Planar SOFC Research Articles

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.

Introduction of Solid Oxide Fuel Cell Research in SICCAS

Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS) has about 19 years’ experience in SOFC study, containing both fundamental research of materials and application research in manufacturing processes, stack assembling and system design. SICCAS is now able to prepare large dimension (from 10x10 to 20x20 cm2) single cells in mass production, to prepare modules for planar type SOFC stacks, and to operate 1kW level system. The research effort for application is now concentrated on intermediate temperature (750 ºC) operating, but the fundamental research covers materials and new cell designs for lower temperature (550 ºC), direct hydrocarbon operation, reversible SOEC/FC and so on. All technologies adopted are cost reasonable and easy for mass production. The latest results are presented in this manuscript.

Integration of Innovative Oxide Materials in an Intermediate Temperature Solid Oxide Fuel Cell

This work aims to associate two innovative promising oxide materials, the perovskite-type BaIn0.3Ti0.7O3-δ (BIT07) electrolyte and the lanthanide nickelate Pr2-xNiO4+δ cathode, in a planar anode-supported SOFC for an efficient operation at 700°C. The cell elaboration protocol will be briefly described, and the results of electrochemical characterizations of complete cells will be discussed. Adding a GDC barrier layer in between BIT07 electrolyte and the cathode decreased the polarization resistance by a factor 4, e.g. for Pr1.97NiO4+δ a competitive value of 0.29 Ω cm² is reached. It also led to increased OCV values, as a small p-type conductivity of BIT07 in oxidizing atmosphere associated with a few microscopic cracks observed in the electrolyte layer could be compensated by an electron-blocking effect of GDC and a filling of the porosities of the electrolyte. This cell was operated beyond 500 hours of under current, with a fairly high extrapolated degradation rate of 27% / kh.

Development and Manufacturing of SOFC-Based Products at SOFCpower SpA

SOFCpower SpA provides efficient energy solutions based on its proprietary planar SOFC technology. The company focuses on products that use natural gas either for heat and power generation (CHP) or for distributed power generation at high electrical and total efficiencies. The company develops and manufactures SOFC power modules in close collaboration with both European heat appliance OEMs and utilities. With several years of operational experience in running its pilot plant in Italy, SOFCpower has consolidated its manufacturing know-how and capabilities and has confirmed the competitiveness of its products, capable to match target market requirements and being sold and operated with grid parity prices. Collaboration with Industrial component suppliers and integrators has largely increased in intensity within the last 2 years, key success factor for reaching the required cost and reliability targets for the stationary market. First units have been installed since 201, operating as sheltered field tests in the North-East of Italy within the frame of the Crisalide Initiative. Those activities will be strongly enlarged with the participation in the FCH-JU demonstration project ENE.FIELD started in October 2012. SOFCpower is also involved on strategic development activities to demonstrate the added value of its product for different applications including high temperature electrolysis (HTE), biogas and waste-to-energy (WTE) applications. The paper provides an update of the stack and system development, including operational results of SOFC-based micro-CHP and the approach to enter the distributed cogeneration market.

Eneramic Power Generator – A Reliable and Cycleable 100W SOFC-System

Off-grid power generation is considered as an attractive market for fuel cells. For this purpose, a compact SOFC battery hybrid system was developed that operates on commercially available fuels. Due to its progressive thermal packaging, the mobile prototype achieves a net efficiency of 22 % with commercial BoP components. It was possible to reduce the cold start time of the system by direct cathode preheating with burner off-gas. Thus, the system is designed for frequent start-up and cycling capability. In consequence the planar SOFC stack was tested with rapid heating rates up to 20 K/min in furnace and hotbox environment. The degradation caused by system cycling is less than 0.1 % per cycle. Finally, the optimization of stack temperature conditions has improved the lifetime of the SOFC stack significantly.

