Gas Diffusion Layer Structure Research Articles

Coating Distribution Analysis on Gas Diffusion Layers for Polymer Electrolyte Fuel Cells by Neutron and X-ray High-Resolution Tomography.

Coating load and distribution in gas diffusion layers (GDLs) for polymer electrolyte fuel cells (PEFCs) have a major influence on mass transport losses. To be able to optimize the coating distribution and get more accurate data about the influence of the coating on the PEFC performance, better characterization techniques are necessary. Common analysis techniques are limited to selected sections of the material, or they are not sensitive to small amounts of coating. We propose a new methodology to get a complete description of the coating distribution and the GDL structure by combining high-resolution X-ray tomography with high-resolution neutron tomography. Using an isotopic gadolinium staining method to enhance the neutron and X-ray absorption contrast, lower quantities of coating can be detected. The combination of both imaging techniques allows for a more detailed analysis of the coating distribution.

Characterization of Gas Diffusion Layers through Computational Fluid Dynamics Modeling and BET Theory

The purpose of this work is to use computation fluid dynamics using mesh simulations to model the flow through gas diffusion layers (GDLs), in order to calculate the fundamental properties of the GDL. The study of porosity, tortuosity, liquid and gas permeability, and pore size distribution for GDL’s are the primary focus points in this work. The GDL model geometries seen in this investigation were obtained through three dimensional reconstructed micro-structures from micro X-ray computed tomography (CT). Figure 1 shows the microstructure and streamlines to model the movement of fluid through the GDL. The simulated GDL’s consist of: EP40T, Sigracet 25BA, Sigracet 25BC, GDS3250, and GDS3260. The tortuosity calculated from simulations is taken at multiple points on the GDL to obtain an averaged value for the sample. The average value is used because the GDL structure is a matrix of random carbon fibers. The liquid and gas phase permeability is important to the calculation of tortuosity because it is a measure of the willingness of a porous media to flow through it. In addition, the simulated results are compared to experimental data through the calculation of the MacMullin number. The MacMullin number is a relationship defined by porosity divided by tortuosity. This number describes the ionic resistance, or resistance to current flow, through the GDL.

Combining Pore-Scale Liquid Water Visualization and Modeling to Understand Water Transport in Operating Fuel Cells

An in-depth understanding of liquid water transport in fuel cell gas diffusion layers (GDLs) has the potential to inform the development of novel water management strategies that lead to improved fuel cell performance at high current densities. Several visualization (1-4) and modeling (5, 6) studies have shed light into some of these mechanisms of liquid water transport. However, due to the heterogenous nature of porous materials within the fuel cells, pore-scale modeling and simulations are required for in-depth analysis. Due to recent advancements in visualization techniques, particularly high-resolution synchrotron X-ray (1-4) and neutron (7) micro-computed tomography, the spatial distribution of liquid water within individual GDL pores during fuel cell operation can be resolved. In addition, pore network modeling (8) has been used to simulate and examine pore-to-pore water transport behavior within a wide range of GDL materials. However, we need to combine pore-network simulations with experimental data of realistic fuel cell conditions to study water transport in representative operating conditions in greater detail. In this study, we combined 3-D computed tomography and pore network modeling to expand on our understanding of liquid water transport within gas diffusion layers of operating fuel cells. First, we used synchrotron X-ray tomography to examine the micron-scale GDL porous structure and in-operando 3-D liquid water distribution using a custom fuel cell specialized for imaging. Then, we simulated water transport within the examined GDL structure using pore network modelling. We then compared the 3-D experimental results to our simulations to gain a deeper understanding of realistic inlet conditions for liquid water transport within the GDL. The study demonstrates a robust strategy to probe, understand, and predict liquid water transport within the complex porous structures in PEM fuel cells. References S. J. Normile, D. C. Sabarirajan, O. Calzada, V. De Andrade, X. Xiao, P. Mandal, D. Y. Parkinson, A. Serov, P. Atanassov and I. V. Zenyuk, Meter. Today Energy., 9 (2018).J. Eller, J. Roth, F. Marone, M. Stampanoni and F. N. Büchi, J. Electrochem. Soc., 164, 2 (2017).S. S. Alrwashdeh, I. Manke, H. Markötter, M. Klages, M. Göbel, J. Haußmann, J. Scholta and J. Banhart, ACS Nano., 11, 6 (2017).P. Krüger, H. Markötter, J. Haußmann, M. Klages, T. Arlt, J. Banhart, C. Hartnig, I. Manke and J. Scholta, J.Power Sources., 196, 12 (2011).C. Y. Wang, Chem. Rev., 104 (2004).P. P. Mukherjee, Q. J. Kang and C. Y. Wang, Energy Environ. Sci., 4, 2 (2011).J. M. LaManna, Y. Yue, T. A. Trabold, J. D. Fairweather, D. S. Hussey, E. Baltic and D. L. Jacobson, Meet. Abstr. - Electrochem.Soc., 32 (2017).J. Gostick, M. Aghighi, J. Hinebaugh, T. Tranter, M. A. Hoeh, H. Day, B. Spellacy, M. H. Sharqawy, A. Bazylak, A. Burns, W. Lehnert and A. Putz, Comput. Sci. Eng., 18, 4 (2016).

