Gas Diffusion Layer Samples Research Articles

Transient, spatially resolved desaturation of gas diffusion layers measured via synchrotron visualization

The transient 3-D visualization of the desaturation process of flooded gas diffusion layers (GDLs) is presented for the first time. Desaturation rates and pathways are reported for two commercial GDL samples, one with a PTFE treatment and the other without. On-the-fly Synchrotron X-ray CT is performed while the GDL sample is subjected to a humidified or dry air flow stream to visualize the transient desaturation. The two humidification conditions assist in separating the convective and evaporative components of the desaturation process, showing a slight contribution from the convective effect, while the majority of the desaturation is due to evaporative removal. The convective removal is found to be insufficient to fully desaturate either GDL, with water remaining trapped underneath the channel rib with the more hydrophobic GDL and within the pore space in the more hydrophilic GDL. Stop-and-shoot Synchrotron X-ray computed tomography (CT) is then used in conjunction with program-assisted segmentation to determine initial saturation water volumes. These are then combined with the desaturation times found from the on-the-fly experiments to determine desaturation rates for both GDLs. The desaturation rate using dry air flow for the more hydrophobic GDL is found to be nearly four times faster than that of the more hydrophilic GDL. Results demonstrate that an evaporative contribution is necessary for either GDL sample to reach full desaturation.

The effects of gas diffusion layers structure on water transportation using X-ray computed tomography based Lattice Boltzmann method

The Gas Diffusion Layer (GDL) of a Polymer Electrolyte Membrane Fuel Cell (PEMFC) plays a crucial role in overall cell performance. It is responsible for the dissemination of reactant gasses from the gas supply channels to the reactant sites at the Catalyst Layer (CL), and the adequate removal of product water from reactant sites back to the gas channels.Existing research into water transport in GDLs has been simplified to 2D estimations of GDL structures or use virtual stochastic models. This work uses X-ray computed tomography (XCT) to reconstruct three types of GDL in a model. These models are then analysed via Lattice Boltzmann methods to understand the water transport behaviours under differing contact angles and pressure differences.In this study, the three GDL samples were tested over the contact angles of 60°, 80°, 90°, 100°, 120° and 140° under applied pressure differences of 5 kPa, 10 kPa and 15 kPa. By varying the contact angle and pressure difference, it was found that the transition between stable displacement and capillary fingering is not a gradual process. Hydrophilic contact angles in the region of 60°<θ < 90° showed stable displacement properties, whereas contact angles in the region of 100°<θ < 140° displayed capillary fingering characteristics.

Effects of compression on water distribution in gas diffusion layer materials of PEMFC in a point injection device by means of synchrotron X-ray imaging

In this study, ex-situ experiments performed with a point injection device are conducted to evaluate water distributions in gas diffusion layer (GDL) materials which serve as porous transport media in polymer electrolyte membrane fuel cells (PEMFCs). In this regard, GDL samples manufactured by SGL Group are placed into the point injection device and visualized by means of synchrotron X-ray radiographic and tomographic imaging. The resulting image data undergoes a coordinate transformation that ascertains water agglomerations in GDL pores with regard to their radial displacements from the injection point. In this way, water transport in two different GDL samples possessing the same structural characteristics, but with unique compression rates, are investigated in terms of in-plane water distribution. The radial displacement analysis indicated that the pore saturation of the compressed GDL is higher in both the micro porous layer (MPL) region and the carbon fiber substrate region than that of the uncompressed GDL. The water agglomerations in the uncompressed GDL are predominantly observed in the vicinity of the injection point, indicating a limited in-plane transport. Conversely, in the compressed case water accumulations are detected far from the injection point, even at the edge of the GDL, pointing out that compression promotes the in-plane transport. Prior to the ex-situ experiments, both GDL samples have undergone an ageing procedure to mimic realistic cell operating conditions.

