The impact of thermal conductivity and diffusion rates on water vapor transport through gas diffusion layers

Proper water management in a hydrogen-fueled polymer electrolyte membrane (PEM) fuel cell is critical for performance and durability. A mathematical model has been developed to elucidate the effect of thermal conductivity and water vapor diffusion coefficient in the gas diffusion layers (GDLs). The fraction of product water removed in the vapor phase through the GDL as a function of GDL properties/set of material and component parameters and operating conditions has been calculated. The current model enables identification of conditions wherein condensation occurs in each GDL component. The model predicts the temperature gradient across various components of a PEM fuel cell, providing insight into the overall mechanism of water transport in a given cell design. The water condensation conditions and transport mode in the GDL components depend on the combination of water vapor diffusion coefficients and thermal conductivities of the GDL components. Different types of GDLs and water transport scenarios are defined in this work, based on water condensation in the GDL and fraction of water that the GDL removes through the vapor phase, respectively.     

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

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

Analysis of transient response of a unit proton-exchange membrane fuel cell with a degraded gas diffusion layer

The transient response characteristics and durability problems of proton-exchange membrane fuel cells are important issues for the application of PEM fuel cells to automotive systems. The gas diffusion layer is the key component of the fuel cell because it directly influences the mass transport mechanism. In this study, the effects of GDL degradation on the transient response of the PEM fuel cell are systematically studied using transient response analysis under different stoichiometric ratios and humidity conditions. With GDLs aged by the accelerated stress test, the effects of hydrophobicity and structural changes due to carbon loss in the GDL on the transient response of PEM fuel cells are determined. The cell voltage is measured according to the sudden current density change. The degraded GDLs that had uneven hydrophobicity distributions cause local water flooding inside the GDL and induce lower and unstable voltage responses after load changes.     

Enhanced mechanical and electrochemical durability of multistage PTFE treated gas diffusion layers for proton exchange membrane fuel cells

Gas Diffusion Layers (GDLs) of Proton Exchange Membrane Fuel Cells (PEMFCs) are usually subject to polytetrafluoroethylene (PTFE) treatment in a single stage. The impact of multistage PTFE treatment on the mechanical and electrochemical durability of GDLs used in PEMFCs, is reported here. With the same total amount of PTFE in the GDL, substrates treated with PTFE in multiple stages are seen to possess distinctly improved mechanical and electrochemical durability compared to GDLs treated with PTFE in a single stage. The difference in structure and hydrophobicity of the GDLs, when they are subjected to the two different PTFE treatment routes, are examined to understand the reasons for the improvements. The results indicate that there is a change in surface morphology, pore size distribution, and hydrophobicity of the GDL samples depending on the treatment route adopted. It is observed that it is possible to establish a gradient in PTFE profile in the GDL by adopting multistage treatment. Such gradients counteract loss in hydrophobicity resulting from compression cycles during cell assembly and carbon corrosion due to electrochemical aging. The results reveal that GDLs subject to multistage PTFE treatment, can have increased lifetime as opposed to conventional single stage PTFE treated GDL.     

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

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

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.     

In-plane gas permeability of proton exchange membrane fuel cell gas diffusion layers

A new analytical approach is proposed for evaluating the in-plane permeability of gas diffusion layers (GDLs) of proton exchange membrane fuel cells. In this approach, the microstructure of carbon papers is modeled as a combination of equally-sized, equally-spaced fibers parallel and perpendicular to the flow direction. The permeability of the carbon paper is then estimated by a blend of the permeability of the two groups. Several blending techniques are investigated to find an optimum blend through comparisons with experimental data for GDLs. The proposed model captures the trends of experimental data over the entire range of GDL porosity. In addition, a compact relationship is reported that predicts the in-plane permeability of GDL as a function of porosity and the fiber diameter. A blending technique is also successfully adopted to report a closed-form relationship for in-plane permeability of three-directional fibrous materials.     

Modeling the original and cyclic compression behavior of non-woven gas diffusion layers for fuel cells

It is established that the compression behavior of gas diffusion layers (GDL) is dependent on the level of the mechanical stress it experienced during its lifetime. As a matter of fact, every cycle of compression induces damages in the GDL, including fibers breakage and/or their spatial reorganization. As observed in the experimental work, the first cycle of compression of GDLs as received from the suppliers is already altered by a previous compression that is applied during the manufacturing process. This paper then presents a model able to predict the cyclic behavior of GDL, considering the existence of this compressive stress applied during the manufacturing process. The experimental mechanical properties of the three main types of non-woven GDL (rolls, sheets and felts) were first measured and then predicted using the proposed model, thereby allowing to separate the influences of the manufacturing process, the type of fibers, the presence of a micro-porous layer and a hydrophobic treatment on the GDL.     