Water Transport in Gas Diffusion Layer of PEFC with Wettability Distribution in Thickness Direction

Polymer electrolyte fuel cell (PEFC) is promising as a next generation power source for vehicles due to the high efficiency. To make the PEFC commercially competitive, the reduction of the cost is necessary. For the reduction of the cost of PEFC, it is required to increase the maximum current density. Under the high current density operation, the water produced in a catalyst layer (CL) accumulates in a gas diffusion layer (GDL) and the supply of oxygen is suppressed. Therefore, the water transport in the GDL is important to increase the maximum current density of PEFC. To improve the water transport in the GDL, the wettability distribution is important factor. Utaka el al. showed that better cell performance is obtained by forming the hydrophilic region in the hydrophobic GDL because the water transport in the GDL is controlled by the hydrophilic region (1). Similarly, Forner-Cuenca et al. showed that the cell performance is improved by the GDL with the hydrophilic region (2). There are some reports which showed the water transport is improved by the control of the wettability of the GDL. However, these reports are focused on the planer distribution in the GDL and the effect of the wettability distribution in the thickness direction is not clear. For the optimization of GDL structure, it is necessary to clarify the effect of the wettability distribution in the thickness direction. Therefore, in this study, the effect of the wettability distribution in the thickness direction was investigated. To evaluate the water transport in the GDL with the wettability distribution in the thickness direction, a lattice Boltzmann method (LBM) was used (3). Figure 1 shows the simulation domain and boundaries. The size of GDL is 80μm × 80μm × 80μm and the porosity of GDL is 70%. The bottom of the simulation domain is assumed the micro-porous layer (MPL) and it is treated as a solid wall. MPL crack as inlet boundary is set in the MPL. The top of the simulation domain is outlet boundary. The four sides of the simulation domain are solid walls. The contact angle of GDL, MPL and side walls is set 130º. The density of the two-phase in the simulation is set identical to water, ρ = 978kg/m3. The interfacial tension between the two-phase is σ = 6.40×10-2N/m, and liquid and gas viscosities are μL = 4.04×10-4Pa∙s and μG = 2.30×10-5Pa∙s respectively. Figure 2 shows simulation results of water transport in GDL. Figure 2 (a) shows the result for the GDL without the wettability distribution. The contact angle of the GDL is uniform (130 º). Figure 2 (b) shows the result for the GDL with the wettability distribution in the thickness direction. The contact angle from the GDL surface to the depth of 16 μm is 50º and the other regions of the GDL is 130º. In these figures, the water is shown in red. With the water transport in the GDL without the wettability distribution (Fig. 2 (a)), the water was inflowed to the large pore in the center of the GDL (0.32ms). The water was spread in the large pore (0.96ms). After the much water was accumulated in the GDL, the water was reached at the GDL surface (1.9ms). With the water transport in the GDL with the wettability distribution in the thickness direction (Fig. 2 (b)), the water was inflowed to the large pore in the center of the GDL and the water accumulated in the GDL as well as the GDL without the wettability distribution (0.32ms, 0.96ms). When the water was reached at the hydrophilic region in the top of GDL, the water in the GDL was pulled up to the GDL surface and the amount of the water in the center of the GDL was reduced (1.2ms). Compared with the GDL without wettability distribution, the time when the water reached the GDL surface was earlier in the GDL with wettability distribution. Therefore, the water transport in the thickness direction is improved by the wettability distribution in the thickness direction. In the final full paper, the large-scale simulation including the rib and channel will be carried out and the effect of the wettability in the thickness direction will be discussed. Reference (1) Y. Utaka, R. Koresawa, J. Power Sources, 363, 227-233 (2017). (2) A. Forner-Cuenca, J. Biesdorf, V. Manzi-Orezzoli, L. Gubler, T. J. Schmidt, P. Boillat, J. Electrochem. Soc., 163 (13), F1389 (2016). (3) S. Sakaida, Y. Tabe, and T. Chikahisa, J. Power Sources, 361, 133-143 (2017). Figure 1

