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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.

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; 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.

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.

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.

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

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.

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.

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 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.

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.

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.

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.

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.

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

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.

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

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

International audience; The electrochemical reactions of hydrogen oxidation and steam reduction have been investigated for a porous cermet electrode made of Ni and 3YSZ. The electrochemical characterizations have been performed over a large range of gas compositions at 700°C. It has been shown that the fuel electrode response is activated by the potential under anodic current while a limiting current density appears under cathodic polarization. Moreover, the impedance diagrams exhibit a shape representative of a kind of finite-length Gerischer element with a low-frequency contribution sensitive to the steam content. To interpret these experimental results, a continuous dynamic model has been developed by describing the mass and charge transfers within the electrode. The reaction has been divided into a sequence of elementary steps considering two scenarios of charge transfer based on the oxygen and hydrogen spillover mechanisms. Three-dimensional reconstructions obtained by synchrotron X-ray nano-holotomography have been used to provide the cermet structural properties for the simulations. The numerical computations have shown that the hydrogen spillover scenario is the most appropriate mechanism to reproduce correctly the experiments. Besides, the electrode response is controlled by the charge transfer at triple phase boundary lengths, the oxygen vacancies migration in the 3YSZ network and a pure chemical surface process depending on the polarization. In fuel cell mode, desorption of water molecules from 3YSZ would co-limit the electrode response, while in electrolysis mode, the steam adsorption on Ni would become one of the rate determining steps. Finally, a sensitivity analysis has shown that surface diffusion would also play a key role in the electrode response.