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La1-xSrxCoy-1FeyO3-δ (LSCF) is a widely used oxygen electrode in Solid Oxide Cells (SOCs). Despite its high electrochemical activity combined with a good mixed ionic and electronic conductivity, it is prone to a phase decomposition upon operation decreasing the global cell performance. However, the underlying mechanisms for the reaction mechanism and the degradation are still not precisely understood and require investigations at atomic scale. Therefore, Density Functional Theory (DFT) calculations are used to better unravel the mechanisms occurring in the electrode. In particular, the interaction of the dioxygen gas with the AO-terminated (100) surface is studied. In this context, the oxygen reduction reaction is decomposed in a series of elementary steps including oxygen vacancy formation, adsorption of the dioxygen molecule on the bare surface and into a surface vacancy accompanied with the formation of intermediate oxygen species, followed by the dissociation and incorporation into the bulk. The calculations have been carried out at two different Cobalt contents (y=0.125 and y=0.25).

Solid oxide cells (SOCs) have attracted a growing attention thanks to their high efficiency and ability to operate in both electrolysis (SOEC) and fuel cell (SOFC) modes. Despite its great potential, the current SOC technology faces significant cell degradation during long-term operation. The degradation phenomena are still not well understood as they involve complex and intricate processes arising at different length scales. Additionally, it has been shown that the degradation rate under SOEC operation is generally higher than in SOFC mode. To address this issue, durability tests were performed in SOEC mode at 750, 800 and 850 °C for 2000 h at -1 A cm-2 in 10/90 vol.% H2/H2O and dry air. The electrochemical performance of the cells was assessed using electrochemical impedance spectroscopy. In addition, advanced characterization techniques with nanometer and atomic resolution have been used to study material degradation after long-term testing.

A long-term test of 2000 h has been carried out on a typical solid oxide cell in electrolysis mode at −1 A.cm−2 and 750 °C. The 3D reconstructions of the pristine and aged cermet have revealed a strong Ni depletion at the electrolyte interface. To explain this result, an electrochemical and phase-field model has been developed to simulate the Ni migration in Ni/YSZ electrode. For this purpose, a mechanism has been proposed that takes into account the impact of polarization on the Ni/YSZ wettability. In this approach, it assumes that the Ni/YSZ interfacial energy is changed by the concentration of oxygen vacancies in the electrochemical double layer. Thanks to the model, the Ni migration has been computed in the same condition than the experiment and complemented by a simulation in reverse condition in SOFC mode. In good agreement with the experiment, the simulations have revealed a strong Ni depletion at the electrolyte interface after operation under electrolysis current. On the contrary, a negligible Ni redistribution with a very slight Ni enrichment has been predicted at the electrolyte interface after SOFC operation. These results tend to prove the relevance of the mechanism.

Solid Oxide Cell (SOC) is a high-temperature electrochemical device that can be operated in both fuel and electrolysis modes. It is constituted of a dense electrolyte in Yttria Stabilized Zirconia (YSZ) sandwiched between two porous electrodes. The hydrogen electrode is classically made of a cermet composed of Nickel and YSZ (Ni-YSZ). Because of system failures in operation (e.g. a fuel shortage or air re-introduction during the system shutdown), the Nickel is liable to be re-oxidized inducing its volume expansion. The Ni swelling within the cermet leads to a mechanical damage in the YSZ backbone [1] resulting in a degradation of the overall cell performances [2]. Therefore, it is still required to enhance the hydrogen electrode robustness for a higher redox tolerance. For this purpose, it is essential to predict accurately the formation and distribution of micro-cracks in the YSZ network during the re-oxidation. Nevertheless, the modeling of the initiation and propagation of cracks in complex microstructures of porous ceramics is still a subject of investigation. Moreover, there is also a lack of data on fracture properties for the porous YSZ material. In this frame, the present study was focused to propose a relevant model for the initiation and the growth of cracks in porous ceramics in order to simulate the fracture induced by Ni re-oxidation. For this purpose, a coupled experimental and modeling approach has been adopted (Fig.1). For the experimental part, the mechanical characterizations have been performed on thin YSZ membranes of different porosities representative of SOC microstructures. The compression test has been chosen as it generates a quasi-uniform stress field in the tested samples. To this end, micro-pillars of few tens of micro-meters in section have been milled in the YSZ membrane using a xenon plasma focused ion beam. For each investigated porosity, the compressive fracture strength has been measured on several pillars. The tested samples have been carefully characterized to assess the damage in the microstructure. As expected, the compressive strength was found to decrease with increasing the porosity. Moreover, a transition has been observed from a brittle behaviour at low porosity towards a local damage at high porosity [3]. All these experimental data have been used to validate a numerical tool to simulate the fracture in porous microstructure. In this study, a model based on the Phase Field Method (PFM) has been developed. In this approach, the crack is modeled implicitly through a smooth scalar damage variable and its width is controlled by a length scale parameter. Therefore, this method is well adapted to solve the fracture problem in complex and heterogeneous materials since it is mesh independent. The implemented model corresponds to the one proposed by C. Miehe et al. [4] who proposed a staggered scheme for the numerical resolution. A special attention has been paid in this work to evaluate the model capability to accurately predict the crack initiation for porous electrode material. For this purpose, the results of the PFM simulations have been compared to the mixed criterion proposed by D. Leguillon [5]. The study has been conducted on notched samples with different opening angles and for macro-cracks blunted by different cavity sizes. It has been highlighted that the two approaches provide consistent predictions for the crack initiation. Moreover, it has been checked that the PFM model is able to reproduce accurately the expected decrease of the fracture toughness when increasing the material porosity. Once these studies completed, the PFM model has been used to simulate the micro-compression test using a multiscale approach. It has been found that the model is able to reproduce correctly the decrease of the measured compressive fracture strength with the sample porosity. Moreover, the distribution of cracks in the simulations has enabled analysing the transition from a brittle to a local damage detected during the experiments. The validated model has been applied to simulate the fracture induced by the Ni re-oxidation. The repartition and density of micro-cracks in the YSZ backbone has been computed during the Ni reoxidation and the results have been confronted with an available tomographic data set. Finally, the impact on the cermet redox tolerance of the Yttrium content in YSZ has been discussed thanks to the modelling. [1] Faes, A. et al. J. Power Sources 193, 55–64. [2] Laurencin, J. et al. J. Power Sources 192, 344–352. [3] Meille, S. Et al . J. Eur. Ceram. Soc. 32, 3959–3967. [4] Miehe, C. et al. Comput. Methods Appl. Mech. Eng. 199, 2765–2778. [5] Leguillon, D. J. Mech. – A/Solids 21, 61–72. Figure 1

International audience; The classical 1D continuum model of oxygen exchange in a porous mixed conductor (bulk path) is presented in terms of Linear Sweep Voltammetry (LSV) and Cyclic Voltammetry (CV). We show that, from a formal point of view, it corresponds to an insertion, diffusion and chemical reaction mechanism. Some general rules for LSV/CV curves are established. Using a dimensionless description, we evidence eight different limiting behaviors of the electrochemical model, which depend on four dimensionless parameters. For an almost full insertion level at equilibrium, these parameters can be reduced to only two, i.e. the dimensionless electrode thickness L and the dimensionless chemical rate constant Lambda. We show that the resulting zone diagram, plotted using the representation log L versus log Lambda, is useful to predict the possible sequences of model behaviors as a function of the potential sweep rate and the electrode thickness. This theoretical analysis is applied to a porous LSCF oxygen electrode under air at high temperature.

