Strontium Migration at the GDC-YSZ Interface of Solid Oxide Cells in SOFC and SOEC Modes

Strontium migration from the oxygen electrode to the interface between the electrolyte and interlayer was investigated in solid oxide cells in fuel cell (SOFC) and electrolysis (SOEC) modes. Four samples were imaged by focused ion beam scanning electron microscopy (FIB-SEM) serial sectioning. After reconstruction, the spatial distribution of the strontium zirconate secondary phase was examined by measuring the volume fractions, phase size distributions, interfacial surface areas and the shape of each detected distinct inclusion. The analysis shows that the accumulation lingers during operation. The results also suggest that the inclusions tend to propagate towards the GDC/YSZ interface. The detrimental effect on the performance was assessed using 3-D finite element transport analysis, by comparing the effective conductivity computed with and without the presence of the secondary phase. The operation in SOFC mode caused a limited decrease of the conductivity, while the effects on the performance are more significant for SOEC operation

Modeling of Local Cell Degradation in Solid Oxide Fuel Cells: Cumulative Effect of Critical Operating Points

A CFD model was developed to predict with accuracy the local electrochemical performance in an operating solid oxide fuel cell. A particular focus was given on the study of performance limitations and degradation sources caused by the design of the fuel cell, by the choice of materials and components, or by improper operating points. The model is used to understand typical degradation effects observed on stacks tested in collaboration with HTCeramix - SOFCpower. The model is able to predict several degradation effects caused by the use of compressive seal materials with remaining open porosity. Diffusion across seals is modeled, as well as the resulting parasitic combustion of air and fuel. The risk of local redox-cycling for the anode supported cell is evaluated locally, as well as an eventual local reduction of the cathode layers. The cracking of electrolyte caused by redox cycles is modeled locally, with increasing gas leakage at each cycle, allowing to model the cumulative effect of successive critical operating points. The results are compared with experiments, showing an overall good agreement

Experimental real-time optimization of a solid oxide fuel cell stack via constraint adaptation

The experimental validation of a real-time optimization (RTO) strategy for the optimal operation of a solid oxide fuel cell (SOFC) stack is reported in this paper. Unlike many existing studies, the RTO approach presented here utilizes the constraint-adaptation methodology, which assumes that the optimal operating point lies on a set of active constraints and then seeks to satisfy those constraints in practice via the addition of a correction term to each constraint function. These correction terms, also referred to as “modifiers”, correspond to the difference between predicted and measured constraint values and are updated at each steady-state iteration, thereby allowing the RTO to iteratively meet the optimal operating conditions of an SOFC stack despite significant plant-model mismatch. The effects of the filter parameters used in the modifier update and of the RTO frequency on the general performance of the algorithm are also investigated

Characterization of 3D Interconnected Microstructural Network in Mixed Ionic and Electronic Conducting Ceramic Composites

The microstructure and connectivity of the ionic and electronic conductive phases in composite ceramic membranes are directly related to device performance. Transmission electron microscopy (TEM) including chemical mapping combined with X-ray nanotomography (XNT) have been used to characterize the composition and 3-D microstructure of a MIEC composite model system consisting of a Ce0.8Gd0.2O2 (GDC) oxygen ion conductive phase and a CoFe2O4 (CFO) electronic conductive phase. The microstructural data is discussed, including the composition and distribution of an emergent phase which takes the form of isolated and distinct regions. Performance implications are considered with regards to the design of new material systems which evolve under non-equilibrium operating conditions

Thermomechanical and Electrochemical Degradation in Anode-Supported Solid Oxide Fuel Cell Stacks

