Étude de Réduction et d’Oxydation de l’Anode d’une Pile à Combustible à Oxyde Solide à Support Anode (SOFC)

Le but de cette étude est de comparer la microstructure de l’anode en fonction des conditions de réduction et lors de cycles redox. Pour réaliser ce projet, un code Mathematica® est développé. Celui-ci permet de traiter et d’analyser des images SEM pour en extraire la proportion des différentes phases, leur taille de grain et leur contiguïté

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.cm2 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 3rd generation stacks of SOFC in the European project “REAL SOFC” or the prototypes in French ANR projects

Duktile Gesteinsdeformation durch zeitabhängige Kataklase

In Gesteinen, die bei schwach- bis mittelgradiger Metamorphose deformiert wurden, sind Mikrostrukturen, wie z.B. undulöse Auslöschung, Subkörner und kristallographische Regelungen, typisch. Sie werden i.a. als das Resultat eines kristallplastischen Deformationsprozesses interpretiert. Das häufige Auftreten dieser Strukturen führte zu der Annahme, daß kristallplastische Deformationsprozesse zu den dominierenden duktilen Deformationsmechanismen während der Erdkrustendeformation gehören.Die vorliegende Arbeit beschäftigt sich mit Experimenten, bei denen die zeitabhängige kata-klastische Deformation polykristalliner, kompaktierter Aggregatproben des sehr gut löslichen, elastisch/spröden Salzes Natriumchlorat (NaClO3) untersucht wurde. Dieses Salz wurde als Analogmaterial gesteinsbildender Minerale wie Quarz und Feldspat ausgewählt. Es wurde nun untersucht, ob und in welcher Weise spröde Deformationsmechanismen in Anwesenheit von Wasser scheinbar kristall-plastische Mikrostrukturen hervorrufen können.Aus den Versuchen ergibt sich nun die Folgerung, daß scheinbar kristallplastisch erzeugte Mikrostrukturen wie Subkörner auch durch spröde zeitunabhängige Risse und subkritische (langsame) Kataklase entstehen können. Diese Erkenntnisse sind wichtig für das rheologische Verhalten der Erdkruste und somit auch für Modellierungen der Erdkrustendeformation. Ihre Betrachtung und Übertragung auf die Deformationsstrukturen der natürlich deformierten kontinentalen, oberkrustalen Gesteine der Erde würde die Interpretation der Mikrostrukturen in diesen Bereichen stark ändern. Dies hätte eine wesentliche Änderung auf den Verlauf der Gesteinsfestigkeitskurven in den Spannungsprofilen der Erdoberkruste zur Folge.Rocks that were deformed under low- to medium-grade metamorphic conditions are characterized by microstructures, such as undulous extinction, subgrains and preferred crystallographic orientation. These structures are generally interpreted as the result of crystal-plastic deformation processes. Due to common occurrence of microstructures crystal-plastic deformation processes are thought to be dominant ductile deformation mechanisms during crustal deformation.This study discusses experiments on time-dependent cataclastic deformation of polycrystalline compact aggregate samples of easily soluble elastic-brittle sodium chlorate (NaClO3) salt, an analogue material to rock forming minerals, e. g. quartz and feldspar. The objective of the experiments was to determine if and under which conditions apparent crystal-plastic microstructures could result from brittle deformation mechanisms in presence of water.In order to evaluate the influence of fluids on deformation and resulting microstructures different experimental setups were realized: (i) triaxial, (ii) in-situ deformation, and (iii) in-situ indentation experiments. Boundary conditions (pressure, temperature and deformation rate) were chosen to allow pressure solution and precipitation.The experiments have shown that crystal-plastically-derived microstructures such as subgrains can also result from time-independent brittle fracturing and subcritical (slow) cataclasis. These observations are critical for our understanding of the rheological behaviour of the crust and, consequently, for modelling of crustal deformation

Stress induced grain boundary migration in very soluble brittle salt

Grain boundary migration (GBM) was studied in-situ at room temperature, atmospheric pressure and an applied diffmfwerential stress of ~9.5 MPa under the optical microscope, in a wet aggregate of an elastic-brittle salt (sodium chlorate). The aggregate was previously deformed predominantly by a combination of grain boundary sliding, pressure solution and cataclastic solution creep. After deformation, but when the sample was still under differential stress, undeformed, fracture-free grains were observed to grow at the cost of deformed, intensely fractured grains. GMB rates typically fell in the range 2--10 [mu]m/day. GBM took place only as long as the sample was under stress. Boundaries stopped to migrate as soon as stress was taken off and started to migrate again as soon as the sample was stressed again. Our observations suggest that GBM in this material and under the prevailing experimental conditions is a stress-induced process, i.e. driven by differences in elastic strain between fractured and unfractured grains

Pressure solution compaction of sodium chlorate and implications for pressure solution in NaCl

