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This paper describes an optimization strategy for operating solid-oxide fuel-cell systems at optimal efficiency. Specifically, we present the experimental validation of a real-time optimization (RTO) strategy applied to a commercial solid-oxide fuel-cell system. The proposed RTO scheme effectively pushes the system to higher levels of efficiency and maintains the system there despite perturbations by tracking active constraints. The optimization approach uses either steady-state measurements, or transient measurements in combination with a dynamic model, and can deal effectively with plant-model mismatch. In the reported experiments, the approach drives the system to the desired power demand at optimal efficiency. The experimental fuel-cell system reached 65% DC electrical efficiency. As such, the proposed RTO scheme is a promising candidate for enforcing optimal micro-CHP operation. In addition, the approach can deal with slow drifts such as degradation without compromising on efficiency. Finally, and important from a practical point of view, we suggest guidelines for safe and optimal operation. (C) 2019 Elsevier Ltd. All rights reserved.

The experimental implementation of real-time optimization (RTO) to a commercial solid-oxide fuel-cell (SOFC) system is reported in this paper. The goal of RTO is to maximize the system efficiency at steady state subject to several operating constraints. The proposed RTO strategy is a constraint-adaptation approach, which consists in adding bias correction terms to the constraints in the optimization problem. These bias terms are estimated during operation using transient measurements in combination with a dynamic model. The scheme enforces plant optimality by continuously detecting and tracking the set of active constraints. This approach drives the fuel-cell system quickly to the desired power demand, while maximizing the efficiency and paying attention to constraint satisfaction. As such, this RTO scheme has the ability to both control and optimize fuel-cell systems. This experimental system reached about 65% efficiency. In addition, it was possible to deal with slow drifts such as degradation without compromising on optimality.

While the global fuel utilization of solid oxide fuel cells (SOFCs) is limited by the stack aging rate, the fuel excess is typically used in a burner, and thus limiting the system electrical efficiency. Further, natural-gas-fueled SOFCs require treated water for the steam reforming process, which increases operational cost. Here, we introduce a novel micro anode off-gas recirculation fan that is driven by a partial-admission (21%) and low-reaction (15%) steam turbine with a tip diameter of 15 mm. The 30 W turbine is propelled by pressurized steam, which is generated from the excess stack heat. The shaft runs on dynamic steam-lubricated bearings and rotates up to 175 krpm. For a global fuel utilization of 75% and a constant fuel mass flow rate, the electrical gross DC efficiency based on the lower heating value was improved from 52 % to 57 % with the anode off-gas recirculation, while the local fuel utilization decreased from 75% to 61%, which is expected to significantly increase stack lifetime. For a global fuel utilization of 85%, gross efficiencies of 66% in part load (4.5 kWe) and 61% in full load (6.3 kWe) were achieved with the anode off-gas recirculation. The results suggest that the steam-driven anode off-gas recirculation can achieve a neutral water consumption.

Thermo-mechanical issues in planar solid oxide fuel cells (SOFC) must be understood and overcome to meet the reliability standards for market implementation. This study presents a modelling framework constituted by a thermo-mechanical and electrochemical model of a SOFC stack. Rate-independent plasticity and creep of the component materials have been considered. Modified periodic boundary conditions have been implemented to simulate the behaviour of a single repeating unit (SRU) in a stack. The analysis starts with the simulation of stack conditioning procedures, which aims at capturing the stress build-up/relaxation throughout the several steps in the stack components production and assembly. The simulated stresses after the initialisation sequence provide the initial stress state for the simulation of stack operation, which consists in importing into Abaqus software the temperature profiles from thermo-electrochemical simulations. The study includes long-term steady-state operation in both co-flow and counter-flow configuration, interrupted by thermal cycling. The temperature profiles implemented in the long-term operation cases correspond to operation under internal steam-methane reforming. The simulation results show that the contact pressure is significantly lower at room temperature, after a first thermal cycle after stack qualification. The contact pressure is lost in large regions, which is expected to alter in the long-term the electrical conductivity of the interface. It also progressively decreases during long-time operation. Thermal cycling after long-term operation worsens the extent of loss of contact pressure at room temperature. Most of the active area is close to no-contact pressure. No critical stresses were found in the sealant joining the cell to the SRU. In contrast, the stresses in the manifold sealants were found to be relevantly high. Similarly to the evolution of contact pressure, long-term operation increases the risks of sealant failure during thermal cycling. Figure 1: On the top side, temperature field in co-flow configuration plotted on the deformed shape of a short stack (deformation along the z-axis magnified by 300:1), state: “Start Operation”. On the bottom side, contact pressure at the interface anode/GDL fuel along the symmetry line of the SRU. Lines 3b, 4 and 5 are referred to the case of long-term operation in co-flow configuration. Figure 1

