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.     

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.     

Non-uniform agglomerate cathode catalyst layer model on the performance of PEMFC with consideration of water effect

We focus on the effect of cathode catalyst layer physical structure on the cell performance of proton exchange membrane fuel cell (PEMFC). At low polarization, high inlet humidification predicts better cell performance because of the more active surface in the CL. As polarization is extended near the mass transfer limited regime, high humidification only renders a flooded electrode and inferior cell performance. Catalyst layer with better capillary water transport parameters performs better than that with inferior water repulsion capability. Permeation in the gas diffusion layer (GDL) is important for efficient oxygen diffusion in mass transfer influenced regime. On the other hand, the permeability in catalyst layer only has secondary effect.The distribution of material properties in the CL is studied for the MEA fabrication strategy. The CL is divided into three sub-layers with changing material properties. With water effect considered, better performance is obtained for higher porosity near the GDL, higher electrolyte fraction in the agglomerate near the membrane. The effect of agglomerate particle size differs in the ohmic and mass transfer controlled regimes. Larger agglomerate size near GDL is preferred in the ohmic limited regime, while smaller size near GDL performs better if operated at mass transfer controlled regime.     

Microstructure reconstruction and characterization of PEMFC electrodes

We reconstruct a proton exchange membrane fuel cell (PEMFC) catalyst layer (CL) and a non-woven carbon paper gas diffusion layer (GDL) by specially-designed stochastic methods. The reconstructed microstructures evidently distinguish all the participating components in the composite GDL/CL. Characterization analyses with respect to the reconstructed GDL/CL give important structural properties such as geometrical connectivity of an individual phase, pore size distribution, and volume-specific interfacial area. Self-developed Lattice Boltzmann (LB) models are established to calculate effective transport physical properties including effective thermal/electric conductivity and effective species diffusivity of the reconstructed GDL/CL, and permeability of the reconstructed GDL. Accordingly, we obtain tortuosity values for pore or solid phase in the reconstructed GDL/CL.     

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.     

Measurement of liquid water content in cathode gas diffusion electrode of polymer electrolyte fuel cell

Water management in cathode gas diffusion electrode (GDE) of polymer electrolyte fuel cell (PEFC) is essential for high performance operation, because liquid water condensed in porous gas diffusion layer (GDL) and catalyst layer (CL) blocks oxygen transport to active reaction sites. In this study, the average liquid water content inside the cathode GDE of a low-temperature PEFC is experimentally and quantitatively estimated by the weight measurement, and the relationship between the water accumulation rate in the cathode GDE and the cell voltage is investigated. The liquid water behavior at the cathode is also visualized using an optical diagnostic, and the effects of operating conditions and GDL structures on the water transport in the cathode GDE are discussed. It is found that the liquid water content in the cathode GDE increases remarkably after starting the fuel cell operation due to the water production at the CL. At a high current density, the cell voltage drops suddenly after starting the operation in spite of a low water content in the cathode GDE. When the GDL thickness is increased, much water accumulates near the cathode CL and the fuel cell shuts down immediately after the operation. In the final section of this paper, the structure of cathode GDL that has several grooves for water removal is proposed to prevent water flooding and improve fuel cell performance. This groove structure is effective to promote the removal of the liquid water accumulated near the active catalyst sites.     

Microporous layer for water morphology control in PEMFC

We have used environmental scanning electron microscope to observe vapor condensation and liquid water morphology and breakthrough in porous layers of polymer electrolyte membrane fuel cell. These suggest presence of large droplets and high liquid saturation at interface of the catalyst layer (CL) and gas diffusion layer (GDL), due to jump in pore size. We develop a model for morphology of liquid phase across multiple porous layers by use of both continuum and breakthrough (percolation) treatments. Using the results of this model we show the liquid morphologies deteriorate the efficiency of electrochemical reactions in CL and increase the water saturation in GDL. Then we show that inserting a microporous layer between CL and GDL reduces both the droplet size and liquid saturation and improves the cell performance.     

Pore scale modeling of a proton exchange membrane fuel cell catalyst layer: Effects of water vapor and temperature

A pore scale model of a polymer electrolyte membrane (PEM) fuel cell cathode catalyst layer is developed which accounts for species transport, electrochemical reactions and thermal transport. Effective transport parameters are computed over a range of operating conditions including the effective oxygen diffusivity, effective water vapor diffusivity, effective proton conductivity, effective electron conductivity and the effective thermal conductivity. In addition, the total amount of oxygen consumption is computed for different operating conditions. Finally, a critical assessment of the impact of assumptions made in the absence of detailed morphological data is presented.     

