A non-sealed solid oxide fuel cell micro-stack with two gas channels

Non-sealed solid oxide fuel cell (NS-SOFC) micro-stacks with two gas channels were fabricated and operated successfully under various CH₄/O₂ gas mixtures in a box-like stainless-steel chamber. The cells with an anode-facing-cathode configuration were connected in serial by zigzag sliver sheets. Each cell consisted of the Ni/yttria-stabilized zirconia (YSZ) anode, the YSZ electrolyte, and the Sm_0.2Ce_0.8O_1.9-impregnated (La_0.75Sr_0.25)_0.95MnO₃ cathode. In this configuration, to ensure the identical gas distribution over the electrode surfaces, two gas channels with small vents flanking the stacks were used as gas channels of methane and oxygen for anodes and cathodes, respectively. The selectivity requirement of both the anode and cathode for the oxidation and reduction of CH₄ and O₂ was lowered and the sheets could extend the residence time of gas flow over the electrode surface. By the direct flame heat with a liquefied petroleum gas burner, the stacks presented a rapid start-up and full utilization of the exhaust gas. Eventually, an open-circuit voltage (OCV) of 1.8 V and maximum power output of 276 mW was produced by a two-cell stack. For a four-cell stack, a maximum power output of 373 mW was obtained.     

An error in solid oxide fuel cell stack modeling

This paper points out an error in the literature and analyzes its effect on electrochemical models of solid oxide fuel cell stacks. A correction is presented.     

Effects of crystallization on the high-temperature mechanical properties of a glass sealant for solid oxide fuel cell

Effects of crystallization on the high-temperature mechanical properties of a newly developed silicate-based glass sealant (GC-9) are investigated for use in planar solid oxide fuel cell (pSOFC). The aged, crystallized GC-9 glass is produced by heat treatment of the original GC-9 glass at 900 °C for 3 h. Not only crystalline phases are formed but the residual glass is also changed in the aged GC-9 glass after the heat treatment. Mechanical properties of the aged GC-9 glass are determined by four-point bending technique at temperature from 25 °C to 750 °C. The glass transition temperature of the given glass is reduced but the softening temperature is increased by such a crystallization heat treatment. The aged GC-9 glass exhibits a greater flexural strength and Young's modulus than the non-aged one at temperature below 650 °C due to the existence of crystalline phases. At temperature of 700 °C and 750 °C, a greater extent of stress relaxation is found in the aged GC-9 glass such that its strength and stiffness are much lower than those of the non-aged one. The changes in the thermal and mechanical properties through the given aging treatment are favorable for application of the GC-9 glass sealant in pSOFC.     

High-temperature mechanical properties of a solid oxide fuel cell glass sealant in sintered forms

High-temperature mechanical properties of a silicate-based glass sealant (GC-9) for planar solid oxide fuel cell have been studied in sintered forms. Ring-on-ring biaxial flexural tests are carried out at room temperature to 800 °C for the sintered GC-9 glass. The results are also compared with those in cast bulk forms. From the force-displacement curves, the glass transition temperature (T_g) of the non-aged, sintered GC-9 glass is estimated to be between 700 °C and 750 °C, while that of the aged one is between 750 °C and 800 °C. Due to a crack healing effect of the residual glass at high temperature, the flexural strength of the sintered GC-9 glass at temperature of 650 °C to T_g point is greater than that at room temperature. At temperature above T_g, the flexural strength and stiffness are considerably reduced to a level lower than the room-temperature one. The sintered GC-9 glass with pores and crystalline phases has a flexural strength lower than the cast bulk one at temperature of 650 °C and below. Due to a greater extent of crystallization, the flexural strength and stiffness of the sintered GC-9 glass are greater than those of the cast bulk one at 700-800 °C.     

Effects of operations and structural parameters on the one-cell stack performance of planar solid oxide fuel cell

A three-dimensional mathematical model coupling the electrochemical kinetics with fluid dynamics is developed to simulate the heat and mass transfer in the one-cell stack of planar solid oxide fuel cells (SOFCs). Based on flow uniformity analysis, the distributions of temperature, current density, overpotential loss and other performance parameters in various operating parameters are obtained using a commercial CFD code (Fluent) coupled with the external subroutines programmed by VC++. Numerical flow data are observed in good agreement with experimental results reported in the literature. Results show that the one-cell stack in counter flow case has the advantages in better uniform current density and temperature distributions of PEN (Positive/Electrolyte/Negative) structure in the width direction, higher power output, fuel utilization factor and fuel efficiency than that in co-flow case. For counter flow case, better thermoelectric characteristics are observed in the temperature gradient, power output, fuel utilization factor and fuel efficiency with the decrease in the fuel inlet flow rate or the anode porosity. Increasing the air inlet flow rate and decreasing the fuel inlet temperature will reduce the temperature gradient; power output, fuel utilization factor and fuel efficiency are enhanced with the increase of the air inlet temperature and the decrease of the anode pore size and thickness.     

