Feasibility of periodic surface models to develop gas diffusion layers: A gas permeability study

A geometric modeling scheme called periodic surface model (PS) is used to construct three dimensional (3D) models of a gas diffusion layer's (GDL) microstructure, which allows for rapid model construction and modification of representative volume elements (RVE) with embedded periodic boundary conditions. The reconstructed PS models are optimized with the help of the genetic algorithm embedded in MATLAB to generate models with refined mesh for computational fluid dynamics (CFD) analysis. The GDL geometry is built in ANSYS/ICEM CFD, automatically, using a customized code that couples MATLAB and ICEM. To verify the validity of the suggested modeling approach the microstructures of the GDLs with different porosity and fiber orientation are generated and the in-plane and through-plane permeability and tortuosity are calculated using ANSYS/FLUENT software. The numerically predicted values of in-plane and through-plane permeability are compared to experimental measurements. Using the genetic algorithm significantly decreases the fibers intersection volume in the RVE, especially as porosity decreases. It has been found that the tortuosity of the GDL is a function of the spatial orientation of the fibers in the RVE, when the fibers are at a small angle, the in-plane tortuosity can be higher than the through-plane tortuosity.     

Effect of GDL permeability on water and thermal management in PEMFCs—I. Isotropic and anisotropic permeability

The performance of proton exchange membrane fuel cells (PEMFCs) with various isotropic and anisotropic permeabilities of the gas diffusion layer (GDL) was investigated using computational fluid dynamics analysis. A three-dimensional, non-isothermal model was employed with a single straight channel; both humidification and phase transportation were included in the model. The total water and thermal management for systems operating at high current densities was obtained. The results showed that the cell performance deteriorated for low isotropic permeability of the GDL. Water removal from the cathode GDL was significantly reduced in systems with low isotropic permeability or anisotropic systems with low permeabilities in both the in-plane and through-plane directions. Moreover, both the in-plane and through-plane permeabilities were found to affect water and thermal management in PEMFCs, especially in the low permeability ranges. Variations in GDL permeability had a greater influence on ohmic losses than on cathode overpotentials because the former losses depend on water and thermal management. In addition, the results showed that water and thermal management was good in systems in which the permeability in at least one direction (in-plane or through-plane) was high, whereas systems with low permeability in both the in-plane and through-plane directions exhibited poor water and thermal management. However, heat removal in PEMFCs was negatively affected by low permeability, leading to higher temperatures in the cell. The present numerical results suggested that modeling with isotropic permeability conditions may overpredict the cell performance, and inaccurately predict the water and thermal management in PEMFCs.     

In-plane and through-plane gas permeability of carbon fiber electrode backing layers

The absolute gas permeability of several common gas diffusion layer (GDL) materials for polymer electrolyte membrane fuel cells was measured. Measurements were made in three perpendicular directions to investigate anisotropic properties. Most materials were found to display higher in-plane permeability than through-plane permeability. The permeability in the two perpendicular in-plane directions was found to display significant anisotropy. Materials with the most highly aligned fibers showed the highest anisotropy and the permeability could differ by as much as a factor of 2. In-plane permeability was also measured as the GDL was compressed to different thicknesses. Typically, compression of a sample to half its initial thickness resulted in a decrease in permeability by an order of magnitude. Since the change in GDL thickness during compression can be converted to porosity, the relationship between measured permeability and porosity was compared to various models available in the literature, one of which allows the estimation of anisotropic tortuosity. The effect of inertia on fluid flow was also determined and found to vary inversely with permeability, in agreement with available correlations. The results of this work will be useful for 3D modeling studies where knowledge of permeability and effective diffusivity tensors is required.     

