Numerical evaluation of a PEM fuel cell with conventional flow fields adapted to tubular plates

In this research a 3D numerical study on a PEM fuel cell model with tubular plates is presented. The study is focused on the performance evaluation of three flow fields with cylindrical geometry (serpentine, interdigitated and straight channels) in a fuel cell. These designs are proposed not only with the aim to reduce the pressure losses that conventional designs exhibit with rectangular flow fields but also to improve the mass transport processes that take place in the fuel cell cathode. A commercial computational fluid dynamics (CFD) code was used to solve the numerical model. From the numerical solution of the fluid mechanics equations and the electrochemical model of Butler-Volmer different analysis of pressure losses, species concentration, current density, temperature and ionic conductivity were carried out. The results were obtained at the flow channels and the catalyst layers as well as in the gas diffusion layers and the membrane interfaces. Numerical results showed that cylindrical channel configurations reduced the pressure losses in the cell due to the gradual reduction of the angle at the flow path and the twist of the channel, thus facilitating the expulsion of liquid water from the gas diffusion layers and in turn promoting a high oxygen concentration at the triple phase boundary of the catalyst layers. Moreover, numerical results were compared to polarization curves and the literature data reported for similar designs. These results demonstrated that conventional flow field designs applied to conventional tubular plates have some advantages over the rectangular designs, such as uniform pressure and current density distributions among others, therefore they could be considered for fuel cell designs in portable applications.     

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.     

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.     

A comprehensive,consistent and systematic mathematical model of PEM fuel cells

This paper presents a comprehensive, consistent and systematic mathematical model for PEM fuel cells that can be used as the general formulation for the simulation and analysis of PEM fuel cells. As an illustration, the model is applied to an isothermal, steady state, two-dimensional PEM fuel cell. Water is assumed to be in either the gas phase or as a liquid phase in the pores of the polymer electrolyte. The model includes the transport of gas in the gas flow channels, electrode backing and catalyst layers; the transport of water and hydronium in the polymer electrolyte of the catalyst and polymer electrolyte layers; and the transport of electrical current in the solid phase. Water and ion transport in the polymer electrolyte was modeled using the generalized Stefan–Maxwell equations, based on non-equilibrium thermodynamics. Model simulations show that the bulk, convective gas velocity facilitates hydrogen transport from the gas flow channels to the anode catalyst layers, but inhibits oxygen transport. While some of the water required by the anode is supplied by the water produced in the cathode, the majority of water must be supplied by the anode gas phase, making operation with fully humidified reactants necessary. The length of the gas flow channel has a significant effect on the current production of the PEM fuel cell, with a longer channel length having a lower performance relative to a shorter channel length. This lower performance is caused by a greater variation in water content within the longer channel length.     

Three-dimensional transport model of PEM fuel cell with straight flow channels

In this work, an isothermal, steady-state, three-dimensional (3D) multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with straight gas channels. The model computational domain, includes anode flow channel, membrane electrode assembly (MEA) and cathode flow channel. The catalyst layer within the domain has physical volume without simplification. A comprehensive set of 3D continuity equation, momentum equations and species conservation equations are formulated to describe the flow and species transport of the gas mixture in the coupled gas channels and the electrodes. The electrochemical reaction rate is modified by an agglomerate model to account for the effect of diffusion resistance through catalyst particle. The activation overpotential is predicted locally in the catalyst layer by separately solving electric potential equations of membrane phase and solid phase. The model is validated by comparison of the model prediction with experimental data of Ticianelli et al. The results indicate the detailed distribution characteristics of oxygen concentration, local current density and cathode activation overpotential at different current densities. The distribution patterns are relatively uniform at low average current density and are severely non-uniform at higher current density due to the mass transfer limitation. The local effectiveness factor in the catalyst layer can be obtained with this model, so the mass transport limitation is displayed from another point of view.     