Impact of gas diffusion layer mechanics on PEM fuel cell performance

During the clamping of a Fuel Cell, the porosity of the gas diffusion layer (GDL) and the contact resistances are continuously reduced. In this work, the optimum clamping pressure is determined for three commonly used GDLs. For this purpose, polarization curves are recorded for contact pressures of 0.1–2.7 N/mm2. For the SGL 29BCE material, the optimum contact pressure range is very narrow, while the Toray TGP-H 060 material performs best in a wider range; the Freudenberg H2315 material is the most robust type, and works best from 0.6 N/mm2 until the maximum (2.7 N/mm2). For the understanding of the mechanical effects on the GDL structure, scanning electron microscopy and nano-computer tomography images are made. In addition, permeability measurements are carried out. To transfer the results, the local pressure distributions are recorded, and their minima and maxima are considered. Ultimately, this information allows the transfer of the results to other flow field geometries.

Mass transport in anode gas diffusion layer of direct methanol fuel cell derived from compression effect

The anode gas diffusion layer plays an important role on mass transport. By adjusting the assembly pressure, the structure of gas diffusion layer can be controlled to investigate the two-phase behavior. The electrochemical test indicates that increasing the assembly pressure causes increase in mass transport resistance. Inhomogeneous compression of gas diffusion layer is observed by three-dimensional imaging technology. Based on the experimental data, a two-phase model is established. The simulated results show that the increment of surface roughness and the reduction of porosity, both derived from compression of gas diffusion layer, exacerbate CO2 blockage of the cell. Lowering polytetrafluoroethylene content of anode gas diffusion layer alleviates CO2 blockage by reducing the difference of surface tension between liquid-solid and gas-solid interface as well as the negative capillary pressure of gas diffusion layer. Thus, the peak power density climbs from 60.61 to 49.94 mW cm−2 to 74.79 and 70.47 mW cm−2 at the assembly pressure of 1.00 and 2.00 MPa, respectively. The optimal assembly pressure locates at where good contact between membrane electrode assembly and bipolar plate is achieved, with the least gas diffusion layer deformation possible.

Effect of Current Density on the Dynamical Evolution of Liquid Water Distribution within the Gas Diffusion Layers of PEMFC Employing a Multi-Timescale Kinetic Monte Carlo Method

The liquid water produced in the cathode reaction layer of a polymer electrolyte membrane fuel cell (PEMFC) can obstruct the transport paths of the reactant gases within the gas diffusion layers (GDL), especially at high current densities. This may lead to fuel starvation, reduced overall cell performance and lifetime. In this study, using a newly developed multi-timescale kinetic Monte Carlo model, the evolution of the liquid water distribution over time within a real GDL structure for different current densities is simulated. The processes included in this model comprise water transport and water generation, which occur on two different timescales. The multi-timescale model allows for sampling both process types by implementing the much slower water production reaction with a defined interval among the faster and more frequent water movement steps. The simulation results reveal how the real cell operating conditions and GDL surface wetting properties can affect the amount and distribution of liquid water clusters within the structure.

