Conductivity Of Gas Diffusion Layer Research Articles

Conduction mechanism analysis and modeling of different gas diffusion layers for PEMFC to improve their bulk conductivities via microstructure design

Increasing the conductivity of gas diffusion layers (GDLs) is an important way to improve the output performance of polymer electrolyte membrane fuel cells (PEMFCs). However, the complex porous fiber structures of GDLs significantly enhances the difficulty of quantitatively altering their conductivity which is determined by the carbon fibers and the conduction characteristics between fibers. In addition, the microstructures of various types of GDLs are different. Thus, it is a considerable challenge to explore the conductive mechanisms of these porous materials and optimize their structures to reduce their bulk resistances. In this work, a mathematical graph theory model that applies to the through-plane (T-P) bulk resistance prediction of two types of commonly used GDLs, carbon paper and carbon felt, is established to explain their different micro conduction mechanisms in depth. In addition to the number of fiber contact points, their distribution, as well as the resistance of the carbon fibers, are all important factors affecting the T-P conductivity. Optimizing fiber density and fiber diameter can significantly improve the T-P conductivity of carbon paper. In comparison, making the structure of carbon felt more compact so that the distribution of its contact points in the T-P direction can be more uniform will be more effective for the reduction of its T-P bulk resistance. Meanwhile, the T-P bulk resistance of carbon paper can also be effectively improved by optimizing the content and distribution of the binders. A method to decline the bulk resistance of carbon paper by aggregating the binders in the in-plane (IP) direction is proposed. The simulation results show that it can reduce the T-P bulk resistance of carbon paper by about 19.9% at a compressive stress of 1.5 MPa. This study provides further guidance for optimizing the structural designs of GDLs to optimize their conduction performance.

Collective enhancement in hydrophobicity and electrical conductivity of gas diffusion layer and the electrochemical performance of PEMFCs

Gas diffusion layer (GDL) with optimized water management capacity facilitates the electrochemical performance of proton exchange membrane fuel cells (PEMFCs). However, the impact of hydrophobicity manipulation on the mass transport capacity of GDL makes it challenging to enhance the electrochemical performance of PEMFCs effectively. In this work, we proposed a facile strategy for collectively optimizing the hydrophobicity and electrical conductivity (EC) of GDL through the hydrothermal deposition followed by a high-temperature sintering process (HDS-GDL) Specifically, surface roughness of GDL and corresponding fiber diameter firstly increases after the hydrothermal deposition (HD-GDL) and then decreases after high-temperature sintering. Surface water contact angle of the HDS-GDL (133°) increases as compared to that of the Toray GDL (CR-GDL, 125°). Meanwhile, in-plane and through-plane ECs of HDS-GDL are simultaneously enhanced as compared to the CR-GDL. Moreover, HDS-GDL with collectively enhanced hydrophobicity and ECs further improve the electrochemical performance of PEMFCs. The polarization curve implies that enhancement of the power density at the Ohmic region under a wide cathode relative humidity (RH 0–80%) is mainly dominated by the optimized hydrophobicity. The findings in this work provide a new strategy for improving the electrochemical performance of PEMFCs by precisely tailoring the water management capacity of GDL.

Numerical investigation of effective thermal conductivity of gas diffusion layer of the PEM fuel cell

A three-dimensional multicomponent multiphase (MCMP) thermally coupled lattice Boltzmann model (LBM) was developed to investigate the effect of water content on the effective thermal conductivity (ETC) of the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC). A stochastic model was then used to reconstruct the 3D GDLs, and the effects of the material conditions on the heat and mass transfer characteristics were examined in detail. The results showed that the more the liquid water was saturated, the higher the estimated ETC of the GDL. When the wettability of the carbon fiber was constant, the distribution of liquid water played a more critical role in predicting the ETC.

Impact of the temperature variation on the thermal conductivity of gas diffusion layers for polymer electrolyte fuel cells

The purpose of this study is to evaluate the thermal conductivity of gas diffusion layers (GDLs) when subjected to different operating temperatures. The measurement apparatus is completely designed and the thermal conductivities are evaluated for five different GDL samples. The amount of catalyst material within the GDLs is also considered in the current study. Based on the obtained measurements, thermal conductivity correlations as a function of the operating temperature are proposed. The correlations consider a confidence interval of 95%, and according to the results, they show good predictivity. In this study, the GDL thermal conductivity coefficient functions are proposed. The mentioned correlations can be used to determine the thermal properties of fuel cells for a range of temperatures from 20 °C to 80 °C.

