Effect of Terphenyl Isomeric Structure on Poly(diallyldialkylammonium)-Functionalized Anion Exchange Membranes for Water Electrolysis

Abstract

The structural design of anion exchange membranes (AEMs) critically affects their ionic conductivity, mechanical strength, and alkaline stability-key requirements for high-performance AEM water electrolyzers (AEMWEs). Herein, we report a backbone isomerism engineering strategy to modulate hydration and transport properties by synthesizing m-terphenyl and p-terphenyl units as rigid aromatic backbones, functionalized with pyrrolidinium groups via cyclopolymerization and covalent crosslinking. Beyond homopolymer studies, physical blends, and random copolymers were systematically compared to investigate how backbone geometry and integration method influence membrane properties. All membranes exhibited low swelling and relatively high mechanical strength under wet conditions, while hydration and conductivity varied depending on structure and polymer type. m-Terphenyl-based AEMs formed a twisted, microporous structure, providing higher free volume, water uptake, and hydroxide conductivity than their p-terphenyl counterparts. In contrast, p-terphenylbased membranes exhibited superior mechanical strength and alkaline stability due to their linear conformation. Blends and copolymers showed intermediate properties, balancing hydration and mechanical robustness. Incorporating water electrolysis performance and durability, these structural variations were found to directly influence practical AEMWE operation. Notably, the m-terphenyl-based membrane achieved high AEMWE performance of 8.6 A/cm² at 2.0 V using nonplatinum group metal anode, outperforming the blend, copolymer, and p-terphenyl-based membranes. This study provides the first comprehensive correlation between terphenyl isomerism and AEM performance, offering molecular-level insights into how backbone architecture can be tailored to enhance electrochemical functionality.