A. Hessler-Wyser

In situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Transmission Electron Microscope

Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500°C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle.

Glass-Forming Exogenous Silicon Contamination in Solid Oxide Fuel Cell Cathodes

Spatially resolved analyses, by energy-dispersive X-ray spectroscopy (EDS) scanning electron microscopy (SEM), allowed the quantification of exogenous Si contamination in a solid oxide fuel cell (SOFC) cathode after operation. The Si quantification, taking into account the endogenous Si impurity level, correlated well with the expectation from the condensation of Si(OH)4 vapor, originating from upstream alloy components and saturated in the hot inlet air. At higher resolution, EDS-transmission electron microscopy (TEM) pointed out the deposition of Si vapor in the form of amorphous SiO2, blocking oxygen incorporation into the electrolyte phase within a composite SOFC cathode.

On Potential Application of Coated Ferritic Stainless Steel Grades K41X and K44X in SOFC/HTE Interconnects

K41X is a ferritic stainless steel grade which was successfully developed in exhaust gas manifold where the temperature could reach 950°C. It contains about 18% wt of chromium and it is stabilized with both titanium and niobium to warranty a good weldability, formability and high temperature corrosion resistance. Moreover, an addition of niobium improves high temperature mechanical properties, in particular the creep resistance. K44X, an enhanced version of K41X with 19%-wt. of Cr plus niobium and molybdenum, was recently developed to be used up to 1000°C. It exhibits better high temperature properties and oxidation resistance. Thanks to their high temperature resistance and their cost competitiveness, these two grades were recently considered as potential candidates to be used as interconnects for Solid Oxide Fuel Cells (SOFC) and High Temperature Electrolysis (HTE), either bare or more certainly coated in order to increase the life duration of the SOFC or HTE systems. This paper will present the high temperature properties of K41X and K44X, in particular oxidation behavior in isothermal and cyclic conditions under operating atmosphere. The positive effect of the addition of a protective coating on these steel grades in terms of oxidation resistance will then be presented. Most of the studied coatings are Mn-Co spinels deposited by sol-gel, atmospheric plasma spray or electroplating, their aim being to limit the chromium evaporation and to fit the severe performance requirements. They lead to low and stable contact resistance, which is a requirement necessary for long-term SOFC/HTE operation: for example a contact resistance of 22 mΩ.cm was obtained after 3500 h at 800°C in air with MnCoFe spinel coating. In this respect, K41X was recently chosen to be tested for the 3 generation stacks of SOFC in the European project “REAL SOFC” or the prototypes in French ANR projects. ECS Transactions, 35 (1) 2481-2488 (2011) 10.1149/1.3570246

Energy-filtered environmental transmission electron microscopy for the assessment of solid-gas reactions at elevated temperature: NiO/YSZ-H2 as a case study.

A novel approach, which is based on the analysis of sequences of images recorded using energy-filtered transmission electron microscopy and can be used to assess the reaction of a solid with a gas at elevated temperature, is illustrated for the reduction of a NiO/ceramic solid oxide fuel cell anode in 1.3mbar of H2. Three-window elemental maps and jump-ratio images of the O K edge and total inelastic mean free path images are recorded as a function of temperature and used to provide local and quantitative information about the reaction kinetics and the volume changes that result from the reaction. Under certain assumptions, the speed of progression of the reaction front in all three dimensions is obtained, thereby providing a three-dimensional understanding of the reaction.

High Temperature Stability of Amorphous Zn-Sn-O Transparent Conductive Oxides Investigated by In Situ TEM and X-ray Diffraction

Transparent conductive oxides (TCOs) are used in a wide variety of technologies, including photovoltaic cells, organic light-emitting diodes, low-emissivity windows and flat-panel displays. While indiumbased compounds are the most widely employed TCOs due to their good optical and electrical properties, they are expensive as a result of the scarcity of indium. Therefore, there is strong interest in the development of alternative oxides that contain only earth-abundant elements. In addition to presenting equivalent electrical and optical properties to those of In-based TCOs, these alternative compounds should also exhibit good thermal stability for compatibility with device fabrication.

Electron energy-loss spectroscopy and energy-filtered TEM imaging for the in situ assessment of reduction-oxidation reactions in Ni-based solid oxide fuel cells

1. Interdisciplinary Centre for Electron Microscopy, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland 2. Photovoltaics and Thin Film Electronics Laboratory, Ecole Polytechnique Fédérale de Lausanne, Neuchâtel, Switzerland 3. Center for Electron Nanoscopy, Technical University of Denmark Lyngby, Denmark 4. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Jülich Research Centre, Jülich, Germany 5. Fuelmat group, Ecole Polytechnique Fédérale de Lausanne, Sion, Switzerland