Experimental study on the effect of temperature and water content on the thermal conductivity of gas diffusion layers in proton exchange membrane fuel cell

The gas diffusion layer is one of the key components of proton exchange membrane fuel cell. Temperature and water content of gas diffusion layers have a significant impact on its thermal conductivity. The thermal conductivity of the dry gas diffusion layer shows an upward trend with increasing temperature, but the change is small. The thermal conductivity of the gas diffusion layer shows a linear increase with the increase of water content. When the water content of the gas diffusion layer is 77%, its thermal conductivity increases with the temperature. Its thermal conductivity increased from 0.3592 W/(m·K) at 25 °C to 0.7871 W/(m·K) at 85 °C.

The Influence of Argon, Air and Hydrogen Gas on Thermal Conductivity of Gas Diffusion Layers and Temperature Gradients in PEMFCS

Many renewable electricity sources (e.g., wind and solar) are intermittent, and in many cases peak production does not align with peak consumption. One way to store this excess renewable energy is by means of hydrogen gas through the process of water electrolysis. The stored energy in hydrogen gas can be recovered with a fuel cell when needed. [1] This way fuel cells can be an energy buffer and help to stabilize power systems based on renewable energy sources. A proton exchange membrane fuel cell (PEMFC) consists of several components. The membrane electrode assembly (MEA) is sandwiched between gas diffusion layers (GDLs) on each side. GDLs in fuel cells have to satisfy a range of conflicting demands. They must deliver good mass transport, but also good electrical and thermal transport. In this work the thermal conductivity of two commercial GDLs was measured with either hydrogen, air, or argon present inside their pores. The results show an increase of up to 19% with regard to the thermal conductivity for the Freudenberg H1410 GDL with hydrogen present in the pores as opposed to measurements with air present. The thermal conductivity in the Sigracet 10BA GDL was also enhanced, with an increase of 15% with hydrogen present in the pores. This correlates with the thermal conductivity of hydrogen gas, which is higher than that of air. Furthermore, the results suggest that the GDL materials have a lower thermal conductivity with argon gas present in the pores. The thermal conductivity for Freudenberg H1410 increased from 0.119 ± 0.011 WK-1m-1 for air to a thermal conductivity of 0.140 ± 0.015 WK-1m-1 for hydrogen gas at a compaction pressure of 10 bar. The thermal conductivity of Sigracet 10BA increased from 0.30 ± 0.05 WK-1m-1 for air to a thermal conductivity of 0.32 ± 0.03 WK-1m-1 for hydrogen gas at a compaction pressure of 10 bar. These results suggest that the gas present in the pores has a significant influence on the thermal conductivity of the GDL. Additionally, a 2D thermal model has been constructed to represent the impact of the results on the temperature distribution inside a fuel cell, see figure 1. The considerable temperature gradients inside the single cells need to be considered when assessing lifetime and degradation effects [1-4]. Judging by the scarcity of data on this in the literature this is not the case as of today. Figure 1: PEMFC land temperature profiles saturated with air, argon, or hydrogen on the anode side. The dashed vertical lines represent the border between different materials in the MEA References [1] O. S. Burheim, Engineering Energy Storage, 1st edition, Academic Press, 2017. [2] R. Bock, A.D. Shum, X. Xiao, H. Karoliussen, F. Seland, I.V. Zenyuk and O.S. Burheim, Thermal conductivity and compaction of GDL-MPL interfacial composite material, J. Electrochem. Soc. 2018 165(7): F514-F525 [3] R. Bock, H. Karoliussen, B. G. Pollet, M. Secanell, F. Seland, D. Stanier and O. S. Burheim, The influence of graphitization on the thermal conductivity of catalyst layers and temperature gradients in proton exchange membrane fuel cells, International Journal of Hydrogen Energy, In-press 2018 [4] R. Bock, A. Shum, T. Khoza, F. Seland, N. Hussain, I. V Zenyuk and O. S. Burheim Experimental Study of Thermal Conductivity and Compression Measurements of the GDL-MPL Interfacial Composite Region, ECS Trans. 2016 75(14): 189-199 Figure 1

