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Solid Oxide Cell (SOC) technology is considered as an efficient electrolysis technology to produce hydrogen at large scale. It can also operate in fuel cell mode using different fuels (carbon-based or non-carbon based like ammonia), and in reversible mode. Though proofs-of concept have been achieved at different relevant scales for those operating modes, some R&D works still need to be performed to improve performance, durability and cost. Improved and upscaled cells and stacks need to be developed, with a methodology combining multiscale and multiphysics modelling, electrochemical characterization in relevant conditions and advanced post-test analysis. Their integration into modules made of several stacks is also a stepping stone in order to reach multi-MW electrolysers. CEA is working on the whole value chain of SOC technology, from cell development and optimisation to module design and operation through stack upscaling. Recent achievements on those aspects will be presented.

Detailed electrochemical and 3D microstructural characterization of Ni/YSZ electrodes confirms that Ni migration also occurs during CO2 electrolysis, most likely driven by the gradient in the Ni–YSZ interfacial energy along the current direction.

International audience; A novel particle-based model is proposed to generate synthetic yet representative 3D microstructures of typical SOC electrodes. The model steps can be related to the real electrode manufacturing routes, classically via powders processing, making it a practical tool for electrodes design optimization. The representativeness of the synthetic microstructures is checked on several two-phase (LSCF, LSC) and three-phase (Ni-YSZ) electrodes reconstructed by synchrotron X-ray and FIB-SEM tomography. The validation shows a very good agreement between the real and synthetic media in terms of metric, topological and physical properties. Furthermore, the model is adapted to simulate the microstructure of a typical Ni-YSZ current collecting layer by taking into account a bimodal pore-size-distribution. In this objective, the macro-pores resulting from the burning-off of specific pore-formers are morphologically separated in the reconstruction from the micro-porosity network. Finally, the geometrical features of the macro-pores are meticulously characterized and successfully emulated by using parametric ellipsoids.

Numerical models are versatile tools to study and predict efficiently the performance of solid oxide cells (SOCs) according to their microstructure and composition. As the main contribution to the cell polarisation is due to the oxygen electrode, a large part of the proposed models has been focused on this electrode. Electrode modelling aims to improve the SOCs performance by serving as a guide for the microstructural optimisation, and helps to better understand the electrochemical reaction mechanisms. For studying the electrode microstructure, three categories of models can be distinguished: homogenised models, simplified geometry based models, and reconstructed microstructure based models. Most models are based on continuum physics, while elementary kinetic models have been developed more recently. This article presents a review of the existing SOCs models for the oxygen electrode. As a perspective, the current challenges of electrode modelling are discussed in views of a better prediction of the performance and durability, and more specifically for the case of thin-film SOCs.

In this review paper, a critical assessment of the main degradation processes in three key components of solid oxide fuel cells and electrolysers (negative and positive electrodes and the interconnect) is undertaken, attempting prioritization of respective degradation effects and recommendation of the best approaches in their experimental ascertainment and numerical modeling. Besides different approaches to quantifying the degradation rate of an operating solid oxide cell (SOC), the latest advancements in microstructural representation (3D imaging and reconstruction) of SOC electrodes are reviewed, applied to the quantification of triple-phase boundary (TPB) lengths and morphology evolution over time. The intrinsic degradation processes in the negative (fuel) electrode and the positive (oxygen) electrode are discussed, covering first the composition and governing mechanisms of the respective electrodes, followed by a comprehensive evaluation of the most important factors of degradation during operation. By this systematic identification of dominant degradation processes, measurement techniques, and modeling approaches, the foundations are laid for the definition of meaningful accelerated stress testing of SOC cells and stacks, which will help the technology achieve the constantly more demanding durability targets in market applications.

International audience; Three long-term experiments have been performed in SOEC and SOFC modes at different operating temperatures. The durability tests confirm a higher degradation in electrolysis mode with respect to fuel cell operation. In addition, a larger increase of the ohmic resistance is observed for the cell operated at higher temperature in electrolysis mode. The oxygen electrodes of the pristine and tested cells have been characterized by synchrotron X-ray micro-diffraction and micro-fluorescence to assess the relation between the material destabilization and the formation of insulating phases due to interlayer diffusion. The analyses of the pristine cell confirm the presence after the electrode sintering of strontium zirconate and a Gd-rich inter-diffusional layer in the electrolyte just below the zirconates. Moreover, evolutions in the LSCF unit cell volume reveal strontium segregation after aging. The associated material destabilization is linked to the accumulation of SrZrO 3 at the barrier layer-inter-diffusional layer interface in operation and both phenomena are found to be thermally-activated and promoted in electrolysis mode. Finally, the crystallographic evolution of the inter-diffusional layer in electrolysis mode has been investigated by X-ray diffraction. A slight increase of the phase peaks intensity detected at the highest temperature is correlated to the largest formation of SrZrO 3 observed in this condition. Based on these preliminary results, it is proposed that the loss of Zr 4+ from the electrolyte due to the zirconates formation could facilitate the inter-diffusion of Gd, reducing the local ionic conductivity and thus significantly contributing to the largest increase in the ohmic resistance observed in this case.

Solid Oxide Fuel Cells (SOFCs) are regarded as a promising technology for economic power generation due to their high efficiency and large fuel flexibility. Durability is a severe hurdle towards their deployment. The near future targets in respect to Degradation Rate (DR) are about 0.1% kh−1, which needs improved monitoring and diagnostics. This work aims at introducing a new approach based on Differential Analysis of the i-V curves, named DiVA. It operates with the Differential Resistance Rd and its evolution during long term testing. Two new performance indicators are introduced. Since derivatives are more sensitive to small deviations, the Differential Resistance Analysis (DRA) ensures increased sensitivity and information capability in respect to degradation monitoring and diagnostics, which is demonstrated on a small stack during thermal cycling conditions – before and after the first thermal cycle, on button cells tested up to 9000 h, as well as on button cells operating in fuel cell and in electrolysis mode. The results show that DRA is several times more sensitive in comparison with the classical DR evaluation based on registration of the voltage decrease at constant current.

