Transport Phenomena Within the Cathode for a Polymer Electrolyte Fuel Cell

A one-dimensional two-phase steady model is developed to analyze the coupled phenomena of cathode flooding and mass-transport limitation for a polymer electrolyte fuel cell. In the model, the liquid water transport in the porous electrode is driven by the capillary force based on Darcy's law, while the gas transport is driven by the concentration gradient based on Fick's law. Furthermore, the catalyst layer is treated as a separate computational domain. The capillary pressure continuity is imposed on the interface between the catalyst layer and the gas diffusion layer. Additionally, through Tafel kinetics, the mass transport and the electrochemical reaction are coupled together. The saturation jump at the interface between the gas diffusion layer and the catalyst layer is captured in the results. Meanwhile, the results further indicate that the flooding situation in the catalyst layer is much more serious than that in the gas diffusion layer. Moreover, the saturation level inside the cathode is largely related to the physical, material, and operating parameters. In order to effectively prevent flooding, one should first remove the liquid water residing inside the catalyst layer and keep the boundary value of the liquid water saturation as low as possible.     

Transport phenomena within the porous cathode for a proton exchange membrane fuel cell

A two-phase, one-dimensional steady model is developed to analyze the coupled phenomena of cathode flooding and mass-transport limiting for the porous cathode electrode of a proton exchange membrane fuel cell. In the model, the catalyst layer is treated not as an interface between the membrane and gas diffusion layer, but as a separate computational domain with finite thickness and pseudo-homogenous structure. Furthermore, the liquid water transport across the porous electrode is driven by the capillary force based on Darcy's law. And the gas transport is driven by the concentration gradient based on Fick's law. Additionally, through Tafel kinetics, the transport processes of gas and liquid water are coupled. From the numerical results, it is found that although the catalyst layer is thin, it is very crucial to better understand and more correctly predict the concurrent phenomena inside the electrode, particularly, the flooding phenomena. More importantly, the saturation jump at the interface of the gas diffusion layer and catalyst layers is captured, when the continuity of the capillary pressure is imposed on the interface. Elsewise, the results show further that the flooding phenomenon in the CL is much more serious than that in the GDL, which has a significant influence on the mass transport of the reactants. Moreover, the saturation level inside the cathode is determined, to a great extent, by the surface overpotential, the absolute permeability of the porous electrode, and the boundary value of saturation at the gas diffusion layer-gas channel interface. In order to prevent effectively flooding, it should remove firstly the liquid water accumulating inside the CL and keep the boundary value of liquid saturation as low as possible.     

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.     

Enhancing under-rib mass transport in proton exchange membrane fuel cells using new serpentine flow field designs

New flow field configurations are developed to improve the performance of polymer electrolyte membrane fuel cells (PEMFCs). The developed designs aim to uniformly distribute the reactants over the reaction area of the catalyst layer surface, boost the under-rib convection mass transport through the gas diffusion layer, decrease the water flooding effect in the gas diffusion layer-catalyst layer interface, and maintain the membrane water content within the required range to augment protonic conductivity. To evaluate the performance parameters of a PEMFC, a comprehensive three-dimensional, two-phase mathematical model has been developed. The model includes the charge transport, electrochemical reactions, mass conservation, momentum, energy, and water transport equations. The results signify that the improved flow field patterns attain a considerable boosting of the output power, the under-rib convection mass transport, improvement of the reactant distribution over the catalyst layer surface and decline of the liquid water saturation in the gas diffusion layer-catalyst layer interface. The developed configurations achieve a higher power density of 0.82 W/cm² at a current density of 1.74 A/cm², compared to the standard serpentine configuration, which attains about 0.67 W/cm² at a current density of 1.486 A/cm².Accordingly, the develop configurations demonstrate a 22.6% enhancement in power density.     