The Influence of Argon, Air and Hydrogen Gas on Thermal Conductivity of Gas Diffusion Layers and Temperature Gradients in PEMFCS

Gas diffusion layers (GDL) in fuel cells have to satisfy a range of conflicting demands. They must deliver good mass transport, but also good electrical and thermal transport. The thermal conductivity of two GDLs was measured with either hydrogen or argon present inside the pores. The results show an increase of up to 19% with regard to the thermal conductivity for the Freudenberg H1410 GDL with hydrogen present in the pores as opposed to measurements with air present. The thermal conductivity in the Sigracet 10BA GDL was also enhanced, with an increase of 15% with hydrogen present in the pores. This correlates with the thermal conductivity of hydrogen gas, which is higher than that of air. Furthermore, the results suggest that the GDL materials have a lower thermal conductivity with argon gas present in the pores. The thermal conductivity for Freudenberg H1410 increased from 0.119±0.011 WK-1m-1 for air to a thermal conductivity of 0.140±0.015 WK-1m-1 for hydrogen gas at a compaction pressure of 10 bar. The thermal conductivity of Sigracet 10BA increased from 0.30±0.05 WK-1m-1 for air to a thermal conductivity of 0.32±0.03 WK-1m-1 for hydrogen gas at a compaction pressure of 10 bar. These results suggest that the gas present in the pores has an influence on the thermal conductivity of the GDL. Additionally, a 2D thermal model has been constructed to represent the impact of the results on the temperature distribution inside a fuel cell.

(Invited) Review: PEMFC Materials' Thermal Conductivity and Influence on Internal Temperature Profiles

The polymer electrolyte membrane fuel cell (PEMFC) is far from isothermal when in operation. This is well established both from experiments and from mathematical modelling. The latter relies on knowledge of heat transport and local heat generation in the different materials and cell design that constitute the PEMFC. Over the last 15 years the knowledge about thermal conductivity and heat generation in the PEMFC materials as a research field has been established and developed. From first looking at through-plane thermal conductivity of only membranes and gas diffusion layers (GDL), the most recent studies include heat transport dependency on non-isotropic properties, PTFE content, water content, compression, heat pipe effects, and material integration. Depending on the fabric, dry GDL thermal conductivity range from 0.15 to 0.9 W K-1 m-1 when compressed at close to 10 bar compaction pressure. The most common fabrics have thermal conductivity between 0.3 and 0.4 W K-1 m-1. Adding water increases the thermal conductivity by between a factor of two and three, up to 1.5 W K-1 m-1, see ref 1. The in-plane thermal conductivity is larger and more than 10 W K-1 m-1, see ref 2-3. Thermal conductivity of fully humidified Nafion is 0.27 W K-1 m-1, see ref 4. Over the past few years, more attention has been given to the thermal conductivity of the microporous layer (MPL) and the catalyst layer (CL) and the way the MPL interact with the GDL, see ref 5-6. Thermal conductivity of these thin layers (CL and MPL) have been measured to be in the range of 0.1 W K-1 m-1, and in the region where the MPL and the GDL interfere to make a composite material, where the thermal conductivity has been estimated to be as high as 10 W K-1 m-1 . These different values and their historical development leads to a natural selection of 5 reference thermal models at 1 A cm-2 and 0.67 V, leading to temperature profiles under the rib of the current collector, shown i the attached figure. The base case consists of dry materials and no consideration of thin layer effects (CL and MPL), whereas the thin layer effects leads to additional temperature elevation. Accounting for the MPL-GDL interferences, on the other hand leads to a lowered temperature elevation inside the PEMFC. Adding water and accounting for these effects, and in addition considering the GDL with the highest thermal conductivity (wet Toray), the maximum temperature elevation under the polarisation plate land is lowered by a factor of 6 compared the base case of the oldest available thermal conductivity values. The attached figure shows the temperature profiles under the polarisation plate land for these five models and also indicate the regions; GDL, GDL-MPL-comp., MPL, Anode CL, Membrane, Cathode CL, MPL, GDL-MPL-Comp., GDL and the thickness, given by distance from the membrane. A richer and more detailed overview of thermal conductivity of several different materials and their effects on modelling thermal gradients inside the PEMFC is presented in the presentation and following paper at this conference. The take home message is that investigating thermal conductivity of PEMFC components is important for modelling and certainly not a closed chapter. References O. Burheim, H. Lampert, J. Pharoah, P. Vie, S. Kjelstrup, Through-plane thermal conductivity of PEMFC porous transport layers, Journal of Fuel Cell Science and Technology 8 (2011) 021013–1–11. N. Zamel, E. Litovsky, S. Shakhshir, X. Li, J. Kleiman, Measurement of in-plane thermal conductivity of carbon paper diffusion media in the temperature range of -20 ◦c to +120 ◦c, Appl. Energy 88 (2011) 3042–3050.E. Sadeghi, N. Djilali, M. Bahrami, A novel approach to determine the in-plane thermal conductivity of gas diffusion layers in proton exchange membrane fuel cells, J. Power Sources 196 (2011) 3565–3571.O. Burheim, P. Vie, J. Pharoah, S. Kjelstrup, Ex-situ measurements of through-plane thermal conductivities in a polymer electrolyte fuel cell, Journal of Power Sources 195 (2010) 249–256.O. S. Burheim, G. A. Crymble, R. Bock, N. Hussain, S. Pasupathi, A. du Plessis, S. le Roux, F. Seland, H. Su, B. G. Pollet, Thermal conductivity in the three layered regions of micro porous layer coated porous transport layers for the pem fuel cell, International Journal of Hydrogen Energy.R. Bock, A. Shum, T. Khoza, F. Seland, N. Hussain, I. V. Zenyuk, O. S. Burheim, Experimental study of thermal conductivity and compression measurements of the gdl-mpl interfacial composite region, ECS Transactions 75 (14) (2016) 189–199. Figure 1

