Effects of gas diffusion layer deformation on the transport phenomena and performance of PEM fuel cells with interdigitated flow fields

In this study, a three-dimensional, non-isothermal, two-phase flow mathematical model is developed and applied to investigate the effect of the GDL deformation on transport phenomena and performance of proton exchange membrane (PEM) fuel cells with interdigitated flow fields. The thickness and porosity of the GDL is decreased after compression, and the corresponding transport parameters (permeability, mass diffusivity, thermal conductivity and electrical conductivity) are affected significantly. The alterations in geometry and transport parameters of the GDL are considered in the mathematical model. The oxygen concentration, temperature, liquid water saturation and volumetric current density distributions of PEM fuel cells without compression are investigated and then compared to the PEM fuel cells with various assembly forces. The numerical results show that the cell performance is considerably improved with increasing assembly forces. However, the pressure drops in the gas flow channels are also substantially increased. It is concluded that the assembly force should be as small as possible to decrease the parasitic losses with consideration of gas sealing concern.     

Three-dimensional multi-phase simulation of PEM fuel cell considering the full morphology of metal foam flow field

It is well-known that flow field design is of primary importance to optimization of proton exchange membrane (PEM) fuel cell. Traditional channel-rib flow fields, e.g. parallel or serpentine channels, always lead to non-uniform distributions of reactant gas, liquid, current density and so on between the channel and rib regions. Metal foam materials with high porosity (>90%) have been proposed as alternative flow fields for PEM fuel cells. In this study, influences of metal foam flow field on the transport phenomena coupled with the electrochemical reactions in PEM fuel cell are investigated using a three-dimensional (3D) multi-phase non-isothermal model. Specifically, the full morphology of metal foam flow field is taken into account in the 3D simulation after validated against experimental permeability data. The full morphology inclusion enables capture of the detailed gas flow from the flow field into the gas diffusion layer (GDL) and the current collection at the metal foam/GDL interface. In addition, compared with the conventional channel-rib flow fields, the metal foam design greatly increases fuel cell performance in the high current density regime. In addition, the oxygen and current density distributions in PEM fuel cell with the metal foam flow field are more uniform than those in the conventional one. Though the current collection area at the GDL surface is much smaller in the metal foam flow field, the relevant Ohmic loss won't increase significantly due to the improved physical contact by the fine pore structure of metal foam over the GDL.     

Assembly pressure and membrane swelling in PEM fuel cells

Assembly pressure and membrane swelling induced by elevated temperature and humidity cause inhomogeneous compression and performance variation in proton exchange membrane (PEM) fuel cells. This research conducts a comprehensive analysis on the effects of assembly pressure and operating temperature and humidity on PEM fuel cell stack deformation, contact resistance, overall performance and current distribution by advancing a model previously developed by the authors. First, a finite element model (FEM) model is developed to simulate the stack deformation when assembly pressure, temperature and humidity fields are applied. Then a multi-physics simulation, including gas flow and diffusion, proton transport, and electron transport in a three-dimensional cell, is conduced. The modeling results reveal that elevated temperature and humidity enlarge gas diffusion layer (GDL) and membrane inhomogeneous deformation, increase contact pressure and reduce contact resistance due to the swelling and material property change of the GDL and membrane. When an assembly pressure is applied, the fuel cell overall performance is improved by increasing temperature and humidity. However, significant spatial variation of current distribution is observed at elevated temperature and humidity.     

Three-dimensional modeling of a PEMFC with serpentine flow field incorporating the impacts of electrode inhomogeneous compression deformation

The effects of compression deformation of gas diffusion layer (GDL) on the performance of a proton exchange membrane fuel cell (PEMFC) with serpentine flow field were numerically investigated by coupling two-dimensional GDL mechanical deformation model based on Finite Element Analysis and three-dimensional two-phase PEMFC model with incorporating the deformation impacts. Emphasis is located on exploring the influences of assembly pressure on the non-uniform geometric deformation and distributions of transport properties in the GDL, flow behaviors and local distributions of oxygen and current density, cell polarization curves and net power densities of the PEMFC. It was indicated that the non-uniform deformation of GDL results in inhomogeneous distributions of porosity and permeability in the GDL due to the presence of rib-channel pattern, and the transport properties in the under-rib region are greatly reduced with increasing the assembly pressure, consequently weakening the gas flow and oxygen transport in the under-rib region and increasing the non-uniformity of local current density distribution. As for the overall cell performance, however, attributed to the tradeoff between the adverse impacts of GDL compression on mass transport loss and positive effects on reducing ohmic loss, the overall cell performance is firstly increased and then decreased with increasing assembly pressure from 0 MPa to 5.0 MPa, and the maximum cell performance can be achieved at the assembly pressure of about 1.0 MPa for all cases studied. As compared with the case for zero assembly pressure, the maximum net power density of the cell can be improved by about 7.7%, 9.9%, 10.5% and 10.7% for the cathode stoichiometry ratios of 2.0, 3.0, 4.0 and 5.0@i_ref = 1 A·cm⁻², respectively. Practically, it is suggested that the assembly pressure is controlled in an appropriate range of 0.5 MPa–1.5 MPa such that the cell net power can be boosted and pressure head requirement for the pump can be maintained in a appropriate level.     

