Non-woven Gas Diffusion Layers Research Articles

Modeling the original and cyclic compression behavior of non-woven gas diffusion layers for fuel cells

It is established that the compression behavior of gas diffusion layers (GDL) is dependent on the level of the mechanical stress it experienced during its lifetime. As a matter of fact, every cycle of compression induces damages in the GDL, including fibers breakage and/or their spatial reorganization. As observed in the experimental work, the first cycle of compression of GDLs as received from the suppliers is already altered by a previous compression that is applied during the manufacturing process. This paper then presents a model able to predict the cyclic behavior of GDL, considering the existence of this compressive stress applied during the manufacturing process. The experimental mechanical properties of the three main types of non-woven GDL (rolls, sheets and felts) were first measured and then predicted using the proposed model, thereby allowing to separate the influences of the manufacturing process, the type of fibers, the presence of a micro-porous layer and a hydrophobic treatment on the GDL.

Effects of Impurities on Pre-Doped and Post-Doped Membranes for High Temperature PEM Fuel Cell Stacks

In this paper, we experimentally investigated two high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) stacks for their response to the presence of reformate impurities in an anode gas stream. The investigation was aimed at characterizing the effects of reformate impurities at the stack level, including in humidified conditions and identifying fault features for diagnosis purposes. Two HT-PEMFC stacks of 37 cells each with active areas of 165 cm2 were used with one stack containing a pre-doped membrane with a woven gas diffusion layer (GDL) and the other containing a post-doped membrane with non-woven GDL. Polarization curves and galvanostatic electrochemical impedance spectroscopy (EIS) were used for characterization. We found that both N2 dilution and impurities in the anode feed affected mainly the charge transfer losses, especially on the anode side. We also found that humidification alleviated the poisoning effects of the impurities in the stack with pre-doped membrane electrode assemblies (MEA) and woven GDL but had detrimental effects on the stack with post-doped MEAs and non-woven GDL. We demonstrated that pure and dry hydrogen operation at the end of the tests resulted in significant recovery of the performance losses due to impurities for both stacks even after the humidified reformate operation. This implies that there was only limited acid loss during the test period of around 150 h for each stack.

Anisotropic properties of gas transport in non-woven gas diffusion layers of polymer electrolyte fuel cells

For this study, gas transport in the gas diffusion layers of polymer electrolyte fuel cells was analyzed in one through-plane and two in-plane directions. Gas transport was calculated using Lattice Boltzmann simulations, with non-woven gas diffusion layers measured both through-plane and in-plane. The micro structure for the transport simulations was based on a stochastic model that can take into account uncompressed and compressed materials. The micro structure of this kind of gas diffusion layers is superposed by fiber bundles. Their impact on the anisotropy of the in-plane permeabilities was then investigated. Finally, the influence of structural inhomogeneities on in-plane flow was analyzed. Compression has a high influence on through-plane and in-plane permeability. The impact of the fibers bundles is smaller than the impact of local variations of the micro structure according to the stochastic geometry model.

Interaction of Pore Size and Hydrophobicity/Hydrophilicity for Improved Oxygen and Water Transport through Microporous Layers

Microporous layers consisting of different ratios of acetylene black and carbon fibers with either a hydrophobic polytetrafluoroethylene (PTFE) or a hydrophilic perfluorosulfonic acid (PFSA) ionomer binder are investigated with regards to oxygen and water transport in PEMFCs. For that, the materials are characterized by scanning electron microscopy and mercury porosimetry, revealing an increase of porosity and pore sizes for an increasing carbon fiber content. MPLs, coated onto a commercial hydrophobized non-woven gas diffusion layer substrate, are examined in H2/air fuel cell tests under differential-flow conditions at various dry and humid operating conditions. For both hydrophobic and hydrophilic MPLs in the presence of significant amounts of liquid water in the diffusion layer substrate, the materials with larger pore sizes, i.e. higher carbon fiber contents, perform superior at 0.6 V and show the lowest oxygen transport resistance. However, at the same carbon composition, hydrophilic MPLs have a lower performance compared to the corresponding hydrophobic MPLs, which is explained by the capillary pressure barriers for different pore properties. At operating conditions relevant for automotive applications, a performance enhancement of 48% could be achieved for a purely carbon fiber based MPL compared to a commercial reference.