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

This paper reports the results from the European project PROSOFC (funded by FCH-JU, FP7). Within the project an innovative methodology for a production and reliability oriented SOFC cell and stack design is developed. In particular the PROSOFC project aims at improving the robustness, manufacturability, efficiency and cost of SOLIDpowers state-of-the-art SOFC stacks so as to reach market entry requirements. The key issues are the mechanical robustness of solid oxide fuel cells (SOFCs), and the delicate interplay between cell properties, stack design, and operating conditions of the SOFC stack. The novelty of the project lies in combining state of the art methodologies for cost-optimal reliability-based design (COPRD) with actual production optimization. To achieve the COPRD beyond state of the art multi-physical modeling concepts were developed and validated for significantly improved understanding of the production and operation of SOFC stacks. The most critical failure modes for stack operation were identified in order to align the experimental investigations to generate suitable data for modelling these failures. The multi-physics model allows a probabilistic approach to consider statistical variations in production, material and operating parameters for the optimization phase. The project provides a methodology for 3D description of spatial distribution of material properties based on a random field models. The key for the whole methodology are validating experiments, homogenized models on multiple levels of the SOFC system (cell, interconnect, sealings) and introduction of extensive test programs specified by the COPRD methodology. The probabilistic models were related to the experimentally obtained properties of base materials to establish a statistical relationship between the material properties and the most relevant load effects in view of potential damage/failure based on multi-physics simulation can be established. Software algorithms for meta models that allow the detection of relationships between input parameters (materials, loading, etc.) and output parameters and to perform a sensitivity analysis were developed and implemented. The capabilities of the methodology for COPRD being developed is illustrated on two practical cases. A procedure based on meta models has been developed to estimate elastic, primary and secondary creep model parameters from 4-point bending tests. The goal is to (i) account for the effects of the friction between the sample and the rollers, anticlastic curvature, contact point shift and large deflection, which can affect the accuracy of the measurements, and (ii) measure properties which cannot be obtained using closed-form analytical solutions. This could be achieved by model-based parameter estimation with a 3-D continuum finite-element (FE) model, at the cost of very high running time. The proposed approach is computationally much more efficient. It starts with a sensitivity analysis to generate distributed meta models of the displacement at the inner rollers during loading-unloading cycles and creep measurements simulated by the 3-D FE model. The optimization problem for parameter estimation is then solved using the meta models. The accuracy of the approach was tested using a set of numerical experiments and comparison with results from 3-D imaging and computational homogenization. The experiments on Ni-YSZ samples were then analyzed. For the stochastic investigation of the thermo-electrochemical behavior of the stack a multi-physics stack model implemented in gPROMS and Fluent was linked and controlled automatically by optiSlang to perform a sensitivity analysis. The outcome is (i) the identification of the most relevant production and operating parameters which can have a significant impact on the stack lifetime and (ii) meta models of the efficiency and of the 3-D temperature distribution generated in the view of optimization. The tests show for instance that the accuracy of the 3-D meta model of the temperature distribution is sufficient to identify the minimum of variations in the spatial distribution of the temperature during cycling between full and part load by using sets of operating conditions for optimization.

To improve the industry benchmark of solid oxide fuel cell (SOFC) systems, we consider anode off-gas recirculation (AOR) using a small-scale fan. Evolutionary algorithms compare different system design alternatives with hot or cold recirculation. The system performance is evaluated through multi-objective optimization (MOO) criteria, i.e., maximization of electrical efficiency and cogeneration efficiency. The aerodynamic efficiency and rotordynamic stability of the high-speed recirculation fan is investigated in detail. The results obtained suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable, including for small power scale (10 kWe).

The reliability of solid oxide fuel cell (SOFC) systems is nowadays close to meet the requirements for market implementation. However, mechanical failures remain salient issues, which often limit the practical lifetime of SOFC stacks and directly impact the costs. To our knowledge, the capability for in-operando monitoring of the risks of their occurrence has not yet been demonstrated. Mechanical issues are typically identified indirectly once a critical failure already occurred, e.g. from cell open-voltage measurements. Therefore, numerical thermo-mechanical investigations are of high interest to understand and predict potential failure modes in SOFC stacks. Finite-element (FE) stack models commonly consider idealized components, i.e. computational domains imported from computer-aided design. In reality, the dimensions and geometry of the produced stack components have statistical variations, because of the imperfections of manufacturing processes and of the costs associated with component quality. In this study, production data shows that the interconnect is a component that is subject to such manufacturing tolerances. The resulting variability in shape, e.g. flatness and/or thickness, is expected to have an impact on the distribution of the contact pressure over the active area. Therefore, a dedicated measurement and simulation workflow has been developed to simulate the effects of component initial deformation. One repeating unit of an SOFC stack is meshed and implemented in the thermo-mechanical model here presented, to maintain the computational time at a few days on a workstation. Modified periodic boundary conditions are imposed on the upper and lower parts of the meshed domain to model the conditions at the middle of a large stack. The mechanical interaction between the components is modelled by softened contact or constrained node displacement, depending upon the interface. Further adjustments of the interface properties are performed during the analysis to account in a simplified manner for the stack conditioning steps, such as sintering of the contact pastes and crystallisation of the glass sealant. The material constitutive laws implemented in the model include (i) rate-independent elasto-plasticity and (ii) combined primary-secondary creep for the interconnect alloy and the anode cermet. Only secondary creep is considered for the materials of the other SRU parts. The elastic properties of the gas-diffusion layer (GDL) materials are estimated by computational homogenization. This allows the modelling of the GDLs as simplified continuum geometries to reduce the stack simulation runtime. The workflow for thermo-mechanical analysis developed in this study starts with the measurement of the deformation profile of several real interconnects, followed by implementation in the stack model, before the simulation of the stack assembly. Then, the stack conditioning procedure is simulated to account for the effects of the stack manufacturing on the stress-state just before operation. After this initialization step, the simulation of stack operation consists in importing temperature profiles from thermo-electrochemical simulations into the FE thermo-mechanical model. The simulated scenarios are either thermal cycling without operation or long-term operation in co-flow configuration at nominal conditions (0.4 A cm-2), interrupted by thermal cycles. For the considered design and for a given amplitude of initial interconnect deformations, the simulation results show that the distribution of the contact pressure over the active area can be already altered at the end of the stack qualification. In such cases, the GDLs cannot fully accommodate for the interconnect initial deformation, resulting in zones subjected to either higher or lower contact pressure, compared to the case of idealized initial components. This may lead to hot spots and locally pronounced irreversible deformations of the GDLs, among others, with potentially severe loss of contact pressure upon thermal-cycling or long-term operation. The stress in the sealant after thermal cycling increases after prolonged operation. In this situation, the regions of the sealants exposed to higher temperatures during long-term operation are subjected to larger stress at room temperature.

