Sulfonated phenolphthalein poly(ether sulfone) (SPES C) with various sulfonation degree were synthesized by reaction of PES C with sulfonic acid as sulfonating agent and solvent.Their proton conductivity and methanol permeability as a function of the degree of sulfonation were investigated.For 70 21% degree of sulfonation,proton conductivity was 3 45×10 -2 S/cm at 100℃,which is closed to or superior to that of Nafion 115 membrane at the same conditions (4 19×10 -2 S/cm).Methanol permeability of SPES C was considerable smaller than that of Nafion 115 membrane.Because of their high conductivity and low methanol permeability,SPES C membranes appear to be excellent candidate for use in DMFC applications.
In order to solve the methanol crossover problem caused by the poor alcohol rejecting ability of the Nafion TM membranes currently used in direct methanol fuel cell (DMFC),polyvinylidene fluoride(PVDF)was blended with Nafion material to prepare a new type of membrane for DMFC and its ionic conductivity and the methanol permeability was investigated.PVDF was used here to regulate the ratio between hydrophilic group and the hydrophobic group exist in the blend membranes according to the hydrophilic hydrophobic balance theory proposed in pervaporation.The adding of PVDF lead to a great improvement of the membrane’s MeOH rejection.When PVDF content was increased from 25% to 75%,the MeOH permeability was reduced from～10 -7 to ～10 -9 cm 2/s,1 to 3 orders of magnitude lower than that of Nafion117.The membrane conductivity decreased,although remaining at high value(～10 -4 S/cm),with the increase of PVDF content.Compared with the Nafion117 membrane,the PVDF Nafion blend membrane with only 25% of Nafion showed an improvement in performance by about 40 times,if the ratio( σ/P ) was adopted to characterize the performance of a membrane for DMFC.Percolation theory,cocontinuous morphology features and phase transition model were applied to explain the behavior of membrane conduction.
