Effective thermal conductivity and thermal contact resistance of gas diffusion layers in proton exchange membrane fuel cells. Part 2: Hysteresis effect under cyclic 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 the determination of the effective thermal conductivity as well as the thermal contact resistance between the GDL and adjacent surfaces/layers. The Part 1 companion paper describes an experimental procedure and a test bed devised to allow separation of the effective thermal conductivity and thermal contact resistance, and presents measurements under a range of static compressive loads. In practice, during operation of a fuel cell stack, the compressive load on the GDL changes.In the present study, experiments are performed on Toray carbon papers with 78% porosity and 5% PTFE under a cyclic compressive load. Results show a significant hysteresis in the loading and unloading cycle data for total thermal resistance, thermal contact resistance (TCR), effective thermal conductivity, thickness, and porosity. It is found that after 5 loading-unloading cycles, the geometrical, mechanical, and thermal parameters reach a “steady-state” condition and remain unchanged. A key finding of this study is that the TCR is the dominant component of the GDL total thermal resistance with a significant hysteresis resulting in up to a 34% difference between the loading and unloading cycle data. This work aims to clarify the impact of unsteady/cyclic compression on the thermal and structural properties of GDLs and provides new insights on the importance of TCR which is a critical interfacial transport phenomenon.     

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.     

Numerical determination of the effective thermal conductivity of fibrous materials. Application to proton exchange membrane fuel cell gas diffusion layers

The knowledge of physical properties, such as the thermal conductivity, plays an important role in the management of the heat transfer in fibrous materials like PEFC gas diffusion layers. Measurement of thermal conductivity by experimental means is not easy (due to the anisotropy and the high porosity), therefore the availability of experimental data is rather limited. In this paper, the numerical determination of the effective thermal conductivity of fibrous materials is investigated using a three-dimensional approach. Two different fiber geometries were studied with randomly generated fiber structures with overlapping and non-overlapping fibers. The corresponding anisotropic thermal conductivities are computed through the solution of the energy transport equation. The results were validated through a comparison with existing experimental data and the influence of different parameters such as fiber orientation, fiber diameter and binding material were investigated.     

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${k^{\prime }}_{\text{eff}}$) has a positive correlation with effective porosity (?) or the pore-area fractal dimension (D_p) when k_s/k_g < 1; whereas it has a negative correlation with ? 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.     

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.     

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.     

Carbon nano-chain and carbon nano-fibers based gas diffusion layers for proton exchange membrane fuel cells

Gas diffusion layers (GDL) for proton exchange membrane fuel cell have been developed using a partially ordered graphitized nano-carbon chain (Pureblack^® carbon) and carbon nano-fibers. The GDL samples’ characteristics such as, surface morphology, surface energy, bubble-point pressure and pore size distribution were characterized using electron microscope, inverse gas chromatograph, gas permeability and mercury porosimetry, respectively. Fuel cell performance of the GDLs was evaluated using single cell with hydrogen/air at ambient pressure, 70 °C and 100% RH. The GDLs with combination of vapor grown carbon nano-fibers with Pureblack carbon showed significant improvement in mechanical robustness as well as fuel cell performance. The micro-porous layer of the GDLs as seen under scanning electron microscope showed excellent surface morphology showing the reinforcement with nano-fibers and the surface homogeneity without any cracks.     

Effective permeability of gas diffusion layer in proton exchange membrane fuel cells

In gas diffusion layers (GDLs) of proton exchange membrane fuel cells (PEMFCs), effective permeability is a key parameter to be determined and engineered. In this study, through-plane (TP) and in-plane (IP) flow behaviors of GDLs are investigated analytically based on a scaling estimate method. The TP permeability and IP permeability of unidirectional fibers are determined first, based on that the minimum distance and the inscribed radius between fibers are adopted as the characteristic lengths for normal and parallel flows, respectively. The permeabilities of two-dimensional (2D) and three-dimensional (3D) GDLs are estimated by a proper mixture of the local TP and IP permeabilities of fiber alignments. The mechanistic model agrees closely with experimental and numerical results over a wide porosity range. With the new model, the influences of porosity and fiber orientation on flow behaviors are analyzed.     

Water permeation through gas diffusion layers of proton exchange membrane fuel cells

Water transport through gas diffusion layer of proton exchange membrane fuels cells is investigated experimentally. A filtration cell is designed and the permeation threshold and the apparent water permeability of several carbon papers are investigated. Similar carbon paper with different thicknesses and different Teflon loadings are tested to study the effects of geometrical and surface properties on the water transport. Permeation threshold increases with both GDL thickness and Teflon loading. In addition, a hysteresis effect exists in GDLs and the permeation threshold reduces as the samples are retested. Moreover, several compressed GDLs are tested and the results show that compression does not affect the breakthrough pressure significantly. The measured values of apparent permeability indicate that the majority of pores in GDLs are not filled with water and the reactant access to the catalyst layer is not hindered.     