On the Limitations of Volume-Averaged Descriptions of Gas Diffusion Layers in the Modeling of Polymer Electrolyte Fuel Cells

As a complement to experimentation, numerical modeling is a key tool to optimize the design of Polymer Electrolyte Fuel Cells (PEFCs). Moreover, it plays an important role for the study of multiphysics, multiphase and multiscale phenomena, which are difficult to explore experimentally due to the small characteristic dimensions of the Membrane Electrode Assembly (MEA). In particular, thorough understanding of the coupled mass, charge and heat transport processes that occur in thin, heterogeneous, anisotropic, mix-wettability Gas Diffusion Layers (GDLs) is essential to develop new MEA designs with improved performance and enlarged durability [1,2]. In the PEFC literature, the most extended approach to model GDLs is volume averaging, where effective properties are used to describe the multiple transport processes that take place in this component. However, the applicability of volume-averaged formulations when modeling thin GDL materials has been questioned many times, and the error introduced in the numerical predictions is unclear. In this work, we explore the limitations introduced by volume-averaged formulations of GDLs under single-phase conditions. To this end, Lattice Boltzmann (LB) simulations were performed on X-ray computed tomography images of various commercial GDLs to assess the existence of a Representative Elementary Volume (REV). The tomographic images of the dry GDLs were acquired at Lawrence Berkeley National Lab’s Advanced Light Source (ALS) synchrotron (Beamline 8.3.2) with a resolution of 1.3 µm/voxel, and then segmented into void and solid phases using the algorithms described in [1,3]; the model was implemented in the open-source LB solver Palabos. Relevant effective transport properties were determined in both the in- and through-plane directions by direct numerical simulation, including effective diffusivity, permeability and effective electrical/thermal conductivity. Figure 1(a) shows the results computed for the through-plane effective diffusivity and thermal conductivity on an untreated Toray® TGP-H-120 sample using boxes of material with different size in the x, y, z directions (L x, L y, L z); the center of the boxes was always taken in the middle position of the original GDL sample. The in-plane size to obtain representative and consistent effective properties was found to be around 1 mm, in agreement with the microstructural analysis reported by Zenyuk et al. [3]. This length scale is comparable to the rib/channel width used in PEFCs, which is typically around 1 mm. Regarding the through-plane direction, there is not a representative size much smaller than the thickness of the material. Numerical results are only similar to previous experimental data when the full thickness (or almost the full thickness) of the material is considered. Therefore, it can be concluded that a REV cannot be rigorously defined in a GDL. In other words, there is not a piece of material with representative properties, whose dimensions are much smaller than the characteristic size of the cell in any direction. Given the impossibility to define a REV in GDLs, the error introduced by volume-averaged formulations was then evaluated by using a non-isothermal, single-phase, steady-state model of a 2x1 mm2 differential PEFC implemented in ANSYS® Fluent. In particular, we compared the predictions of a fully volume-averaged model with those of a pore-scale model accounting for the GDL microstructure. Only the core region of the GDL was included, and a macroscopic description was considered for the remaining components of the MEA (catalyst layers, microporous layers and membrane). The operating conditions were established at 80 ○C, 50% relative humidity, 1.5 bar and 0.5 V. As shown in Figure 1(b), the average results predicted by both models are similar, even though the spatial distributions differ significantly. The impact of the GDL microstructure on the current density distribution arises from the lack of separation of scales discussed above. The preliminary results show that GDL volume-averaged models are a good approximation to determine overall cell performance, as it has been traditionally assumed in the literature. This holds, at least, when the effect of interfacial regions is not considered. However, the local distributions predicted by volume-averaged models can be quite different from the actual scenario at the pore-scale level, in which heterogeneities can lead, e.g., to the formation of hotspots that may affect MEA durability. Further implications of the use of GDL volume-averaged models under multiphase conditions should also be investigated in future work.

Numerical Predicting of Liquid Water Transport inside Gas Diffusion Layer for PEMFC Using Lattice Boltzmann Method

