Modeling Nafion Ionomer During Catalyst Layer Fabrication Process of Polymer Electrolyte Fuel Cell Using Molecular Dynamics Method
Date
2022-05-11
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Abstract
One of the most popular types of fuel cells is the polymer electrolyte fuel cell (PEFC) with hydrogen as fuel. This generation of the fuel cell is used extensively to convert chemical energy of hydrogen fuel to electricity. It can be used in various applications, from stationary applications to vehicles, due to its small size, low temperature, and pressure. However, the cost and durability of PEFCs remain a barrier to large-scale commercialization. The central R&D target for large-scale PEFC commercialization focuses on lowering the cost without compromising its performance. A significant part of the price and performance of PEFC is linked directly to the catalyst layers. The performance of the catalyst layer is strongly influenced by the ionomer, an ionically charged polymer. This is due to the undeniable influence of ionomers on material transport (oxygen and proton). The ionic moiety, located on the ionomer side chains, helps in proton transport. In addition, oxygen molecules diffuse through the ionomer agglomerates. Therefore, the ionomer local structure has a noticeable influence on ion and oxygen transport. Since the fabrication process of the catalyst layer can affect the ionomer structure in electrodes of PEFCs, this process has a significant impact on the performance of the catalyst layers. As a result, the catalyst layer fabrication process has a considerable effect on the performance of PEFCs. Instead of empirically guided electrode fabrication, a theory-based knowledge of ionomer morphology alteration throughout the catalyst layer fabrication process is required for establishing a manufacturing guideline for high-performance electrodes for PEFCs. For this purpose, the Nafion ionomer, the most commercialized fluorinated ionomer for PEFCs, is modelled during the catalyst layer fabrication steps by the molecular dynamics method (MD) in this thesis. Herein, we study the Nafion ionomer morphology changes at three main stages of catalyst layer fabrication, which are Nafion structure in dispersion media, Nafion orientation in catalyst ink, and evolution of Nafion ionomer morphology during the drying of catalyst ink in the transformation process of catalyst ink to the catalyst layer. First, the molecular assembly of Nafion ionomer in a variety of solvents with various dielectric constants (ε=2.38 to 109) is modelled using all atomistic molecular dynamics to study the aggregate structure of Nafion in practical dispersion systems. Our investigation demonstrates that a range of morphologies exists based on the solvent polarity, unlike the common assumption that Nafion aggregates have an elongated form in all dispersion media. The presence and clustering of ion pairs in Nafion dispersions, which is not previously observed in PFSA ionomer dispersions, is demonstrated in this work. A new aggregation phase diagram for different Nafion dispersion systems is introduced for the first time. Based on this diagram, Nafion can have three different morphology types depending on the dielectric constant of the applied solvent. I. In the low polar solvents (εr ≤ 45), the ionic clusters of side chains cause cross-linked ionomer aggregations. II. In medium polar solvents (εr ~ 57), Nafion has a two-dimensional lamella-like structure. III. The high polar solvents (εr ≥ 78) generates tightly packed elongated aggregates. Also, a novel phenomenon of solvent microphase segregation in water/IPA mixture caused by PFSA's surfactant-like hydrophilic/hydrophobic properties is discovered. We also studied the assembly of the Nafion ionomer on planar Pt, planar graphite, and graphite/Pt nanoparticles substrates in water, IPA, and water/IPA to gain a better understanding of the macroscopic, microscopic structures of Nafion ionomer found in popular catalyst inks. The Pt surface causes the creation of ion-pairs on the substrate in high dielectric solvents such as water, while sulfonic acid is fully dissociated in the water dispersion system. Herein, we showed that the interaction of ionomers with graphite is solvent independent. Nonetheless, ionomer coverage for planar Pt varies considerably, ranging from 72 percent for pure water to 28 percent for pure IPA. Furthermore, increasing the aqueous concentration results in more homogeneous ionomer coverage on the Pt surface. Adding Pt NPs to the surface of graphite reduces the ionomer coverage of graphite in IPA and water/IPA environments by 12 to 15%. Pt NPs have a lower ionomer coverage than planar Pt in aqueous media. As a result, while modelling catalyst ink or catalyst layer problems, the combination of graphite/Pt and the surface shape of the catalyst must be explored. In IPA, some NPs are completely covered by ionomer; however, some are bare. By adding water to the environment, the ionomer coverage of Pt NPs becomes more homogeneous. In the last part of this work, we used fully atomistic molecular dynamics to simulate the dynamic changing of the ionomer structure during solvent evaporation. The examined solvents in this study are pure water, a 50/50 blend of water and IPA, and pure IPA. To investigate the structure of ionomers on the catalyst, we simulated a drying process of ionomer dispersion on a platinum (111) substrate. This work examines ionomer surface coverage, sulfonic group abundance at the interface, ionic clusters' existence and their sizes, and polymer density variation from Pt/ionomer to polymer/fluid interfaces throughout the drying process. Surface coverage analyses show that the ionomer has the lowest coverage (20-48 percent) on the platinum substrate in pure IPA during and after drying. The ionomer surface coverage changes dramatically (from 25% to 82%) in the water/IPA sample because of a change in the nature of the solvent since IPA is more volatile compared to water.
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Keywords
Molecular Dynamics, Polymer electrolyte Fuel cell, Nafion ionomer
Citation
Tarokh, A. (2022). Modeling Nafion Ionomer During Catalyst Layer Fabrication Process of Polymer Electrolyte Fuel Cell Using Molecular Dynamics Method (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.