Effects of porosity gradient in gas diffusion layers on performance of proton exchange membrane fuel cells

A three-dimensional, two-phase, non-isothermal model has been developed to explore the interaction between heat and water transport in proton exchange membrane fuel cells (PEMFCs). Water condensate produced from the electrochemical reaction may accumulate in the open pores of the gas diffusion layer (GDL) and retard the oxygen transport to the catalyst sites. This study predicts the enhancement of the water transport for linear porosity gradient in the cathode GDL of a PEMFC. An optimal porosity distribution was found based on a parametric study. Results show that a optimal linear porosity gradient with ?₁ = 0.7 and ?₂ = 0.3 for the parallel and z-serpentine channel design leads to a maximum increase in the limiting current density from 10,696 Am⁻² to 13,136 Am⁻² and 14,053 Am⁻² to 16,616 Am⁻² at 0.49 V, respectively. On the other hand, the oxygen usage also increases from 36% to 46% for the parallel channel design and from 55% to 67% for the z-serpentine channel design. The formation of a porosity gradient in the GDL enhances the capillary diffusivity, increases the electrical conductivity, and hence, benefits the oxygen transport throughout the GDL. The present study provides a theoretical support for existing reports that a GDL with a gradient porosity improves cell performance.     

Gas diffusion through differently structured gas diffusion layers of PEM fuel cells

Proton exchange membrane fuel cell (PEMFC) gas diffusion layers (GDLs) play important parts in diffusing gas, discharging liquid water, and conducting electricity, etc. When liquid water is discharged through GDL to gas channel, there will be some pores of GDLs occupied by liquid water. In this study, based on a one-dimensional model, the distribution of liquid water phase saturation is analyzed for different GDL structures including GDL with uniform porosity, GDL with sudden change porosity (GDL with microporous layer (MPL)) and GDL with gradient porosity distribution. The effect on gas diffusion of the changes of porosity and liquid saturation due to water remaining in GDL pores is calculated. The conclusions are that for uniform porosity GDL, the gas diffusion increases with the increase of porosity and contact angle and increases with the decrease of the thickness of GDL; for GDL with MPL, the larger the MPL porosity and the thinner the MPL thickness are, the stronger the gas diffusion is; for gradient change porosity GDL with the same average equivalent porosity, the larger the porosity gradient is, the more easily the gas diffuses. The optimization for GDL gradient structure shows that the GDL with a linear porosity distribution of 0.4x+0.4$0.4x+0.4$ is the best of the computed cases.     

Numerical study on permeability of gas diffusion layer with porosity gradient using lattice Boltzmann method

In this paper, the lattice Boltzmann method (LBM) has been employed to explore the permeability and internal fluid flow behavior of the gas diffusion layer (GDL). Three different non-uniform porosity distributions are designed as linear type, stepped type, and transitional type and compared with constant porosity samples. Results show that the linear porosity gradient distribution leads to higher permeability values compared with the other two types. For samples with total porosity of 0.65 and 0.75, optimal porosity gradient distributions bring about an enhancement of permeability have been found. The impact of porosity gradient distribution on the velocity field is presented. Dependencies of permeability with porosity and tortuosity are demonstrated through several fitted equations.     

Modeling of gas diffusion layers with curved fibers using a genetic algorithm

This paper presents a methodology for modeling microstructures of fibrous porous media with curved fibers. The developed methodology utilizes implicit periodic surface model coupled with the genetic algorithm (GA) optimization to construct the porous microstructures. The fibers profile is represented by the periodic implicit surfaces and their orientation and curvature are determined by GA optimization. To reconstruct the microstructures with higher resemblance to the actual porous media GA is utilized to minimize the fibers stored strain energy and their intersection volumes. Coupling the image processing techniques to the geometry construction procedure the morphological and transport properties of the constructed microstructures are also determined. To verify the feasibility and the accuracy of the proposed methodology the microstructure of Freudenberg H2315 GDL is constructed and characterized. The presented methodology enables a parametric design approach. Thus, the effects of the microstructure's properties e.g., fibers diameter, fibers orientation and porosity of the porous structure on the transport properties of the fibrous media are investigated.     

