Permeability Of Gas Diffusion Layer Research Articles

Effect of through-plane and in-plane permeability of gas diffusion layers on the performance of PEM fuel cell

A three-dimensional model with a complete set of governing equations for all components of the PEM fuel cell is developed to investigate the permeability effects on the transport of reactive species and the flooding phenomenon in PEM (Proton Exchange Membrane) fuel cell. This model couple fluid flow distribution, species, electric potential, electric current density, and saturation level in the flow channel, gas diffusion porous layer, catalyst layer, and fuel cell polymer membrane. The electrodes and channels are modeled as two-phase. The effects of isotropic and non-isotropic permeability have been studied and the simulation results show that in order to obtain higher efficiency, the permeability of the gas diffusion layer should be high in the flow direction(in-plane) and perpendicular to the main flow (through plane). High permeability in the direction of the main flow and Low permeability in the direction perpendicular to the main flow will also have a high yield. The current study shows that by optimizing the permeability of the gas diffusion layer, a higher efficiency can be achieved in the fuel cell.

Methodology of the PEM FC Gas Diffusion Layer Permeability Determination and Its Description Related to the Fuel Cell Flow Field Design

Hydrogen fuel cells represent an important part of the new energy scheme for low emission society. Development of the more efficient, durable and energy dense systems requires precise information on the numerous aspects, especially on the local distribution of physio-chemical quantities inside the fuel cells stacks. It is extremely difficult to obtain such information experimentally. Therefore, the mathematical modelling represents a feasible alternative. It offers an efficient and versatile way for the acquisition of required information. Accuracy of mathematical model, however, is primarily determined by the accuracy and reliability of the experimental data available. In this particular case, permeability of gas diffusion layer for reactants and products is of interest. Permeability values in porous environment are available experimentally. It is, however, important to keep in mind that permeability value is strongly dependent on the material porosity and thus on a degree of its compression inside the stack. Therefore, permeability value is varying with position over the electrode. Suitable approach, making possible to describe smoothly variation of the gas diffusion layer permeability with degree of its compression, is desirable. Traditional approach is to use corresponding semi-empirical equations. It is a purpose of this work to verify applicability of the Kozeny-Carman equation for this particular case and thus to fill in this gap in the current knowledge.Prior the experimental validation of the Kozeny-Carman equation accuracy, methodology of obtaining sufficiently accurate permeability data needs to be developed. In the first step of this process, permeability cells were designed, characterized by the homogeneous gas flow distribution in the channel, used for positioning of the gas diffusion layer sample, to be characterized. A special attention was paid to the absence of any turbulence domains close to the pressure reading ports, which could cause potential error in the observed pressure values. The figure provided documents results of the cell geometry optimization. Mathematical model calculated gas stream lines shown document fulfilling above stated conditions and thus ensuring reliability of the experimental data obtained. The cell lid determined the channel height itself. A series of lids was produced, allowing study of gas diffusion layers permeability at the different degrees of compression, i.e. porosities. The channel depth accuracy was required to be within tolerance of 2 mm. Pressure drop on the channel was determined by varying the streaming gas flow rate. Hydrogen and nitrogen (simulating air) were used as a test gasses. Permeability of gas diffusion layers were determined based on pressure drops using Darcy’s law. This set of data allowed comparing experimental data with Kozeny-Carman equation behavior. The agreement obtained was sufficient to justify the usage of this type of semi-empirical equation for the flow field mathematical modelling. Corresponding conclusion was in the last step of the study verified by means of the model serpentine flow field attached at the gas diffusion layer at the different degrees of compression. Experimental data obtained have shown very good agreement with the model calculations. It can thus be concluded, that Kozeny-Carman provides sufficient approximation of the gas diffusion permeability values to be used in the mathematical models for design of fuel cells and fuel cells stacks for geometry optimization.The work was supported from European Regional Development Fund-Project "Fuel Cells with Low Platinum Content" (No. CZ.02.1.01/0.0/0.0/16_025/0007414). Figure 1