Measurement of relative permeability of fuel cell diffusion media

Gas diffusion layer (GDL) in PEM fuel cells plays a pivotal role in water management. Modeling of liquid water transport through the GDL relies on knowledge of relative permeability functions in the in-plane and through-plane directions. In the present work, air and water relative permeabilities are experimentally determined as functions of saturation for typical GDL materials such as Toray-060, -090, -120 carbon paper and E-Tek carbon cloth materials in their plain, untreated forms. Saturation is measured using an ex situ gravimetric method. Absolute and relative permeability functions in the two directions of interest are presented and new correlations for in-plane relative permeability of water and air are established.     

A fractal permeability model for the gas diffusion layer of PEM fuel cells

In this paper a fractal permeability model for the gas diffusion layer (GDL) of PEM fuel cells (PEMFCs) is presented. The model accounts for the actual microstructures of the GDL in terms of two fractal dimensions, one relating the size of the capillary flow pathways to their population and the other describing the tortuosity of the capillary pathways. In addition, the gas molecule effect is considered by using the Adzumi equation. The fractal permeability model is found to be a function of the tortuosity fractal dimension, pore area fractal dimension, sizes of pore and the effective porosity of porous medium without any empirical constants. mercury-intrusion porosimetry was used to measure the microstructures of the GDL. Based on scanning electron microscope (SEM) images, two fractal dimensions are determined by the box-counting method. To verify the validity of the model, the predicted permeability data of the present fractal model were compared with the experimental data supplied by Toray Inc. It is found that the permeability prediction of the model was in accordance with experimental data. This verifies the validity of the present fractal permeability model for the GDL.     

High-density and low-density gas diffusion layers for proton exchange membrane fuel cells: Comparison of mechanical and transport properties

Gas diffusion layers (GDL) play multi-roles in proton exchange membrane fuel cells, including gas-water transport, thermal-electron conduction and mechanical support. Mechanical strength and transport properties are essential for GDLs. In this work, high-density (paper-type) and low density (felt-type) GDLs are scanned and reconstructed using X-ray computed tomography. Porosities under different compression ratios are compared and discussed. Effective diffusivity and liquid water permeability are calculated using pore-scale modeling and lattice Boltzmann method. Mechanical strength, anisotropic thermal-electrical resistivity for two types of GDLs are obtained using compression tests and thermal-electrical conductivity measurements. Results show that the porosity, diffusivity, permeability, and through-plane thermal-electrical conductivity of felt-type GDL are significantly higher than that of paper-type GDL owing to the higher porosity and fiber-clusters oriented along the through-plane direction. The in-plane electrical resistivity of paper-type GDL is lower than that of felt-type GDL. The mechanical strength of felt-type GDL is much lower, but the fibers of paper-type GDL are more easily to be broken because of its lower elasticity. The results obtained may guide microstructure optimization and performance improvement of GDLs.     

In-plane gas permeability of proton exchange membrane fuel cell gas diffusion layers

A new analytical approach is proposed for evaluating the in-plane permeability of gas diffusion layers (GDLs) of proton exchange membrane fuel cells. In this approach, the microstructure of carbon papers is modeled as a combination of equally-sized, equally-spaced fibers parallel and perpendicular to the flow direction. The permeability of the carbon paper is then estimated by a blend of the permeability of the two groups. Several blending techniques are investigated to find an optimum blend through comparisons with experimental data for GDLs. The proposed model captures the trends of experimental data over the entire range of GDL porosity. In addition, a compact relationship is reported that predicts the in-plane permeability of GDL as a function of porosity and the fiber diameter. A blending technique is also successfully adopted to report a closed-form relationship for in-plane permeability of three-directional fibrous materials.     

A fractal model for predicting permeability and liquid water relative permeability in the gas diffusion layer (GDL) of PEMFCs

In this study, a fractal model is developed to predict the permeability and liquid water relative permeability of the GDL (TGP-H-120 carbon paper) in proton exchange membrane fuel cells (PEMFCs), based on the micrographs (by SEM, i.e. scanning electron microscope) of the TGP-H-120. Pore size distribution (PSD), maximum pore size, porosity, diameter of the carbon fiber, pore tortuosity, area dimension, hydrophilicity or hydrophobicity, the thickness of GDL and saturation are involved in this model. The model was validated by comparison between the predicted results and experimental data. The results indicate that the water relative permeability in the hydrophobicity case is much higher than in the hydrophilicity case. So, a hydrophobic carbon paper is preferred for efficient removal of liquid water from the cathode of PEMFCs.     

