Title

Composite membranes and membrane electrode assemblies for fuel cells

Date of Completion

January 2002

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) operating at elevated temperatures (>100°C) enhance the CO-tolerance of the anode catalyst and improve the system efficiency. However, Nafion®, the most widely used membrane, suffers high resistance (Nafion® 1135 > 0.6 Ω-cm2 at 120°C and 30% relative humidity) due to dehydration. Nafion®-Teflon® -Zr(HPO4)2 composite membranes with lower resistance (<0.3 Ω-cm2 at 120°C and 30% relative humidity) were prepared. The reduction of membrane thickness and the additional proton exchange sites for proton conduction both contributed to improve the resistance. The main performance loss at elevated temperature was from low protonic conductivity and oxygen permeability in the cathode catalyst layer and dehydration of the membrane. ^ Two major obstacles limit the wide spread commercial applications of Direct Methanol Fuel Cells (DMFCs) in the portable power supplies: high methanol crossover (permeation) through the Nafion® membrane and low activity of methanol oxidation catalysts. Operating DMFCs at elevated temperatures reduces methanol crossover and accelerates the anode reaction rate. To overcome the high resistance of Nafion® membranes at high temperature, solid acid Zr(HPO4)2 was mixed with Nafion® to form a composite membrane with improved protonic conduction at low relative humidity. A decrease in methanol crossover by a factor of five occurs when the temperature is increased from 80°C to 105°C. However, fuel cell performance at 105°C was not as good as that at 60°C mainly due to the “dry-out” of the cathode catalyst layer at elevated temperature. ^ Tri-layer membranes that are composed of one central methanol-barrier layer and two conductive layers have been developed to suppress the methanol crossover through the polymeric membrane in liquid-fed DMFCs. Nafion ®-PVDF was used as a central methanol-barrier layer with two outside Nafion® layers. Both proton conduction and methanol diffusion were regulated by the properties of the PVDF-Nafion layer. The thickness of the barrier layer and the PVDF content affects methanol crossover and proton conductivity of the tri-layer membrane. Good MEA performance occurred because of lower methanol crossover and the presence of a good membrane and electrode interface for proton transfer. ^