Poly(ether sulfone)-Based Anion Exchange Membranes Containing Dense Quaternary Ammonium Cations and Their Application for Fuel Cells

Poly(ether sulfone)-based anion exchange membranes (AEM)s containing dense quaternary ammonium cations were prepared for anion exchange membrane fuel cell applications. Poly(ether sulfone)s, XMePh-Z, derived from a new aromatic diol, 4,4′-dihydroxy-2,2′,6,6′-tetra(p-tolyl)diphenyl ether containing four tolyl substituents, 4,4′-(hexafluoroisopropylidene)diphenol (HFDP), and bis(4-fluorophenyl)sulfone (BFS), were prepared and functionalized via photoinitiated bromination on tolyl substituents followed by quaternization. The photoinitiated bromination is so selective that the reaction occurred only on the methyl groups of tolyl substituents without polymer main chain degradation. The IEC values of poly(ether sulfone)-based AEMs, from 1.21 to 2.11 mmol/g, can be controlled by the degree of bromination. These membranes with water uptake from 40 to 120% exhibited hydroxide conductivity at 80 °C from 13 to 45 mS/cm. By using different ratios of monomers with and without tolyl substituents, the distribution of cationic groups can be controlled. AEMs with a smaller amount of tolyl-substituted monomer had their cationic sites located more closely when IEC values were the same. This allowed membranes with the same IEC values exhibited lower water uptake, better dimensional stability, and higher hydroxide conductivity. These AEMs also demonstrated good alkaline stability. In the case of 40MePh-1.72 with an IEC value of 1.72 mmol/g, the hydroxide conductivity and IEC remained at 78 and 81% of their original values, respectively, after 720 h of an alkaline stability test. A single fuel cell based on 40MePh-1.72 exhibited an open circuit voltage of 1.03 V and peak power density of 291 mW/cm2 at 60 °C using platinum loading of 0.5 mg/cm2. It performed better than a single fuel cell based on commercially available FAA-3 with an open circuit voltage of 1.02 V and peak power density of 226 mW/cm2 under the same testing conditions.