A Comprehensive Fractal Approach in Determination of the Effective Thermal Conductivity of Gas Diffusion Layers in Polymer Electrolyte Membrane Fuel Cells

The challenges in the fuel cell industry is to produce the efficient thermal and water management for accurate determination the effectiveness thermal conductivity of gas diffusion layers (GDL) used in polymer electrolyte membrane fuel cells (PEMFC’s). This is one of the factors affecting the durability of a fuel cell and need to get a solution to minimize costs and optimize the use of electrodes and cells. The main objective of this research is focusing on the capability of the fractal approach for estimation effectiveness thermal conductivity of gas diffusion media. Moreover, on this research also to propose modified fractal equations in determination of the effective thermal conductivity of GDL in PEMFCs based on previous study. Other objectives in this study are demonstrated the thermal conductivity of GDL treated with PTFE contents by using through-plane thermal conductivity experiment method. The through plane method (experimental method) has been used in estimating thermal conductivity of the GDL. Thermal resistance for GDL also has been investigating under compression pressure 0.1 Mpa until 1.0 MPa. In fractal equation, the determination of tortuous and pore fractal dimension can be done by using Scanning Electron Microscopy (SEM) method. Determination of effectiveness thermal conductivity using of fractal equation with slightly modified. In findings, it was found that fractal equation have been modified measured on the GDL parameter characteristics shows the effectiveness thermal conductivity of GDL for Toray carbon paper 060 and 090 is 0.3218 W/mK and 0.3306 W/mK respectively, compared to through plane thermal conductivity experimental value which is 0.3378 W/mK and 0.3236 W/mK respectively. For Sigracet 35 AA and 35 BA by using fractal approach technique is 0.1619 W/mK and 0.1182 W/mK respectively, compared to through plane thermal conductivity experimental value which is 0.1594 W/mK and 0.2752 W/mK respectively.

알칼리용액에서 산소환원 및 발생반응에 대한 La0.8Sr0.2CoO3전극의 기체확산층 영향

Various commercially available gas diffusion layers (GDLs) from different manufacturers were used to prepare an air electrode using <TEX>$La_{0.8}Sr_{0.2}CoO_3$</TEX> perovskite (LSCP) as the catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in an alkaline solution. Various GDLs have different physical properties, such as porosity, conductivity, hydrophobicity, etc. The ORR and OER of the resulting cathode were electrochemically evaluated in an alkaline solution. The electrochemical properties of the resulting cathodes were slightly different when compared to the physical properties of GDLs. Pore structure and conductivity of GDLs had a prominent effect and their hydrophobicities had a minor effect on the electrochemical performances of cathodes for ORR and OER.