Woven gas diffusion layers for polymer electrolyte membrane fuel cells: Liquid water transport and conductivity trade-offs

Gas diffusion layers (GDLs) provide pathways for water removal in a polymer electrolyte membrane (PEM) Fuel Cell. Woven GDLs, have shown higher capability to retain water and improve performance under humid conditions compared to non-woven GDLs. In this work, we investigate water transport, distribution and location of breakthrough in woven GDLs using fluorescent microscopy. GDLs with no coating, 30, and 55 wt% fluorinate ethylene propylene (FEP) were investigated. FEP increases hydrophobicity and affects thermal and electrical conductivities. The results show that the FEP-treated GDLs have higher breakthrough pressures and water contact angles than non-treated GDLs. For untreated samples, water breakthrough occurs in non-compressed regions; whereas, for FEP-treated samples emergence occurs in the compressed regions. Furthermore, water was observed to first cover visible pores inside the GDLs prior to breakthrough. Increasing FEP loading promotes the propagation of water inside the GDLs. Thermal conductivity is found to improve with FEP coating and attains a maximum at 30 wt% FEP loading, whereas electrical conductivity decreases with increasing FEP loading. This analysis shows more pores are engaged in water transport with higher FEP loading. Implementation of woven GDLs in fuel cell design requires a balancing of the water and heat transport benefits with the reduced electrical conductivity.

Stochastic reconstruction of carbon fiber paper gas diffusion layers of PEFCs: A comparative study

A 3D microstructure of the non-woven gas diffusion layers (GDLs) of polymer electrolyte fuel cells (PEFCs) is reconstructed using a stochastic method. For a commercial GDL, due to the planar orientation of the fibers in the GDL, 2D SEM image of the GDL surface is used to estimate the orientation of the carbon fibers in the domain. Two more microstructures with different fiber orientations are generated and compared. The method is verified by comparing the commercial GDL (Toray TGP-H-060) model properties with other simulations or real GDL data. Three different reconstructed models are compared in terms of permeability, and the 3D pore size distribution of the models is also investigated. Through-plane (TP) and in-plane (IP) tortuosity, and the effects of binder addition on tortuosity are also discussed. For the TGH-H-060, tortuosity is derived to be 0.93, 1.50, and 1.42 in IP-x, IP-y, and TP-z directions, respectively. It is shown that adding binders to the fibrous skeleton increases the tortuosity of the pore phase.

Impact of compression on gas transport in non-woven gas diffusion layers of high temperature polymer electrolyte fuel cells

Gas transport in non-woven gas diffusion layers of a high-temperature polymer electrolyte fuel cell was calculated with the Lattice Boltzmann method. The underlying micro structure was taken from two sources. A real micro structure was analyzed in the synchrotron under the impact of a compression mask mimicking the channel/rib structure of a flow field. Furthermore a stochastic geometry model based on synchrotron X-ray tomography studies was applied. The effect of compression is included in the stochastic model. Gas transport in these micro structures was simulated and the impact of compression was analyzed. Fiber bundles overlaying the micro structure were identified which affect the homogeneity of the gas flow. There are significant deviations between the impact of compression on effective material properties for this type of gas diffusion layers and the Kozeny-Carman equation.

A dedicated compression device for high resolution X-ray tomography of compressed gas diffusion layers

We present an experimental approach to study the three-dimensional microstructure of gas diffusion layer (GDL) materials under realistic compression conditions. A dedicated compression device was designed that allows for synchrotron-tomographic investigation of circular samples under well-defined compression conditions. The tomographic data provide the experimental basis for stochastic modeling of nonwoven GDL materials. A plain compression tool is used to study the fiber courses in the material at different compression stages. Transport relevant geometrical parameters, such as porosity, pore size, and tortuosity distributions, are exemplarily evaluated for a GDL sample in the uncompressed state and for a compression of 30 vol.%. To mimic the geometry of the flow-field, we employed a compression punch with an integrated channel-rib-profile. It turned out that the GDL material is homogeneously compressed under the ribs, however, much less compressed underneath the channel. GDL fibers extend far into the channel volume where they might interfere with the convective gas transport and the removal of liquid water from the cell.

