Modelling of coupled electron and mass transport in anisotropic proton-exchange membrane fuel cell electrodes

As one of the key components of proton-exchange membrane fuel cells, the gas-diffusion layer (GDL) that is made of carbon fibres usually exhibits strong structural anisotropy. Nevertheless, the GDL has traditionally been simplified as a homogeneous porous structure in modelling the transport of species through the GDL. In this work, a coupled electron and two-phase mass transport model for anisotropic GDLs is developed. The effects of anisotropic GDL transport properties due to the inherent anisotropic carbon fibres and caused by GDL deformations are studied. Results indicate that the inherent structural anisotropy of the GDL significantly influences the local distribution of both cathode potential and current density. Simulation results further indicate that a GDL with deformation results in an increase in the concentration polarization due to the increased mass-transfer resistance in the deformed GDL. On the other hand, the ohmic polarization is found to be smaller in the deformed GDL as the result of the decreased interfacial contact resistance and electronic resistance in the GDL. This result implies that an optimum deformation needs to be achieved so that both concentration and ohmic losses can be minimized.     

Investigation of the effects of the anisotropy of gas-diffusion layers on heat and water transport in polymer electrolyte fuel cells

The aim of this work is to study the effects of gas-diffusion layer (GDL) anisotropy and the spatial variation of contact resistance between GDLs and catalyst layers (CLs) on water and heat transfer in polymer electrolyte fuel cells (PEFCs). A three-dimensional, two-phase, numerical PEFC model is employed to capture the transport phenomena inside the cell. The model is applied to a two-dimensional cross-sectional PEFC geometry with regard to the in-plane and through-plane directions. A parametric study is carried out to explore the effects of key parameters, such as through-plane and in-plane GDL thermal conductivities, operating current densities, and electronic and thermal contact resistances. The simulation results clearly demonstrate that GDL anisotropy and the spatial variation of GDL/CL contact resistance have a strong impact on thermal and two-phase transport characteristics in a PEFC by significantly altering the temperature, water and membrane current density distributions, as well as overall cell performance. This study contributes to the identification of optimum water and thermal management strategies of a PEFC based on realistic anisotropic GDL and contact-resistance variation inside a cell.     

Effect of deformation on electrical properties of carbon fibers used in gas diffusion layer of proton exchange membrane fuel cells

In proton exchange membrane fuel cells, the stacks of anode, cathode, and membrane layers including gas diffusion layer (GDL) are held together by a compressive force applied through a bipolar plate. In this work, we studied the electrical properties of a carbon fiber of a GDL under deformation using four-point measurement methods inside a scanning electron microscope (SEM). We found out that through bending deformation the electrical resistivity of carbon fibers will be reduced. The drop in resistance during deformation may be the result of increasing conduction channels in the carbon fiber and parallel transport through them. Our finding offers a new insight on the effect of deformation on tuning the electrical properties of GDL materials.     

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.     

Numerical investigation on the sensitivity of endplate design and gas diffusion material models in quantifying localized interface and bulk electrical resistance

A localized non-intuitive relationship between electrical interface contact resistance and bulk properties such as bulk electrical resistance and permeability in the fuel cell gas diffusion layer (GDL) is reported. A numerical method is adopted to investigate contact pressure and hence the interface contact resistance at the interfaces of bipolar plate (BPP)|GDL and GDL|Polymer electrolyte membrane (PEM). The results are observed to be sensitive to GDL material models as well as endplate designs. This means, endplates designed to improve the electrical contact resistance or contact pressure at the BPP|GDL interface may not necessarily assure an improvement in bulk properties, in fact, it is observed in this study that these properties are inversely related. Further, a differential deformation in GDL along with consolidation effect is predicted with compressible version of hyperelastic material model. More importantly, it is revealed that the selection of material models plays a significant role in the deformation behaviour of the GDLs irrespective of the clamping design adopted.     

Influence of clamping force on the performance of PEMFCs

The objective of the present paper is to investigate the effect of clamping force on the performance of proton exchange membrane fuel cell (PEMFC) with interdigitated gas distributors considering the interfacial contact resistance, the non-uniform porosity distribution of the gas diffusion layer (GDL) and the GDL deformation. The clamping force between the GDL and the bipolar plate is one of the important factors to affect the performance and efficiency of the fuel cell system. It directly affects the structure deformation of the GDL and the interfacial contact electrical resistance, and in turn influences the reactant transport and Ohmic overpotential in the GDL. Finite element method and finite volume method are used, respectively, to study the elastic deformation of the GDL and the mass transport of the reactants and products. It is found that there exists an optimal clamping force to obtain the highest power density for the PEMFC with the interdigitated gas distributors.     