3D thermomechanical behaviour of solid oxide fuel cells operating in different environments

Hermetic sealing and long-term structural reliability of fuel cell stacks depend strongly on the thermomechanically induced stress–strain behaviour. These are especially affected by the environment; the fuel cell is operating in. Most of the research and development studies, as well as laboratory studies are conducted within electrically heated furnaces rather than operating in an insulated system environment. The thermomechanical comparison of them is not fully understood, yet. The present study utilises a previously developed full scale three dimensional planar type 6-cell SOFC short stack model to shed light on the thermomechanical response of high temperature fuel cells operating in system and furnace environments. The physically resolved coupled computational fluid dynamics and computational structural mechanics model has been improved, accounting for the rate dependent creep strain, as well as including the furnace domain and thermal radiation to fully describe the thermal and deformation behaviour of the stack. The non-linear elastoplastic behaviour of the metal components as a function of temperature is considered. The results are validated using creep strain data from the literature and in-house post-mortem images. The study gives an insight about the critical regions prone to failure due to creep strain operating in different environments and the long-term fuel cell behaviour. Moreover, the critical locations appear to be prone to high creep strain after 1000 h operation time.

Thermal cycling of Crofer22APU-sealant-anode supported electrolyte joined structures for planar SOFCs up to 3000 h

Thermal cyclic tests in air were performed for Crofer22APU/glass-ceramic/Anode-Supported-Electrolyte (ASE) joined samples for a total period of 3000h. Different cooling rates from the SOFC operating temperature of 800°C to room temperature were carried out. Large and elongated voids were observed to develop just below the Cr2O3/Cr–Mn spinel layer of the Crofer22APU. Diffusion of Al to the areas just below the protective oxide layer led to formation of Al2O3 on the surface of the voids. The emergence of CaAl2Si2O8 as a new phase in the glass–ceramic was attributed to the diffusion of Ca away from the interfacial areas thus decreasing the proportion of Ca to Al and Si. The glass-ceramic sealant maintained good adherence with the Crofer22APU and the ASE without presence of any crack even after 3000h of exposure.

Study on paper-structured catalyst for direct internal reforming SOFC fueled by the mixture of CH4 and CO2

Inorganic fiber network including YSZ fiber which acts as catalyst support was created by the simple paper-making process, and novel Ni-loaded paper-structured catalysts (PSCs) with excellent catalytic activity for the dry reforming of methane were designed and developed. The PSCs exhibited high fuel conversion comparable to the conventional powdered catalysts with less than one-tenth catalyst weights. The significant advantages of the PSCs are their high mechanical flexibility and material workability. So far, a functionally-graded catalytic reaction field which leads to uniform temperature distribution during biogas reforming resulting in stable operation of planar SOFC was successfully developed by the PSC array based on the kinetic simulation model built in this research.

Effect of contact area and depth between cell cathode and interconnect on stack performance for planar solid oxide fuel cells

Effect of contact area and depth between cell cathode and interconnect on output power density and degradation of stack for planar SOFCs has been investigated systematically. The results indicate that the maximum output power density (MOPD) of repeating units inside stack increases firstly and then decreases slightly with the increasing interface contact area and depth, respectively, showing an approximate convex parabolic relation of power density to interface contact area and depth. The degradation rate of repeating units decreases gradually for 972 h' operation under 0.75 V unit-cell voltage, 0.476 A cm−2 current density and 41.8% fuel utilization with different contact area. At the optimum value of the interface contact area, the repeating unit inside stack appears no degradation under operation for 1060 h under 0.8 V unit-cell voltage, 0.444 A cm−2 current density, and 78% fuel utilization efficiency.

A Novel High Temperature Metal - Air Battery

Based on the planar SOFC and SOE technology, a high temperature metal – air battery was operated at temperatures between 700°C and 800°C. It reached charging and discharging capacities of about 2 hours at power densities of more than 250mW/cm². A Siemens stack concept was developed and validated in numerous lab experiments as well as in several stack demonstrators of more than 1kW power output. The storage material is a proprietary development consisting of a iron/iron oxide combination which can be produced economically. It showed very low degradation rates of 1 - 2% in 1,000 charging / discharging cycles. More than 10,000 cycles were reached in 10x10 cm² short stacks. System efficiencies for the upper MW range of > 60% and a cycle life of about 10,000 cycles will be achieved.

