Hyperbranched-type anion exchange membranes with electrostatic interactions for high performance anion exchange membrane water electrolysis

Abstract

Poly (aryl piperidinium) (PAP) has been widely employed in anion exchange membrane water electrolysis (AEMWE) because of its high ion exchange capacity and superior chemical stability. PAP-based anion exchange membranes (AEMs) equipped with hyperbranched structures have recently garnered significant attention as they contain multiple reactive sites, thus exhibiting high molecular weights and enhanced mechanical properties. Herein, hyperbranched poly (p-terphenyl N-methyl piperidinium) (QPTP) polymers using triphenylamine (b-Nm-QPTP) and triphenylmethane (b-Cm-QPTP) as hyperbranching units were fabricated and compared, notably with respect to the hyperbranching units. A linear QPTP polymer with no hyperbranched structures was also synthesized and used to fabricate a QPTP-based AEM for comparison. Both b-Nm-QPTP and b-Cm-QPTP achieved a higher viscosity (>1.4 dL/g) than the linear QPTP, and the b-Nm-QPTP- and b-Cm-QPTP-based AEMs exhibited enhanced mechanical properties (>30 MPa in terms of stress) compared to the QPTP-based AEM. Further, b-N5-QPTP, comprising 5 % triphenylamine, demonstrated the most pronounced microphase separation; this was attributed to nitrogen–water electrostatic interactions, as confirmed by molecular dynamics simulations. Thus, this membrane exhibited not only well-defined ion channels and improved ionic conductivity (157.68 mS/cm at 80 °C) but also remarkable chemical stability, with an ionic conductivity retention of over 96 % in 3 M KOH at 60 °C. Additionally, the AEMWE single-cell performance of b-N5-QPTP, 6.313 A/cm² at 2.0 V, was significantly higher than that of the commercial PiperION membrane (4.806 A/cm² at 2.0 V) and remained high (4.438 A/cm² at 2.0 V) even when non-noble metal catalysts were used, demonstrating its high feasibility for AEMWE applications.