Study on PEFC Gas Diffusion Layer with Designed Wettability Pattern Tolerant to Flooding

For reduction of the cost of a polymer electrolyte fuel cell (PEFC), it is desired to increase the maximum current density. Under high current density operation, it is important to control water drainage for smooth supply of oxygen. A gas diffusion layer (GDL) which is placed between separator and CL plays an important role for the water transport. Therefore, the improvement of GDL design is important to increase the maximum current density. Recently, there is a report which shows that better cell performance was obtained using the GDL with designed hydrophobic and hydrophilic lines (1). However, the water transport in the GDL is still unclear because the detail observation of liquid water behavior in GDL is difficult due to the quite complex structure and the very small pores (pore diameter: approximately 10 mm). Therefore, there is some room for improving the GDL structure. The objective of this study is to investigate the water transport in GDL with designed wettability pattern and to develop the ideal GDL. To evaluate the water transport in the GDL, a lattice Boltzmann method (LBM) is applied. The LBM is a simulation method to analyze the fluid behavior by tracking movements of large number of particle ensembles. The particle density is expressed by distribution functions, and the distribution functions are calculated by a simple law of collision and transition ensuring the continuity equation and the Navier-Stokes equations for incompressible fluids. Due to the simplicity of the algorithm, it has an advantage of adaptability for complex boundary geometries. Therefore, the LBM is suitable model to simulate the water behavior in the GDL. For high speed calculation, the LBM treating two-phases as having the same density is applied in this study although the densities of air and water are very different. The applicability of this LBM model for the simulation in GDL has been confirmed in the previous study (2). The density of the two-phase in the simulation is set identical to water, ρ = 978kg/m3. The interfacial tension between the two-phase is σ = 6.40×10-2N/m, and liquid and gas viscosities are μL = 4.04×10-4Pa∙s and μG = 2.30×10-5Pa∙s respectively. Figure 1 shows the simulation domain and boundaries. The lower half of simulation domain is GDL layer, on which rib and channel space are placed. Size of GDL is 240μm × 240μm × 100μm and the porosity of GDL is 75%. Sizes of rib and channel are both 120μm × 240μm × 160μm. The bottom of the simulation domain is assumed the micro-porous layer (MPL) and it is treated as a solid wall. Commonly, much water stays in the GDL region under rib. Therefore, to focus on the water behavior in GDL under rib, MPL crack as inlet boundary is set at MPL under rib. Top of the channel is outlet boundary. Four side boundaries are solid walls. The contact angle of GDL, MPL and side walls is set 130º. The contact angle of rib is set 50º. Figure 2 shows simulation results of water transport in GDL without designed wettability pattern. In the simulation, the water is transported randomly and water spreads in the x-y plane of GDL. Finally, the water is drained to channel from pores which is slightly away from the rib. Figure 3 shows simulation results of water transport in GDL with designed wettability pattern. The pattern is set in the center of GDL structure and the size is 50μm × 240μm × 100μm. The contact angle of designed wettability pattern is 50º. Here, the GDL structure is same as Fig. 2. In the simulation, the water is accumulated in the designed wettability pattern of GDL and the smooth water transport from the GDL region under rib to the GDL region under channel is achieved. It is known that water tends to retain in the GDL under ribs and smooth removal of the water is important to avoid flooding. In Ref. (1), it seems that the cell performance using the GDL with designed wettability pattern was improved by the smooth water transport from GDL under rib to GDL under channel. In the final full paper, the effect of the structural characteristics of the GDL such as the contact angle of the pattern and the pore distributions will be discussed. Figure 1

Effect of Current Density and Operating Parameters on the Dynamical Evolution of Liquid Water Distribution within the Gas Diffusion Layers of PEMFC Using a Multiscale Kinetic Monte Carlo Method

