Proton exchange membrane fuel cells (PEMFCs) could one day equip a new generation of automobiles that emit only water. This would require replacing the current perfluorinated and sulfonated membrane (Nafion®) with a more effective material in terms of cost, environmental impact and mechanical resistance (above 80°C). Alternatives to Nafion®, such as sPEEK (sulfonated polyether ether ketone), have excellent mechanical properties but suffer from a shorter lifetime and lower performance. Despite its excellent thermomechanical properties, sPEEK is in fact oxidized in the fuel cell during operation, which drastically reduces its lifetime to a few hundred hours (instead of tens of thousands of hours for Nafion®).

The Irig researchers therefore chose sPEEK to rapidly evaluate their chemical membrane stabilization strategy. They developed a sol-gel phase based on MPTS (3-mercaptopropyltrimethoxysilane) within sPEEK membranes in order to protect its hosts from oxidizing species generated during the fuel cell operation.

The scientists studied the complex structure of these new hybrid membranes in relation to their functional properties and durability. They observed nanostructures at ranges from a few nm to a few hundred nm, using a combination of atomic force microscopy (AFM), focused ion beam scanning electron microscopy (3D FIB-SEM), small-angle neutron scattering (SANS) and wide-angle x-ray scattering (WAXS).

The AFM images show that the sol-gel phase is distributed in large spherical domains ranging from 100 to 200 nm in diameter depending on the sol-gel content. The sol-gel nanoparticles likely grow in the less compact spaces (interbundles) of the host sPEEK membrane. SANS results reveal that the sol-gel phase has a hierarchical organization and that the "ion channels" are only compressed by the sol-gel phase insertion, which helps preserve the ionic conduction of the sPEEK.

This multiscale study has made it possible to specify the localization of the sol-gel phase associated with the chemical stabilization of the membrane.

This work is being continued with the development of stabilizing and regenerative sol-gel phases, which holds great promise for increasing the lifetime of the membranes.

The neutron scattering experiments were conducted at the Institut Laue-Langevin in Grenoble (D22 beam line).