Effect of the cathode gas diffusion layer on the water transport behavior and the performance of passive direct methanol fuel cells operating with neat methanol

The passive operation of a direct methanol fuel cell with neat methanol requires the water that is produced at the cathode to diffuse through the membrane to the anode to compensate the methanol oxidation reaction (MOR). Hence, the anode performance of this type of fuel cell can be limited by the water transport rate from the cathode to the anode. In this work we theoretically show that the water transport from the cathode to the anode depends primarily on the design of the cathode gas diffusion layer (GDL). We investigate experimentally the effects of the design parameters of the cathode GDL, including the PTFE (polytetrafluoroethylene) content in the backing layer (BL), and the carbon loading and the PTFE content in the microporous layer (MPL) on the water transport and the performance of the passive DMFC with the help of a reference electrode. The results indicate that on one hand, these parameters can be adjusted to decrease the water concentration loss of the anode performance, but on the other hand, they can also cause an increase in the oxygen concentration loss of the cathode performance. Hence, an optimal balance in minimizing the both concentration losses is the key to maximize the cell performance.     

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

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

Numerical study of the effect of the GDL structure on water crossover in a direct methanol fuel cell

A two-dimensional two-phase non-isothermal mass transport model is developed to numerically investigate the behavior of water transport through the membrane electrode assembly (MEA) of a direct methanol fuel cell. The model enables the visualization of the distribution of the liquid saturation through the MEA and the analysis of the distinct effects of the three water transport mechanisms: diffusion, convection and electro-osmotic drag, on the water-crossover flux through the membrane. A parametric study is then performed to examine the effects of the structure design of the gas diffusion layer (GDL) on water crossover. The results indicate that the flow-channel rib coverage on the GDL surface and the deformation of the GDL can cause an uneven distribution of the water-crossover flux along the in-plane direction, especially at higher current densities. It is also found that both the contact angle and the permeability of the cathode GDL can significantly influence the water-crossover flux. The water-crossover flux can be reduced by improving the hydrophobicity of the cathode GDL.     

Effects of the electrical resistances of the GDL in a PEM fuel cell

In most PEM fuel cell models, the electrical resistance of the gas diffusion layers (GDL) is neglected under the assumptions that the GDL electrical conductivity is orders of magnitude higher than the ionic conductivity of the membrane. Recently some modeling efforts have taken the effects of electrical resistance of the GDL into consideration [H. Meng, C.Y. Wang, Electron transport in PEFCs, J. Electrochem. Soc. 151 (2004) A358–A367; B.R. Sivertsen, N. Djilali, CFD-based modeling of proton exchange membrane fuel cells, J. Power Sources 141 (2005) 65–78] and some of the results showed that under certain conditions, this effect was significant enough to alter the characteristics of current density distributions under the gas channels and the land areas. If these results are applicable to real-life fuel cells, the present design criteria and optimization procedures must be significantly changed to incorporate the effect of GDL electrical resistance. To examine this issue closely, a three-dimensional fuel cell model incorporating electron transport in the GDL is developed and employed to investigate the effect of electrical resistance through the GDL. In this model, the anisotropic nature of the GDL is taken into consideration by using different electrical conductivities in the through-plane and in-plane directions. The modeling results show that when realistic electrical conductivities for the GDL are used, the effect of the electrical resistance of GDL is slight and can be neglected for all industrial applications. It is believed that the over-estimations of the GDL resistance were mainly caused by neglecting the anisotropic nature of the GDL and/or lumping the contact resistance indiscriminately into the GDL, thus overestimating the electrical resistance of the GDL in the in-plane direction. Besides taking into consideration of the electrical resistance of GDL, the present model also take into consideration of the electron transport in the catalyst layers. When realistic values of electrical conductivities are used for both the GDL and catalyst layers, there is no significant change in the characteristics of current density distribution across the land and channel.     

