Anode-supported Planar SOFC Research Articles

Three-dimensional modeling of anode-supported planar SOFC with different shapes of corrugated electrolytes

Three-dimensional modeling of anode-supported planar SOFC with different shapes of corrugated electrolytes

CFD modelling and experimental validation of cell performance in a 3-D planar SOFC

Fuel cells can be used to provide power for most electrical or electronic devices designed for operation from batteries or from conventional utility power sources. In this study, a three dimensional Computational Fluid Dynamics (CFD) simulation model has been developed and experimentally tested for an anode-supported planar SOFC that has bipolar plated for corrugation which serving as a gas channel and current collector. Experiments were performed on planar cross-flow type at different reactant flow rates, cell temperatures and pressures. In the experimental analysis, values varied from 0.12 L/min to 2 L/min for reactant and from 700 °C to 800 °C SOFC cell temperature. Thereby divergent operating parameters about cell parameters have been addressed. The conservation equations of momentum, energy and mass types are solved with the ANSYS FLUENT software in the proposed model. The maximum power density measured as 6 kW/m2 under optimum working conditions. The results also show that the current density and the inlet velocity of fuel gassed are the main parameters that drive the fuel utilization and the total conversion efficiency. All the experimental and numerical findings, which were in good agreement with each other, showed that for Current density – Potential difference characteristic of SOFC cell graphs.

Chromium Poisoning of Cathodes in Solid Oxide Fuel Cells and its Mitigation Employing CuMn1.8O4 Spinel Coatings on Interconnects

Intermediate temperature solid oxide fuel cells (IT-SOFCs) enable the use of chromia-forming alloys in interconnects and balance-of-plant (BoP) materials. However, depending on the SOFC operating conditions, the chromium (Cr) containing species can transport and deposit in the SOFC cathodes and deteriorate their performances. In order to investigate the Cr-poisoning phenomena, anode-supported planar SOFCs with Sr-doped LaMnO3 (LSM) + yttria-stabilized zirconia (YSZ) cathode active layer and LSM cathode current collector layer were fabricated and electrochemically tested in contact with Crofer interconnect at 800 °C. The cathode atmosphere (dry air or 10% humidified air) and current condition (no current or constant cathodic current) were varied in tests on identical cells, and the performance degradations under different test conditions were compared. It was found that both humidity and cathodic current can promote Cr poisoning. Microstructure characterizations also confirmed major amounts of Cr-containing deposits at the cathode/electrolyte interfaces of the cell tested with cathodic current and/or humidity. Free energy minimization calculations and thermogravimetric experiments were performed to determine the chromium vapor species that form over the chromia-forming alloy interconnect and result in the chromium deposition. Based on these results, the roles of humidity and cathodic current in chromium poisoning are evaluated, and a performance degradation mechanism associated to chromium vapor species dissociation at the cathode/electrolyte interface is proposed. In order to mitigate the effect of Cr-poisoning, a protective Cu-Mn spinel coating was applied on the interconnect surface. The Cu-Mn spinel coating has good thermal compatibility, high stability and electronic conductivity at the SOFC operating temperature. The candidate interconnect material (Crofer22H meshes) with no protective coating, those with commercial CuMn2O4 spinel coating and the ones with lab-developed CuMn1.8O4spinel coating were investigated. The lab-developed CuMn1.8O4spinel coating was deposited on the Crofer22H mesh by electrophoretic deposition and densified by a reduction and re-oxidation process. Anode-supported SOFCs with LSM based cathode with different Crofer22H meshes (bare, commercially coated CuMn2O4, and lab-coated CuMn1.8O4) were electrochemically tested at 800 °C with a constant cathodic current (0.5 A/cm2) for total durations of up to 216 hours, and the cell performances were characterized by current-voltage measurements every 24 hours. Cross sections of the Crofer22H meshes and the SOFC cathodes were examined before and after the cell tests by scanning electron spectroscopy and energy dispersive X-ray spectroscopy. The results of the mitigating effects of the two types of Cu-Mn spinel interconnect coatings on Cr-poisoning of the SOFC cathodes were compared. It was observed that the performance of the denser lab-developed Cu-Mn spinel coating was distinctly better showing significantly less chromium containing deposits at the cathode/electrolyte interface after the test.