The purpose of this work is to use a numerical simulation applying the Lattice Boltzmann Method (LBM) [1-2] technique to understand the two-phase transport of liquid water and air inside the gas diffusion layers (GDLs) used in polymer electrolyte membrane fuel cells (PEMFC). The liquid water breakthrough dynamics described by the capillary pressure across the GDLs with and without a microporous layer (MPL) are the major focus in this work. The model geometries of GDLs and MPLs used in this study were obtained by three dimensional reconstructed micro-structure from both Nano and Micro X-ray computed tomography (CT). The overall predictions of water breakthrough pressure agree with the data observed in the experiment. They also reveal that the breakthrough pressure and liquid water evolution inside the GDL samples are different when the wetting properties of GDL and/or MPL are changed. Figure 1 gives an example of model geometries for the simulation. It consists of both GDL and MPL those were taken from Nano X-ray CT. This also includes a crack that was observed from the image. Figure 2 shows an example of the predictions of water evolution inside the GDL and MPL. When the liquid water was pushed from MPL to GDL, liquid water spread all over the MPL before it moves toward the GDL. This is because the pressure required to push the liquid water thru the MPL structure or the crack in MPL is higher than the pressure used for liquid water spreads on the top of MPL [3].

Numerical Predicting of Liquid Water Transport inside Gas Diffusion Layer for PEMFC Using Lattice Boltzmann Method

The purpose of this work is to use the Lattice Boltzmann Method (LBM) a numerical simulation technique to understand the two-phase transport of liquid water and air inside the gas diffusion layers (GDLs) used in polymer electrolyte membrane fuel cells (PEMFC). The major focus of this work is the local saturation of liquid water under breakthrough dynamics across the GDLs with and without a microporous layer (MPL). The model geometries of GDLs and MPLs used in this study were obtained by three dimensional reconstructed micro-structure from Micro X-ray computed tomography (CT). The overall predictions of water breakthrough pressure agree with the data observed in the experiments. They also reveal that the breakthrough pressure and liquid water evolution inside the GDL samples are different when the wetting properties of GDL and/or MPL are changed.

Experimental and pore-level numerical investigation of water evaporation in gas diffusion layers of polymer electrolyte fuel cells

The liquid water generated by the electrochemical reaction in a polymer electrolyte fuel cell (PEFC) can greatly affect the cell performance and/or efficiency. Phase change of the water due to evaporation is therefore an important process in understanding the water and flow behavior in the porous gas diffusion layers (GDLs) of PEFCs. In this work a 3D lattice Boltzmann (LB) methodology is adopted to model the flow and mass diffusion in a binary air/water mixture. A modeling framework is designed so as to focus on the understanding of the phase change processes of stationary water inside a GDL with a thickness less than 0.25mm. Consistent models are employed to simulate evaporation at pore scale and on the irregular water-gas interface. The numerically obtained evaporation fluxes are compared against experimental data for the exact same 3D porous GDL sample. The pore-scale simulations are first shown to be in good agreement with the measurements for global evaporation rates in GDLs. Numerical simulations are then performed inside and outside the GDL to investigate the physics of evaporation, including the interaction between the diffusion at pore scale and the external convection in the channel of the PEFC. The role of evaporation-induced flows is finally studied by comparing the computed magnitudes of the vertical velocities obtained with and without inclusion of evaporation.

Micro-Scale Analysis of Liquid Water Breakthrough inside Gas Diffusion Layer for PEMFC Using X-ray Computed Tomography and Lattice Boltzmann Method

The main objective of this work is to predict the breakthrough pressure of liquid water transport through the gas diffusion layer (GDL) and/or micro porous layer (MPL) used in polymer electrolyte membrane fuel cells. The integration of structural GDL and MPL with Lattice Boltzmann Method is primary focused. The numerical predictions are also compared with experimental data. The interaction between liquid phase and different surface treatments of solid structures controls the evolution of liquid water and the change of capillary pressure. The geometries of GDLs and MPLs were obtained by three dimensional reconstructed micro-structure images from both nanometer and micrometer-scaled high spatial resolution X-ray computed tomography (CT). The predictions of water breakthrough pressure agree with the data observed in the experiment. They also reveal that the breakthrough pressure and liquid water evolution inside the GDL samples are different when the wetting properties of GDL and/or MPL are changed. The detailed microporous property can be obtained using high spatial resolution image from nanometer-scaled X-ray CT, a.k.a. Nano X-ray CT. Meanwhile, images from micrometer-scaled X-ray CT, a.k.a. Micro X-ray CT, give proper field of view to cover complete vision of porous materials, including cracks in the MPL.

