Werner Lehnert

Modelling of gas transport phenomena in SOFC anodes

Internal steam reforming in SOFC cells leads to inhomogeneous temperature distributions according to the fast reforming reaction kinetics. This results in thermal induced stresses and may lead therefor to mechanical failure of the material. A one-dimensional numerical simulation program has been developed to describe the transport of gases inside the SOFC anode due to diffusion and permeation as well as the kinetic of the reforming reaction and the electrochemistry. Simulations with experimentally determined reaction rates and structural properties of the anode materials have been performed. In order to reduce the methane conversion rate, a sensitivity analysis has been performed. It can be shown, e.g., that a reduction of the structural parameter ψ which is the ratio of porosity e to tortuosity δ of 26.28% compared to standard material leads to a lowering of the methane conversion rate of 12.24%. Finally, produced cermets are screened in view of a conversion lowering effect.

Investigation of water evolution and transport in fuel cells with high resolution synchrotron x-ray radiography

The authors report on in situ investigations of liquid water evolution and transport in an undisturbed operating fuel cell at the microscopic level. Synchrotron x-ray radiography enhances the spatial resolution by two orders of magnitude compared to the state-of-the-art techniques in this field. The primary spots of liquid water formation, their growth, and transport inside the porous gas diffusion material were analyzed; correlations between operating conditions and the dynamics of droplet formation are described. Previous findings from modeling and simulation approaches are confirmed and the applicability for the description of in situ processes of a recently proposed model has been proven.

Cross-sectional insight in the water evolution and transport in polymer electrolyte fuel cells

The evolution of liquid water and its transport through the porous gas diffusion media in an operating fuel cell were investigated applying an experimental setup for high spatial resolution of 3μm. Fundamental aspects of cluster formation in hydrophobic/hydrophilic porous materials as well as processes of multiphase flow are addressed. The obtained water distributions provide a detailed insight in the membrane electrode assembly and the porous electrode with regard on the existence and transport of liquid water. In addition, the results approve transport theories used within the framework of percolation theory and demonstrate the need for adapted modeling approaches.

Quasi–in situ neutron tomography on polymer electrolyte membrane fuel cell stacks

Quasi–in situ neutron tomography is applied to polymer electrolyte membrane fuel cell stacks for a cell-by-cell detection of liquid water agglomerates. Water distributions in the corresponding anodic and cathodic flow fields are analyzed separately. The influence of the membrane thickness as well as effects of the electro-osmotic drag and of back-diffusion from the cathode to the anode on the water distribution are investigated. Furthermore, the well-known engineering problem of the anomalous behavior of the outermost cells in long multistacks is addressed. The suitability of neutron tomography to support the development of fuel cells is shown.

Modeling of Mass and Heat Transport in Planar Substrate Type SOFCs

A mathematical model is presented that incorporates the mass transport by diffusion in the porous structure of thick substrate type solid oxide fuel cells ~SOFCs!. On the basis of the mean transport pore model a multidimensional study allows for an optimization of the structural parameters of the substrates with respect to cell performance. Next to the mass transport in the porous substrates the electrochemical kinetics, methane/steam reforming and shift reaction, and energy equations are integrated in the model and boundary as well as operation conditions can be varied. Two-dimensional simulations for both anode as well as cathode substrate describe the extension of the existing model to two dimensions. The analysis emphasizes mass transport in porous electrodes, especially in the problematical zone below the interconnect ribs. Furthermore, the cathode substrate concept is considered as well, which has not been done before. Model The model was developed for a SOFC based on a planar sub- strate type concept. The fuel cell is considered to operate on either hydrogen or a ~prereformed! methane/steam mixture as fuel and oxygen from air as oxidant. Next to the description of the electro- chemical reactions, the methane/steam reforming and shift reactions, the heat-transfer and the mass-transfer processes are included in the model. Distinct from previous models, the fuel and oxidant flow, i.e., multicomponent transport, were developed for a realistic fuel cell comprising porous elements, based on a finite-volume-based computational fluid dynamics ~CFD! approach. For this purpose the model calculations were performed using the commercial CFD- package FLUENT. Model assumptions.—Ideal gas mixtures, incompressible and laminar flow due to small gas velocities, and pressure gradients are assumed. The electrodes have a homogeneous, isotropic structure and no gradients within mechanical properties. The electrolyte is regarded as an infinite thin layer between anode and cathode. In the model the mass and heat sources according to the electrochemical conversion arise in the boundary cells of the anode next to the elec- trolyte. The thickness of these boundary cells depends on the trun- cation grade of the anode.

