Preparation and characterization of composite proton exchange membranes for fuel cell applications

Date of Completion

January 2000

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

Various composite membranes were prepared and characterized by X-ray diffraction (XRD), AC impedance spectroscopy, differential scanning calorimetry (DSC), equilibrium contact angles, and methanol transport measurements to obtain better membranes for use either in a direct methanol fuel cell or in a hydrogen fuel cell. Results showed that methanol crossover rates for the composite membranes containing various particles decreased with increasing particle content. However, the resistance increased dramatically with increasing particle content especially for kaolin particles due to preferential orientation of clay materials resulting in significant contact resistance. A polymer blend containing the PVDF and Nafion® prepared by casting a PVDF/Nafion ® solution led to a methanol crossover reduction and resistance increase with increasing PVDF content. The equilibrium contact angles of water drops on the Nafion®/PVDF blend membranes as a function of the PVDF content were similar to the plot of the advancing angle versus the percentage of the lower-surface-energy phase. X-ray diffusion on studies indicated that these two polymers crystallized separately when blended and cast from their solution, and the crystallization behavior was equivalent to that of the unblended state. DSC revealed that when PVDF was mixed with Nafion® in their solution forms, an inter-diffusion or other interaction takes place at the interface between their non-crystalline regions. ^ Zirconium hydrogen phosphate, a protonic conductive solid, was impregnated into the porous structure of Nafion® by in-situ formation between zirconyl cation and phosphoric acid. Nafion ®-zirconium hydrogen phosphate (NZHP) membranes resulted in good conductivity and superior characteristics at elevated temperatures (over 100°C). Membranes containing phosphotungstic acid particles (a proton conductor) bound by Nafion® in the porous structure of Teflon® were found to maintain good conductance at elevated temperature, significantly better than Nafion® 117. This elevated temperature results in an improvement of the kinetics of the anode reactions and a simultaneous reduction of carbon monoxide poisoning. ^ Membrane-electrode assemblies (MEAs) for membranes were characterized and fabricated using various techniques. Single cell testing results of various MEAs were conducted and the results were compared. ^

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