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

Swift heavy 56 MeV 15N3+ ions were generated with particle fluences of 0, 3×106, 3×107, 3×108, 3×109 ions/cm2 to form a latent track zone in a 25-μm-thick film of polytetrafluoroethylene (iPTFE). Styrene (St) was then grafted onto the iPTFE films by UV-irradiation or pre-γ-irradiation, and after sulfonation iPTFE-based proton-conducting membranes were obtained, here called, iPTFE-g(UV)-PStSA and iPTFE-g(γ)-PStSA membranes, respectively, which had a straight cylindrical damage zone around the ion path. The degree of grafting was found to be about 7.5% with a particle fluence of 3×107 ions/cm2 and with either the UV-method or the γ-method. The ion-exchange capacity, proton conductivity in the 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, 0.15×10−6 cm2/s, 50 MPa and 600%, in contrast to 0.06 mmol/g, 0.06 S/cm, 0.35×10−6 cm2/s, 19 MPa and 210% for the iPTFE-g(γ)-PStSA membrane, respectively. In comparison, the Nafion 112 measured in our laboratory exhibited an ion-exchange capacity of 0.91 mmol/g, a proton conductivity of 0.06 S/cm, a MeOH permeability of 1.02×10−6 cm2/s, a tensile strength of 35 MPa and an elongation at break of 295%. It can be concluded from these data that the lower crossover of MeOH, the same proton conductibility, the lower ion-exchange capacity, and the superior mechanical properties of the UV-grafted proton-conducting membranes compared to the Nafion make them promising materials for widespread application in direct methanol fuel cells. On the other hand, the tests of mechanical strength showed that the PTFE base film is subject to degradation by the ion-beam irradiation as well as the γ-irradiation.