GDL and MPL Characterization and Their Relevance to Fuel Cell Modelling

Abstract

PEM fuel cells (PEMFC) are a promising technology for electrochemical energy conversion. However, especially for applications requiring high current density, the gas diffusion layer (GDL) deserves further attention and efforts. Liquid water saturation of pores in the GDL limits the flow of reactants to the catalyst and creates starvation areas with inhomogeneous current density distribution. This leads to accelerated material degradation in these areas and to poor overall performance. Traditional fuel cell models, e.g. CFD simulation modules, make simplifying assumptions, i.e. by modelling the GDL fibre structure as homogeneous porous material, which may cause current density distributions to appear more uniform than they really are. This presentation will give a survey on relevant parameters for GDL characterization, useful methods for investigations and on implications of these results to GDL modelling. The GDL structure inside of an assembled fuel cell can be characterized using X-ray based methods, especially synchrotron or µ-CT based radiography and tomography [1, 2]. Radiographic methods allow for a relatively quick investigation on the integral water distribution within the different MEA layers, especially the GDL substrate and - to some extent - also for the membrane, depending on the selection of material and operating parameters. On the other hand, tomographic methods allow for 3D resolved water distribution analysis, especially for the water distribution within the GDL substrate. Moreover, the different GDL structures due to flow field compression under-land and under-channel are subject of these investigations. Such measurements are performed with sub-μm pixel size at ZSW/HZB to account for the effects of fine pores and fiber structures in detail. The structural information obtained can be used for advanced modelling techniques determining reactant concentrations in the gaseous media employing single-phase approaches. Liquid water effects caused by pore volume reduction may be directly used as model input at this stage. To perform a reasonable modelling of the water distribution itself, not only structural, but also surface parameters must be known. These have to be additionally determined by ex situ techniques. Ideally, the surface properties inside the porous structure would be known down to a sub µm scale. Unfortunately, no generally applicable methods are known until today. First approaches are e.g. the STXM method [3]. But at least integral surface property data can be determined requiring significantly less effort. Different approaches to characterize the wettability of porous media will be discussed. As a very efficient method to determine the inner contact angle of the GDL, inverse gas chromatography techniques may be used. This inner contact angle has been successfully used for a modelling technique (Monte Carlo (MC) modelling) describing the behavior of liquid water inside the porous fibre structure [4]. Thus, using both structural and surface properties, water distribution and GDL transport parameters may be modelled (MC, CFD at µm to mm level) and introduced into models on the macro-scale (e.g. CFD at cell level with segmented GDL areas over rib and over channel). Furthermore, GDL material characteristics regarding transport properties require special attention. As relevant parameters for ex-situ characterization, rating of GDL materials and as important modelling input, structure and material related physical parameters as Leverett function, electrical and heat conductivity, mechanical parameters as E module and shear modulus G can be determined by both modelling on the basis of accurate structure data, e.g. from µ-CT imaging, and – on the macro scale – by direct measurement. Specially, the compression force cycle behavior is mainly obtained from direct measurements. Also, permeability data (in and through plane) may be measured directly. Besides permeation, gas-phase diffusion of the relevant species in the substrate and – on a much smaller scale involving different diffusion mechanisms – in the MPL pores is not trivial to characterize. First results obtained by recently designed setups employing a special low turbulence cell in combination with molar flow analysis for determination of diffusion coefficients and a method to determine the Leverett function will be presented. The ex situ methods recommended will be listed and described in the presentation.