Ion track grafting: A way of producing low-cost and highly proton conductive membranes for fuel cell applications

International audienceKeywords: Ion track grafting Radiografting Swift heavy ions Proton conductivity Polymer electrolyte membrane Proton exchange membrane fuel cell a b s t r a c t Proton conductive individual channels through a poly(vinyl di-fluoride) PVDF matrix have been designed using the ion track grafting technique. The styrene molecules were radiografted and further sulfonated leading to sulfonated polystyrene (PSSA) domains within PVDF. The grafting process all along the cylindrical ion tracks creates after functionalisation privileged paths perpendicular to the membrane plane for proton conduction from the anode to the cathode when used in fuel cells. Such ion track grafted PVDF-g-PSSA membranes have low gas permeation properties against H 2 and O 2. A degree of grafting (Y w) of 140% was chosen to ensure a perfect coverage of PSSA onto PVDF-g-PSSA surface minimizing interfacial ohmic losses with the active layers of the Membrane Electrolyte Assembly (MEA). A three-day fuel cell test has been performed feeding the cell with pure H 2 and O 2 , at the anode and cathode side respectively. Temperature has been progressively increased from 50 to 80 • C. Polarisation curves and Elec-trochemical Impedance Spectroscopy (EIS) at different current densities were used to evaluate the MEA performance. From these last measurements, it has been possible to determine the resistance of the MEA during the fuel cell tests and, thus the membrane conductivity. The proton conductivities of such membranes estimated during fuel cell tests range from 50 mS cm −1 to 80 mS cm −1 depending on the operating conditions. These values are close to that of perfluorosulfonated membrane such as Nafion ® in similar conditions

Caractérisation de la structure des membranes ionomères (NAFION^{\mbox{\tiny\textregistered}}) par diffusion de rayons X aux petits angles

Ionomer membranes, like Nafion^{\mbox{\tiny\textregistered}} used in fuel cell, present a nano- phase separation between domains with different ionic concentrations. Up to now models describe the ionic domains as spheres of about 40 Å diameter. Small angles X-ray scattering studies over a large range of wave vectors, lead to a new assumption for the Nafion structure, describing the polymer aggregation as elongated objects surrounding by the ionic charges.Les membranes ionomères de type Nafion^{\mbox{\tiny\textregistered}} utilisées en pile à combustible, sont caractérisées par une nano-séparation de phases entre des domaines plus ou moins riches en sites ioniques. Les modèles proposés pour décrire ces domaines ioniques, les représentent généralement sous forme de sphères de 40 Å de diamètre. L'étude en diffusion de rayons X que nous avons menée récemment, sur une large gamme de vecteurs d'ondes, nous permet de proposer une vision différente de la structure du Nafion en considérant une agrégation de polymères sous formes d'objets très allongés, avec en surface les charges ioniques

Elaboration of Nanostructured and Highly Proton Conductive Membranes for PEMFC by Ion Track Grafting Technique.

International audienceDespite 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