The effect of high-temperature proton exchange membranes with microphase separation structure on phosphoric acid loss

Phosphoric acid (PA) leakage of PA-doped high-temperature proton exchange membranes (HT-PEMs) is pivotal for its practical application under working conditions. Here, to enhance the PA retention of HT-PEMs, the concept of microphase separation regulated by flexible long aliphatic side-chain was introduced into poly(isatin-co-p-terphenyl-imidazole) matrix (PITI-CmQ90) via direct superacid-catalyzed copolymerization from N-alkylated isatin monomer. The designed PITI-CmQ90 membranes illustrate obvious microphase separation and larger ionic domain confirmed via Small-angle X-ray scattering (SAXS) analysis suggesting wider proton transport channels. Notably, PA/PITI-C6Q90 membrane exhibits the best PA retention ability under relative humidity (RH) cycle tests at different temperatures, which mainly attributed to the excellent PA interaction through moderate ion aggregation in the microphase-separated PEM matrix verified by solid 31P NMR. As a result, the corresponding MEA displays the most durable single-cell performance under a step-V procedure from 1.0 V to 0.1 V cycle experiments at different temperatures. Therefore, the idea of microphase separation structure on PA retention mentioned in this work helps to study the ionic-pairs interactions from the perspective of membrane microstructure and provide a new strategy to enhance the durability of PEMs.