Lattice Boltzmann simulations of anisotropic permeabilities in carbon paper gas diffusion layers

The multiple-relaxation-time (MRT) lattice Boltzmann method (LBM) with multi-reflection solid boundary conditions is used to study anisotropic permeabilities of a carbon paper gas diffusion layer (GDL) in a fuel cell. The carbon paper is reconstructed using the stochastic method, in which various porosities and microstructures are achieved to simulate different samples. The simulated permeability and tortuosity show anisotropic characteristics of the reconstructed carbon papers with in-plane permeability higher than through-plane, and in-plane tortuosity lower than through-plane. The calculated permeabilities are in good agreement with existing measurements. The relationship between the permeability and the porosity is fitted with empirical relations and some fitting constants are determined. Furthermore, the obtained relationship of tortuosity and porosity is used in a fractal model for permeabilities. The results indicate that the fractal model and the Kozeny–Carman equation provide similar predictions on the through-plane permeability of the carbon paper GDL.     

Effect of hydrophobic gas diffusion layers on the performance of the polymer exchange membrane fuel cell

This study uses fuel cell gas diffusion layers (GDLs) fabricated in the laboratory from carbon fiber cloth with different concentrations of hydrophobic agents in proton exchange membrane fuel cells (PEMFCs), and investigates the relationship between the hydrophobic agent content of the carbon fiber cloth and fuel cell performance.The paper examines the effect of hydrophobic agent content on GDL thickness, contact angle, air permeability, and surface and through-plane resistivity. Carbon fiber cloth is impregnated with hydrophobic agent concentrations of 0, 3, 5, 10, 30, and 50 wt%, and the resulting GDLs are subjected to performance tests. When the test piece area is 25 cm², the test temperature 80 °C, the gasket thickness 0.36 mm, and the hydrophobic agent content 5 wt%, a fuel cell using the GDL has a current density of 1430 mA cm⁻² at 0.3 V.     

Comparing through-plane diffusibility correlations in PEFC gas diffusion layers using the lattice Boltzmann method

One of the key elements in a polymer electrolyte fuel cell (PEFC) is the gas diffusion layer (GDL). The GDL offers mechanical support to the cell and provides the medium for diffusing the reactant gases from the flow plates to the electrolyte enabling the electrochemical reactions, and therefore the energy conversion. At the same time, it has the task of transporting the electrons from the active sites, near to the electrolyte, towards the flow plates.Describing the fluid flow and mass transport phenomena through the GDLs is not an easy task not only because of their complex geometries, but also because of these phenomena occur at microscale levels. Most of the PEFC models at cell scale make assumptions about certain microscale transport parameters, assumptions that can make a model less close to the reality. The purpose of this study is to analyze five different proposed correlations to estimate the through-plane (TP) diffusibility of digitally created GDLs and using lattice Boltzmann (LB) models. The correlations are ranked depending on their precision, accuracy and symmetry. The results show that the best estimation is given when the porosity and gas-phase tortuosity are taken into account in the correlation.     

Recent progress of the gas diffusion layer in proton exchange membrane fuel cells: Material and structure designs of microporous layer

Proton exchange membrane fuel cells (PEMFCs) have become the most attractive power supply units for stationary and mobile applications. The operation, design characteristics, as well as performance of PEMFCs, are closely related to the multiphase transport of mass, heat, and electricity in the cell, a critical of which is the gas diffusion layer (GDL). It is very important to guarantee the transmission of water and gasses under high current density, and which is the weakness of PEMFCs at present. Microporous layer (MPL) is considered to be the key variable for mass transfer, so varieties of works focus on modification of MPL materials and its structure design. However, there is still a lack of special review to summarize and prospect the progress of MPL in recent years. This review article therefore focuses on the insights and comprehensive understanding of four critical issues of the MPL, the porosity, pore size distribution, wettability, structural design and the durability of MPL. At last, the conclusion and recommendations section summarized the future prospects and recommendations for possible research opportunities.     