In this study, a kinetic Monte Carlo (KMC) model is employed to observe the variation of the liquid water distribution within the gas diffusion layers (GDL) of polymer electrolyte membrane fuel cells (PEMFCs) with time and current density. The application of the model yields deeper understanding on the dependence of the liquid water agglomerations quantity and dispersal on the surface characteristics, the thermodynamic boundary conditions and the current density. This work, which focuses on the evolution of the system with time, represents a mayor enhancement to the previous works on the Grand Canonical Monte Carlo (GCMC) model 1–3, in which the behavior of the system was investigated after a steady-state had been reached. Particularly, by direct integration of the current density into the simulations, in addition to the local temperature and relative humidity, another step forward towards a complete representation of the real operating conditions of a running fuel cell is reached.In the simulations, the real GDL structure is mapped onto a 3D grid featuring an appropriate voxel size, which is chosen to adequately represent the structural details of the respective material. As an initial state at the time zero, either a certain amount of liquid water can be randomly distributed within the open pores, or the steady-state water distribution obtained from a previous simulation with the standard GCMC model in the same GDL structure can be used. In the KMC model, the dynamic of the system is characterized by subsequent transitions from one state to another. Escaping from a certain state can occur along a number of pathways. Each of these pathways has its own rate constant, which corresponds to the Metropolis rate. All the states which preceded the actual state of the system are not important. This is the defining property of Markov chains. After each movement, a time increment is computed, which is a statistical estimation of the time needed for this event to take place in reality. Having obtained the time step, the amount of water produced as the result of the current density can be calculated. The water is added from the CL side and is removed from the opposite side, when it reaches the boundary region of the simulation box. The KMC simulation results show how different current densities and boundary conditions effect the amount of liquid water in the structure, and how water distributes and moves in the GDL depending on the structural and wetting properties. Figure 1 shows an example of the simulation results at four different times after starting from a random water distribution: t = 4.6×10-8 s, t = 0.29 s, t = 0.75 s and t = 1.26 s, respectively. The initial water content corresponds to 20% occupation of the available pore space. In this simulation the current density was set to a constant value of 0.9 A cm-2 and the voxel size had a value of 4.348 μm. The local temperature and relative humidity within the structure were obtained from a previous CFD study. The surface contact angle of the GDL to water is considered to have a value of 94°. The results show the change in size, amount and distribution of water agglomerations with time. As expected for a hydrophobic surface, the water tends to fill the bigger pores within the structure.

Effect of Current Density and Operating Parameters on the Dynamical Evolution of Liquid Water Distribution within the Gas Diffusion Layers of PEMFC Using a Multiscale Kinetic Monte Carlo Method

Gas transport paths within the gas diffusion layers (GDL) of polymer electrolyte membrane fuel cells (PEMFC) could become obstructed by liquid water agglomerations, especially at high current densities, which can lead to fuel starvation, reduced overall cell performance and lifetime. In this study, using a multiscale kinetic Monte Carlo model, the evolution of the liquid water distribution with time within a real GDL structure for different current densities is simulated. The processes covered in this model comprise water transport and water generation, which involve two different timescales. The multiscale model allows for sampling both types of processes by implementing the much slower water production reaction with a defined interval among the faster and more frequent water movement steps. The simulation results reveal how the operating conditions and GDL surface wetting properties effect the amount and distribution of the liquid water clusters within the structure.

Change of Preferential Liquid Breakthrough Pathways in PEMFC Gas Diffusion Layers: Evidence of Pressure-Induced Dynamic Transport

Polymer Electrolyte Membrane (PEM) fuel cells are considered as promising clean sources for automotive applications. A key challenge to reduce the cost of this technology is to increase the power density by operating PEM fuel cells at high current densities. One strategy is to improve the 3D structure of the gas diffusion layer (GDL) located between the channel and the catalyst layer (CL). At the cathode side, this GDL is used to evacuate the liquid water toward the channel while providing access to the oxygen to the CL [1]. Although innovative GDL structures have been proposed, a better understanding of the liquid water would help to design better GDL. In particular, few works have investigated the liquid water transport after several hours of steady state fuel cell operating conditions although changes in liquid water preferential pathway have been observed [2]. Such changes are not predicted in theory, and a physical explanation of these mechanisms is still required. In this presentation, we investigate ex situ the change of liquid water preferential pathways through PEM fuel cell GDLs. Liquid water breakthough from the GDL to the fuel cell channels is mimicked using a microfluidic PDMS device with embedded GDLs. Liquid water is injected through the GDL to the microchannel, and changes in liquid water pathways are observed after several hours of operations. These changes in liquid water pathway are attributed to the dynamic of droplet eruption in fuel cell microchannel. To validate this assumption, a small capillary network with is built. It is shown that after the liquid water breakthrough, the pressure variation due to the growth of the water droplets in the microchannel enables the liquid water to invade smaller capillaries which at the end can change the preferential path. The experimental observations reported in this work show that the invasion percolation theory can be used to predict the preferential pathway at the onset of the liquid water breakthrough, but pressure-induced dynamic transport [3] due the water droplet eruption in the channel has also to be taken into account to predict the preferential pathway after long time of fuel cell operation.