Analyzing oxygen transport resistance and Pt particle growth effect in the cathode catalyst layer of polymer electrolyte fuel cells

The oxygen transport resistance in the cathode catalyst layer (CL) of polymer electrolyte fuel cells (PEFCs) has been reported to be significantly higher than expected, especially when the platinum (Pt) loading is low and/or the degree of CL degradation is severe. In this paper, the oxygen transport resistance behavior in the cathode CL is numerically analyzed under various CL design and operating conditions. Particular emphasis is placed on the aged CL wherein Pt particle growth and active Pt surface area loss are observed. For this study, a previously developed micro-scale catalyst model is improved upon to account for Pt particle size. The new model includes calculations of catalyst activity and electrochemically active surface areas, as well as various transport resistances through the ionomer and liquid films. After coupling the micro-scale CL model with a three-dimensional PEFC model, multi-scale simulations are carried out under various PEFC catalyst designs (varying Pt loading, ionomer fraction, oxygen permeation rate through the ionomer film) and operating conditions (drying or flooding of the electrode, high or lower current density). The simulation results agree well with experimental oxygen transport resistance data and further indicate that CL design with low Pt loading is more susceptible to degradation. Providing extensive multi-dimensional contours of species concentration, temperature, and current density inside the PEFC, this study provides a comprehensive understanding of oxygen transport resistance in the cathode CL in different PEFC situations.     

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.     

Two-phase transport in the cathode gas diffusion layer of PEM fuel cell with a gradient in porosity

Two-phase transport in the cathode gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC) is studied with a porosity gradient in the GDL. The porosity gradient is formed by adding micro-porous layers (MPL) with different carbon loadings on the catalyst layer side and on the flow field side. The multiphase mixture model is employed and a direct numerical procedure is used to analyze the profiles of liquid water saturation and oxygen concentration across the GDL as well as the resulting activation and concentration losses. The results show that a gradient in porosity will benefit the removal rate of liquid water and also enhance the transport of oxygen through the cathode GDL. The present study provides a theoretical support for the suggestion that a GDL with porosity gradient will improve the cell performance.     

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

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

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

Review of gas diffusion layer for proton exchange membrane-based technologies with a focus on unitised regenerative fuel cells

Proton exchange membrane (PEM) based technologies (fuel cells and electrolysers) offer promising sustainable power generation and storage solutions for a diverse range of stationary and mobile applications. Unitised regenerative fuel cell (URFC) is an electrochemical cell that can operate both as a fuel cell (FC) and an electrolyser (E). However, for a widespread commercialisation, further improvements are required that address the durability, performance, and cost limitations. One of the main challenging components in developing URFCs is the gas diffusion layer (GDL) as it plays different vital roles, some of which are paradoxical in FC and E-modes. Therefore, in this paper, the published research on GDL of PEM-URFCs as well as relevant studies on PEM fuel cells and electrolysers are critically reviewed. The materials and novel methods to address the corrosion in E-mode are discussed. This is followed by presenting and discussing different properties of GDLs affecting the performance in FC and E-modes: i.e. porosity, thickness, pore size, transport properties, thermal and electrical conductivity, and the GDL compressibility. Finally, the main modifications of the GDLs, such as hydrophobisation and microporous layer application, to improve the performance of a URFC are analysed and discussed.     

A pore network study on the role of micro-porous layer in control of liquid water distribution in gas diffusion layer

In proton exchange membrane fuel cell (PEMFC), a hydrophobic micro-porous layer (MPL) is usually placed between catalyst layer (CL) and gas diffusion layer (GDL) to reduce flooding. Recent experimental studies have demonstrated that liquid water saturation in GDL is drastically decreased in the presence of MPL. However, theoretical studies based on traditional continuum two-phase flow models suggest that MPL has no effect on liquid water distribution in GDL. In the present study, a pore network model with invasion percolation algorithm is developed and used to investigate the impacts of the presence of MPL on liquid water distribution in GDL from the viewpoint at the pore level. A uniform pressure and uniform flux boundary conditions are considered for liquid water entering the porous layer in PEMFC. The simulation results reveal that liquid water saturation in GDL is reduced in the presence of MPL, but the reduction depends on the condition of liquid water entering the porous layer in PEMFC.     