International audience; An original 3D stochastic model, based on the truncated plurigaussian random fields, has been adapted to simulate the complex microstructure of SOC electrodes. The representativeness of the virtual microstructures has been checked on several synchrotron X-ray and FIB-SEM tomographic reconstructions obtained on typical LSCF, LSC and Ni-YSZ electrodes. The validation step has been carried out by comparing numbers of electrode morphological properties as well as the phase effective diffusivities. This analysis has shown that the synthetic media mimic accurately the complex microstructure of typical SOC electrodes. The model capability to simulate different types of promising electrode architectures has also been investigated. It has been shown that the model is able to generate virtual electrode prepared by infiltration resulting in a uniform and continuous thin layer covering a scaffold. With a local thresholding depending on the position, continuous graded electrodes can be also produced. Finally, the model offers the possibility to introduce different correlation lengths for each phase in order to control the local topology of the interfaces. All these cases illustrate the model flexibility to generate various SOC microstructures. This validated and flexible model can be used for further numerical microstructural optimizations to improve the SOC performances.

Solid Oxide Fuel Cells (SOFCs) are a promising technology that can provide efficient and clean energy production. The general barriers hindering their market entry are durability, i.e. resistance to aging, and costs. In parallel to the deeper insight into the different degradation sources and improved understanding of ageing factors and their interactions, work towards higher accuracy for the assessment and monitoring of real-world fuel cell ageing in necessary. The requirements for operational stability formulate the parameter “degradation rate” (DR). Most often long term durability tests are performed at constant current load and the decrease of the voltage is used for its definition. In this work a new approach based on analysis of the volt-ampere characteristics, named Differential Resistance Analysis (DRA), is presented. It operates with the differential resistance, i.e. with the derivative of the voltage in respect to the current (dU/dI = Rd) which is more sensitive to small deviations and thus increases the sensitivity of the analysis. Two performance indicators are derived (Rd, min and ∆U*) with differing selectivity: ∆U* is more sensitive to activation losses and Rd, min - to transport hindrances. The application of the DRA is demonstrated on examples from measurements in fuel cell and in reverse (fuel cell/electrolyzer) mode, as well as on modeling data. The results show that the method is at least 10 times more sensitive to DR evaluation in comparison with the classical approach.

A set of long-term tests (t ≥ 1000 h) have been carried out in fuel cell and electrolysis modes on typical Ni-YSZ//YSZ//LSCF-CGO cells. The degradation rates were found to be higher in electrolysis than in fuel cell operation. Post-test analyses have revealed that Sr diffusion and formation of SrZrO3 at YSZ/CGO interface occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode. As a consequence, LSCF destabilization is found to be not involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. An in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF demixing mechanism. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in LSCF material (while the fuel cell condition results in an increase in the concentration of oxygen vacancies). It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation is detailed.

Ceramic high-temperature fuel cells and electrolysers are efficient energy conversion systems for electrical power generation and hydrogen production. Their core components are constituted by a stack of electroactive Solid Oxide Cells (SOCs) in which the electrochemical reactions take place. Thanks to their flexibility, the same stack can be alternatively operated in both fuel cell and electrolysis modes. However, the insufficient durability of SOCs still constitutes a major limitation for the technology. The present study addresses this issue and aims to bring some new insights on the effect of the Solid Oxide Fuel Cell (SOFC) versus Solid Oxide Electrolysis Cell (SOEC) operating modes on degradation of a typical Ni-YSZ//YSZ//CGO//LSCF-CGO cell. The electrochemical degradations are generally attributed to several underlying phenomena such as electrode microstructural evolution, material chemical decomposition or electroactive sites poisoning by contaminants. Among them, it is generally considered that Ni agglomeration in the Ni-YSZ cermet and Lanthanum Strontium Cobalt Ferrite (LSCF) material destabilization are two prevalent mechanisms involved in the cell performance deterioration. Therefore, these two mechanisms have been specifically investigated by a coupled approach of long-term testing in both SOFC and SOEC modes (1000 ≤ t (h) ≤9000) and post-test characterizations. The experimental results have then been analyzed in the frame of an in-house multi-scale model with the purpose to interpret them and to quantify the effect of material ageing on cell performances. The extent of Ni agglomeration has been characterized by three-dimensional electrode reconstructions obtained by X-ray nano-holotomography at European Synchrotron Radiation Facility (ESRF) (on the new Nano-Imaging beamline ID16A-NI). The new set-up and protocol enable the reconstructions of valuable 3D volumes (Fig. 1) with a large field of view (~50 µm) along with a high spatial resolution (~50 nm). The electrode morphological properties, which have been measured on the 3D volumes, have revealed a substantial Ni coarsening over time at 850°C and 750°C, whereas no Ni depletion was detected at the electrolyte interface. The increase of the Ni particle size is found to induce a decrease in both (i) the density of Triple Phase Boundary (TPBs) lines and (ii) the interfacial surface area between Ni and gas. Moreover, it was found that the Ni/YSZ interfacial surface area does not evolve during the experiments. This statement indicates that the ceramic backbone in the cermet prevents a massive Ni agglomeration at the SOFC/SOEC operating temperature. The compilation of all experimental data have allowed fitting the parameters of a physically-based law for Ni coarsening that was introduced in the modelling framework. The simulations have revealed that Ni agglomeration explains around 20-25% of the electrochemical degradation at 850°C after 1000 hrs of operation. However, the electrode microstructural evolution is found not to be affected by the cell polarizations. Therefore, the mechanism cannot explain the higher degradation rates recorded in electrolysis mode compared to the fuel cell ones. To explain the impact of the operating modes (SOFC or SOEC) on the degradation rates, several post-test analyses (i.e. Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray µfluorescence and µdiffraction techniques) have been employed to investigate the phase reactivity in the region of the CGO barrier layer. The characterizations have revealed that Sr diffusion across the barrier layer and formation of SrZrO3 secondary phase occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode (Fig. 2). As a consequence, LSCF destabilization is found not to be involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. The post-test analyses have also revealed a diffusion and an accumulation of Co in the region of the barrier layer which is concomitant with the formation of SrZrO3. The formation of these Co-rich segregates in contact with SrZrO3 grains have been identified as cobalt-ferrite type compound. The in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF destabilization mechanism. The cell polarization curves and the local quantities within the O2 electrode have been computed in both fuel cell and electrolysis modes. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in the LSCF material. It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation has been detailed. Figure 1