Evolution of gas diffusion layer structures for aligned Pt nanowire electrodes in PEMFC applications

Gas diffusion layer (GDL), consisting of a microporous layer (MPL) and a carbon fibre substrate, is one of the major components in proton exchange membrane fuel cells (PEMFCs). In gas diffusion electrodes (GDEs) with in-situ grown aligned Pt nanowire (NW) catalysts, the GDL can also provide an important function in controlling the growth and distribution of the Pt nanowires. In this work, a systematic investigation is conducted to evaluate the evolution of the GDL structure on the PtNW growth process to prepare GDEs. The influence mechanisms including carbon loading, carbon composition and polytetrafluoroethylene (PTFE) loading in the MPL and PTFE in the carbon fibre substrate on the electrode power performance are studied in detail. An optimum structure for MPL, 4 mg cm−2 carbon loading with an equal amount of carbon black (CB) and acetylene black (AB), plus 5% PTFE loading, is deserved. This GDL structure can provide suitable substrate coverage, reasonable surface nucleation sites and required hydrophobicity for the in-situ growth of PtNWs. The results indicate that the GDL features play a significant role in the growth and distribution of the obtained Pt nanowires to achieve high performance GDEs for PEMFC application.

Apparent contact angles of liquid water droplet breaking through a gas diffusion layer of polymer electrolyte membrane fuel cell

The lattice Boltzmann method is used to simulate the three-dimensional dynamic process of liquid water breaking through the gas diffusion layer (GDL) in the polymer electrolyte membrane fuel cell. An accurate method is introduced to analyze asymmetric droplet shape. Ten micro-structures of Toray GDL were built based on a stochastic geometry model. It was found that asymmetric droplets are produced on the GDL surfaces. Their local apparent contact angles vary with different view angles and geometries. They are different to the idealized contact angles by symmetric simplification. It was concluded that the apparent contact angles are influenced by GDL structures and view angles. This information can help to bridge the gap between mesoscale and cell-scale simulations in the field of fuel cell simulation.

Three-dimensional lattice-Boltzmann model for liquid water transport and oxygen diffusion in cathode of polymer electrolyte membrane fuel cell with electrochemical reaction

Polymer electrolyte membrane (PEM) fuel cells have higher efficiency and energy density and are capable of rapidly adjusting to power demands. Effective water management is one of the key issues for increasing the efficiency of PEMFC. In the current study, a three-dimensional (3D) lattice Boltzmann model is developed to simulate the water transport and oxygen diffusion in the gas diffusion layer (GDL) of PEM fuel cells with electrochemical reaction on the catalyst layer taken into account. In this paper, we demonstrate that this model is able to predict the liquid and gas flow fields within the 3D GDL structure and how they change with time. With the two-phase flow and electrochemical reaction coupled in the model, concentration of oxygen through the GDL and current density distribution can also be predicted. The model is then used to investigate the effect of microporous layer on the cell performance in 2D to reduce the computational cost. The results clearly show that the liquid water content can be reduced with the existence of microporous layer and thus the current density can be increased.

Two-Phase Flow Dynamics in the Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells: Volume of Fluid Modeling and Comparison with Experiment

This paper proposes a three-dimensional (3D) volume of fluid (VOF) study to investigate two-phase flow in the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells and liquid water distribution. A stochastic model was adopted to reconstruct the 3D microstructures of Toray carbon papers and incorporate the experimentally-determined varying porosity. The VOF predictions were compared with the water profiles obtained by the X-ray tomographic microscopy (XTM) and the Leverett correlation. It was found local water profiles are similar in the sample's sub-regions under the pressure difference Δp = 1000 Pa between the two GDL surfaces, but may vary significantly under Δp = 6000 Pa. The water-air interfaces inside the GDL structure were presented to show water distribution and breakthrough.