Impact of Contact Pressure Cycling on Non-Woven GDLs of HT-PEM Fuel Cells

The Gas Diffusion Layer (GDL) plays an important role in the performance of a fuel cell, but it also strongly controls the durability of the membrane-electrode-assembly (MEA). Mechanical damages like fiber intrusion at high contact pressure can lead to hydrogen crossover and electrical short circuit formation. In this paper a non-woven GDL is analyzed by electrochemical methods in a fuel cell test station under contact pressure cycling between 0.2 and 1.5 MPa. These results are compared ex-situ with the microscopic structural changes in the material investigated by micro computed tomography in combination with a specially designed compression tool. The optical measurements allow the identification of GDL defects. A clear correlation between the loss of internal resistance and hydrogen crossover current and the emerging of small cracks and fiber intrusions could be established.

Impact of Contact Pressure Cycling on Non-Woven GDLs of HT-PEM Fuel Cells

The Gas Diffusion Layer (GDL) plays an important and critical role within High Temperature Polymer Electrolyte Membrane fuel cells (HT-PEMFC). GDLs usually consist of woven (cloth) or non-woven (e.g. felt or paper) arrays of carbon fibers. The main tasks of a GDL are:Gas transport and distribution of reactants and reaction productsElectrical conductivityHeat transportationWater managementSupport of Membrane-Electrode-Assemblies (MEA). The influence of the GDL on the performance of a fuel cell is an extensively discussed topic nowadays (1, 2). The impact on the durability also needs attention. Physical damages, like for example fiber intrusion through the catalyst layer into the membrane, due to high contact pressures have been confirmed prior to this (3, 4). For this reason, ex-situ measurements with an ad hoc designed compression device (5) have been performed. In this work we will examine in more detail the dependency on compression and the behavior of non-woven carbon cloth GDL material. Therefore the compression device, used before, had to be optimized and rescaled furthermore. This allows a better geometrical resolution while utilizing the tool in micro-computed tomography (µ-CT). Results with the new device will be shown within this work.In addition to the ex-situ investigations of the GDL defects based on compression forces, in-situ electrochemical measurements of MEAs like polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) will be carried out. Therefore, MEAs were physically stressed with specific contact pressure cycling tests. One cycle starts at 0.2 MPa up to 1.5 MPa (steps: 0.2, 0.5, 1.0 and 1.5 MPa) and will be repeated 3 times with conditioning or relaxation time over night for each contact pressure and electrochemical characterization during day time.Supplementary, GDLs will be analyzed separately with the help of the new compression device and the µ-CT. Focus of this work lies on the investigation of the porosity of the non-woven GDL material, which is utilized in the investigated MEAs. Within bipolar plates (BPP) with their various flow fields, the porosity of a GDL typically shows a non-uniform behavior (6). Due to the channel and land areas of the serpentine flow fields utilized in this study, the compression on the carbon fibers is locally different. Under the land areas the GDL will be squeezed and within the channels a dilation of the GDL material will be observed (4, 7). To analyze reversible and irreversible defects, the in-situ contact pressure cycling will be imitated completely ex-situ with pure non-woven GDL material. With help of the reconstructed and binary µ-CT data, the porosity of the GDL will be determined and illustrated within 3-dimensional models for each contact pressure (Figure 1). Figure 1: 3D-Model of an uncompressed non-woven GDL