International audience; An elementary kinetic model was developed to predict the electrochemical response of porous LSCF and LSCF-CGO electrodes. The model was validated thanks to experiments performed on symmetrical cells using a three-electrode setup. After the model calibration on polarization curves, it has been shown that the model is able to simulate accurately the experimental impedance diagram at OCP and under polarization without additional fitting. Moreover, the evolution of the electrode polarization resistance with the oxygen partial pressure is well reproduced by the model. The electrodes reaction mechanism was thoroughly analyzed and it has been shown that the transition from the bulk path to the surface path depends on the temperature, the polarization and the oxygen partial pressure. The rate-determining steps for the LSCF electrode have been identified at OCP as function of the oxygen partial pressure. Finally, a sensitivity analysis has been performed to study the impact of LSCF demixing on the electrode performances. For a given decomposition, it has been highlighted that the surface passivation would be more impacting than the decrease of the ionic conductivity. Moreover, the impact of the LSCF decomposition would be more detrimental for the electrode performances evaluated in electrolysis mode.

This work reports a durability study in galvanostatic mode at 700 °C for 900–960 h related to structural and microstructural evolution of LaPrNiO4+δ (LPNO) oxygen electrode. The performances of the electrode, deposited by electrostatic spray deposition (ESD) and screen printing (SP), are not changed when operated under electrolysis current whereas a degradation is observed in fuel cell mode. Small quantities of secondary phases (La,Pr)3Ni2O7-δ in the fine ESD microstructure and Pr6O11 on both ESD and SP layers are detected in the pristine electrode by synchrotron μ-X-ray diffraction and fluorescence. The destabilization of LPNO electrode, which slightly occurs in the ESD layer after thermal aging in air, is strongly accelerated under anodic and cathodic currents. Indeed, the formation of a new phase (La,Pr)4Ni3O10-δ on both ESD and SP layers occurs with the extension of (La,Pr)3Ni2O7-δ in the SP layer, and a significant increase in the amount Pr6O11 at high current density. Nevertheless, the electrode microstructure, determined by FIB-SEM tomography, remains stable under current whereas severe delamination is observed after aging in fuel cell mode. Based on these characterizations, the mechanisms of electrode aging have been discussed to explain the different behavior in fuel cell and electrolysis modes.

International audience; Two types of solid oxide cells with different Ni-YSZ cermet microstructures have been aged in electrolysis and fuel cell modes for operating times ranging from 1000 to 15000 hours. The pristine and aged cermets have been reconstructed by synchrotron X-ray holotomography. Nickel agglomeration has been observed in the bulk of the operated samples inducing a significant loss of triple phase boundary lengths. The inspection of the microstructural properties has confirmed the stabilizing role of YSZ on Ni coarsening. Furthermore, the gradients of properties quantified at the electrolyte interface have revealed a depletion of Ni only in the electrochemically active region of the electrode. The process is strongly promoted for a coarse cermet microstructure when operated under electrolysis current. The evolution of the microstructural properties has been implemented in an in-house multiscale model. The simulations have shown that the loss of performance is dominated by the depletion of Ni in case of a coarse microstructure. Thanks to the computations, it has been shown that the Ni depletion is controlled by the cathodic overpotential. To explain this dependency, it has been proposed that the accumulation of oxygen vacancies in the double layer could deteriorate the Ni/YSZ interface and trigger the Ni depletion.

Chapter 11: Cell, Stack, and System Modeling and Simulation; International audience; As it plays a key role on the performances and durability of solid oxide cells, the electrode microstructure has to be finely characterized and modelled. In this frame, a plurigaussian random field model and an original sphere packing algorithm have been developed to simulate a large variety of electrode microstructures. The two stochastic geometrical models have been validated on various electrode reconstructions obtained by synchrotron X-ray holotomography including the microstructures of thin functional layers and a thick current collector. Afterwards, semi-analytical microstructural correlations have been proposed and validated on a large dataset of representative synthetic microstructures. These equations allow establishing the links between the microstructural properties controlling the performances (Triple Phase Boundary lengths density and specific surface areas) to the ‘basic’ geometric attributes of the electrode (composition, porosity and particle size distribution). These correlations have been used to model the loss of active TPBl due to the Ni agglomeration.

International audience; Rare earth nickelate, LaPrNiO4+δ (LPNO), is one of the most suitable oxygen electrode materials for Intermediate Temperature Solid Oxide Cells (IT-SOC). By using Electrostatic Spray Deposition (ESD) and Screen Printing (SP), various architectural designs of electrodes are fabricated on Ce0.9Gd0.1O2-δ (GDC) electrolyte. Three electrode configurations are investigated based on a single Active Functional Layer (AFL), a double layer of the same composition which is an AFL topped by a current collecting layer (CCL) and a triple layer which is mainly the double layer with an LPNO/GDC composite interface. For each configuration, the electrode response and properties are analyzed by using Electrochemical Impedance Spectroscopy (EIS), X-ray diffraction and Scanning Electron Microscopy (SEM). The electrochemical properties are discussed as a function of the electrode design, microstructure, and composition with the presence of a higher order nickelate such as (La,Pr)3Ni2O7-δ. The electrochemical performance is enhanced thanks to better adhesion of the electrode onto the electrolyte, the presence of an LPNO/GDC composite interface, a more homogeneous distribution of fine grains, and a flatter electrode surface. A significant decrease in the polarization resistance (Rpol) is measured from 0.72, 0.50, and down to 0.20 Ω cm2 at 600 °C from single to triple layer, respectively.

International audience; The reaction mechanism and the impact of microstructure on performances for a porous LSCF‐CGO composite used as O2 electrode in SOCs have been investigated. For this purpose, an integrated approach coupling (i) electrochemical testing, (ii) advanced 3D characterizations, and (iii) modeling was proposed. In this frame, a symmetrical cell was tested in a three‐electrode setup and the microstructure of the LSCF‐CGO working electrode was reconstructed by FIB‐SEM tomography. The experimental polarization curves and the impedance diagrams along with the extracted microstructural parameters were used to validate an in‐house microscale electrochemical model. This model considers two parallel reaction pathways with (i) an oxidation/reduction at TPBls (surface path) and (ii) an oxygen transfer at the gas/LSCF interface (bulk path). It was shown that the LSCF‐CGO electrode reaction mechanism is mostly controlled by the charge transfer at TPBls whatever the polarization. Finally, the impact of electrode composition, porosity and particles size on the cell polarization resistance was investigated by using the electrochemical model in combination with a large dataset of synthetic microstructures generated by a geometrical stochastic model. The effect of each microstructural parameter on the electrode performance was analyzed in order to provide useful guidelines for the cell manufacturing.

International audience; La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) electrode deposited on Gadolinium Doped Ceria (GDC) has been characterized by cyclic voltammetry and impedance spectroscopy in the temperature range of 300 to 700 °C. We demonstrate that the LSCF microstructure has a strong influence on the shape of the voltammograms and on the variation of the series resistance, Rs, measured on the impedance diagrams as a function of the dc bias. LSCF was deposited with two different microstructures, either porous layer (~10 μm thick) by screen-printing (SP) or almost dense layer (~2 μm thick) by Electrostatic Spray Deposition (ESD). For the denser film, one cathodic peak and one reverse anodic peak were evidenced under air at 300 and 500°C. The Rs value was also found to increase with the dc cathodic bias. On the contrary, no peak was observed with the porous film under the same conditions and Rs was independent on the applied potential from 300 to 700°C. Decreasing the oxygen partial pressure allowed the peaks to be evidenced and Rs to vary. The results are discussed in terms of oxygen exchange rate at the LSCF/gas interface, which depends on the LSCF specific surface area and the oxygen partial pressure.