Degradation of gas diffusion layers through repetitive freezing

This work investigates the degradation of an individual gas diffusion layer (GDL) by repeated freezing cycles. The pore size distribution, gas permeability, surface structure, and contact angle on the surface of the GDL were measured in four different types of GDL: SGL paper with a microporous layer (MPL); SGL paper with 5 wt% of polytetrafluoroethylene (PTFE) loading; Toray paper without PTFE loading; and Toray paper with 20 wt% of PTFE loading. After repeated freezing cycles, the porosity of the GDL without PTFE was reduced by 27.2% due to the volumetric expansion of the GDL. The peak of the log differential intrusion moved toward a smaller pore diameter slightly because of the repeated freezing process. The crack of the MPL increased in its width and length after repeated freezing cycles. The through-plane gas permeability of the GDL with the MPL doubled after repeated freezing cycles due to the growth of the crack in the MPL, but was very small for the GDLs with Toray paper. Besides, the GDLs with PTFE loading showed a relatively larger decrease in the contact angle on the surface than the GDL without PTFE loading due to the separation of PTFE from the carbon fiber during the repeated freezing process.     

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.     

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.     

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.     

Compressive stress and its impact on the gas diffusion layer: A review

The gas diffusion layer is subjected to several mechanical stresses during the manufacturing and application processes of fuel cell. The deformation of gas diffusion layer under compression affects the mass transfer and electron migration between the flow channel and reaction area. This paper mainly reviews the published research on stress distribution of the gas diffusion layer, the effects of compressive stress on the bulk and pore characteristics of the gas diffusion layer, and the performance of fuel cell. Furthermore, the relationship between stress and strain of the gas diffusion layer is summarized. The impacts of compression on the gas diffusion layer present the characteristics of multi-parameters, non-linearly, small variation and strong coupling. This review suggests that the compression affects the pore and bulk characteristics of gas diffusion layer should be comprehensively investigated.     

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 polytetrafluoroethylene-treatment and microporous layer-coating on the in-plane permeability of gas diffusion layers used in proton exchange membrane fuel cells

The in-plane permeability has been experimentally estimated for a number of carbon substrates and microporous layer (MPL)-coated gas diffusion layers (GDLs) as used in proton exchange membrane (PEM) fuel cells. The results show that the in-plane permeability of the tested carbon substrates decreases with increasing polytetrafluoroethylene (PTFE) loading and, in contrast, the greater is the PTFE loading in the MPL, the greater is the permeability. It has been shown that the in-plane permeability of the carbon substrates is reduced by an order of magnitude if they are coated with MPLs. Further, the permeability is different from one in-plane principal direction to another by a factor of about two. Finally, ignoring the inertial terms (for the reported flow rates) and the compressibility of the flowing air results in significant errors in the obtained values of the permeability.     

Effects of porosity distribution variation on the liquid water flux through gas diffusion layers of PEM fuel cells

Flooding of the membrane electrode assembly (MEA) and dehydrating of the polymer electrolyte membrane have been the key problems to be solved for polymer electrolyte membrane fuel cells (PEMFCs). So far, almost no papers published have focused on studies of the liquid water flux through differently structured gas diffusion layers (GDLs). For gas diffusion layers including structures of uniform porosity, changes in porosity (GDL with microporous layer (MPL)) and gradient change porosity, using a one-dimensional model, the liquid saturation distribution is analyzed based on the assumption of a fixed liquid water flux through the GDL. And then the liquid water flux through the GDL is calculated based on the assumption of a fixed liquid saturation difference between the interfaces of the catalyst layer/GDL and the GDL/gas channel. Our results show that under steady-state conditions, the liquid water flux through the GDL increases as contact angle and porosity increase and as the GDL thickness decreases. When a MPL is placed between the catalyst layer and the GDL, the liquid saturation is redistributed across the MPL and GDL. This improves the liquid water draining performance. The liquid water flux through the GDL increases as the MPL porosity increases and the MPL thickness decreases. When the total thickness of the GDL and MPL is kept constant and when the MPL is thinned to 3 μm, the liquid water flux increases considerably, i.e. flooding of MEA is difficult. A GDL with a gradient of porosity is more favorable for liquid water discharge from catalyst layer into the gas channel; for the GDLs with the same equivalent porosity, the larger the gradient is, the more easily the liquid water is discharged. Of the computed cases, a GDL with a linear porosity 0.4x + 0.4 is the best.     