Lattice Boltzmann Modeling of the Effective Thermal Conductivity of an Anisotropic Gas Diffusion Layer in a Polymer Electrolyte Membrane Fuel Cell with Residual Water

In the present work, the anisotropic effective thermal conductivity of the gas diffusion layer (GDL) of a polymer electrolyte membrane (PEM) fuel cell is determined using a thermal lattice Boltzmann model. Using saturation patterns obtained from three-dimensional (3-D) pore network modeling simulations with invasion percolation, the thermal conductivity of the GDL containing liquid water is determined. For a GDL modeling domain with a total saturation of 24.4% (flooded inlet condition), increases of 20.8% and 5.4% are noted for the through-plane and in-plane thermal conductivities, respectively. As the GDL plays an important role in thermal and water management within a PEM fuel cell, the simulated thermal conductivities determined in this work can provide insight into the effect of the GDL on the thermal management required for improved PEM fuel cell performance.

Non-isothermal multi-dimensional direct methanol fuel cell model with micro-porous layers mitigating water/methanol crossover

A two-phase three-dimensional direct methanol fuel cell (DMFC) model with the capability of saturation jump is developed in order to investigate the effect of micro-porous layers (MPL) on the water/methanol crossover in a DMFC. It is found that hydrophobic MPL on the cathode side helps cathode catalyst layer reserve water whereas hydrophobic MPL on the anode side blocks liquid flow to the cathode side, which effectively reduces water/methanol crossover. As the water transport in a DMFC occurs in two-phase, energy equation considering the heat-pipe effect with latent heat is solved together. Calculation result revealed thermal conductivity of gas diffusion layer has a strong influence on both temperature and water transport. Finally, a large-scale simulation of 28 cm2 cell is conducted to examine current density and fuel concentration distribution.

Effects of anisotropic permeability and electrical conductivity of gas diffusion layers on the performance of proton exchange membrane fuel cells

A 3-dimensional model for an in-house proton exchange membrane (PEM) fuel cell with serpentine channels has been developed in order to investigate the sensitivity of the fuel cell performance to the anisotropic gas permeability and electrical conductivity of gas diffusion layers (GDLs). For a realistic range of transport properties being investigated, the fuel cell performance was found to be very sensitive to the electrical conductivity but almost insensitive to the gas permeability of the GDL. For the given operating conditions, the current density was found to be a maximum in the vicinity of the edge between the flow channel and the rib of the current collector. Since the most common GDL materials present a rather significant anisotropy in the in-plane directions, the effects of such anisotropy has been evaluated. Given that the through-plane conductivity is maintained constant for all the cases investigated, for a realistic range of the in-plane electrical conductivity, the fuel cell performance was found to be almost insensitive to this parameter. Therefore such anisotropy can be practically ignored. Finally, for single phase operating conditions, the U-bend in the serpentine channel has no effect on the overall performance of the fuel cell. Hence, only a straight channel of the fuel cell may be modelled and used as a quick performance indicator.

Determination of the effective thermal conductivity of gas diffusion layers in polymer electrolyte membrane fuel cells: a comprehensive fractal approach

Accurate determination of the effective thermal conductivity of gas diffusion layers used in polymer electrolyte membrane fuel cells is very critical for efficient thermal and water management. In this paper, effective thermal conductivity of a diffusion medium material with and without polytetrafluoroethylene (PTFE) was estimated using a fractal approach. Fractal dimensions were obtained using the measured pore size distributions of the samples. The predicted through-plane thermal conductivities were in good agreement with the experimentally measured data. It was demonstrated that such a fractal technique can be employed, with slight modifications, to estimate in-plane thermal conductivity. Besides, application of a random multifractal simulation to obtain the required tortuous fractal dimension is introduced, which can be helpful in the absence of scanning electron microscopy images. It was shown that the tortuous fractal dimension obtained is in good agreement with the reported value in the literature. Finally, variation of the tortuous fractal dimensions with PTFE contents was also determined, which is of practical importance when estimating thermal conductivity of diffusion media with different amount of PTFE contents. Copyright © 2011 John Wiley & Sons, Ltd.