A typical La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) electrode was reconstructed by X-ray nanotomography. The new Nano Imaging beamline ID16A-NI of the European Synchrotron Radiation Facility (ESRF) was used to conduct the investigation. Particular attention was paid to prepare samples with a well-adapted shape for the tomographic experiments. The optimized sample preparation and new experimental set-up enables an improved spatial resolution of about 50 nm to be obtained in the electrode reconstruction of 51.2 x 25.6(2) x pi mu m(3). The LSCF microstructural properties were quantified in the 3D volume and used as input data in a dynamic micro scale electrochemical model. The numerical tool includes two parallel reaction pathways with an oxygen exchange at the LSCF/gas surface and a charge transfer at the electrode TPB. Electrochemical impedances were computed in the time domain at OCP, as well as under anodic and cathodic polarizations. Simulations allowed the microstructural parameters to be linked to the basic mechanisms of electrode operation according to the electrode polarization. A microstructural sensitivity analysis was performed on the single-phase LSCF and LSCF-CGO composite electrodes in order to identify the parameters that impact most the electrode response. It was found that the LSCF-CGO composite presents much higher performances compared to the LSCF single phase electrode especially in anodic polarization. (C) 2016 Elsevier B.V. All rights reserved.

Improving the oxygen electrode performances of the single solid oxide electrolysis cell (SOEC) is of peculiar interest as it contributes to a large extent to the cell polarization resistance. The present study focuses on alternative oxygen electrode materials: the oxygen overstoichiometric rare-earth nickelate oxides Ln 2 NiO 4+δ (Ln = La, Pr or Nd). Their electrochemical properties are measured on three-electrode symmetrical half-cells under zero dc conditions and under anodic polarization. The electrochemical characteristics are compared to those of the usual perovskite La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ (LSFC) electrode. Under polarization, in the temperature range from 600 °C to 800 °C, the Pr 2 NiO 4+δ -based cells exhibit the highest performances. At the same anodic overpotential of 0.15 V/Pt/air, the current densities measured through Pr 2 NiO 4+δ nickelate-based cells are more than ten times larger than those obtained with a cell based on LSCF. The modeling of the anodic overpotential phenomena (activation and concentration terms) occurring at the oxygen electrode is achieved and discussed.

International audience; For many industrial applications, hydrogen must be pressurized before being used or stored. Because compressing liquid water is less energetic than the first levels of gaseous H2 compression, High Temperature Steam Electrolysis (HTSE) performed under pressure might represent an advantageous way for hydrogen production. With the goal of improving the electrolysis efficiency, an experimental and modeling approach has been adopted in order to better understand the basic underlying mechanisms of pressurized electrolysis operation. Experiments were carried on two different single commercial solid oxide cells at 800 °C in the pressure range of 1–10 bar. As a first result, according to the i–V curves, two main pressure effects have been observed. First, as expected, the Open Circuit Voltage is higher under pressure. Then, the limiting current density is increased with increasing the pressure, meaning that the hydrogen production can be improved. The electrochemical model, which has been adjusted on the experimental i–V curves obtained at atmospheric conditions, has been validated for the pressurized operation. Simulations have shown that the improvement of the limiting current is related to the decrease of cathode's concentration overpotential with pressure. Moreover, an optimal pressure can be defined depending on the cell polarization. Higher pressures than the optimal one lead to slightly decrease the hydrogen production rate, mainly due to Open Circuit Voltage increase that cannot be balanced enough by the cathode's concentration overpotential decrease. Finally, this study demonstrates that cell performances under pressure are less sensitive to the variation of cermet-support microstructural properties.

Microstructure of a cermet Ni–YSZ bi-layer is analysed on the basis of three dimensional reconstructions obtained on both functional layer and cell support. Microstructural parameters of gas, ionic and electronic phases are determined in terms of phase connectivity, mean particles diameter, particles size distribution, specific surface area, tortuosity factor and density of TPBls. Microstructural properties are introduced in an SOEC cathode micro model that takes into account the specific configuration of the Ni–YSZ composite bi-layer. Simulations show that the extent of the electrochemical reaction in the support is very limited. Moreover, it is found that electrode apparent activation energy is a combination of effective ionic conduction and charge transfer in the active functional layer.

International audience; The performance of advanced functional materials for fuel cell applications are closely linked to the material composition and morphology at the micro and nano-scales. 3D characterization techniques that can provide bulk information at these fine scales are therefore essential for microstructure optimization of these materials. Here, the X-ray nano-holotomography technique is used to image various multi-phase and absorbing solid oxide fuel cell electrodes. Different porous structures for typical commercial cells and innovative electrode designs obtained using a freeze-casting process are studied. Taking advantage of the geometrical setup and the use of high energy X-rays, both large reconstructions (field of view: 150 A mu m) and local tomography at higher resolution (field of view: 50 A mu m) can be performed on the same sample to have a multi-scale approach. This produces highly representative sample volumes with a size/resolution ratio that allows the geometric and physical properties of the materials to be calculated, e.g., connectivity of each phase, mean particles diameters, specific surface area, particle size distributions, tortuosity factors, and densities of triple boundary lengths.

International audience; X-ray computed nanotomography (nano-CT) and scanning electron microscopy (SEM) have been applied to characterize the microstructure of a Solid Oxide Fuel Cell (SOFC) anode. A direct comparison between the results of both methods is conducted on the same region of the microstructure to assess the spatial resolution of the nano-CT microstructure, SEM being taken as a reference. A registration procedure is proposed to find out the position of the SEM image within the nano-CT volume. It involves a second SEM observation, which is taken along an orthogonal direction and gives an estimate reference SEM image position, which is then refined by an automated optimization procedure. This enables an unbiased comparison between the cell porosity morphologies provided by both methods. In the present experiment, nano-CT is shown to underestimate the number of pores smaller than 1 mu m and overestimate the size of the pores larger than 1.5 mu m. (C) 2013 Elsevier Inc. All rights reserved.

A methodology, based on synchrotron X-ray nano-tomography measurements, is proposed to obtain 3D reconstructions of porous supports for Solid Oxide Fuel Cell (SOFC). The methodology has been applied to investigate the microstructure of a typical Ni–8YSZ cermet substrate of an Anode Supported Cell (ASC). Experimental conditions have been optimised so that a 3D reconstruction of 42 μm × 42 μm × 105 μm has been computed with a voxel size of 60 nm. The 3D reconstruction has been numerically analysed showing a quasi isotropy over the medium. A special attention has been paid to compute the morphological properties controlling the gas diffusion through the support. In this frame, it has been found that a volume as large as 35 μm × 35 μm × 35 μm is required to be statistically representative for the whole substrate. More particularly, the tortuosity factor τ has been calculated on the basis of finite element simulations. The effect of the boundary conditions taken for these simulations has been investigated. In the direction perpendicular to the anode/electrolyte interface, the tortuosity factor has been determined to τ z = 2.8 − 0.2 + 0.3 .

