Yen-Chun Chen


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Chen

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Yen-Chun

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0000-0003-2161-9198

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2021 - 20236
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Publications 1 - 6 of 6
  • Catalyst Aggregate Size Effect on the Mass Transport Properties of Non-Noble Metal Catalyst Layers for PEMFC Cathodes
    Item type: Journal Article
    Ünsal, Seçil; Bozzetti, Michele; Chen, Yen-Chun; et al. (2023)
    Journal of the Electrochemical Society
    Non-noble metal catalysts (NNMCs) are regarded as a promising alternative to the costly Pt-based materials required to catalyze the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes. However, the large diversity of NNMC synthesis approaches reported in the literature results in materials featuring a wide variety of particle sizes and morphologies, and the effect of these properties on these catalysts' PEMFC performance remains poorly understood. To shed light on this matter, in this work we studied the physical and electrochemical properties of NNMC layers prepared from materials featuring broadly different aggregate sizes, whereby this property was tuned by ball milling the precursors used in the NNMCs' synthesis in the absence vs presence of a solvent. This led to two NNMCs featuring similar Fe-speciations and ORR-activities, but with vastly different aggregate sizes of >5 μm vs ≈100 nm, respectively. Following the extensive characterization of catalyst layers (CLs) prepared with these materials via electron microscopy and X-ray tomography, PEMFC tests at different loadings unveiled that the smaller aggregate size and ≈20% higher porosity of the CL prepared from the wet-milled sample resulted in an improvement of its mass transport properties (as well as a ≈2-fold enhancement of its peak power density under H2/air operation) over the dry-milled material.
  • Characterizing Water Transport in the Microporous Layers of Polymer Electrolyte Fuel Cells
    Item type: Doctoral Thesis
    Chen, Yen-Chun (2023)
    Microporous layers (MPLs) are an essential part of the gas diffusion layers (GDLs), which improve polymer electrolyte fuel cell (PEFC) performance, particularly at high current density and humid operating conditions. One of the ways to reach future PEFC performance and cost target is through better design and integration of this layer. To achieve this, understanding of the water transport in MPLs is important. This thesis aims at obtaining insight to the morphology of and water transport in the MPLs by means of X-ray tomographic microscopy (XTM). The opening three chapters provide the general background of this thesis study. Chapter 1 lays out the motivation for this work. Chapter 2 gives a basic overview of PEFC thermodynamics, efficiency and major loss mechanisms. Chapter 3 summarizes the methods used in this thesis work, including XTM principles and fundamentals, illustration of the laboratory computed tomography (CT) and beamline XTM setups and the image processing steps. First, MPL properties and water saturation within are characterized in an ex situ setup (Chapter 4 and Chapter 5). In Chapter 4, the use of polychromatic XTM is extended for the quantitative determination of water saturation in MPLs through establishing a calibration relation between grayscale values and linear attenuation coefficients. From there on, MPL properties and porosity are characterized based on laboratory CT imaging. The results show that the porosity distribution of MPLs is not homogeneous and therefore, for accurate determination of the spatial water saturation distribution, porosity distribution must be obtained first. In Chapter 5, fifteen commercially available MPL materials are characterized for their total porosity, the microporosity, the crack volume, thickness and porosity heterogeneity. Some of these properties (especially the porosity and its heterogeneity) were previously unknown. This provides a new insight to the properties of MPLs and reveals the diversity of commercial MPLs with respect to thickness, porosity and morphology, which cannot be summarized in a single morphological category as often done previously. Operando water transport in MPLs and GDLs is characterized in Chapter 6 and Chapter 7. In Chapter 6, high porosity MPLs with different pore size distribution (PSD) are investigated, and the work is focused on the first minute fuel cell operation from dry state as this allows for easier characterization of the water transport modes. Data show that the addition of both kind of high porosity MPLs (with different PSD) resulted in enhanced vapor transport at the beginning of fuel cell operation. Therefore, it was hypothesized that the (low) thermal conductivity of MPLs is one of the important properties determining water transport mechanisms. In Chapter 7, water transport in MPLs of the same material but with different morphology (sheet and intruding) is investigated. Here, results show that an intruding MPL morphology leads to lower liquid water saturation in the GDL, especially the MPL/GDL mixed region. This is proposed to be the reason for the better performance of the PEFC with at highly humid and high current density conditions. Chapter 8 summarizes the results and conclusions from this work and motivates further XTM studies on water transport in PEFCs, operated at even more realistic operating conditions with further developed MPLs and GDLs.
  • Analysis of the MPL/GDL Interface: Impact of MPL Intrusion into the GDL Substrate
    Item type: Journal Article
    Berger, Anne; Chen, Yen-Chun; Gatzemeier, Jacqueline; et al. (2023)