Three-Dimensional Computational Fluid Dynamics Modelling for a Planar Solid Oxide Fuel Cell of a New Design

The aim of the work is to investigate the electrochemical processes of a planar Solid Oxide Fuel Cell and to estimate the performance of a novel SOFC design. The design involves cross-flow bipolar plates. Each of the bipolar plates has an air channel system on one side and a fuel channel system on the other side. One pair of channels in adjacent plates is used for the air flow along the cathode, the other for the fuel flow along the anode of each cell. A three-dimensional Computational Fluid Dynamics model for the anode-supported planar SOFC with complex bipolar plates has been developed. The distributions of temperature and gas flow through the channels and MEA structure were studied. The results identified the most susceptible areas for significant increase of the temperature at high current density, which can lead to hot spots formation and destruction of the electrodes.

Three-dimensional CFD modeling of transport phenomena in multi-channel anode-supported planar SOFCs

In this study, a three-dimensional computational fluid dynamics (CFD) model is developed and applied for anode-supported planar SOFC involving multi-channels. The developed model is first validated in agreement with the experimental data obtained at same conditions. Three different flow arrangements (co-, counter- and cross-flow) are simulated and compared in terms of cell overall performance and various transport phenomena occurred inside the SOFC single cell functional components. Local distribution of temperature, mass flow rate, current density, gas concentrations of reactants and products in both fuel and air sides under different flow arrangements is predicted and presented. It is found that the co-flow and counter-flow arrangements have a better performance than that of the cross-flow arrangement at the same operating conditions. It is also found that the temperature for the three flow arrangements is unevenly distributed and the significant temperature gradients exist along the length of the cell. The mass flow rate of fuel at the inlet of each channel is uniform, however its difference between the side channel and the channel at the center is increasing along the fuel flow direction, which reaches a maximum value at the outlet region. It is also predicted that the maximum current density is located at the interfaces between the channels, ribs and the electrodes resulting in a large over-potential and a heat source in the electrodes, which is harmful to the cell overall performance and working life time.

Three-dimensional CFD modeling of transport phenomena in anode-supported planar SOFCs

In this study, a three-dimensional computational fluid dynamics model has been developed for an anode-supported planar SOFC. The conservation equations of mass, momentum, species/charges and thermal energy are solved by finite volume method for a complete unit cell consisting of 13 parallel channels in both anode and cathode. The simulation results of the developed model are well in agreement with the experimental data obtained at same conditions. In this study, the co-flow arrangement with hydrogen utilization of 60 % and operating voltage of 0.7 V is used as the base case, and compared with the counter-flow arrangement. The predicted results reveals that the maximum temperature obtained in the counter-flow arrangement is about 10 A degrees C lower than that of co-flow, but the counter-flow arrangement has a higher temperature gradient between the respective anodes and cathodes in a cross-section normal to the main flow direction, especially in the air inlet region of the cell (x = 0.04 m),which is very harmful to the lifetime of materials. The current density is very unevenly distributed along and normal to the flow direction for both the co- and counter-flow arrangements, and the maximum values occur at junctions of the electrodes, channels and ribs, which causes higher over-potentials and ohmic heating.

Three-Dimensional CFD Modeling of Transport Phenomena in a Cross-Flow Anode-Supported Planar SOFC

In this study, a three-dimensional computational fluid dynamics (CFD) model is developed for an anode-supported planar SOFC from the Chinese Academy of Science Ningbo Institute of Material Technology and Engineering (NIMTE). The simulation results of the developed model are in good agreement with the experimental data obtained under the same conditions. With the simulation results, the distribution of temperature, flow velocity and the gas concentrations through the cell components and gas channels is presented and discussed. Potential and current density distributions in the cell and overall fuel utilization are also presented. It is also found that the temperature gradients exist along the length of the cell, and the maximum value of the temperature for the cross-flow is at the outlet region of the cell. The distribution of the current density is uneven, and the maximum current density is located at the interfaces between the channels, ribs and the electrodes, the maximum current density result in a large over-potential and heat source in the electrodes, which is harmful to the overall performance and working lifespan of the fuel cells. A new type of flow structure should be developed to make the current flow be more evenly distributed and promote most of the TPB areas to take part in the electrochemical reactions.