International audience; A 2D multi-physic in-house-model has been developed to analyse the performances of Solid Oxide Electrolysis Cell (SOEC) stack This model encompasses a combined electrochemical and thermal description of the electrolyser An analytical solution for multi-species diffusion across the porous cathode has been implemented in the model This numerical tool is useful to provide all the important parameters of the stack operation distribution of temperature heat fluxes local current densities gas concentrations and overpotentials Simulations show that thermal equilibrium of the stack is strongly dependant on radiative heat losses whereas convective heat transfers are limited In the exothermic operation mode the cell warming depends on the radiation efficiency A parametric study has been carried out to analyse SOEC irreversible losses It is found that the anode activation overpotential is significant whereas the polarisation due to the cathode activation remains much more limited Anode concentration overpotentials are found to be insignificant whatever the operating condition Cathode concentration overpotentials are found to be moderate in the case of Electrolyte Supported Cell (ESC) while they can increase drastically with current density in the case of Cathode Supported Cell (CSC) Addition of diluent gases into the H(2) H(2)O mixture is found to increase the concentration overpotentials

International audience; Highly porous ceramics are critical components of Solid Oxide Fuel Cells. Nevertheless, their mechanical properties are poor and not fully understood. Herein, Discrete Element simulations are used to quantitatively assess the relation between the particulate microstructure of highly porous ceramics and their mechanical properties. Partially sintered ceramics are numerically generated with complex microstructures including pore formers and bilayers. These microstructures are then tested in tension and compression to obtain their elastic and fracture behavior. Compiling experimental data from the literature and simulations results, an Orowan-Petch type relation between strength and particle size is proposed for highly porous ceramics. It is based on the local fracture model at the length scale of solid bonds between sintered particles.

International audience; Typical microstructures of LSM/YSZ NiO/YSZ Ni/YSZ composite electrodes are simulated by the discrete element method The numerical microstructures are generated by taking into account in a realistic manner the sintering process This allows complex microstructures such as Mayers or microstructures containing pore formers to be obtained NiO particles in the NiO/YSZ composite electrodes are reduced to Ni Reduction is carried out with the discrete element formalism which allows particle rearrangement to be taken Into account We show that the mechanical percolation of the YSZ phase plays an important role during the reduction of NiO The various numerical microstructures generated by sintering and reduction are analyzed to evaluate important microstructural features such as macroscopic porosity pore surface area and Triple Phase Boundary length (c) 2010 Elsevier BV All rights reserved

International audience; The effects of Ni-YSZ cermet re-oxidation in anode supported Solid Oxide Fuel Cells (SOFCs) have been investigated. Damage mechanisms have been studied in both cases of direct oxidation in air (i.e., fuel shutdown) or by an ionic current (i.e., fuel starvation). Direct oxidation tests show that the electrolyte cracks for a conversion degree of Ni into NiO ranging between similar to 58 and similar to 71%. This failure mode has been modelled considering both the bulk expansion of the cermet induced by the transformation of the Ni phase and the change of mechanical stresses in the multilayered cell. In the case of fuel starvation, a thin layer of the cermet was electrochemically re-oxidised at 800 degrees C and then reduced under a hydrogen stream. This 'redox' cycle was repeated until the degradation of the cell. The evolution of the impedance diagrams recorded after each cycle suggests that the cermet damages in an area close to anode/electrolyte interface. The mechanical modelling states that a delamination can occur along the interface between the Anode Functional Layer(AFL) and the Anode Current Collector (ACC) substrate. This theoretical result confirms the experimental trends observed by impedance spectroscopy. (C) 2009 Elsevier B.V. All rights reserved.

This thesis was dedicated to the development of a relevant numerical tool to predict the fracture in 3D porous ceramics such as the electrodes of Solid Oxide cells (SOCs). For this purpose, the so-called phase-field method (PFM) was adopted. Indeed, this model offers many advantages to take into account the characteristics of real complex electrode microstructures. Because of the lack of fracture properties data on porous Yttria-Stabilized Zirconia (YSZ), micro-mechanical characterizations were carried out to provide experimental results for the model validation. A dedicated protocol from the specimen fabrication and compression up to post-mortem characterization was developed. The compressive fracture strength was measured as a function of porosity and a transition in the fracture mode from a brittle behavior to diffuse damage was detected when increasing the porosity. Moreover, the capacity of the PFM model to accurately predict the crack initiation when the fracture is controlled by a mixed stress-energy criterion was validated. For this purpose, the PFM simulations were compared to the well-established predictions given by the coupled criterion on ideal geometries. Besides, the regularization length scale parameter in the PFM model formulation was found to contain information on the material fracture properties and to depend on the type of the local geometry from which the crack initiates. The relevance of the PFM model to predict the fracture in real porous ceramic microstructures was then assessed by simulating the apparent fracture toughness and the compressive fracture strength as a function of porosity. A good agreement with experimental data was obtained, thus validating the PFM model. Finally, as an illustration of the PFM capability to study the mechanical damage of porous SOCs electrodes, the YSZ skeleton micro-cracking induced by the Ni re-oxidation was simulated. The critical degree of oxidation was calculated considering two typical cermets (Ni-3YSZ and Ni-8YSZ). From this study, the PFM model appears as an efficient tool to analyze the effect of material and microstructural properties on the cermet redox tolerance and, more generally, on the mechanical robustness of porous ceramic-based electrodes., Cette thèse a été dédiée au développement d'un outil numérique pertinent pour prévoir la rupture des céramiques poreuses telles que les électrodes des cellules à oxyde solide. Pour ce faire, la méthode dite par champ de phase (PFM) a été adoptée. Cette méthode permet de tenir compte des caractéristiques des microstructures complexes des électrodes. En raison du manque de données des propriétés à rupture de la zircone poreuse stabilisée à l'yttrium (YSZ), des caractérisations micromécaniques ont été réalisées afin de fournir des résultats expérimentaux pour la validation du modèle. Un protocole dédié depuis la fabrication et la compression de l'échantillon jusqu'à la caractérisation post-mortem a été développé. La contrainte à rupture en compression a été mesurée en fonction de la porosité. Une transition dans le mode de rupture d’un comportement fragile à un endommagement diffus a été détecté avec l’augmentation de la porosité. En outre, la capacité du modèle PFM à prévoir précisément l’amorçage d’une fissure, notamment quand la rupture est contrôlée par un critère mixte en énergie et contrainte a été validée. Pour ce faire, les simulations par la méthode par champ de phase ont été comparées aux prédictions obtenues par le critère couplé sur des géométries idéales. De plus, il s’est avéré que le paramètre de régularisation introduit dans le modèle PFM contient les informations sur les propriétés à rupture du matériau et dépend du type de la géométrie locale à partir de laquelle la fissure s’amorce. La pertinence du modèle PFM à prévoir la rupture dans des microstructures réelles de céramiques poreuses a été ensuite évaluée en simulant la ténacité apparente et la contrainte à rupture en compression en fonction de la porosité. Un bon accord avec les résultats expérimentaux a été obtenu validant ainsi le modèle PFM. Finalement, pour illustrer la capacité du modèle PFM à étudier l’endommagement mécanique des électrodes poreuses des cellules à oxyde solide, la microfissuration du squelette zircone induite par la réoxydation du Ni a été simulée. Le degré de réoxydation critique a été calculé sur deux cermets typiques en Ni-3YSZ et Ni-8YSZ. Cette étude a montré que le modèle PFM est un outil efficace pour analyser l’effet des caractéristiques des matériaux et de leurs microstructures sur la tolérance du cermet à la réoxydation, et plus généralement, sur la robustesse mécanique des électrodes à base de céramiques poreuses.

