Date of Completion

8-19-2014

Embargo Period

8-19-2015

Keywords

Alkaline Anion Exchange Membranes, Ionic Conductivity, Transmission X-ray Microscopy, in situ Nickel Oxidation

Major Advisor

Wilson K. S. Chiu

Associate Advisor

Brice N. Cassenti

Associate Advisor

Ugur Pasaogullari

Associate Advisor

Michael T. Pettes

Associate Advisor

Michael W. Renfro

Field of Study

Mechanical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Fuel cell systems are capable of providing power for a wide range of applications. In order to improve the performance and lifetime of fuel cell systems, transport induced losses are considered. For portable power applications, alkaline anion exchange membrane (AEM) fuel cells are one option. However the ion transport through the membrane is a significant performance loss. Understanding how these membranes operate and how to increase ion mobility is important for minimizing transport losses. One factor which affects AEM performance is the local hydration in the membrane and water-membrane diffusion coefficient. By studying water transport using water flux measurements, predictions can be made for effective ion-membrane diffusion coefficients and ionic conductivity as functions of the local hydration in the membrane. Comparing the ionic conductivity results to experimental measurements, equilibrium constants can be calculated for different ionic species in the membrane to the fixed side chain groups. Understanding how dissociation of mobile ions affects transport through the membrane can be useful for designing new membranes with higher ionic conductivities.

Transport losses also affect stationary power systems such as solid oxide fuel cells (SOFCs) which suffer from degradation mechanisms such as oxidation of the electronic conductor nickel in the anode. When nickel oxidizes, there is a significant decrease in the material electronic conductivity and as well as an increase in the particle volume. With the volume expansion, the stresses between solid phases will increase and can initiate cracks, which can lead to cell failure. The cracks will increase transport lengths for different species and decrease fuel cell performance. To investigate this degradation mechanism, nickel oxidation is investigated using a new technique in a synchrotron based transmission X-ray microscope (TXM). Using the TXM, the nickel particles can be imaged in situ at the nanoscale while the reaction is occurring in SOFC operating conditions. The images can then be analyzed to measure reaction rates and activation energy.

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