Constructal flow distributor as a bipolar plate for proton exchange membrane fuel cells

A plate-type constructal flow distributor is implemented as a gas distributor for a proton exchange membrane fuel cell. A 3D complete model is simulated using CFD techniques. The fuel cell model includes the gas flow channels, the gas diffusion layers and the membrane-electrode assembly (MEA). The governing equations for the mass and momentum transfer are solved including the pertinent source terms due to the electrochemical reactions in the different zones of the fuel cell. Three constructal flow configurations were studied; each pattern is a fractal expansion of the original design, therefore, the only difference between them is the number of branches in the geometry. It was found that the number of branches is the key parameter in the performance of a fuel cell when using the constructal distributors as flow channels. The performance of the fuel cell is reported in I-V curves, power curves, and overpotential curves in order to determine which irreversibility is the main cause of energy losses. In terms of flow analysis, it was found that the constructal flow distributor presents a low pressure drop for a wide range of Reynolds number conditions at the inlet, as well as an excellent uniformity of flow distribution. Regardless of the outstanding hydrodynamic performance of the constructal distributors and the large current density values obtained, the implementation of these designs as flow patterns for PEMFCs need further optimization; first, the manufacturing of the plates have to be addressed in an efficient way; and secondly, the application in stacks will require an elaborate design to accomplish this task.     

A general model of proton exchange membrane fuel cell

In this study, a general model of proton exchange membrane fuel cell (PEMFC) was constructed, implemented and employed to simulate the fluid flow, heat transfer, species transport, electrochemical reaction, and current density distribution, especially focusing on liquid water effects on PEMFC performance. The model is a three-dimensional and unsteady one with detailed thermo-electrochemistry, multi-species, and two-phase interaction with explicit gas–liquid interface tracking by using the volume-of-fluid (VOF) method. The general model was implemented into the commercial computational fluid dynamics (CFD) software package FLUENT^® v6.2, with its user-defined functions (UDFs). A complete PEMFC was considered, including membrane, gas diffusion layers (GDLs), catalyst layers, gas flow channels, and current collectors. The effects of liquid water on PEMFC with serpentine channels were investigated. The results showed that this general model of PEMFC can be a very useful tool for the optimization of practical engineering designs of PEMFC.     

A two-phase flow and transport model for PEM fuel cells

A two-phase flow and multi-component mathematical model with a complete set of governing equations valid in different components of a PEM fuel cell is developed. The model couples the flows, species, electrical potential, and current density distributions in the cathode and anode fluid channels, gas diffusers, catalyst layers and membrane, respectively. The modeling results of typical concentration distributions are presented. The coupling of oxygen concentration, current density, overpotential and potential are shown in the membrane electrode assembly (MEA). The model predicted fuel cell polarization curves for different cathode pressures compared well with our experimental data.     

Multiple concentric spirals for the flow field of a proton exchange membrane fuel cell

The present analysis considers a three-dimensional non-isothermal model in a single phase of a PEM fuel cell with a flow field path in the shape of 1, 2, 3, 4, 6, and 8 concentric spirals. The current density contours, the water content and the entropy generated in all zones of the fuel cell are predicted. The analysis of the three-dimensional model includes the gas flow channels in the six geometric shapes mentioned above, the current collectors, gas diffusion layers, catalyst layers on both sides of the model, anode and cathode, and a proton exchange membrane in between. The energy equation, mass conservation, and transport of species equations are solved, including source terms that take into account the electrochemical effects occurring inside the cell. Also, the entropy generation equation is added to the governing equations of the model. The results allow a comparison to help to decide which of the 6 analyzed configurations improve the performance of the fuel cell, increasing the current density produced, reducing the pressure drop and producing the most uniform current density. The entropy generation analysis reveals the effects that cause the most significant losses (irreversibilities) in the cell. The Bejan number and the Π number are used to compare the irreversibilities produced by the matter flow and by the heat transfer for each one of the six models.     

