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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.

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. Today's state-of-the-art cathodes are LaxSr1–xMnO3–? for SOFC operating at high temperature i.e. 800–1000°C, or mixed conducting LaxSr1–xCoyFe1–yO3–? for intermediate temperature operation, i.e. 600-800°C. Among the variety of alternative materials, SmxSr1–xCoO3–? and BaxSr1–xCoxFe1–xO3–? 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.

Since the mid-1970s, microencapsulation has become increasingly popular in food, detergent, cosmetic and pharmaceutical industries to protect active agents from degradation or facilitate their controlled release or targeted delivery. Here we report on a synthesis route of a novel class of hollow inorganic microcapsules with unique microstructural and mechanical properties. The method is based on the adsorption of calcium aluminate particles at the interface of water droplets of an oil-continuous emulsion. Upon contact with water, these particles hydrate and form a mechanically stable, porous capsule shell. After solvent evaporation, hollow microcapsules can be harvested with diameters between 30 and 200 μm and yields of up to 75%. The mechanical characterization of entire capsules is accomplished using a uniaxial, micromechanical compression setup installed in a scanning electron microscope. We show that these inorganic calcium aluminate microcapsules are highly crack tolerant owing to their porous shell microstructure. Such a behavior is in strong contrast to the one of hollow aluminosilicate cenospheres, which feature dense shells and show therefore brittle failure in our compression tests. [ABSTRACT FROM AUTHOR]

Particle-stabilized liquid foams provide a general route for producing low-density macroporous materials from melt-processable and intractable thermoplastic polymers. In this paper, we demonstrate how these liquid foams can be used to design macroporous polymers with tailored microstructures and properties by adjusting the various processing parameters. By varying the size, concentration, and wettability of the particles in the colloidal suspensions and controlling the frothing, drying, and sintering conditions, macroporous materials with porosities between 33 and 95% and median pore sizes (D50) between 13 and 634 μm were obtained. This foaming process is applicable to a wide range of hydrophobic materials and is demonstrated here on commercially available polymeric powders of poly(tetrafluoroethylene) (PTFE), poly(vinylidene fluoride) (PVDF), poly(ether imide) (PEI), and poly(ether ether ketone) (PEEK). [ABSTRACT FROM AUTHOR]

In this communication, we describe a general, straightforward route to produce highly porous bulk materials from melt-processable and intractable polymers using particle-stabilized liquid foams. [ABSTRACT FROM AUTHOR]

Four compositions of Ba1−xSrxCo1−yFeyO3−δwere studied for phase, oxygen uptake–release, and transition metal (TM) oxidation states after solid state processing and with in situheating from 300 to 1273 K in air. X-Ray diffraction showed that all compositions except one had the cubic perovskite structure at all temperatures; that with x, y= 0.2 was a mixture as prepared, becoming predominantly cubic at high temperature. Thermogravimetry showed a reversible oxygen absorption–desorption of approximately ±1% from 700 to 1273 K. X-Ray absorption and Mössbauer spectroscopy showed a majority TM3+valence, with at most 40% TM4+. Up to a temperature of 1073 K, the TM4+was reduced to TM3+. Further heating of the composition with x, y= 0.2 to 1233 K resulted in the reduction of Co3+to Co2+. Results from room temperature measurements confirm the thermally activated carrier hopping mechanism with charge fluctuations, while the high temperature delocalized carrier conductivity occurs with a small amount of TM reduction and without phase change for the initially cubic samples. [ABSTRACT FROM AUTHOR]

Foams and emulsions stabilized by colloidal particles can lead to new materials with unique structures and properties. In this Highlight article, we describe the underlying mechanisms of this new enabling technology, highlighting some of the processing routes to obtain capsules and porous structures for a variety of applications. [ABSTRACT FROM AUTHOR]

Abstract The effect of cobalt oxide as sintering aid for CeO2 and Ce0.9Gd0.1O1.95 powders was studied as a function of initial powder particle size. The sintering effect of cobalt oxide, measured as the difference in temperature of maximum shrinkage rate between undoped and 1 cat% cobalt oxide doped powders, decreased with initial particle size. Almost no effect was found for the smallest particle sizes investigated (d < 10 nm) whereas the maximum shrinkage rate temperature decreases by more than 200°C for particles d > 100 nm. For the smaller particles, only doping of much higher concentration produces significant decreases in the temperature of maximum shrinkage rate. This observation suggests that the effectiveness of sintering depends on the specific surface area of the starting powder and points at the decisive role of the doping method used. Dense microstructures with average grain sizes smaller than 100 nm are obtained by doping very fine powders with cobalt oxide. [ABSTRACT FROM AUTHOR]

Abstract Spray pyrolysis has been applied to deposit a wide variety of thin films. These films were used in various devices such as solar cells, sensors, and solid oxide fuel cells. It is observed that often the properties of deposited thin films depend on the preparation conditions. An extensive review of the effects of spray parameters on film quality is given to demonstrate the importance of the process of optimization. The substrate surface temperature is the most critical parameter as it influences film roughness, cracking, crystallinity, etc. Processes involved in the spray pyrolysis technique are discussed in this review as well. [ABSTRACT FROM AUTHOR]