The durability and efficiency of Solid Oxide Fuel Cells or Electrolysis Cells (SOFC/SOEC) depend on the reaction mechanisms taking place within the electrodes. These mechanisms combine a series of elementary steps including mass and charge transfers along with chemical and electrochemical reactions. These processes are still not precisely understood as they involve complex physical phenomena occurring at the atomic scale. Therefore, it is still required to carry out some fundamental studies to better unravel the basic mechanisms involved in the material properties such as the electronic or ionic conductivity, etc. For such basic investigations, the ab-initio modeling is nowadays recognized as a powerful and relevant method. In this context, this work aims to provide deeper insights into the mechanisms at atomic scale by using Density Functional Theory (DFT) calculations. La1-xSrxCoyFe1-yO3-δ (LSCF) and LSCF-based materials are Mixed Ionic-Electronic Conductors, which are commonly used as oxygen electrode for the SOFCs/SOECs [1]. LSCF is a perovskite oxide that contains two transition metals, i.e. cobalt and iron. Owing to the complexity of this system, our work was first devoted to the parent material La1-xSrxFeO3 (LSF), which was here studied by DFT+U calculations. A special focus was paid to finely describe the localization of holes and the formation of oxygen vacancies as a function of the oxygen partial pressure. Electronic structure calculations were carried out at different strontium contents x (x=0.0, x~0.1, x~0.4, x~0.6). Progressive changes during Sr substitution were evaluated in the Density Of State (DOS). The probability densities of the gap-state wave functions were visualized in order to describe the spatial extension of the localized electron holes. The oxygen vacancy concentration, which is related to the oxygen diffusion coefficient, was calculated and coupled to a defect model in order to give at any temperature and partial oxygen pressure the defect concentration. This work will be extended to the typical LSCF material to provide some essential insights into the chemistry of this imperfect solid. [1] S. P. Jiang, International Journal of Hydrogen Energy, 2019, 44, 7448 – 7493

Due to the mixed ionic electronic conductive properties of the Lanthanum Strontium Cobalt Ferrite (LSCF) La0.6Sr0.4Co0.2Fe0.8O3−δ compound, it is of great scientific and technological interest. Especially in the Solid Oxide Fuel Cell (SOFC) technology, this compound has receiving great attention as a cathode material. However, its chemical reactivity with the Yttria-stabilized Zirconia (YSZ) electrolyte still remains one of the main challenges, which demands a comprehension in the μm and sub-μm range. In order to address the reactivity issues locally in the micrometre scale range, 2D and 3D X-ray μ-diffraction and μ-fluorescence analysis have been performed on a pristine LSCF cathode layer. The cathode was deposited on a dense YSZ electrolyte substrate spaced by a thin Gadolinium doped Ceria Oxide (CGO) barrier layer in between LSCF and YSZ to limit the reactivity. The present approach offers a larger field of view in comparison to electron microscopy techniques. The method can provide a more representative information and may offer some insights on the reactivity distribution along the interfaces. The formation of micro SrZrO3 inclusions in LSCF layer is then indubitably identified, as well as in the CGO/YSZ interface.

In this paper the potential of time-of-flight secondary ion mass spectroscopy combined with focused ion beam technology to characterize the composition of a solid oxide fuel cell (SOFC) in three-dimension is demonstrated. The very high sensitivity of this method allows even very small amounts of elements/compounds to be detected and localized. Therefore, interlayer diffusion of elements between porous electrodes and presence of pollutants can be analyzed with a spatial resolution of the order of 100 nm. However, proper element recognition and mass interference still remain important issues. Here, we present a complete elemental analysis of the SOFC as well as techniques that help to validate the reliability of obtained results. A discussion on origins of probable artifacts is provided.

This work aims at investigating through a modeling approach the difference in thermal and electrochemical responses between high temperature H 2 O electrolysis and H 2 O+CO 2 co-electrolysis in solid oxide cells. The study has been conducted by considering a typical planar stack configuration with cathode supported cells. The influence of the local temperature on the polarization curve is discussed. Operating maps are simulated for both electrolysis modes depending on cell voltage and inlet gas flow rate, covering a complete range of gas conversion rates. The optimum domains of operating conditions combining high performances and reasonable temperature elevations are identified. In overall, higher performances are found in steam electrolysis. Indeed, the co-electrolysis process is found to be strongly limited by mass transport through the thick cathode. However, co-electrolysis exhibits an easier thermal management. Finally, the composition of the syngas produced by co-electrolysis is found to be highly flexible through adjustments of the operating parameters.