Micro-tubular, solid oxide fuel cell stack operated under single-chamber conditions

A micro-tubular, solid oxide fuel cell stack has been developed and operated under single-chamber conditions. The stack, made of three single-cells, arranged in triangular configuration, was operated between 500 and 700 °C with varying methane/air mixtures. The results show that the operating conditions for the stack differ significantly than the single-cell operation reported in our earlier study. The stack operated at 600 °C with methane/oxygen mixture of 1.0 gives stable performance for up to 48 h, whereas for the single-cell, this mixing ratio was not suitable. The increase in the inert gas flow rate improves the stack performance up to a certain extent, beyond that; the power output by the stack reduces due to extensive dilution of the reactants. It is concluded that both, the operating conditions and the addition of inert gas, need to be tuned according to the number of cells present within the stack.     

Monolithic flat tubular types of solid oxide fuel cells with integrated electrode and gas channels

A new monolithic solid oxide fuel cell (SOFC) design stacked with flatten tubes of unit cells without using metallic interconnector plate is introduced and evaluated in this study. The anode support is manufactured in a flat tubular shape with fuel channel inside and air gas channel on the cathode surface. This design allows all-ceramic stack to provide flow channels and electrical connection between unit cells without needing metal plates. This structure not only greatly reduces the production cost of SOFC stack, but also fundamentally avoids chromium poisoning originated from a metal plate, thereby improving stack stability. The fuel channel was created in the extrusion process by using the outlet shape of mold. The air channel was created by grinding the surface of pre-sintered support. The anode functional layer and electrolyte were dip-coated on the support. The cathode layer and ceramic interconnector were then spray coated. The maximum power density and total resistance of unit cell with an active area of 30 cm² at 800 °C were 498 mW/cm² and 0.67 Ωcm², respectively. A 5-cell stack was assembled with ceramic components only without metal plates. Its maximum power output at 750 °C was 46 W with degradation rate of 0.69%/kh during severe operation condition for more than 1000 h, proving that such all-ceramic stack is a strong candidate as novel SOFC stack design.     

Thermal stress analysis of planar solid oxide fuel cell stacks: Effects of sealing design

A three-dimensional multi-cell model based on a prototypical, planar solid oxide fuel cell (pSOFC) stack design using compliant mica-based seal gaskets was constructed in this study to perform comprehensive thermal stress analyses by using a commercial finite element analysis (FEA) code. Effects of the applied assembly load on the thermal stress distribution in the given integrated pSOFC stack with such a compressive sealing design were characterized. A comparison was made with a previous study for a similar comprehensive multi-cell pSOFC stack model but using only a rigid type of glass-ceramic sealant instead. Simulation results indicate that stress distributions in the components such as positive electrode-electrolyte-negative electrode (PEN) plate, PEN-supporting window frame, nickel mesh, and interconnect were mainly governed by the thermal expansion mismatch rather than by the applied compressive load. An applied compressive load of 0.6 MPa could eliminate the bending deformation in the PEN-frame assembly plate leading to a well joined structure. For a greater applied load, the critical stresses in the glass-ceramic and mica sealants were increased to a potential failure level. In this regard, a 0.6 MPa compressive load was considered an optimal assembly load. Changing the seal between the connecting metallic PEN-supporting frame and interconnect from a rigid type of glass-ceramic sealant to a compressive type of mica gasket would significantly influence the thermal stress distribution in the PEN plate. The critical stress in the PEN was favorably decreased at room temperature but considerably increased at operating temperature due to such a change in sealing design. Such differences in the stress distribution could be ascribed to the differences in the constrained conditions at the interfaces of adjacent components under various sealing designs.     

固体氧化物燃料电池与燃气轮机混合发电系统

基于固体氧化物燃料电池系统的高效率、环保性以及排气废热的巨大利用潜能,将其与燃气轮机组成混合发电装置,是一种极有前景的分布式发电方案.文章以SWP公司的加压型SOFC-小型燃气轮机混合循环系统为例,对固体氧化物燃料电池及燃气轮机混合循环系统的原理及发展现状作了分析,为我国固体氧化物燃料电池-燃气轮机混合循环系统的研制提供参考.     

