Novel acid–base blend membranes for application in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) were synthesized and characterized. The acid–base blend membranes demonstrated high thermal and excellent chemical stabilities in terms of oxidative weight loss. Small changes in the molecular weight distribution after immersion in Fenton's solution were determined with gel permeation chromatography (GPC). Scanning electron microscope (SEM) images showed the outstanding integrity of the acid–base blend membranes after a 24 hour-long Fenton's test. In contrast to pure polybenzimidazole (PBI) and AB-PBI membranes the new acid–base membranes exhibited long-term stability in phosphoric acid (PA) at 130 °C. Ionic cross-linking between acid and base blend polymers improved the stability and integrity of the membranes. The in situ conductivities of several acid–base blend membranes were higher than that of pure AB-PBI membranes with drastically reduced acid uptake. Membrane electrode assemblies (MEAs) based on these blend membranes were prepared showing good fuel cell performance at practical relevant operation conditions. This study proves that acid–base blends are a suitable alternative to pure PBI and AB-PBI as membranes for HT-PEMFCs.

Ionic cross-linking between the SAC component and (a) F₆PBI by imidazole group protonation or (b) N-methylmorpholine after quaternization.

Anion exchange blend membranes (AEBMs) were prepared for use in Vanadium Redox Flow Batteries (VRFBs). These AEBMs consisted of 3 polymer components. Firstly, PBI-OO (nonfluorinated PBI) or F6-PBI (partially fluorinated PBI) were used as a matrix polymer. The second polymer, a bromomethylated PPO, was quaternized with 1,2,4,5-tetramethylimidazole (TMIm) which provided the anion exchange sites. Thirdly, a partially fluorinated polyether or a non-fluorinated poly (ether sulfone) was used as an ionical cross-linker. While the AEBMs were prepared with different combinations of the blend polymers, the same weight ratios of the three components were used. The AEBMs showed similar membrane properties such as ion exchange capacity, dimensional stability and thermal stability. For the VRFB application, comparable or better energy efficiencies were obtained when using the AEBMs compared to the commercial membranes included in this study, that is, Nafion (cation exchange membrane) and FAP 450 (anion exchange membrane). One of the blend membranes showed no capacity decay during a charge-discharge cycles test for 550 cycles run at 40 mA/cm² indicating superior performance compared to the commercial membranes tested.

In this review the research of the author's group regarding the optimization of the chemical stability and properties of aromatic ion-exchange polymers by suitable and systematic synthesis of electron-deficient monomer building blocks and by their (co) polymerization into highly stable aromatic ionomers are presented. Moreover, the preparation of physically or covalently cross-linked ion-exchange membranes by blending these ionomers with each other and with suitable commercial polymers to finally afford ion-exchange membranes with properties tailored for electromembrane applications such as fuel cells, electrolyses, and redox-flow batteries are described.

Both cation-exchange membranes and anion-exchange membranes are used as ion conducting membranes in vanadium redox flow batteries (VRFBs). Anion-exchange membranes (AEMs) are applied in vanadium redox flow batteries due to the high blocking property of vanadium ions via the Donnan exclusion effect. In this study, novel anion-exchange blend membranes (AEBMs) were prepared, characterized, and applied in VRFBs. Bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide), poly[(1-(4,4′-diphenylether)-5-oxybenzimidazole)-benzimidazole] (PBI-OO) and sulfonated polyether sulfone polymer were combined to prepare 3-component AEBMs with 1,2,4,5-tetramethylimidazole (TMIm) for quaternization. 3-component AEBMs showed significantly enhanced chemical and mechanical properties compared with those of 2-component AEBMs, resulting in an improved performance in VRFBs. The compositions of the anion-exchange polymers in 3-component AEBMs were systematically varied to optimize the AEBMs for the redox-flow battery application. While the 3-component AEBMs showed comparable efficiencies with Nafion^® 212 membranes, they displayed improved vanadium ions cross-over as was confirmed by open circuit voltage tests and capacity fade tests conducted in VRFBs. In addition, one of the synthesized 3-component AEBM had a superior coulombic efficiency and capacity retention in a charging–discharging test over 300 cycles at a current density of 40 mA/cm². It can thus be concluded that 3-component AEBMs are promising candidates for long-term operation in VRFBs.