Effect of TiO[sub 2] Surface Properties on Performance of Nafion-Based Composite Membranes in High Temperature and Low Relative Humidity PEM Fuel Cells

Nafion/10% (mass % is used in this paper) TiO 2 composite membranes were studied in a H 2 /O 2 proton exchange membrane (PEM) fuel cell over a range of relative humidity (RH) from 26 to 50% at temperatures of 80 and 120°C.According to the scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) structural analysis, the composite Nafion/TiO 2 membranes had a two-layer structure, one layer enriched with TiO 2 particles, and the other dominated by the Nafion polymer. Although the TiO 2 particles were mainly concentrated on one side of the composite membrane, sufficient hydration was apparently achieved for the whole membrane. Two TiO 2 (rutile) powders used for the preparation of the composite membranes differed in specific surface area (SSA), surface zeta potential, and particle morphology. A TiO 2 powder with five times higher SSA, 22 mV higher zeta potential (at the low-pH limit), and distinctly different individual particle and particle aggregate morphologies resulted in a four-times increase of current density (at 0.6 V) when the composite membranes were made and tested in PEM fuel cell at temperature of 120°C and relative humidity of 26%. We speculate that a greater number of protonated sites per unit mass of powder in the membrane and a higher density of the protonated sites contribute to the enhanced fuel cell performance.