Optimization of assembly clamping pressure on performance of proton-exchange membrane fuel cells

The compression induced by the assembly of proton-exchange membrane (PEM) fuel cells causes partial deformation of the gas-diffusion layers (GDLs) and affects the characteristics of the porous media and, consequently, influences the performance of PEM fuel cells. The objective of the present study is: (1) to develop a three-dimensional model to investigate the effect of assembly clamping pressure on the GDL properties and thus on the performance of PEM fuel cells, and (2) to determine the optimum clamping pressures when the cell is operated under different operating voltages. The optimum clamping pressures under different operating voltages are explored by using a global searching method, namely, the simultaneous perturbation stochastic algorithm (SPSA) method. The simulation results indicate that a clamping pressure of 1 or 1.5 MPa improves the fuel cell performance when the cell is operated under high operating voltages, and causes the cell performance to decrease when it is operated under low-voltage conditions. The optimum clamping pressures increase when the operating voltage increases.     

Three‐dimensional optimisation of a fuel gas channel of a proton exchange membrane fuel cell for maximum current density

Proton exchange membrane (PEM) fuel cells operated with hydrogen and air offer promising alternative to conventional fossil fuel sources for transport and stationary applications because of its high efficiency, low‐temperature operation, high power density, fast start‐up and potable power for mobile application. Power levels derivable from this class of fuel cell depend on the operating parameters. In this study, a three‐dimensional numerical optimisation of the effect of operating and design parameters of PEM fuel cell performance was developed. The model computational domain includes an anode flow channel, membrane electrode assembly and a cathode flow channel. The continuity, momentum, energy and species conservation equations describing the flow and species transport of the gas mixture in the coupled gas channels and the electrodes were numerically solved using a computational fluid dynamics code. The effects of several key parameters, including channel geometries (width and depth), flow orientation and gas diffusion layer (GDL) porosity on performance and species distribution in a typical fuel cell system have been studied. Numerical results of the effect of flow rate and GDL porosity on the flow channel optimal configurations for PEM fuel cell are reported. Simulations were carried out ranging from 0.6 to 1.6 mm for channel width, 0.5 to 3.0 mm for channel depth and 0.1 to 0.7 for the GDL porosity. Results were evaluated at 0.3 V operating cell voltage of the PEM fuel cell. The optimisation results show that the optimum dimension values for channel depth and channel width are 2.0 and 1.2 mm, respectively. In addition, the results indicate that effective design of fuel gas channel in combination with the reactant species flow rate and GDL porosity enhances the performance of the fuel cell. The numerical results computed agree well with experimental data in the literature. Consequently, the results obtained provide useful information for improving the design of fuel cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Numerical study of assembly pressure effect on the performance of proton exchange membrane fuel cell

The performance of the fuel cell is affected by many parameters. One of these parameters is assembly pressure that changes the mechanical properties and dimensions of the fuel cell components. Its first duty, however, is to prevent gas or liquid leakage from the cell and it is important for the contact behaviors of fuel cell components. Some leakage and contact problems can occur on the low assembly pressures whereas at high pressures, components of the fuel cell, such as bipolar plates (BPP), gas diffusion layers (GDL), catalyst layers, and membranes, can be damaged. A finite element analysis (FEA) model is developed to predict the deformation effect of assembly pressure on the single channel PEM fuel cell in this study. Deformed fuel cell single channel model is imported to three-dimensional, computational fluid dynamics (CFD) model which is developed for simulating proton exchange membrane (PEM) fuel cells. Using this model, the effect of assembly pressure on fuel cell performance can be calculated. It is found that, when the assembly pressure increases, contact resistance, porosity and thickness of the gas diffusion layer (GDL) decreases. Too much assembly pressure causes GDL to destroy; therefore, the optimal assembly pressure is significant to obtain the highest performance from fuel cell. By using the results of this study, optimum fuel cell design and operating condition parameters can be predicted accordingly.     

