Relationships between microstructure and transport properties of proton-conducting porous PVDF membranes

Fluorinated polymers are today investigated as possible alternatives to Nafion™ in PEM Fuel Cells. In this paper, we study the relationships between the microstructure and the proton transport of porous PVDF homopolymer membranes swollen by 11 M aqueous solution of H3PO4. The analysis is performed on membranes with different nominal pores size values, dp. The membranes are thermally stable at least up to 250 °C. A dependence of the proton transport on the pores size has been found, that is particularly evident for low dp values and at low relative humidity (R.H.). Conductivity values exceeding 0.1 S cm-1 are obtained at 80 °C even at 10% R.H. for dp ≥ 0.22 μm. We show that the behaviour of the transport properties cannot be simply rationalized in terms of dp, but it requires an accurate knowledge of the membrane microstructure (tortuosity, pores interconnections and size distributions). 31P NMR spectroscopy also shows that anisotropic interactions take place between the components of the solution and at a lower extent, between the solution and the pores walls, also for dp values in the micrometer range. In order to make a preliminary check of the suitability of these membranes for applications in Direct Methanol Fuel Cells (DMFCs), studies of methanol crossover and diffusion through the membrane have been carried out and compared with those of Nafion™