Sodium chloride (NaCl) has been extensively used as a material to develop, test and improve pressure solution (PS) rock deformation models. However, unlike silicate and carbonate rocks, NaCl can deform plastically at very low stresses (~0.5 MPa). This could mean that NaCl is less suitable for use as an analogue for rocks that do not deform plastically at conditions where PS is important. In order to test the reliability of NaCl as a rock analogue, we carried out a series of uniaxial compaction experiments on sodium chlorate (NaClO3) at room pressure and temperature (P-T) conditions and applied effective stresses of 2.4 and 5.0 Mpa. NaClO3 is a very soluble, elastic-brittle salt, that cannot be deformed plastically at room P-T conditions. The results were compared with experiments on NaCl at similar conditions and show that NaClO3 behaves in a strikingly similar way to NaCl, despite its brittleness. Like NaCl, it most likely compacts by a grain boundary diffusion controlled PS mechanism. Mechanical data were fitted to a power law in the form: (with volumetric strain ratevolumetric strain [var epsilon], effective stress [sigma] and grain size d). A reasonable fit was obtained, with [alpha]=2 to 4, n=1.6±0.5, and m=2.8±0.5. The similarity in mechanical behaviour of the two materials (NaCl plastic, NaClO3 brittle) suggests that plasticity does not play a key role in PS compaction deformation of NaCl. This means that its plasticity is not a drawback for its use as a PS analogue for rocks or for deriving PS creep laws for salt from compaction experiments

Simulation of a high temperature electrolyzer

International audienceBased on Solid Oxide Fuel Cell (SOFC) technology, Solid Oxide Electrolysis Cell (SOEC) offers an interesting solution for mass hydrogen production. This study proposes a multiphysics model to predict the SOEC behavior, based on similar charge, mass, and heat transport phenomena as for SOFC. However, the mechanism of water steam reduction on Nickel/Yttria-Stabilized Zirconia (Ni/YSZ) cermet is not yet clearly identified. Therefore, a global approach is used for modeling. The simulated results demonstrated that a Butler–Volmer’s equation including concentration overpotential provides an acceptable estimation of the experimental electric performance under some operating conditions. These simulations highlighted three thermal operating modes of SOEC and showed that temperature distribution depends on gas feeding configurations

Electrical Interaction Model of Planar SOFC Stack Fed with Natural Gas

International audienceSolid oxide fuel cells (SOFC) are energy conversion devices thatproduce electricity and heat directly from a fuel such asnatural gas. Little attention has been paid to the electricalbehaviour of a SOFC stack. This paper presents an electricalinteraction model of a planar anode supported intermediatetemperature SOFC stack with direct internal reforming. The SOFCstack model is built up of multiple single repeat units stackedon top of each other and takes into account, amongst otherparameters, contact resistances and gas flow. This assembly issandwiched between two end plates. A combined model with mass,charge and heat balances has been developed with a specialattention to the description of electrical behavior. Such anapproach can provide a picture of the two dimensionaldistribution of potential, current density, and temperature.Simulations are performed to analyse the impact of changes inmaterial conductivities, electrical configuration and operation conditions

Optimization of SOFC interconnect design using Multiphysic computation

International audienceThe aim of this work is to optimize an interconnect design. A three-dimensional model have been developed in order to investigate the effect of interconnect design on electrical performance and degradation process. Oxygen concentration, potential, current density and temperature distribution in interconnect and SOFC cathode have been calculated. Cathode degradation has been supposed to be due to temperature gradient non-uniformity. Our studies have demonstrated the impact of cathode/interconnect contact on thermal and electrical behavior. Thus, an optimization of the cathode/interconnect contact using COMSOL Multiphysics® software has been investigated. In this investigation, the effects of the two geometrical parameters are considered. This paper presents the modification of cathode/interconnect contact area and electrical collecting pins size. Simulations show a decreasing power density and a reduction of temperature gradient for an increasing contact area. With a decreasing size of collecting pins, better temperature homogeneity and power density are recorded

Methods for quantification of anode support microstructure

Degradation observed on long term stack tests may be explained in part by microstructural changes of the electrodes. Particularly at the anode side, the coarsening of nickel particles will affect its performances. Increasing the metal particle size will first diminishes the Three-Phase Boundary (TPB) where fuel oxidation takes place, and secondly decrease the electronic conductivity [1]. In order to quantify the changes in anode microstructure, SEM images analysis is used. Some techniques can be time consuming therefore sample preparation, image treatment and analysis have been optimized to be more efficient. The application of different SEM sample preparation and analysis methods on HTceramix anode supports each show advantages and drawbacks.. One quick method consists in polishing the anode without infiltration and analyzing images manually [2]. A second method requires more elaborate preparation, with image analysis carried out using a Mathematica code. These methods are applied on as sintered and aged (in- stack test) cells based on two different HTceramix anode support structures. 1. Simwonis, D., Tietz, F., and Stoever, D., 2000, Nickel coarsening in annealed Ni/8YSZ anode substrates for solid oxide fuel cells: Solid State Ionics, v. 132, p. 241-251. 2. Monachon, C., Hessler-Wyser, A., Faes, A., Van herle, J., and Tagliaferri, E., 2007, A quick method for characterizing Nickel-Yttria stabilized Zirconia cermet microstructure by scanning electron microscopy: to be submitted