Title

Mechanical behavior, modeling, strength and failure analysis of polyelectrolyte membranes

Date of Completion

January 2007

Keywords

Applied Mechanics|Engineering, Mechanical

Degree

Ph.D.

Abstract

Currently, ionomer polymeric membranes are used in a variety of specialized applications. Such applications include, but are not limited to, dialysis, electrolysis, membrane separators, dryers/humidifiers, and the most promising application: polymer electrolyte membrane fuel cells. Although their use is widespread, significant gaps in understanding the mechanical behavior of these materials still remain. Many ionomer membranes change their material structure, and in turn, their mechanical properties in response to applied thermal and moisture conditions that are functions of spatial position. It has been observed that constrained materials subjected to changing environmental conditions exhibit unusual behavior. In some cases, mechanical failure is seen in the absence of external applied mechanical loads. The overall goal of this investigation is to fully characterize and model the mechanical behavior of PEM fuel cell membranes (in particular Nafion®111), including the presence of hygro-thermal environments. Experimental results from mechanical tests under controlled environment show a strong influence of temperature and water content on the nonlinear response of the membrane. Data recovered in these tests have been used in finite element models to predict the behavior of materials used in certain applications or geometries. Creep, stress-rupture, stress relaxation and degradation of Nafion® membranes is studied as part of this work. Two major degradation mechanisms have been investigated for the membrane electrode assembly (MEA): chemical degradation due to open circuit voltage (OCV) in a PEM cell, and mechanical degradation due to cyclic hydration. With this information, failure modes can be determined which are necessary for durability modeling and life prediction. Based on the ex-situ experimental results of the accelerated degradation tests a failure function is proposed and a strength evolution damage concept is introduced to estimate failure of the membrane electrode assembly (MEA) under accelerated conditions. ^