Flexibly linked and isomeric piperidinium-based anion exchange membrane with enhanced alkaline stability for durable alkaline water electrolysis

A critical challenge in developing anion exchange membrane (AEM) water electrolysis is to design high-performance AEMs with chemical and mechanical stability under harsh alkaline environments. Herein, we report the rational design and synthesis of QP(T-3-Pip) featuring flexibly linked isomeric piperidinium cations via a Friedel-Crafts polyhydroxyalkylation between terphenyl and commercial 3-piperidinaldehyde. Two key molecular engineering strategies are employed: (i) introducing a methylene linker to relieve ring strain, and (ii) strategically positioning the quaternary nitrogen at the 3-position of the piperidinium ring to suppress β-hydrogen elimination. Compared to the benchmark quaternized poly(terphenyl piperidinium) (QPTP), QP(T-3-Pip) demonstrates substantially improved water uptake (77 %), comparable ionic conductivity, and superior tensile strength (∼20 MPa). Notably, QP(T-3-Pip) retains over 78 % of its cationic functionality after 40 days in 5 M KOH at 80 °C, while QPTP shows pronounced degradation after 24 days. Electrolyzer tests using non-noble metal catalysts, QP(T-3-Pip) reveal a high current density of 2.3 A cm−2 at 1.8 V, and exceptional operational durability over 750 h at 1.0 A cm−2. This work highlights the critical role of piperidinium isomerism and linker engineering in tuning alkaline stability and mechanical integrity, offering a robust and scalable platform for next-generation AEMs for cost-effective alkaline water electrolyzers.