Effective thermal conductivity and thermal contact resistance of gas diffusion layers in proton exchange membrane fuel cells. Part 1: Effect of compressive load

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a PEM fuel cell. The analysis of this process requires determination of the effective thermal conductivity as well as the thermal contact resistance associated with the interface between the GDL and adjacent surfaces/layers.In the present study, a custom-made test bed that allows the separation of effective thermal conductivity and thermal contact resistance in GDLs under vacuum and ambient conditions is described. Measurements under varying compressive loads are performed using Toray carbon paper samples with a porosity of 78% for a range of thicknesses. The measurements are complemented by compact analytical models that achieve good agreement with experimental data. A key finding is that thermal contact resistance is the dominant component of the total thermal resistance; neglecting this phenomenon may result in significant errors in evaluating heat transfer rates and temperature distributions.     

Through-plane thermal conductivity of the microporous layer in a polymer electrolyte membrane fuel cell

Understanding the thermal properties of the microporous layer (MPL) is critical for accurate thermal analysis and improving the performance of proton exchange membrane (PEM) fuel cells operating at high current densities. In this study, the effective through-plane thermal conductivity and contact resistance of the MPL have been investigated. Gas diffusion layer (GDL) samples, coated with 5%-wt. PTFE, with and without an MPL are measured using the guarded steady-state heat flow technique described in the ASTM standard E 1225-04. Thermal contact resistance of the MPL with the iron clamping surface was found to be negligible, owing to the high surface contact area. Effective thermal conductivity and thickness of the MPL remained constant for compression pressures up to 15 bar at 0.30 W/m°K and 55 μm, respectively. The effective thermal conductivity of the GDL substrate containing 5%-wt. PTFE varied from 0.30 to 0.56 W/m°K as compression was increased from 4 to 15 bar. As a result, GDL containing MPL had a lower effective thermal conductivity at high compression than the GDL without MPL. At low compression, differences were negligible. The constant thickness of the MPL suggests that the porosity, as well as heat and mass transport properties, remain independent of the inhomogeneous compression by the bipolar plate. Despite the low effective thermal conductivity of the MPL, thermal performance of the GDL can be improved by exploiting the excellent surface contact resistance of the MPL.     

Analytic determination of the effective thermal conductivity of PEM fuel cell gas diffusion layers

Accurate information on the temperature field and associated heat transfer rates are particularly important in devising appropriate heat and water management strategies in proton exchange membrane (PEM) fuel cells. An important parameter in fuel cell performance analysis is the effective thermal conductivity of the gas diffusion layer (GDL). Estimation of the effective thermal conductivity is complicated because of the random nature of the GDL micro structure. In the present study, a compact analytical model for evaluating the effective thermal conductivity of fibrous GDLs is developed. The model accounts for conduction in both the solid fibrous matrix and in the gas phase; the spreading resistance associated with the contact area between overlapping fibers; gas rarefaction effects in microgaps; and salient geometric and mechanical features including fiber orientation and compressive forces due to cell/stack clamping. The model predictions are in good agreement with existing experimental data over a wide range of porosities. Parametric studies are performed using the proposed model to investigate the effect of bipolar plate pressure, aspect ratio, fiber diameter, fiber angle, and operating temperature.     

A novel approach to determine the in-plane thermal conductivity of gas diffusion layers in proton exchange membrane fuel cells

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a proton exchange membrane (PEM) fuel cell. The analysis of this process requires determination of the effective thermal conductivity. This transport property differs significantly in the through-plane and in-plane directions due to the anisotropic micro-structure of the GDL.A novel test bed that allows separation of in-plane effective thermal conductivity and thermal contact resistance in GDLs is described in this paper. Measurements are performed using Toray carbon paper TGP-H-120 samples with varying polytetrafluoroethylene (PTFE) content at a mean temperature of 65-70 °C. The measurements are complemented by a compact analytical model that achieves good agreement with experimental data. The in-plane effective thermal conductivity is found to remain approximately constant, k ≈ 17.5 W m⁻¹ K⁻¹, over a wide range of PTFE content, and its value is about 12 times higher than that for through-plane conductivity.     