Influence of Binder Porosity on Gdl Gas Phase Transport

In polymer electrolyte fuels (PEFC) carbon fiber based gas diffusion layers (GDL) are used for fine distribution of reactant gases from the flow field channels to the catalyst layer (CL). It is a key component for PEFC as it serves also for water removal from the CL as well as heat and electricity transport between CL and the flow field plates. GDL materials can be classified as woven, felts, papers, where the later type requires a binder to mechanically stabilize the fiber arrangement. While most GDL binders are solid, the binder of SGL GDLs appears porous and it’s influence on gas phase transport is unclear [1]. In SEM surface images of SGL, GDL binder domains of appear as flake like fine-structure with particles of about 100 to 300 nm (see Fig. 1a). Here, we apply multi-dimensional X-ray imaging techniques and multi-scale numerical simulations to clarify possible contributions of the binder porosity to overall GDL gas transport. Absorption contrast X-ray tomographic microscopy measurements at the TOMCAT beamline of the Swiss Light Source (SLS) with voxel sizes of minimum 0.16 µm revealed a high porosity (59 %) of SGL 24 BA binder domains of (Fig. 1b). Computational transport simulations on the segmented binder structures showed, that the binder domains provide isotropic effective diffusivity of 0.30. In order to quantify the contribution of the porous binder to the overall fluid transport in the GDL, the obtained binder diffusivity values were used in multi-scale transport simulations of ternary segmented XTM data of representative GDL samples with 2.2 µm voxel size. If the binder was treated as solid, through-plane effective diffusivity of the GDL of 0.1 was obtained. If we applied a multi-scale simulation approach, that treats the binder domain as porous and uses the transport parameters of the binder domain as input values for the coarse scale simulations of the whole GDL domain, the through-plane effective diffusivity increased to 0.4 (+300 %). These numbers will be verified by X-ray ptychograptic computed tomography measurements of the binder fine structure at the cSAXS beamline of the SLS that proved resolutions of up to 16 nm [2].

Effect of the pore size variation in the substrate of the gas diffusion layer on water management and fuel cell performance

The gas diffusion layer (GDL) is a key component of a proton exchange membrane (PEM) fuel cell due to its role as a pathway for fuel, air, and water. GDL determines the mass balance and water management in the PEM fuel cell. Thus, an investigation of the optimal structural characteristics of the GDL with improved water management is important to ensure high performance of the PEM fuel cell. In this study, a PEM fuel cell model that considers the structural characteristics of GDL is developed, and the various the GDL structural characteristics are analyzed and validated. Specifically, the effects of pore size variation in the substrate of the GDL on water management and cell performance are investigated. Two GDL samples with different pore sizes are evaluated according to the porosity, pore size, thickness, and internal contact angle to understand the basic structural characteristics of the GDL. The GDL structural parameters with the basic characteristics are incorporated into the developed model. The cell performance is predicted to be relative to the two-phase mass transport inside the GDL. The characteristics of the micro-porous layer (MPL) and MPL penetration part are found to be affected by the variation in the macro-pore size of the substrate. The water management capability of the GDL varies with these differences with respect to water retention and removal characteristics. Under the condition of relative humidity (RH) 100%, the averaged saturation value of the MPL and MPL penetration part of the GDL with small macro-pore in the substrate is 18.8% higher than that of the GDL with large macro-pore in the substrate. The cell performance is also affected by the operating conditions of relative humidity and current load. The voltage of the GDL with small macro-pore in the substrate at the current density of 1.6A/cm2 is 10.3% lower than that of the GDL with large macro-pore in the substrate under RH 100%.

The Effect of Clamping Pressure on Gas Diffusion Layer Performance in Polymer Electrolyte Fuel Cells

AbstractClamping pressure applied in polymer electrolyte fuel cell (PEFC) assembly is known to have a significant effect on performance. Compression applied on the membrane electrode assembly (MEA) deforms the gas diffusion layer (GDL) and causes a decrease in the GDL thickness, porosity and electrical resistance. These changes in the GDL properties have a significant influence on the MEA performance. In this study, three sets of GDL samples were tested in situ under varied clamping pressure levels to demonstrate the difference in the GDL behaviour with compression, and to optimize the MEA clamping pressure.The paper focuses on the change in the performance of MEAs with various types of GDLs, and relates the properties of the GDL to the behaviour of the MEA under compression. The study discusses the change in GDL compression behaviour with the change in the GDL thickness, density and structure.