An elementary kinetic model was developed to predict the electrochemical response of porous LSCF and LSCF-CGO oxygen electrodes used in Solid Oxide Cells (SOC). For this purpose, an integrated approach coupling (i) electrochemical testing, (ii) advanced 3D characterizations, and (iii) modeling was proposed. The microstructural properties extracted from the 3D electrodes volumes obtained by FIB-SEM tomography were implemented in the model as input parameters. The model was validated on a large set of experiments performed under different operating conditions by testing symmetrical cells in a three-electrode configuration. The reaction mechanism of both studied electrodes was thoroughly analyzed and the rate-determining steps for the electrodes have been identified. The LSCF model was also extended to simulate the cyclic voltammetry curves. The resulting simulations were compared with semi-analytical solutions to check the relevance of this simplified approach to assess the voltammetry response of porous LSCF electrodes. From the experimental point of view, long-term tests and post-test characterizations were performed at the electrode and complete cell levels as experimental evidences to support a proposed degradation mechanism for the LSCF demixing. In parallel, the experimental data for the validation of the micro model for the hydrogen electrode have been collected. A special effort have been paid to assemble a steam generator system capable to produce a stable flow over a large range of water partial pressure. Finally, a sensitivity analysis has been performed with the LSCF full elementary model to study the impact of LSCF decomposition on the electrode response., Un modèle cinétique élémentaire a été développé pour prédire la réponse électrochimique des électrodes poreuses composées de LSCF et de LSCF-CGO pour des cellules à oxyde solide. Dans ce but, une approche couplant des i) tests électrochimiques, ii) des caractérisations avancées et de la iii) modélisation a été proposée. Grâce à la tomographie FIB-SEM des reconstructions 3D des électrodes ont été obtenues. Ces dernières ont permis d’extraire les propriétés microstructurales qui ont été implémentés comme paramètres d’entrée du modèle. Ce modèle a été validé grâce à des expériences réalisées dans de nombreuses conditions en testant des cellules symétriques dans une configuration à trois électrodes. Les mécanismes réactionnels des deux électrodes ont été minutieusement étudiés et les étapes cinétiques déterminantes des deux électrodes ont pu déterminées. Le modèle pour le LSCF a été également étendu pour simuler les courbes de voltammétrie cyclique. Les simulations obtenues ont été comparées à des solutions semi - analytiques pour vérifier la pertinence de l’approche simplifiée. D’un point de vue expérimental, des tests de longue durée et des caractérisations post-tests ont été réalisés à l’échelle de l’électrode et de la cellule complète. Ceux-ci ont permis de confirmer expérimentalement un mécanisme de décomposition du LSCF. En parallèle, les données expérimentales pour la validation du micro-modèle pour l’électrode à hydrogène ont été collectées. Une attention toute particulière a été donnée pour assembler un système de génération de vapeur capable de produire un débit constant pour une large plage de valeurs de pression partielle d’eau. Finalement, une analyse de sensitivité a été réalisée avec le modèle élémentaire du LSCF afin d’étudier l’impact de la décomposition du LSCF sur la réponse de l’électrode.

This work was dedicated to the analysis of the degradation of solid oxide cells operated in electrolysis and fuel cell modes. A threefold methodology has been applied by coupling (i) electrochemical tests, (ii) advanced post-test analyses and (iii) multi-scale modeling. Long-term durability experiments have been carried out on standard cells (Ni-YSZ/YSZ/GDC/LSCF) to investigate the performance loss as a function of the operating conditions. Specimens, which have been extracted from the pristine and aged samples, have been characterized to evaluate the microstructural and physico-chemical evolutions occurring at the two electrodes. In parallel, detailed micro-kinetic models for the hydrogen electrode (Ni-YSZ cermet) and for the oxygen electrode (LSCF/LSCF-GDC) have been developed and experimentally validated before being integrated into a macro-scale model for the complete cell. The multi-scale numerical tool has been used to propose a better understanding of the underlying forces driving the degradation. Moreover, the impact on the cell performances has been simulated and discussed as a function of the operating conditions. Based on the results presented in this work, it has been confirmed that the degradation of SOCs is significantly larger in electrolysis mode with respect to fuel cell operation under H2. On the one hand, it has been shown that the difference in durability behavior can be ascribed to the effect of the cathodic overpotential on the nickel instability in the hydrogen electrode. On the other hand, it has been observed and demonstrated that the destabilization of the LSCF is favored by both the anodic current and the high operating temperature., Ce travail a été dédié à l’étude de la dégradation des piles à oxyde solide fonctionnant en mode électrolyse et pile à combustibles. Une approche intégrée couplant (i) des caractérisations électrochimiques, (ii) des analyses post-test avancées et (iii) une modélisation multi-échelle a été adoptée. Des tests de longue durée ont été menés avec des cellules standards (Ni YSZ/YSZ/GDC/LSCF) pour évaluer la dégradation des performances en fonction des conditions de fonctionnement. Des échantillons, extraits de cellules initiales et après test, ont été finement caractérisés pour déterminer les évolutions microstructurales et physico-chimiques des deux électrodes en fonctionnement. En outre, des modèles de cinétiques élémentaires ont été développés et validés expérimentalement pour l’électrode à hydrogène (cermet Ni-YSZ) et pour l’électrode à oxygène (LSCF/LSCF-GDC) avant d’être intégrés dans un modèle de cellule à l'échelle macroscopique. L'outil numérique multi-échelle a été utilisé pour étudier les mécanismes sous-jacents contrôlant les phénomènes de dégradation. Par ailleurs, leurs effets sur les performances de la cellule en fonction des conditions de fonctionnement ont été évalués et discutés grâce au modèle. Sur la base des résultats présentés dans ce travail, il a été confirmé que la dégradation des SOCs est nettement plus importante en mode électrolyse par rapport à un fonctionnement en pile à combustibles fonctionnant sous H2. Cette différence peut être attribuée d’une part, à l'effet des surtensions cathodiques sur l'instabilité du nickel dans l’électrode à hydrogène. D’autre part, il a été observé et démontré que la déstabilisation du LSCF est favorisée sous fort courant anodique à haute température.