Effect of weave tightness and structure on the in-plane and through-plane air permeability of woven carbon fibers for gas diffusion layers

In this study, woven gas diffusion layers (GDLs) with varying weave type and tightness are investigated. Plain and twill weave patterns were manufactured in-house. The in-plane and through-plane air permeability of the woven samples were tested, and mercury intrusion porosimetry (MIP) tests were performed to study the pore structure. It was found that the twill weave has a higher permeability than the plain weave, which is consistent with literature. Like non-woven carbon papers, woven GDLs have higher in-plane permeability than through-plane permeability; however it has been shown that it is possible to manufacture a GDL with higher through-plane permeability than in-plane permeability. It was also concluded that the percentage of macropores in the weave is the driving factor in determining the through-plane air permeability. This work lays the groundwork for future studies to attempt to characterize the relationship between the weave structure and the air permeability in woven GDLs.     

Effect of the anisotropic thermal conductivity of GDL on the performance of PEM fuel cells

Gas diffusion layers (GDLs) are one of the main components in proton exchange membrane (PEM) fuel cells. In this paper, the effect of anisotropic thermal conductivity of the GDL is numerically investigated under different operating temperatures. Furthermore, the sensitivity of the PEM fuel cell performance to the thermal conductivity of the GDL is investigated for both in-plane and through-plane directions and the temperature distributions between the different GDL thermal conductivities are compared. The results show that increasing the in-plane and through-plane thermal conductivity of the GDL increases the power density of PEM fuel cells significantly. Moreover, the temperature gradients show a greater sensitivity to the in-plane thermal conductivity of the GDL as opposed to the through-plane thermal conductivity.     

Effective permeability of gas diffusion layer in proton exchange membrane fuel cells

In gas diffusion layers (GDLs) of proton exchange membrane fuel cells (PEMFCs), effective permeability is a key parameter to be determined and engineered. In this study, through-plane (TP) and in-plane (IP) flow behaviors of GDLs are investigated analytically based on a scaling estimate method. The TP permeability and IP permeability of unidirectional fibers are determined first, based on that the minimum distance and the inscribed radius between fibers are adopted as the characteristic lengths for normal and parallel flows, respectively. The permeabilities of two-dimensional (2D) and three-dimensional (3D) GDLs are estimated by a proper mixture of the local TP and IP permeabilities of fiber alignments. The mechanistic model agrees closely with experimental and numerical results over a wide porosity range. With the new model, the influences of porosity and fiber orientation on flow behaviors are analyzed.     

Effect of fiber orientation on Liquid–Gas flow in the gas diffusion layer of a polymer electrolyte membrane fuel cell

In this study, the lattice Boltzmann method was used to simulate the three-dimensional intrusion process of liquid water in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC). The GDL was reconstructed by the stochastic method and used to investigate fiber orientation's influence on liquid water transport in the GDL of a PEMFC. The fiber orientation can be described by the angle between a single fiber and the in-plane direction; three different samples were simulated for three different fiber orientation ranges. The simulated permeability correlated well with the anisotropic characteristics of reconstructed carbon papers. It was concluded that the fiber orientation had a significant effect on the liquid invasion pattern in the GDL by changing the pore shape and distribution of the GDL. The results indicated that the stochastically reconstructed GDL, taking into account the fiber orientation, better demonstrates the mass transport properties of the GDL.     

