Michel Prestat

Solid oxide fuel cells: Systems and materials

A solid oxide fuel cell (SOFC) is a solid-state energy conversion system that converts chemical energy into electrical energy and heat at elevated temperatures. Its bipolar cells are electrochemical devices with an anode,electrolyte, and cathode that can be arranged in a planar or tubular design with separated gas chambers for fuel and oxidant. Single chamber setups have bipolar cells with reaction selective electrodes and no separation between anode and cathode compartments. A nickel/yttria-stabilized-zirconia (YSZ) cermet is the most investigated and currently most widespread anode material for the use with hydrogen as fuel. In recent years, however, doped ceria cermet anodes with nickel or copper and ceria as the ceramic phase have been introduced together with ceria as electrolyte material for the use with hydrocarbon fuels. The state-of-the-art electrolyte material is YSZ of high ionic and nearly no electronic conductivity at temperatures between 800-1000 °C. In order to reduce SOFC system costs, a reduction of operation temperatures to 600-800 °C is desirable and electrolytes with higher ionic conductivities than YSZ are aimed for such as bismuth oxide, lanthanum gallate or mixed conducting ceria and the use of thin electrolytes. Proton conducting perovskites are researched as alternatives to conventional oxygen conducting electrolyte materials. At the cathode, the reduction of molecular oxygen takes place predominantly on the surface. Todays state-of-the-art cathodes are La x Sr 1 - x MnO 3 - δ for SOFC operating at high temperature i.e. 800-1000 °C, or mixed conducting La x Sr 1 - x Co y Fe 1 - y O 3 - δ for intermediate temperature operation, i.e. 600-800 °C. Among the variety of alternative materials, Sm x Sr 1 - x CoO 3 - δ and Ba x Sr 1 - x Co x Fe 1 - x O 3 - δ are perovskites that show very good oxygen reduction properties. This paper reviews the materials that are used in solid oxide fuel cells and their properties as well as novel materials that are potentially applied in the near future. The possible designs of single bipolar cells are also reviewed.

The influence of constrictivity on the effective transport properties of porous layers in electrolysis and fuel cells

The aim of the present investigation is to define microstructure parameters, which control the effective transport properties in porous materials for energy technology. Recent improvements in 3D-imaging (FIB-nanotomography, synchrotron X-ray tomography) and image analysis (skeletonization and graph analysis, transport simulations) open new possibilities for the study of microstructure effects. In this study, we describe novel procedures for a quantitative analysis of constrictivity, which characterizes the so-called bottleneck effect. In a first experimental part, methodological tests are performed using a porous (La,Sr)CoO3 material (SOFC cathode). The tests indicate that the proposed procedure for quantitative analysis of constrictivity gives reproducible results even for samples with inhomogeneous microstructures (cracks, gradient of porosity). In the second part, 3D analyses are combined with measurements of ionic conductivity by impedance spectroscopy. The investigations are preformed on membranes of electrolysis cells with porosities between 0.27 and 0.8. Surprisingly, the tortuosities remain nearly constant (1.6) for the entire range of porosity. In contrast, the constrictivities vary strongly and correlate well with the measured transport resistances. Hence, constrictivity represents the dominant microstructure parameter, which controls the effective transport properties in the analysed membrane materials. An empirical relationship is then derived for the calculation of effective transport properties based on phase volume fraction, tortuosity, and constrictivity.

3D Microstructure Effects in Ni-YSZ Anodes: Prediction of Effective Transport Properties and Optimization of Redox Stability

