Development and characterization of proton conductive membranes and membrane electrode assemblies for fuel cells

Date of Completion

January 2005

Keywords

Chemistry, Polymer|Engineering, Chemical|Energy

Degree

Ph.D.

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs), including hydrogen fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), are considered as attractive electrical power sources. However, there are some technical obstacles that impede the commercialization of PEMFCs. For instance, in H 2-PEMFCs, carbon monoxide (CO) poisoning of the anode catalyst causes serious performance loss; in DMFCs, methanol crossover through the membrane reduces the overall fuel cell efficiency. This work focused on: (1) developing high performance membrane electrode assemblies (MEAs) and investigating their behavior at higher temperature H2-PEMFC with H2+CO as the fuel; (2) improving DMFCs efficiency by preparing low methanol crossover/good proton conductivity membranes based on Nafion® matrix; (3) synthesizing and modifying low cost sulfonated hydrocarbon (SPEEK) membranes for both H2-PEMFCs and DMFCs applications. ^ High performance membrane electrode assemblies (MEAs) with composite Nafion®-Teflon®-Zr(HPO 4)2 membranes were prepared, optimized and characterized at higher temperature (> 100°C)/lower relative humidity (< 100% RH) condition, using H2 or H2+CO as the fuel. Effects of CO concentration, temperature, relative humidity to the CO poisoning on H 2-PEMFC were studied by applying various electrochemical techniques. The electrochemical oxidation mechanism of H2/CO in higher temperature PEMFC was investigated and simulated. ^ Two type of membranes based on Nafion® matrix were prepared: silica/Nafion® membrane and palladium impregnated Nafion® (Pd-Nafion®) membrane. The composite silica/Nafion® membrane was developed by in-situ sol-gel reaction followed by solution casting, while the Pd-Nafion® was fabricated via a supercritical fluid CO2 (scCO 2) route. Reduced methanol crossover and enhanced efficiency was observed by applying each of the two membranes to DMFCs. In addition, the research demonstrated that scCO2 is a promising technique for modifying membranes or depositing nano-particle electrocatalysts onto electrolyte. ^ Sulfonated poly(ether ether ketone) (SPEEK) was synthesized by a sulfonation reaction using poly(ether ether ketone) (PEEK). Multilayer structure SPEEK membranes with methanol barriers were fabricated and showed enhanced membrane stability in DMFCs. Improved MEA performance was obtained due to lower methanol crossover and the presence of a good membrane/electrode interface for facilitating proton transfer. ^

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