Fractal model for predicting the effective binary oxygen diffusivity of the gas diffusion layer in proton exchange membrane fuel cells

We propose an analytical model to predict the effective binary oxygen diffusivity of the porous gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs). In this study, we consider the fractal characteristics of the porous GDL as well as its general microstructure, and we adopt the Bosanquet equation to derive effective diffusivity. The fractal characterization of GDL enables us to model effective diffusivity in a continuous manner while taking into account the effect of pore size distribution. Comparison to two other theoretical models that are generally accepted in the simulation of PEMFCs shows similar trends in all three models, indicating that our proposed model is well founded. Furthermore, the predicted effective binary oxygen diffusivities of two samples show that after treatment with polytetrafluoroethylene (PTFE), the effective binary diffusivity of the GDL decreases. Based on the parametric effect analysis, we conclude that effective binary diffusivity is negatively correlated with tortuosity fractal dimension but positively correlated with the fractal dimension of pore area, porosity, or mean pore diameter. The proposed model facilitates fast prediction of effective diffusivity as well as multi-scale modeling of PEMFCs and thus facilitates the design of the GDLs and of PEMFCs.     

Characterisation of mechanical behaviour and coupled electrical properties of polymer electrolyte membrane fuel cell gas diffusion layers

Local compression distribution in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC) and the associated effect on electrical material resistance are examined. For this purpose a macroscopic structural material model is developed based on the assumption of orthotropic mechanical material behaviour for the fibrous paper and non-woven GDLs. The required structural material parameters are measured using depicted measurement methods. The influence of GDL compression on electrical properties and contact effects is also determined using specially developed testing tools. All material properties are used for a coupled 2D finite element simulation approach, capturing structural as well as electrical simulation in combination. The ohmic voltage losses are evaluated assuming constant current density at the catalyst layer and results are compared to cell polarisation measurements for different materials.     

Stochastic modeling of polymer electrolyte membrane fuel cell gas diffusion layers – Part 1: Physical characterization

Stochastic modeling of GDL structures requires a detailed characterization of the constituent elements of the material. In this work, a variety of imaging methods, including optical microscopy, microscale computed tomography, and scanning electron microscopy were used to characterize seven commercially available gas diffusion layers (GDLs). The result is a catalogue of the following geometrical characteristics: fiber diameter, fiber pitch and co-alignment, areal weight and volume, and microporous layer (MPL) crack size and frequency. This catalogue, when combined with previous GDL characterizations, is expected to provide enough information to create representative, predictive, stochastic models of the GDL.     

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.     

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.     

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.     

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.     

Influence of anisotropic bending stiffness of gas diffusion layers on the electrochemical performances of polymer electrolyte membrane fuel cells

The effects of gas diffusion layer’s (GDL’s) anisotropic bending stiffness on the electrochemical performances of polymer electrolyte membrane fuel cells have been investigated for carbon fiber-felt and -paper GDLs. The bending stiffness values of all GDLs in the machine direction are higher than those in the cross-machine direction. We have prepared GDL sheet samples such that the machine direction of GDL roll is aligned with the major flow field direction of a metallic bipolar plate at angles of 0° (parallel: ‘0° GDL’) and 90° (perpendicular: ‘90° GDL’). The I–V performances of all the 5-cell stacks with 90° GDLs are higher than those with 0° GDLs. All the 5-cell stacks with 90° GDLs show lower values of high-frequency resistance (HFR) than those with 0° GDLs. However, the gas pressure differences at both anode and cathode of 5-cell stacks with 90° GDLs appear to be similar to or slightly lower than those with 0° GDLs, making the lower HFR as a dominant factor for the improved I–V performances. This may result from the reduced intrusion of 90° GDLs into gas channels than 0° GDLs as observed by less thickness reduction under compression of 90° GDLs. A 45° GDL (skew alignment) also shows better performances than the 0° GDL.     

Steady saturation distribution in hydrophobic gas-diffusion layers of polymer electrolyte membrane fuel cells: A pore-network study

A pore-network model is developed to simulate liquid water transport in a hydrophobic gas-diffusion layer (GDL) during the operation of polymer electrolyte membrane fuel cells (PEMFCs). The steady saturation distribution in GDLs is determined through a numerical procedure using a pore-network model combined with invasion-percolation path-finding and subsequent viscous two-phase flow calculation. The simulation results indicate that liquid water transport in hydrophobic GDLs is a strongly capillary-driven process that almost reaches the pure invasion-percolation limit with zero capillary number. A uniform flux condition is found to better reflect the actual phenomenon occurring at the inlet boundary for liquid water entering a GDL than a uniform pressure condition. The simulation further clarifies the effect of the invaded pore fraction at a uniform-flux inlet boundary in modifying water transport in GDLs. Finally, the effect of the GDL thickness on the steady saturation distribution is investigated.     

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.     

Anode water removal and cathode gas diffusion layer flooding in a proton exchange membrane fuel cell

Anode water removal (AWR) is studied as a diagnostic tool to assess cathode gas diffusion layer (GDL) flooding in PEM fuel cells. This method uses a dry hydrogen stream to remove product water from the cathode, showing ideal fuel cell performance in the absence of GDL mass transfer limitations related to water. When cathode GDL flooding is limiting, the cell voltage increases as the hydrogen stoichiometry is increased. Several cathode GDLs were studied to determine the effect of microporous layer (MPL) and PTFE coating. The largest voltage gains occur with the use of cathode GDLs without an MPL since these GDLs are prone to higher liquid water saturation. Multiple GDLs are studied on the cathode side to exacerbate GDL flooding conditions to further confirm the mechanism of the AWR process. Increased temperature and lower cathode RH allow for greater overall water removal so the voltage improvement occurs faster, though this leads to quicker membrane dehydration.