The impact of flow distributors on the performance of planar solid oxide fuel cell

This paper presents a newly established testing rig for planar solid oxide fuel cell. Two sets of nearly identical single-cell stacks except using different designs of flow distributors are measured to show how exactly the cell performance of such single-cell stacks would vary with a change in the degree of flow uniformity. It is found that by using small guide vanes around the feed header of commonly used rib-channel flow distributors to improve effectively the degree of flow uniformity, the power density of the single-cell stack can be increased by 10% as compared to that without using guide vanes under exactly the same experimental conditions. Also discussed are the start-up procedure and effects of hydrogen and air flow rates varying from 0.4 slpm to 1 slpm on cell performance of these two single-cell stacks which are measured over a range of the operating temperature varying from 650 °C to 850 °C. After 100 h of continuous cell operation, the examination of the reduction and oxidation stability of the anodic surface reveals that the improvement of flow uniformity in flow distributors is useful to achieve a more balanced use of the anodic catalyst.     

Experimental characterization of glass-ceramic seal properties and their constitutive implementation in solid oxide fuel cell stack models

This paper discusses experimental determination of solid oxide fuel cell (SOFC) glass-ceramic seal material properties and seal/interconnect interfacial properties to support development and optimization of SOFC designs through modeling. Material property experiments such as dynamic resonance, dilatometry, flexure, creep, tensile, and shear tests were performed on PNNL's glass-ceramic sealant material, designated as G18, to obtain property data essential to constitutive and numerical model development. Characterization methods for the physical, mechanical, and interfacial properties of the sealing material, results, and their application to the constitutive implementation in SOFC stack modeling are described.     

Fabrication and operation of a 6 kWe class interconnector-type anode-supported tubular solid oxide fuel cell stack

A 6 kW class interconnector-type anode-supported tubular solid oxide fuel cell (ICT SOFC) stack is fabricated and operated in this study. An optimized current-collection method, which the method for current collection at the cathode using the winding method and is the method for the connection between cells using interconnect, is suggested to enhance the performance of the fabricated cell. That method can increase the current collection area because of usage of winding method for cell and make the connection between cells easy. The performance of a single cell with an effective electrode area of 205 cm² exhibits 51 W at 750 °C and 0.7 V. To assemble a 1 kW class stack, the prepared ICT SOFC cells are connected in series to 20 cells connected in parallel (20 cells in series × two in parallel, 20S2P). Four modules are assembled for a 6 kWe class stack. For one module, the prepared ICT SOFC cells are connected in series to 48 cells, in which one unit bundle consists of two cells connected in parallel. The performance of the stack in 3% humidified H₂ and air at 750 °C exhibits the maximum electrical power of 7425 W.     

New interconnector design optimization to balance electrical and mechanical performance of solid oxide fuel cell stack

Mechanical stability and integrity are the pre-requisites for the long-term stable high power output of solid oxide fuel cell (SOFC) stacks. However, most of the previous research concentrated on improving the electrochemical performance of SOFC stacks, while the mechanical stability is rarely studied. In this study, a three-dimensional electro-thermo-mechanical coupled model is established to study the impact of interconnector (IC) structure on electrical performance and mechanical stability of SOFC simultaneously. It reveals that IC design with discrete ribs can enhance the maximum power density by up to 12.96%. The maximum principal stress value of positive electrode-electrolyte-negative electrode (PEN) is slightly influenced by IC design, while the stress distribution characteristic is obviously dominated by geometrical structure of IC. Compared with symmetrically arranged ICs at anode and cathode side, the unsymmetrical IC design with regularly discrete cubic, staggered discrete cubic, and discrete cylindrical ribs at cathode side and traditional IC design at anode side can respectively decrease the thermal stress of IC by 19.31%, 6.39%, and 12.09%, while the thermal stress of IC can be further released by 29.44% and 16.44% by rounding the corners of regularly arranged, and staggered distributed cubic ribs. By using new IC design, the failure probability of PEN is reduced by up to 28.97%, while increased by 8.37% only for the case with traditional IC at anode side and staggered cubic ribs at cathode side. To balance the electrical power output and mechanical stability, the discrete cylindrical ribs and discrete cubic ribs with rounded corners are better choices.     

Effect of thermal cycling on durability of a solid oxide fuel cell stack with external manifold structure

A 5-cell stack with external manifold is thermal cycled between room temperature and 750 °C fifteen times. The electric performances after each cycle are measured and compared. The stack has an initial peak output of 328.44 W and shows excellent stability in thermal cycling. The average operating voltage degradation rate is only 0.8% corresponding each thermal cycle. A cell from the stack is randomly chosen for electrochemical evaluation. Its performance is found to be comparable to a cell which is not thermal cycled. Post-test examination shows deterioration of cathode contact materials at points of contact and cracks throughout the oxide layer between corrugated and bipolar plates to be the main causes of the degradation.     

