Proton-conducting phosphotungstic acid/sulfonated fluorinated block copolymer composite membrane for polymer electrolyte fuel cells with reduced hydrogen permeability

摘要

A sulfonated fluorinated block copolymer (SFBC) consisting of sulfone and ether bridges was synthesized via nucleophilic substitution polymerization, and then incorporated with 10, 20 or 30 wt% of phosphotungstic acid (PWA) using facile solution casting approach to fabricate composite membranes. The monomer sulfonation was carried out for SFBC to avoid the random sulfonation that can degrade the mechanical strength of polymer chains. A higher local concentration of acidic moieties within molecular frameworks of PWA and good mechanical properties allow for an unprecedented approach to tailor the proton conductivity as well as mechanical strength of composite membrane, as evidenced by alternating current (AC) impedance and dynamic mechanical (DMA) analyses. The surface morphological properties and roughness of membranes were investigated by field emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM); the composite membranes exhibited even and denser dispersion of PWA in polymer matrix. Compared to pristine SFBC-50, the water uptake, hydrophilicity and ion exchange capacity of SFBC-50/PWA-X membranes were significantly improved. The peak proton conductivity of SFBC-50/PWA-30 membrane at 90 A degrees C under 100% relative humidity (RH) is 105.22 mS/cm, which is 2.55 folds higher than that of pristine SFBC-50 (41.11 mS/cm) and 1.2 folds lower than that of Nafion-117 membrane (127.12 mS/cm). The peak power density delivered by the PEFC containing SFBC-50/PWA-30 membrane is 377.83 mW/cm(2) at a load current of 864.29 mA/cm(2) while operating the cell at 60 A degrees C under 100% RH. In contrast, under identical condition, the pristine SFBC-50 membrane delivered the peak power density of 147.37 mW/cm(2) at a load current of 353.52 mA/cm(2), a 2.56-fold lower performance compared to composite membrane. Furthermore, composite membrane exhibited much lower H-2 permeability compared to that of SFBC-50 and Nafion-117 membrane.