Understanding ion transport in fuel cell catalyst layers.
Sabarirajan, Dinesh Chandra Kumar.
2019
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Polymer
electrolyte fuel cells are low temperature fuel cells that convert chemical energy to
electricity with zero emissions. The major hurdles in commercializing PEFCs is the cost
associated with catalyst (Pt) and the losses due to proton transport and sluggish oxygen
reduction reaction (ORR). Ionomer plays a crucial role in aiding proton transport in
conventional catalyst layers, as its ... read moreacid groups help transport protons. Ionomer
distribution in catalyst layers is not well-known, and its morphological properties such
as volume fraction and tortuosity are generally back-calculated. To reduce cost lower
loadings of Pt need to be used, then each Pt nano-particle has to be as efficient as
possible to catalyze oxygen. Ionomer, although essential for proton transport can
inhibit ORR by poisoning Pt surface. Thus a delicate balance in ionomer content has to
be reached for optimal PEFC catalyst layer design. To optimize ionomer content in this
thesis I developed novel diagnostic tools for precise measurement of ionomer tortuosity
within the catalyst layers. I compared a well-studied AC technique to a novel
interpretation of DC method and found that AC technique measures local proton transport
and thus actual proton conductivity, whereas DC technique accounts for ionomer
tortuosity and measures effective conductivity. This thesis demonstrates that both
techniques can be used to calculate ionomer tortuosity within the catalyst layer as a
function of relative humidity and applied potential. The findings suggest that ionomer
content plays a significant role in ionomer distribution and tortuosity and also the
type of carbon material plays a role too. The developed method can be used in the fields
of supercapacitors, batteries and fuel cells that do not contain precious metals.
Furthermore to understand ionomer local interaction with Pt electrocatalyst, I utilized
CO-displacement technique. This method is uniquely positioned to measure surface
coverage of Pt under different environments. The findings suggest that coverage scales
with Pt loading, even when normalized per electrochemical surface area. The implication
is that at low loading Pt might have sufficient ionomer coverage, which can result in
poor ionic conductivity. Ion transport mechanisms in ionomer-free electrodes were
studied by the use of model electrodes with well-described shape-factor using atomic
layer deposition (ALD). Using techniques like cyclic voltammetry, CO-stripping are used
to elucidate transport mechanisms with respect to temperature and relative humidity. To
provide better understanding of the ionomer effects for transport, the performance of
ALD electrodes with and without ionomer were compared. Overall, in this thesis I combine
novel diagnostics to understand ionic conductivity in ionomer-free and ionomer
containing catalyst layers. The knowledge of ionomer distribution and its properties can
significantly aid in the design of electrode with better morphology and electrochemical
properties, and also help reduce Pt loading.
Thesis (Ph.D.)--Tufts University, 2019.
Submitted to the Dept. of Mechanical Engineering.
Advisor: Iryna Zenyuk.
Committee: Michael Zimmerman, Fiorenzo Omenetto, and Adam Weber.
Keyword: Mechanical engineering.read less - ID:
- hh63t7935
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