Trifluoromethylated and Sulfonated Polyphenylenes as Proton Exchange Membranes for Fuel Cells

Sulfonated polyphenylenes (SPPs) are regarded as one of the most promising next-generation proton exchange membranes (PEMs) due to excellent thermal and chemical stability. In this study, a series of sulfonated polyphenylene ionomers with various IECs (3.0–4.0 mequiv g–1), SPP-TP-13tfm (“13” indicating substitution position of CF3) containing trifluoromethylphenylene, are designed and synthesized. Notwithstanding the intrinsic rigidity of the polyphenylene backbone, the introduction of trifluoromethyl provides excellent membrane-forming capacity because it importantly decreases the persistence length of the polyphenylene backbone. The proton conductivity of the SPP-TP-13tfm-4.0 membrane reaches as high as 563.0 mS cm–1 at 80 °C, which is significantly higher than that of the Nafion NRE 211 membrane (160.8 mS cm–1). Furthermore, under the conditions of 30% RH and 80 °C with supplying H2/air, the maximum power density of the SPP-TP-13tfm-4.0 membrane (846 mW cm–2) is increased by 16.1%, compared to that of the Nafion NRE 211 membrane (729 mW cm–2, 109 mΩ cm2), due to the low ohmic resistance (71 mΩ cm2). Therefore, SPPs modified with trifluoromethyl provide a strategy to be the next-generation PEMs.