Diffusion parameter correlations for PEFC gas diffusion layers considering the presence of a water-droplet

A polymer electrolyte fuel cell (PEFC) produces electrical energy according to the electrochemical reactions carried out inside the cell. During the energy conversion, water molecules are also produced at the cathode side, which affects the gas diffusion layer (GDL) diffusion parameters. The generated water-drops from the reaction may give partial or total blockage of the reactant gases and also material oxidation. The mentioned phenomena influence the performance of the PEFCs. This paper aims to describe and quantify the impact on diffusion parameters of GDLs when the size of the formed water-drops inside the layer is varying. This study considers digitally generated GDLs, in which the porosity, gas-phase tortuosity and diffusibility are studied. The fluid flow behavior through the three-dimensional porous domain representing the GDL is obtained with the lattice Boltzmann method (LBM). Depending on the water-drop size, the impact of the mentioned parameters can be computed. For the current study a spherical water-drop whose radius varies between the 15 and 35% of the size of the domain was considered. The studied parameters showed a dependency of the water-drop radius, each changing independently and several correlations to predict the behavior of the mentioned diffusion transport parameters are proposed.     

A parametric study of the cathode catalyst layer of PEM fuel cells using a pseudo-homogeneous model

A pseudo-homogeneous model for the cathode catalyst layer performance in PEM fuel cells is derived from a basic mass–current balance by the control volume approach. The model considers kinetics of oxygen reduction at the catalyst/electrolyte interface, proton transport through the polymer electrolyte and oxygen diffusion through porous media. The governing equations, a two-point boundary problem, are solved using a relaxation method. The numerical results compare well with our experimental data. Using the model, influences of various parameters such as overpotential, proton conductivity, catalyst layer porosity, and catalyst surface area on the performance of catalyst layer are quantitatively studied. Based on these results, cathode catalyst layer design parameters can be optimized for specified working conditions.     

Investigation of two-phase flow in the compressed gas diffusion layer microstructures

This study aims to investigate how multiple parameters affect the two-phase flow in compressed gas diffusion layer (GDL). A stochastic model is adopted to reconstruct the GDL microstructures. Solid mechanics simulations on the reconstructed GDL microstructures are performed, based on the finite element method (FEM). Various pore morphologies and distributions of compressed GDLs are observed. Two-phase flow in GDL is simulated using a volume of fluid (VOF) model. Corner droplet (on the GDL surface) and water flow (emerging from GDL bottom) are considered. It is found that two-phase flow in the GDL is highly influenced by compression, fiber diameter, porosity, and GDL thickness. The results indicate that a larger fiber diameter or higher porosity contributes to the water transport due to larger average pore size. Furthermore, water removal from a thicker GDL is more difficult, whereas water transport in the lower part of a compressed thick GDL is easy.     

PEMFC电极孔隙率的优化研究

对质子交换膜燃料电池单体建立了三维稳态电化学模型,考察了气体扩散层孔隙率对电池性能的影响,验证了扩散层孔隙率及层厚的变化反映从气体通道到扩散层和催化剂层的反应气体扩散量,进而影响电化学反应的活跃程度;以膜与阴极催化剂层界面处获得的最大电压为目标函数,采用鲍威尔搜索法对气体扩散层孔隙率进行数值优化,得到了扩散层孔隙率和层厚的最优值。通过优化前后氧气浓度和电流密度的对比显示,这些参数可以显著改善电极的传质性能,使燃料电池获得最佳性能。     

Predictions of phase temperatures in a porous cathode of polymer electrolyte fuel cells using a two-equation model

In this study, both solid-phase and fluid-phase temperatures inside a porous cathode of a polymer electrolyte fuel cell in contact with an interdigitated gas distributor are predicted numerically. The porous cathode consists of a catalyst layer and a diffusion layer. The heat transfer in the catalyst layer is coupled with species transports via a macroscopic electrochemical model. On the other hand, in the diffusion layer, the energy equations based on the local thermal non-equilibrium (LTNE) are derived to resolve the temperature difference between the solid phase and the fluid phase. As for the species transports, the Bruggemann model is employed to describe the effective diffusivities of the oxygen and water vapor in the porous cathode. Results show that the wall temperature decreases with increasing the intrinsic heat transfer coefficient. As the intrinsic heat transfer coefficients increase, the porous electrode becomes local thermal equilibrium with a strong thermal interaction (heat transfer) between the solid and fluid phases. Under the conditions of high intrinsic heat transfer coefficients, the temperature difference between the solid matrices and the reactant fluids are negligible.