Gas Diffusion Layer: The Critical Player in Gases Distribution in the Proton Exchange Membrane Fuel Cell

Within the last decades, fuel cells have become the center of attention of research community, industry, and the general public. This is due to the emphasis on the sustainability of the society and the emerging concept of a hydrogen economy. Proton exchange membrane fuel cells (PEM FCs) represent a mature and established fuel cell technology. The flexibility of operation, the high degree of fuel utilization, and the output power density make them suitable for mobile applications such as hydrogen-fueled vehicles or on-site energy generation. Membrane electrode assembly (MEA) is the mainstay of the PEM FCs. The MEA consists of the proton exchange membrane, catalytic layers, and gas diffusion layers (GDLs). Single MEAs are interconnected by bipolar plates with flow field channels homogenously distributing reactants and realizing easy removal of the generated water. Individual cells are compressed together, forming the fuel cell stack. The GDL plays a three-fold role in the performance of PEM fuel cells. It has to offer good electrical conductivity, ensure good reactant distribution along the reaction zone, and facilitate the removal of the water produced by the fuel cell operation. The optimal combination of these three aspects of GDL allows the FC to achieve high performance. Furthermore, the homogeneous distribution of the reactants is crucial for the durability and lifespan of the FC. The inhomogeneous reactants distribution leads to a local starvation of the electrode, leading to high local overvoltage and thus shortening the lifespan of overloaded parts. Although GDLs play a crucial role in the operation of PEM fuel cells, so far, they have received little attention. Hence, the objective of our study is to compare commercially available GDLs with respect to their properties, namely permeability under conditions close to the operating fuel cell.The approach of combining advanced mathematical modeling with extensive experimental characterization was used to achieve this task. Testing cells allowing precise determination of the in-plane permeability of various GDLs were designed based on the computational fluid dynamics modeling. According to the contemporary literature, two different carbon based GDLs were examined, namely carbon-cloth and carbon-paper, which principally differ in the carbon-fiber orientation in their internal structure. The investigated properties of GDLs thus include thickness, porosity, internal structure, and the presence or absence of microporous layer.The achieved experimental results show that at given compression, thicker carbon paper GDLs are superior to carbon cloth samples in terms of permeability and deformation resistance. The obtained permeability values of GLDs at different degrees of compression were compared with predictions based on the Carman-Kozeny equation to assess the accuracy of this semi-empirical prediction. Finally, the importance of the optimal flow field and GDL combination in terms of reactant distribution in the fuel cell stack of industrial conditions is documented via mathematical modelling based on the obtained experimental data.This work was supported by the European Regional Development Fund-Project “Fuel Cells with Low Platinum Content” (No. CZ.02.1.01/0.0/0.0/16_025/0007414).

Water management of the proton exchange membrane fuel cells: Optimizing the effect of microstructural properties on the gas diffusion layer liquid removal

The formation of water columns inside the gas diffusion layer (GDL) of the proton exchange membrane fuel cell (PEMFC), which is harmful phenomenon, can be controlled by the GDL's microstructure and material. Using computational fluid dynamics (CFD), a three-dimensional model is developed to monitor the impacts of the GDL's porosity and permeability on the maximum GDL liquid removal. In this regard, twenty-four different cases are simulated at the GDL contact angle of 110°. Results indicate that higher permeabilities and porosities improve the GDL liquid removal and the performance of the system. Obtaining the simulation data, an artificial neural network (ANN) model is trained at the current density of 0.41 A/ cm2 and the voltage of 0.6 V to predict the maximum GDL liquid removal in 300000 points and to perform the optimization. The ANN model is trained with four neurons with the respective mean squared error values 6.32422e-6, 1.00637e-5, and 4.12086e-6 for the training, validation, and testing, which approves the accuracy of the model. Using a fitted curve and the ANN model, the optimum values of the porosity and the permeability are computed to be 0.9 and 1.481e-11 (m2), respectively, to reach the maximum GDL liquid removal of 0.373 (kg/m3s).