Characterization of Liquid Water Invasion in Gdls Using X-Ray Tomographic Microscopy

The accumulation of liquid water in the gas diffusion layer (GDL) of low temperature polymer electrolyte fuel cells (PEFC) results in increased mass transport limitations [1] and eventually in a significant potential and efficiency penalty. To achieve efficient water removal and maintain high performance, it is necessary to understand the morphological (porosity, pore and throat sizes) and chemical (surface tension) properties of the GDL, which control the water transport [2]. An ex-situ capillary pressure (pc) – saturation setup was developed in the framework of the IMPALA project [3] for imaging experiments using X-ray tomographic microscopy (XTM) at the Swiss Light Source synchrotron facility as it offers the advantage of well defined capillary pressure boundary conditions. This approach allows a quantitative analysis of the water transport through the GDL by linking the local saturation at the achieved pc levels with the microstructure of the invaded pores and throats.Within the setup, a hydrophilic membrane implements a homogeneous boundary condition at the bottom of the GDL. At the top of the GDL either a hydrophobic membrane or a flow field was placed. While the hydrophobic membrane setup allows to study complete liquid wetting of the pore space, the flow field structure allows to implement an anisotropic compression simulating PEFC conditions and to study its consequences on water transport. GDL samples of different suppliers have been tested. The breakthrough pressure, which corresponds to the capillary pressure when water reaches the opposite side of the injection surface, is a function of the materials. Correlation with the dry GDL microstructure shows that pore (and throat) size is the major controlling parameter. SGL 24BA shows a lower breakthrough pressure than Toray TGP-H060 due to larger throats and exhibits a stronger channel rib deviation of pore and throat sizes as well as of saturation.

Study of effective transport properties of fresh and aged gas diffusion layers

Gas diffusion layers (GDLs) play an important role in proton exchange membrane fuel cells (PEMFCs) for the diffusion of reactant and the removal of product water. In the current study fresh and aged GDLs (Sigracet® GDL34BC) were investigated by X-ray computed tomography to obtain a representative 3D image of the real GDL structure. The examined GDL samples are taken from areas located under the flow channel and under the land. Additionally, a brand new Sigracet® GDL34BC was taken as a reference sample in order to find out the impact of fuel cell assembly on GDL. The produced 3D image data were used to calculate effective transport properties such as thermal and electrical conductivity, diffusivity, permeability and capillary pressure curves of the dry and partially saturated GDL. The simulation indicates flooding by product water occurs at contact angles lower than 125° depending on sample porosity. In addition, GDL anisotropy significantly affects the permeability as well as thermal and electrical conductivities. The calculated material bulk properties could be next used as input for CFD modelling of PEM fuel cells where GDL is usually assumed layer-like and homogeneous. Tensor material parameters allow to consider GDL anisotropy and lead to more realistic results.

X-ray Tomographic Analysis of Porosity Distributions in Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells

This paper describes a method to characterize the structure of polytetrafluoroethylene (PTFE) treated gas diffusion layers (GDLs) with and without microporous layers (MPLs) using 3D X-ray micro computed tomographic (μCT) microscopy. In this work, the structure of single and dual layer GDLs is evaluated via μCT for various GDL samples (such as Toray TGP-H-060 and AvCarb EP40) loaded with different MPLs. A new method is presented for separating, or segmenting, the various phases of the GDL, i.e., void space, carbon fiber (including binder and PTFE), and MPL. Through analysis, it was found that the variation in bulk porosity and the average pore diameter of the GDLs depends highly on the GDL series manufacturing and treatment processes. Using advanced image analysis techniques, routines were developed to accurately segment the GDL fibers (including binder/PTFE) and the MPL. The percentage of the intruding MPL material into the carbon fiber paper as a function of the GDL thickness was successfully found for dual layer GDLs, with varying PTFE content and areal weight loading in the MPL. This analysis provides invaluable insight into the physical microstructure of paper-based GDLs, emphasizing the heterogeneous porosity distribution of single layer GDLs and the interaction of the MPL with the carbon fiber paper of dual layer GDLs.