Effect of GDL permeability on water and thermal management in PEMFCs—II. Clamping force

In proton exchange membrane fuel cells (PEMFCs), the permeability of the gas diffusion layer (GDL) differs between the in-plane and through plane directions, and the overall permeability in the shoulder region is typically lower than that in the channel region due to the clamping force applied through the bipolar plates. Here, we conducted a numerical investigation of GDLs with different isotropic or anisotropic permeabilities in the channel and shoulder regions for PEMFCs. A three-dimensional, non-isothermal model was employed with a single straight channel. We found that the water and thermal management in PEMFCs depend on the permeability characteristics of the GDL, especially in the shoulder region. Moreover, the ohmic loss and cathode overpotential varied depending on the difference in isotropic permeability between the channel and shoulder regions. In the study on GDLs with anisotropic permeabilities in the channel and shoulder regions, however, we found that variations in the anisotropic permeabilities in the channel and shoulder regions, had little effect on the cathode overpotential at the shoulder region, but caused significant changes in ohmic loss as the ohmic loss depended on water and thermal management. In addition, we found that the cell temperature was much higher in GDLs with low anisotropic permeabilities due to hindering of the water removal process.     

Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells: 2. Absolute permeability

This is the second in a series of papers in which we present methods demonstrated in our group for the estimation of transport properties in gas diffusion layers (GDLs) for proton exchange membrane fuel cells (PEMFCs). Here we describe a method for determining separately the in-plane (x, y-directions) and through-plane (z-direction), viscous and inertial permeability coefficients of macro-porous substrates and micro-porous layers by controlling the direction of the gas flow through the porous sample. The method is applied initially to the macro-porous substrate of the GDL alone and subsequently to the macro-porous substrate with different micro-porous layers applied on it. The permeability coefficients of the micro-porous layer are calculated from the two measurements. The permeability coefficients are calculated from the Darcy–Forchheimer equation by application of the method of least squares. The method was applied to GDLs having different contents of polytetrafluoroethylene (PTFE) and carbon types. The samples with a higher PTFE content have in-plane and through-plane viscous permeability coefficients higher than those of the samples with lower PTFE content. The in-plane and through-plane viscous permeability coefficients also depend on the carbon type.     

Fractal model for prediction of effective hydrogen diffusivity of gas diffusion layer in proton exchange membrane fuel cell

This paper presents a novel prediction model of the effective hydrogen diffusivity for the gas diffusion layer (GDL) in proton exchange membrane fuel cell (PEMFC) by using fractal theory to characterize microstructure. With the consideration of pore-size distribution and Knudsen diffusion effect, a relationship between micro-structural parameters and effective hydrogen diffusivity of GDL is deduced. The prediction of effective hydrogen diffusivities of two samples shows that Knudsen diffusion effect makes the effective diffusivity value decrease, and after being treated with polytetrafluoroethylene (PTFE), carbon paper, a basal material of the GDL, exhibits a lower effective diffusivity value due to the decrease in the pore space and porosity. From the parametric effect study, it can be concluded that effective diffusivity has a positive correlation with pore area fractal dimension D_p or porosity ?, whereas it has a negative correlation with tortuosity fractal dimension D_t.     

A novel approach to determine the in-plane thermal conductivity of gas diffusion layers in proton exchange membrane fuel cells

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a proton exchange membrane (PEM) fuel cell. The analysis of this process requires determination of the effective thermal conductivity. This transport property differs significantly in the through-plane and in-plane directions due to the anisotropic micro-structure of the GDL.A novel test bed that allows separation of in-plane effective thermal conductivity and thermal contact resistance in GDLs is described in this paper. Measurements are performed using Toray carbon paper TGP-H-120 samples with varying polytetrafluoroethylene (PTFE) content at a mean temperature of 65-70 °C. The measurements are complemented by a compact analytical model that achieves good agreement with experimental data. The in-plane effective thermal conductivity is found to remain approximately constant, k ≈ 17.5 W m⁻¹ K⁻¹, over a wide range of PTFE content, and its value is about 12 times higher than that for through-plane conductivity.     