High-density and low-density gas diffusion layers for proton exchange membrane fuel cells: Comparison of mechanical and transport properties

Gas diffusion layers (GDL) play multi-roles in proton exchange membrane fuel cells, including gas-water transport, thermal-electron conduction and mechanical support. Mechanical strength and transport properties are essential for GDLs. In this work, high-density (paper-type) and low density (felt-type) GDLs are scanned and reconstructed using X-ray computed tomography. Porosities under different compression ratios are compared and discussed. Effective diffusivity and liquid water permeability are calculated using pore-scale modeling and lattice Boltzmann method. Mechanical strength, anisotropic thermal-electrical resistivity for two types of GDLs are obtained using compression tests and thermal-electrical conductivity measurements. Results show that the porosity, diffusivity, permeability, and through-plane thermal-electrical conductivity of felt-type GDL are significantly higher than that of paper-type GDL owing to the higher porosity and fiber-clusters oriented along the through-plane direction. The in-plane electrical resistivity of paper-type GDL is lower than that of felt-type GDL. The mechanical strength of felt-type GDL is much lower, but the fibers of paper-type GDL are more easily to be broken because of its lower elasticity. The results obtained may guide microstructure optimization and performance improvement of GDLs.

Effects of high-temperature operating conditions on the through-plane gas permeability of gas diffusion layers used in PEFCs

In order to simulate the effects of high-temperature operating conditions on the through-plane gas permeability of gas diffusion layers (GDLs) used in polymer electrolyte fuel cells, uncoated and coated GDLs were heated at various temperatures (i.e. 200, 500 and 800 °C). The results show that the through-plane gas permeability of the uncoated GDLs generally increases after higher temperature treatment. However, the coated GDL displays a different trend: the through-plane gas permeability increases with increasing temperature treatment to 200 and 500 °C, but then decreases after heat treatment at 800 and 1000 °C. With the assistance of SEM images, the above results are discussed.

Effect of composition and structure of gas diffusion layer and microporous layer on the through‐plane gas permeability of PEFC porous media

The through-plane permeability of the gas diffusion media (GDM) is investigated experimentally with regard to the microporous layer (MPL) composition and the gas diffusion layer (GDL) composition and structure. The MPL composition is held constant at 80% carbon powder and 20% polytetrafluorethylene (PTFE) (by weight) for various carbon loadings. The decrease or increase in GDM permeability was found to be dependent on the structure of the GDL used in conjunction with the type of carbon powder. It was found that low-surface-area carbon powder (Vulcan XC-72R) forms thin, dense MPLs with small cracks when compared to high-surface-area powder (Ketjenblack EC-300J), which creates thick, rough MPLs with large cracks with increased carbon loadings. For most cases, the permeability decreases with increasing carbon loading; however, the non-woven, straight fibre carbon papers using Ketjenblack EC-300J show the lowest permeability at the lowest carbon loading. Furthermore, the percentage reduction from the GDL substrate permeability appears to be predictable for similar structures with increasing carbon loading. The increase in PTFE loading from 10% to 30% in the GDL was shown to have a significant impact on the percentage reduction from the original GDL permeability of ~9% to 15% for the GDM composed of Vulcan XC-72R as carbon powder; however, such effects are insignificant when using Ketjenblack EC-300J carbon powder.

(Invited) Multi-Scale Analysis of Transport in Dry and Partially-Saturated Porous Media