Coupling fuel cell and electrolysis technologies provide an attractive solution to absorb the fluctuations induced by the deployement of the intermittent renewable energy sources. In this frame, the technology based on the "Solid Oxide Cells" (SOCs) appears as a promising solution as the same device can be alternatively used in both the electrolysis and fuel cell modes. The SOCs are based on an oxide ceramic electrolyte operating at a high temperature (800-1000 °C) allowing to reach very good efficiency. Hovewer, their durability is still insufficient to envisage their economic deployment. An important issue limiting the SOCs lifetime is the chemical and mechanical destabilizations of the oxygen electrode, as well as its chemical reactivity with the electrolyte upon operation. Moreover, the high operating temperature of SOCs leads to severe constraints on materials assembling and on fabrication processes.The objectives of this PhD thesis are to better understand the role of microstructure and the reaction mechanisms of oxygen electrode in SOCs operating in intermediate temperature (650-800 °C) and to investigate the aging in electrolysis and fuel cell modes through an integrated approach coupling electrochemical and physicochemical characterizations with modeling. In this work, architecturally designed LaPrNiO4+δ, referred to as LPNO, oxygen electrode has been prepared on Ce0.9Gd0.1O2-δ electrolyte by taking advantage of the complementary properties of La2NiO4+δ and Pr2NiO4+δ extremes, i.e. larger chemical stability of La2NiO4+δ and larger electrochemical activity of Pr2NiO4+δ. The performances of SOCs are not only due to the intrinsic properties of materials but also to the association of functional structured materials and the properties of their interfaces. In this frame, novel architectures based on an active functional layer (AFL) fabricated by electrostatic spray deposition (ESD) and topped by an optimized current collecting layer (CCL) deposited by screen-printing (SP) have been optimized aiming to decrease the operating temperature in order to improve the performances and the lifetime. A unique coral-like nanostructured AFL with large porosity and good adhesion on the electrolyte has been proved to be of great importance in the oxygen electrode reactions. The electrode performance and durability in symmetrical SOCs and in a complete electrolysis cell have been investigated in detail in thermal aging and in galvanostatic mode. These data have been thoroughly discussed in relationships with structural and microstructural evolution of LPNO oxygen electrode thanks to post-mortem characterizations using scanning electron microscopy, 3D reconstruction by focused ion beam-SEM, laboratory X-ray diffraction and synchrotron X-ray µ-diffraction and µ-fluorescence.; Dans le cadre du déploiement des énergies renouvelables par nature intermittentes, le couplage des technologies de piles à combustibles et d'électrolyseurs offre une solution innovante pour absorber les pics de production ou de consommation électrique. Dans ce cadre, la technologie basée sur des "cellules à oxydes solides" (SOCs) apparaît comme une solution attractive permettant de remplir les deux fonctions avec le même objet. Ces convertisseurs électrochimiques spécifiques sont constitués d'un électrolyte céramique oxyde fonctionnant à haute température (800 – 1000 °C) permettant d’atteindre des rendements très importants. Néanmoins, la durabilité des SOCs reste à ce jour insuffisante. Dans ce cadre, un des verrous technologiques important est la déstabilisation chimique et mécanique de l'électrode à oxygène ainsi que sa réactivité chimique avec l’électrolyte en fonctionnement. De plus, la température de fonctionnement élevée des SOFC entraîne des contraintes sévères sur l'assemblage des matériaux et sur les processus de fabrication.Les objectifs de cette thèse sont de mieux comprendre les mécanismes réactionels et le rôle de la microstructure des électrodes à oxygène dans les cellules à oxydes solides fonctionnant à température intermédiaire (650 - 750 °C) et d'étudier le vieillissement à travers une approche couplant caractérisations électrochimiques, physico-chimiques et de modélisation. Dans ce travail, l’électrode à oxygène architecturée LaPrNiO4+δ, nommée LPNO, a été déposée sur l'électrolyte Ce0.9Gd0.1O2-δ en vue de tirer parti des propriétés complémentaires des phases extrêmes, à savoir La2NiO4+δ de plus grande stabilité chimique et Pr2NiO4+δ, de plus grande activité électrochimique. Par ailleurs, les performances des SOCs dépendent non seulement des propriétés intrinsèques des matériaux mais aussi de l'association des matériaux fonctionnels et des propriétés de leurs interfaces. Par conséquent, de nouvelles architectures d’électrodes, à base d’une couche fonctionnelle active (AFL) fabriquée par atomisation électrostatique (ESD) et surmontée d'une couche de collecteur de courant optimisée (CCL) déposée par sérigraphie (SP), ont été optimisées pour diminuer la température de fonctionnement afin d'améliorer les performances et la durée de vie des cellules. Une couche fonctionnelle active nanostructurée avec une microstructure unique de type corail présentant une grande porosité et une bonne adhérence sur l'électrolyte s'est avérée d'une grande importance dans les réactions des électrodes à oxygène. Les performances et la durabilité des électrodes dans les SOC symétriques et dans une cellule d'électrolyse complète ont été étudiées en détail en vieillissement thermique et en mode galvanostatique. Ces données ont été discutées en relation avec l'évolution structurale et microstructurale de l'électrode à oxygène LPNO grâce à des caractérisations post-mortem utilisant la microscopie électronique à balayage, la reconstruction 3D par faisceau d'ions focalisés MEB, la diffraction des rayons X en laboratoire et la -diffraction et µ-fluorescence par rayonnement X synchrotron.

Coupling fuel cell and electrolysis technologies provide an attractive solution to absorb the fluctuations induced by the deployement of the intermittent renewable energy sources. In this frame, the technology based on the "Solid Oxide Cells" (SOCs) appears as a promising solution as the same device can be alternatively used in both the electrolysis and fuel cell modes. The SOCs are based on an oxide ceramic electrolyte operating at a high temperature (800-1000 °C) allowing to reach very good efficiency. Hovewer, their durability is still insufficient to envisage their economic deployment. An important issue limiting the SOCs lifetime is the chemical and mechanical destabilizations of the oxygen electrode, as well as its chemical reactivity with the electrolyte upon operation. Moreover, the high operating temperature of SOCs leads to severe constraints on materials assembling and on fabrication processes.The objectives of this PhD thesis are to better understand the role of microstructure and the reaction mechanisms of oxygen electrode in SOCs operating in intermediate temperature (650-800 °C) and to investigate the aging in electrolysis and fuel cell modes through an integrated approach coupling electrochemical and physicochemical characterizations with modeling. In this work, architecturally designed LaPrNiO4+δ, referred to as LPNO, oxygen electrode has been prepared on Ce0.9Gd0.1O2-δ electrolyte by taking advantage of the complementary properties of La2NiO4+δ and Pr2NiO4+δ extremes, i.e. larger chemical stability of La2NiO4+δ and larger electrochemical activity of Pr2NiO4+δ. The performances of SOCs are not only due to the intrinsic properties of materials but also to the association of functional structured materials and the properties of their interfaces. In this frame, novel architectures based on an active functional layer (AFL) fabricated by electrostatic spray deposition (ESD) and topped by an optimized current collecting layer (CCL) deposited by screen-printing (SP) have been optimized aiming to decrease the operating temperature in order to improve the performances and the lifetime. A unique coral-like nanostructured AFL with large porosity and good adhesion on the electrolyte has been proved to be of great importance in the oxygen electrode reactions. The electrode performance and durability in symmetrical SOCs and in a complete electrolysis cell have been investigated in detail in thermal aging and in galvanostatic mode. These data have been thoroughly discussed in relationships with structural and microstructural evolution of LPNO oxygen electrode thanks to post-mortem characterizations using scanning electron microscopy, 3D reconstruction by focused ion beam-SEM, laboratory X-ray diffraction and synchrotron X-ray µ-diffraction and µ-fluorescence., Dans le cadre du déploiement des énergies renouvelables par nature intermittentes, le couplage des technologies de piles à combustibles et d'électrolyseurs offre une solution innovante pour absorber les pics de production ou de consommation électrique. Dans ce cadre, la technologie basée sur des "cellules à oxydes solides" (SOCs) apparaît comme une solution attractive permettant de remplir les deux fonctions avec le même objet. Ces convertisseurs électrochimiques spécifiques sont constitués d'un électrolyte céramique oxyde fonctionnant à haute température (800 – 1000 °C) permettant d’atteindre des rendements très importants. Néanmoins, la durabilité des SOCs reste à ce jour insuffisante. Dans ce cadre, un des verrous technologiques important est la déstabilisation chimique et mécanique de l'électrode à oxygène ainsi que sa réactivité chimique avec l’électrolyte en fonctionnement. De plus, la température de fonctionnement élevée des SOFC entraîne des contraintes sévères sur l'assemblage des matériaux et sur les processus de fabrication.Les objectifs de cette thèse sont de mieux comprendre les mécanismes réactionels et le rôle de la microstructure des électrodes à oxygène dans les cellules à oxydes solides fonctionnant à température intermédiaire (650 - 750 °C) et d'étudier le vieillissement à travers une approche couplant caractérisations électrochimiques, physico-chimiques et de modélisation. Dans ce travail, l’électrode à oxygène architecturée LaPrNiO4+δ, nommée LPNO, a été déposée sur l'électrolyte Ce0.9Gd0.1O2-δ en vue de tirer parti des propriétés complémentaires des phases extrêmes, à savoir La2NiO4+δ de plus grande stabilité chimique et Pr2NiO4+δ, de plus grande activité électrochimique. Par ailleurs, les performances des SOCs dépendent non seulement des propriétés intrinsèques des matériaux mais aussi de l'association des matériaux fonctionnels et des propriétés de leurs interfaces. Par conséquent, de nouvelles architectures d’électrodes, à base d’une couche fonctionnelle active (AFL) fabriquée par atomisation électrostatique (ESD) et surmontée d'une couche de collecteur de courant optimisée (CCL) déposée par sérigraphie (SP), ont été optimisées pour diminuer la température de fonctionnement afin d'améliorer les performances et la durée de vie des cellules. Une couche fonctionnelle active nanostructurée avec une microstructure unique de type corail présentant une grande porosité et une bonne adhérence sur l'électrolyte s'est avérée d'une grande importance dans les réactions des électrodes à oxygène. Les performances et la durabilité des électrodes dans les SOC symétriques et dans une cellule d'électrolyse complète ont été étudiées en détail en vieillissement thermique et en mode galvanostatique. Ces données ont été discutées en relation avec l'évolution structurale et microstructurale de l'électrode à oxygène LPNO grâce à des caractérisations post-mortem utilisant la microscopie électronique à balayage, la reconstruction 3D par faisceau d'ions focalisés MEB, la diffraction des rayons X en laboratoire et la -diffraction et µ-fluorescence par rayonnement X synchrotron.