The local electrochemistry is studied in a SOFC repeat-element using a test setup that allows a local control of the polarization. During long-term tests, the local current densities evolve, following activation and degradation periods. Electrochemical impedance spectroscopy (EIS) measurements performed at regular intervals are used to analyze the activation and degradation of the individual electrochemical processes. By computing the distribution of relaxation times (DRT) from the EIS spectra, a total of 6 electrochemical processes are revealed and their evolution studied separately. It is found that not only the activation/degradation rates but also their spatial evolution are strongly influenced by the choice of materials and by the operating conditions. In particular, the degradation of the anode propagates over the active area with important local differences, and in a wave-like manner that differs from one test to the other. Results are presented and analyzed for several long-term tests operated with hydrogen or steam-reformed methane as fuel.

An optimized stack design was developed by SOFCpower-HTceramix group since 2011 and validated in the pilot production level in early 2013, to be integrated into the HoTbox™ module which includes all other hot components, a compact and thermally efficient solution, mainly but not exclusively for µCHP applications. The proprietary planar stack has been tested under severe conditions, with several H2/N2 mixtures and several type of reformate for both natural gas and biogas, allowing high fuel utilization for high electrical efficiency, up to 70% under steam reforming of natural gas at 750°C and S/C ratio of 2, whit realistic current density (j>0,3 A/cm2) and single-pass configuration. The optimized stack demonstrated also very good robustness, surviving to more than 50 full thermal cycles from operation to ambient temperature, without significant degradation.

In this study, a physically-based model for the interpretation of the impedance spectra of an anode-supported LSCF/GDC/YSZ/Ni:YSZ solid oxide fuel cell is presented. The model locally describes transport and reaction phenomena within the cell components through mass conservation equations. The microstructural properties of the electrodes are predicted through numerical three-dimensional reconstruction of the microstructure, with input parameters obtained from the analysis of SEM pictures of each layer. Simulations show that the model reproduces impedance spectra obtained in different operating conditions with the same set of fitting parameters, comprising material-specific kinetic constants and electrochemical capacitances, which fairly agree with independent literature data and a previous analysis of the spectra through DRT. The model allows for the deconvolution and quantification of the characteristic resistance and frequency of the different physical processes that build up the impedance of the cell. In particular, 7 processes are identified: charge-transfer reactions between LSCF/GDC, GDC/YSZ and Ni/YSZ interfaces appear in the high-frequency range, the medium-frequency feature is due the oxygen reduction reaction and the gas diffusion in the anode, while the low-frequency arc is mainly due to the gas conversion in the anodic channel. An additional low frequency contribution (

This work aims to compare the effect of combined chromium and sulfur contaminating conditions to the Cr contamination alone on the Cr poisoning mechanisms in (La,Sr)MnO3-(Zr,Y)O-2 solid oxide fuel cell (SOFC) cathodes. Whereas Cr2O3 and (Cr,Mn)(3)O-4 are found at active triple phase boundaries under the Cr-poisoning condition, the formation of SrCrO4 is promoted under combined Cr and S contaminating conditions, where Cr accumulations act as getters incorporating sulfur, to form Sr(Cr,S)O-4 compounds. The identification of this phase is validated on the synthesized and simulated species by scanning/transmission electron microscopy (SEM/TEM) techniques; its possible formation is predicted by thermodynamic analysis of the stability of perovskite compounds in the presence of combined Cr and S polluting conditions. In contrast, sulfur alone is not found to poison active sites in these composite cathodes. These findings suggest that the Cr poisoning degradation mechanism is altered when (La,Sr)MnO3 is exposed to Cr vapors in the presence of sulfur contamination; the access to electrochemical active sites may be hindered by the formation of Sr(Cr,S)O-4 in a similar manner to a Cr-getter effect in (La,Sr)(Co,Fe)O-3 cathodes. (C) 2011 Elsevier B.V. All rights reserved.

One of the major limitations of the nickel (Ni) - yttria-stabilized zirconia (YSZ) anode support for solid oxide fuel cells (SOFC) is its low capability to withstand transients between reducing and oxidizing atmospheres ("RedOx" cycle), owing to the Ni-to-NiO volume expansion. This work presents results on different anode supports fabricated by tape casting. Three compositions are prepared, as the outcome of a preceding design of experiment approach. The NiO proportion is 40, 50 and 60 wt% of the anode composite.

Solid Oxide Fuel Cells (SOFCs) are likely candidates for electricity production with high efficiency and promise reasonable fuel tolerance. While fuel impurities are known anode poisons, less attention is brought to air-purity, especially to air-side contaminants from exogenous sources, which are studied here. This work aims to quantify pollutants and identify their sources during SOFC testing in stack configuration. Post-analyses of a long-term test have shown that performance degradation was mainly due to cathode pollutants originated upstream of the cell, therefore their source identification is crucial. The compressed air system, feeding the airflow to the cathode, was investigated by filtering and subsequent chemical analysis of the filters. Hot-air-sampling was redone in situ at the cathode air entry during a new test run to assess the contaminant concentrations in air in SOFC test conditions. In addition, the behavior of SOFC proximal system components, i.e. alloy oxidation, was characterized separately. The study focused on chromium contamination from Cr-containing high temperature alloys used in Balance-of-Plant (BoP) components, i.e. heat-exchangers, tubing and flanges in direct contact with the airflow at high temperature. Concentrations of volatile Cr-species in air under SOFC testing conditions were compared to Cr-accumulation on the tested cell as well as to Cr-evaporation rates from BoP alloys which were individually characterized regarding oxidation behavior. Evaporated Cr quantities were found to saturate the air with Cr-vapors at the cathode air-inlet, as confirmed by the in-situ measurement of volatile species in the hot airflow. Moreover, these amounts are well correlated to accumulated Cr in the cell after long term testing. Si-containing sub-oxide scale layers of heat-resistant BoP alloys were identified as silicon contamination source besides sub-micron dust from environmental air. The analyses of laboratory air quality revealed sulfur in sufficient quantity to act as a cathode pollutant. The results of this study suggest guidelines to reduce air side contamination of SOFC stacks from exogenous sources.

