Doctoral Dissertations

Date of Award

8-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Matthew M. Mench

Committee Members

Kivanc Ekici, David J. Keffer, Douglas Scott Aaron

Abstract

Energy production is central to the climate challenge, as a significant portion of greenhouse gases responsible for trapping heat in the Earth's atmosphere arises from burning fossil fuels to generate electricity and heat. To mitigate the adverse effects of climate change, emissions must be reduced by almost half by 2030 and achieve net-zero emissions by 2050. Polymer electrolyte fuel cells (PEFCs) offer numerous advantages compared to traditional internal combustion engines in vehicles. Fuel cell electric vehicles are known for their exceptional operating efficiency (over 60%), impressive driving range (more than 400 miles), and quick refueling times (under 5 minutes).

Automotive PEFCs undergo extensive load cycling, often reaching hundreds of thousands of cycles in regular use. However, the dissolution of platinum (Pt) catalyst and carbon-support oxidation under these conditions are significant barriers to their widespread adoption. The U.S. Department of Energy has set durability benchmarks at 8,000 hours for light-duty vehicles and 30,000 hours for heavy-duty vehicles. Consequently, PEFC catalyst degradation has received significant attention in the past decade. Still, our understanding of catalyst degradation remains incomplete, particularly regarding heterogeneity in catalyst growth and its localized performance impact.

This dissertation employs a combination of advanced characterization techniques to study the heterogeneous aging patterns in PEFCs and their localized impact. These techniques include synchrotron micro-X-ray diffraction, high-resolution neutron imaging, spatially and temporally resolved current and temperature measurements via a segmented cell, and a variety of electrochemical and analytical methods. This research presents a novel approach by providing a spatially-resolved comparison of current and Pt particle size distributions over a larger active area, relevant to fuel cell stack applications. Furthermore, these techniques have been utilized to identify interactions between different degradation modes in PEFCs and to elucidate path-dependent degradation. Finally, this research correlates heterogeneous catalyst growth with liquid water distribution within a PEFC, offering new insights into the factors influencing performance and durability.

A deeper understanding of local performance responses to aging and the interactions of different degradation mechanisms explored in this dissertation will enable the design of highly durable fuel cells capable of withstanding thousands of load/unload and start-up/shutdown cycles.

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