Uwe Reimer

Stochastic Aspects of Mass Transport in Gas Diffusion Layers

The relationship between the 3D morphology of gas-diffusion layers (GDL) of HT-PEFCs and their functionality is analyzed. A stochastic model describing the microstructure of paper-type GDL is combined with the Lattice-Boltzmann method (LBM) to simulate gas transport within the GDL microstructure. Virtual 3D microstructures representing paper-type GDL are generated by a stochastic model, where the binder morphology is systematically modified. On these structures, single phase single component gas flow is computed by the LBM. Quality criteria evaluating the spatial homogeneity of gas supply are introduced and related to the binder morphology. The spatial homogeneity of the gas supply is analyzed by a parametrized stochastic model describing the gas flow at the exit of the GDL. This approach gives insight into the spatial structure of the gas flow at the GDL exit. The quality of gas supply is quantified by characterizing size and arrangement of regions with high gas supply. This stochastic gas flow model predicts the quality of gas supply for further binder morphologies. Analyzing the quality criteria and the stochastic evaluation of the spatial structure of the gas flow field at the GDL exit, it is found that the binder morphology has an essential influence on the gas supply.

Stability Issues for Fuel Cell Models in the Activation and Concentration Regimes

Code stability is a matter of concern for three-dimensional (3D) fuel cell models operating both at high current density and at high cell voltage. An idealized mathematical model of a fuel cell should converge for all potentiostatic or galvanostatic boundary conditions ranging from open circuit to closed circuit. Many fail to do so, due to (i) fuel or oxygen starvation causing divergence as local partial pressures and mass fractions of fuel or oxidant fall to near zero and (ii) nonlinearities in the Nernst and Butler-Volmer equations near open-circuit conditions. This paper describes in detail, specific numerical methods used to improve the stability of a previously existing fuel cell performance calculation procedure, at both low and high current densities. Four specific techniques are identified. A straight channel operating as a (i) solid oxide and (ii) polymer electrolyte membrane fuel cell is used to illustrate the efficacy of the modifications. (Less)

Hochtemperatur-Polymerelektrolyt-Brennstoffzellen

Phosphorsaure ist ein thermisch stabiler und preiswerter Elektrolyt fur die HT-PEFC. Weiterhin ist sie ungiftig, besitzt einen sehr geringen Dampfdruck (geringe Verluste durch Gasstromung) und benotigt aufgrund der hohen Betriebstemperatur kein aktives Management von flussigem Wasser. Jedoch andert sich die Leitfahigkeit als Funktion des Wassergehaltes. Demzufolge andert sich der Widerstand mit den Betriebsbedingungen. Diese Anderung kann durch die Wahl eines geeigneten Flowfield-Designs der Brennstoffzelle gunstig beeinflusst werden. Die Medienfuhrung hat auch einen wesentlichen Einfluss auf die Stromdichteverteilung von Zellen bei Betrieb mit CO-haltigem Brenngas. Vorteilhaft ist eine Fuhrung der Gase im Gegenstrom mit gleichzeitiger Gleichstromfuhrung des Kuhlmediums. Grose Gradienten in der Stromdichteverteilung bedingen eine beschleunigte Alterung. Aufgrund der Betriebstemperatur von ca. 160 °C stellt die HT-PEFC nur geringe Anforderungen an die Reinheit von Wasserstoff als Brenngas. Es konnen CO-Gehalte bis 3 Vol.-% am Brenngaseingang toleriert werden. Geringe Methan-Konzentrationen (bis 1 Vol.-%) haben keinen Einfluss auf die Leistung.

Stochastic Analysis of the Gas Flow at the Gas Diffusion Layer/Electrode Interface of a High-Temperature Polymer Electrolyte Fuel Cell

In polymer electrolyte fuel cells of the types PEFC, DMFC and HT-PEFC, the gas diffusion layer (GDL) connects the electrodes with the feeding channels of the bipolar plate. The GDL is typically composed of materials based on carbon fibers, e.g., paper, woven or non-woven textiles. Efficient fuel cell operation requires that the electrodes are sufficiently supplied by gaseous fluids from the channels. Also, reaction products must be transported away from the electrodes. The GDL also has to provide electronic contact to the bipolar plates, but its major task is the mass transport of fluids. The gas transport in through-plane direction is simulated in the porous structure of the GDL, represented by stochastic geometries equivalent to the real structure. In order to support multi-scale simulation, effective properties can be calculated from these mesoscale simulation results to provide model parameters for continuum approaches in cell-scale simulations. In this paper, the resulting gas flow is analyzed with statistical methods with the focus on the interface between GDL and electrode. This approach provides the opportunity to detect quantitative relationships between functionality and microstructure and to design virtual GDL materials with improved transport properties. The evaluation of the interface with stochastic methods provides substantiated properties suitable for connecting regions representing fuel cell components of different spatial scales.

Irreversible Losses in Fuel Cells

Abstract The term “irreversible losses” is used to summarize the major effects that lead to a deviation of the observed cell voltage compared with the value calculated from thermodynamic theory. The aim of this chapter is to serve two purposes. First, an overview of the main loss mechanisms coming from electrode kinetics, ohmic resistance, and mass transport is provided. Second, this chapter demonstrates how the underlying physical model in our mind influences the method and result of calculations. Throughout the chapter, several modeling approaches are compared, and their implications on the model parameters are discussed. This is of great practical importance for the interpretation of experimental work and the comparison with literature data. The examples describe the performance of half cells, single cells, and fuel cell stacks.