Performance analysis of a PEM fuel cell cathode with multiple catalyst layers

The cathode catalyst layer (CL) of a PEM fuel cell (PEMFC) plays an important role in the performance of the cell because of the rate limiting mechanisms that take place in it. For enhancing the performance of a PEMFC, the use of multiple, ultra thin CLs instead of a single CL is considered in the present work. Since the concentration of oxygen decreases in a CL from the diffusion medium-CL interface towards the polymer membrane, the CL adjacent to the diffusion medium should be of higher porosity than the other CLs. Similarly, the CL adjacent to the polymer membrane should contain more ionomer than the other CLs. Furthermore, liquid water should be removed without causing significant mass transport and/or ohmic losses. Therefore, the design parameters of a CL can be varied spatially to minimize losses in a PEMFC. However, such a continuously graded CL is difficult to manufacture due to lack of commercially available techniques and associated costs. As an alternative, a combination of layers can be synthesized where each layer is manufactured with different design parameters. This approach provides the opportunity to optimize the design parameters of each layer. With this objective in mind, a detailed steady state model of a PEMFC cathode with multiple layers is developed. The model considers liquid water in all the layers. The catalyst layer microstructure is modeled as a network of spherical agglomerates. For improved water management, a thin micro-porous layer is considered between the gas diffusion layer (GDL) and the first catalyst layer. The performance curves for various combinations of the design parameters are shown and the results are analyzed. The results show that there exists an optimum combination of design parameters for each catalyst layer that can significantly improve the performance of a PEMFC.     

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.     

Transient analysis for the cathode gas diffusion layer of PEM fuel cells

A one-dimensional, non-isothermal, two-phase transient model has been developed to study the transient behaviour of water transport in the cathode gas diffusion layer of PEM fuel cells. The effects of four parameters, namely the liquid water saturation at the interface of the gas diffusion layer and flow channels, the proportion of liquid water to all of the water at the interface of the cathode catalyst layer and the gas diffusion layer, the current density, and the contact or wetting angle, on the transient distribution of liquid water saturation in the cathode gas diffusion layer are investigated. Especially, the time needed for liquid water saturation to reach steady state and the liquid water saturation at the interface of the cathode catalyst layer and gas diffusion layer are plotted as functions of the above four parameters. The ranges of water vapour condensation and liquid water evaporation are identified across the thickness of the gas diffusion layer. In addition, the effects of the above four parameters on the steady state distributions of gas phase pressure, water vapour concentration, oxygen concentration and temperature are also presented. It is found that increasing any one of the first three parameters will increase the water saturation at the interface of the catalyst layer and gas diffusion layer, but decrease the time needed for the liquid water saturation to reach steady state. When the liquid water saturation at the interface of the gas diffusion layer and flow channels is high enough (≥0.1), the liquid water saturation at steady state is almost uniformly distributed across the thickness of the gas diffusion layer. It is also found that, under the given initial and boundary conditions in this paper, evaporation takes place within the gas diffusion layer close to the channel side and is the major process for water phase change at low current density (<2000 A m⁻²); condensation occurs close to the catalyst layer side within the gas diffusion layer and dominates the phase change at high current density (>5000 A m⁻²). For hydrophilic gas diffusion layers, both the time needed for liquid water saturation to reach steady state and the water saturation at the interface of the catalyst layer and gas diffusion layer will increase when the contact angle increases; but for hydrophobic gas diffusion layers, both of them decrease when the contact angle increases.     

