Numerical investigation of multi-layered porosity in the gas diffusion layer on the performance of a PEM fuel cell

A mathematical model is developed to investigate the influence of porosity configurations in the gas diffusion layer (GDL) of the cathode on the electrochemical performance characteristics of a 3-D high-temperature proton exchange membrane (PEM) fuel cell. Four different non-uniform porosity configurations are defined through step functions and analyzed with uniform porosity case. The results are presented in terms of the cell performance characteristics viz. Current density, power density, vorticity magnitude, oxygen molar concentration, overpotential, and total power dissipation density. Our study reveals that oxygen molar concentration, current density, power density are found to be maximum when the stepwise porosity in GDL decreases in the streamwise direction. However, these parameters observed to be the least when the stepwise porosity in GDL increases along the streamwise direction. Additionally, the highest total power dissipation density is observed when the porosity in GDL varies across cross-stream wise direction among other configurations considered. However, it is found to be the least when porosity varies in a streamwise direction. The overpotential becomes the least when stepwise porosity decreases in the streamwise direction although the same is found to be maximum when the porosity in GDL increases along the streamwise direction. The performance is found to be optimal when porosity is maximum at cathode gas channel inlet and GDL-cathode gas channel interface.     

Measurement of effective gas diffusion coefficients of catalyst layers of PEM fuel cells with a Loschmidt diffusion cell

In this work, using an in-house made Loschmidt diffusion cell, we measure the effective coefficient of dry gas (O₂-N₂) diffusion in cathode catalyst layers of PEM fuel cells at 25 °C and 1 atmosphere. The thicknesses of the catalyst layers under investigation are from 6 to 29 μm. Each catalyst layer is deposited on an Al₂O₃ membrane substrate by an automated spray coater. Diffusion signal processing procedure is developed to deduce the effective diffusion coefficient, which is found to be (1.47 ± 0.05) × 10⁻⁷ m² s⁻¹ for the catalyst layers. Porosity and pore size distribution of the catalyst layers are also measured using Hg porosimetry. The diffusion resistance of the interface between the catalyst layer and the substrate is found to be negligible. The experimental results show that the O₂-N₂ diffusion in the catalyst layers is dominated by the Knudsen effect.     

Multi-dimensional liquid water transport in the cathode of a PEM fuel cell with consideration of the micro-porous layer (MPL)

A multi-dimensional two-phase PEM fuel cell model, which is capable of handling the liquid water transport across different porous materials, including the catalyst layer (CL), the micro-porous layer (MPL), and the macro-porous gas diffusion medium (GDM), has been developed and applied in this paper for studying the liquid water transport phenomena with consideration of the MPL. Numerical simulations show that the liquid water saturation would maintain the highest value inside the catalyst layer while it possesses the lowest value inside the MPL, a trend consistent qualitatively with the high-resolution neutron imaging data. The present multi-dimensional model can clearly distinguish the different effects of the current-collecting land and the gas channel on the liquid water transport and distribution inside a PEM fuel cell, a feature lacking in the existing one-dimensional models. Numerical results indicate that the MPL would serve as a barrier for the liquid water transport on the cathode side of a PEM fuel cell.     

Analysis of liquid water transport in cathode catalyst layer of PEM fuel cells

The performance of a polymer electrolyte membrane (PEM) fuel cell is significantly affected by liquid water generated at the cathode catalyst layer (CCL) potentially causing water flooding of cathode; while the ionic conductivity of PEM is directly proportional to its water content. Therefore, it is essential to maintain a delicate water balance, which requires a good understanding of the liquid water transport in the PEM fuel cells. In this study, a one-dimensional analytical solution of liquid water transport across the CCL is derived from the fundamental transport equations to investigate the water transport in the CCL of a PEM fuel cell. The effect of CCL wettability on liquid water transport and the effect of excessive liquid water, which is also known as “flooding”, on reactant transport and cell performance have also been investigated. It has been observed that the wetting characteristic of a CCL plays significant role on the liquid water transport and cell performance. Further, the liquid water saturation in a hydrophilic CCL can be significantly reduced by increasing the surface wettability or lowering the contact angle. Based on a dimensionless time constant analysis, it has been shown that the liquid water production from the phase change process is negligible compared to the production from the electrochemical process.     

