Elaboration of Nanostructured and Highly Proton Conductive Membranes for PEMFC by Ion Track Grafting Technique.
Résumé
Despite some serious drawbacks (high cost, conductivity losses at high temperature, water swelling shortening the membrane durability), Nafion® is still the reference as membrane for PEMFC. In order to develop a new type of proton conductive membrane, our strategy is based on the utilization of swift heavy ions (SHI) grafting process to create nanometric cylindrical proton conductive pathways, in the thickness direction of the membrane, to enhance proton conduction from the anode to the cathode. The mechanical and dimensional stability of the proton exchange membrane was insured by the pristine PVDF matrix. In particular, a poly(vinyl di-fluoride) (PVDF) matrix was irradiated with SHI to obtain radically active latent tracks in the polymer film. Styrene was then radiografted and further sulfonated into these irradiated cylindrical regions, leading to sulfonated polystyrene (PVDF-g-PSSA) domains within PVDF. The role of the grafting degree and fluence of irradiation of the PVDF matrix on PVDF-g-PSSA membranes properties (chemical composition, ion exchange capacity, water uptake) was investigated. Then, a membrane-electrode assembly (MEA) was prepared and fuel cell tests have been performed. The cell temperature was progressively increased from 50°C to 80°C. Polarization curves and electrochemical impedance spectroscopy (EIS) at different current densities were used to evaluate the membrane-electrode assembly (MEA) performances. Our results clearly show that PVDF-g-PSSA membranes with a grafting degree of about 140% (PVDF-g-PSSA 140%), obtained after irradiation at a fluence of 1010 ions/cm2, lead to proton conductivities ranging from 30 mS/cm to 60 mS/cm depending on the operating conditions. These values are close to those of a Nafion® membrane tested in the same conditions. However, the durability of these membranes is limited to about 70 hours due to high stiffness of the membrane that weakens mechanical properties during fuel cell operation. To increase the durability, one solution was to decrease the fluence. The decrease of the fluence leads to membranes with lower grafting yield (about 45%). However, despite the lower grafting degree and the lower amount of sulfonated groups, fuel cell performances are similar to those of (PVDF-g-PSSA 140%) membrane. This result indicates that the cylindrical nanocomposite structure plays a key role in the enhancement of the proton conductivity. Moreover, the good fuel cell performances are associated to adequate mechanical properties that improve the durability of the membrane. In conclusion, our work demonstrates that SHI grafting is a powerful and low cost (about 200 US$/m2 of membrane) technique to obtain a specific and controlled nano-scale structure allowing a good trade-off between adequate mechanical stability and high proton conductivity.