Water Distribution In Gas Diffusion Layer Research Articles

The Role of Thermal Conductivity on Liquid Water Distribution in GDLs

Operating polymer electrolyte fuel cells (PEFCs) at increasingly higher current density and efficiency necessitates overall improvements in fuel cell water management [1]. A crucial role facilitating these improvements is played by the gas diffusion layer (GDL) within the membrane electrode assembly (MEA) [2]. The GDL is responsible for distributing the reactant gases from the flow field towards the catalyst layer as well as removing reaction products from the catalyst layer to the flow fields. For example, at the cathode, the electrochemical reaction of oxygen ions with protons and excess electrons yields water and heat, which must be removed through the GDL to the flow field. The removal of water through the GDL can lead to blocked pores in the GDL [2], thereby restricting the reactants’ gas flow towards the catalyst layer causing mass transport losses. GDL design is crucial towards improving the performance of PEFCs, especially at high current densities, such that it minimizes these transport losses. Additionally, removal of the excess heat generated at the cathode catalyst layer through the GDL indicates that the GDL thermal conductivity (k) plays a crucial role affecting the temperature distribution within the MEA [3].In this work, two GDLs varying in thermal conductivity were selected and assembled in MEAs which were then imaged in-operando using X-ray tomographic microscopy [4]. In-Operando imaging allowed the evaluation of the liquid water distribution at steady state over a range of operational conditions such as temperature and current density. Water distribution results for a range of current densities will be presented for selected cell temperatures between 40 and 70 °C. For GDL I with the lower thermal conductivity, the channels are completely dry at 40 °C. For GDL II with the higher thermal conductivity, a wet-dry transition was observed in between 50 and 70°C. High saturations were observed in both the channel and the land regions at 50 °C, while at 70 °C the channels appear to be dry with liquid water only being present under the lands (see Figure 1). These results provide experimental evidence of a major influence of GDL thermal conductivity on liquid water distribution and overall water management in fuel cells. Keywords – operando, X-ray tomographic microscopy, GDL, water visualization Acknowledgement Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Ballard Power Systems, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Canada Research Chairs.

Measurement of liquid water distribution in GDL under cross-flow-inducing parallel flow field using operando synchrotron X-ray radiography

Baffles in flow field channels for air in a polymer electrolyte fuel cell are known to enhance the performance by inducing convective flow through the gas diffusion layer (GDL) with smaller pressure loss than interdigitated flow fields. This work experimentally correlates performance enhancement with the amount of liquid water in a GDL. A parallel flow field (PFF) that has different inlet and outlet opening sizes is applied to induce cross-flow in the GDL under the rib between adjacent air channels. Operando synchrotron X-ray radiography experiments are conducted to compare the amount of water in the GDL with that for a PFF having the same inlet and outlet sizes. The performance enhancement by application of different opening sizes increases with decreasing relative humidity and increasing air flow rate. A significant performance enhancement is observed when the amount of water in the GDL substrate under the rib becomes almost zero. No performance enhancement is observed under over-humidified conditions, although a decrease in the amount of water in the GDL is still observed, which suggests that the performance becomes insensitive to the difference in the liquid water saturation as the saturation increases.

Investigation of Liquid Water Distribution under Temperature Gradient in PEFC Using in-situ X-ray Visualization

Polymer electrolyte fuel cells (PEFCs) are one of promising electric generating devices. It is necessary to understand the relationships between the water distribution and the temperature distribution in gas diffusion layers (GDLs) to improve the performance at high current density. To reveal the effects of cell temperature on liquid water accumulation inside PEFCs, in-situ X-ray measurement and numerical simulation were performed at 40, 60, 70, and 80°C. The measurement was carried out using a fuel cell having a serpentine-type flow field, ribs and flow channels of 300 μm width, and 1.5×3.0mm2 active area. Current density was kept 1.5 A/cm2 for 300 secs in all cases. Both the experimental results and the numerical results showed that the liquid water distribution in GDLs strongly depends on cell temperature. In the case of lower temperature, liquid water accumulated in a region near CL and MPL. While in the case of higher temperature, liquid water accumulated in a region near rib. In addition, the experimental results indicated that the vapor crossover depended on existence of liquid water in MPL, and it was shown that the amount of vapor crossover increased at lower temperature condition.