Mechanical Characteristics of Electrolytes Assessed with Resonant Ultrasound Spectroscopy

AbstractElastic properties and residual stresses are critical for the assessment of the reliability of ceramics components. The thin electrolyte layer of anode supported SOFCs is under a state of high compressive residual stress. In the present study, the elastic properties of this layer as well as the residual stress distribution induced in the anode‐supported planar SOFC cell were examined by using the resonant ultrasound spectroscopy (RUS). A modal analysis by finite element simulation demonstrated that the RUS has a great potential to determine these mechanical characteristics. The Young's modulus can be determined from the natural frequencies of the layered sample whilst the residual stress can be determined using the sample with a semi‐hole. Estimates of the Poisson's ratio are possible. In the experiment, various individual resonant oscillations were successfully identified using the present RUS system. Hence, by comparison of experimental results and simulations, the elastic properties as well as the residual stress were obtained. Although the proposed method is still experimentally challenging, it could be developed to become a powerful tool to simultaneously determine residual stresses and elastic constants of thin‐layered systems.

3D transient thermomechanical behaviour of a full scale SOFC short stack

Hermetic sealing of planar solid oxide fuel cell components is a critical issue. The long term operation and structural reliability of the fuel cell stacks depend strongly on the thermomechanically induced stress–strain behaviour of the fuel cell stack. These are especially affected through the thermal transients, which the fuel cell stack is subjected to, over time. Hence, the thermomechanical characterisation of the fuel cell stack during thermal cycling is indispensable. The current paper elucidates a fully three dimensional thermomechanical analysis of a planar type SOFC short stack over a whole thermal cycle. A coupled computational fluid dynamics and computational structural mechanics analysis has been performed. Typical stack components i.e., cell component, wire-mesh, metal frame, interconnector plates and sealant materials have been considered. The model represents the physical resolution of the air channels and the manifold regions. The non-linear elasto-plastic behaviour of the metal components as a function of temperature is considered. The study gives an insight about the transient thermal behaviour of a full scale fuel cell stack and its thermomechanical response, determining the mechanisms that trigger the thermomechanically induced stress during the heating-up, operation and shut-down stages.

Theoretical studies on the electrochemical and mechanical properties and microstructure optimization of micro-tubular solid oxide fuel cells

A comprehensive thermal fluid electrochemistry model for the analysis of the electrochemical performance and a thermal mechanical model for the analysis of the mechanical behavior of micro-tubular solid oxide fuel cell (mtSOFC) are presented. Material properties for the models are determined by the available theory and experiment and linked to the compositions and microstructures of the materials. Good agreement between the theoretical and experimental I–V relations is obtained. The mode is used to examine the effects of various parameters on the electrochemical and mechanical performance of mtSOFC. Collecting current from both sides of the anode is found to significantly increase the cell output. Increasing the Ni content or reducing the Ni particle size is generally helpful for improving the electrochemical performance, but the increased Ni content reduces the mechanical stability. The suitable Ni content is thus obtained. The LSM content is inconsequential to the mechanical stability and should be determined by achieving high electrochemical performance. Properly designed mtSOFCs are shown to provide high current outputs. Therefore, mtSOFC is a promising technology with both the benefits of planar SOFC for high current density and tubular SOFC for thermal cycling endurance.

Investigation of Impactors on Cell Degradation Inside Planar SOFC Stacks

AbstractThe impactors on cell degradation inside planar SOFC stacks were investigated using both coated and uncoated Fe–16Cr alloys as the interconnects under stable operating conditions at 750 °C and thermal cycling conditions from 750 to 200 °C. It was found that cell degradation inside the stack is primarily dependent on the interfacial contact between the cathode current‐collecting layer and the interconnect. Additionally, cell degradation is found to be independent of the high‐temperature oxidation and Cr vaporization of the interconnects during stack operation, as the stacks are well sealed. The coating on the interconnect can further improve the contact between the cell cathode and the interconnect when the latter is properly embedded into the current‐collecting layer.

