A numerical approach to predict the SOFC fracture : the case of an anode supported cell.

International audienceThe purpose of this work is focused on the calculation of the stress field inside planar anode supported cells. The cell fracture was estimated through the statistical approach of Weibull. After elaboration, a high residual compressive stress was calculated in the thin electrolyte layer. A slight tensile stress was pointed out in the anode in a region close the anode/electrolyte interface. For high electrolyte thickness (>20 mu m), this tension leads to low survival probabilities of the anode. At SOFC operating temperature, the elaboration stress is partially relaxed. In this condition, the thermal cycling between the room temperature to the SOFC operating one should not induced any cell degradation. The first cermet reoxidation step was also analyzed. This study has shown that the cathode is damaged as soon as the anodic expansion reaches values between 0.05-0.09%. The electrolyte fracture is predicted to occur for anodic expansion ranging between 0.12-0.15%

Carbon deposition in CH4/CO2 operated SOFC: Simulation and experimentation studies

ENERGIE+KGRDue to their high operating temperatures, SOFCs can be directly fed with biogas, mainly composed of CH4 and CO2. In this work, experiments was performed with a classical Ni-YSZcermet//YSZ//LSM cell fed either with a synthetic simulated biogas (CH4/CO2 ratio equal to 1 with 6% humidity), or with humidified H-2. In both cases, the performances are found to be very similar, which confirms the ability of SOFCs to operate with internal reforming of biogas. Nevertheless, carbon formation in these operating conditions needs to be considered because of durability concerns. Thermodynamic calculations and modelling are carried out to evaluate the risk of carbon deposition depending on operating parameters. In the ternary diagram C-H-O, the limits for carbon deposition are plotted, allowing the determination of "safe" operating conditions in terms of CH4 inlet flow rate and cell voltage. First experiments confirm these modelling results

A Numerical Tool to estimate SOFC Mechanical Degradation: Case of the Planar Cell Configuration

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Activated Slip Systems and Localized Straining of Irradiated Zr Alloys in Circumferential Loadings

International audienceDuring the plastic deformation of irradiated Zr alloys, specific slip systems are activated. Indeed, the dislocation loops of <a> type are strong obstacles for the dislocations gliding on the classical prismatic planes. An increase in yield strength is observed. This allows the activation of other slip systems. The basal slip, having a <a> Burgers vector can interact with the irradiation-induced dislocation loops. This interaction leads to the annealing of the loops by various mechanisms and leads to the formation of dislocation free bands after limited plastic strain (channeling). TEM examinations of Zry-4 samples strained after irradiation have been performed. High density of basal slip has been observed on channels. With such a mechanism, the irradiated Zr alloys exhibit a strain-softening behavior for loading orientations allowing this kind of interaction. A modeling of the interaction between the gliding dislocations and the loops has been performed to analyze the channel formation kinetics. Fine element (FE) computation of strain-softening materials has shown strong effects on the plastic zone shape ahead of crack tips. The results are discussed, focusing on the impact of this behavior on the mechanical properties of irradiated cladding. Specific attention is given on hoop straining with respect to PCI, RIA, or axial splitting

Solid Oxide Fuel Cells damage mechanisms due to Ni-YSZ re-oxidation: Case of the Anode Supported Cell.

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

Impact of Nickel Agglomeration on Solid Oxide Cell Operated in Fuel Cell and Electrolysis Modes

International audienceLong-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