    Journal of the Electrochemical Society
    Interfaces are crucial for the water management in polymer electrolyte membrane fuel cells (PEMFCs). The introduction of a microporous layer (MPL) had a revolutionary effect on the water distribution by improving the interface between the catalyst layer and the gas diffusion layer substrate (GDL-S). Hence, it is vital to maximize the improvement by further characterizing and advancing the properties of the interfaces, in this case the MPL/GDL-S interface. This study aims at fabricating a GDL with an MPL that intrudes into the GDL-S, analyzing the impact on the GDL-S structure and on PEMFC performance. Mercury intrusion porosimetry (MIP) and ex situ X-ray tomography (XTM) show that the intrusion of the MPL into the hydrophobic GDL-S proceeds via the preferential filling of the GDL-S macropores, thereby reducing their size and volume fraction in the GDL-S. While an intruding MPL leads to a small performance increase under wet PEMFC operating conditions, this improvement could only be achieved by a careful management between the extent of MPL intrusion and the partial macropore blocking in the GDL-S. Furthermore, the impact of MPL intrusion on the liquid water saturation of the GDL was quantified by operando XTM. The results provide design guidelines for improved GDLs.
  • Determination of the porosity and its heterogeneity of fuel cell microporous layers by X-ray tomographic microscopy
    Item type: Journal Article
    Chen, Yen-Chun; Karageorgiou, Chrysoula; Eller, Jens; et al. (2022)
    Journal of Power Sources
    Advancing polymer electrolyte fuel cell technology includes the rational design of the microporous layer (MPL) coating on the gas diffusion layer (GDL), where the porosity and morphology on an operation-relevant size scale are still largely undetermined and hinder further developments. Here, 15 commercially available GDLs with MPL coatings from three major manufacturers (seven Freudenberg, four Sigracet (R) SGL and four CeTech materials) were characterized by X-ray tomographic microscopy. An extensive set of structural parameters for the MPLs are presented, including MPL total porosity, microporosity, porosity heterogeneity and thickness heterogeneity. The analyses show that the CeTech GDLs tend to have MPLs with the lowest porosity, while the Sigracet (R) GDLs have MPL with the highest porosity. Furthermore, Freudenberg H23 materials have the lowest porosity heterogeneity, and the Freudenberg CX materials' porosity are most heterogeneous. Many of the commercial MPLs, expected to be homogeneous, show a gradient of MPL microporosity in the thickness direction. The characterized MPLs are classified into five distinct classes based on thickness and porosity heterogeneities. This classification and the detailed data presented support the understanding of fuel cell performances with different MPL types. The comprehensive set of data also serve as realistic input values for material and fuel cell modeling studies.
  • On the water transport mechanism through the microporous layers of operando polymer electrolyte fuel cells probed directly by X-ray tomographic microscopy
    Item type: Journal Article
    Chen, Yen-Chun; Dörenkamp, Tim; Csoklich, Christoph; et al. (2023)
    Energy Advances
    Product water transport via the microporous layer (MPL) and gas diffusion layer (GDL) substrate during polymer electrolyte fuel cell (PEFC) operation was directly and quantitatively observed by X-ray tomographic microscopy (XTM). The liquid water distribution in two types of MPLs with different pore size distributions (PSDs) was characterized as a function of the inlet gas relative humidity (RH) and current density under humid operating conditions at 45(degrees)C. During the first minute of PEFC operation, liquid water mainly accumulated at the catalyst layer (CL)/MPL interface and in the GDL substrate close to the flow fields. Furthermore, under all tested conditions, saturation in the MPL was low (<25%), whereas under the rib, the saturation in the GDL was up to ca. 70%. Based on these XTM results, it is confirmed that in the high porosity MPLs, vapor transport was non-negligible even at high humidity conditions. Therefore, on top of the widely discussed MPL pore size and its distribution, it is proposed that the lower thermal conductivity from the high porosity of MPLs can also be a main cause of promoted vapor transport, reducing water saturation near the CL.
  • A Method for Spatial Quantification of Water in Microporous Layers of Polymer Electrolyte Fuel Cells by X-ray Tomographic Microscopy
    Item type: Journal Article
    Chen, Yen-Chun; Berger, Anne; De Angelis, Salvatore; et al. (2021)
    ACS Applied Materials & Interfaces
    A microporous layer (MPL) is typically added to the gas diffusion layer of polymer electrolyte fuel cells (PEFCs) to promote cell performance and water management. The transport mechanism of the water through the MPL is, however, not well understood due to its small pores (20-500 nm). Here, we demonstrate that polychromatic X-ray tomographic microscopy (XTM) can be used to determine the porosity and the spatial distribution of water in nanoporous materials and can quantitatively map the liquid water saturation of MPLs. The presented technique requires no a priori knowledge of the composition of the MPL and has a field of view on the millimeter scale, which is orders of magnitude larger than conventional electron microscopy techniques for nanoscale materials. The available field of view is compatible with existing operando cells for X-ray tomography, paving the way for the analysis of water transport in MPLs during operation.
Publications 1 - 6 of 6