Effect of gas diffusion layer compression on PEM fuel cell performance

The gas diffusion layer (GDL) plays a very important role in the performance of Proton Exchange Membrane (PEM) fuel cells. The amount of compression on the GDL affects the contact resistance, the GDL porosity, and the fraction of the pores occupied by liquid water, which, in turn, affect the performance of a PEM fuel cell. In order to study the effects of GDL compression on fuel cell performance a unique fuel cell test fixture was designed and created such that, without disassembling the fuel cell, varying the compression of the GDL can be achieved both precisely and uniformly. Besides, the compression can be precisely measured and easily read out. Using this special fuel cell fixture, the effects of GDL compression on PEM fuel cell performance under various anode and cathode flow rates were studied. Two different GDL materials, carbon cloth double-sided ELAT and TORAY™ carbon fiber paper were used in these studies. The experimental results show that generally the fuel cell performance decreases with the increase in compression and over-compression probably exists in most fuel cells. In the low current density region, generally there exists an optimal compression ratio.     

An analytical analysis on the cross flow in a PEM fuel cell with serpentine flow channel

A serpentine flow channel can be considered as neighboring channels connected in series, and is one of the most common and practical channel layouts for polymer electrolyte membrane (PEM) fuel cells, as it ensures the removal of liquid water produced in a cell with good performance and acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross directly to the neighboring channel via the porous gas diffusion layer (GDL) due to the high‐pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area resulting in a substantially lower amount of pressure drop in an actual PEM fuel cell compared with the case without cross flow. In this study, an analytical solution is obtained for the cross flow in a PEM fuel cell with a serpentine flow channel based on the assumption that the velocity of cross flow is linearly distributed in the GDL between two successive U‐turns. The analytical solution predicts the amount of pressure drop and the average volume flow rate in the flow channel and the GDL. The solution is validated over a wide range of the thickness and permeability of the GDL by comparing the results with experimental measurements and 3‐D numerical simulations in literature. Excellent agreement is obtained for the permeability less than 10^?9 m², which covers the typical permeability values of the GDLs in actual PEM fuel cells. The solution presents an accurate and efficient estimation for cross flow providing a useful tool for the design and optimization of PEM fuel cells with serpentine flow channels. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance improvement of proton exchange membrane fuel cell by using annular shaped geometry

A complete three-dimensional and single phase CFD model for a different geometry of proton exchange membrane (PEM) fuel cell is used to investigate the effect of using different connections between bipolar plate and gas diffusion layer on the performances, current density and gas concentration. The proposed model is a full cell model, which includes all the parts of the PEM fuel cell, flow channels, gas diffusion electrodes, catalyst layers and the membrane. Coupled transport and electrochemical kinetics equations are solved in a single domain; therefore no interfacial boundary condition is required at the internal boundaries between cell components.This computational fluid dynamics code is used as the direct problem solver, which is used to simulate the three-dimensional mass, momentum and species transport phenomena as well as the electron- and proton-transfer process taking place in a PEMFC that cannot be investigated experimentally. The results show that the predicted polarization curves by using this model are in good agreement with the experimental results. Also the results show that by increasing the number of connection between GDL and bipolar plate the performance of the fuel cell enhances.     

Cross-leakage flow between adjacent flow channels in PEM fuel cells

In polymer electrolyte membrane (PEM) fuel cells, serpentine flow channels are used conventionally for effective water removal. The reactant flows along the flow channel with pressure decrease due to the frictional and minor losses as well as the reactant depletion because of electrochemical reactions in the cells. Because of the short distance between the adjacent flow channels, often in the order of 1 mm or smaller, the pressure gradient between the adjacent flow channels is very large, driving part of reactant to flow through the porous electrode backing layer (or the so-called gas diffusion layer)—this cross-leakage flow between adjacent flow channels in PEM fuel cells has been largely ignored in previous studies. In this study, the effect of cross-flow in an electrode backing layer has been investigated numerically by considering bipolar plates with single-channel serpentine flow field for both the anode and cathode side. It is found that a significant amount of reactant gas flows through the porous electrode structure, due to the pressure difference, and enters the next flow channel, in addition to a portion entering the catalyst layer for reaction. Therefore, mixing occurs between the relatively high concentration reactant stream following the flow channel and the relatively low reactant concentration stream going through the electrode. It is observed that the cross-leakage flow influences the reactant concentration at the interface between the electrode and the catalyst layer, hence the distribution of reaction rate or current density generated. In practice, this cross-leakage flow in the cathode helps drive the liquid water out of the electrode structure for effective water management, partially responsible for the good PEM fuel cell performance using the serpentine flow channels.     

