Well-designed polyphenylene PEMs with high proton conductivity and chemical and mechanical durability for fuel cells

For highly proton conductive and durable proton exchange membranes, we designed and synthesized a new series of sulfonated polyphenylene ionomers (SPP-TFP) containing trifluoromethyl substituents with different ion exchange capacities (IEC). The resulting ionomers had high molecular weight (M_n = 51.2-123.4 kDa and M_w = 96.1-556.1 kDa) with reasonable polydispersity (3.8-5.4). The ionomers were highly soluble in some organic solvents such as DMSO and ethanol and provided bendable and ductile membranes by solution casting. The SPP-TFP-3.5 membrane exhibited the best balanced properties as proton exchange membranes; the proton conductivity was 7.5 mS cm⁻¹ at 20% RH and 80 °C and the maximum strain was 155 ± 5%. The fuel cell performance of the SPP-TFP-3.5 membrane was comparable with that of the Nafion NRE 211 membrane. Furthermore, in the accelerated combined chemical and mechanical durability test based on wet/dry cycling at open circuit voltage (OCV) at 90 °C supplying H₂ (anode) and air (cathode), the SPP-TFP-3.5 membrane (9847 cycles and 46.5 h) outperformed Nafion NRE 211(8788 cycles and 41.5 h). Furthermore, compared with Nafion NRE 211 (46.7 μV h⁻¹ of average decay), SPP-TFP-3.5 showed negligible change in the cell voltage at 0.15 A cm⁻² under 90 °C and 30% RH (air/H₂) for 300 h. Such high durability and performance in practical fuel cells have not been reported for aromatic ionomer membranes.