Preparation of PTFE-based fuel cell membranes by combining latent track formation technology with graft polymerization

Yoshida, Masaru*; Kimura, Yosuke*; Chen, J.; Asano, Masaharu; Maekawa, Yasunari

The swift heavy ions of 56 MeV Nwere irradiated with particle fluences of 3 10 ions/cm to form the latent track zone for a 25 m-thick film of polytetrafluoroethylene (iPTFE). The styrene (St) was grafted onto the iPTFE films by UV-irradiation and pre--irradiation and the resulting iPTFE-based proton conducting membranes were obtained after sulfonation, namely, iPTFE-g(UV)-PStSA and iPTFE-g()-PStSA membranes, which have a straight cylindrical damage zone around the ion path. The degree of grafting was obtained to be about 7.5% in both the UV-method and the -method. The ion exchange capacity, proton conductivity in thickness direction, MeOH permeability, tensile strength and elongation at break of the obtained iPTFE-g(UV)-PStSA membrane were 0.50 mmol/g, 0.06 S/cm, 0.15 10 cm/s, 50 MPa and 600%, in contrast to 0.06 mmol/g, 0.06 S/cm, 0.35 10 cm/s, 19 MPa and 210% for the iPTFE-g()-PStSA membrane, respectively. For comparison, the Nafion 112 measured in our laboratory showed an ion exchange capacity of 0.91 mmol/g, a proton conductivity of 0.06 S/cm, a MeOH permeability of 1.02 10 cm/s, a tensile strength of 35 MPa and an elongation at break of 295%. It could conclude from these data that the lower crossover of MeOH, proton conductibility of the Nafion corresponding even in lower ion exchange capacity and higher mechanical properties of the UV-grafted proton-conducting membranes make them promising materials for widespread application in direct methanol fuel cell.