Numerical study on the compression effect of gas diffusion layer on PEMFC performance

A numerical method is developed to study the effect of the compression deformation of the gas diffusion layer (GDL) on the performance of the proton exchange membrane fuel cell (PEMFC). The GDL compression deformation, caused by the clamping force, plays an important role in controlling the performance of PEMFC since the compression deformation affects the contact resistance, the GDL porosity distribution, and the cross-section area of the gas channel. In the present paper, finite element method (FEM) is used to first analyze the ohmic contact resistance between the bipolar plate and the GDL, the GDL deformation, and the GDL porosity distribution. Then, finite volume method is used to analyze the transport of the reactants and products. We investigate the effects of the GDL compression deformation, the ohmic contact resistivity, the air relative humidity, and the thickness of the catalyst layer (CL) on the performance of the PEMFC. The numerical results show that the fuel cell performance decreases with increasing compression deformation if the contact resistance is negligible, but an optimal compression deformation exists if the contact resistance is considerable.     

An integrated model of the water transport in nonuniform compressed gas diffusion layers for PEMFC

Water transport in gas diffusion layer (GDL) is a very important issue for high power density Proton Exchange Membrane Fuel Cell (PEMFC). During the GDL and bipolar plate (BPP) assembly process, the water transport behavior is greatly influenced by the nonuniform compression on the GDL, which leads to uneven distribution of the internal mass transport pores. In this study, an integrated model is developed to predict the water transport in nonuniform compressed GDL. Firstly, a GDL compression deformation model is built to obtain the relationship between the GDL deformation and assembly clamping force based on energy method. Then, a water transport model is established by considering the probability density function (PDF) of the pore size for the compressed GDL. The accuracy of the integrated model has been verified by comparing with the finite element method (FEM) and the computational fluid dynamics (CFD) simulation results. The influence of assembly clamping force, GDL thickness and channel geometry are analyzed based on the integrated model. Drainage pressure increases monotonically with the assembly clamping force and is divided into three stages. For the baseline case, 0.2 mm of GDL thickness and small rib-channel ratio is conducive to improving drainage capacity. It provides the guidance for matching of GDL/BPP assembly condition and performance prediction of PEMFC.     

The influence of anode gas diffusion layer on the performance of low-temperature DMFC

The influence of the anode gas diffusion layers (GDLs) on the performances of low-temperature DMFCs, and the properties of mass transport and CO₂ removal on these anode GDLs were investigated. The membrane electrode assembly (MEA) based on the hydrophilic anode GDL, which consisted of the untreated carbon paper and hydrophilic anode micro-porous layer (comprised carbon black and 10 wt.% Nafion), showed the highest power density of 13.4 mW cm⁻² at 30 °C and ambient pressure. The performances of the MEAs tended to decline with the increase of the PTFE content in the anode GDLs due to the difficulty of methanol transport. The contact angle measurements revealed that the wettabilities of the anode GDLs decreased as the increase of PTFE content. The wettabilities of the GDLs were improved by addition of hydrophilic Nafion ionomer to the GDLs. From the visualizations of CO₂ gas bubbles dynamics on the anodes using a transparent cell, it was observed that uniform CO₂ gas bubbles with smaller size formed on hydrophilic anode GDLs. And bubbles with larger size were not uniform over the hydrophobic anode GDLs. It was believed that adding PTFE to the anode GDL was not helpful for improving the CO₂ gas transport in the anode GDL of the low-temperature DMFC.     

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.     

Modeling the Influence of GDL and flow-field plate parameters on the reaction distribution in the PEMFC cathode catalyst layer

In this paper, a two-dimensional cross-the-channel model was applied to investigate the influence of gas diffusion layer (GDL) property and flow-field geometry on the local reaction rate in the PEMFC cathode catalyst layer. The model predictions show that the rate of consumption of oxygen or current density under the land area may differ considerably from that under the channel. Simulation results indicate that for a fixed channel width, increasing the channel-to-land width ratio results in improved water transport and positively impacts the overall reaction rates at high overpotential. However, too high ratio may retard electron transport and thus lead to worse cathode performance. Numerical simulation results revealed that GDL electrical conductivity and thickness have a strong influence on how the chemical species and electrons are transported to the active catalyst sites. Depending on the electrical conductivity of GDL, the region of higher reaction rate may occur either under the land or under the channel. Consideration of orthotropic electrical conductivity of GDL affects the simulation results significantly highlighting the need to improve our understanding of GDL transport coefficients. Simulation of GDL compression effects showed that the total current density is not affected significantly but current density distribution is. The extent of reactant bypass through the GDL from one channel to the other depends on the GDL permeability and results in a significant enhancement in reaction rates.     

