Title

The effects of cationic contamination on the physio-chemical properties of perfluoroionomer membranes

Date of Completion

January 2008

Keywords

Energy|Engineering, Materials Science

Degree

Ph.D.

Abstract

Proton Exchange Membrane (PEM) technology cannot meet fuel cell and electrolyzer durability standards for stationary and transportation applications. Cell designs are not of sufficient maturity to demonstrate more than several thousand hours of invariant performance. One of the limiting factors is the operational lifetime of membrane electrode assemblies (MEA's) because of pin-holing, dry-out, mechanical breeches, chemical attack and contamination. This program investigated the role of contamination on the degradation of perfluorinated membranes in fuel cell and electrolysis environments. Tests were conducted to develop an understanding of the effects of cationic contaminants on fundamental design parameters for these membranes including water content, ion exchange capacity, gas diffusion, ionic conductivity, and mechanical properties. ^ Tests showed that cations rapidly transport into the membrane and disperse throughout its structure achieving high equilibrium concentrations. Ion charge density appears to govern membrane water content with small ions demonstrating the highest water content. Permeability studies showed transport in accordance with Fick's law in the following order: H2>O2>N 2>H2O. Cations negatively affect gas and water transport, with charge density affecting transport rates. Unique diffusion coefficients were calculated for each contaminating species suggesting that the contaminant is an integral participant in the transport process. AC resistance measurements showed that size of the ion charge carrier is an important factor in the conduction mechanism and that membrane area specific resistance correlates well with water content. ^ Increases in membrane yield strength and the modulus of elasticity were demonstrated with increased contamination. Tensile tests showed that cation size plays an important role in determining the magnitude of this increase, indicating that larger ions interfere more with strain than smaller ones. Contaminants reduced strain to break with smaller ions showing the greatest effect. Ultimate tensile strength increased slightly with all contaminants except lithium, which effected a reduction in this property, reflecting a relationship with contaminant size. ^ This study has produced key data and relationships concerning the effects of cationic contamination on perfluoroionomer membranes such as Nafion 117. This provides a framework for the determination of empirical coefficients that are essential to developing multiphysics models relating to the effects of cationic contaminants on perfluoroionomer membranes. ^