Parametric study of the porous cathode in the PEM fuel cell

The electrochemical behavior and the reactant transport in the porous gas diffusion layer (GDL) and catalyst layer (CL) are controlled by a large number of parameters such as porosity, permeability, conductivity, catalyst loading, and average pore size, etc. A three‐dimensional polymer electrolyte membrane fuel cell model is developed. The model accounts for the mass, fluid, and thermal transport processes as well as the electrochemical reaction. Using this model, the effects of the various porous electrode design parameters including porosity, solid electronic conductivity, and thermal conductivity of cathode GDL, and the catalyst loading, average pore size of cathode CL are investigated through parametric study. The model is shown to agree well with the experimental data of some porous electrode specifications. In addition, the model shows promise as a tool for optimizing the design of fuel cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Recent progress of gas diffusion layer in proton exchange membrane fuel cell: Two-phase flow and material properties

Proton exchange membrane (PEM) fuel cells are a promising candidate as the next-generation power sources for portable, transportation, and stationary applications. Gas diffusion layers (GDL) coated with microporous layers (MPL) are a vital component of PEM fuel cells, providing multiple functions of mechanical support, reactant transport, liquid water removal, waste heat removal, and electron conductance. In this review, we explain several most important aspects in the research and development (R&D) of this fuel cell component, including material characterization, liquid water detection/quantitation, structure reconstruction, fundamental modeling, transport properties, and durability. Specially, the commonly used microstructure reconstruction methods for GDLs are presented and discussed. Visualization techniques for liquid water detection in the GDL and MPL microstructures are described. Major modeling approaches, such as the multiphase mixture (M²) formulation, pore networks model (PNM), lattice Boltzmann method (LBM) and volume of fluid (VOF) approach, are reviewed and explained. Important material properties and parameters that greatly influence two-phase flow and fuel cell performance, and GDL-related material degradation issues are discussed and summarized to further advance on the GDL material design and development.     

Measurement of the water transport rate in a proton exchange membrane fuel cell and the influence of the gas diffusion layer

Water management in a PEM fuel cell significantly affects the fuel cell performance and durability. The gas diffusion layer (GDL) of a PEM fuel cell plays a critical role in the water management process. In this short communication, we report a simple method to measure the water transport rate across the GDL. Water rejection rates across a GDL at different cathode air-flow rates were measured. Based on the measurement results, the fuel cell operating conditions, such as current density, temperature, air stoichiometry and relative humidity, corresponding to membrane drying and flooding conditions were identified for the particular GDL used. This method can help researchers develop GDLs for a particular fuel cell design with specific operating conditions and optimize the operation conditions for the given PEM fuel cell components.     

Performance of a metallic gas diffusion layer for PEM fuel cells

A novel metallic porous medium with improved thermal and electrical conductivities and controllable porosity was developed based on micro/nano technology for its potential application in PEM fuel cells. In this work to demonstrate its applicability, the gas diffusion medium, made of 12.5 μm thick copper foil, was tested in an operational fuel cell. The small thickness and straight-pore feature of this novel material provides improved water management even at low flow rates. The performance does not decline at lower flow rates, unlike conventional gas diffusion layers. It has been shown that the performance can be further enhanced by increasing the in-plane transport. The improvements of such gas diffusion layer, including pore shape, porosity, and surface properties, are fully discussed.     

Two-phase flow and evaporation in model fibrous media: Application to the gas diffusion layer of PEM fuel cells

Two-phase flow dominated by capillary effects in model fibrous media is studied combining pore-network simulations and visualisations on transparent micromodels. It is shown that the process of liquid water invasion in a hydrophobic medium can be simulated using the classical invasion percolation algorithm provided that the contact angle (measured in air, which is the wetting phase) is sufficiently far below 90°. For contact angles approaching 90°, changes in the interface local growth mechanisms lead to changes in the invasion pattern.     

A comprehensive assessment on the durability of gas diffusion electrode materials in PEM fuel cell stack

Polymer electrolyte membrane (PEM) fuel cell is the most promising among the various types of fuel cells. Though it has found its applications in numerous fields, the cost and durability are key barriers impeding the commercialization of PEM fuel cell stack. The crucial and expensive component involved in it is the gas diffusion electrode (GDE) and its degradation, which limits the performance and life of the fuel cell stack. A critical analysis and comprehensive understanding of the structural and functional properties of various materials involved in the GDE can help us to address the related durability and cost issues. This paper reviews the key GDE components, and in specific, the root causes influencing the durability. It also envisages the role of novel materials and provides a critical recommendation to improve the GDE durability.     