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.

Effective Thermal Conductivity of Gas Diffusion Layers Used in PEMFC: Measured with Guarded-Hot-Plate Method and Predicted by a Fractal Model

An experimental setup to determine thermal conductivity and thickness of a gas diffusion layer (GDL) as a function of compression was developed and calibrated. The actual thermal conductivities, based on compressed thickness, of plain Toray TGP-H gas diffusion layer (GDL) at compression pressures of 0.04 to 1.5 MPa and over temperature range of 25 °C to 75 °C are reported in this paper. The thermal conductivity of TGP-H GDL was found to decrease with temperature and to increase with compression. An effort was made to numerically account for the effective thermal conductivity by using theoretical models. Krischer model was found to be able to match with the experimental data, but this model relied on a priori knowledge of the thermal properties of the solid matrix. A new analytical model based on fractal analysis of GDL microstructure is proposed to account for GDL thermal conductivity as well as its variation with compression without using any empirical parameter.

Effective Thermal Conductivity of Gas Diffusion Layers Used in PEMFC: Measured with Guarded-Hot-Plate Method and Predicted by a Fractal Model

not Available.

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.

Analysis of Heat Transport in a Proton Exchange Membrane (PEM) Fuel Cell

In this study a two-phases, single-domain and non-isothermal model of a Proton Exchange Membrane (PEM) fuel cell has been studied to investigate thermal management effects on fuel cell performance. A set of governing equations, conservation of mass, momentum, species, energy and charge for gas diffusion layers, catalyst layers and the membrane regions are considered. These equations are solved numerically in a single domain, using finite-volume-based computational fluid dynamics technique. Also the effects of four critical parameters that are thermal conductivity of gas diffusion layer, relative humidity, operating temperature and current density on the PEM fuel cell performance is investigated. In low operating temperatures the resistance within the membrane increases and this could cause rapid decrease in potential. High operating temperature would also reduce transport losses and it would lead to increase in electrochemical reaction rate. This could virtually result in decreasing the cell potential due to an increasing water vapor partial pressure and the membrane water dehydration. Another significant result is that the temperature distribution in GDL is almost linear but within membrane is highly non-linear. However at low current density the temperature across all regions of the cell dose not change significantly. The cell potential increases with relative humidity and improved hydration which reduces ohmic losses. Also the temperature within the cell is much higher with reduced GDL thermal conductivities. The numerical model which is developed is validated with published experimental data and the results are in good agreement.

Fractal model for prediction of effective thermal conductivity of gas diffusion layer in proton exchange membrane fuel cell

The process of heat transfer within porous media is usually considered as a transport through large numbers of straight channels with uniform pore sizes. For the prediction of effective thermal conductivity of gas diffusion layer (GDL), morphological properties such as the tortuosity of channels and pore-size distribution of this porous layer should be considered. Thus in this article, novel parallel and series-parallel prediction models of effective thermal conductivity for the GDL in proton exchange membrane fuel cell (PEMFC) have been derived by fractal theoretical characterization of the real microstructure of GDL. The prediction of fractal parallel model for carbon paper, a basal material of the GDL, is in good agreement with the reference value supplied by Toray Inc. The prediction results from the proposed models are also reasonable because they are distributed between the upper and lower bounds. Parametric effect has been investigated by using the presented models in dimensionless formalism. It can be concluded that dimensionless effective thermal conductivity ( k ′ eff ) has a positive correlation with effective porosity ( ɛ) or the pore-area fractal dimension ( D p) when k s/ k g < 1; whereas it has a negative correlation with ɛ or D p when k s/ k g > 1 and with tortuous fractal dimension ( D t) whether k s/ k g < 1 or not. Furthermore, these fractal models have been modified by considering the effect of polytetrafluoroethylene (PTFE) incorporated into the pore spaces of carbon paper, and the corresponding model prediction shows that there is an increase in the effective thermal conductivity due to the filling of PTFE that has high thermal conductivity.