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.

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.

This study aims to quantify in a holistic approach chromium (Cr) contamination in solid oxide fuel cell (SOFC) stack testing using adapted tools: i) a hot gas sampling method to analyze volatile Cr species in the air flux at the cathode inlet location; ii) a rapid quantification method for Cr as condensed matter in cathode material of post-test samples. The hot air sampling method reveals itself as a reproducible and time-resolved quantification technique for Cr trace amounts in a gas flow; this technique is seen as a promising evaluation tool for Cr contamination issues in SOFC systems. The quantification method reveals severe Cr-poisoning in a cell. The combined findings indicate that Cr contamination generated by system components located upstream the cell must be suppressed by hindering the access of Cr pollutants to the cathode compartment.

The locally-resolved degradation behavior was studied during 1900 hours in a SOFC repeat-element. In-situ measurements of local electrochemical performance were made on 18 locations over a segmented anode-supported cell. The evolution of local current densities, overpotentials and area-specific resistances was studied, showing a reorganization of the electrochemical reaction with time. The extent and the spatial distribution of degradation were established for different electrochemical reactions steps using impedance spectroscopy. The low-frequency cathode contribution was the mostly altered process, followed by the charge transfer reaction on anode side. Post-experiment analyses allowed to identify three major pollutants on the cathode side (chromium, silicon and sulfur), whose spatial distributions corresponded to the observed local degradation. Chromium was present only near the air inlet, while silica was found to affect a larger area, with decreasing amounts along the flow. Sources of pollutants were identified in system components as well as within the stack repeat-element.

Structural stability issues in planar solid oxide fuel cells arise from the mismatch between the coefficients of thermal expansion of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells accumulate residual stresses due to the sintering phase during the manufacturing process. Further, the load applied during assembly of the stack to ensure electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. Finally, thermal gradients cause additional stresses in operation. The temperature profile generated by a thermo-electrochemical model implemented in an equation-oriented process-modelling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the distribution of stress and contact pressure on all components of a standard solid oxide fuel cell repeat unit. The different layers of the cell, i.e. anode, electrolyte, cathode and compensating layer were considered in the analysis by using the sub-modelling capabilities of the finite-element tool. Both steady-state and dynamic simulations were performed, with an emphasis on the cycling of the electrical load. The study includes two different types of cells, operation under both thermal partial oxidation and internal steam-methane reforming and two different initial thicknesses of the air and fuel compressive sealing gaskets. The results generated by the models are presented in two papers: Part I, focuses on the assessment of the risks of failure of the cell, which was performed by Weibull analysis, while the issues related to the other components are discussed in Part II. Only the anode support contributed to the probability of failure, since the other layers underwent compressive stresses independently of the operating conditions. The cell at room temperature after the reduction procedure was revealed as a critical case. Thermal gradients and the shape of the temperature profile generated during transient operation induced high probabilities of failure. The computed reliability is incompatible with commercialisation, but the scatter induced by the experimental data covers several orders of magnitude. Alternatively, the computed required strength of the anode material to fulfil a probability of failure of 10−2 in a 50-cells stack during steady-state operation appears achievable. Finally, extreme care is required when using the maximum thermal gradient or temperature difference over the SRU as an indicator for cell cracking.

Structural stability issues in planar solid oxide fuel cells arise from the mismatch between the coefficients of thermal expansion of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells accumulate residual stresses due to the sintering phase during the manufacturing process. Further, the load applied during assembly of the stack to ensure electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. Finally, thermal gradients cause additional stresses in operation. The temperature profile generated by a thermo-electrochemical model implemented in an equation-oriented process modelling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the distribution of stress and contact pressure on all components of a standard solid oxide fuel cell repeat unit. The different layers of the cell in exception of the cathode, i.e. anode, electrolyte and compensating layer were considered in the analysis to account for the cell curvature. Both steady-state and dynamic simulations were performed, with an emphasis on the cycling of the electrical load. The study includes two different types of cell, operation under both thermal partial oxidation and internal steam–methane reforming and two different initial thicknesses of the air and fuel compressive sealing gaskets. The results generated by the models are presented in two papers: Part I focuses on cell cracking. In the present paper, Part II, the occurrences of loss of gas-tightness in the compressive gaskets and/or electrical contact in the gas diffusion layer were identified. In addition, the dependence on temperature of both coefficients of thermal expansion and Young's modulus of the metallic interconnect (MIC) were implemented in the finite-element model to compute the plastic deformation, while the possibilities of thermal buckling were analysed in a dedicated and separate model. The value of the minimum stable thickness of the MIC is large, even though significantly affected by the operating conditions. This phenomenon prevents any unconsidered decrease of the thickness to reduce the thermal inertia of the stack. Thermal gradients and the shape of the temperature profile during operation induce significant decreases of the contact pressure on the gaskets near the fuel manifold, at the inlet or outlet, depending on the flow configuration. On the contrary, the electrical contact was ensured independently of the operating point and history, even though plastic strain developed in the gas diffusion layer.

