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.