The solid oxide fuel cell (SOFC) is a direct energy conversion device, which allows the production of electricity with high efficiency while maintaining pollutant emissions at a low level. It offers better fuel flexibility and lower vulnerability to impurities in the oxidising and fuel gases than other fuel cells. The lack of reliability and durability of SOFC devices currently impedes their large-scale commercialisation. The mitigation of mechanical and electrochemical degradation that together limit the lifetime, requires the precise understanding of the phenomena. The underlying processes act and interact at different spatial and temporal scales, which complicates characterisation and, consequently, the identification of the dominant contributions to the observed overall degradation in field conditions. This thesis has developed a modelling framework to investigate electrochemical and mechanical reliability and durability issues in planar, intermediate-temperature SOFC stacks based on anode-supported cells. The approach consisted in coupling thermo-electrochemical and thermo-mechanical continuum models, spanning from the electrode micro-scale to the stack macro-scale, to include in the analysis the detrimental interactions between the different phenomena that provoke failure after combined prolonged operation and cycling conditions. To achieve this aim, the existing cleavages between the research fields had to be bridged. At each sub-scale and for each aspect of interest, data was first gathered to identify the needed and achievable level of complexity of the description, and to calibrate the models using parameter estimation. Then, specific studies to understand the key dependences on the local conditions and limitations of the proposed approaches were carried out. Finally, the sub-models were implemented together in SRU/stack models to capture the multi-factorial and progressive nature of immediate or delayed failures in SOFCs. The implementation of a calibrated electrochemical model with degradation phenomena in SOFC stack models shed light on the micro- and macro-scale interactions that cause the progressive activation of the electrochemical degradation phenomena. Dynamic optimisation identified the critical decision variables, in terms of operating conditions, whereas disparities in the electrochemical and mechanical properties of the materials, stack design and constraints from the system were further included, to propose case-specific mitigation procedures. The results ascertained the predominant effect of overpotential, rather than current density, on the electrochemical degradation, as it governs the chromium contamination and the formation of undesirable insulating phases in the cathode. In the most striking cases, the lifetime could be extended by a factor of up to five, by the sole adjustments of the operating conditions. Counter-flow configuration, with low methane conversion in the reformer, is more favourable than co-flow. The thermo-mechanical contact model enlarged the analysis with insights into the complex failure modes, which ultimately cause the mechanical failure of the cell directly or indirectly, through a succession of deleterious events. The model explains the difficulty to ensure the integrity of the cells, the electrical contact and gas-tightness of the compartments, while preventing thermal buckling, during load following and thermal cycling. The effects of design and history were analysed in light of thermo-electrochemical and mechanical degradation, combined with rate-independent plasticity and creep, and stacking conditions. For the first time, to our knowledge, the modelling framework developed here has encompassed both electrochemical and mechanical degradation, along with calibration procedures, for lifetime predictions and identification of complex failure modes. This capability is expected to contribute significantly to improve the durability of SOFC devices in the future, since most of the modelled issues are currently addressed by progressive empirical adjustments

Compilation of mechanical properties for the structural analysis of solid oxide fuel cell stacks. Part II: Metallic interconnect, sealants and gas diffusion layers.

Solid oxide fuel cell (SOFC) stacks are extremely vulnerable to mechanical failures in any of their constituents, which ultimately lead to cell cracking. Like all devices, distinct failure sequences arise from different events or design particularities. The understanding of the interactions between the different components is crucial to effectively mitigate their possible adverse effects. This study comprises two papers, which aim at compiling data on the mechanical properties of the materials used in intermediate-temperature, anode-supported SOFC stacks, in the view of stress analysis. Part I focuses on the ceramic cell materials. This Part II covers the metallic interconnects, the glass-ceramic sealants or compressive gaskets and gas-diffusion layers. A selection is performed among the possible solutions found in stack designs, depending on the availability of the data. The reported properties of the materials, namely thermal expansion, strength, elastic or elasto-plastic and creep behaviour, are discussed in terms of modelling frameworks

Sensitivity of Stresses and Failure Mechanisms in SOFCs to the Mechanical Properties and Geometry of the Constitutive Layers

A model based on the Euler-Bernoulli theory is used to assess the sensitivity of residual stresses in solid oxide fuel cells to the mechanical properties and geometry of the constituents. It considers different cell configurations, characterised by the presence or not of a compensating layer, and a cathode based on either lanthanum strontium manganite (LSM) or lanthanum strontium cobaltite ferrite (LSCF). The implementation of creep in the model provides insights into the parameters that affect the zero-stress temperature and behaviour during ageing. The amount of irreversible deformation generated in the cell layers after the sintering step depends on the mechanical properties of the layers, type of cell and to some extent, cooling rate. X-ray diffraction measurements from literature are used to verify the prediction. Depending on the mechanical properties, the stress state in the LSM cathode changes from tensile to compressive with respect to temperature. During combined ageing and thermal cycling, tensile stress might arise in the compatibility layer of LSCF-based cells, due to the relief of the initial compressive stress at operating temperature. The Weibull analysis provides the assessment of mechanical failure. A simplified approach is used for buckling-driven delamination, but the propagation of cracks is predicted for unlikely large preexisting defects

Current State of Models for the Prediction of Mechanical Failures in Solid Oxide Fuel Cells

Structural reliability and durability is a hurdle for the large-scale commercialisation of solid oxide fuel cell (SOFC). Because of the multiphysics nature of the problem, modelling is required to first gain knowledge and then alleviate mechanical failures. This chapter places the emphasis on stress analysis at the SOFC stack scale. A brief overview on the mechanical behaviour of the cell, sealing and gas diffusion layer materials is provided, along with a discussion on their implementation in finite-element software. Modelling approaches to handle the variety of situations that arises from stack design and application specificities are exposed. The current modelling capabilities are illustrated with results from studies available in literature, which enables to identify the most stringent model limitations that must be overcome to efficiently improve the reliability and durability of SOFC devices in future