The ageing behaviour of glass-ceramic seal materials and of alloys is a key element for a durable performance of SOFC stacks. Post-experiment analyses performed on a stack operated for more than 4000h at 750°C and at high fuel utilization (80%) reveal significant differences in ageing behaviour of such materials when comparing fuel inlet and fuel outlet areas. Two samples were cut from the fuel inlet and outlet manifolds of an operated SOFC stack. The glass-ceramic sealing material and the alloy were exposed to a dual atmosphere (external air and internal fuel stream). The fuel composition was of dry H2/N2 (60:40 vol. %) at the inlet and of H2O/H2/N2 (48:12:40 vol. %) at the outlet. The stack was operated in co-flow configuration, with a large excess of air. The paper is focused on the sealant, on the structural steel and on the interfaces between both materials. The following figure schematically describes the samples and the zones of interest. Both samples were mounted in epoxy-resin, polished and observed by optical microscopy and SEM-EDX. It was found that the interfaces between the seal and the alloy were significantly affected by the difference in fuel composition, with more severe degradation at the fuel outlet side. Further, an increased porosity of the sealant was observed on the same location, while the inlet sample remained less affected. Finally, the sealant exposed to the outgoing air was significantly polluted by chromium generated in the stack and transported by the air stream. These investigations therefore reveal that the long-term evolution of the sealing and of the metallic support depends on the local conditions in the stack, as well as on additional interactions with other degradation phenomena. Acknowledgement: 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. Figure 1

Results on solid oxide fuel cell stacks tested at 800° C with H2 fuel and using planar Ni-zirconia anode supported cells, 80x80x0.2 mm in size, are presented. Modeling and numerical simulation is used to interpret observed results and develop improved designs. Where neccessary, the models are calibrated with additional experimental data. Emphasis is placed on the critical issue of nickel anode reoxidation, related to the fuel flow field. Consideration is also given to gradients of temperature and current density developing over the cells and predicted by the models; local current density could be validated by measurement. Flow distribution within stacks is also illustrated by both experimental and modeling results.

International audience; After 10,000 h solid oxide fuel cell (SOFC) stack operation, the Cr-poisoning situation in (La0.6Sr0.4) (Co0.2Fe0.8)O-3 (LSCF)-based cathode material is depicted in this work. Systematic Cr profiling by energy-dispersive X-ray spectroscopy (EDS), from post-operation samples taken at different locations within the air flow field, reveals Cr accumulation in electrochemically active cathode regions, although the major amount of Cr is trapped in inactive surface-proximal cathode regions; the 20 m LSCF current collector does not fully impede the Cr access to the functional cathode. The distribution of Cr within the flow field reports on the severity of the sources and causes for Cr contamination: 1) Cr preferentially accumulates at sealing-proximal locations; this is explained by fuel-leakage through the sealing with subsequent water vapor generation in the cathode compartment, aggravating local Cr-poisoning via the Cr-oxyhydroxide vapor species; 2) high Cr amounts at air inlet regions point to system components located upstream of the cell to contribute to local Cr contamination; 3) the remaining main part of the cell experiences low Cr-poisoning; the protective solution to prevent Cr evaporation from the metallic interconnects thus appears to be adequate. The detected low overall amounts of Cr contamination, partially correlated to the observed low performance degradation, encouragingly indicate 40,000 h operation, a prerequisite for stationary SOFC application, to be in reach for LSCF-based stacks. (C) 2012 Elsevier BM

On-line control and optimization can improve the efficiency of fuel cell systems, whilst simultaneously ensuring that the operation remains within a safe region. Also, fuel cells are subject to frequent variations in their power demand. This paper investigates the realtime optimization (RTO) of a solid oxide fuel cell (SOFC) stack. An optimization problem maximizing the efficiency subject to operating constraints is defined. Due to inevitable model inaccuracies, the open-loop implementation of optimal inputs evaluated off-line may be suboptimal, or worse, infeasible. Infeasibility can be avoided by controlling the constrained quantities. However, the constraints that determine optimal operation might switch with varying power demand, thus requiring a change in the regulator structure. In this paper, a control strategy that can handle plant-model mismatch and changing constraints in the face of varying power demand is presented and illustrated. The strategy consists in the integration of RTO and model predictive control (MPC). A lumped model of the SOFC is utilized at the RTO level. The measurements are not used to re-estimate the parameters of the SOFC model at different operating points, but to simply adapt the constraints in the optimization problem. The optimal solution generated by RTO is implemented using MPC that uses a step-response model in this case. Simulation results show that near-optimality can be obtained, and constraints are respected despite model inaccuracies and large variations in the power demand. Fil: Marchetti, Alejandro Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Centro Internacional Franco Argentino de Ciencias de la Información y Sistemas; Argentina. Universidad Nacional de Rosario; Argentina Fil: Gopalakrishnan, A.. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Chachuat, B.. Imperial College London; Reino Unido Fil: Bonvin, D.. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Tsikonis, L.. Laboratoire d; Suiza Fil: Nakajo, A.. Laboratoire d; Suiza Fil: Wuillemin, Z.. Laboratoire d; Suiza Fil: Van Herle, J.. Laboratoire d; Suiza

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.

Planar SOFC stack technology based on a unique concept (SOFConnex™) uses structured gas distribution layers between unprofiled metal sheet interconnects and thin Ni-YSZ anode supported electrolyte cells. The layers are flexible both in material and design and allow to implement new configurations relatively simply; manifolding can be internal, external, or combined. Together with thin stack components, independent of the supplier, the SOFConnex™ stacking approach allows compact planar assembly with low cost potential and adequate power density. Different cell and flow designs have been realized. With a basic flow configuration, short stacks (50cm2 cell active area) were assembled and tested, power density at 800°C reaching 0.5W∕cm2 at 0.7V average cell voltage (1.5kWe∕L, 0.36Ωcm2 area specific resistance), for 65% fuel utilization and 35% lower heating value electrical efficiency. Short stacks were thermally cycled and operated with both hydrogen and syngas. Degradation was essentially Ohmic (confirmed from impedance spectroscopy on stacks) and at first mainly due to the cathode-electrolyte interfacial reaction, performance loss was subsequently strongly reduced after cathode replacement. Using multiple voltage probes with additional interconnects allowed to separately monitor current collection losses during polarization. With an improved design in terms of sealing, postcombustion control and flow field, stacks up to 1kWe have been operated.