Effects of hydrophobic agent content in macro-porous substrates on the fracture behavior of the gas diffusion layer for proton exchange membrane fuel cells

Although the adhesion between the macro-porous substrate (MPS) and micro-porous layer (MPL) of a gas diffusion layer (GDL) is a critical factor that affects the reliability and durability of proton exchange membrane fuel cells, systematic studies quantifying the interfacial fracture energy of GDL have not yet been reported. Therefore, in this study, the interfacial fracture energy of GDLs with different contents of hydrophobic agents in the MPS is quantitatively measured. GDL samples with 0, 5, 10, and 20 wt% of hydrophobic agent content are tested using double cantilever beam fracture mechanics tests. It is observed that the interfacial fracture energy of the GDLs increases as the content of hydrophobic agent increases, due to more favorable interactions between the hydrophobic agents of the MPL and MPS. Optical microscope, scanning electron microscope, and energy-dispersive X-ray spectroscope analyses are performed on the bare and delaminated surfaces in order to investigate the mechanism of the interfacial fracture energy increase of the GDLs.

Pore-network modeling of liquid water flow in gas diffusion layers of proton exchange membrane fuel cells

Abstract Fluid flow through the gas diffusion layer (GDL) of fuel cells is numerically studied using a pore network modeling approach. The model is developed based on an experimental visualization technique (fluorescence microscopy). The images obtained from this technique are analyzed to find patterns of flow inside the GDL samples with different hydrophobicity. Three different flow patterns are observed: initial invasion, progression, and pore-filling. The observation shows that liquid water flows into the majority of available pores on the boundary of the untreated GDL and several branches are segregated from the initial pathways. For the treated GDL, however, a handful of boundary pores are invaded and the original pathways extend toward the other side of the medium with minimum branching. The numerical model, developed based on an invasion percolation algorithm, is used to study the effects of GDL hydrophobicity and thickness on the flow configuration and breakthrough time as well as to determine the flow rate and saturation in different GDL samples. During the injection of water into the samples, it is numerically shown that the flow rates are monotonically decreasing for both treated and untreated samples. For the treated sample, however, the injection flow rate is constantly lower than that of the untreated sample, resulting in a lower overall water saturation at breakthrough. The numerical results also suggest that hydrophobic treatment of thick samples has minor effects on water management and overall performance. The developed model can be used to optimize the GDL properties for designing porous medium with effective transport characteristics.

Deformation of PEM fuel cell gas diffusion layers under compressive loading: An analytical approach

In the PEM fuel cell stack, the fibrous porous gas diffusion layer (GDL) provides mechanical support for the membrane assembly against the compressive loads imposed by bipolar plates. In this study, a new mechanistic model is developed using fundamental beam theory that can accurately predict the mechanical deflection of GDL under compressive loads. The present analytical model is built on a unit cell approach, which assumes a simplified geometry for the complex and random GDL microstructure. The model includes salient microstructural parameters and properties of the fibrous porous medium including: carbon fiber diameter, fiber elastic modulus, pore size distribution, and porosity. Carbon fiber bending is proved to be the main deformation mechanism at the unit cell level. A comprehensive optical measurement study with statistical analysis is performed to determine the geometrical parameters of the model for a number of commercially available GDL samples. A comparison between the present model and our experimental stress–strain data shows a good agreement for the linear deformation region, where the compressive pressure is higher than 1 MPa.

Analytical Modeling of PEM Fuel Cell Gas Diffusion Layers Deformation Under Compression: Part 2 - Nonlinear Behaviour Region