The analyses of the start-up process of a planar, anode-supported solid oxide fuel cell using three different start-up procedures

Three start-up procedures for an anode-supported planar SOFC are proposed and investigated numerically in the present study. The first is to introduce the inlet fuel at the operation temperature after the heat-up process is completed. The second is to incorporate the anode-recycling mechanism into the start-up process. The third is to fix the difference between the inlet-fuel temperature and the cell minimum temperature. The numerical results obtained from the present study show that the effective maximum absolute temperature-gradient is exhibited in the early stage of the start-up process. For the present investigated SOFC configuration, the required start-up time for the case using methane is 3.2-fold longer than that using hydrogen. The effective maximum absolute temperature-gradient for the case using hydrogen is 2.2-fold larger than that utilizing methane. The endothermic internal reforming reaction of methane has a positive effect on the accommodation of the temperature uniformity during the start-up process. The anode-recycling mechanism significantly reduces the start-up time For the fixed-temperature-difference start-up procedure, a properly selected temperature difference may lead to a smaller effective maximum absolute temperature-gradient in the early stage and a shorter start-up time by accelerating the start-up pace in the later stage.

Performance Evaluation of Anode-supported Planar SOFC with Precisely-simulated Reformate Gases

Performance of a novel simulated-reformate generator was evaluated for testing SOFC cell/stacks performance with ease, no handling of pure carbon monoxide, flexibility for various steam-reforming conditions. Gas composition of synthesized simulated reformate gases with the generator agreed by ± 0.5 mol.% to equilibrium ones for each reforming condition (S/C= 2-3.5, representative fuels such as methane and biodiesel). Furthermore, an anode-supported planar cell was tested with the generator. As long as fuels including biodiesel are reformed in a reformer to equilibrium and SOFC has internal reforming capability, cell performance and DC electrical efficiency would be almost the same at a common S/C despite fuel species. For kerosene reformates, change in S/C (2.0-4.0) affected cell voltages at 80% fuel utilization by ± 10 mV, or approximately ± 1.2%. The gas conversion impedance for the kerosene reformates coincided with calculated values (e.g. 0.25 ± 0.01 Ω cm−2). Therefore, it is concluded that cell performance at various reforming conditions can be evaluated with the generator easily, precisely, and safely without pure CO handling.

On the Thermodynamic Stability and the Kinetic Activity of SOFC Materials

Materials research in solid oxide fuel cells has been driven by the recognition of developing novel constituents which enable faster kinetics and greater power, in order to reduce the cost of SOFC systems (e.g. $/kW). When an SOFC is operating with very high current density, the physical properties of its constituents may not be the same as these at the conditions of low current density or open circuit voltage. This is particularly true for the SOFC cathode, the reduction electrode which dissociates oxygen molecules to oxygen ions, because the cathode is generally considered to be the cell component that limits the electrochemical performance of anode-supported planar SOFCs. The aimed of this paper is to address a fundamental question: how do fast kinetics and high current in an operating SOFC affect the thermodynamic states of its material constituents.

The analyses of the heat-up process of a planar, anode-supported solid oxide fuel cell using the dual-channel heating strategy

In the present study, a finite-volume (FV) model has been developed to investigate the thermal behavior, the heat-up time and the corresponding temperature gradient for an anode-supported planar SOFC during the heat-up process. A methane burner is employed for the heat-up of the SOFC. Effects of the burner power and the flow configuration on the temperature distribution, the effective maximum-temperature-gradient, the heat-up time and the required energy in the heat-up process are investigated. The numerical results obtained from the present study show that the single-channel mode is impractical for the SOFC heat-up due to the lengthy heat-up time. For a fixed-power burner, the required heat-up time for the counter-flow configuration is about 25% less than that of the co-flow configuration. For the counter-flow configuration, the temperature gradient is averagely about 17% larger than that for the co-flow configuration. The total energy required for the counter-flow configuration is about 20% less than that for the co-flow configuration. The counter-flow configuration is superior to the others as far as the heat-up time and the required energy are concerned, although it yields a relatively higher maximum-temperature-gradient.

Anode-supported planar SOFC with high performance and redox stability

Solid oxide fuel cells with full ceramic anodes have recently attracted increasing attention, because the conventional Ni/YSZ cermet anodes may fail during practical operation due to their weak mechanical stability in the case of re-oxidation of the nickel. However, until now the reported fuel cells based on ceramic anodes have been fabricated only as small pellet-sized cells and electrochemical performance has been barely satisfactory, making it difficult to evaluate these attempts with respect to commercial feasibility. Herein, we report single cells based on Y-substituted SrTiO 3 anode substrates. These planar cells have outer dimensions of 50 × 50 mm 2, which has not been reached for a ceramic anode-supported cell before. They show power densities of 0.7–1.0 W cm − 2 at 0.7 V and 800 °C, which are sufficient for technical applications. The cells survived 200 anode-gas changes between fuel and air (redox cycles), providing a new direction for the development and commercialisation of anode-supported solid oxide fuel cells.