A three-dimensional agglomerate model for the cathode catalyst layer of PEM fuel cells

In this work, a three-dimensional, steady-state, multi-agglomerate model of cathode catalyst layer in polymer electrolyte membrane (PEM) fuel cells has been developed to assess the activation polarization and the current densities in the cathode catalyst layer. A finite element technique is used for the numerical solution to the model developed. The cathode activation overpotentials, and the membrane and solid phase current densities are calculated for different operating conditions. Three different configurations of agglomerate arrangements are considered, an in-line and two staggered arrangements. All the three arrangements are simulated for typical operating conditions inside the PEM fuel cell in order to investigate the oxygen transport process through the cathode catalyst layer, and its impact on the activation polarization. A comprehensive validation with the well-established two-dimensional “axi-symmetric model” has been performed to validate the three-dimensional numerical model results. Present results show a lowest activation overpotential when the agglomerate arrangement is in-line. For more realistic scenarios, staggered arrangements, the activation overpotentials are higher due to the slower oxygen transport and lesser passage or void region available around the individual agglomerate. The present study elucidates that the cathode overpotential reduction is possible through the changing of agglomerate arrangements. Hence, the approaches to cathode overpotential reduction through the optimization of agglomerate arrangement will be helpful for the next generation fuel cell design.     

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.     

Effect of the porous carbon layer in the cathode gas diffusion media on direct methanol fuel cell performances

The effect of cathode gas diffusion media with microporous layers (MPLs) on direct methanol fuel cell (DMFC) performances is studied by combining electrochemical analysis and physicochemical investigation. The membrane electrode assemblies (MEAs) using MPL-modified cathode gas diffusion layers (GDLs, GDL-1) showed slightly better performances (117 mW cm⁻²) at 0.4 V and 70 °C than commercial GDL (SIGRACET^® product version: GDL-35BC, SGL Co.) DMFC MEAs (110 mW cm⁻²). This might be due to high gas permeability, uniform pore distributions, and low water transport coefficient including methanol crossover. For GDL-1, the air permeability was 31.0 cm³ cm⁻² s⁻¹, while the one for SGL 35BC GDLs was 21.7 cm³ cm⁻² s⁻¹. Also, the GDL-1 in the pore-size distribution diagrams had distinct peaks due to more uniform distributions of macropores and micropores with smaller holes between aggregates of carbon particles compared to GDL-35 BC as confirmed by SEM images. Furthermore, the MEA using GDL-1 for the cathode had a lower water transfer coefficient compared to an MEA with a commercial 35 BC GDL.     

Experimental investigation of liquid water formation and transport in a transparent single-serpentine PEM fuel cell

Liquid water formation and transport were investigated by direct experimental visualization in an operational transparent single-serpentine PEM fuel cell. We examined the effectiveness of various gas diffusion layer (GDL) materials in removing water away from the cathode and through the flow field over a range of operating conditions. Complete polarization curves as well as time evolution studies after step changes in current draw were obtained with simultaneous liquid water visualization within the transparent cell. The level of cathode flow field flooding, under the same operating conditions and cell current, was recognized as a criterion for the water removal capacity of the GDL materials. When compared at the same current density (i.e. water production rate), higher amount of liquid water in the cathode channel indicated that water had been efficiently removed from the catalyst layer.

Visualization of the anode channel was used to investigate the influence of the microporous layer (MPL) on water transport. No liquid water was observed in the anode flow field unless cathode GDLs had an MPL. MPL on the cathode side creates a pressure barrier for water produced at the catalyst layer. Water is pushed across the membrane to the anode side, resulting in anode flow field flooding close to the H₂ exit.     

Liquid water breakthrough pressure through gas diffusion layer of proton exchange membrane fuel cell

The dynamic behavior of liquid water transport through the gas diffusion layer (GDL) of the proton exchange membrane fuel cell is studied with an ex-situ approach. The liquid water breakthrough pressure is measured in the region between the capillary fingering and the stable displacement on the drainage phase diagram. The variables studied are GDL thickness, PTFE/Nafion content within the GDL, GDL compression, the inclusion of a micro-porous layer (MPL), and different water flow rates through the GDL. The liquid water breakthrough pressure is observed to increase with GDL thickness, GDL compression, and inclusion of the MPL. Furthermore, it has been observed that applying some amount of PTFE to an untreated GDL increases the breakthrough pressure but increasing the amount of PTFE content within the GDL shows minimal impact on the breakthrough pressure. For instance, the mean breakthrough pressures that have been measured for TGP-060 and for untreated (0 wt.% PTFE), 10 wt.% PTFE, and 27 wt.% PTFE were 3589 Pa, 5108 Pa, and 5284 Pa, respectively.     