In-situ measurements of GDL effective permeability and under-land cross-flow in a PEM fuel cell

When reactant gases flow along a channel in serpentine flow field of a proton exchange membrane (PEM) fuel cell, there is a pressure difference between the adjacent channels and it produces an under-land cross-flow (or under-rib convection) from the higher pressure side to the lower pressure side through the gas diffusion layer (GDL). A unique experimental setup is developed for in-situ measurement of this cross-flow and the GDL effective permeability at the cathode side of a PEM fuel cell under dry and realistic humidified gas conditions. The non-Darcy effect, defined as a function of the Forchheimer number is studied and compared for both 1 mm and 2 mm land widths and both dry and humidified air conditions. Finally, a dimensional analysis is performed and the non-dimensional cross-flowrate is shown to increases linearly with the increase of the non-dimensional pressure difference.     

Uneven gas diffusion layer intrusion in gas channel arrays of proton exchange membrane fuel cell and its effects on flow distribution

Intrusion of the gas diffusion layer (GDL) into gas channels due to fuel cell compression has a major impact on the gas flow distribution, fuel cell performance and durability. In this work, the effect of compression resulting in GDL intrusion in individual parallel PEMFC channels is investigated. The intrusion is determined using two methods: an optical measurement in both the in-plane and through-plane directions of GDL, as well as an analytical fluid flow model based on individual channel flow rate measurements. The intrusion measurements and estimates obtained from these methods agree well with each other. An uneven distribution of GDL intrusion into individual parallel channels is observed. A non-uniform compression force distribution derived from the clamping bolts causes a higher intrusion in the end channels. The heterogeneous GDL structure and physical properties may also contribute to the uneven GDL intrusion. As a result of uneven intrusion distribution, severe flow maldistribution and increased pressure drop have been observed. The intrusion data can be further used to determine the mechanical properties of GDL materials. Using the finite element analysis software program ANSYS, the Young's modulus of the GDL from these measurements is estimated to be 30.9 MPa.     

Effect of gas diffusion layer modulus and land-groove geometry on membrane stresses in proton exchange membrane fuel cells

The electrical functionality of PEM fuel cells is facilitated by minimizing the contact resistances between different materials in the fuel cell, which is achieved via compressive clamping. The effect of the gas diffusion layer (GDL) modulus on the in-plane stress in the membrane after clamping is studied via numerical simulations, including both isotropic and anisotropic GDL properties. Furthermore, the effect of cell width and land-groove width ratio on the in-plane stress in the membrane subjected to a single hygro-thermal cycle is investigated for aligned and alternating gas channel geometries. The results from varying the GDL properties suggest that the in-plane stress in the membrane after clamping is due to a non-linear and coupled interaction of GDL and membrane deformation. The results of the geometric studies indicate that when the gas channels are aligned, the cell width and land-groove width ratio affect the in-plane stress distribution, but do not significantly affect the stress magnitudes. However, when the gas channels are alternating, the cell width and land-groove width ratio have significant effect on the membrane in-plane stresses. The effect of land-groove geometry is qualitatively verified by a series of experimental compression tests.     

Characterization of flooding and two-phase flow in polymer electrolyte membrane fuel cell stacks

A partially flooded gas diffusion layer (GDL) model is proposed and solved simultaneously with a stack flow network model to estimate the operating conditions under which water flooding could be initiated in a polymer electrolyte membrane (PEM) fuel cell stack. The models were applied to the cathode side of a stack, which is more sensitive to the inception of GDL flooding and/or flow channel two-phase flow. The model can predict the stack performance in terms of pressure, species concentrations, GDL flooding and quality distributions in the flow fields as well as the geometrical specifications of the PEM fuel cell stack. The simulation results have revealed that under certain operating conditions, the GDL is fully flooded and the quality is lower than one for parts of the stack flow fields. Effects of current density, operating pressure, and level of inlet humidity on flooding are investigated.     

A numerical investigation of the effects of GDL compression and intrusion in polymer electrolyte fuel cells (PEFCs)

The purpose of this work is to numerically investigate the effects of non-uniform compression of the gas diffusion layer (GDL) and GDL intrusion into a channel due to the channel/rib structure of the flow-field plate. The focus is placed on accurately predicting two-phase transport between the compressed GDL near the ribs and uncompressed GDL near the channels, and its associated effects on cell performance. In this paper, a GDL compression model is newly developed and incorporated into a comprehensive three-dimensional, two-phase PEFC model developed earlier. To assess solely the effects of GDL compression and intrusion, the new fuel cell model is applied to a simple single-straight channel fuel cell geometry. Numerical simulations with different levels of GDL compression and intrusion are carried out and simulation results reveal that the effects of GDL compression and intrusion considerably increase the non-uniformity, particularly, the in-plane gradient in liquid saturation, oxygen concentration, membrane water content, and current density profiles that in turn results in significant ohmic and concentration polarizations. The present three-dimensional GDL compression model yields realistic species profiles and cell performance that help to identify the optimal MEA, gasket, and flow channel designs in PEFCs.     