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

Modeling plays a very important role in the development of fuel cells and fuel cell systems. The aim of this work is to investigate the electrochemical processes of a Solid Oxide Fuel Cell (SOFC) and to evaluate the performance of the proposed SOFC design. For this aim a three-dimensional Computational Fluid Dynamics (CFD) model has been developed for an anode-supported planar SOFC with corrugated bipolar plates serving as gas channels and current collector. The conservation of mass, momentum, energy and species is solved by using the commercial CFD code FLUENT in the developed model. The add-on FLUENT SOFC module is implemented for modeling the electrochemical reactions, loss mechanisms and related electric parameters throughout the cell. The distributions of temperature, flow velocity, pressure and gaseous (fuel and air) concentrations through the cell structure and gas channels is investigated. The relevant fuel cell variables such as the potential and current distribution over the cell and fuel utilization are calculated and studied. The modeling results indicate that, for the proposed SOFC design, reasonably uniform distributions of current density over the active cell area can be achieved. The geometry of the cathode gas channel has a substantial effect on the oxygen distribution and thus the overall cell performance. Methods for arriving at improved cell designs are discussed.     

A mathematical model for simulating methanol permeation and the mixed potential effect in a direct methanol fuel cell

A mathematical model for simulating methanol permeation and the pertinent mixed potential effect in a direct methanol fuel cell (DMFC) is presented. In this model a DMFC is divided into seven compartments namely the anodic flow channel, the anodic diffusion layer, the anodic catalyst layer, the proton exchange membrane (PEM), the cathodic catalyst layer, the cathodic diffusion layer and the cathodic flow channel. All compartments are considered to have finite thickness, and within every one of them a set of governing equations are given to stipulate methanol transport and oxygen transport. For the flow channels, fluid dynamics, which could substantially lower the local methanol concentration within catalyst layers is taken into account. With the knowledge of local concentrations of the species, the electrochemical reaction rates within both catalyst layers can be quantified by a kinetic Tafel expression. For the anodic catalyst layer the local external current generated by methanol oxidation is computed; for the cathodic catalyst layer, in addition to the local external current generated by oxygen reduction, the local internal current as a result of methanol permeation is also computed. With the information of the local internal current, the mixed potential effect, which is responsible for adversely lowering the cell voltage can be analyzed.     

Three‐dimensional modelling of catalyst layers in PEM fuel cells: Effects of non‐uniform catalyst loading

Improving the performance of polymer‐electrolyte membrane (PEM) fuel cells depends on the optimization of catalyst layer composition and structure for large active surfaces. Modelling studies provide a valuable tool for investigating the effects of catalyst layer composition and structure on the electrochemical and physical phenomena occurring in PEM fuel cells. Previous modelling studies have shown that the distribution of electrochemical reactions in catalyst layers is highly dependent on the complex interaction of activation and ohmic effects as well as contributions from transport limitations and variations in local and overall current densities. In this paper, three‐dimensional, multicomponent and multiphase transport computations are performed using a computational fluid dynamics (CFD) code (FLUENT^TM) with a new PEM fuel cell module, which has been further improved by taking into account the detailed composition and structure of the catalyst layers using the multiple thin‐film agglomerate model. The detailed modelling of reactions in the catalyst layers is used to determine methods of improving the effectiveness of catalyst layers for a given platinum loading. First, available data on catalyst layer composition and structure are used in CFD computations to predict reaction rate distributions. Based on these results, spatial variations in catalyst loading are then implemented in CFD computations for the same overall catalyst loading to investigate possible performance gains. It is found that grading catalyst loading towards the membrane in the anode and the gas channel inlet in the cathode provides the most beneficial effects on the fuel cell performance. Thus the results suggest that significant savings in cost can be attained by reducing the platinum loading in underutilized regions of the catalyst layers, while at the same time improving the performance. Copyright © 2009 John Wiley & Sons, Ltd.     