Numerical modeling of the mechanical behavior of proton exchange membrane fuel cell performance: Design of experiment study and optimization

The fundamental magnitude which can be associated with the performance of a fuel cell is the contact pressure. The contact pressure sustained by the GDL will directly impact the electrical performance of the fuel cell, in particular the contact resistance. This contact resistance can be modeled in two different ways: analytically from the mechanical model and electrically from a mechanical - electrical coupling model. We opted for the analytical resolution. The contact resistance was calculated analytically, based on the mechanical model. Note that the contact resistance is influenced by several mechanical parameters such as the clamping pressure, the porosity of the GDL and the dimensions of two components GDL and BPP. This porosity decreases during compression in order to make waterproofing. In our study, to model the porosity of the GDL, two approaches were presented: pore network approach (used in the case of low porosity) and continuous approach (used in the case of high porosity). It is necessary to quantify and verify the influence of three factors: the porosity of the GDL, the bending radius of the bipolar plate and the thickness of the GDL on the contact pressures. To do this, we conducted two experimental plans on the stack: one corresponding to low porosity and the other one to high porosity. The optimal parameters having been identified, we found a good correlation between the numerical results and the experimental results found in the bibliography.     

Mass balance research for high electrochemical performance direct methanol fuel cells with reduced methanol crossover at various operating conditions

Mass balance research in direct methanol fuel cells (DMFCs) provides a more practical method in characterizing the mass transport phenomena in a membrane electrode assembly (MEA). This method can be used to measure methanol utilization efficiency, water transport coefficient (WTC), and methanol to electricity conversion rate of a MEA in DMFCs. First, the vital design parameters of a MEA are recognized for achieving high methanol utilization efficiency with increased power density. In particular, the structural adjustment of anode diffusion layer by adding microporous layer (MPL) is a very effective way to decrease WTC with reduced methanol crossover due to the mass transfer limitation in the anode. On the other hand, the cathode MPL in the MEA design can contribute in decreasing methanol crossover. The change of structure of cathode diffusion layer is also found to be a very effective way in improving power density. In contrast, the WTC of DMFC MEAs remains virtually constant in the range of 3.4 and 3.6 irrespective of the change of the cathode GDL. The influence of operating condition on the methanol utilization efficiency, WTC, and methanol to electricity conversion rate is also presented and it is found that these mass balance properties are strongly affected by temperature, current density, methanol concentration, and the stoichiometry of fuel and air.     

Bubble behaviors in direct methanol fuel cell anode with parallel design

In this paper, the Volume of Fluid (VOF) method for tracking the gas-liquid interface is employed to investigate the carbon dioxide (CO₂) behaviors inside the anode of a direct methanol fuel cell (DMFC). The CO₂ bubble emergence processes from the catalyst layer (CL) to the gas diffusion layer (GDL) and then to the flow channels are studied with two different strategies. In the first strategy, the CL and GDL are modeled as a uniform porous layer; in the second strategy, they are modeled as a well-ordered-path GDL and a uniform CL. The simulation results show that the second modeling strategy can better capture and match the fundamental phenomena of CO₂ bubble formation and evolvement observed from the experiments inside a DMFC anode.     

Contact resistance prediction and structure optimization of bipolar plates

The objective of this work is to investigate the effect of clamping force on the interfacial contact resistance and the porosity of the gas diffusion layer (GDL) in a proton exchange membrane fuel cell (PEMFC). An optimal rib shape for the bipolar plate is developed to analyze the electrical contact resistance. We found that the electrical contact resistance is determined by both the clamping force and the contact pressure distribution. A minimum contact resistance can be obtained in the case of a constant contact pressure distribution. The porosity of the GDLs underneath the rib of the bipolar plate decreases with increasing the clamping force, and the void volume is changed with the deformation of the GDLs. It is found that there exists an optimal rib width of the bipolar plates to obtain a reasonable combination of low interfacial contact resistance and good porosity for the GDL.