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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

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.

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.

Microstructural evolution of Nickel in Ni-YSZ fuel electrodes is one of the main limiting degradation mechanisms in solid oxide cells [1]. The redistribution of Nickel is ascribed to its high mobility and low wettability with YSZ, related to interfacial surface tensions. Practically, the degradation manifests as the agglomeration of Ni, potentially accompanied by its migration, mainly reported in electrolysis mode. Extensive efforts have been made to understand Ni agglomeration and relocation. In this frame, advanced imaging techniques have been employed to characterize the microstructure of electrodes after aging in a wide range of conditions. In order to support these observations and gain fundamental insights, a number of computer simulations methods such as: multistate kinetic Potts-Monte Carlo models, cellular automata, phase field, and topological boundary dynamics have been successfully employed in modelling grain growth and recrystallization phenomena. Nevertheless, questions still remain on the exact underlying mechanisms of Ni migration. In the shadow of this lack of clear understanding, a morphological modelling approach aiming at mimicking Ni depletion based on the most fundamental definition of chemical potential and surface curvature minimization would be very helpful to guide researchers into gaining clearer insights of this phenomenon. Moreover, the morphological simulations are very fast compared to the physically based models. Therefore, they can offer the possibility to emulate large database of numerical microstructures in order to study the impact of Ni evolution on the microstructural properties [2]. The current work falls within this framework and aims at modelling the two main Ni degradation phenomena, namely coarsening and migration. For this purpose, a novel model based on morphological operations from mathematical morphology [3] has been developed and adapted to the Ni-YSZ specificities. The considered driving force for Ni evolution is the minimization of its surface curvature, and implicitly its chemical potential, which strongly depends on the local operating conditions. After calibration, the model showed a capability to predict, starting from an initial 3-D microstructure as input data, Ni relocation that was in agreement with that observed in a large dataset of 3-D reconstructions from pristine and aged cells. This helps understanding the associated microstructural evolutions which are linked to the cell Area Specific Resistance (ASR) through an adapted in-house model. This coupling of microstructural and electrochemical models allows giving clearer insights and practical recommendations for the design of fuel electrodes with improved stability and electro-catalytic activity. [1] M. Hubert, J. Laurencin, P. Cloetens, B. Morel, D. Montinaro, F. Lefebvre-Joud, Journal of Power Sources, 397 (2018) 240-251. [2] H. Moussaoui, R.K. Sharma, J. Debayle, Y. Gavet, G. Delette, J. Laurencin, Journal of Power Sources, 412 (2019) 736-748. [3] J. Serra, Image Analysis and Mathematical Morphology: Theoretical Advances, Academic Press, Cornell University (1988). Figure 1