The optimization of Ni-YSZ anodes is an important step in obtaining electrically efficient anode supported SOFCs. Despite the continuous studies concerning optimization of the anode reduction conditions, degradation phenomena, oxidation-reduction cycling behaviour and microstructural changes, the anode reduction continues to be an active field for deeper insight [1,2], since as a final stage of the cell preparation, it offers an opportunity for optimization in respect to mechanical stability, electronic conductivity, catalytic activity and gas permeation. This is additionally complicated due to the counter-flow of the fuel and exhaust water vapour. This study combines two approaches – impedance spectroscopy and gas permeability studies that ensure experimental information about the main processes occurring during anode reduction: (i) formation of a new microstructure influencing the diffusive mass transfer and (ii) development of an electrically conductive network. The gas permeability studies [3] are carried out on anode pellets at room temperature before and after reduction performed at different conditions (temperature, reduction atmosphere). The observed changes are discussed and compared with SEM microstructural analysis. The impedance measurements are carried out directly during the reduction of button cells, segmented cells and NiO-YSZ anode pellets and thus the reduction process is studied during its evolution. The influence of the reduction conditions, including the duration of the process, is discussed in respect to the formation of an optimal Ni network with good electrical connectivity and catalytic activity. The presented figure shows two impedance diagrams measured on a segment from segmented fuel cell in different stages of the reduction process . Acknowledgement: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. References S.-W. Baek, J. Bae, Internat. J. Hydrogen energy 36 (2011) 689 B. Liu, Y. Zhang, B. Tu, Y. Dong, M. Cheng, J. Power Sources 165 (2007) 14 E. Mladenova, D. Vladikova, Z. Stoynov, A. Chesnaud, A. Thorel, M. Krapchanska, Bulg. Chem. Communic. 3 (2013) 366 Figure 1

Abstract Solid Oxide Fuel Cells (SOFCs) are promising candidates for future energy conversion devices that transform chemical energy of fuel into electricity [1]. One of the main challenges in improving the performance and cost-effectiveness of SOFC is the control of the degradation processes, such as Sr diffusion and chemical interactions which can contribute to the overall reduction of the cell performance [2-4]. Several authors have evaluated the importance of cation diffusivities for surface segregation of Sr and thus for a major degradation mechanism of SOFC cathodes [5, 6]. In the present work, a 3000 hour degradation test was carried out at 780 ºC in a short stack under real time operation conditions. After the test, a cell from this stack was disassembled and samples from nine different areas were analyzed, looking for evidences of degradation phenomena taking place. A pristine sample was also analysed as reference. The stack consist on coated ferritic stainless steel, anode supported cells and seal. The cells are supportend on a Ni-YSZ cermet anode with yttria-stabilized zirconia (YSZ) as electrolyte and a lanthanum strontium cobalt ferrite (LSCF) oxide as cathode material. A gadolinia-doped ceria (GDC) is used as barrier layer between cathode and electrolyte to prevent the formation of poorly conducting secondary phases, such as SrZrO3 or La2Zr2O7[7]. The aim of this work is to study the Sr diffusion, the effectiveness of GDC barrier layer and the evolution of LSCF cathode during operation. The deterioration of the performance measured in the cell is correlated with degradation mechanisms observed in post mortem experiments carried out in pristine and aged cells. The evolution of the Sr and other species in the cathode is examined by X-ray diffraction (XRD), confocal laser Raman spectroscopy, electron probe micro analyzer (EPMA-WDS), scanning electron microscopy equipped with an Energy-dispersive X-ray analyzer. Besides, the concentrations of pollutants in cells were obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) after dissolution in acid. A local microstructural and phase distribution study is carried out by means of transmission electron microscopy (TEM) and a high resolution scanning electron microscope coupled with electron energy-loss spectroscopy (EELS). The results throw light on the evolution of the cathode/barrier layer/electrolyte system of SOFC cells working under real conditions after long operating time (Figure 1). Figure 1. SEM images of the cross section and EPMA elemental distribution maps of the LSCF/GDC/YSZ in pristine and aged cells. References [1] K. Hilpert, W. J. Quaddakers, L. Singheiser, in “Handbook of Fuel Cells- Fundamentals, Technology and Applications”, ed. W. Vielstich, H. A. Gasteiger and A. Lamm, John Wiley & Sons, New Jersey, USA, vol. 4, (2003) 1037-1051. [2] R. Knibbe, J. Hjelm, m. Menon, N. Pryds, M. Sogaard, H. J. Wang, K. Neufeld, J. Am. Ceram. Soc., 93 [9] (2010) 2877-2883. [3] D. E. Vladikova, Z. B. Stoynov, A. Barbucci, M. Viviani, P. Carpanese, J. A. Kilner, S. J. Skinner, R. Rudkin, Electrochimica Acta, 53 (2008) 7491-7499. [4] F. Wang, M- E. Brito, K. Yamaji, D-H. Cho, M. Nishi, H. Kishimoto, T. Horita, H. Yokokawa, Solid State Ionics 262 (2014) 454-459. [5] M. Kubicek, G. M. Rupp, S. Huber, A. Penn, A. K. Opitz, J. Bernardi, M. Stöger-Pollach, H. Hutter, J. Fleig, Phys. Chem. Chem. Phys., 16 (2014) 2715-2726. [6] J. S. Hardy, J. W. Templeton, D. J. Edwards, Z. Lu, J. W. Stevenson, J. Power Sources, 198 (2012) 76-82 [7] A. Arregui, L. M. Rodriguez-Martinez, S. Modena, M. Bertoldi, J. van Herle, V. M. Sglavo, Electrochimia Acta, 58 (2011) 312-321. Acknowledgements 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. Figure 1