Parametric study of the porous cathode in the PEM fuel cell

The electrochemical behavior and the reactant transport in the porous gas diffusion layer (GDL) and catalyst layer (CL) are controlled by a large number of parameters such as porosity, permeability, conductivity, catalyst loading, and average pore size, etc. A three‐dimensional polymer electrolyte membrane fuel cell model is developed. The model accounts for the mass, fluid, and thermal transport processes as well as the electrochemical reaction. Using this model, the effects of the various porous electrode design parameters including porosity, solid electronic conductivity, and thermal conductivity of cathode GDL, and the catalyst loading, average pore size of cathode CL are investigated through parametric study. The model is shown to agree well with the experimental data of some porous electrode specifications. In addition, the model shows promise as a tool for optimizing the design of fuel cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of polytetrafluoroethylene-treatment and microporous layer-coating on the electrical conductivity of gas diffusion layers used in proton exchange membrane fuel cells

The purpose of this study is to investigate the effect of ploytetrafluoroethylene (PTFE)-treatment and microporous layer (MPL)-coating on the electrical conductivity of gas diffusion layers (GDLs), as used in proton exchange membrane fuel cells (PEMFCs). The results show that, for PTFE-treated GDLs, the electrical conductivity in orthogonal in-plane directions is almost invariant with the PTFE loading. On the other hand, the in-plane conductivity of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be higher than that of the counterpart SGL 10BC (25% PTFE) and this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Further, the conductivity of each GDL sample was measured in two perpendicular in-plane directions in order to investigate the in-plane anisotropy. The results show that the electrical conductivity of the GDL sample in one direction is different to that in the other direction by a factor of about two. The contact resistance, the main factor affecting the through-plane conductivity, of PTFE-treated GDLs shows a different trend to the corresponding in-plane conductivity, namely it increases as the PTFE loading increases. On the other hand, the contact resistance of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be lower than that of the counterpart SGL 10BC (25% PTFE) and again this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Also, it was noted that the MPL coating appears to have a positive effect in reducing the contact resistance between the GDL and the bipolar plate. This is most likely due to the compressibility of the MPL layers that allows them to fill in the ‘gaps’ that exist in the surface of the bipolar plates and therefore establishes a good contact between the latter plates and the GDLs. Finally, good curve fitting of the contact resistance as a function of the clamping pressure has been achieved.     

Effect of the anisotropic thermal conductivity of GDL on the performance of PEM fuel cells

Gas diffusion layers (GDLs) are one of the main components in proton exchange membrane (PEM) fuel cells. In this paper, the effect of anisotropic thermal conductivity of the GDL is numerically investigated under different operating temperatures. Furthermore, the sensitivity of the PEM fuel cell performance to the thermal conductivity of the GDL is investigated for both in-plane and through-plane directions and the temperature distributions between the different GDL thermal conductivities are compared. The results show that increasing the in-plane and through-plane thermal conductivity of the GDL increases the power density of PEM fuel cells significantly. Moreover, the temperature gradients show a greater sensitivity to the in-plane thermal conductivity of the GDL as opposed to the through-plane thermal conductivity.     

Measurement of in-plane thermal conductivity of carbon paper diffusion media in the temperature range of −20 °C to +120 °C

Carbon paper is commonly used as the gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells as it exhibits high chemical and mechanical durability. This diffusion medium is also anisotropic, which directly affects its transport properties and specifically the thermal conductivity. In this study, the in-plane thermal conductivity of the carbon paper GDL was determined using thermal diffusivity measurements for a temperature range from −20 to +120 °C and four Teflon loadings (0, 5, 20 and 50 wt.%). It is important to understand the effect of temperature on the thermal conductivity since PEM fuel cells are designed to operate under various temperatures depending on the application of use. Further, Teflon is used to change the hydrophobic properties of the carbon paper GDL with 20 wt.% as the most widely used percentage. In this study, the Teflon loadings were chosen to gain a comprehensive understanding of the thermal resistance due to Teflon. In this study, a quasi-steady method was used to measure the thermal properties of the carbon paper; hence, the phase transformation in the presence of PTFE was investigated. The thermal conductivity decreases with an increase in temperature for all samples. The addition of as little as 5 wt.% Teflon resulted in high thermal resistance decreasing the overall thermal conductivity of the sample. Further addition of Teflon did not have major effects on the thermal conductivity. For all treated samples, the thermal conductivity lies in the range of 10.1–14.7 W/mK. Finally, empirical relations for the thermal diffusivity and conductivity with temperature were deduced.     

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.     