Effects of electrode wettabilities on liquid water behaviours in PEM fuel cell cathode

Liquid water transport is one of the key challenges for water management in a proton exchange membrane (PEM) fuel cell. Investigation of the air–water flow patterns inside fuel cell gas flow channels with gas diffusion layer (GDL) would provide valuable information that could be used in fuel cell design and optimization. This paper presents numerical investigations of air–water flow across an innovative GDL with catalyst layer and serpentine channel on PEM fuel cell cathode by use of a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different static contact angles (hydrophilic or hydrophobic) were applied to the electrode (GDL and catalyst layer). The results showed that different wettabilities of cathode electrode could affect liquid water flow patterns significantly, thus influencing on the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown, several gas flow problems were observed, and some useful suggestions were given through investigating the flow patterns.     

Study of the characteristics of temperature rise and coolant flow rate control during malfunction of PEM fuel cells

In actual PEM fuel cell systems, the coolant flow rate is generally controlled to maintain a preset temperature at the coolant outlet. This implies that a change in coolant supply flow rate is a good early indicator of a malfunctioning PEM fuel cell stack and system components. In this study, various fuel cell malfunctions are simulated based on the practical coolant flow control strategy by using a three-dimensional, two-phase, multiscale PEM fuel cell model developed in our previous studies. The focus is on analysis of the characteristics of coolant flow rate change along with voltage degradation in various fuel cell malfunction cases. The model predictions show that in general, the coolant flow rate tends to increase proportionally with the degree of voltage degradation, but the increase in temperature inside the membrane electrode assembly (MEA) is not always related to the voltage drop and is influenced more directly by local current density distribution. Although the present numerical comparison between the normal and malfunctioning cases is conducted at the low current density of 0.3 A cm^?2, the general cell behavior will not be altered at higher current densities due to inverse relationship between cell performance and waste heat generation. The present work elucidates the complex interplay among increase in coolant flow rate, increase in MEA temperature, voltage drop, and change in local current density distribution when a PEM fuel cell malfunctions.     

Increasing the efficiency of a portable PEM fuel cell by altering the cathode channel geometry: A numerical and experimental study

Portable fuel cells are receiving great attention today mainly because their energy density is higher than any portable battery solution. Among other types, portable polymer electrolyte membrane (PEM) fuel cells are an established technology where research on increasing their efficiency is leading product development and manufacturing. The objective of this work was to study and evaluate the redesign of a commercial portable fuel cell, improving its efficiency. A three-dimensional model of the original PEM fuel cell with parallel plus a transversal flow channel design was developed using Comsol Multiphysics, including the effects of liquid water formation and electric current production. Using this model, the effects of different channel geometries and respective cathode flow rates on the cell’s performance, including the local transport characteristics, were studied. Laboratory tests with various fuel cell stacks using the new channels structure were effectuated for an evaluation of the fuel cell’s performance, showing improvements in its efficiency of up to 26.4%.     

A novel constitutive stress-strain law for compressive deformation of the gas diffusion layer

Substantial compressive deformation occurs in the gas diffusion layer (GDL) under the pressure applied during the fuel cell assembly. The GDL deformation has a direct impact on the efficiency and performance of the fuel cell since it leads to the alteration of the GDL microstructure and porosity. This makes the accurate characterization of the GDL compressive behavior crucial for analyzing the fuel cell performance and its optimal design. In this paper, analytical, experimental, and numerical methods have been employed to comprehensively study the constitutive law of the GDL under compression. Starting from the recently developed stress-density relations, the constitutive stress-strain equations are derived for the GDL and the relation between the stress-density and stress-strain laws are revealed. Experimental compression tests have been performed on GDL samples and the capability of the proposed constitutive law in capturing the real behavior of the material has been proved. It has been observed that the simplifying assumption of constant zero Poisson's ratio in the through-plane direction made in many previous studies cannot accurately represent the GDL material behavior and a modification is proposed. The developed constitutive law has been successfully implemented in a finite element model of the GDL-bipolar plate assembly in the fuel cell structure and the variations of the GDL porosity, density, and through-plane Young's modulus and Poisson's ratio have been investigated for different vertical displacements of the bipolar plate.