This work aims at better understanding the impact of Solid Oxide Cells (SOC) microstructure on their performance, with an illustration on an LSCF-CGO electrode. A coupled 3D stochastic geometrical and electrochemical modeling approach has been adopted. In this frame, a plurigaussian random field model and an in-house sphere packing algorithm have been adapted to simulate the microstructure of SOCs. The geometrical models have been validated on different electrodes reconstructed by synchrotron X-ray nano-holotomography or focused ion-beam tomography. Afterwards, semi-analytical microstructural correlations have been proposed and validated on a large dataset of representative synthetic microstructures. These relationships allow establishing the link between the electrode ‘basic’ parameters (composition, porosity and grain size), to the ‘key’ electrochemical parameters (Triple Phase Boundary length density and Specific surface areas), and are particularly useful for cell manufacturers who can easily control the first set of parameters. Concerning the electrochemical part, a reference symmetrical cell made of LSCF-CGO has been tested in a three-electrode setup. This enabled the validation of an oxygen electrode model that links the electrode morphological parameters to its polarization resistance, taking into account the thermodynamic data. Finally, the coupling of the validated models has enabled the investigation of the impact of electrode composition, porosity and grain size on the cell electrochemical performance, and thus providing useful insights to cell manufacturers., Ce travail porte sur la compréhension de l’impact de la microstructure sur les performances des Cellules à Oxyde Solide (SOC), avec une illustration sur l’électrode à oxygène en LSCF-CGO. Une approche couplant de la modélisation géométrique et électrochimique a été adoptée pour cet effet. Le modèle des champs aléatoires plurigaussiens et un autre basé sur des empilements de sphères ont été développés et adaptés pour les microstructures des SOCs. Ces modèles 3D de géométrie stochastique ont été ensuite validés sur différentes électrodes reconstruites par nano-holotomographie aux rayons X au synchrotron ou par tomographie avec un microscope électronique à balayage couplé à une sonde ionique focalisée. Ensuite, des corrélations semi-analytiques ont été proposées et validées sur une large base de microstructures synthétiques. Ces relations permettent de relier les paramètres ‘primaires’ de l’électrode (la composition, la porosité et les diamètres des phases) aux paramètres qui pilotent les réactions électrochimiques (la densité de points triples, les surfaces spécifiques interphases) et sont particulièrement pertinents pour les équipes de mise-en-forme des électrodes qui ont plus de contrôle sur ce premier ensemble de paramètres. Concernant la partie portant sur l’électrochimie, des tests sur une cellule symétrique en LSCF-CGO ont permis de valider un modèle déjà développé au sein du laboratoire, et qui permet de simuler la réponse électrochimique d’une électrode à oxygène à partir des données thermodynamiques et de microstructure. Finalement, le couplage des deux modèles validés a permis d’étudier l’impact de la composition des électrodes, leur porosité ou encore taille des grains sur leurs performances. Ces résultats pourront guider les équipes de mise-en-forme des électrodes vers des électrodes plus optimisées.

This work aims at a better understanding of the high temperature Solid Oxide Cells degradation. An approach based on electrochemical tests, advanced post-test characterizations and multi-scale models has been used to investigate the links between the performances, the electrodes microstructure and their degradation. In that goal, long-term durability tests have been performed over thousand hours in different operating conditions. Electrode microstructures have been reconstructed by X-ray nano-holotomography for the pristine and the aged cells. It is worth noting that a special attention has been paid to improve both the process reliability for the tomographic experiments as well as the spatial resolution of the 3D reconstructed images. Thanks to the valuable 3D volumes, the Ni-YSZ microstructural properties of the H2 electrode have been quantified for the fresh and the aged samples. Then, a physically-based model for Nickel particle agglomeration has been adjusted on the microstructural parameters obtained by the 3D analysis and implemented in an in-house multi-scale modelling framework. Beforehand, it has been necessary to enrich the available numerical tool with a specific module dedicated to the oxygen electrode made in Mixed Ionic Electronic Conducting materials. Once validated on polarisation curves, the completed model has been used to quantify the contribution of Nickel agglomeration on the experimental degradation rates recorded in fuel cell and electrolysis modes., Ce travail porte sur l’étude de la dégradation des convertisseurs électrochimiques haute température à oxydes solides. Une approche couplant des tests électrochimiques, des caractérisations post-mortem avancées et une modélisation multi-échelle a été mise en place afin d’établir les liens entre les performances, la microstructure des électrodes et leur dégradation. Dans ce but, des essais de durabilité de plus de mille heures ont été menés dans différentes conditions opératoires. La microstructure des électrodes a été reconstruite par nano-holotomographie des rayons X pour la cellule de référence avant et après vieillissement. Une attention particulière a été apportée à la mesure de la résolution spatiale et à la fiabilisation du protocole expérimental. Grâce aux volumes 3D, les propriétés microstructurales de l’électrode H2 en Ni-YSZ ont été quantifiées pour les cellules à l’état initial et vieillies. Un modèle physique d’agglomération des particules de Nickel a ensuite été ajusté sur les analyses tridimensionnelles et intégré dans une structure de modélisation multi-échelle développée au laboratoire. Il a auparavant été nécessaire de compléter l’outil numérique avec un module spécifique dédié aux matériaux composant l’électrode à oxygène fait avec un conducteur mixte ionique-électronique. Une fois le modèle validé sur des courbes de polarisation expérimentales, il a été utilisé pour quantifier la contribution de l’agglomération du Nickel sur les pertes de performances mesurées expérimentalement en mode pile à combustible et électrolyse.

