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@PHDTHESIS{Olbrich:984249,
author = {Olbrich, Wolfgang},
othercontributors = {Eikerling, Michael and Zander, Brita Daniela and Thiele,
Simon},
title = {{S}tructure-based modeling of catalyst layers for proton
exchange membrane fuel cells},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2024-03965},
pages = {1 Online-Ressource : Illustrationen},
year = {2024},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2024, Kumulative Dissertation},
abstract = {Fuel cells are expected to play an integral role in a
future energy system where they act as efficient energy
converters generating electricity from renewable hydrogen.
Proton exchange membranes fuel cells (PEMFCs) are favored
for their robustness, efficiency and compactness and are
currently transitioning from prototyping stage and niche
applications to large-scale commercial deployment. Ongoing
development efforts have to meet ambitious targets in terms
of durability, performance and cost effectiveness.
Therefore, the overall fuel cell device, and in particular
the cathode catalyst layer (CCL), must be further improved
and optimized from atomic scale to system level. CCL
materials development hinges on comprehensive mathematical
models covering the entire causal chain from fabrication
parameters during ink stage, over the resulting
microstructure, to macroscopic properties, and finally to
the operative performance metrics. The scope of this work
covers three major interconnected subjects of research:
firstly, structure formation during ink stage is lacking a
quantitative approach that rationalizes the cause-effect
relations between ink parameters and the resulting
agglomerated microstructure; secondly, the crucial relations
between molecular-to-mesoscale characteristics of the
catalyst microstructure and water-related properties
(wetting behavior, water uptake, Platinum utilization, and
the CCLs susceptibility to flooding) require a
comprehensive, scale-bridging approach to overcome the
limitations of a merely empirical consideration, especially
with regard to the objective of further lowering the
Platinum loading in PEMFCs; and thirdly, experimental
literature data indicate a significant impact of ionomer
morphology on proton conductivity within the CCL, which is
commonly neglected by models available in literature but has
gained importance together with efforts to enhance the
current density of the cell. Methodologically, these
research gaps were addressed by a profoundly structure-based
modeling approach that allows to comprehensibly trace the
cause-effect-chain from molecular to macroscopic scale.
Starting with the CCL composition model to study structure
formation during ink stage, the incremental self-assembly of
solvated ionomer and Pt/C particles was analyzed in an
analytical-mechanistic approach, from which two key
parameters were derived: an ionomer dispersion parameter
that captures the tendency of ionomer to self-aggregation or
film formation, and the initial ionomer film thickness. The
model solution includes pore size distribution and ionomer
morphology within the final CCL microstructure and was
seamlessly employed as input to the models for wetting
behavior and proton transport. To model the wetting behavior
of the CCL composite material, a novel conceptual approach
is proposed based on an extensive literature review: the
degree of alignment describes the state of molecular
ordering of sidechains and backbones in the ionomer thin
film and was directly linked to its wetting properties. From
this rationalization, macroscopic wetting behavior of the
CCL can be deduced. The analysis of the model results
supports a crucial hypothesis: lowering the Platinum loading
renders the CCL more prone to flooding, caused by a
successive shift of molecular alignment as Platinum particle
concentration on the support surface is reduced, which
eventually triggers an inversion from hydrophobic to
hydrophilic properties. Extending the wettability model, the
common approach of considering water uptake via capillary
condensation was refined by including adsorption processes
at Platinum nanoparticles, carbon surface, and ionomer.
After deconvoluting water uptake, the application of a
percolation-based approach could partially reproduce the
experimentally observed behavior but also revealed a
particular mismatch with the model results for low-Pt-loaded
materials. Thus, this works hints, in accordance with a
hypothesis from the literature, that connection to the
proton supply network might be provided in thin films of
adsorbed water, even when the pore space is not fully
flooded. Proton transport properties were calculated from
direct numerical simulations for a wide parameter space
covering various ionomer morphologies. Based on the
statistical information provided from the composition model,
a stochastical image generation process was developed.
Essential structural features shaping proton conductivity
were extracted and coined into an analytical approach based
on percolation theory. The model-based analysis of
experimental data from the literature attributed deviating
trends at identical CCL composition to different regimes of
ionomer morphology. A list of design principles for fuel
cell developers was compiled from the results of the
different models, thereby proposing concrete levers and
measures to tweak and improve fuel cell performance.
Additionally, disruptive approaches to overcome limitations
regarding Platinum loading, e.g, the chemical modification
of carbon support surface, were devised and were found to
align with efforts from recent experimental works. With
regard to the ongoing extensive efforts in PEMFC research
and development, the insights obtained from model-based
analysis of experimental data exemplify how structure-based
models can guide the interpretation and design of
experiments, and ultimately enable acceleration and steering
of iterative laborious materials development cycles.
Eventually, the thorough comparison of model results and
experimental data revealed emerging gaps in the
understanding of structure-property relation in PEMFC
catalyst layers, such as an suspected enhanced proton
mobility in ionomer thin films and the ambiguous mechanism
for Platinum utilization by water in low-Pt-loaded cells,
and thereby lays the groundwork for future experimental and
theoretical works.},
cin = {526810 / 520000},
ddc = {620},
cid = {$I:(DE-82)526810_20191118$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2024-03965},
url = {https://publications.rwth-aachen.de/record/984249},
}
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