Effect of gas diffusion layer modulus and land-groove geometry on membrane stresses in proton exchange membrane fuel cells

The electrical functionality of PEM fuel cells is facilitated by minimizing the contact resistances between different materials in the fuel cell, which is achieved via compressive clamping. The effect of the gas diffusion layer (GDL) modulus on the in-plane stress in the membrane after clamping is studied via numerical simulations, including both isotropic and anisotropic GDL properties. Furthermore, the effect of cell width and land-groove width ratio on the in-plane stress in the membrane subjected to a single hygro-thermal cycle is investigated for aligned and alternating gas channel geometries. The results from varying the GDL properties suggest that the in-plane stress in the membrane after clamping is due to a non-linear and coupled interaction of GDL and membrane deformation. The results of the geometric studies indicate that when the gas channels are aligned, the cell width and land-groove width ratio affect the in-plane stress distribution, but do not significantly affect the stress magnitudes. However, when the gas channels are alternating, the cell width and land-groove width ratio have significant effect on the membrane in-plane stresses. The effect of land-groove geometry is qualitatively verified by a series of experimental compression tests.     

Evaluating breakthrough pressure in gas diffusion layers of proton exchange membrane fuel cells

This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer （GDL） of a pro- ton exchange membrane fuel cell （PEMFC）. An ex-situ testing was conducted on a transparent test cell to visu- alize the water droplet formation and detachment on the surface of different types of GDLs through a CCD cam- era. The breakthrough pressure, at which the liquid water penetrates the GDL and starts to form a droplet, was measured. The breakthrough pressure was found to be different for the GDLs with different porosities and thick- nesses. The equilibrium pressure, which is defined as the minimum pressure required maintaining a constant flow through the GDL, was also recorded. The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL. A pore network model was modified to further study the relationship between the breakthrough pressure and the GDL properties and thicknesses. The breakthrough pressure increases for the thick GDL with smaller micro-pore size.     

The influence of graphitization on the thermal conductivity of catalyst layers and temperature gradients in proton exchange membrane fuel cells

As the proton exchange membrane fuel cell (PEMFC) has improved its performance and power density, the efficiency has remained unchanged. With around half the reaction enthalpy released as heat, thermal gradients grow. To improve the understanding of such gradients, PEMFC component thermal conductivity has been increasingly investigated over the last ten years, and the catalyst layer (CL) is one of the components where thermal conductivity values are still scarce. CLs in PEMFC are where the electrochemical reactions occur and most of the heat is released. The thermal conductivity in this region affects the heat distribution significantly within a PEMFC. Thermal conductivities for a graphitized and a non-graphitized CL were measured for compaction pressures in the range of 3 and 23 bar. The graphitized CL has a thermal conductivity of 0.12 ± 0.05 WK^–1m^–1, whilst the non-graphitized CL conductivity is 0.061 ± 0.006 WK^–1m^–1, both at 10 bar compaction pressure. These results suggest that the graphitization of the catalyst material causes a doubling of the thermal conductivity of the CL. This important finding bridges the very few existing studies. Additionally, a 2D thermal model was constructed to represent the impact of the results on the temperature distribution inside a fuel cell.     

X-ray computed tomography of gas diffusion layers of PEM fuel cells: Calculation of thermal conductivity

Three commercially available gas diffusion layers were investigated by 3D X-ray computed tomography (CT). The carbon fibers and the 3D structure of the gas diffusion layers were clearly resolved by this lab-based technique.     

Effective transport coefficients in PEM fuel cell catalyst and gas diffusion layers: Beyond Bruggeman approximation

The Bruggeman approximation has widely been used for estimating the effective conductivity and diffusivity of both the catalyst and gas diffusion layers of polymer electrolyte membrane (PEM) fuel cells. This approximation is based on the Bruggeman’s Effective Medium Theory [Bruggeman D. Berechnung verschiedener physikalischer konstanten von heterogenen substanzen. Ann Phys (Leipzig) 1935;24:636–79], which provides empirical correlation for the effective properties of a composite system. Since it is an empirical correlation, a unique correlation based on the Bruggeman approximation does not always hold for the PEM fuel cell effective properties. Rather, the Bruggeman correlation is a cell specific and experiment dependent correlation that depends on structure, phase composition, water saturation, experimental parameters, etc. Further, this correlation needs to be combined with other correlations to estimate the effective diffusivities. In this article, a set of mathematical formulations has been proposed for the effective transport properties in both the catalyst and gas diffusion layers of a PEM fuel cell. The effective conductivity and diffusivity expressions are derived from the mathematical formulations of the Hashin Coated Sphere model [Hashin Z. The elastic moduli of heterogeneous materials. J Appl Mech 1962;29:143–50], which provides an identical mathematical foundation for each of these effective properties rather than an empirical correlation and avoid to use of multiple correlations together. The present model formulations agree well with the results available in literature for the limiting case. Hence, the proposed formulations for the effective transport properties will be a useful estimating tool in the numerical modeling of PEM fuel cells.     