This paper shows how X-ray computed nanotomography (CNT) can be correlated with focused ion beam time-of-flight secondary ion mass spectrometry (FIB-TOF-SIMS) tomography on the same sample to investigate both the morphological and elemental structure. This methodology is applicable to relatively large specimens with dimensions of several tens of microns whilst maintaining a high spatial resolution of the order of 100 nm. However, combining X-ray CNT and FIB-TOF-SIMS tomography requires innovative sample preparation protocols to allow both experiments to be conducted on exactly the same sample without chemically or structurally modifying the sample between measurements. Moreover, dedicated algorithms have been developed for effective data fusion that is biased with nine degrees of freedom. This methodology has been tested using a porous and heterogeneous solid oxide fuel cell (SOFC) that has features varying in size by three orders of magnitude - from hundreds of nanometre large pores and grains to tens of micron wide functional layers.

The longer lifetimes of solid oxide cells (SOC) – obtained and still required, both in fuel cell and electrolysis operation – exacerbate the need for accelerated stress tests, since neither reliable degradation models are currently available nor validated methodologies to evaluate expected stack durability in a practical timeframe. The project AD ASTRA ( “HArnessing Degradation mechanisms to prescribe Accelerated Stress Tests for the Realization of SOC lifetime prediction Algorithms”) started operations to tackle this on January 1st 2019, funded by the European Fuel Cells and Hydrogen Joint Undertaking. Generally, a great number of operational parameters influence SOC degradation during lifetime, each contributing according to characteristic times and intensities, often in convoluted or even contrasting fashion. Aggravation of test input parameters can thus lead to non-representative accelerated degradation of a component where concurring effects are differently influenced, leading to a biased end state of the component investigate. To systematically address, harness and accelerate the selected failure modes, a matrix was set up of degradation mechanisms, test items, test procedures and characterization methods, that was translated to protocols in such a way that consistent measurement data are generated, helping subsequent analysis and correlation of phenomena and parameters by the project’s modelling and simulation activities. The testing approach consists of a dual-focus campaign targeting: • macroscopic stack testing procedures, where short stacks will be operated in situ, in aggravated conditions with the objective of identifying test settings that will stress the stack in a representative way but minimizing testing time and resource expenditure • specific component ageing tests, to reproduce (ex-situ) the degraded condition of critical stack components or interfaces in a faster time, based on analysis of the components extracted from stacks that have been field-tested. A radical innovation of the project consists in the integration of these two methods: components specifically aged ex situ assembled into otherwise “new” stacks for in-situ testing, so that their effect on stack performance can be easily isolated from other components that in real-world conditions would also degrade. These tests allow to observe from the start (t=0) the end-of-service stack degradation, radically reducing in-situ testing time and providing powerful means to validate models simulating the long-term effects of component degradation in a stack.

To manage the grid with renewable electrical sources and ensure their storage, hydrogen chains have to be considered. However, hydrogen must be pressurized before being used or stored. The first levels of compression are less energetic for liquids than for the hydrogen gas, therefore the electrolysis of steam directly under pressure has to be studied. On one hand, with the solid oxide technology, the Open Circuit Voltage (OCV) presents the drawback of increasing with pressure. On the other hand, the pressure might improve the reactive species supply to the electrochemical sites and reduce the related losses. Thus, what are the electrolysis performances under pressure? And is there an energetic gain at the system level to produce pressurized hydrogen? In this context, experimental and modelling approaches have been adopted in order to understand the basic underlying mechanisms of pressurized electrolysis operation. In this frame, SOEC performance tests were realized under pressure in the range of 1 to 10 bars on two types of single commercial cells. They were based on conventional materials and differentiated by their microstructure. Concerning the set-up, the test platform requirements were achieved by developing an innovative mechanical design associated to a specific steam management. Moreover, a previously proposed electrochemical model of solid oxide electrolysis cell was adapted to take into account the pressure effects. The model allows computing all the most important operation parameters such as local current densities, local molar fraction, overpotentials and also the cell microstructural parameters [1]. According to the complete I-V curves from 0 to -1.3 A/cm2 at 800°C with high steam conversion, it has been observed that OCVs respect the Nernst relation under pressure while the limit current appears at higher current values. This work shows that the hydrogen production can be increased thanks to a 10 bars operation at the autothermal voltage. Moreover, these trends have been analyzed with the model. The impact of pressure on the electrochemical performances is much more important in the range of 1 to 5 bars than above. In our experimental conditions, the diffusion improvement in the porous cathode alone can explain this behavior. Finally, the impact of each microstructural parameter on the cell performances with pressure is discussed. The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement 621173. [1] J. Laurencin, D. Kane, G. Delette, J. Deseure, F. Lefebvre-Joud, Modelling of solid oxide steam electrolyser : impact of the operating conditions on hydrogen production, 2011, Journal of Power Sources, 196, 2080-2093. Figure 1

This work highlights an experimental and modeling approach devoted to a better understanding of H2O and CO2 co-electrolysis mechanisms at 800 °C. A standard Cathode Supported Cell (CSC) was used in this study. Through numerical adjustments on experimental polarization curves, the cathode microstructural parameters and exchange current densities for H2O and CO2 reductions were determined and subsequently implemented in an in-house co-electrolysis model. Additionally, micro gas chromatography (μGC) analyses were performed in co-electrolysis operating mode for different cell polarizations (from i = 0 to i = −1.75 A cm−2). μGC analyses at Open Circuit Voltage (OCV) were used to validate the kinetic constants of the Water Gas Shift (WGS) reaction implemented in the model. Predictions of both co-electrolysis polarization curves and outlet gas compositions were then compared to the experimental measurements. The good agreement between simulated and experimental data proves the relevance of the macroscopic representation of electrochemical processes through a “surface ratio” that takes into account the H2O and CO2 electrolyzes competition. A sensitivity analysis was performed to ensure a better understanding of co-electrolysis mechanisms and further investigate the influence of the reverse WGS reaction over CO production.

