Fluorinated Polyarylene with Branched Units for Enhanced Proton Exchange Membrane Stability in Fuel Cells

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The proton exchange membrane (PEM) is a critical component of proton exchange membrane fuel cells (PEMFCs), for which its durability and performance are of paramount importance. This study presents a series of four-armed, star-shaped branched polymers (SPTP-T x ) with tunable branching degrees, synthesized via a straightforward two-step method, for application in PEMFCs. The branching moiety, tetraphenylethylene (TPM), imparts an increased free volume and a well-defined microphase-separated structure to the polymers, enabling a high proton conductivity of up to 195 mS cm –1 at 80 °C. Furthermore, the multiple branching sites in TPM monomers facilitate an increase in the polymer’s weight-average molecular weight to 113 kg mol –1, which in turn yields a high tensile strength of 67.3 MPa. The membrane also demonstrates outstanding oxidative stability, retaining over 77.2% of its mass after 12 h of immersion in Fenton’s reagent at 68 °C. In H 2 /air fuel cell tests, the membrane electrode assembly (MEA) based on SPTP-T 0.10 exhibits a very low hydrogen crossover flux of 1.46 × 10 –9 mmol cm –2 s –1 yet achieves a high peak power density of 648 mW cm –2, both values being markedly superior to those of the linear SPTP membrane. Furthermore, during a 100 h accelerated stress test at 80 °C and 30% relative humidity, the open-circuit voltage (OCV) decay rate was only 0.667 mV h –1 . This work demonstrates that incorporating TPM as a branched unit is an effective strategy for enhancing the critical PEM properties, including oxidative stability and proton conductivity, offering valuable insights for developing high-performance PEMFCs.