3D microstructure modeling of compressed fiber-based materials

A novel parametrized model that describes the 3D microstructure of compressed fiber-based materials is introduced. It allows to virtually generate the microstructure of realistically compressed gas-diffusion layers (GDL). Given the input of a 3D microstructure of some fiber-based material, the model compresses the system of fibers in a uniaxial direction for arbitrary compression rates. The basic idea is to translate the fibers in the direction of compression according to a vector field which depends on the rate of compression and on the locations of fibers within the material. In order to apply the model to experimental 3D image data of fiber-based materials given for several compression states, an optimal vector field is estimated by simulated annealing. The model is applied to 3D image data of non-woven GDL in PEMFC gained by synchrotron tomography for different compression rates. The compression model is validated by comparing structural characteristics computed for experimentally compressed and virtually compressed microstructures, where two kinds of compression – using a flat stamp and a stamp with a flow-field profile – are applied. For both stamps types, a good agreement is found. Furthermore, the compression model is combined with a stochastic 3D microstructure model for uncompressed fiber-based materials. This allows to efficiently generate compressed fiber-based microstructures in arbitrary volumes.

A novel method for preparing proton exchange membrane fuel cell electrodes by the ultrasonic-spray technique

A novel ultrasonic-spray method for preparing gas diffusion electrodes (GDEs) for proton exchange membrane fuel cell (PEMFC) is described. Platinum (Pt) loaded on Nafion®-bonded GDEs were prepared by the ultrasonic-spray method on various commercial woven and non-woven gas diffusion layers (GDLs) at several Pt loadings in the range of 0.40–0.05mgcm−2. The ultrasonic-sprayed GDEs were tested and compared to commercial and hand-painted GDEs. It was found that the GDEs prepared by the ultrasonic-spray method exhibited better performances compared to those prepared by the hand-painting technique, especially at low Pt loadings. GDEs fabricated by the ultrasonic-spray method with a platinum loading of 0.05mgcm−2 exhibited a peak power rating of 10.9Wmg−1 compared to 9.8Wmg−1 for hand-painted GDEs. For all experiments using various GDLs, Sigracet SGL 10BC exhibited the best performance with a peak power of 0.695Wcm−2.

Characterisation of mechanical behaviour and coupled electrical properties of polymer electrolyte membrane fuel cell gas diffusion layers

Local compression distribution in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC) and the associated effect on electrical material resistance are examined. For this purpose a macroscopic structural material model is developed based on the assumption of orthotropic mechanical material behaviour for the fibrous paper and non-woven GDLs. The required structural material parameters are measured using depicted measurement methods. The influence of GDL compression on electrical properties and contact effects is also determined using specially developed testing tools. All material properties are used for a coupled 2D finite element simulation approach, capturing structural as well as electrical simulation in combination. The ohmic voltage losses are evaluated assuming constant current density at the catalyst layer and results are compared to cell polarisation measurements for different materials. The results show that the largest part of the polarisation difference found between roll-good and batch type materials with wide channel flowfields is well captured by the simulation and is due to additional electrical losses in the locally low compressed GDL. Thus, for the first time a broader understanding of the significant performance impact of diffusion layer mechanical properties is generated. However, at higher loads an interaction of compression with electrical and additional heat and mass transport effects occurs, which will be included in the next part of the study. This part is limited to structural mechanics and coupled electrical transport effects.

Modeling of Two-Phase Behavior in the Gas Diffusion Medium of PEFCs via Full Morphology Approach

A full morphology (FM) model has been developed for studying the two-phase characteristics of the gas diffusion medium in a polymer electrolyte fuel cell (PEFC). The three-dimensional (3D) fibrous microstructure for the nonwoven gas diffusion layer (GDL) microstructure has been reconstructed using a stochastic technique for Toray090 and SGL10BA carbon papers. The FM model directly solves for the capillary pressure-saturation relations on the detailed morphology of the reconstructed GDL from drainage simulations. The estimated capillary pressure-saturation curves can be used as valuable inputs to macroscopic two-phase models. Additionally, 3D visualization of the water distribution in the gas diffusion medium suggests that only a small number of pores are occupied by liquid water at breakthrough. Based on a reduced compression model, the two-phase behavior of the GDL under mechanical load is also investigated and the capillary pressure-saturation relations are evaluated for different compression levels.