TEM investigation on zirconate formation and chromium poisoning in LSM/YSZ cathode

Cell durability is a crucial technological issue for SOFC commercialization, and considerable progress has been made in recent years. A number of degradation pathways have been established, amongst which microstructural changes, poisoning effects and formation of less conductive phases. In this study, transmission electron microscopy was used to observe submicron-scale effects on selected cathode zones of an anode supported cell tested in SOFC stack repeat element configuration. The test has been performed with a dedicated segmented test bench, at 800 A degrees C for 1900 h, which allowed to spatially resolve degradation processes, and therefore to improve their correlation with localized post-test analysis. Evidence is presented of reaction products (mainly SrZrO(3)) at the LSM/YSZ interfaces as well as of contaminants, in particular Cr, but also Si. A polarized cell segment is compared to an unpolarized one, to assess any influence of cathode polarization

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

Towards the Next-Generation of Solid Oxide Fuel Cell Systems

To improve the industry benchmark of solid oxide fuel cell systems (SOFC), we consider anode off-gas recirculation using a blower as an add-on to our next-generation SOFC system. Evolutionary algorithms compare the different design alternatives, i.e. co-flow or counter-flow stack operation with hot or cold recirculation. The system performance is evaluated through multiobjective optimization criteria, i.e. maximization of electrical efficiency and cogeneration efficiency. The results obtained suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable

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

Experimental and modeling investigations on local performance and local degradation in solid oxide fuel cells

This thesis focuses on performance, reliability and degradation in solid oxide fuel cells (SOFC), which currently represent the three key technical challenges for the deployment of this technology. By its development of dedicated modeling and experimental tools, this thesis offers a new access to, and comprehension of, local electrochemical performance, degradation and reliability. The experimental tools developed have, for the first time, allowed in-situ measurements of local performance degradation with time, and this in a real SOFC prototype. Giving access to local electrochemistry on 18 measurement points in a repeat-element of 200 cm2 active area, the spatial distribution of the electrochemical reaction, as well as its degradation with time, could be monitored and analyzed. The intrinsically local character of degradation could clearly be revealed, a point of central importance for future investigations on stack degradation and for the interpretation of post-experiment analyses. Using impedance spectroscopy, it was possible to identify the affected electrochemical processes and to study the spatial distribution of their degradation. The result was put in relation with post-experiment analyses, allowing to identify pollutants on the air side as major source of degradation. To understand the highly coupled phenomena leading to performance, degradation and reliability issues, a 3D computational fluid dynamic model (CFD) was developed. Based on the key idea to include the non-ideal properties of the used components and materials in the model, it was possible to obtain an excellent match between experimental observations and modeling outputs. Besides the identification of performance limitations, one result of crucial importance was in addition obtained by the identification of the principal cause of failure for the prototypes tested in the laboratory. The model revealed the presence of detrimental local redox-cycling of the cells upon changes of the operating point, as a result from an inadequate combination of slightly porous seal materials and certain aspects of the stack construction. This analysis, validated by experimental observations, led to solutions permitting an important gain in efficiency and reliability. Based on the identified and analyzed performance limitation, degradation and failure sources obtained from the tools developed in this thesis, two stack prototypes were successively designed, manufactured and tested in collaboration with the industrial partner HTceramix-SOFCpower. Starting from a predecessor design limited at 250 Wel and an electrical efficiency (LHV) inferior to 40%, the first of the designed prototypes attained a power output of 1.1 kWel (72 cells of 50 cm2), as well as a maximal efficiency of 53% in short stack configuration. The second designed stack, which represents the major achievement of this thesis, had, at the time of writing, reached a power output of 1.84 kWel and a maximal efficiency of 53% in a 20-cell stack configuration (200 cm2 cells). Both results were obtained using dilute hydrogen as fuel; in other words, future operation on reformed natural gas should lead to an electrical efficiency exceeding 60% (LHV). With the successful resolution of the main failure source and a demonstrated gain in efficiency, the chosen design iterations confirmed the predicting capabilities and the accuracy of the CFD model for a design towards the mandatory reliability and the high performance expected from SOFC stacks. Finally, the degradation issue, which was found to be strongly correlated with pollution from different sources, was addressed in a prospective study showing the capability of the CFD model to predict the internal generation, transport and deposition of pollutants inside of a stack. The good match obtained with experimental observations supports therefore the development of such types of models, both for model-based diagnostics and for future design iterations

Impact of Random Geometric Distortions on the Performance and Reliability of an SOFC

