Ionization, hydration and intermolecular H-bonding in fuel cell membranes

Proton exchange membrane fuel cell (PEMFC) is a promising technology for powering automobiles and small portable electronics. One major challenge is to improve the proton-conducting polymers that constitute the heart of the fuel cell, the membrane-electrode assembly. Antagonist properties are desired: thermal and mechanical stability on the one side, excellent proton conductivity on the other side. The current technical standard of choice for these materials is the family of polymers known as perfluorosulfonic acid (PFSA) ionomers, among them the benchmark material, Nafion [1]. Recently new original aromatic multi-blocs copolymers functionalized with the same ionic function as the one in Nafion have been synthetized [2,3]. The combination of the AILES beamline and the home-made hydration cell, allow to perform in situ hydration sequences (hydration/dehydration including drying) of mesoporous samples, with real-time acquisition of FIR and MIR spectra. It has allowed to i) elucidate the mechanism of sulfonic acid ionization and concomitant hydronium ion formation, ii) establish the state and properties of adsorbed water molecules, iii) probe the existence of hydration thresholds related to the solvation of SO3-/H3O+ species and formation of a multi-connected H-bond network and iv) to highlight the mechanism which govern the proton conductivity.