New hybrid inorganic-organic proton conducting membranes based on Nafion and a [(ZrO2)•(Ta2O5)0.119] oxide core-shell nanofiller

The current state-of-the-art proton-conducting membranes for application in fuel cells and electrolysers are perfluorinated ionomers such as Nafion® and Aquivion®. These materials feature excellent chemical and electrochemical stability and very high proton conductivity in fully-hydrated conditions. However, perfluorinated ionomers do not yield good performances in a dry state and at temperatures higher than 80-90°C. To overcome these deficiencies, a new family of hybrid inorganic-organic membranes based on Nafion and a “core-shell” nanofiller consisting of ZrO2 nanoparticles acting as the “core” covered by a thin layer of Ta2O5 as the “shell” is proposed. Six hybrid membranes with a nanofiller content of 3, 5, 9, 11, 13 or 15 wt% are prepared by a standard solvent-casting procedure. These systems are extensively characterized by thermoanalytical techniques (water uptake determination, HR-TGA, MDSC, DMA), vibrational spectroscopy (FT-IR ATR), SEM, HR-TEM, and broadband electric spectroscopy (BES). In general, the addition of nanofiller reduces the water uptake and improves the mechanical properties of the hybrid membranes as compared to pristine Nafion. Promising conductivities are obtained for the hybrid membranes at T=115°C (7.5x10-2 Scm-1 at 9wt% nanofiller vs. 3.3x10-2 Scm-1 for Nafion). The interplay between the structure and the proton conduction mechanism is investigated and reveals the presence of two electric polarizations associated with structural inhomogeneities of the materials. Finally, the best performing hybrid membrane (9 wt% nanofiller) is used to manufacture a membrane-electrode assembly (MEA) which is tested in single fuel cell configuration. The results demonstrate an improved fuel cell performance at low hydration levels in comparison with pristine recast Nafion.