A Study on the Effect of Morphology and Wettability on Water Saturation Parameters of Gas Diffusion Layers of PEMFCs

Abstract

Water transport within a Polymer Electrolyte Membrane Fuel Cell (PEMFC) is important for performance and lifetime optimization. Specifically within the porous structure of the Gas Diffusion Layer (GDL), liquid water agglomerations can impede gas transport to the catalyst layer and thus affect the electrochemical reactions, finally resulting in reduced cell performance. Consequently, it is important to understand the formation of water agglomerations within the pores both from an experimental point of view and for modelling purposes. Concerning the latter, correct predictions on cell behaviour and performance require a reliable description of the processes relevant for water and gas transport. In this study, we tackle this issue both with experimental and modelling techniques. The results of the different approaches clearly indicate the need of an overall description comprising all relevant processes. Employing a dedicated, custom-built experimental setup, the GDL specific water saturation parameters under varying hydrostatic pressure can be measured using an X-ray tomography apparatus. The results obtained can be used to parametrize a water saturation model for Computational Fluid Dynamics (CFD) simulations and thereby lead to more precise results of such simulations, since a realistic CFD simulation of water transport in the PEMFC requires the information of water saturation of the GDL depending on the generated water pressure. In a CFD simulation software such as ANSYS Fluent®, the water transport within the GDL is often described by a model based on the capillary pressure – water saturation relationship. This model was firstly developed by Leverett1 for oil reservoirs in sandstone and was later complemented by Udell2. However, the standard parameters of the Leverett-function suggested by Udell are material-dependent constants and not only might differ from those of recent state-of-the-art GDL materials in general, but also should to be determined individually for each GDL type. In this study, the dependence of the water saturation level of GDLs on the applied water pressure is investigated employing X-ray tomography. This method also allows for a locally resolved analysis of the liquid water distribution within the GDL pores. For the detection of water droplets with a resolution of down to 2.5 µm, a new apparatus has been developed which permits applying water pressure up to 500 mbar on the surface of the porous material while acquiring the tomography data. In Figure 1, such a tomogram of a GDL (through plane and in plane cross section) recorded under 100 mbar water pressure is shown. Following another path, cell- or single-channel level CFD calculations employing such integral models for the GDL can yield the local temperature and relative humidity distributions within the porous material, which can then be used as inputs for Monte Carlo (MC) simulations which work on locally resolved structural data of the µm-scale pores. This technique is used to study the dependency of pore saturation of the GDL on structural and operating parameters. In this work, MC simulations were employed to determine the liquid water distribution in the GDL pores under realistic cell operating conditions. This voxel-based MC model (Figure 2) takes into account not only the movements of droplets subject to the surface properties of the solid and liquid phases, but also their evaporation and condensation. These simulations are based on real GDL structures including the different material compressions under rib and channel, surface characteristics and operating conditions. Previous results obtained with the same technique have shown good reliability when compared to in-situ experiments3. The present results indicate that to obtain a realistic description of the water distribution in a GDL under real operating conditions, both pressure-driven liquid water transport and condensation effects should be reflected together with the structure and wettability of the porous material. This study was supported by the German Federal Government within the project Optigaa II (03ET6015F). The aim of this project is to enhance the performance of PEMFCs by systematic GDL optimization References M. C. Leverett, Trans. AIME, 142, 152–168 (1941).K. S. Udell, Int. J. Heat Mass Transfer, 28, 485–495 (1985).K. Seidenberger et al., J. Power Sources, 239, 628–641 (2013). Figure 1