Composite Membranes with Sulfonic and Phosphonic Functionalized Inorganics for Reduced Relative Humidity PEM Fuel Cells

Nafion/inorganic (inorganic = S-ZrO2, SiO2-SO3H, SBA-15-SO3H, MCM-41-SO3H, and P-Si gels) composite membranes were synthesized with the goal of increasing the proton exchange membrane (PEM) fuel cell performance at elevated temperatures up to 120°C and reduced relative humidity (RH) down to 25%. Inorganic materials were incorporated within the Nafion by three different techniques. The conductivity of composite membranes with sulfonated inorganic materials was not higher than the conductivity of the recast Nafion in the whole range of inorganic loading. Conductivity of the Nafion/P-Si gel membranes was affected by the surface of inorganic powders and was higher for membranes prepared by in situ formation techniques. At 120°C and 70% RH, the conductivity of some composite membranes reached 100 mS cm−1. Effect of the inorganic phase on the membrane performance in a fuel cell was much stronger than on its conductivity probably due to substantial improvement in membrane water retention properties. At 120°C and 50% RH, the cell resistance of the P-Si composite membrane was found to be 40% lower than that of Nafion. By applying the modified rule of mixtures, the composite proton conductivity was calculated and compared with experimental data.