Improvements in imaging techniques, such as micro- and nano-computer tomography (CT) and focus ion beam scanning electron microscopy (FIB-SEM), have enabled the reconstruction of complex porous media that can then be analyzed by computer simulation to estimate effective transport properties, e.g., ref. [1, 2]. Experimental setups have also been developed to measure dry and partially-saturated gas diffusivity and permeability of gas diffusion layers (GDLs) and catalyst layer (CLs), e.g., ref. [3, 4]. Based on the available literature, a comparison between micro-scale simulation results and experimental data must be performed to assess the accuracy of direct numerical simulation CT image analysis for dry and partially-saturated transport property estimation. In the later case, the validity of computational methods to predict water intrusion, such as full morphology/morphological image opening [1] and watershed segmentation [5], should also be assessed by comparing mercury intrusion experiments to numerical predictions, and by direct comparison of numerical results to CT images. Finally, these transport properties should be integrated into complete fuel cell models where volume-averaging techniques are used to properly estimate channel-porous media interactions. Such interactions however are seldom studied in literature.The proposed contribution aims at first studying the validity of two popular methods for numerical estimation of effective transport properties from three-dimensional reconstructed porous media, i.e., direct numerical simulation (DNS) and pore network modeling (PNM). Two fuel cell gas diffusion media and an electrolyzer porous transport layer are analyzed by CT and characterized by measuring mercury intrusion porosimetry (MIP), and dry permeability and diffusivity. A comparison of numerical and experimental results shows that DNS tools in OpenFCST [6] are capable of accurately predicting intrusion, and transport properties without using any fitting parameters. Accurate predictions are also achieved with the PNM implementation in OpenPNM [7] when the inscribed diameter method is used to estimate the pore size distribution, and the equivalent diameter is used to estimate pore transport properties. These tools open an avenue for computational design of porous materials.Effective transport properties must be integrated into volume-averaged fuel cell models where porous media-channel interactions are of paramount importance. These interactions however, as well as the method used for volume averaging fuel cell equations, are seldom studied in detail. Therefore, the second contribution of the talk aims at analyzing channel-porous media interactions by developing a compressible volume-averaged channel-porous media model in OpenFCST, and validating it with respect to permeability and diffusion bridge experiments in the literature. Comparison of numerical and experimental results show that, depending on the volume-averaging methodology used, experimentally obtained transport properties do not correspond to the input parameters needed in volume-averaged models. Finally, the numerical model is used to estimate flow by-pass in serpentine and inter-digitated channels, as well as compressibility effects [8].

An analytical model for the transverse permeability of gas diffusion layer with electrical double layer effects in proton exchange membrane fuel cells

An analytical model is presented for the transverse permeability of gas diffusion layer (GDL) based on an ordered array of parallel charged circular cylinders at the steady state. The formula of calculating the permeability of the transverse direction is given by solving the fluid momentum equation in a unit cell. In the present approach, the proposed model is explicitly related to the porosity and fiber radius of fibrous porous media, the zeta potential, and the physical properties of the electrolyte solution. Besides, the effects of these parameters (the porosity, unit cell aspect ratio, fiber radius, and molar concentration) on the transverse permeability are discussed detailedly. The model predictions are compared with the previous studies in the available literature, and good agreement is found.

Study on Assembly Mechanics and Its Effect on the Performance of Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell (PEMFC) is an energy conversion device, which converts the chemical energy of fuels into electrical energy directly through electrochemical reactions, it has been considered as one of the most promising clean energy conversion devices widely because of its wide range of fuel sources, clean and free pollution, high operating current, can start-up quickly at room temperature and other advantages. All of components of a PEM fuel cell need to be assembled by exerting pressure from the outside, since the gas diffusion layer (GDL) is a porous structure and its elastic modulus is relatively small, during the process of exerting pressure, if the assembly pressure is too large, it will cause the GDL over deformation or even irreversible damage, thus reducing the gas transmission channel, increasing the mass transfer resistance, and even will damage the fuel cell components, shorten the lifetime of the fuel cell; in contrast, if the assembly pressure is too small, due to the poor contact between the bipolar plate and GDL, the contact resistance will increase, which will lead the reduce of the fuel cell efficiency, and the leak tightness of gas will not be guaranteed, there will be a risk of fuel gas leakage. In this paper, the numerical simulation method is used to simulatethe deformation of GDL under different assembly pressures, and the influence of transmission parameters such as GDL porosity and permeability caused by assembly pressure and its influence on the performance of PEMFC are studied in order to get the optimum pressure, making the best performance of PEMFC. The main work and achievements are as follows: The single channel PEMFC geometric model is established, and the numerical simulation is used to simulate, and the parameters such as porosity and permeability of PEMFC under different assembly pressures are calculated according to the empirical formula. When the assembly pressure from 0 to 3.0 MPa, the porosity decreased from the initial value of 0.78 to about 0.38; permeability also has an order of magnitude change; and the contact resistance decreases as the assembly pressure increases, but it decreases gradually; The finite element analysis (FEA) is used to analyze the effect of different assembly pressures on GDL deformation. The results show that, under the action of assembly pressure, the part of GDL, which under the bipolar plate ribs, will be deformed in different degrees, and the deformation on both sides is similar and obvious, the degree of deformation increases as the assembly pressure increases, while the part of GDL, which under the channel, is almost unchanged; Through the simulation, the distribution of the components concentration in the single channel PEMFC flow channel and the polarization curves under the assembly pressure are analyzed, and compares with different assembly pressures. Since the gas in the flow channel reacts in the catalyst layer, the gas concentration decreases along the flow direction under different assembly pressures. For the polarization curves, when the assembly pressure is between 0.5 to 1.0 MPa, the same operating voltage, the fuel cell current density is greater than the case of other pressures, and the fuel cell with the highest power under this pressure. Therefore, when the assembly pressure between 0.5-1.0 MPa, the single channel PEMFC performance is optimal.