Wire rod coating process of gas diffusion layers fabrication for proton exchange membrane fuel cells

Gas diffusion layers (GDLs) were fabricated using non-woven carbon paper as a macro-porous layer substrate developed by Hollingsworth & Vose Company. A commercially viable coating process was developed using wire rod for coating micro-porous layer by a single pass. The thickness as well as carbon loading in the micro-porous layer was controlled by selecting appropriate wire thickness of the wire rod. Slurry compositions with solid loading as high as 10 wt.% using nano-chain and nano-fiber type carbons were developed using dispersion agents to provide cohesive and homogenous micro-porous layer without any mud-cracking. The surface morphology, wetting characteristics and pore size distribution of the wire rod coated GDLs were examined using FESEM, Goniometer and Hg porosimetry, respectively. The GDLs were evaluated in single cell PEMFC under various operating conditions (temperature and RH) using hydrogen and air as reactants. It was observed that the wire rod coated micro-porous layer with 10 wt.% nano-fibrous carbon based GDLs showed the highest fuel cell performance at 85 °C using H₂ and air at 50% RH, compared to all other compositions.     

Pore-network modeling of liquid water flow in gas diffusion layers of proton exchange membrane fuel cells

Fluid flow through the gas diffusion layer (GDL) of fuel cells is numerically studied using a pore network modeling approach. The model is developed based on an experimental visualization technique (fluorescence microscopy). The images obtained from this technique are analyzed to find patterns of flow inside the GDL samples with different hydrophobicity. Three different flow patterns are observed: initial invasion, progression, and pore-filling. The observation shows that liquid water flows into the majority of available pores on the boundary of the untreated GDL and several branches are segregated from the initial pathways. For the treated GDL, however, a handful of boundary pores are invaded and the original pathways extend toward the other side of the medium with minimum branching. The numerical model, developed based on an invasion percolation algorithm, is used to study the effects of GDL hydrophobicity and thickness on the flow configuration and breakthrough time as well as to determine the flow rate and saturation in different GDL samples. During the injection of water into the samples, it is numerically shown that the flow rates are monotonically decreasing for both treated and untreated samples. For the treated sample, however, the injection flow rate is constantly lower than that of the untreated sample, resulting in a lower overall water saturation at breakthrough. The numerical results also suggest that hydrophobic treatment of thick samples has minor effects on water management and overall performance. The developed model can be used to optimize the GDL properties for designing porous medium with effective transport characteristics.     

Effects of porosity gradient in gas diffusion layers on performance of proton exchange membrane fuel cells

A three-dimensional, two-phase, non-isothermal model has been developed to explore the interaction between heat and water transport in proton exchange membrane fuel cells (PEMFCs). Water condensate produced from the electrochemical reaction may accumulate in the open pores of the gas diffusion layer (GDL) and retard the oxygen transport to the catalyst sites. This study predicts the enhancement of the water transport for linear porosity gradient in the cathode GDL of a PEMFC. An optimal porosity distribution was found based on a parametric study. Results show that a optimal linear porosity gradient with ?₁ = 0.7 and ?₂ = 0.3 for the parallel and z-serpentine channel design leads to a maximum increase in the limiting current density from 10,696 Am⁻² to 13,136 Am⁻² and 14,053 Am⁻² to 16,616 Am⁻² at 0.49 V, respectively. On the other hand, the oxygen usage also increases from 36% to 46% for the parallel channel design and from 55% to 67% for the z-serpentine channel design. The formation of a porosity gradient in the GDL enhances the capillary diffusivity, increases the electrical conductivity, and hence, benefits the oxygen transport throughout the GDL. The present study provides a theoretical support for existing reports that a GDL with a gradient porosity improves cell performance.     

Water permeation analysis on gas diffusion layers of proton exchange membrane fuel cells for Teflon-coating annotation

This study presents an analysis of water permeation of a polytetrafluoroethylene (PTFE)-coated gas diffusion layer (GDL) to determine the influence of hydrophobic treatment on the GDL for diagnosis of water flooding. It is found that the behaviour of water drainage is controlled by the pore configuration instead of the hydrophobicity in GDL. Better water drainage is achieved by the action of the Teflon coating in modulating the GDL pore configuration to give both a larger average pore size and a wider distribution of pore size. The results show that water penetration through the GDL must overcome a threshold surface tension defined by the largest pore range. A 30 wt.% PTFE coating of a GDL is shown to generate a satisfactory pore configuration, explaining the improved cell polarization performance with a lower driven pressure (∼1.91 kPa) and a higher rate of water drainage.     

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

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

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.