It is now common knowledge that flow uniformity within repeat-elements - as well as among them in stack configuration - is of major importance for the performance and reliability of solid oxide fuel cells (SOFCs). At first, a design optimization of the configuration of the gas diffusion layer (GDL) may appear sufficient to ensure that the fuel is converted uniformly. However, in practice, the precision of assembly and manufacturing tolerances are known to have a large impact on the quality of the fuel distribution. Hence, the purpose of this study is to evaluate the impact of random geometric distortions on the performance and reliability of an SOFC. The methodology is based on Monte Carlo simulations (MCS) by using CFD tools. The idea is to compute quality indicators for a set of randomly deformed GDLs. From that point, several sensitivity analyses are performed to forecast the GDL quality and highlight the best GDL geometries: comparison of different GDL configurations, sensitivity to variations of mean and standard deviations of tolerances, impact on maximum fuel utilization, and resulting production yield, etc. The scope of the current work is limited to standardised distortions and a simple model of the GDL. Both planar and vertical deformations can be applied to a reference geometry. Amplitudes of the deformations follow a normal distribution whereas the position and extent of the affected area are uniformly distributed over a specified interval. Those random geometric distortions are generated with a MATLAB routine which is used to post-process the original mesh free from distortions. The CFD simulations are then carried out with FLUENT and finally post-processed using MATLAB. The methodology and routines developed for this study can be used as a decisional tool to conduce the design optimization phase of the GDL and manifolds

Simulation of Thermal stresses in anode-supported solid oxide fuel cell stacks

Structural stability issues in planar solid oxide fuel cells (SOFC) arise from the mismatch between the coefficients of thermal expansion (CTE) of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells undergo residual stresses due to the sintering phase during the manufacturing process. Furthermore, the load applied during the assembly of the stack to ensure the electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. In operation, finally, thermal gradients cause additional stresses. The temperature profile generated by a thermo-electro-chemical model implemented in an equation oriented process modeling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the stress distribution in all components of a representative SOFC repeat element. An uncoupled approach was used, since no direct feedback from the stress calculation to the thermo-electro-chemical model exists. The thermal stresses in the components of the repeat element were simulated in both steady-state and dynamic operations. Particular conditions such as current load shutdown, and cooling to room temperature after operation, were investigated as well. The different layers of the cell, i.e. anode, electrolyte, cathode, compensating layer and compatibility layer, were considered in the analysis by using the submodelling capabilities of the finite-element tool. Assessment of the risks of failure was performed by the widely used Weibull analysis. The occurrence of plastic deformation and the dependence on temperature of both CTE and Young’s modulus of the metallic parts as well as the orthotropic nature of the compressive sealant were implemented in the finite-element model. The residual stresses were dominating the stress state in the cell, except in severe operation conditions. Thus the cell at room temperature after the reduction procedure was revealed as the most critical case. On the contrary, thermal gradients induced irreversible deformation of the metallic interconnector in the area submitted to the highest temperature

The Effects of Component Tolerances on the Thermo-Mechanical Reliability of SOFC Stacks

The reliability of solid oxide fuel cell (SOFC) systems is closing the gap to meet the requirements for market implementation. Finite-element (FE) stack models, typically used for stack reliability analyses, commonly consider idealized components. In reality, dimensions and geometry of the produced stack components have statistical variations. The resulting variability in shape is expected to have an impact on the distribution of the contact pressure over the cell active area. In this study, the initial deformation of the metallic interconnect (MIC) is implemented in the stack model. The simulation starts with the simulation of the stack production and qualification. The results of this first calculation are used to simulate either thermal cycling or prolonged continuous operation. The distribution of the simulated contact pressure on the active area is found to evolve differently during thermal cycling and operation, if the initial deformation of the MIC is included in the stack model

Electrochemical Model of Solid Oxide Fuel Cell for Simulation at the Stack Scale; Part I. Calibration Procedure on Experimental Data

Lifetime prediction and improvement of solid oxide fuel cell (SOFC) devices require a reliable electrochemical model that supports the implementation of degradation phenomena. This study comprises two parts. This Part I describes the calibration of an electrochemical model based on physical principles for simulation at the stack scale. Part II presents the further implementation of degradation models. A distinction is made between the two most common cathode materials, lanthanum strontium manganite and lanthanum strontium cobalt ferrite. The experimental data used for the parameter estimations was gathered by two segmented setups. The calibrations enabled to reproduce adequately the measurements over a wide range of operating conditions. The optimal values of the physical parameters were inside the ranges reported in literature. Unambiguous discrimination could not be achieved between variations (i) in the choice of electrode rate-determining steps, (ii) data on the properties of the materials found in literature and (iii) empirical relations for the steam-methane reforming reaction. However, these model variations do not affect significantly the predicted magnitudes and distributions of the field variables assumed to govern the degradation processes at the SRU scale, compared with the uncertainties on the degradation phenomena to be implemented in Part II. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3596433] All rights reserved

Glass-Forming Exogenous Silicon Contamination in Solid Oxide Fuel Cell Cathodes

Spatially resolved analyses, by energy-dispersive X-ray spectroscopy (EDS) scanning electron microscopy (SEM), allowed the quantification of exogenous Si contamination in a solid oxide fuel cell (SOFC) cathode after operation. The Si quantification, taking into account the endogenous Si impurity level, correlated well with the expectation from the condensation of Si(OH)4 vapor, originating from upstream alloy components and saturated in the hot inlet air. At higher resolution, EDS-transmission electron microscopy (TEM) pointed out the deposition of Si vapor in the form of amorphous SiO2, blocking oxygen incorporation into the electrolyte phase within a composite SOFC cathode