Development of Ether-Free Polyarylene-Based Anion Exchange Membranes for Alkaline Fuel Cells through Crosslinking, Copolymerization, and Blending Strategies

Abstract

Developing high-performance anion exchange membranes (AEMs) with well-balanced properties is essential for advancing AEM fuel cell technology. However, current AEMs face performance and durability limitations due to the trade-off challenge between different properties. Often, optimizing one characteristic, such as the swelling ratio, compromises another, like hydroxide conductivity. This challenge persists despite recent progress in AEM development. Furthermore, maintaining chemical stability in highly alkaline environments at elevated temperatures presents a significant hurdle. This work addresses these challenges through various approaches, including crosslinking, copolymerization, and blending.

Firstly, we developed crosslinked ether-free polyfluorene-based AEMs using a crosslinker containing potential cationic groups. Optimizing the crosslinking degree improved swelling ratio (<15.9%), water uptake (<78.0%), and mechanical properties (>35 MPa), while simultaneously enhancing hydroxide conductivity (>144.6 mS cm^⁻¹). This enhancement was attributed to an improved microphase-separated morphology. Moreover, the resulting membranes exhibited exceptional chemical stability, retaining over 93-95% of their hydroxide conductivity, ion exchange capacity (IEC), and tensile strength in 3 M NaOH at 80 °C, representing one of the best chemical stability results reported to date. Secondly, we developed copolymer AEMs using bibenzyl (flexible) and dimethylfluorene (rigid) monomers. We demonstrated that optimizing the monomer weight percentages allows for fine-tuning AEM properties, resulting in membranes with well-balanced properties. Finally, we fabricated blend AEMs by combining two ether-free polyarylene-based polymer phases with identical backbones but different IECs. This strategy produced AEMs with a highly microphase-separated morphology, achieving a remarkable hydroxide conductivity of 262.9 mS cm^⁻¹. This ion conductivity represents a new benchmark. In AEM fuel cell tests, the blend AEM reached a peak power density of 1.36 W cm^⁻² and demonstrated stable operation at 0.5 A cm^⁻² for 150 hours, with a voltage decay rate of only 0.26 mV h^⁻¹. This performance surpasses that of the leading commercial PiperION^™ AEM under identical testing conditions.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Minhua SHAO (Supervisor) & Gholamreza Pircheraghi (Supervisor)