Error of Darcy's law for serpentine flow fields: An analytical approach

Serpentine flow fields and other flow fields with partial under-land cross-flow are commonly used in various energy devices, such as proton exchange membrane (PEM) fuel cells and redox flow batteries, due to their higher mass transfer rate to reaction sites and better product removal capability. Accurately predicting the under-land cross-flow rate and pressure drop in such flow fields is crucial in flow field design optimizations. Darcy's law is the most commonly used model in predicting the under-land cross-flow and pressure drop in such flow fields. However, since the Darcy's law neglects inertial effect, its validity in different designs and operating conditions needs to be carefully studied. In this work, mathematical models for a serpentine flow field are developed based on both the Darcy's law and a modified Darcy's law that includes the inertial effect. Both models are solved and analytical solutions are obtained. The predicted pressure drops and under-land cross-flow rates from the two models are compared with experimental data and the results show that under some conditions, both the Darcy's law and the modified Darcy's law can predict pressure drop and under-land cross-flow rate reasonably well. However, under other conditions the Darcy's law can result in significantly large errors in predicting both pressure drop and under-land cross-flow rates. Further studies provide the variations of errors from the Darcy's law with different parameters, including channel length, gas diffusion layer (GDL) thickness, land width, inlet flow rate, GDL permeability and GDL inertial coefficient.

Through-plane gas permeability of gas diffusion layers and microporous layer: Effects of carbon loading and sintering

Knowledge of the absolute permeability for the various porous layers is necessary to obtain accurate profiles for water saturation within the membrane electrode assembly (MEA) in a two-phase model of a polymer electrolyte membrane fuel cell (PEMFC). In this paper, the gas permeability of gas diffusion layers (GDLs) coated with microporous layers (MPLs) of various carbon loadings for two different carbon blacks have been experimentally measured. The permeability of the GDL was found to decrease by at least one order of magnitude after the MPL-coating. Also, the permeability of the MPLs was shown to be lower than that of the carbon substrate by 2–3 orders of magnitude. Further, it was found that the gas permeability of the MPLs changes significantly from one carbon loading to another despite the use of a single weight composition for all the MPLs coated, namely 20% PTFE and 80% carbon black. This signifies the possible inaccuracy in estimating the MPL permeability through employing the cross-section SEM images as they do not resolve the MPL penetration into the carbon substrate. Finally, the MPL sintering was found to slightly decrease the permeability of the GDL.