The effects of gas diffusion layers structure on water transportation using X-ray computed tomography based Lattice Boltzmann method

The Gas Diffusion Layer (GDL) of a Polymer Electrolyte Membrane Fuel Cell (PEMFC) plays a crucial role in overall cell performance. It is responsible for the dissemination of reactant gasses from the gas supply channels to the reactant sites at the Catalyst Layer (CL), and the adequate removal of product water from reactant sites back to the gas channels.Existing research into water transport in GDLs has been simplified to 2D estimations of GDL structures or use virtual stochastic models. This work uses X-ray computed tomography (XCT) to reconstruct three types of GDL in a model. These models are then analysed via Lattice Boltzmann methods to understand the water transport behaviours under differing contact angles and pressure differences.In this study, the three GDL samples were tested over the contact angles of 60°, 80°, 90°, 100°, 120° and 140° under applied pressure differences of 5 kPa, 10 kPa and 15 kPa. By varying the contact angle and pressure difference, it was found that the transition between stable displacement and capillary fingering is not a gradual process. Hydrophilic contact angles in the region of 60°<θ < 90° showed stable displacement properties, whereas contact angles in the region of 100°<θ < 140° displayed capillary fingering characteristics.

Study on Gas Diffusion Layer Structure Tolerant to Flooding in PEFC by Scale Model Experiment and LBM Simulation

To improve the performance of a polymer electrolyte fuel cell (PEFC), it is essential to control liquid water behavior in the cell. Gas diffusion layer (GDL) plays an important role for the water transport. As the structure of GDL is quite complex and small with the scale order of 200 mm thickness, measurement of liquid water behavior in the GDL is difficult, and only a few results are reported with X-ray measurement and pore-network simulation. Therefore, the water transport in GDL is still unclear and there may be some room for improving the structure of GDL. The objective of this study is to evaluate the effects of different degree of isotropic fiber directions and uneven hydrophobic and hydrophilic structure of GDL for the water transport. For the analysis enlarged scale model experiment and high-speed Lattice Boltzmann Method (LBM) are applied. In the previous research of the authors, the effects of the GDL fiber direction on the cell performance was investigated using anisotropic GDL, and the results of the experiments showed that the cells with perpendicular fibers are more tolerant to flooding than cells with fibers parallel to the channel direction (1). The fiber direction is important factor for the water control in the GDL. The fiber direction perpendicular to the channel discharged liquid water under libs more smoothly. Similarly, there is a report which shows that better cell performance was obtained with the GDL with cyclic hydrophobic and hydrophilic lines (2). The fiber direction and wettability of GDL are important factors for the water control in the GDL, but the water behavior in the structures have not been observed and the effect of these factors on water transport is not clear yet. The authors proposed a scale model experiment with similarity to the actual behavior. In the experiment silicon oil is used for the liquid water and water for air. With this combination of the fluids, buoyancy effects are set similar with the actual system in enlarged models with 300 times of the actual size. Additionally, the authors have developed LBM model with high speed computation to make it possible to simulate the space covering the GDL width and channel. Figure 1 is a result comparing the scale model and LBM simulation. The figure shows the liquid water distribution in GDL. Figure 1 (a) is experimental results with an enlarged scale model and Fig. 1 (b) is simulation result by LBM. The two figures show similar water distribution. Figure 2 shows two different fiber structures with LBM simulation. Figure 2 (a) is the structure of anisotropic fiber and Fig. 2 (b) is the one of isotropic fiber. The fibers have a diameter of 8μm and orientation is set to give based on electrical resistivity with the actually measured GDLs with different isotropy. Sizes of the each GDL structure is 270μm × 270μm × 200μm and porosities is 75%. Liquid water is introduced from a hole placed at the bottom center of the boundary, which corresponds to MPL boundary. The contact angle of solid wall (GDL fibers, MPL, side wall) is set 130º. Figure 3 shows the difference in water distributions in two different fiber structures with LBM simulation. Figure 3 (a) shows the result for anisotropic and Fig.3 (b) shows the result for isotropic fiber. The water tends to spread along with fiber direction in the anisotropic GDL, whereas water transports upwards randomly in the isotropic GDL. It is known that water tends to retain in the GDL under ribs and smooth removal of the water is important to avoid flooding. The anisotropic GDL may guide the liquid water toward channel direction from under the rib, when the fiber direction is placed perpendicular to the rib direction. In the final full paper, the water transport characteristics with different fiber orientations and wettability will be demonstrated in detail. Reference (1) K. S. S. Naing, Y. Tabe, and T. Chikahisa, J. Power Sources, 196 (2011) 2584-2594. (2) A. Forner-Cuenca, J. Biesdorf, V. Manzi-Orezzoli, L. Gubler, T. J. Schmidt and P. Boillat, J. Electrochemical Society, 196 (13) F1389-F1398 (2016). Figure 1