Increasing the electrocatalytic activity of the oxygen electrode is a possible wayto improve SOFCs performance. It can be achieved either by searching for new materials, orby working on the shaping technique and microstructure of electrodes prepared with wellknownmaterials.This thesis developed the latest approach by applying the infiltration technique to thenickelates materials La2NiO4+δ and Pr2NiO4+δ, known for their mixed electronic and ionicconduction properties. The benefits of the infiltration method over more usual techniquessuch as screen printing was first demonstrated on La2NiO4+δ electrodes. The thoroughanalysis of electrochemical measurements highlighted the extensive role of both thegas/electrode (specific area) and the electrode/electrolyte interfaces on the efficiency of theelectrode.The study of Pr2NiO4+δ-based electrodes, and especially of the secondary phases arisingfrom its decomposition, allowed discovering the remarkable electrocatalytic properties of thesimple oxide Pr6O11, leading to very low polarization resistance values (Rp = 0.028 Ω·cm2 at600 °C). The integration of such electrodes in SOFC single cells led to power densities up to825 mW·cm-2 at only 600 °C, with a remarkable stability measured during 800 h at 600 °Cand 0.5 A·cm-2., L’amélioration des performances des piles à combustible à oxyde solide(SOFC) passe en partie par l’augmentation de l’activité électrocatalytique de l’électrode àoxygène. Cet objectif peut être atteint soit en recherchant des matériaux plus performants,soit en travaillant sur la mise en forme et la microstructure d’électrodes de matériaux connus.Cette thèse développe cette dernière approche en considérant surtout la technique originalede mise en forme par infiltration appliquée aux matériaux La2NiO4+δ et Pr2NiO4+δ, connuspour leurs propriétés de conduction mixte électronique et ionique. L’optimisation desparamètres a été effectuée sur des électrodes à base de La2NiO4+δ, et a conduit à lapréparation d’électrodes performantes. L’analyse approfondie des résultats de mesuresélectrochimiques a permis de démontrer l’importance primordiale des interfacesgaz/électrode (surface spécifique) et électrode/électrolyte sur l’efficacité de l’électrode.L’étude d’électrodes à base de Pr2NiO4+δ et des phases secondaires issues de sadécomposition ont mis en lumière les propriétés électrocatalytiques remarquables de l’oxydesimple Pr6O11, conduisant à des résistances de polarisation très faibles (Rp = 0,028 Ω·cm2 à600 °C). L’intégration de ce type d’électrode dans une cellule SOFC complète a permis demesurer une densité de puissance élevée de 825 mW·cm-2 à seulement 600 °C etremarquablement stable après 800 h de fonctionnement à 600 °C et 0,5 A·cm-2.

The thermal, electrochemical and mechanical response of a high temperature steam electrolyzer (HTSE) has been analyzed in this work. To this end, a multi-physics and multi-scale modelling approach has been employed: • A local model, at the microstructure scale of the electrodes, has been used to analyze the apparent electrochemical behavior of the electrodes related to the studied electrolysis cell. System operation, in a stack of several cells, has been then analyzed using a thermoelectrochemical model at the macroscopic scale of the HTSE. An element of experimental validation of the model comes with the results. • A thermomechanical model for the calculation of the stress state of the HTSE has been developed. In this model, the intrinsic physical phenomena of the cell, of its operation under current at high temperatures and those ascribable to the mechanical interactions between the cell and its environment have been considered. The unknown data required for the models have been obtained by the characterization and homogenization calculations of the three-dimensional microstructure of the electrodes. Besides, the viscoplastic behavior of the cathode material has been determined by a four-point bending creep test. The study made it possible to define an optimal operating zone, ensuring both high electrochemical performances and acceptable temperature levels. Proposals aiming to reduce the mechanical damage of the system have been also produced., La réponse thermique, électrochimique et mécanique d'un électrolyseur de la vapeur d'eau à haute température (EVHT) a été analysée dans ce travail. Pour ce faire, une approche de modélisation multi-physique et multi-échelle a été employée : • Un modèle local, à l'échelle de la microstructure des électrodes, a été utilisé pour analyser le comportement électrochimique apparent des électrodes de la cellule d'électrolyse étudiée. Le fonctionnement du système au sein d'un empilement de plusieurs cellules a ensuite été analysé grâce à un modèle thermoélectrochimique à l'échelle macroscopique de l'EVHT. Un élément de validation expérimentale du modèle accompagne les résultats. • Un modèle thermomécanique pour le calcul de l'état de contrainte de l'EVHT a été développé. Celui-ci tient compte des phénomènes physiques intrinsèques à la cellule et à son fonctionnement sous courant à hautes températures et à ceux imputables aux interactions mécaniques entre la cellule et son environnement. Les données manquantes nécessaires à l'exécution des modèles ont été obtenues par la caractérisation et par des calculs d'homogénéisation de la microstructure tridimensionnelle des électrodes. Par ailleurs le comportement viscoplastique du matériau de la cathode a été mis évidence par des essais de fluage en flexion quatre points. L'étude a permis de définir un domaine de fonctionnement optimal garantissant des performances électrochimiques élevées avec des niveaux de température acceptables. Des propositions visant à réduire l'endommagement mécanique du système ont également été produites.

This work investigates the high temperature co-electrolysis of H2O and CO2 in Solid OxideCells. A detailed model was developed, encompassing electrochemical, chemical, thermal andmass transfer phenomena, and introducing a macroscopic representation of the co-electrolysismechanism. This model allows predicting the performances and outlet compositions in singlecell and stack environments. An experimental validation protocol was implemented on twotypes of commercial Cathode Supported Cells, ranging from polarization curves, obtained insingle and co-electrolysis modes, to micro gas analyses. These tests aimed both at determiningthe different exchange current densities, representative of the kinetics of electrochemicalreactions, and validating the simulated cell global behavior and mechanism proposed.Comprehensive analysis of the simulations led to the identification of limiting processes andpaths for optimization, as well as to the establishment of co-electrolysis operating maps., Cette étude porte sur la co-électrolyse de H2O et CO2 à 800°C dans une cellule à oxydessolides. Un modèle détaillé a été développé afin de rendre compte des phénomènesélectrochimiques, chimiques, thermiques et de transferts de matière, et introduisant unereprésentation macroscopique du mécanisme de co-électrolyse. Il permet d’estimer lesperformances et les compositions en sortie de cellule. Un protocole expérimental, visant àvalider les principales hypothèses de ce modèle, a été appliqué à deux types de cellulecommerciale à cathode support. À partir de courbes de polarisations, obtenues en électrolyseet en co-électrolyse, ainsi que d’analyses gaz, les densités de courant d’échange, illustrant lescinétiques électrochimiques, ont pu être estimées, et le mécanisme proposé a pu être validé.L’analyse des simulations a permis l’identification des processus limitant la co-électrolyse, laproposition de voies d’optimisation et l’établissement des cartographies de fonctionnement.