ガス拡散層の相対透過率による固体高分子形燃料電池内水輸送への影響

It is important to understand liquid water transport phenomena in through-plane direction in an operating polymer electrolyte fuel cell (PEFC) for improving power generation efficiency. As water accumulation affects the mass transport, relative permeability has been investigated as a function of water saturation. However, the values are much different in each model in particular for lower water saturation condition. The purpose of this study is to clarify water transport phenomena and effect of relative permeability of gas diffusion layer (GDL) on water transport phenomena in a PEFC. Three-dimensional mass transfer analysis in a GDL with multiphase mixture model was carried out. Numerical results using two different relative permeability models and experimental results were compared. Liquid water were distributed uniformly in the through-plane direction using Wang's model. Using hussaini's model, much water accumulate at reactive surface. Better results could be obtained using Hussaini's model of the relative permeability for predicting the water distribution.

The effects of the composition of microporous layers on the permeability of gas diffusion layers used in polymer electrolyte fuel cells

The effects of the composition of the microporous layer (MPL) on the through-plane permeability of the gas diffusion layers (GDLs) used in polymer electrolyte fuel cells (PEFCs) have been thoroughly experimentally investigated in this paper. For a given PTFE loading in the MPL, the GDL permeability was found to decrease with increasing carbon loading and this is due to the increase in the thickness of the MPL. For all the investigated carbon loadings of the MPL, the permeability values of the GDLs were found to have common trends for the PTFE loadings ranging from 10 to 50% (by weight): the GDL permeability increases when the PTFE loading in the MPL is increased from 20 to 50%; the GDL permeability decreases when the PTFE loading in the MPL is increased from 10 to 20%; and the GDL permeability is a minimum at 20% PTFE loading present in the MPL. On the other hand, the permeability of the GDL was found to depend on the carbon loading of the MPL in the PTFE range 0–10%. The effects of the MPL composition on the MPL permeability were found to be similar to those on the GDL permeability. However, the permeability values of the MPLs of the same composition, which were supposed to be ideally the same, were found to significantly vary. This was attributed to the MPL penetration into the body of the carbon substrates.

Water Transport Simulation in a Gas Diffusion Layer of PEMFC Using Lattice Boltzmann Method

The water transport in a gas diffusion layer (GDL) has been investigated using two-dimensional lattice Boltzmann (LB) simulation. The LB model is developed to simulate the dynamic behavior of liquid water and enables to visualize the water-invasion process through micro-pores in GDL. To investigate the effect of rib structure on water invasion process in GDL, two different cases (i.e., with and without rib structure) are compared. The numerical model is verified by the comparison of the flow permeabilities in GDL. The validation result indicates that the LB model can properly predict the permeability of GDL and enables to simulate the water transport behavior in the GDL. The reconstruction of GDL is established by randomly placing the particles in GDL and ignoring the GDL deformation due to clamping force. The results of LB simulation confirm that the liquid water transport inside GDL is strongly governed by capillary force and the rib structure greatly impacts on the water transport behavior. The rib structure influences on the location of water breakthrough by comparing the simulation results of two different cases. This is due to the higher resistance force underneath the rib, resulting in the change of flow path which preferentially selects the lower resistance force. The water saturation level under the channel is higher than that under the rib caused by the suppression of growth of water cluster. After water breakthrough, the liquid water distribution under the channel has little change, whereas that under the rib keeps stretching for a while. The result indicates that a careful control of rib structure would enhance the water removal from the GDL. Therefore further studies for the optimum design of rib structure are needed. In an operating PEMFCs, the mechanism of water transport and wetting characteristics play an important role on flooding behavior. Therefore the results of the present study would contribute to the novel design for better water removal and flooding alleviation from the GDL.

Determination of Permeability of the Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells (PEMFCs)

One vital component in the Proton Exchange Membrane Fuel Cells (PEMFCs) is the Gas Diffusion Layer (GDL). This layer is a pathway for the reactants to reach the reaction site and for the by-products to be removed. Due to this application, the permeability of this layer has been investigated experimentally and numerically [1]. This layer has a highly porous structure, and hence identifying a geometry that resembles best the GDL for modeling flow through this layer is very important. In this study, the 3D image of a GDL sample has been obtained with high resolution imaging, X-ray microtomography, and is used as a model to simulate flow through this layer. This layer typically treated with micro-porous layer (MPL). In this paper, MPL was separated and permeability through the MPL is investigated separately using an image-processing method developed in-house. The results indicate that MPL reduces the permeability of GDL considerably. This work facilitates the study of the effect of MPL on the permeability of the GDL, which is impossible to be determined in experimental approaches [2]. [1] J. Pharoah, "On the permeability of gas diffusion media used in PEM fuel cells," J. Power Sources, vol. 144, pp. 77-82, 2005. [2] J. Ihonen, M. Mikkola and G. Lindbergh, "Flooding of gas diffusion backing in PEFCs physical and electrochemical characterization," J. Electrochem. Soc., vol. 151, pp. A1152-A1161, 2004.