Two-dimensional two-phase mass transport model for methanol and water crossover in air-breathing direct methanol fuel cells

A two-dimensional two-phase mass transport model has been developed to predict methanol and water crossover in a semi-passive direct methanol fuel cell with an air-breathing cathode. The mass transport in the catalyst layer and the discontinuity in liquid saturation at the interface between the diffusion layer and catalyst layer are particularly considered. The modeling results agree well with the experimental data of a home-assembled cell. Further studies on the typical two-phase flow and mass transport distributions including species, pressure and liquid saturation in the membrane electrode assembly are investigated. Finally, the methanol crossover flux, the net water transport coefficient, the water crossover flux, and the total water flux at the cathode as well as their contributors are predicted with the present model. The numerical results indicate that diffusion predominates the methanol crossover at low current densities, while electro-osmosis is the dominator at high current densities. The total water flux at the cathode is originated primarily from the water generated by the oxidation reaction of the permeated methanol at low current densities, while the water crossover flux is the main source of the total water flux at high current densities.     

An approach for determining the liquid water distribution in a liquid-feed direct methanol fuel cell

In determining the liquid water distribution in the anode (or the cathode) diffusion medium of a liquid-feed direct methanol fuel cell (DMFC) with a conventional two-phase mass transport model, a current-independent liquid saturation boundary condition at the interface between the anode flow channel and diffusion layer (DL) (or at the interface between the cathode flow channel and cathode DL) needs to be assumed. The numerical results resulting from such a boundary condition cannot realistically reveal the liquid distribution in the porous region, as the liquid saturation at the interface between the flow channel and DL varies with current density. In this work, we propose a simple theoretical approach that is combined with the in situ measured water-crossover flux in the DMFC to determine the liquid saturation in the anode catalyst layer (CL) and in the cathode CL. The determined liquid saturation in the anode CL (or in the cathode CL) can then be used as a known boundary condition to determine the water distribution in the anode DL (or in the cathode DL) with a two-phase mass transport model. The numerical results show that the water distribution becomes much more realistic than those predicted with the assumed boundary condition at the interface between the flow channel and DL.     

The gas diffusion layer in polymer electrolyte membrane fuel cells: A process model of the two-phase flow

A two-phase flow process model for the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell, considering also the cathode catalyst layer (CL), is presented. For this purpose, a systematic analysis of the factors affecting flooding and drying, including the liquid accumulation in the gas channel (CH), was performed using a one-dimensional reference model for the GDL and a compact channel model. The treatment proposed for the CH-GDL interface was compared with other boundary conditions in the literature. It was concluded that the liquid accumulation in the channel is determinant for estimating the steady state and transient GDL flooding, but that predicting the saturation level in the CL can help for determining operation policies for precluding flooding in the GDL-CL composite, in the absence of an adequate channel model. Bifurcation behavior, associated with the water phase change, was identified by means of the compact model.     

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.     

The effects of the cathode flooding on the transient responses of a PEM fuel cell

This study investigates the effects of the flooding of the gas diffusion layer (GDL), as a result of liquid water accumulation, on the performance of a proton exchange membrane fuel cell (PEMFC). The transient profiles of the current generated by the cell are obtained using the numerical resolution of the transport equation for the oxygen molar concentration in the unsteady state. The dynamics of the system are captured through the reduction of the effective porosity of the GDL by the liquid water which accumulates in the void space of the GDL. The effects of the GDL porosity, GDL thickness and mass transfer at the GDL–gas channel interface on the evolution with time of the averaged current density are reported. The effects of the current collector rib on the evolution of the molar concentration of oxygen are also examined in detail.     