Numerical study for in-plane gradient effects of cathode gas diffusion layer on PEMFC under low humidity condition

A numerical study about in-plane porosity and contact angle gradient effects of cathode gas diffusion layer (GDL) on polymer electrolyte membrane fuel cell (PEMFC) under low humidity condition below 50% relative humidity is performed in this work. Firstly, a numerical model for a fuel cell is developed, which considers mass transfer, electrochemical reaction, and water saturation in cathode GDL. For water saturation in cathode GDL, porosity and contact angle of GDL are also considered in developing the model. Secondly, current density distribution in PEMFC with uniform cathode GDL is scrutinized to design the gradient cathode GDL. Finally, current density distributions in PEMFC with gradient cathode GDL and uniform cathode GDL are compared. At the gas inlet side, the current density is higher in GDL with a gradient than GDL with high porosity and large contact angle. At the outlet side, the current density is higher in GDL with a gradient than GDL with low porosity and small contact angle. As a result, gradient cathode GDL increases the maximum power by 9% than GDL with low porosity and small contact angle. Moreover, gradient cathode GDL uniformizes the current density distribution by 4% than GDL with high porosity and large contact angle.     

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.     

PEM fuel cell performance improvement through numerical optimization of the parameters of the porous layers

A numerical model for a PEM fuel cell has been developed and used to investigate the effect of some of the key parameters of the porous layers of the fuel cell (GDL and MPL) on its performance. The model is comprehensive as it is three-dimensional, multiphase and non-isothermal and it has been well-validated with the experimental data of a 5 cm² active area-fuel cell with/without MPLs. As a result of the reduced mass transport resistance of the gaseous and liquid flow, a better performance was achieved when he GDL thickness was decreased. For the same reason, the fuel cell was shown to be significantly improved with increasing the GDL porosity by a factor of 2 and the consumption of oxygen doubled when increasing the porosity from 0.40 to 0.78. Compared to the conventional constant-porosity GDL, the graded-porosity (gradually decreasing from the flow channel to the catalyst layer) GDL was found to enhance the fuel cell performance and this is due to the better liquid water rejection. The incorporation of a realistic value for the contact resistance between the GDL and the bipolar plate slightly decreases the performance of the fuel cell. Also the results show that the addition of the MPL to the GDL is crucially important as it assists in the humidifying of the electrolyte membrane, thus improving the overall performance of the fuel cell. Finally, realistically increasing the MPL contact angle has led to a positive influence on the fuel cell performance.     

Estimation of contact resistance in proton exchange membrane fuel cells

The contact resistance between the bipolar plate (BPP) and the gas diffusion layer (GDL) is an important factor contributing to the power loss in proton exchange membrane (PEM) fuel cells. At present there is still not a well-developed method to estimate such contact resistance. This paper proposes two effective methods for estimating the contact resistance between the BPP and the GDL based on an experimental contact resistance–pressure constitutive relation. The constitutive relation was obtained by experimentally measuring the contact resistance between the GDL and a flat plate of the same material and processing conditions as the BPP under stated contact pressure. In the first method, which was a simplified prediction, the contact area and contact pressure between the BPP and the GDL were analyzed with a simple geometrical relation and the contact resistance was obtained by the contact resistance–pressure constitutive relation. In the second method, the contact area and contact pressure between the BPP and GDL were analyzed using FEM and the contact resistance was computed for each contact element according to the constitutive relation. The total contact resistance was then calculated by considering all contact elements in parallel. The influence of load distribution on contact resistance was also investigated. Good agreement was demonstrated between experimental results and predictions by both methods. The simplified prediction method provides an efficient approach to estimating the contact resistance in PEM fuel cells. The proposed methods for estimating the contact resistance can be useful in modeling and optimizing the assembly process to improve the performance of PEM fuel cells.     

Effect of anisotropic thermal conductivity of the GDL and current collector rib width on two-phase transport in a PEM fuel cell

A two-dimensional two-phase model based on the classical two-fluid model is used to analyze electrochemical and thermal transport in a PEMFC. The model is extended to account for the dependence of interfacial area density on liquid volume fraction. At a given fixed voltage, the fuel cell generates maximum current density for low through-plane and high in-plane thermal conductivities at high humidity operating conditions. It is also predicted that for low humidity operating conditions, the fuel cell generates maximum current density if the GDL is tailored to have high through-plane thermal conductivity near the inlet and progressively decreasing through-plane thermal conductivity at distances away from the inlet. At fully humidified cathode inlet conditions, narrower current collector ribs generate higher current densities at all voltages by reducing the resistance to diffusion of reactants and products through the GDL. In order to maximize the current density at low humidities, ribs must be wider near the inlet and narrower away from the inlet. The proposed methodology for tailoring GDL through-plane thermal conductivities and rib widths reduces the risk of membrane dehydration near inlet and also reduces the possibility of excessive liquid accumulation in the region away form the inlet.     