A parametric study of multi-phase and multi-species transport in the cathode of PEM fuel cells

In this study, a mathematical model is developed for the cathode of PEM fuel cells, including multi-phase and multi-species transport and electrochemical reaction under the isothermal and steady-state conditions. The conservation equations for mass, momentum, species and charge are solved using the commercial software COMSOL Multiphysics. The catalyst layer is modeled as a finite domain and assumed to be composed of a uniform distribution of supported catalyst, liquid water, electrolyte and void space. The Stefan–Maxwell equation is used to model the multi-species diffusion in the gas diffusion and catalyst layers. Owing to the low relative species' velocity, Darcy's law is used to describe the transport of gas and liquid phases in the gas diffusion and catalyst layers. A serpentine flow field is considered to distribute the oxidant over the active cathode electrode surface, with pressure loss in the flow direction along the channel. The dependency of the capillary pressure on the saturation is modeled using the Leverette function and the Brooks and Corey relation. A parametric study is carried out to investigate the effects of pressure drop in the flow channel, permeability, inlet relative humidity and shoulder/channel width ratio on the performance of the cell and the transport of liquid water. An inlet relative humidity of 90 and 80% leads to the highest performance in the cathode. Owing to liquid water evaporation, the relative humidity in the catalyst layer reaches 100% with an inlet relative humidity of 90 and 80%, resulting in a high electrolyte conductivity. The electrolyte conductivity plays a significant role in determining the overall performance up to a point. Further, the catalyst layer is found to be important in controlling the water concentration in the cell. The cross-flow phenomenon is shown to enhance the removal of liquid water from the cell. Moreover, a shoulder/channel width ratio of 1:2 is found to be an optimal ratio. A decrease in the shoulder/channel ratio results in an increase in performance and an increase in cross flow. Finally, the Leverette function leads to lower liquid water saturations in the backing and catalyst layers than the Brooks and Corey relation. The overall trend, however, is similar for both functions. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

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.     

Microscale and Macroscale Aspects of Water Management Challenges in PEM Fuel Cells

Water management is critical to the successful implementation of proton exchange membrane (PEM) fuel cells in the automotive transportation sector. Liquid water appears in the fuel cells not only from the water generated at the cathode catalyst layer but also as a result of condensation of water vapor within the humidified gases. Topics of intense current interest include the microscopic flow of liquid water through the membrane, catalyst layers, and gas diffusion layers; the macroscopic interaction between water and the gas flow at the gas diffusion layer interface; and the two-phase, multicomponent flow through the gas channels. Recent work published in this area is reviewed and recommendations for future work are outlined.     

Parametric study of the cathode and the role of liquid saturation on the performance of a polymer electrolyte membrane fuel cell—A numerical approach

A two-dimensional two-phase steady state model of the cathode of a polymer electrolyte membrane fuel cell (PEMFC) is developed using unsaturated flow theory (UFT). A gas flow field, a gas diffusion layer (GDL), a microporous layers (MPL), a finite catalyst layer (CL), and a polymer membrane constitute the model domain. The flow of liquid water in the cathode flow channel is assumed to take place in the form of a mist. The CL is modeled using flooded spherical agglomerate characterization. Liquid water is considered in all the porous layers. For liquid water transport in the membrane, electro-osmotic drag and back diffusion are considered to be the dominating mechanisms. The void fraction in the CL is expressed in terms of practically achievable design parameters such as platinum loading, Nafion loading, CL thickness, and fraction of platinum on carbon. A number of sensitivity studies are conducted with the developed model. The optimum operating temperature of the cell is found to be 80-85 °C. The optimum porosity of the GDL for this cell is in the range of 0.7-0.8. A study by varying the design parameters of the CL shows that the cell performs better with 0.3-0.35 mg cm⁻² of platinum and 25-30 wt% of ionomer loading at high current densities. The sensitivity study shows that a multi-variable optimization study can significantly improve the cell performance. Numerical simulations are performed to study the dependence of capillary pressure on liquid saturation using various correlations. The impact of the interface saturation on the cell performance is studied. Under certain operating conditions and for certain combination of materials in the GDL and CL, it is found that the presence of a MPL can deteriorate the performance especially at high current density.     

