Preparation And Fuel Cell Application Of Vinyl-addition Type Sulfonated Polynorbornene Membranes And Its Phosphotungstic Acid/Silica Hybrid Membranes

Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising technology for clean and efficient power generation in the twenty-first century. Proton exchange membranes (PEMs) are the key components in fuel cell system. The researchers have focused to reach the proton exchange membrane with high proton conductivity, low electronic conductivity, low permeability to fuel, excellent chemical/thermal stability, good mechanical properties and low cost. Current PEMFC technology is based on expensive perflourinated proton-exchange membranes (Nafion membran) that operate effectively only under fully hydrated conditions and possess high methanol crossover. To settle the problems mentioned above, fluorine-free materials with properties comparable to those of Nafion, based on sulfonated aromatic polymers, irradiation graft polymers, and cross-linked and blend polymers, were successfully proposed.Compares to the Nafion membrane, vinyl-addition type polynorbornene membrane has lower production costs, thermal stability, high glass transition temperature and superior methanol barriers properties, it is believed to be more suitable for eventual application in direct methanol fuel cells (DMFCs). Novel sulfonated poly(2-butoxymethylene norbornene-co-2-(6-phenoxy-hexylox-ymethylene)-5-norbornene [sP(BN/PhHN)] were prepared successfully through vinyl addition type polymerization and then sulfonated with concentrated sulfuric acid (98%) as sulfonating agent in a component solvent. The sP(BN/PhHN)-40with the max. Degree of sulfonation (DS) of40%can be obtained by controlling the sulfonating reaction time from8h to20h, and a proton conductivity of3.35×10-3S cm-1was achieved at70℃. The methanol permeabilities of these membranes were in the range from0.26to6.58×10-7cm2s-1, which were remarkably lower than Nafion (2.36×10-6cm2s-1). TEM analysis revealed that these side-chain type membranes have a microphase separated structure composed of hydrophilic side-chain domains and hydrophobic polynorbornene main chain domains. Sulfonated polynorbornene containing soft spacers displayed better properties, such as lower water uptake, high thermal properties, mechanical properties and low methanol permeability.A series of novel hybrid sulfonated polynorbornene-silica (PBN-SiO2) proton conducting membranes doped with different weight ratio of phosphotungstic acid (PWA) was prepared by the casting procedure. The PWA is an acid catalyst for sol-gel process and the hydrated PWA is helpful to enhance proton conductivity. The chemical structures of the hybrid membranes were confirmed by FT-IR spectroscopy and transmission electron microscopy (TEM), the results indicated that PWA particles were well dispersed in these membranes. The proton conductivity of the composite membrane containing40wt.%PWA reaches the maximum of6.1×10-2S cm-1and increased gradually with PWA content and temperature elevating, while the methanol permeabilities (3.52-9.39×10-7cm2s-1) of these membranes were much lower than that of Nafion117(2.36×10-6cm2s-1). Operating at80℃and under constant air flux, the cell delivers a power density of63mW cm-2and a current density of132.3mW cm-2, the membranes also exhibited excellent thermally stable, mechanical properties and oxidation stability. These results imply that the PBN/SiO2/PWA hybrid membranes are promising materials in the direct methanol fuel cells (DMFC) applications.