Influence of Pore Size Distribution on Operando Gas Diffusion Layer Liquid Saturation

Hydrogen fed polymer electrolyte fuel cells (PEFCs) have drawn attention as key technology for future decarbonized energy systems [1], in particular in the mobility sector, due to their high current density at low operating temperatures and rapid start-up [2]. The liquid water generated during high current density operation may saturate the pores of gas diffusion layers (GDLs), hence impede the gas transport to the catalyst layer, leading to performance losses. Higher performance requires, improved water management in GDLs. In this work, the liquid water distribution in GDLs has been studied with an operando X-ray tomographic microscopy (XTM) setup at the TOMCAT beamline of the Swiss Light Source (SLS) at Paul Scherrer Institut (PSI) [3,4]. XTM scans with a duration of 0.1 s and a voxel edge length of 2.75 um were employed to investigate PEFCs assembled with three types of GDL materials (Freudenberg I6, Toray TGPH-060 and SGL 24BA) with increasing pore size (see Figure 1). Properties of the operando liquid saturation were determined between OCV and 1.25 A/cm2 at current steps of 0.25 A/cm2 under H2/air operation with 100%/0% and 100%/100% humidified gases at start-up temperature of 25 oC. The influence of the GDL substrate pore size, current density and gas humidity on liquid water saturation and connectivity in the GDL will be presented.

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

A Monte Carlo Study on the Effect of Structural and Operating Parameters on the Water Distribution within the Microporous Layer and the Catalyst Layer of PEM Fuel Cells

Depending on the operating conditions, the overall performance and durability of polymer electrolyte membrane fuel cells (PEMFC) can be strongly influenced by liquid water saturation of pores within the gas diffusion layer (GDL). Water agglomerations in the GDL may occur due to material morphology, surface characteristics and local boundary conditions. In the catalyst layer (CL) and microporous layer (MPL) these parameters are considerably different than those of the fibre substrate. The MPL and CL feature by orders of magnitude smaller pores than the substrate and the boundary conditions, e.g. temperature and relative humidity, are different than those in the substrate near the bipolar plate. Thus, different physical behavior within each layer can be observed, e.g. local liquid sorption. Therefore, finding an optimal strategy regarding water management requires knowledge on virtually all the factors involved. To approach this target, we are using and further developing a 3D Monte Carlo (MC) model which simulates water distribution within the GDL, employing the real GDL structure and operating conditions, exploited respectively from tomography and computational fluid dynamics (CFD) studies1. The 3D images are obtained from an assembled fuel cell and therefore, the results include compression and inhomogeneity information of the GDL. Furthermore, recent progress of the model has enabled simulations on the length scales relevant for the MPL and CL in addition to the fibre substrate. GDL substrates usually have an average pore size in the micrometer scale, whereas this value for the MPLs and CLs may go down to the nanometer scale. Therefore, a complete simulation of the water distribution in GDL includes working on at least two scales. Although obtaining accurate results on all three sub-layers is equally important, MPL and CL are the more challenging ones from the point of view of both structure and surface properties acquisition. Furthermore, simulating relevant domains within the material requires expensive computational effort. In this study, we report first results on MC simulations of the water distribution within the MPL and CL pores for different operating conditions. The 3D reconstruction of the material is obtained from the focused ion beam - scanning electron microscope (FIB-SEM) tomography2. By using atomic layer deposition (ALD) of ZnO, a good material contrast between pores and solid particles is achieved and therefore a reliable binarization can be obtained3. The voxel size is set to 45 nm in order to achieve both, an accurate computation with respect to the applied equations and an appropriately sized simulation domain. Our MC model provides the water distribution within the actual MPL and CL structures based on the local boundary conditions. The results show how the water distribution depends on parameters such as contact angle, porosity, local temperature and relative humidity. An example of simulation results is illustrated in the Figure 1.

Water Distribution in GDL near Optimal Humidification

In order to better understand the two phase properties of the porous GDL, the liquid water pattern and local saturation in the Toray 060 gas diffusion layer (GDL) is visualized in-situ in cells close to the optimal performance point at 80 {degree sign}C by X-ray tomographic microscopy. A recording time of about 10 s is achieved. At the optimal performance point (dew point of reactant gases 74 {degree sign}C) the water volume fraction observed depends on the rib width. A variation of {plus minus} 8 {degree sign}C of the feed gas dew point leads to significant saturation and performance changes. For different current densities the water volume fraction and permeability of the liquid phase of the cathode are determined and analyzed.

A pore network study on the role of micro-porous layer in control of liquid water distribution in gas diffusion layer

In proton exchange membrane fuel cell (PEMFC), a hydrophobic micro-porous layer (MPL) is usually placed between catalyst layer (CL) and gas diffusion layer (GDL) to reduce flooding. Recent experimental studies have demonstrated that liquid water saturation in GDL is drastically decreased in the presence of MPL. However, theoretical studies based on traditional continuum two-phase flow models suggest that MPL has no effect on liquid water distribution in GDL. In the present study, a pore network model with invasion percolation algorithm is developed and used to investigate the impacts of the presence of MPL on liquid water distribution in GDL from the viewpoint at the pore level. A uniform pressure and uniform flux boundary conditions are considered for liquid water entering the porous layer in PEMFC. The simulation results reveal that liquid water saturation in GDL is reduced in the presence of MPL, but the reduction depends on the condition of liquid water entering the porous layer in PEMFC.