Numerical investigation of the optimal Nafion ionomer content in cathode catalyst layer: An agglomerate two-phase flow modelling

A two dimensional, across the channel, isothermal, two-phase flow model for a proton exchange membrane fuel cell is presented. Reactant transport in porous media, water phase transfer and water transport through the membrane are included. The catalyst layer is modelled as a spherical agglomerate structure. Liquid water occupies the secondary pores of the cathode catalyst layer to form a liquid water coating surrounding the agglomerate. The thickness is calculated by coupling the two-phase flow model with the agglomerate model. Ionomer swelling is associated with the non-uniform distribution of water in the ionomer determined from several processes occurring simultaneously, namely (1) water phase transfer between the vapour, dissolved and liquid water; (2) membrane/ionomer water content depending on the water vapour pressure; (3) a water film covering the catalyst agglomerate; (4) water transport through the membrane via electro-osmotic drag, back diffusion and hydraulic permeation. The model optimises the initial dry ionomer content in the cathode catalyst layer. The simulation results indicate that, to achieve the best fuel cell performance, the initial dry ionomer volume fraction should be controlled around 10%, corresponding to 0.3 mg cm⁻². By considering the effect of ionomer swelling on the reduction in CCL porosity and the increase in oxygen mass transport resistance, the accuracy of the model prediction is improved, especially at higher current densities.     

Liquid and oxygen transport through bilayer gas diffusion materials of proton exchange membrane fuel cells

In proton exchange membrane fuel cells (PEMFCs), a hydrophobic micro-porous layer (MPL) is usually placed between the catalyst layer (CL) and the conventional gas diffusion layer (GDL) to relieve the flooding. In this paper, a pore network model is developed to investigate how the MPL structure affects the liquid and oxygen transport properties of the bilayer gas diffusion material (GDM) consisting of fine MPL and coarse GDL. The regular three-dimensional pore network constructed to represent the bilayer GDM are composed of the cubic pores that are connected by the narrow throats of square cross section. Based on this model, the capillary pressure, liquid permeability, and oxygen effective diffusivity as a function of GDM liquid saturation are determined. Parameter studies are performed to elucidate the influences of MPL thickness and of MPL crack width. Also analyzed are the liquid distributions in different structural GDMs at the moment of breakthrough. The results reveal a liquid saturation jump at the MPL/GDL interface in the plain bilayer GDM, but a liquid saturation drop in the defective bilayer GDM.     

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.     

Microscopic Observations of Freezing Phenomena in PEM Fuel Cells at Cold Starts

In polymer electrolyte membrane (PEM) fuel cells, the generated water transfers from the catalyst layer to the gas channel through microchannels of different scales in a two phase flow. It is important to know details of the water transport phenomena to realize better cell performance, as the water causes flooding at high current density conditions and gives rise to startup problems at freezing temperatures. This article presents specifics of the ice formation characteristics in the catalyst layer and in the gas diffusion layer (GDL) with photos taken with an optical microscope and a cryo scanning electron microscope (cryo-SEM). The observation results show that cold starts at –10°C result in ice formation at the interface between the catalyst layer and the microporous layer (MPL) of the GDL, and that at –20°C most of the ice is formed in the catalyst layer. Water transport phenomena through the microporous layer and GDL are also a matter of interest, because the role of the MPL is not well understood from the water management angle. The article discusses the difference in the water distribution at the interface between the catalyst layer and the GDL arising from the presence of such a microporous layer.     

A two dimensional partial flooding model for PEMFC

Proton exchange membrane fuel cells (PEMFCs) have attracted much attention in these years. In PEMFCs, liquid/gas two-phase flow is a common phenomenon, which has great influence on fuel cell performance. However, the liquid water transport process has not been satisfactorily modeled yet. In this work, a two dimensional partial flooding model was developed, in which the pore size distribution of the gas diffusion layer (GDL) is taken into consideration in the explanation of fuel cell flooding for the first time. Liquid water produced is considered to flood a fraction of the GDL hydrophobic pores with diameter greater than the capillary condensation threshold diameter, and the unflooded pores will serve as passageway for gas transportation and the corresponding catalyst area is available for electrochemical reaction. Use this model, it is simple to explain membrane dehydration and electrode flooding. Different operation conditions have been studied with the model and the model polarization curves show reasonable accordance with the experimental results.     