Numerical studies of liquid water behaviors in PEM fuel cell cathode considering transport across different porous layers

In this paper, a two-phase two-dimensional PEM fuel cell model, which is capable of handling liquid water transport across different porous materials, is employed for parametric studies of liquid water transport and distribution in the cathode of a PEM fuel cell. Attention is paid particularly to the coupled effects of two-phase flow and heat transfer phenomena. The effects of key operation parameters, including the outside cell boundary temperature, the cathode gas humidification condition, and the cell operation current, on the liquid water behaviors and cell performance have been examined in detail. Numerical results elucidate that increasing the fuel cell temperature would not only enhance liquid water evaporation and thus decrease the liquid saturation inside the PEM fuel cell cathode, but also change the location where liquid water is condensed or evaporated. At a cell boundary temperature of 80 °C, liquid water inside the catalyst layer and gas diffusion media under the current-collecting land would flow laterally towards the gas channel and become evaporated along an interface separating the land and channel. As the cell boundary temperature increases, the maximum current density inside the membrane would shift laterally towards the current-collecting land, a phenomenon dictated by membrane hydration. Increasing the gas humidification condition in the cathode gas channel and/or increasing the operating current of the fuel cell could offset the temperature effect on liquid water transport and distribution.     

The effect of operating parameters on water transport in PEM fuel cells

Water content in the membrane and the presence of liquid water in the catalyst layers (CL) and the gas diffusion layers (GDL) play a very important role in the performance of a PEM fuel cell. To study water transport in a PEM fuel cell, a two‐phase flow mathematical model is developed. This model couples the continuity equation, momentum conservative equation, species conservative equation, and water transport equation in the membrane. The modeling results of fuel cell performances agree well with measured experimental results. Then this model is used to simulate water transport and current density distribution in the cathode of a PEM fuel cell. The effects of operating pressure, cell temperature, and humidification temperatures on the net water transfer through the membrane, liquid water saturation, and current density distribution are studied. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(2): 89–100, 2006; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20107     

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.     

Modeling the phenomena of dehydration and flooding of a polymer electrolyte membrane fuel cell

A one-dimensional, two-phase, transient PEM fuel cell model including gas diffusion layer, cathode catalyst layer and membrane is developed. The electrode is assumed to consist of a network of dispersed Pt/C forming spherically shaped agglomerated zones that are filled with electrolyte. Water is modeled in all three phases: vapor, liquid and dissolved in the ionomer to capture the effect of dehydration of the ionomer as well as flooding of the porous media. The anode is modeled as a sophisticated spatially reduced interface. Motivated by environmental scanning electron microscope (ESEM) images of contact angles for microscopic water droplets on fibers of the gas diffusion layer, we introduce the feature of immobile saturation. A step change of the saturation between the catalyst layer and the gas diffusion layer is modeled based on the assumption of a continuous capillary pressure at the interface. The model is validated against voltammetry experiments under various humidification conditions which all show hysteresis effects in the mass transport limited region. The transient saturation profiles clearly show that insufficient liquid water removal causes pore flooding, which is responsible for the oxygen mass transport limitation at high current density values. The simulated and measured current responses from chronoamperometry experiments are compared and analyzed.     

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.     

A hybrid model of cathode of PEM fuel cell using the interdigitated gas distributor

A two-dimensional (2D), single- and two-phase, hybrid multi-component transport model is developed for the cathode of PEM fuel cell using interdigitated gas distributor. The continuity equation and Darcy's law are used to describe the flow of the reactant gas and production water. The production water is treated as vapor when the current density is small, and as two-phase while the current density is greater than the critical current density. The advection–diffusion equations are utilized to study species transport of multi-component mixture gas. The Butler–Volmer equation is prescribed for the domain in the catalyst layer. The predicted results of the hybrid model agree well with the available experimental data. The model is used to investigate the effects of operating conditions and the cathode structure parameters on the performance of the PEM fuel cell. It is observed that liquid water appears originally in the cathodic catalyst layer over outlet channel under intermediate current and tends to be distributed uniformly by the capillary force with the increase of the current. It is found that reduction of the width of outlet channel can enhance the performance of PEM fuel cell via the increase of the current density over this region, which has, seemingly, not been discussed in previous literatures.     