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.     

Investigation of sintered stainless steel fiber felt as gas diffusion layer in proton exchange membrane fuel cells

The feasibility of using sintered stainless steel fiber felt (SSSFF) as gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs) is evaluated in this study. The SSSFF is coated with an amorphous carbon (a-C) film by closed field unbalanced magnetron sputter ion plating (CFUBMSIP) to enhance the corrosion resistance and reduce the contact resistance. The characteristics of treated SSSFF, including microscopic morphology, mechanical properties, electrical conductivity, electrochemical behavior and wettablity characterization, are systematically investigated and summarized according to the requirements of GDL in PEMFC. A membrane electrode assembly (MEA) with a-C coated SSSFF-15 GDL is fabricated and assembled with a-C coated stainless steel bipolar plates in a single cell. The initial peak power density of the single cell is 877.8 mW cm⁻² at a current density of 2324.9 mA cm⁻². Lifetime test of the single cell over 200 h indicates that the a-C coating protects the SSSFF-15 GDL from corrosion and decreases the performance degradation from 30.6% to 6.3%. The results show that the SSSFF GDL, enjoying higher compressive modulus and ductility, is a promising solution to improve fluid permeability of GDL under compression and PEMFC durability.     

A micro-scale model for predicting contact resistance between bipolar plate and gas diffusion layer in PEM fuel cells

Contact resistance between the bipolar plate (BPP) and the gas diffusion layer (GDL) in a proton exchange membrane (PEM) fuel cell constitutes a significant portion of the overall fuel cell electrical resistance under the normal operation conditions. Most current methods for contact resistance estimation are experimental and there is a lack of well developed theoretical methods. A micro-scale numerical model is developed to predict the electrical contact resistance between BPP and GDL by simulating the BPP surface topology and GDL structure and numerically determining the status for each contact spot. The total resistance and pressure are obtained by considering all contact spots as resistances in parallel and summing the results together. This model shows good agreements with experimental results. Influences of BPP surface roughness parameters on contact resistance are also studied. This model is beneficial in understanding the contact behavior between BPP and GDL and can be integrated with other fuel cell simulations to predict the overall performance of PEM fuel cells.     

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.     

Combined effects of microstructural characteristics on anisotropic transport properties of gas diffusion layers for PEMFCs

Gas diffusion layer (GDL) plays a key role in proton exchange membrane fuel cells, which provides multi-functions for gas transport, thermal-electrical conduction and mechanical support. Coupling manipulation of different microstructural characteristics could potentially improve transport properties of GDLs. This work proposes an approach to reconstruct heterogenous GDLs and conduct pore-scale modeling to evaluate the anisotropic transport properties. The models are reconstructed using X-ray computed tomography, stochastically reconstruction methods and morphological processing techniques, which consider different fiber diameter, GDL thickness and local porosity distribution type. Combined effects of microstructure characteristics on tortuosity, diffusivity, thermal-electrical conductivity and anisotropic ratios are investigated comprehensively. The results show that the diffusivity with fiber diameter of 7 μm is approximately 7% lower, and the conductivity is 8% higher than that of 9 μm. The anisotropic ratios of diffusivity, thermal conductivity and electrical conductivity range from 1.25 to 1.65, 5 to 20, and 20 to 55, respectively. Local porosity distribution of uniform-fluctuated type, fiber diameter of 7 μm and GDL thickness of 126 μm are suggested to balance diffusivity and thermal-electrical conductivity simultaneously. The methods and results can guide microstructure design of other porous electrodes with higher performance.     

A dynamic model of air-breathing polymer electrolyte fuel cell (PEFC): A parametric study

A dynamic model for an air-breathing PEFC has been built to investigate the transient response of the fuel cell to load changes. The sensitivities of the dynamic response, as well as the steady state performance, to: the ambient temperature and relative humidity; the thickness and the thermal conductivity of the cathode GDL; and the fuel utilisation, have been studied. A previously-developed steady-state model of the fuel cell was linked to the dynamic model to feed the latter with the data of the cell temperature as it changes with the current density. It was found that, when there are sudden changes to high loads, there exist optimum values for the ambient temperature and GDL thickness at which the overshoots are mitigated and the steady state performance is improved. Further, the transient and steady state performance were found to improve with increasing the ambient relative humidity and GDL thermal conductivity. Finally, the fuel utilisation was found to have no impact on the dynamic response of the fuel cell. All the above findings have been presented and discussed in the paper.