Platinum group metal-free electrocatalysts for cathodic oxygen reduction for use in high-temperature proton exchange membrane fuel cells: spectroscopic and mass transport studies.
Permanent URL:
http://hdl.handle.net/2047/D20261117
Budil, David E. (Committee member)
Lopez, Steven (Committee member)
Zhang, Ke (Committee member)
Currently, Pt-based materials are the state-of-the-art catalysts for the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) in acid fuel cells, and are responsible for a large portion of the cell stack cost in commercial systems. However, due to the sluggish kinetics and higher overpotentials associated with ORR compared to HOR, a higher Pt catalyst loading is traditionally required. Pt-based materials are inherently more susceptible to poisons, reducing their power output over the lifetime of the system. Therefore, the cathode of the fuel cell has been traditionally studied for Pt alternatives, whether that be through the reduction of Pt loading or through substitution of Pt with platinum group metal (PGM)-free materials. PGM-free catalysts traditionally have been synthesized through the mixture of a metal and nitrogen precursors in the presence of a carbon support material, followed by heat treatments to form metal-nitrogen coordinated materials on a carbon support.
This dissertation will focus on two major aspects related to PGM-free materials in proton exchange membrane fuel cells. Chapter 1 will give the necessary background into fuel cell technology, and the numerous systems that are currently studied for various applications, as well as providing the electrochemical background for the methodologies used to study these materials. Chapter 2 will demonstrate a combined experimental and theoretical approach to gaining increased understanding of mass transport effects within a PGM-free catalyst layer in the PEMFC environment. Experiments using low concentration oxygen cathode fuels combined with a previously-developed model allow for mass transport resistances to be calculated based on limiting current scenarios. Chapters 3 and 4 will introduce several PGM-free materials developed through different techniques, including a Sacrificial Support Method (SSM) and the formation of Metal-Organic-Framework (MOF) structures, for their use in high-temperature PEMFCs (HT-PEMFCs), which have potential in stationary power generation applications. In addition to rotating ring disk electrode (RRDE) studies to evaluate the ORR pathway associated with these catalysts, these materials will demonstrate immunity to phosphate poisoning at low temperatures. Additionally, they will be evaluated for performance and durability in the HT-PEMFC environment. Chapter 5 will evaluate a potential alternative to a pure PGM-free cathode, where PGM-free materials are combined with low loadings of Pt catalysts in an effort to match the performance of high loading Pt membrane electrode assemblies (MEAs). These systems would allow for at least a 70% reduction in the total platinum content within the MEA, increasing the commercial viability of these systems. Chapter 6 will summarize the findings in Chapters 2-5 while also discussing the ongoing and future efforts of this work.
HT-PEMFC
mass transport
platinum group metal-free catalysis
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