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Sulfur poisoning in a strontium doped lanthanum cobaltite (LSC) cathode current collection layer was revealed in a solid oxide fuel cell (SOFC) tested in repeat-element and stack configuration. Sources of sulfur contamination, other than trace SOx in air, were identified. Strontium sulfate (SrSO4) and strontium chromate (SrCrO4) enriched with sulfur were found at the interface between LSC and air channels. Understanding degradation mechanisms is a major issue in the development of SOFCs. In particular, in repeat-element and stack configuration, an important coupling of different degradation processes exists, with internal sources at the stack level and exogenous sources coming from system components. A specific diagnostic test station was developed to allow locally-resolved measurements of electrochemical performance and degradation in a repeat-element. Large differences in local degradation behavior were observed, affecting different electrochemical processes. Post-mortem analysis, mainly done by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), allowed to identify the pollutant species responsible for degradation. Complete results are published elsewhere [1-2]. In this study, an anode-supported cell, with an active strontium doped lanthanum manganite (LSM) - yttria stabilized zirconia (YSZ) composite cathode and a LSC current collection layer, was tested over 1900 h around 1073 K. Chromium and sulfur were found as major pollutant species on the cathode side; both are dependent on upstream conditions and components. While chromium poisoning is well known for SOFC cathodes, sulfur contamination of the cathode has received more attention recently. Sulfur poisoning is a major limitation for wide use of perovskite catalysts for treatment of auto exhaust gas [3]. Sulfur pollution from trace SOx in air and involving sulfate formation was reported by Xu et al. [4], where perovskite-type ceramics were used for oxygen permeation membranes. Yokokawa et al. identified sulfur as impurity after long-term operation of SOFC stacks in [5], and Xiong et al. reported and studied sulfur poisoning of different cathode materials just recently [6]. In the present experiment, a commercial vulcanized polymer tube for air inlet and an insulating high temperature sealing paste were identified as potential sulfur sources. Figure 1 shows the LSC microstructure before and after exposure to sulfur containing air. Strontium sulfate growth on porous LSC surface leads to an almost dense sulfate layer. Strontium sulfate was found over the whole LSC thickness without affecting the LSM/YSZ cathode. The more pronounced strontium sulfate formation in LSC compared to LSM is explained by the higher activity of strontium oxide (SrO) in LSC [7]. Chromium on the LSC surface was only found at the air-inlet, indicating that Cr was principally evaporated from upstream system components. SEM/EDX analysis allowed to identify sulfur-rich strontium chromate with a composition close to Sr(Cr0.85S0.15)O4. The chromate structure is changing along the airflow direction, from an almost dense structure to isolated particles, while maintaining the same composition. Further analyses are ongoing to confirm sulfate and chromate crystalline structures.

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State-of-the-art nickel-based SOFC anode-supported cells are highly sensitive to reoxidation of the metal phase at the temperature of utilization. This work presents results on redox stable nickel–YSZ (yttria- stabilized zirconia) anode-supported cells, for both smaller and larger scale cells. A 55cm2 cell was mounted in a SOFC stack repeat element and tested over 40 full redox cycles. Performances and elec- trochemical impedance results of the repeat element are compared to the smaller sized cells of similar anode support structure.

The properties of sealing materials are important for the performance and reliability of solid oxide fuel cells (SOFCs). Even if the properties of a sealing material can be studied separately, it remains difficult to quantify the effect of an imperfect seal on the repeat-element behavior. In this study, simulation is used to investigate the effects of an imperfect seal behavior on the performance and reliability of SOFCs. Diffusion through the sealing material and inherent local combustion of fuel are added to the computational fluid dynamics (CFD) repeat-element model, which also allows us to compute the flow field, the electrochemical reactions, and the energy equations. The results are in good agreement with experiments. The zones of parasitic combustion and local overheating are well reproduced. Furthermore, the model predicts a risk of reoxidation under polarization that is well observed. The model also shows the necessity to take into account the diffusion transport for the development of compressive seal materials, hence verifying the hypotheses made by other groups. The modeling approach presented here, which includes the imperfections of components, allows us to reproduce experiments with good accuracy and gives a better understanding of degradation processes. With its reasonable computational cost, it is a powerful tool for a design of SOFC based on reliability.

Use of intermediate temperature solid oxide fuel cells for power generation is attractive, due to the highest achievable electrical efficiencies in the low power generation range. Nonetheless, there still exists a potential to improve the industry benchmark SOFC systems, which use natural gas or biogas as fuel and consider steam methane reforming, with external steam supply, for syngas production and usage in the stack. Anode off-gas recirculation using a blower is the add-on to our next-generation SOFC system. Since the recirculating feed contains steam produced in the stack, no external steam supply is needed for reforming. This eliminates the use of expensive water de-ionisation sets. Further, it allows for high overall fuel utilization at low diffusion losses. System performance is evaluated through multi-objective optimization criteria, i.e. maximization of electrical efficiency and cogeneration efficiency. Evolutionary algorithms compare the different design alternatives, i.e. co-flow or counter-flow stack operation with hot or cold recirculation. The system flowsheet which includes models for BOP components and an inhouse experimentally validated SOFC stack model, is solved using the commercial software Belsim VALI. The design variables identified for this system are 1. oxygen to carbon ratio before the external reformer 2. external to internal methane reforming fraction 3. reducing fuel species molar fraction at anode outlet 4. air-fuel equivalent ratio in the burner and 5. blower inlet temperature. Successive generations of population are obtained by reproduction and mutation of the existing population. Following the ‘survival of the fittest’ rule, the iteration is stopped when a non-dominated solution set represented by a Pareto-optimal front is obtained. The results obtained (eg. figure 1) suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable. Figure 1

The increasing quality and durability of solid oxide fuel cells (SOFCs) 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 H-2 partial pressures, temperatures and bias current, effectively demonstrating the direct impact of each studied variable on the cell performance and degradation behavior. Segmented cell testing has been proven to be an effective strategy to achieve better 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. (C) 2010 Elsevier B.V. All rights reserved.