This study investigates the influence of microstructure on the effective ionic and electrical conductivities of Ni-YSZ (yttria-stabilized zirconia) anodes. Fine, medium, and coarse microstructures are exposed to redox cycling at 950 °C. FIB (focused ion beam)-tomography and image analysis are used to quantify the effective (connected) volume fraction (Φeff), constriction factor (β), and tortuosity (τ). The effective conductivity (σeff) is described as the product of intrinsic conductivity (σ0) and the so-called microstructure-factor (M): σeff = σ0 × M. Two different methods are used to evaluate the M-factor: (1) by prediction using a recently established relationship, Mpred = εβ0.36/τ5.17, and (2) by numerical simulation that provides conductivity, from which the simulated M-factor can be deduced (Msim). Both methods give complementary and consistent information about the effective transport properties and the redox degradation mechanism. The initial microstructure has a strong influence on effective conductivities and their degradation. Finer anodes have higher initial conductivities but undergo more intensive Ni coarsening. Coarser anodes have a more stable Ni phase but exhibit lower YSZ stability due to lower sintering activity. Consequently, in order to improve redox stability, it is proposed to use mixtures of fine and coarse powders in different proportions for functional anode and current collector layers.

Fuel Cell Modeling and Simulations

Abstract: Fundamental and phenomenological models for cells, stacks, and complete systems of PEFC and SOFC are reviewed and their predictive power is assessed by comparing model simulations against experiments. Computationally efficient models suited for engineering design include the (1+1) dimensionality approach, which decouples the membrane in-plane and through-plane processes, and the volume-averaged-method (VAM) that considers only the lumped effect of pre-selected system components. The former model was shown to capture the measured lateral current density inhomogeneities in a PEFC and the latter was used for the optimization of commercial SOFC systems. State Space Modeling (SSM) was used to identify the main reaction pathways in SOFC and, in conjunction with the implementation of geometrically well- defined electrodes, has opened a new direction for the understanding of electrochemical reactions. Furthermore, SSM has advanced the understanding of the COpoisoning- induced anode impedance in PEFC. Detailed numerical models such as the Lattice Boltzmann (LB) method for transport in porous media and the full 3-D Computational Fluid Dynamics (CFD) Navier-Stokes simulations are addressed. These models contain all components of the relevant physics and they can improve the understanding of the related phenomena, a necessary condition for the development of both appropriate simplified models as well as reliable technologies. Within the LB framework, a technique for the characterization and computer- reconstruction of the porous electrode structure was developed using advanced pattern recognition algorithms. In CFD modeling, 3-D simulations were used to investigate SOFC with internal methane steam reforming and have exemplified the significance of porous and novel fractal channel distributors for the fuel and oxidant delivery, as well as for the cooling of PEFC. As importantly, the novel concept has been put forth of functionally designed, fractal-shaped fuel cells, showing promise of significant performance improvements over the conventional rectangular shaped units. Thermo-economic modeling for the optimization of PEFC is finally addressed. Keywords: Multidimensional simulations of fuel cells · Porous electrode structure characterization · State-space modeling of electrochemical reactions · Thermo-economic optimization

Residual stress and buckling patterns of yttria-stabilised zirconia thin films for micro-solid oxide fuel cell membranes

Introduction Micro-solid oxide fuel cells (micro-SOFCs) have drawn much attention in the last 5 years as promising energy conversion systems for powering small portable electronic devices [1-4]. Micro-SOFCs consist of a thin freestanding cathode-electrolyte-anode assembly (typically <1 μm) prepared by microfabrication techniques used in silicon technologies (Fig. 1). One crucial factor is the thermomechanical stability of these thin-film membranes during microfabrication and device operation. In this study, the thermomechanical properties of freestanding yttria-stabilised-zirconia (YSZ) thin-film membranes deposited by pulsed laser deposition (PLD) for application in micro-SOFCs are investigated and the experimental observations regarding the buckling patterns are simulated.

Grain and Grain Boundary Conductivities in Nanocrystalline Yttria-Stabilized-Zirconia Thin Films

Nanocrystalline yttria-stabilized-zirconia thin films with grain sizes smaller than 15 nm are fabricated by spray pyrolysis. Impedance spectroscopy is performed perpendicular to the thin film between room temperature and 600 °C. For a grain size of 13 nm, the grain and grain boundary contributions of the electrical conductivity can be discerned but only between 200 °C and 400 °C. For smaller grains, and higher or lower temperatures, the grain contribution cannot be resolved.