Water Electrolysers

In anion exchange membrane water electrolyzers (AEMWEs), the construction of efficient triple-phase boundaries (TPBs) within the catalyst layer (CL) is often hindered by catalyst agglomeration and the spatial distribution of ionomers, particularly for metal oxide-based catalysts that are inherently prone to aggregation. Achieving an ideal TPB requires a well-interpenetrated network of catalyst aggregates, ionomer, and pore space. However, current understanding of how the chemical structure of anion exchange ionomers (AEIs) governs catalyst agglomerate dispersion and ionomer penetration into these aggregates remains limited. Here, we systematically evaluate the influence of three representative commercial AEIs (PiperION A5, Sustainion XA-9, and Fumion FAA-3) on CL morphology and systematically investigate AEMWE performance. The results demonstrate that ionomers with flexible hydrophilic structures are more favorable for penetrating catalyst aggregates to form internal binding networks, therefore minimizing film-like coverage, enhancing active-site exposure, reducing charge-transfer resistance, and ultimately promoting effective TPB formation. Moreover, reducing the ionomer content further optimizes TPB construction within the CL, leading to a substantial improvement in AEMWE performance. Building on these findings, prioritizing essential alkaline stability, aryl piperidinium ionomers are engineered with a flexible meta-terphenyl to adjust rigidity, while triphenylbenzene branching is incorporated to suppress swelling. This synergistic modification greatly improves the overall ionomer performance, reaching an operating voltage of 1.60 V@1 A cm^-2. These findings offer valuable insights into the relationship between AEI structure and CL morphology, providing practical design principles for next-generation anodic AEIs.