International audience; Long-term experiments have been carried out to investigate the impact of Nickel (Ni) coarsening on the performance of Solid Oxide Cell. Durability tests have been performed with H2 electrode supported cells at 850 °C and 750 °C in fuel cell and electrolysis modes. Microstructural changes in the composite electrode of Nickel and Yttria Stabilized Zirconia (YSZ) have been characterized by synchrotron X-ray nanotomography. Analysis of the reconstructions have revealed that Ni coarsening induces a significant decrease of both the density of Triple Phase Boundary lengths (TPBls) and the Ni/gas specific surface area. However, the contact surface between Ni and YSZ is not changed upon operation, meaning the Ni sintering is inhibited by the YSZ backbone. Moreover, the Ni coarsening rate is independent of the electrode polarization. The evolution of TPBls in operation has been fitted by a phenomenological law implemented in an electrochemical model. Simulations have shown that microstructural changes in the H2 electrode explain 30% of the total degradation in fuel cell mode and 25% in electrolysis mode at 850 °C after 1000-2000 h. Moreover, it has been highlighted that the temperature at which the degradation is estimated after the durability experiment plays a major role on the result.

International audience; The relevance of micro Laue diffraction tomography (mu-LT) to investigate heterogeneous polycrystalline materials has been studied. For this purpose, a multiphase solid oxide fuel cell (SOFC) electrode composite made of yttria-stabilized zirconia and nickel oxide phases, with grains of about a few micrometres in size, has been analyzed. In order to calibrate the Laue data and to test the technique's sensitivity limits, a monocrystalline germanium sample of about 8 x 4 mu m in cross-section size has also been studied through mu-LT. The SOFC and germanium Laue diffraction pattern analyses are compared and discussed. The indexing procedure has been successfully applied for the analysis of the germanium Laue data, and the depth-resolved two-dimensional cartographies of the full deviatoric strain tensor components were obtained. The development and application of an original geometrical approach to analyze the SOFC Laue data allowed the authors to resolve grains with sizes of about 3 mu m and to identify their individual Laue patterns; by indexing those Laue patterns, the crystalline phases and orientations of most of the grains identified through the geometrical approach could be resolved

Session 1:Image Analysis - SPIE : Society of Photo-Optical Instrumentation Engineers; International audience; A strong correlation exists between the performance of Solid Oxide Cells (SOCs), working either in fuel cell or electrolysis mode, and their electrodes microstructure. However, the basic relationships between the three-dimensional characteristics of the microstructure and the electrode properties are not still precisely understood. Thus, several studies have been recently proposed in an attempt to improve the knowledge of such relations, which are essential before optimizing the microstructure, and hence, designing more efficient SOC electrodes. In that frame, an original model has been adapted to generate virtual 3D microstructures of typical SOCs electrodes. Both the oxygen electrode, which is made of porous LSCF, and the hydrogen electrodes, made of porous Ni-YSZ, have been studied. In this work, the synthetic microstructures are generated by the so-called 3D Gaussian ‘Random Field model’. The morphological representativeness of the virtual porous media have been validated on real 3D electrode microstructures of a commercial cell, obtained by X-ray nano-tomography at the European Synchrotron Radiation Facility (ESRF). This validation step includes the comparison of the morphological parameters like the phase covariance function and granulometry as well as the physical parameters like the ‘apparent tortuosity’. Finally, this validated tool will be used, in forthcoming studies, to identify the optimal microstructure of SOCs.

Microstructural evolution of a Solid Oxide Electrolyser Cell (SOEC) Ni–YSZ cermet cathode is investigated using three dimensional electrode characterisations. 3D reconstructions are obtained on a reference and two long-term tested cells, which were maintained at −0.5 and −0.8 A cm−2 for 1000 h at 800 °C. During the long term tests, air was fed at the anode and a mixture of 10% H2–90% H2O was fed at the cathode. In this framework, reconstructions have been obtained from synchrotron X-ray nano-tomography technique. Microstructural properties extracted from the 3D reconstructions exhibit an evolution during the tests. Triple Phase Boundary length is decreasing from 10.49 ± 1.18 μm−2 for the reference cell to 6.18 ± 0.6 μm−2 for the long term tested cell at −0.8 A cm−2. Evolutions of morphological parameters were introduced in an in-house multi-scale model to evaluate their impacts on the electrode degradation, and hence, on the global SOEC performance.

This work details an in-house 2D electrochemical and thermal model of solid oxide electrolysis cell (SOEC) dedicated to the simultaneous electrolysis of steam and carbon dioxide. The apparent exchange current densities for both H2O and CO2 electrochemical reductions have been fitted on single electrolysis polarization curves obtained on a standard cathode supported cell (CSC). By using these values, predictive simulations have been performed in a co-electrolysis mode. The results have been compared to experimental data obtained in the same conditions. A good agreement is found between the simulated points and the experimental polarization curves, meaning that the model stands to predict accurately the SOEC response in co-electrolysis mode. Simulations have been performed to investigate co-electrolysis mechanisms in terms of partial pressures, current densities, and overpotentials distributions along the cell. The relative influence of both water gas shift (WGS) reaction and CO2 electrolysis on the global CO production is also discussed.