CFD investigating the effects of different operating conditions on the performance and the characteristics of a high-temperature PEMFC

The effects of different operating conditions on the performance and the characteristics of a high-temperature proton exchange membrane fuel cell (PEMFC) are investigated using a three-dimensional (3-D) computational fluid dynamics (CFD) fuel-cell model. This model consists of the thermal-hydraulic equations and the electrochemical equations. Different operating conditions studied in this paper include the inlet gas temperature, system pressure, and inlet gas flow rate, respectively. Corresponding experiments are also carried out to assess the accuracy of this CFD model. Under the different operating conditions, the PEMFC performance curves predicted by the model correspond well with the experimentally measured ones. The performance of PEMFC is improved as the increase in the inlet temperature, system pressure or flow rate, which is precisely captured by the CFD fuel cell model. In addition, the concentration polarization caused by the insufficient supply of fuel gas can be also simulated as the high-temperature PEMFC is operated at the higher current density. Based on the calculation results, the localized thermal-hydraulic characteristics within a PEMFC can be reasonably captured. These characteristics include the fuel gas distribution, temperature variation, liquid water saturation distribution, and membrane conductivity, etc.     

Two-phase flow modeling of liquid-feed direct methanol fuel cell

A two-phase flow model was developed for liquid-feed methanol fuel cells (DMFC) to evaluate the effects of various operating parameters on the DMFC performance. In this study, a general homogenous two-dimensional model is described in details for both porous layers and fluid channels. This two-dimensional general model accounts for fluid flow, electrochemical kinetics, current density distribution, hydrodynamics, multi-component transport, and methanol crossover. It starts from basic transport equations including mass conservation, momentum transport, energy balance, and species concentration conservation in different elements of the fuel cell sandwich, as well as the equations for the phase potential in the membrane and the catalyst layers. These governing equations are coupled with chemical reaction kinetics by introducing various source terms. It is found that all these equations are in a very similar form except the source terms. Based on this observation, all the governing equations can be solved using the same numerical formulation in the single domain without prescribing the boundary conditions at the various interfaces between the different elements of the fuel cell. The numerical simulation results, such as velocity field, local current density distribution, and species concentration variation along the flow channel, under various operation conditions are computed. The performance of the DMFC affected by various parameters such as temperature, pressure, and methanol concentration is investigated in this paper. The numerical results are further validated with available experimental data from the published literatures.     

Performance analysis of a proton exchange membrane fuel cell using tree-shaped designs for flow distribution

The performance of a proton exchange membrane (PEM) fuel cell is directly associated to the flow channels design embedded in the bipolar plates. The flow field within a fuel cell must provide efficient mass transport with a reduced pressure drop through the flow channels in order to obtain a uniform current distribution and a high power density. In this investigation, three-dimensional fuel cell models are analyzed using computational fluid dynamics (CFD). The proposed flow fields are radially designed tree-shaped geometries that connect the center flow inlet to the perimeter of the fuel cell plate. Three flow geometries having different levels of bifurcation were investigated as flow channels for PEM fuel cells. The performance of the fuel cells is reported in polarization and power curves, and compared with that of fuel cells using conventional flow patterns such as serpentine and parallel channels. Results from the flow analysis indicate that tree-shaped flow patterns can provide a relatively low pressure drop as well as a uniform flow distribution. It was found that as the number of bifurcation levels increases, a larger active area can be utilized in order to generate higher power and current densities from the fuel cell with a negligible increase in pumping power.     

Parameter sensitivity examination for a complete three-dimensional,two-phase,non-isothermal model of polymer electrolyte membrane fuel cell

Parameter sensitivity analysis is carried out for a complete three-dimensional, two-phase, non-isothermal model of polymer electrolyte membrane (PEM) fuel cell with a parallel flow field design. The model couples the two-phase flow of the multi-component reactants and liquid water, species transport, electrochemical reactions, proton and electron transport, and the electro-osmosis transport, back diffusion of water in the membrane, and energy transport. Twenty nine parameters, which are classified into the structural or transport parameters of porous layers (tortuosity, porosity, permeability, proton conductivity, electron conductivity, and thermal conductivity) as well as the electrochemical parameters (anodic and cathodic exchange current densities, anodic and cathodic transfer coefficients for anode and cathode reactions), are used to implement individual parameter investigation. The results show the parameters can be divided in to strongly sensitive, conditional sensitive and weak sensitive parameters according to its effect on the cell polarization curve. The optimization of parameters of cathode gas diffusion layer (GDL) and catalyst layer (CL) is more important to improve cell performance than that of anode GDL and CL because liquid water transport and removal affect significantly membrane hydration and reactant transport. Electrochemical parameters determine the activation potential and the slope of ohmic polarization hence these parameters can be used to fit experimental polarization curve more effectively than the other parameters.     

