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.

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

Zacharie Wuillemin

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.

EPFL2009

Solid oxide fuel cell stack simulation and optimization, including experimental validation and transient behavior

Diego Larrain

This thesis presents the development of models for the simulation and optimization of the design of a planar solid oxide fuel cell (SOFC) stack. Fuel cells produce electric power directly from a fuel by electrochemical reactions. The high efficiencies demonstrated make them a promising technology for energy conversion. The main challenges lie with reliability and cost reduction. Some applications demand at the same time strong requirements on the compactness of the system and its ability to load following. The models have been developed to represent the novel stack proposed by HTceramix SA (Yverdon, Switzerland) which is tested and partly developed at the Laboratoire d'Energétique Industrielle (LENI). The model has been created in a way which allows its use for design optimization: this requires detailed and validated outputs to gain insight in the behavior of a new stack design and computational efficiency to allow sensitivity studies and optimization. Electrochemistry, mass and heat transfer phenomena are combined with a 2D fluid motion description to obtain a generalized model which can be applied to a large range of geometries. An efficient stack modeling approach is proposed. Validation of the model has been carried out with measurements and a 3D computational fluid dynamics model. A methodology based on parameter estimation has been used to identify kinetic parameters and other uncertain parameters. Local temperature measurements and a local current density measurement have been performed and also used for model validation. The 2D model has been successfully validated showing good agreement with both the experiments and the detailed 3D model. Simulation of the novel stack geometry (counter-flow) has allowed to identify the main problems arising from this compact geometry where the non-homogeneous velocity field creates stagnant zones which limit the operation at high efficiency. The simulated temperatures are characterized by important gradients and excessive level values (>850°C) for an intermediate temperature SOFC (700-800°C). This motivated to work on an alternative geometry, which based on simulation results, solves most of the problems previously identified. The thesis presents several examples of the influence of design on the system performance and reliability. Transient simulations have been performed and the design choice had only a small impact on the transient behavior which presents intrinsically an important thermal inertia. On the contrary, degradation behavior is dependent on the design. Stack degradation has been simulated by including the metal interconnect degradation into the stack model. The approach has allowed to identify a new criterion to express degradation consistently for different test conditions. To assist stack design, new approaches are necessary. The geometry of a stack was initially determined by a number of decision variables (such as cell area, thickness of the channels and interconnects) on which extensive sensitivity analysis were conducted. This method is of limited use as each of the objectives on stack design led to different solutions. To overcome this limitation, multi-objective optimization has been applied to the stack design problem. Application of this method is new in this field and different optimization strategies are tested. The results from the optimization allow to identify a clear trade-off between the compactness of the stack and the temperature level (and therefore the degradation).

EPFL2005

Segmented cell testing for cathode parameter investigation

Christos Comninellis, Josef Andreas Schuler, Pietro Tanasini, Jan Van Herle, Zacharie Wuillemin

The increasing quality and durability of Solid Oxide Fuel Cells (SOFC) state-of-the-art materials renders the long-term testing of fuel cells difficult since considerably long equipment times are needed to obtain valuable results. Moreover, reproducibility issues are common due to the high sensitivity of the performance and degradation on the testing conditions. An original segmented cell configuration has been adopted in order to carry out four tests in parallel, thus decreasing the total experimental time and ensuring the same operating conditions for the four segments. The investigation has been performed on both anode-supported cells and symmetrical Lanthanum-Strontium Manganite – Yttria-Stabilized Zirconia (LSM-YSZ) electrolyte-supported cells. In separate tests, the influence of variables like cathode thickness, current density and cathode composition on performance and degradation have been explored on anode supported cells. Furthermore, the effect of chromium poisoning has been studied on electrolyte-supported symmetric cells by contacting one segment with a chromium-iron interconnect material. Long-term polarization of the segments is controlled with a multi-channel galvanostatic device designed in-house. Electrochemical characterization has been performed through Electrochemical Impedance Spectroscopy (EIS) at different H2 partial pressures, temperatures and bias current, effectively demonstrating the direct impact of each studied variable on the cell performance and degradation behaviour. Segmented cell testing has been proven to be an effective strategy to achieve high reproducibility for SOFC measurements since it avoids the inevitable fluctuations found in a series of successively run tests. Moreover, simultaneous testing increased n-fold the data output per experiment, implying a considerable economy of time.

European Fuel Cell Forum 20102010