Synthesis and characterization of hybrid inorganic-organic proton conducting membranes for PEMFCs

The scientific community and industry show a keen interest towards proton exchange membrane fuel cells (PEMFCs) owing to their low environmental impact, high conversion efficiency and energy density, and applicability in a wide range of systems including light-duty vehicles and portable electronic devices. The proton exchange membrane (PEM) is a critical component of PEMFCs as it allows the transport of the H3O+ ions evolved at the anode to the cathode, where oxygen is reduced to water. Today’s reference PEMs feature perfluorinated main chains functionalized with perfluoroether side chains tipped with highly acidic -SO3H groups. These materials (e.g., DupontTM Nafion®, Asashi Aciplex®, Dow®, 3M and Flemion®) are characterized by a high chemical, thermal and mechanical stability and an excellent proton conductivity at high hydration levels. This latter requirement limits the widespread commercial application of conventional PEMs, which have inadequate proton conductivity at temperatures above 90°C and at low hydration levels. PEMFCs capable of operating above 120°C and at low levels of hydration would not require bulky and expensive water management modules, simplify thermal management and reduce the impact of catalyst poisons. In an effort to overcome the above-mentioned limitations of conventional PEMs, this work overviews the synthesis and characterization of new alternatives to standard perfluorinated ionomers. The materials are prepared according to two distinct strategies: 1) doping Nafion to improve its thermo-mechanical properties and proton conductivity or extend its operating conditions to temperatures above 100°C and anhydrous conditions; 2) synthesis and characterization of PEMs based on polybenzimidazole and polysulfone as an alternative to perfluorinated polymers. In the first strategy two different systems are obtained, by doping a Nafion membrane with the [(ZrO2)(Ta2O5)0.119] inorganic “core-shell” nanofiller or with two different proton conducting ionic liquids, triethylammonium methanesulfonate and triethylammonium perfluorobutanesulfonate. The second strategy is focused on the study of new PEMs alternative to perfluorinated polymers such as polybenzimidazole and sulfonated poly(p-phenylenesulfone) membranes, whose properties are modulated by the addition of phosphoric acid and an hybrid filler or poly(1-oxotrimethylene) and silica, respectively. All the membranes are extensively characterized in terms of their thermal, mechanical, structural and electrical properties to highlight the interactions between the different components. These interactions play a crucial role to determine the macroscopic properties of the membranes and affect strongly their behavior under operating conditions in fuel cells.