The effects of porosity distribution variation on PEM fuel cell performance

Gas diffusion layers (GDL) are one of the important parts of the PEM fuel cell as they serve to transport the reactant gases to the catalyst layer. Porosity of this layer has a large effect on the PEM fuel cell performance. The spatial variation in porosity arises due to two effects: (1) compression of the electrode on the solid landing areas and (2) water produced at the cathode side of gas diffusion layers. Both of these factors change the porosity of gas diffusion layers and affect the fuel cell performance. To implement this performance analysis, a mathematical model which considers oxygen and hydrogen mass fraction in gas diffusion layer and the electrical current density in the catalyst layer, and the fuel cell potentials are investigated. The porosity variation in the GDL is calculated by considering the applied pressure and the amount of the water generated in the cell. The validity of the model is approved by comparing the computed results with experimental data. The obtained results show that the decrease in the average porosity causes the reduction in oxygen consumption, so that a lower electrical current density is generated. It is also shown that when the electrical current density is low, the porosity variation in gas diffusion layer has no significant influence on the level of polarization whereas at higher current density the influence is very significant. The porosity variation causes non-uniformity in the mass transport which in turn reduces the current density and a lower fuel cell performance is obtained.     

直接液体甲醇燃料电池的数学模型

描述了一个用于模拟直接甲醇燃料电池特性的垂直于流道的二维数学模型。模型同时考虑了电化学动力学、水动力学和多组分传递。计算了电地内反应物浓度的分布、电流密度分布、甲醇窜流以及电压—电流特性曲线等。结果表明：集流板前的催化层内反应物浓度非常低；流道边缘附近电流密度比平均电流密度大许多倍。图5表l参9     

Modeling and simulation of a proton exchange membrane fuel cell using computational fluid dynamics

An isothermal, three dimensional, single phase model was presented to evaluate a proton exchange membrane fuel cell (PEMFC) with serpentine flow. The mass, momentum and electrochemical equations were solved simultaneously for the steady state condition using computational fluid dynamics (CFD) software based on the finite element method. The model considered reactions as mass source/sink terms, and electron transport in the catalyst layers and GDLs. To validate the model, the numerical results were compared to the experimental data collected from the fabricated membrane electrode assemblies. The exchange current density parameter of the catalysts was fitted by the model to calibrate the results. The model showed good agreement with experimental data and predicted a higher current density for the catalyst with a higher surface area and Pt content. The oxygen, hydrogen and water mass fraction distribution, velocity magnitude and pressure distribution were estimated by the model. Moreover, the effect of pressure and temperature, as two important operating conditions, on the current density was predicted by the validated model.     

Innovative gas diffusion layers and their water removal characteristics in PEM fuel cell cathode

Liquid water transport is one of the key challenges regarding the water management in a proton exchange membrane (PEM) fuel cell. Conventional gas diffusion layers (GDLs) do not allow a well-organized liquid water flow from catalyst layer to gas flow channels. In this paper, three innovative GDLs with different micro-flow channels were proposed to solve liquid water flooding problems that conventional GDLs have. This paper also presents numerical investigations of air–water flow across the proposed innovative GDLs together with a serpentine gas flow channel on PEM fuel cell cathode by use of a commercial computational fluid dynamics (CFD) software package FLUENT. The results showed that different designs of GDLs will affect the liquid water flow patterns significantly, thus influencing the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown. Several gas flow problems for the proposed different kinds of innovative GDLs were observed, and some useful suggestions were given through investigating the flow patterns inside the proposed GDLs.