SOFC electrochemical performances are strongly dependent on the micro and nanostructure of the electrodes. 3D image analysis is an essential tool to characterise these microstructures. However, the studied volumes have to be sufficiently large to be representative of the heterogenous medium. In this study, quantitative phase contrast X-ray nano-holotomography is performed on large volumes of SOFC anode consisting by a Ni/YSZ cermet. Both the electrode functional layer and the current collector of anode supported cell are characterised in 3D. The setup and data processing adaptation needed for such an absorbing materials are described. The X-ray nano-holotomography was performed with an isotropic voxel size of 25 × 25 × 25 nm3 and a field of view of about 50 μm. This technique is non-destructive and was used in local tomography mode. After filtering and thresholding, the segmentation of the 3D reconstructions allows precisely separating the network of each phase of the electrode (metallic, ceramic and porosity), giving representative features of the microstructure.

Microstructural evolution of a Solid Oxide Electrolyser Cell (SOEC) La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) anode is investigated using 3D reconstruction techniques both on a reference and a long-term tested cell. In this framework, reconstructions have been obtained from focused ion beam-scanning electron microscopy and synchrotron X-ray nano-tomography techniques. Microstructural properties extracted from the 3D reconstructions exhibit a slight evolution during the test. Synchrotron X-ray Absorption Spectroscopy (XAS) was also performed, and a modification of the cobalt oxidation state is found. LSCF conductivity decrease due to the change in Co mixed valence has been discussed. Evolutions of both morphological parameters and electrical conductivity were introduced in an in-house micro model to evaluate their impacts on the electrode degradation.

Thermo-mechanical properties of SOFC/SOEC electrodes have been computed by homogenization from three-dimensional reconstructions of their microstructure. The support and the functional layer of a Ni/YSZ supported cell and the oxygen electrode made of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF) have been studied. Values of the effective Young's modulus obtained for the thick porous Ni/YSZ support are in good agreement with the experimental data ( E = 35 GPa). Besides, for thin functional layers, the methodology supplies original values that are difficult to obtain by conventional experimental methods ( E = 105 GPa for the Ni/YSZ Functional Layer and E = 55 GPa for the LSCF electrode). Furthermore, it has been shown that the effective elastic parameters are influenced by the following morphological parameters: the porosity and the formation factor that could also be related to the manufacturing process. Theses morphological parameters have however negligible effect on the effective thermal expansion.

An in-house micro model has been built to describe the electrochemical mechanisms governing both H2 and O2 electrodes operating in SOEC mode. A special attention has been paid to take into account the microstructure properties of the ionic, electronic and gas phases as well as the processes occurring therein. A commercial LSM–YSZ symmetrical cell has been tested at 700, 750 and 850 °C in air. Simulations have been carried out to interpret the experimental data. It is suggested that the kinetic of O2 formation is controlled by a single charge transfer. A sensitivity analysis has been performed using the micro model to quantify the role of the microstructure in the electrode behaviour. Transport of oxygen ions in the functional layer has a strong impact on the cell response since it governs the delocalization of the electrochemical reactions. The density of TPB length is also a key parameter controlling the electrode efficiency. Evolutions of the microstructural parameters in operation have been associated to the degradation of the electrochemical performances. The decrease in TPB length due to Ni agglomeration has a moderate impact whereas the decrease in ionic conductivities of 8YSZ or LSFC could explain a large amount of the cell degradation.

Tomography is a standard and invaluable technique that covers a large range of length scales. It gives access to the inner morphology of specimens and to the three-dimensional (3D) distribution of physical quantities such as elemental composition, crystalline phases, oxidation state, or strain. These data are necessary to determine the effective properties of investigated heterogeneous media. However, each tomographic technique relies on severe sampling conditions and physical principles that require the sample to be adequately shaped. For that purpose, a wide range of sample preparation techniques is used, including mechanical machining, polishing, sawing, ion milling, or chemical techniques. Here, we focus on the basics of tomography that justify such advanced sample preparation, before reviewing and illustrating the main techniques. Performances and limits are highlighted, and we identify the best preparation technique for a particular tomographic scale and application. The targeted tomography techniques include hard X-ray micro- and nanotomography, electron nanotomography, and atom probe tomography. The article mainly focuses on hard condensed matter, including porous materials, alloys, and microelectronics applications, but also includes, to a lesser extent, biological considerations.

International audience; Experiments have been performed in pressurized co-electrolysis mode at 800 °C on a typical Ni-YSZ//YSZ//CGO-LSCF cell. The polarization curves and the composition of the produced syngas have been measured at 1 bar and 10 bar. It has been found that the cell performances are improved under pressure at 1.3 V. The gas analyses have revealed that the methane formation is only activated under polarization and pressure. These experimental results have been used to validate a model which encompasses a chemical and electrochemical description of the co-electrolyser combined with a mass transport module. It has been found that the model is able to predict accurately the polarization curves as well as the syngas compositions at the cell outlet. Once validated, the model has been used to analyze the operating mechanisms in pressurized co-electrolysis. The impact of pressure on the mass transfer, the electrochemical and chemical reactions has been discussed. The close interaction between the electrochemical and chemical reactions for the internal production of CH4 has been specifically highlighted. Finally, operating maps have been computed at 10 bar from 700 °C to 800 °C. These simulations have shown that formation of CH4 in the co-electrolyser remains limited at 700 °C.

