Liquid Water In Gas Diffusion Layer Research Articles

Numerical study on the effect of microporous layer crack changes on water management in gas diffusion layer of proton exchange membrane fuel cell

The existence of the microporous layer (MPL) enhances the transport of reactive gas and liquid water in the gas diffusion layer (GDL), which is a crucial part of the proton exchange membrane fuel cell (PEMFC). However, it is still unclear how the shape and size of MPL surface cracks affect the transmission of liquid water in GDL. Therefore, a three-dimensional volume of fluid (VOF) model is developed to study the relationship between MPL crack and GDL liquid water transport. First, the random direction method is used to reconstruct the carbon fiber layer of GDL, which is in good agreement with the experimental data. Secondly, the morphological changes of MPL are analyzed, and two kinds of cracks with different topological shapes are proposed. The parameters of crack area, width and depth of the numerical model are designed according to the MPL porosity. Finally, the relationship between MPL crack variation and liquid water saturation/variation in GDL is investigated by numerical modeling. The results show that the shape, area, width and depth of MPL crack affect the water fraction in GDL. The impact of crack on the transfer path of liquid water in GDL is mainly in the early stage of water inflow. With the increase of water inflow, this impact is mainly related to the structure of GDL.

High Resolution Observation of Liquid Water in Polymer Electrolyte Fuel Cell Using X-Ray Nano Computed Tomography

Polymer electrolyte fuel cells (PEFCs) are attracting attention as energy device of low environmental impact because of their high generation efficiency and their emitting only water. The gas supplied to the PEFC is humidified, and water is produced at the cathode catalytic layer by the electrochemical reaction between oxygen and protons. The gas diffusion layer (GDL) and micro porous layer (MPL) in PEFC plays an important role in the transport of oxygen and water. When liquid water accumulates in the GDL, the supply of oxygen to the cathode is inhibited, resulting in poor generation performance. In order to maintain the performance of PEFCs, it is important to manage the water in the cell.Attempts have been made to visualize the liquid water in GDLs and MPLs for better GDL and MPL design; X-ray micro computed tomography (micro-CT) is one powerful tool for this. Visualization of liquid water in GDLs and MPLs using micro-CT has so far revealed the behavior of the produced water[1,2]. However, in previous studies, it has been difficult to visualize liquid water in the catalyst layer, which is the reaction field where the water is produced. This is because the spatial resolution of micro-CT is larger than a µm, which is insufficient for analyzing liquid water in the catalytic layer. X-ray CT measurements with a resolution of at least 150 nm are needed to measure the distribution of liquid water in the cross section of the catalyst layer. In this study, a measurement system to reveal the distribution of liquid water in the catalyst layer was developed using a 150 nm-spatial-resolution X-ray CT with a synchrotron radiation x-ray microscope called X-ray nano-CT.A GDL specimen prepared cut to a disk shape with 1 mm diameter. The GDL specimen mounted on a copper stage with high thermal conductivity connected Peltier element module. The micro-CT and nano-CT measurements were carried out in BL20XU beamline at SPring-8, Japan. An X-ray beam energy was 30 keV with a double crystal monochromator. The nano-CT measurements were performed using Zernike phase contrast with voxel size of 38.4 nm. We measured the micro-CT and nano-CT of the dry condition of the GDL specimen. Then, the GDL specimen injected liquid water was scanned. Injection of liquid water into the GDL specimen is not easy because of water-repellent of GDLs. Helium gas saturated water vapor at 40 °C was sprayed to the GDL at 30 ml/min, then the GDL cooled to 10 °C using a Peltier element module and the vapor was condensed inside of the GDL. The measurement setup is shown in Figure 1. This water injection in GDL method was based on the method reported by Kato et al [2].Figure 2 shows the optical system of the nano-CT used in this study. The optical system of this nano-CT provides a condenser zone plate for sample illumination, a Fresnel zone plate for enlarged image, and phase ring for Zernike phase contrast. These optics allow CT with a high spatial resolution of 150 nm to be measured even for materials with low X-ray absorption.A three-dimensional structure of GDL was revealed by CT measurements in each scale order. The distribution of liquid water in GDL were visualized with nanoscale spatial resolution (Figure 3). We have successfully visualized the three-dimension distribution of liquid water in the GDL with nm order scale using nano-CT. The newly established nano-CT with a spatial resolution of 150 nm order is extremely useful for elucidating liquid-water behavior in PEFCs containing catalyst layers, which was not clear before. Acknowledgements This work is based on results obtained from a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). This study was partially supported by the Synchrotron radiation experiments performed at BL20XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal numbers 2021A2009, 2021B1015).

Numerical Analysis Considering the Effects of Liquid Water on PEMFC Unsteady Performance

A three-dimensional, non-isothermal, and unsteady numerical model considering liquid water transport inside a gas diffusion layer (GDL) has been developed to investigate its effect on cell performance. Liquid water in GDL had extremely wide size distributions, as visualized by synchrotron X-ray. It was assumed that liquid water in GDL form droplets with different diameters, which would affect the thermal conductivity and diffusion coefficient. On the other hand, liquid water in the flow channel should not be discounted as it has larger liquid water mass compared with that in GDL. In this paper, the impact of liquid water in the flow channel and droplets in GDL on the steady and unsteady responses of proton exchange membrane fuel cells (PEMFC) power generating characteristics was discussed. It was found that liquid water in the entire system has a significant influence on voltage responses.

Progress in In Situ X-Ray Tomographic Microscopy of Liquid Water in Gas Diffusion Layers of PEFC

Water management is an important factor for optimizing polymer electrolyte fuel cells (PEFC) under high current density conditions as required for the automotive application. The characteristics of the local liquid saturation of the gas diffusion layer (GDL) is of particular interest. Here we report on the development of in-situ X-ray tomographic microscopy (XTM) with a pixel sizes in the order of 2 μm and sensitivity for carbon and liquid water for the quantitative analysis of liquid water in GDLs. In-situ XTM of PEFC is a major experimental challenge. A complete cell needs to be operated under realistic conditions in the constraint space of the small field of view on the beamline sample stage. Further phase segmentation of the images is required to successfully analyze the quantitative properties of the different phases. For this a workflow, applying differential images between dry and wet structures has been developed. Cells with Toray TGP-H-060 GDLs were analyzed in-situ. Droplets that appear on the GDL surface are connected to a significant water structure inside the GDL. Further the water cluster size distribution in the GDL shows that while small droplets (<100 pl) are numerous, most of the water is contained in few larger clusters.