The effects of wetproofing on the capillary properties of proton exchange membrane fuel cell gas diffusion layers

In the analysis of proton exchange membrane (PEM) fuel cell components, the capillary pressure vs. saturation (P_C(S_L)) curve is an increasingly popular tool for understanding the interaction of liquid water with the porous gas diffusion layer (GDL) material. In this study, hysteretic water/air P_C(S_L) measurements were combined with mercury intrusion porosimetry (MIP) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging to quantify the effects of fluoropolymer loading on GDL samples. Commercially wetproofed carbon fiber papers with 0-40 wt.% Teflon loading were investigated. MIP showed a slight reduction in characteristic pore radii and a significant loss of pore volume at the highest Teflon loading. Water/air P_C(S_L) measurements showed a significant reduction in water wetting between samples with 0 and 5 wt.% Teflon loading, but negligible additional wetproofing at loadings from 10 to 40 wt.%. ToF-SIMS imaging, a technique that is sensitive to monolayer surfaces coverages, found that GDL materials with 5 wt.% Teflon loading displayed nearly complete fluoropolymer coverage on the carbon substrate, confirming P_C(S_L) measurements showing that all of the wetproofing occurs in a narrow range of Teflon loadings. Results for P_C(S_L) measurements were fitted using a bundle-of-capillaries model. The apparent water intrusion contact angles fell between 130° and 133° in the rough Teflonated pore space (regardless of loading), whereas the apparent gas intrusion contact angles fell between 66° and 70° for the same materials.     

Effect of compression on liquid water transport and microstructure of PEMFC gas diffusion layers

This work explores how the degradation of the gas diffusion layer (GDL) under compression contributes to the formation of preferential pathways for water transport. Fluorescence microscopy is used to provide ex situ visualization of liquid water transport through the GDL placed beneath an optically transparent clamping plate. Transient image data obtained with a CCD camera indicates that areas of compression in the GDL coincide with preferential pathways for water transport and break-through. Preferential flow of water through the smaller pores resulting from GDL compression is contrary to the expected behaviour in a hydrophobic medium, and this suggests a loss of hydrophobicity. Scanning electron microscopy (SEM) is used to investigate the effect of compression on the morphology of the GDL. These SEM images show that compressing the GDL causes the breakup of fibers and, indeed, deterioration of the hydrophobic coating.     

An analytical model for the transverse permeability of gas diffusion layer with electrical double layer effects in proton exchange membrane fuel cells

An analytical model is presented for the transverse permeability of gas diffusion layer (GDL) based on an ordered array of parallel charged circular cylinders at the steady state. The formula of calculating the permeability of the transverse direction is given by solving the fluid momentum equation in a unit cell. In the present approach, the proposed model is explicitly related to the porosity and fiber radius of fibrous porous media, the zeta potential, and the physical properties of the electrolyte solution. Besides, the effects of these parameters (the porosity, unit cell aspect ratio, fiber radius, and molar concentration) on the transverse permeability are discussed detailedly. The model predictions are compared with the previous studies in the available literature, and good agreement is found.     

Pore network modeling of fibrous gas diffusion layers for polymer electrolyte membrane fuel cells

A pore network model of the gas diffusion layer (GDL) in a polymer electrolyte membrane fuel cell is developed and validated. The model idealizes the GDL as a regular cubic network of pore bodies and pore throats following respective size distributions. Geometric parameters of the pore network model are calibrated with respect to porosimetry and gas permeability measurements for two common GDL materials and the model is subsequently used to compute the pore-scale distribution of water and gas under drainage conditions using an invasion percolation algorithm. From this information, the relative permeability of water and gas and the effective gas diffusivity are computed as functions of water saturation using resistor-network theory. Comparison of the model predictions with those obtained from constitutive relationships commonly used in current PEMFC models indicates that the latter may significantly overestimate the gas phase transport properties. Alternative relationships are suggested that better match the pore network model results. The pore network model is also used to calculate the limiting current in a PEMFC under operating conditions for which transport through the GDL dominates mass transfer resistance. The results suggest that a dry GDL does not limit the performance of a PEMFC, but it may become a significant source of concentration polarization as the GDL becomes increasingly saturated with water.