The ‘redox’ tolerance of a typical anode supported cell was evaluated for three temperatures of re-oxidation. For this purpose, an experimental work has been coupled to a modelling approach to estimate the risk of electrolyte failure during re-oxidation. A special attention has been paid to take into account both (i) the heterogeneity of oxidation and (ii) the cermet visco-plasticity in operation. Data required for the simulations – i.e. the oxidation kinetics rates, the cermet expansions and Young’s modulus – were determined at T = 600, 700 and 800 °C. It has been found that the activation energy related to the kinetics of re-oxidation encounters a modification at high temperature (700–750 °C). This modification has been ascribed to a transition from a homogeneous oxidation process to a heterogeneous one. Local X-ray measurements have confirmed that an oxidation gradient in the cermet arises at T = 800 °C. Mechanical analysis has shown that the presence of an oxidised front at T = 800 °C strongly impacts the cell ‘redox’ tolerance. Indeed, this phenomenon induces a significant cell bending, which adds a compressive stress component to the thin electrolyte. Simulations have been carried out to determine both critical degree of oxidation and durations before electrolyte cracking. The effect of the cermet creep during operation on the cell ‘redox’ tolerance is also discussed.

A methodology is proposed in this study to investigate the creep properties of porous Ni-8YSZ cermet. Creep experiments have been conducted under reducing atmosphere at the typical SOFC operating temperatures. Specimens have been loaded in a four-point bending test bench. A special attention has been paid in this work to the analytical and numerical modelling of the mechanical test. It has been highlighted that Ni-8YSZ exhibits substantial creep strain rates even at relative low temperatures (700 °C High-temperature plastic strains of both Ni and 8YSZ phases have been estimated through the local stress acting on the cermet particles. This analysis indicates that creep behaviour of the Ni-8YSZ composite is not influenced by the metallic phase, but is controlled by the deformation of the 8YSZ matrix. It is also proposed that cermet creep mechanism involves Zr4+cations diffusion at the surface rather than in the bulk of the 8YSZ material. Impact of the Ni-8YSZ cermet creep on the internal stresses distribution in SOFC is discussed considering the anode supported cell (ASC) design. It is shown that cermet creep strain can induce a substantial stress decrease in the thin electrolyte.

Nickel with yttria stabilised zirconia (Ni-YSZ) is usually used as anode material for the solid oxide fuel cells (SOFCs) technology. Upon cyclic reduction and oxidation of Ni-YSZ ceramic–metal (cermet), the induced cell degradation constitutes one of the main limitations to the SOFCs lifetime. In this study, the effect of anode reduction and oxidation cycling on typical electrolyte supported cell (ESC) has been investigated. The electrochemical degradation has been followed by impedance spectroscopy. The cell damage has been characterised after testing with scanning electron microscopy (SEM). It has been shown that cells can withstand several ‘redox’ cycles without major decrease in electrochemical performances. The ‘redox’ cycling induces only a slight degradation rate estimated to ∼70 mΩ cm2 per cycle. The microstructural observations are found to be consistent with the impedance diagram analysis. Both reveal that the damage is restricted to bulk cermet microstructure change, whereas no cracking is initiated at the anode/electrolyte interface.

An in-house-model has been developed to study the thermal and electrochemical behaviour of a planar SOFC fed directly with methane and incorporated in a boiler. The usual Ni-YSZ cermet has been considered for the anode material. It has been found that methane reforming into hydrogen occurs only at the cell inlet in a limited depth within the anode. A sensitivity analysis has allowed establishing that anode thicknesses higher than ∼400–500 μm are required to achieve both the optimal methane conversion and electrochemical performances. The direct internal reforming (DIR) mechanisms and the impact of operating conditions on temperature gradients and SOFC electrical efficiencies have been investigated considering the anode supported cell configuration. It has been shown that the temperature gradient is minimised in the autothermal mode of cell operation. Thermal equilibrium in the stack has been found to be strongly dependent on radiative heat losses with the stack envelope. Electrochemical performance and cell temperature maps have been established as a function of methane flow rates and cell voltages.