Study of the mechanical behavior of paper-type GDL in PEMFC based on microstructure morphology

As the softest part in a proton exchange membrane fuel cell (PEMFC), the gas diffusion layer (GDL) could have a large deformation under assembly pressure imposed by bipolar plate, which would have an impact on the cell performance. So, there is an urgent need to clearly reveal the mechanical behavior of GDL under certain pressure. In this paper, the mechanical behavior of paper-type GDL of PEMFC is studied, considering the complex contact environment in the fibrous layered structure. The microstructure of GDL is reconstructed stochastically, then the stress-strain relationship of GDL is explored from the perspective of solid mechanics by using the finite element method. Based on microstructure morphology, it is found that contact pairs and pore space of microstructure are two key factors determining the nonlinearity of the compressive curve. The equivalent Young's modulus increases with the decrease of porosity and carbon fiber diameter but it is not very sensitive to the carbon paper thickness. The results indicate that with the increase in acting pressure, the average porosity of the carbon paper decreases, and the nonuniformity of porosity along the through-plane direction increases. Furthermore, a reasonable explanation for the increase of concentration loss and the decrease of ohmic loss is given from the microstructure findings of the present study.     

Microstructure reconstruction and characterization of PEMFC electrodes

We reconstruct a proton exchange membrane fuel cell (PEMFC) catalyst layer (CL) and a non-woven carbon paper gas diffusion layer (GDL) by specially-designed stochastic methods. The reconstructed microstructures evidently distinguish all the participating components in the composite GDL/CL. Characterization analyses with respect to the reconstructed GDL/CL give important structural properties such as geometrical connectivity of an individual phase, pore size distribution, and volume-specific interfacial area. Self-developed Lattice Boltzmann (LB) models are established to calculate effective transport physical properties including effective thermal/electric conductivity and effective species diffusivity of the reconstructed GDL/CL, and permeability of the reconstructed GDL. Accordingly, we obtain tortuosity values for pore or solid phase in the reconstructed GDL/CL.     

Estimation of through-plane and in-plane gas permeability across gas diffusion layers (GDLs): Comparison with equivalent permeability in bipolar plates and relation to fuel cell performance

The present work focusses on measuring the permeability across gas diffusion layers (GDLs) first in a dedicated cell and later in PEM fuel cell configuration with varying bi-polar plate designs. Eight carbon paper-based GDLs with and without the microporous layer (MPL), have been tested. An in-house designed dedicated cell allowed measuring pressure drop depending on flow rate, for i) through-plane and ii) in-plane direction. Further, transport measurements were conducted in 25 cm² bi-polar plates (BPs) in fuel cell configuration having single or multiple serpentine channels, by stacking the GDL inside. The results show that gas permeability in the dedicated cell for through-plane and in-plane can be estimated by using Darcy's law. However, for BPs, the flow is affected additionally by inertial contribution (Darcy-Forchheimer). Finally, the efficiency allowed by selected GDLs installed in a fuel cell under operation shows a relationship between the equivalent permeability and the fuel cell performance.     

Investigation of the effects of the anisotropy of gas-diffusion layers on heat and water transport in polymer electrolyte fuel cells

The aim of this work is to study the effects of gas-diffusion layer (GDL) anisotropy and the spatial variation of contact resistance between GDLs and catalyst layers (CLs) on water and heat transfer in polymer electrolyte fuel cells (PEFCs). A three-dimensional, two-phase, numerical PEFC model is employed to capture the transport phenomena inside the cell. The model is applied to a two-dimensional cross-sectional PEFC geometry with regard to the in-plane and through-plane directions. A parametric study is carried out to explore the effects of key parameters, such as through-plane and in-plane GDL thermal conductivities, operating current densities, and electronic and thermal contact resistances. The simulation results clearly demonstrate that GDL anisotropy and the spatial variation of GDL/CL contact resistance have a strong impact on thermal and two-phase transport characteristics in a PEFC by significantly altering the temperature, water and membrane current density distributions, as well as overall cell performance. This study contributes to the identification of optimum water and thermal management strategies of a PEFC based on realistic anisotropic GDL and contact-resistance variation inside a cell.