Development and characterization of sulfonated polysulfone membranes for direct methanol fuel cells

Non perfluorinated ionomer membranes based on thermostable sulfonated aromatic polymers like polyethersulfones have been proposed as alternative materials at the traditional perfluorosulfonic membranes. The modification of polymer has been carried out through a mild sulfonation process that is useful to prepare proton conducting membranes with different sulfonation level and interesting electrochemical performance. The sulfonated polysulfone (SPSf) membranes were synthesized using trimethyl silyl chlorosulfonate as sulfonating agent in homogeneous solution of chloroform. The silyl sulfonate polysulfone is modified in sulfonate form by sodium methoxide reagent. The proton conducting polymer with a degree of sulfonation, varying from 50 to 70%, was easily obtained by changing the mole ratio between the sulfonating agent and the monomer unit of polymers. The sulfonate polysulfone membranes were prepared starting from polymer solution in dimethyl acetamide (DMAc) by conventional casting process. Proton conductivities of about 5·10-2 S·cm-1 at 90°C were found rendering these membranes interesting for a possible use in direct methanol fuel cell applications. The effect of cell temperature on fuel cell performance for a sulfonated polysulfone membrane based MEA (membrane & electrode assembly) in the presence of methanol/O2 and/or air was investigated. A maximum power density of 170 mW cm-2 was recorded at 120°C with 2M methanol/oxygen and a sulfonate polysulfone membrane. Further studies are in progress to improve their structure, thermal stability, proton conductivity and the as well as to the fuel cell performance and life time stability of the membranes.