Introduction The integration of renewable energies to the global energy market implies electricity storage. However, the available technologies for electricity storage have a low energy capacity and a short time-span. In this context, H2 or carbon molecules produced via water and/or water+CO2 electrolysis open up excellent perspectives for storing renewable electricity, especially the High Temperature Electrolysis (HTE). This technology, based on Solid Oxide Cell (SOC), consumes less electrical energy than low temperature electrolysis because of the energy demand is significantly lowered by the vaporization of water1. The required electrical power is also reduced at high temperature since the steam electrolysis is increasingly endothermic with temperature. Moreover HTE can also directly convert a mix of water and CO2 into syngas, (CO+H2) (co-electrolysis), which can be converted into methane or other hydrocarbon molecules. Moreover, this type of co-electrolyzer can operate under pressure which increases the energy efficiency of the system, reducing the compression costs of the downstream chemical processing unit and increasing cells performance.2 In addition, as nickel present in SOC is known to catalyze CO and CO2 (COx) methanation and water gas shift reactions3, a pressure increase in the electrolyser leads to higher rates of these reactions. HTE cells can also be used in fuel cell mode to generate electricity from methane and water4. In this case, nickel catalyzes steam reforming leading to syngas which can be used to produce electricity. However, the contribution of these catalytic reactions are not well-known in co-electrolysis and fuel cell modes. In this view, we propose to evaluate their impact in a SOC fed with carbonated gases by establishing a kinetic model necessary for a complete electrochemical SOC modelling. This work was carried out on two different SOCs to establish kinetic models for co-electrolysis and steam reforming at atmospheric pressure and then under pressure. Catalytic tests The cells studied come from two different suppliers (A and B). In order to study the catalytic reactions, induced by nickel in the hydrogen electrode, other layers have been removed by sandblasting. COX methanation catalytic tests were performed at 400°C with the cells reduced into powder with different inlet mixtures (CO and/or CO2 with H2 with COx/H2 molar ratio around 0.3). Tests were carried out with a total flow rate of 35NmL.min-1 (equivalent to 346NmL.min-1.cm-2 cell and 245NmL.min-1.cm-2 cell for cells A and B, respectively). These flowrates are far from those commonly used for electrochemical tests, but they allow to distinguish the catalytic activity of the two cells. The results showed that cells are not equivalent to each other regarding COx conversion whatever the conditions used (CO and/or CO2) (Figure 1). It also can be noticed that the CO2 hydrogenation activity is reduced after exposure to a CO/H2mixture (Figure. 2) due to carbon deposition blocking the active sites. Secondly, catalytic tests were achieved to demonstrate the influence of water at the inlet (up to 15mol.% of water). The temperature was varied from 400 to 800°C after initial stabilization at 400°C. These tests confirmed the negative effect of water due to a less favorable thermodynamic. Finally, methane steam reforming catalytic tests were carried out to simulate fuel cell feeding conditions (H2O/CH4 molar ratio around 2) with a total flow rate of 5.3NmL.min-1 (equivalent to 52NmL.min-1/cm-2 cell and 37NmL.min-1.cm-2 cell for cells A and B, respectively). These conditions make it possible to approach the operating conditions of an electrolyser. The temperature was varied from 600 to 800°C after initial stabilization at 600°C. It appears that the A cell is close to thermodynamic equilibrium up to 700°C followed by a decrease at higher temperature whereas the B cell does not exhibit this decrease (Figure 5). Conclusions and outlooks The cells exhibit similar catalytic activities with respect to COX methanation and methane reforming. Simulations carried out using the COMSOL software allowed to select three kinetic models for atmospheric SOC operation5 , 6. To discriminate these models and propose a quantitative kinetic model for the SOCs operating under pressure, catalytic and electrochemical tests will be carried out up to 15 bars. Simulations will then be performed (COSMOL) to simulate the functioning of a cell by coupling catalysis, electrochemistry, gaseous and thermal flows. 1. Ni M. et al. Int. J. Hydrogen Energy 33,2337–2354 (2008). 2. Bernadet L. et al. Int. J. Hydrogen Energy 40,12918–12928 (2015). 3. Gao J. et al. RSC Adv. 5,22759–22776 (2015). 4. Er-rbib H. et al. Energy 75,81–88 (2014). 5. Wang Y. et al. Int. J. Hydrogen Energy 34,3885–3893 (2009). 6. Peters R. et al. J. Power Sources 106, 238–244 (2002). Figure 1

Mixed Ionic Electronic Conductors (MIEC) are increasingly used as oxygen electrode of Solid Oxide Cells (SOCs). The main advantage of these materials lies in their electrochemical reactivity which is not restricted to the Triple Phase Boundary lines (TPBls) but can extend to the whole MIEC surface. This results in a significant decrease in activation overpotentials. Several studies have been dedicated to cathodic polarization (fuel cell mode) to understand the MIEC reactive mechanisms. In anodic polarization (electrolysis mode), much fewer studies are available and the electrochemical process occurring remains unclear. Moreover, the role of the electrode microstructure in the reactive mechanisms has to be clarified. Thanks to a coupled modelling and experimental approach, the present study intends to analyze the predominant reaction mechanisms in both polarizations and to investigate the impact of electrode microstructure on the cell performance. The electrode material is of La0.6Sr0.4Co0.2Fe0.8O3-δ compound. In order to obtain relevant morphological properties, a 3D reconstruction was performed by X-ray nano-holotomography at the European Synchrotron Radiation Facility (ESRF) on the Nano-Imaging beamline ID16A. This new beamline has been designed to investigate large volumes of materials with a high spatial resolution. The obtained 3D reconstruction of a sample prepared with a plasma Focused Ion Beam (PFIB) presents a large field of view of 50 µm and a voxel size of 25 nm (Fig. 1). These characteristics allow being representative of the porous medium and thus computing accurately all the electrode morphological properties such as the specific surface area, TPBls, tortuosity factors, etc… These morphological properties are introduced as input data in a specific electrochemical model developed in both anodic and cathodic polarizations. The model takes into account the oxygen reactivity by surface reactions at the gas/MIEC interface as well as the direct oxidation of oxygen adsorbates at the TPBls. This model has been validated on the basis of experimental results performed on symmetrical cells (with a three electrode set-up) (Fig. 2). Simulations have allowed determining the rate-limiting steps in each polarization. As expected in fuel cell mode, the reactive pathway is found to be controlled by the oxygen adsorption and incorporation followed by solid state diffusion. In electrolysis mode, it appears that the mechanism is governed by the direct oxidation at the TPBls followed by the adsorbates diffusion on the surface of MIEC particles. In addition, the specific role of microstructure on the electrode performance has been investigated in both polarizations. The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207 and 621173. Figure 1

Creep properties of a commercial Ni-8YSZ cermet have been investigated at the SOFC operating temperatures. Experiments have shown that Ni-8YSZ cermet exhibits substantial creep strain rates. The creep activation energy and the parameters of the Norton's law have been determined from the experimental data. Impact of such behavior on cell 'redox' tolerance has been discussed for both planar SOFC configurations. Considering unused Anode Supported Cells (ASC), oxidation tests have shown that the electrolyte cracking arises as soon as ~65% of Ni is transformed into NiO. A mechanical analysis, which takes into account the cermet creep, suggests that this threshold could be lowered after a long term operation. In the case of the Electrolyte Supported Cell (ECS), the electrochemical degradation has been recorder upon several full 'redox' cycles. This degradation is found to be well correlated with the viscoplastic deformation of the cermet.