The effect of gas diffusion layer compression on gas bypass and water slug motion in parallel gas flow channels

Water slugs form in the gas flow channels of polymer electrolyte membrane fuel cells (PEMFCs) which hinder reactant transport to the catalyst layer. We report a study correlating video images of slug formation and motion with pressure/flow measurements in parallel gas flow channels. Slugs move when the differential gas pressure exceeds the force to advance the contact lines of the slug with the channel walls. Water slugs can divert the gas flow through the gas diffusion layer (GDL) beneath the ribs to adjacent channels. The flow diversion can cause slugs to stop moving. Slug size and motion has been correlated with in situ GDL permeabilities as functions of GDL compression. Compression reduces the GDL permeability under the ribs much more than the GDL permeability under the channel. A model is presented to describe the spatio‐temporal location of slugs in a PEMFC flow field. © 2014 American Institute of Chemical Engineers AIChE J, 61: 355–367, 2015

Measurements of serpentine channel flow characteristics for a proton exchange membrane fuel cell

Flow characteristics at Re = 660–3000 in a serpentine channel are measured. A scale-up model whose channel hydraulic diameter is 50 times as large as that for a proton exchange membrane fuel cell (PEMFC) is used for the measurements. The flow conditions correspond to operating conditions for PEMFCs of 25–40 cm2 at current density of 1–3 A/cm2 when the fuel utilisation ratio is 0.75 and air is used for the O2 supply. Two different porous media are used to simulate the gas diffusion layer (GDL). The results suggest that although the leakage flow rate is rather insensitive to the total flow rate, it increases significantly depending on the increase of the GDL permeability. Increasing the flow rate or the permeability enhances the sectional secondary flows and is expected to enhance mass transfer on the GDL. It is confirmed that the flow becomes turbulent around the bend even at Re = 660.

Fractal geometry model for through-plane liquid water permeability of fibrous porous carbon cloth gas diffusion layers

The short communication presents an improved fractal model to predict through-plane liquid water permeability of the gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs). The porous structure of a GDL is modeled as a combination of parallel and perpendicular channels to the fluid flow direction. The through-plane permeability equation is derived by a hybrid of fractal longitudinal permeability and transverse permeability in parallel with the former. This model is validated by comparing with the referenced data. It is found that the proposed fractal model is preferred for through-plane permeability predictions of GDL.

Development of a porosity-graded micro porous layer using thermal expandable graphite for proton exchange membrane fuel cells

In this study, a porosity-graded micro porous layer (MPL) was prepared using the double coating method to enhance the water removal ability of the gas diffusion layer (GDL). In the double MPL, the porosity of each layer was controlled using thermal expandable graphite (TEG), which could produce pores in MPL through thermal expansibility. The porosity of the inner layer of the porosity-graded MPL was smaller than the outer layer, so the gradient direction in porosity was from the MPL/catalyst layer interface to the gas diffusion backing layer (GDBL)/MPL interface. In addition, the pore forming ability of TEG and the water removal ability of porosity-graded MPL were characterized. The performance of the porosity-graded MPL was evaluated and compared to the single layer conventional MPL. The porosity-gradient structure in MPL increased the water permeability of GDL and the performance of the single cell in the high current density region. Since the porosity-graded MPL increased the water removal ability of GDL, concentration loss due to water flooding in the high current density region was decreased. These results demonstrate that porosity-graded MPL was beneficial to PEMFC, which has to operate in the high current density region.