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.

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.

Our SOFC stack development technology is based on the unique SOFConnexTM concept, using flexible gas distribution layers between metal sheet interconnect and thin ASE cells. Flexibility is given both in material and design. This ensures proper electrical contact over the whole cell (50 cm2 active), without necessitating restrictive cell fabrication tolerances, and allows easy adaptation and evolution of the flowfields. With the presently used configuration, several multiple cell stacks were assembled and tested. Reproducible stack power density (H2 fuel, λ = 1.5-2, 800°C maximum local temperature) is 0.5 W/cm2 at 0.7 V average cell voltage (1.5 kWe/L), for 67% fuel utilisation (35% LHV electrical efficiency). Performance with simulated POX-syngas (H2/CO/N2) was close to that with H2. Degradation is the focus of attention now that adequate power density and efficiency using the SOFConnex™ approach have been established and reproduced.

There still exists a large gap between simulation work and industrial applications in the context of control and optimization of solid-oxide fuel-cell (SOFC) systems. In an effort to bridge this gap, this study describes the experimental implementation of steady-state real-time optimization (RTO) to an SOFC system that consists of both hardware and software components. The proposed adaptive optimization scheme uses an approximate steady-state model of the fuel-cell system and corrects it “appropriately” so that it becomes “excellent” for optimization. This way, the plant can be steered efficiently toward optimality, while meeting the varying electric power demand. In these experiments, the plant efficiency was increased from 55% to 62% through application of RTO. Furthermore, although the SOFC system is characterized by slow thermal dynamics that may take a few hours to settle to steady state, it has been possible to reduce the time necessary to reach the power setpoint from 1 h to about 5 min thanks to the use of transient measurements and a dynamic model. This experimental work has shown that it is possible, not only to control the SOFC system at a desired operating point, but also to operate it near optimality despite changes in power demand.

The Laboratory for Applied Mechanical Design (LAMD) designed, manufactured, and experimentally tested a novel steam-driven anode off-gas recirculation (AOR) fan for 10 kWel solid oxide fuel cell (SOFC) systems. Due to the dynamic steam-lubricated bearings, the AOR unit is expected to have a high lifetime, even at elevated rotational speeds and temperatures. Additional, the unit is oil-free, explosion-proof, very compact, and cheap to manufacture. The AOR fan diameter is at 19.2 mm and the nominal rotational speed is 175 krpm. The unit was coupled to a 6 kWel SOFC system, reaching electrical gross DC efficiencies, based on the fuel LHV, of 66 % in part load (4.5 kWel gross DC) and 61 % in full load (6.3 kWel) for a global fuel utilization of 85 %. To the best of the authors' knowledge, this was the first time that a steam-driven AOR fan was demonstrated in-situ with an SOFC system.

Our planar SOFC stacking technology uses unprofiled metallic interconnects (MIC) and thin cells of tape cast anode supported YSZ. The key element is the gas diffusion layer (GDL) between cell and MIC, which consists of so-called SOFConnex™. Using square cells with internal manifolds, 0.5 W/cm2 stack power density (800°C) can be obtained on short stacks. However, this open design configuration limits the assembly of large stacks and the durability of operation, owing to postcombustion and redox cycling occurring at unprotected cell edges. A new design, inspired from modeling work and the adaptability of the SOFConnex™ GDL, led to oblong-shaped cells, assembled in a closed stack casing with external air manifolding and fuel recovery manifolding, avoiding postcombustion. While stack power density in both designs remains similar, the operation at increased fuel utilization has become more stable in the 2nd design. Furthermore, a correlation of performance homogeneity during stack testing was drawn to assembly quality control. A 36-cell stack in dilute H2 at 800°C achieved 625 Wel (28% LHV efficiency, 0.35 W/cm2) under continuous polarisation, with all 6 clusters of 6 cells showing coincident i-V-output.

Powder Injection Molding (PIM) gives the possibility to produce, at an industrial rate, ceramic parts with fine details. It is thus a possible approach to reduce the fabrication costs of Solid Oxide Fuel Cells (SOFCs). This work presents fabrication and electrochemical characterization results of injection-molded structured anode-supported SOFCs. Planar anode-supported SOFCs with fine details have been produced by injection molding of a mixture of nickel oxide (NiO) and yttria-stabilized zirconia (YSZ). The channeling structure and support porosity ensure gas transport on the fuel side. Initial anode support characterizations like sintering shrinkage, porosity and electrical conductivity have been done on simple Ni-YSZ test bars. Thin YSZ electrolyte has been deposited by spin-coating on the PIM green body before half-cell co-sintering. Electrochemical testing was carried out with a lanthanum-strontium manganite (LSM)-YSZ cathode. The performance is comparable to tape-cast anode-supported cells, with 0.45 W cm(-2) obtained at 0.6 V and 810 degrees C. Medium-term galvanostatic testing shows a degradation rate of about 1.1% kh(-1). Post-test analysis with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) attribute this mainly to cathode degradation due to Cr and S poisoning. These results are therefore promising for using a PIM fabrication process in the SOFC field. (C) 2012 Elsevier B.V. All rights reserved.