Development of 1 kW class solid oxide fuel cell stack using anode-supported planar cells

We have developed a 1 kW class solid oxide fuel cell (SOFC) stack composed of 50 anode-supported planar 120-mm-diameter SOFCs. Intermediate plates, which exhibited negligible deformation under operating conditions, were placed in the stack to cancel out the cumulative error related to the position and angle of the stack parts. The stack provided an electrical conversion efficiency of 54% (based on the lower heating value (LHV) of the methane used as a fuel) and an output of 1120 W when the fuel utilization, current density, and operating temperature were 67%, 0.28 A cm⁻², and 1073 K, respectively. The stack operated stably for almost 700 h.     

Development of a high power density 2.5 kW class solid oxide fuel cell stack

We have developed a 2.5 kW class solid oxide fuel cell stack. It is constructed by combining 70 power generation units, each of which is composed of an anode-supported planar cell and separators. The power generation unit for the 2.5 kW class stack were designed so that the height of the unit were scaled down by 2/3 of that for our conventional 1.5 kW class stack. The power generation unit for the 2.5 kW class stack provided the same output as the unit used for the conventional 1.5 kW class stack, which means that power density per unit volume of the 2.5 kW class stack was 50% greater than that of the conventional 1.5 kW class stack.     

Multi-scale design simulation of a novel intermediate-temperature micro solid oxide fuel cell stack system

This paper presents a multi-scale simulation technique for designing a novel intermediate-temperature planar-type micro solid oxide fuel cell (mSOFC) stack system. This multi-scale technique integrates the fundamentals of molecular dynamics (MD) and computational fluid dynamics (CFD). MD simulations are carried out to determine the optimal composition of a potential electrolyte that is capable of operation in the intermediate-temperature region without sacrifice in performance. A commercial CFD package plus a self-written computational electrochemistry code are employed to design the fuel and air flow systems in a planar five-cell stack, including the preheating manifold. Different samarium-doped ceria (SDC) electrolyte compositions and operating temperatures from 673 K to 1023 K are investigated to identify the maximum ionic conductivity. The electrochemical performance simulation using an available 5-cell yittria-stablized-zirconia (YSZ) mSOFC stack shows good agreement with our experimental results. The same stack design is used to predict a novel SDC-mSOFC performance. Feasibiulity studies of this intermediate-temperature stack are presented using this multi-scale technique.     

Flow uniformity optimization for large size planar solid oxide fuel cells with U-type parallel channel designs

The U-type parallel channels for large size planar solid oxide fuel cell (pSOFC) are systematically optimized with the computational fluid dynamics (CFD) method. The CFD calculations are based on realistic 3D gas channel models and typical pSOFC working parameters. The optimized geometric parameters include the height of interconnect ribs, aspect ratios of the inlet-header and outlet-header cross-sections, the sum of inlet- and outlet-header widths and the ratio of the outlet-header width to the inlet-header width (α). Detailed CFD calculations show that a suitable α and a relatively large header width are necessary for the flow uniformity of both air and fuel in large size pSOFCs. In particular, α is demonstrated to be a key parameter for the flow uniformity of large size pSOFCs with U-type parallel channel designs and a proper choice of α is of critical engineering importance. The physical origin for the importance of α on the flow distribution is analyzed.     

Performance analysis of a solid oxide fuel cell with reformed natural gas fuel

In the present study a two‐dimensional model of a tubular solid oxide fuel cell operating in a stack is presented. The model analyzes electrochemistry, momentum, heat and mass transfers inside the cell. Internal steam reforming of the reformed natural gas is considered for hydrogen production and Gibbs energy minimization method is used to calculate the fuel equilibrium species concentrations. The conservation equations for energy, mass, momentum and voltage are solved simultaneously using appropriate numerical techniques. The heat radiation between the preheater and cathode surface is incorporated into the model and local heat transfer coefficients are determined throughout the anode and cathode channels. The developed model has been compared with the experimental and numerical data available in literature. The model is used to study the effect of various operating parameters such as excess air, operating pressure and air inlet temperature and the results are discussed in detail. The results show that a more uniform temperature distribution can be achieved along the cell at higher air‐flow rates and operating pressures and the cell output voltage is enhanced. It is expected that the proposed model can be used as a design tool for SOFC stack in practical applications. Copyright © 2009 John Wiley & Sons, Ltd.