Effects of liquid water on the pore structure and transport coefficients in the cathode catalyst layer of PEM fuel cells

We present a pore-scale simulation of the capillary condensation of water in the cathode catalyst layer (CCL) of proton exchange membrane fuel cells by the lattice Boltzmann method. Based on the reconstructed CCL, the capillary condensation process in CCL is simulated under different humidity conditions, and the effects of porosity and especially wettability on the liquid water distribution in CCL are studied. The influence of liquid water on the void pore size distribution and pore connectivity in CCL is evaluated, and the results show that the hydrophilic CCL is more prone to be flooded. Subsequently, the effective transport coefficients of oxygen and proton in partially saturated CCL are investigated. The results reveal that the hydrophobic CCL is beneficial for reducing the gas transport tortuosity but simultaneously causes a higher Knudsen diffusion resistance. By comprehensively considering the changes in tortuosity and Knudsen resistance caused by liquid water, a more practical correlation of effective diffusivity for the partially saturated CCL is proposed. Moreover, this work proves the vital role of liquid water in the proton conduction in CCL. The simulated effective proton conductivity in CCL is more agree with the measurements if the contribution of liquid water to proton transport is considered.     

Simulation on water transportation in gas diffusion layer of a PEM fuel cell: Influence of non-uniform PTFE distribution

Proton exchange membrane fuel cells (PEMFCs) are promising clean power sources with high energy conversion efficiency, fast startup, and no pollutant emission. The generated water in the cathode can cause water flooding of the catalyst layer (CL), which in turn can significantly decrease the fuel cell performance. To address this significant issue of PEMFC, a new gas diffusion layer (GDL) with non-uniform distribution of PTFE is proposed for water removal from the CL. The feasibility of this new GDL design is numerically evaluated by a Lattice-Boltzmann Method (LBM)-based two-phase flow model. The porous structure of the new GDL design is numerically reconstructed, followed by LBM simulations of the water transport in GDL. Three types of different wetting conditions are considered. It is found that liquid water transported 7.87% more with a single row of wetted solids and 13.36% more with two rows of wetted solids. The results clearly demonstrate that the liquid water can be effectively removed from the GDL by proper arrangement of hydrophilic solids in the GDL.     

Liquid water distribution patterns featuring back-diffusion transport in a PEM fuel cell with neutron imaging

Back-diffusion in PEM fuel cells is the water transport mechanism contributing to balance the water content profile in the membrane (in the through-plane direction), transporting water molecules from the cathode electrode towards the anode side of the membrane. In this technical note, neutron radiographs are presented for a 50 cm² N-117 fuel cell with serpentine flow field, where the effect of the back diffusion transport mechanism is clearly identified, in the form of crossed patterns following the cross-flow layout of the flow field. The back diffusion water transport is evident despite the high thickness of the N-117 membrane.     

Tomographic analysis of porosity variation in gas diffusion layer under freeze-thaw cycles

Synchrotron X-ray micro-computed tomography (X-ray μCT) is employed to measure the volume variation of gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). In the present study, 3D structures are reconstructed by merging orthogonal-plane images. Using the 3D reconstruction, the variation of structural parameters such as the porosity in GDL is investigated under freeze-thaw cycles. The freez-thaw cycles are established using cryo system and light source, respectively. As a result, a structural transformation is observed at the interface between GDL and micro porous layer (MPL). In addition, the porosity is critically changed with irreversible transition under freeze-thaw cycles.     

Simulation of species transport and water management in PEM fuel cells

A single phase computational fuel cells model is presented to elucidate three-dimensional interactions between mass transport and electrochemical kinetics in proton exchange membrane (PEM) fuel cells with straight gas channels. The governing differential equations are solved over a single computational domain, which consists of a gas channel, gas diffusion layer, and catalyst layer for both the anode and cathode sides of the cell as well as the solid polymer membrane. Emphasis is placed on obtaining a basic understanding of how three-dimensional flow and transport phenomena in the air cathode impact the electrochemical process in the flow field. The complete cell model has been validated against experimentally measured polarization curve, showing good accuracy in reproducing cell performance over moderate current density interval. Fully three-dimensional results of the flow structure and species profiles are presented for cathode flow field. The effects of pressure on oxygen transport and water removal are illustrated through main axis of the flow structure. The model results indicate that oxygen concentration in reaction sites is significantly affected by pressure increase which leads to rising fuel cells power.