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%0 Thesis
%A Olbrich, Wolfgang
%T Structure-based modeling of catalyst layers for proton exchange membrane fuel cells
%I Rheinisch-Westfälische Technische Hochschule Aachen
%V Dissertation
%C Aachen
%M RWTH-2024-03965
%P 1 Online-Ressource : Illustrationen
%D 2024
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
%Z Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2024, Kumulative Dissertation
%X 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.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2024-03965
%U https://publications.rwth-aachen.de/record/984249