Local Structural Characteristics of Pore Space in GDLs of PEM Fuel Cells Based on Geometric 3D Graphs

Physical properties affecting transport processes inside the gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fu cells mainly depend on the microstructure of its pore space. The presented characterization of the complex structure of the po space is based on geometric three-dimensional (3D) graphs, which are marked to display transport-related properties such as po diameters. This representation of the open volume allows for an investigation of local structural characteristics by considering local tortuosity characteristics, pore sizes, and connectivity characteristics, respectively. The notion of local shortest path leng through the pore space of the GDL is introduced and the probability distribution of this random variable is computed. Its mean value is related to the (physical) tortuosity, which is given by the ratio of the mean effective path length through the GDL and i thickness. The developed methods are applied to simulated and to real (experimentally measured) 3D data. The used stochastic 3 model for the GDL is an extended version of the multilayer model proposed by Thiedmann et al. [J. Electrochem. Soc., 155, B39 (2008)], including a more flexible modeling of binder. The numerical results show the sensitivity of the proposed local chara teristics to varying binder modeling.

Characterization of water exchange and two-phase flow in porous gas diffusion materials by hydrogen-deuterium contrast neutron radiography

Liquid water exchange in two-phase flows within hydrophobic porous gas diffusion materials of polymer electrolyte membrane fuel cells was investigated spatially resolved with H–D contrast neutron radiography. A commonly used one-phase model is sufficient to describe water exchange characteristics at low water production rates. At higher rates, however, a significantly higher exchange velocity is found than predicted by a simple model. A new model for the water transport is derived based on an eruptive mechanism guided by Haines jumps, which is supported by recent experimental findings and leads to a very good agreement with the experiments.

Stochastic 3D Modeling of the GDL Structure in PEMFCs Based on Thin Section Detection

We propose a mathematical model to describe the microstructure of the gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs) based on tools from stochastic geometry. The GDL is considered as a stack of thin sections. This assumption is motivated by the production process and the visual appearance of relevant microscopic images. The thin sections are modeled as planar [two-dimensional (2D)] random line tessellations which are dilated with respect to three dimensions. Our 3D model for the GDL consists of several layers of these dilated line tessellations. We also describe a method to fit the proposed model to given GDL data provided by scanning electron microscopy images which can be seen as 2D projections of the 3D morphology. In connection with this, we develop an algorithm for the segmentation of such images which is necessary to obtain the required structural information from the given grayscale images.

Degradation of Solid Oxide Fuel Cell Anodes Due to Sintering of Metal Particles Correlated Percolation Model

Variation with time of the chemically active three-phase boundary, due to the spontaneous sintering of metal particles, is simulated with the help of a correlated percolation model. Different modes of variation are obtained depending on the relative composition and porosity and the conditions of sintering. They range from a moderate degradation (when the three-phase boundary still penetrates through the whole volume of the anode) to a catastrophic degradation (when the three-phase boundary is localized within a thin layer near the current collector). More specific situations, including non-monotonic variation, are also revealed, as well as a rare case where the three-phase boundary increases when the sintering leads to the opening of new pores. Optimization of composition for a stable anode performance is discussed.

OpenPNM: A Pore Network Modeling Package

Pore network modeling is a widely used technique for simulating multiphase transport in porous materials, but there are very few software options available. This work outlines the OpenPNM package that was jointly developed by several porous media research groups to help address this gap. OpenPNM is written in Python using NumPy and SciPy for most mathematical operations, thus combining Pythons ease of use with the performance necessary to perform large simulations. The package assists the user with managing and interacting with all the topological, geometrical, and thermophysical data. It also includes a suite of commonly used algorithms for simulating percolation and performing transport calculations on pore networks. Most importantly, it was designed to be highly flexible to suit any application and be easily customized to include user-specified pore-scale physics models. The framework is fast, powerful, and concise. An illustrative example is included that determines the effective diffusivity through a partially water-saturated porous material with just 29 lines of code.