In the Proton exchange membrane PEM fuel cell stack, transport characteristics of the porous gas diffusion layer (GDL) change due to the GDL microstructural parameters variation in a result of compressive loading during the fuel cell assembly and operation. The GDL is a carbon fiber based highly porous media in the form of paper or cloth. In addition to the reactants and reactions products transportation, GDL provides mechanical support for the membrane assembly against the compressive loads imposed by bipolar plates. GDL mechanical behaviour can be divided into two regions; nonlinear and linear. In the compressive loads smaller than a critical pressure, GDL has a nonlinear mechanical behaviour and its compressive modulus is a function of deformation level. Using assumption of existing gap between the fibers in GDL micro-structure, the analytical model proposed in first part of this study for linear region, based on the unit cell approach, is extended to predict the nonlinear mechanical behaviour of GDL. The present unit cell model covers salient microstructural parameters and properties of the fibrous porous medium including: carbon fiber diameter, elastic modulus, pore size distribution, and gap size distribution between fibers. Bending of carbon fibers and closing the gaps between fibers are proved to be the main deformation mechanism of GDL deformation in nonlinear region. The statistical parameters of the gap distribution between carbon fibers in the GDL structure are calculated using experimental stress-strain data for a number of commercially available GDL samples. A comparison between the present model and the GDL stress-strain data shows that the model can predict accurately the mechanical behaviour of the GDL material. In addition to the mechanical behaviour, the proposed model provides useful information about the microstructural properties of the GDL during compression such as gap distribution between fibers that can be used in the GDL transport properties prediction. Keywords: Gas Diffusion Layer, Porous media, Fuel cell,analytical modeling, mechanical behaviour, compression

Analytical Modeling of PEM Fuel Cell Gas Diffusion Layers Deformation Under Compression: Part 1 - Linear Behaviour Region

In the Proton exchange membrane PEM fuel cell stack, transport characteristics of the porous gas diffusion layer (GDL) change due to the GDL microstructural parameters variation in a result of compressive loading during the fuel cell assembly and operation. The GDL is a carbon fiber based highly porous media in the form of paper or cloth. In addition to the reactants and reactions products transportation, GDL provides mechanical support for the membrane assembly against the compressive loads imposed by bipolar plates. According to the results of compression tests, GDL mechanical behaviour can be divided into regions; nonlinear and linear. In the compressive loads smaller than a critical pressure, GDL has a nonlinear mechanical behaviour and its compressive modulus is a function of deformation level. After reaching the critical pressure, the compressive strain increases linearly with the applied stress. Using unit cell approach and assuming a simplified geometry for the complex and random microstructure of the GDL, a new mechanistic model is developed that can accurately predict the mechanical behaviour of GDL. The unit cell model covers salient microstructural parameters and properties of the fibrous porous medium including: carbon fiber diameter, elastic modulus, and pore size distribution. Bending of carbon fibers is proved to be the main deformation mechanism of GDL deformation in linear region. A comprehensive optical measurement study with statistical analysis was performed to determine the effective geometrical parameters of the model for a number of commercially available GDL samples. A comparison between the present model and the GDL stress-strain data shows that the model can predict accurately the mechanical behaviour of the GDL material. The nonlinear behaviour of GDL will be addressed in the second part of this study. Keywords: Gas Diffusion Layer, Porous media, Fuel cell,analytical modeling, mechanical behaviour, compression

Characterization of Micro-Porous Layer of Gas Diffusion Layer of a PEM Fuel Cell Via X-Ray Tomographic Microscopy

The gas diffusion layer (GDL) performs key functions within the fuel cell, providing access for hydrogen and oxygen gas to flow to and from the catalyst layer, conducting electricity and heat, and removing water byproduct. The GDL is commonly treated with a micro-porous layer (MPL) on the catalyst side to minimize contact resistance by reducing the roughness of the contact surface. The MPL also reduces mass transport losses and hence improves water management in the cell operated at high current densities [1].This paper describes a method to characterize the GDL/MPL porous structure and their interface using X-ray micro-computed tomographic (µ-CT) microscopy. µ-CT is a relatively new tool that is used for acquiring the internal three-dimensional structure of both fully dense and porous materials [2]. In this work, the structure of the MPL/GDL is evaluated via µ-CT for various GDL samples (such as Toray TGP060 and EP40) loaded with different MPL loadings. Figure 1 shows a 3D image of the EP40 T12 GDL without the MPL (CBT 18). While previous studies have also examined the MPL layer using this technique (e.g. [1,3]), this is the first to examine the GDL/MPL interface, as well as the surface characteristics of the MPL. A new method is presented for separating, or segmenting, various phases (GDL, PTFE, and MPL). Using this methodology, the surface roughness of the MPL, and the porosity within the GDL is identified. The results show that there is considerable overlap between the GDL and MPL. Also, while the MPL has a smooth surface in comparison to the GDL, it remains relatively rough. The correlation between PTFE content, thickness of the MPL, and porosity will be discussed.