Dynamic analysis of gas transport in cathode side of PEM fuel cell with interdigitated flow field

The dynamic performance of PEM fuel cell is one of the most important criteria in its design with application to mobile systems and portable devices. To analyze the features, this work employs an unsteady model about single phase transport in cathode side of PEM fuel cell with interdigitated flow field, which considers both convection and diffusion processes. Two types of dynamic performances, start-up and state-to-state operations, are analyzed. The effects of channel width fraction, porosity of the gas diffusion layer, pressure drop and the surface overpotential of the catalyst layer on the dynamic performance are investigated in detail. Predicted results found that the response time is generally quite fast, less than 0.1 s, to reach the 90% response. The time interval from the start-up to a steady state or from one steady state to another steady state mainly depends on the end condition.     

Modelling and experimental validation of high performance low platinum multilayer cathode for polymer electrolyte membrane fuel cells (PEMFCs)

The aim of this paper is to demonstrate numerically the feasibility of using a sputtering technique for the design of Polymer Electrolyte Membrane Fuel Cell cathodes having low platinum loading and offering high performance. A two-dimensional steady state model accounting for electrochemical reactions, momentum, saturation, species and charge conservation equations have been developed. It has been applied to three different cathode configurations where the platinum is assumed to be sputtered one, two and three times on the same number of carbon–Nafion layers (CNL). The predicted polarization curves were validated with the experimental data obtained in our laboratory. Our results showed that the best performance is obtained with a three Pt sputtered layer cathode. A parametric study proved the dependence of the PEM fuel cell cathode performance on the CNL porosity, the (Pt/C) agglomerate radius and the thickness of this agglomerate structure.     

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.     

Model prediction of effects of operating parameters on proton exchange membrane fuel cell performance

The performance of a proton exchange membrane (PEM) fuel cell is greatly affected by the operating parameters. Appropriate operating parameters are necessary for PEM fuel cells to maintain stable performance. A three-dimensional multi-phase fuel cell model (FCM) is developed to predict the effects of operating parameters (e.g. operating pressure, fuel cell temperature, relative humidity of reactant gases, and air stoichiometric ratio) on the performance of PEM fuel cells. The model presented in this paper is a typical nine-layer FCM that consists of current collectors, flow channels, gas diffusion layers, catalysts layers at the anode and the cathode as well as the membrane. A commercial Computational Fluid Dynamics (CFD) software package Fluent is used to solve this predictive model through SIMPLE algorithm and the modeling results are illustrated via polarization curves including I–V and I–P curves. The results indicate that the cell performance can be enhanced by increasing operating pressure and operating temperature. The anode humidification has more significant influences on the cell performance than the cathode humidification, and the best performance occurs at moderate air relative humidity while the hydrogen is fully humidified. In addition, the cell performance proves to be improved with the increase of air stoichiometric ratio. Based on these conclusions, several suggestions for engineering practice are also provided.     

Understanding the role of cathode structure and property on water management and electrochemical performance of a PEM fuel cell

Water management is vital for the successful development of PEM fuel cells. Water should be carefully balanced within a PEM fuel cell to meet the conflicting requirements of membrane hydration and cathode anti-flooding. In order to understand the key factors that can improve water management and fuel cell performance, the cathodes with different structures and properties are prepared and tested in this study. The experimental results show that even though no micro-porous layer (MPL) is placed between the cathode catalyst layer (CCL) and macro-porous substrate (MaPS), a hydrophobic CCL is effective to prevent cathode flooding and keep membrane hydrated. The impedance study and the analysis of the polarization curves indicate that the optimized hydrophobic micro-porous structure in the MPL or the hydrophobic CCL could be mainly responsible for the improved water management in PEM fuel cells, which functions as a watershed to provide wicking of liquid water to the MaPS and increase the membrane hydration by enhancing the back-diffusion of water from the cathode side to the anode side through the membrane.