Investigation of PEMFC Performance and Property of the Gas Diffusion Layers Utilizing the Numerical Model

The overpotential at the cathode is significantly large in the proton exchange membrane fuel cells; therefore, the oxygen concentration in the oxygen reduction reaction (ORR) area is vital. Condensed water in the flow field and the gas diffusion layer (GDL) reduces oxygen transport to the ORR area. The enhancement of the mass transport of liquid water and oxygen is critical to improve fuel cell performance particularly at the high current density operations. It is required to reduce the total active area in the fuel cell stack and also it is leading to the significant cost reduction. Experimental investigation of oxygen transport has been limited by an inability to resolve water saturation-dependent properties. The two-phase thermos-fluid dynamics model of GDL was developed with Lattice Boltzmann Method (LBM) and it was integrated with computational fluid dynamics (CFD) based proton exchange membrane fuel cell (PEMFC) model to simulate the fuel cell performance with given design parameters and operational conditions [1, 2]. This work, utilizing the developed model, investigates the correlations between fuel cell performance and GDL structures and material properties. Some empirical studies were pursued and structure and property of the micro porous layer (MPL) in the GDL is correlated to the fuel cell performance particularly high current densities [3-5]. The model simulates the fuel cell performance with various porosity and permeability of MPL in the cathode GDL to investigate those correlations. Also the two-phase model can show the presence of residual liquid water in the cathode GDL which effects on the fuel cell performance. It can distinguish the residual liquid water in the GLD underneath of the flow field channel ribs and under the channels. Figure 1 gives an concept of integrated CFD based PEMFC model with micro-scale GDL model with LBM. Figure 2 shows residual liquid water in the cathode GDL with various operating conditions. In this fuel cell design, residual liquid water is correlated with cathode conditions, relative humidity, rather than the current density. It shows the potential capability of a model-based investigation of mass-transport to find solution of design and operational conditions in the PEMFC.

Evaporation of Water from Gas Diffusion Layers

Evaporation of water from gas diffusion layers (GDL) is an important process for water management in polymer electrolyte fuel cells. At temperatures around 80 °C and locally under-saturated conditions, evaporation can have a significant impact on the GDL saturation. However, little is known about the evaporation rates of water in gas diffusion layers. First ex-situ results at room temperature and for low saturations have been reported in [1]. But for a better understanding of the evaporation rates under real operating temperatures, generic data is required at different temperatures and saturation levels. Using a special setup, which is applicable also for X-ray imaging of water saturation, water surface and GDL structure, evaporation rates are determined for well-controlled gas speeds (over the surface of the GDL),, temperatures of up to 80 °C and relevant liquid saturations. Figure 1 shows data (including reproducibility) for Toray TGPH 060 material as function of temperature up to 80 °C, at a capillary pressure of 10 mbar and gas velocity of 6 m/s. [1] I. V. Zenyuk, A. Lamibrac, J. Eller, D. Y. Parkinson, F. Marone, F. N. Büchi, and A. Z. Weber, J. Phys. Chem. C, 2016, 120 (50), pp 28701–28711 Figure 1: Evaporation rates from Toray TGPH 060 gas diffusion layer material a) as function of temperature at a gas velocity of 6.0 m/s; b) as function of gas velocity on the GDL surface at 60 °C; both for a dry gas stream and capillary pressure of 10 mbar; data shown based on preliminary set-up. Figure 1

Multiscale Numerical Model for Contact Resistance Prediction in the Proton Exchange Membrane Fuel Cell

Electrical contact resistance (ECR) between gas diffusion layer (GDL) and bipolar plate (BPP) contributes a significant part in the ohmic loss of proton exchange membrane (PEM) fuel cell. A multi-scale model is developed to predict the ECR by combination of micro “mechanical-electrical” response and macro contact pressure distribution between BPP and GDL. The “mechanical-electrical” constitutive relation is introduced by a micro model with consideration of fiber structure and large deformation of GDL. It is then incorporated into the macro model, in which contact pressure distribution is calculated with channel geometry, to predict the whole ECR between GDL and BPP. Then, contact resistance is measured by experiments to validate the model. At last, influences of micro topography and macro channel feature are discussed based on the numerical model. This model bridges the gap between micro topography effect and macro feature effect on the ECR in the fuel cell.