Doctoral Dissertations

Date of Award

12-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

David J. Keffer, Stephen J. Paddison

Committee Members

Shengting Cui, Brian J. Edwards, James R. Morris

Abstract

The development of novel polymer electrolyte membrane (PEM) materials which operate at high temperature (i.e. > 100˚C) and low humidity conditions and efficiently transport protons has been a major focus for PEM fuel cell technology. The motivation behind a high temperature PEM fuel cell is based on the fact that, at high temperature, the catalysts used in the fuel cell are more active and less susceptible to poisoning due to impurities in the feed stream. The challenge lies in the fact that as the temperature is increased, the membrane loses water and its ability to transport protons. The successful design and synthesis of high-performance PEMs would benefit from a fundamental, molecular-scale understanding of how polymer chemistry, hydration levels, and morphology affect proton mobility within the membrane.

Additionally, substantially less work has concentrated on the molecular-level details of proton transport at the multi-phase interfaces among the PEM, vapor, water, electrodes, and catalyst surface. The electrochemical processes occurred at such interfaces dictate the performance of the PEM fuel cells. Understanding the structural and dynamic properties at these interfaces is, therefore, crucial for the optimization of current energy devices. All such information cannot come from experimental investigations alone, but requires knowledge of multiscale simulations which are successful in bridging distinct time and length scales, providing insights into the morphology and structure through analysis of the molecular processes.

The first objective of this work is to use molecular dynamic (MD) simulations to investigate the nanophase-segregated structure in the PEM as a function of polymer chemistry and hydration levels. The variables probed to define polymer chemistry include (1) side chain length, (2) equivalent weight, and (3) molecular weight. We examine the structure in attempts to establish a relationship between the polymer chemical composition and the hydrated morphology and transport properties. The second objective is to use MD simulations to generate the structure of the interfaces involving the PEM within the Membrane-Electrode Assemblies (MEAs). These interfaces include (1) the PEM/vapor interface, (2) the PEM/vapor/catalyst interface, and (3) the PEM/vapor/carbon electrode interface. We examine these interfaces in order to establish an understanding of the structure of these interfaces as a function of water content.

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