Electrochemical performance and thermal cyclicability of industrial-sized anode supported planar solid oxide fuel cells

Industrial-sized planar anode-supported SOFC cells with a cell dimension of 15 × 15 × 0.1 cm and an active area of 13 × 13 cm2 are fabricated through a processing route of tape casting-screen printing-cofiring. Electrochemical performance and thermal cyclicability of the cell are evaluated in a single cell stack at 750 °C with H2 as the fuel and air as the oxidant. With a gas flow rate of 3 L min−1, the initial open circuit voltage (OCV) is 1164 mV, and the stack demonstrates a power density of 380 mW cm−2 at 473 mA cm−2 with an extrapolated maximum power output of 100 W. After three thermal cycles, the reduction of the stack output voltage is around 5%, and the OCV is maintained at a level above 1145 mV. Based on the obtained results, issues related to stack performance, such as cell quality, contact resistance and sealing, are discussed.

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.

Preparation of dual-pore anode supported Sc2O3-stabilized-ZrO2 electrolyte planar solid oxide fuel cell by phase-inversion and dip-coating

In this paper, we report a dual-pore anode supported Sc2O3-stabilized-ZrO2 (ScSZ) electrolyte planar SOFC with the cell configuration of Ni-YSZ|Ni-ScSZ|ScSZ|LSM-ScSZ|LSM prepared by a combination of phase-inversion and dip-coating. The Ni-YSZ anode substrate is fabricated by phase-inversion, exhibiting an asymmetric dual-pore structure. A Ni-ScSZ anode functional layer (AFL) is introduced between the ScSZ electrolyte and the Ni-YSZ anode substrate by dip-coating, a process aims at increasing the three-phase boundaries at the anode/electrolyte interface. A single cell with this unique anode structure is successfully fabricated, demonstrating maximum power densities of 0.76, 1.04, 1.32 and 1.97 W cm−2 at 650, 700, 750, and 800 °C, respectively, with humidified (3vol% H2O) hydrogen as the fuel and oxygen as the oxidant. The microstructure morphologies of the SOFC are examined by SEM and comparisons of gas permeability for dual-pore and conventional anodes are also investigated.

High performance tubular solid oxide fuel cells with BSCF cathode

Compared with planar SOFCs, tubular SOFCs have the advantages of facile sealing and easy scale up. In this study, a tubular anode-supported thin-film YSZ electrolyte SOFC was successfully fabricated by extrusion of the anode substrate in combination with wet powder spraying of the electrolyte layer. The fuel cell performance was tested by applying either hydrogen or methane as the fuel and ambient air as the cathode atmosphere. Peak power densities of 432 and 145 mW cm−2 were achieved respectively at 800 and 600 °C for a cell with conventional LSM and silver wire as the cathode and the current collector. The performance was further improved by adopting BSCF cathode with an SDC buffer layer. EIS demonstrated the large electrode polarization resistance is the main source of cell resistance, while the contact resistance is also not negligible. The performance of the cell directly operating on methane fuel was also investigated. A special operation mode by alternatively operating on methane and hydrogen fuels was adopted, which can effectively increase the cell operational stability.

Quantitative Analysis of Gas Flow in a Planar SOFC

Computational Fluid Dynamics (CFD) method was adopted to establish the three dimensional model for cathode gas channel of planar SOFC and simulate the gas flow in the channel of a single cell. A quantitative analysis method was presented to research the status of gas flow. The characteristics and diagram of velocity change were obtained by using the quantitative analysis method. Furthermore, it was found that gas leak has a negative effect on the uniformity of gas flow, which is more obvious in the region near the leak side.

J056056 燃料極支持型SOFC発電ユニットの燃料極側集電/ガス供給構造の検討

In power generation with large-electrode SOFC, not only its cell property but also a function of current-collecting and gas-feeding to its three-phase boundary is one of the dominant factors for its power-generating performance. In this study, we focused on current-collecting and gas-feeding structure of anode side that composed of porous metal with a thickness of 0.3 mm and 1.3 mm in planar anode-supported SOFC with electrode area of 95cm2. In this paper, we firstly report on 2-D flow-velocity distribution in the current-collecting and gas-feeding structure with a fuel inlet at the center of 120-mm-diameter electrode. It is clarified that gas flow velocity is larger than 1 mm/s at more than 99% of its electrode area. Moreover, we report on power generating properties with the present structure. It is clarified that the current-collecting and gas-feeding structure of anode side composed of 1.3-mm-thick porous nickel is capable of maintaining a power generating efficiency of 48% (DC, LHV) and an output power of 40W for a thousand hours.