Polysulfonated Fluoro-oxyPBI Membranes for PEMFCs: An Efficient Strategy to Achieve Good Fuel Cell Performances with Low H3PO4Doping Levels

Polybenzimidazoles (PBIs) are promising materials to replace Nafion as the electrolyte in polymer electrolyte membrane fuel cells (PEMFCs). The challenge with these materials is to achieve a good compromise between the H 3PO4 doping level and membrane stability. This can be obtained by a proper monomer design, which can lead to better performing membrane electrode assemblies (MEAs), in terms of durability, acid leaching, and electrode safety. Here the easy and inexpensive synthesis of hexafluoropropylidene oxyPBI (F6-oxyPBI) and bisulfonated hexafluoropropylidene oxyPBI (F6-oxyPBI-2SO3H) is reported. The membranes based on F6-oxyPBI-2SO3H are more stable in an oxidative environment and more mechanically resistant than standard PBI and F6-oxyPBI. Whereas the attainable doping levels are low because of fluorine-induced hydrophobicity, polysulfonation allows high proton conductivity, and fuel cell performances better than those reported for MEAs with F6PBI- or PBI membranes with much higher doping levels. In the case of MEA with a F6-oxyPBI-2SO3H membrane, a peak power density of 360 mW cm-2 is measured. Fuel cell performances of 604 mV at 0.2 A cm-2 are maintained for 800 h without membrane degradation. Low H2 permeability is measured, which remains almost unaffected during a 1000 h life-test.