Use of Electroless Anode Active Layer in Anode-supported Planar SOFC

In SOFC, a novel Ni-YSZ cermet developed through electroless technique is used as an anode as well as anode active layer (AAL). In the present investigation, thickness of such AAL (varied in the range 15 - 140 micrometer) is optimized sequentially for fabricating high performance single cell. The fabrication technique involves tape casting followed by room temperature lamination to form the half cells. Effect of sintering temperature of half cells on the electrochemical performance has been carried out in the range of 1300 degC to 1400 degC. A typical I-V characteristics of coupon cell (active area of ~ 0.3 cm2) sintered at 1400 degC with an optimum AAL thickness show current density of ~ 3 A/cm2 and power density of ~ 2 W/cm2 at 0.7 V and 800 degC. Electrochemical performances of single cells using only electroless anode are also evaluated for comparison. Microstructures of these single cells are correlated with the electrochemical performances.

Three-dimensional Modeling of Anode-supported Planar SOFC with Direct Internal Reforming

This paper presents a three-dimensional model of an anode-supported planar SOFC with corrugated bipolar plates serving as gas channels and current collector above the active area of the cell, based on the direct internal reforming reaction of methane and the electrochemical reaction of hydrogen. A co-flow system with gas mixture of methane, water vapor and a small amount of hydrogen as anode gas and air as cathode gas fed at an inlet temperature of 973K was modeled at a single cell unit level. A simple equation for the cell resistance with measured values for the quasi ohmic resistance is used for the calculation of the current density. The modeling results show the current density distribution and temperature profiles in the cell and gas concentrations profiles along the length of the cell channel. Furthermore, the temperature gradient inside the cell was investigated.

WITHDRAWN: Characterization of an intermediate temperature anode-supported planar SOFC with La0.6Sr0.4CoO3- cathode material

This article has been withdrawn at the request of the author's and Editor. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy .

Use of Electroless Anode Active Layer in Anode-supported Planar SOFC

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Three-dimensional Modeling of Anode-supported Planar SOFC with Direct Internal Reforming

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Montagem e caracterização elétrica de pilhas a combustível de óxido sólido (PaCOS)

This paper is focused on a review of the design features and the electrochemistry characterization of anode-supported planar SOFC. Studies and results of metallic alloy interconnectors and recovery for protection against corrosion and for contact layer are showed. Moreover a discussion of examples of measurements of impedance spectrometry, according to the literature and our experimental results are made. For the anode supported fuel cells the power density varies from 0.1 to 0.5 Wcm2, according to results in the literature (showed in this paper). For electrolyte supported fuel cell the power density can be 10 Wcm-2 for high temperatures. An English-Portuguese glossary of most used terms in SOFC stack is given for greater clarity and to introduce new terms to the reader.

Development of Anode-supported Planar SOFC using Inexpensive and Simple Processing Technique

In the anode-supported planar SOFC, conventionally the porous anode support is fabricated first, over which a thin layer of electrolyte (5-50 µm) is developed to form the half-cell. Additionally, functional layers in the anode side (AFL) and cathode side (CFL) are used. The present contribution reports use of inexpensive and simple processing techniques to fabricate anode-supported planar SOFC where no AFL is used. The process involves room temperature lamination of several layers of tapecast NiO-YSZ together with a top layer of YSZ followed by co-firing. The half cells thus fabricated, having thin electrolyte (~ 20µm), are screen-printed with La(Sr)MnO3 (LSM)-YSZ CFL and LSM cathode layer . Single cells of dimensions up to 10 cm × 10 cm × 1.5 mm have been fabricated. Coupon cells show high current density between 1.0 and 1.5A/cm2 at 8000C at 0.7V. The electrochemical performances of these cells are size independent and show excellent repeatability.

Manufacturing of Anode-Supported SOFC's: Processing Parameters and their Influence

Commercialization of SOFC technology is strongly relying on the establishment of low cost, highly reliable and reproducible manufacturing processes. A major challenge in the production chain is the manufacture of single cells. Anode-supported planar SOFC's are layer structured, where each layer contains a complex microstructure. Successful manufacture of such a cell and the achievement of optimal final properties are very much dependant on each of the processing steps and their interdependence. Therefore, in order to improve the cell performance as well as reducing the processing costs, it is necessary to consider the process chain holistically. Several thousand anode-supported SOFCs are manufactured annually at the Risoe National Laboratory in collaboration with Topsoe Fuel Cell A/S. As a result a large database exists, which enables a statistical analysis of the various controlling parameters. In the present work, some of the critical manufacturing elements are discussed along with their impact on product quality.