Analysis of capillary flow driven model for water transport in PEFC cathode catalyst layer: Consideration of mixed wettability and pore size distribution

The solid matrix of the porous cathode catalyst layer (CCL) of a polymer electrolyte fuel cell is made of two different materials (carbon with supported Pt and ionomer), which are characterized by different wettability (i.e. contact angles). This paper discusses the need for considering the combined consideration of the mixed wettability and the distributed pore structure of CCL in modelling the transport of liquid water and oxygen gas. A simple 1-D model that considers two different pore size distributions, derived from experimental capillary pressure–saturation literature data, for the hydrophobic and hydrophilic pores is presented. The results indicate that for water to be transported in liquid-state through the CCL, the liquid saturation is such that only very small hydrophobic pores remain available for gas transport such that Knudsen diffusion will dominate and must be considered in CCL models.     

A parametric study of cathode catalyst layer structural parameters on the performance of a PEM fuel cell

This paper is a computational study of the cathode catalyst layer (CL) of a proton exchange membrane fuel cell (PEMFC) and how changes in its structural parameters affect performance. The underlying mathematical model assumes homogeneous and steady-state conditions, and consists of equations that include the effects of oxygen diffusion, electrochemical reaction rates, and transport of protons and electrons through the Nafion ionomer (PEM) and solid phases. Simulations are concerned with the problem of minimizing activation overpotential for a given current density. The CL consists of four phases: ionomer, solid substrate, catalyst particles and void spaces. The void spaces are assumed to be fully flooded by liquid water so that oxygen within the CL can diffuse to reaction sites via two routes: within the flooded void spaces and dissolved within the ionomer phase. The net diffusive flux of oxygen through the cathode CL is obtained by incorporating these two diffusive fluxes via a parallel resistance type model. The effect of six structural parameters on the CL performance is considered: platinum and carbon mass loadings, ionomer volume fraction, the extent to which the gas diffusion layer (GDL) extends into the CL, the GDL porosity and CL thickness. Numerical simulations demonstrate that the cathode CL performance is most strongly affected by the ionomer volume fraction, CL thickness and carbon mass loading. These results give useful guidelines for manufactures of PEMFC catalyst layers.     

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.     

Lattice Boltzmann simulations of water transport in gas diffusion layer of a polymer electrolyte membrane fuel cell

The effect of wettability on water transport dynamics in gas diffusion layer (GDL) is investigated by simulating water invasion in an initially gas-filled GDL using the multiphase free-energy lattice Boltzmann method (LBM). The results show that wettability plays a significant role on water saturation distribution in two-phase flow in the uniform wetting GDL. For highly hydrophobicity, the water transport falls in the regime of capillary fingering, while for neutral wettability, water transport exhibits the characteristic of stable displacement, although both processes are capillary force dominated flow with same capillary numbers. In addition, the introduction of hydrophilic paths in the GDL leads the water to flow through the hydrophilic pores preferentially. The resulting water saturation distributions show that the saturation in the GDL has little change after water breaks through the GDL, and further confirm that the selective introduction of hydrophilic passages in the GDL would facilitate the removal of liquid water more effectively, thus alleviating the flooding in catalyst layer (CL) and GDL. The LBM approach presented in this study provides an effective tool to investigate water transport phenomenon in the GDL at pore-scale level with wettability distribution taken into consideration.     

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

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

Effects of an MPL on water and thermal management in a PEMFC

In this study, the effects of adding a microporous layer (MPL) as well as the impact of its physical properties on polymer electrolyte fuel cell (PEMFC) performance with serpentine flow channels were investigated. In addition, numerical simulations were performed to reveal the effect of relative humidity and operating temperature. It is indicated that adding an extra between the gas diffusion layer (GDL) and catalyst layer (CL), a discontinuity in the liquid saturation shows up at their interface because of differences in the wetting properties of the layers. In addition, results show that a higher MPL porosity causes the liquid water saturation to decrease and the cell performance is improved. A larger MPL thickness reduces the cell performance. The effects of MPL on temperature distribution and thermal transport of the membrane prove that the MPL in addition to being a water management layer also improves the thermal management of the PEMFC.