Water Transport In Gas Diffusion Layer Research Articles

Simulation on water transportation in gas diffusion layer of a PEM fuel cell: Influence of non-uniform PTFE distribution

Proton exchange membrane fuel cells (PEMFCs) are promising clean power sources with high energy conversion efficiency, fast startup, and no pollutant emission. The generated water in the cathode can cause water flooding of the catalyst layer (CL), which in turn can significantly decrease the fuel cell performance. To address this significant issue of PEMFC, a new gas diffusion layer (GDL) with non-uniform distribution of PTFE is proposed for water removal from the CL. The feasibility of this new GDL design is numerically evaluated by a Lattice-Boltzmann Method (LBM)-based two-phase flow model. The porous structure of the new GDL design is numerically reconstructed, followed by LBM simulations of the water transport in GDL. Three types of different wetting conditions are considered. It is found that liquid water transported 7.87% more with a single row of wetted solids and 13.36% more with two rows of wetted solids. The results clearly demonstrate that the liquid water can be effectively removed from the GDL by proper arrangement of hydrophilic solids in the GDL.

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

Investigation of the Effective Transport Properties of Gas Diffusion Layer on PEM fuel cell

Abstract Fundamental understanding of the transport properties in gas diffusion layers (GDLs) is a critical issue for the optimal design of polymer electrolyte membrane (PEM) fuel cell. The utilization of the Bruggeman correlation, the Leverett-J function and the Wyllie’s model in the description of the gaseous effective diffusivity, capillary characteristics and liquid water relative permeability in GDLs has been doubted in accurately estimating the effective diffusivities of the GDLs. In this study, a three-dimensional, two-phase flow PEM fuel cell model was developed to investigate how this inaccuracy is reflected on the fuel cell performance. The numerical results show that the gas diffusion and liquid water transport in GDLs are strongly determined by the effective diffusivities, which are closely correlated with the cell performance. The gas diffusion exhibits a more significant effect on the cell performance than the liquid water transport and water relative permeability. The water relative permeability is the dominant factor in determining the liquid water transport compared with the capillary characteristics.

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

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

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

In a polymer electrolyte fuel cell (PEFC), it is important to control water drainage to supply oxygen smoothly to the catalyst layer (CL). Gas diffusion layer (GDL) plays an important role for the water management and it is significant to develop GDL’s that achieves ideal water transport. In this paper, as first step to develop the optimal GDL, the water transport in GDL’s with oriented fiber distribution and with periodic hydrophobic-hydrophilic pattern was investigated. Analysis was made by the lattice Boltzmann method (LBM) and the scale model experiment satisfying similarity conditions for the flow in GDL. In the oriented GDL, the water under rib was drained to the channel smoothly along with oriented fibers placed perpendicular to the rib. In the GDL with hydrophobic-hydrophilic pattern, the water was transported to the channel along hydrophilic band. In this case the hydrophilic region was saturated, while hydrophobic region was left water free.

Lattice Boltzmann Simulation on Water Transport in Gas Diffusion Layer of Polymer Electrolyte Membrane Fuel Cells

The effect of rib on the dynamic behavior of liquid water transport in gas diffusion layer (GDL) is studied. The mechanism of water transport dynamics investigated using multiphase lattice Boltzmann method (LBM). Simulation with a simplified two-dimensional model is used to predict liquid water transport processes in local regions, instead of pursuing a three-dimensional numerical analysis for the entire domain. The observation of lattice Boltzmann (LB) simulation shows that the rib changes the water transport behavior and significantly affects the water transport in GDL.

Water transport in gas diffusion layer of a polymer electrolyte fuel cell in the presence of a temperature gradient. Phase change effect

The gas diffusion layer (GDL) is a crucial component as regards the water management in proton exchange membrane fuel cells. The present work aims at discussing the mechanisms of water transport in GDL on the cathode side using pore network simulations. Various transport scenarios are considered from pure diffusive transport in gaseous phase to transport in liquid phase with or without liquid–vapor phase change. A somewhat novel aspect lies in the consideration of condensation and evaporation processes in the presence of a temperature gradient across the GDL. The effect of thermal gradient was overlooked in previous works based on pore network simulations. The temperature gradient notably leads to the possibility of condensation because of the existence of colder zones within the GDL. An algorithm is described to simulate the condensation process on a pore network.

Visualization of water transport in GDL and flow channel of PEMFC

AbstractLiquid water penetrating the gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC) was studied. The gas diffuse layer (GDL) has a great impact on PEMFC's performanc3e, and is an important component of a PEMFC. An ex‐situ test was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera. 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 GDLs with different porosities and thicknesses. The equilibrium pressure, which is defined as the minimum pressure required for 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. Also the drain performance using three kinds of different bipolar plates were compared in this paper. According to the result of experiment, the average diameters of porous GDLs were found determining the penetrating pressure. Serpentine flow channel proved the best pattern for drainage. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/htj.20330

Determination of Local GDL Saturation on the Pore Level by In Situ Synchrotron Based X-ray Tomographic Microscopy

In-situ synchrotron-based tomographic microscopy (SRXTM) with a spatial resolution in the order of 1μm and sensitivity for carbon and liquid water, has the potential to provide fundamental information for the understanding of the wetting properties of gas diffusion layer (GDL) materials on the pore level. This is important for the understanding of the solid-water interactions in the porous structures since water transport in GDLs is considered a key transport mechanism polymer electrolyte fuel cells (PEFC). However SRXTM of PEFC is a major experimental challenge. To obtain quantitative results, a complete cell needs to be operated under realistic conditions in the constrained space of the small field of view on the beamline sample stage without disturbing the sample rotation.

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.

Pore-network analysis of two-phase water transport in gas diffusion layers of polymer electrolyte membrane fuel cells

A pore-network model was developed to study the water transport in hydrophobic gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs). The pore structure of GDL materials was modeled as a regular cubic network of pores connected by throats. The governing equations for the two-phase flow in the pore-network were obtained by considering the capillary pressure in the pores, and the entry pressure and viscous pressure drop through the throats. Numerical results showed that the saturation distribution in GDLs maintained a concave shape, indicating the water transport in GDLs was strongly influenced by capillary processes. Parametric studies were also conducted to examine the effects of several geometrical and capillary properties of GDLs on the water transport behavior and the saturation distribution. The proper inlet boundary condition for the liquid water entering GDLs was discussed along with its effects on the saturation distribution.