Engineering porous materials for fuel cell applications

Porous materials play an important role in fuel cell engineering. For example, they are used to support delicate electrolyte membranes, where mechanical integrity and effective diffusivity to fuel gases is critical; they are used as gas diffusion layers, where electronic conductivity and permeability to both gas and water is critical; and they are used to construct fuel cell electrodes, where an optimum combination of ionic conductivity, electronic conductivity, porosity and catalyst distribution is critical. The paper will discuss these characteristics, and introduce the materials and processing methods used to engineer porous materials within two of the leading fuel cell variants, the solid oxide fuel cell and the polymer electrolyte membrane fuel cell.     

Symmetric shear test of glass-ceramic sealants at SOFC operation temperature

Glass-ceramics of the BaO–CaO–Al₂O₃–SiO₂ system are frequently used in planar solid oxide fuel cells (SOFC) stacks to seal the fuel and air compartments and to join non-conductively the individual components. Due to the thermal mismatch of the ceramic and metallic materials in the stack, the seals experience predominantly shear stresses. A symmetric shear test has been developed to characterize the critical shear stress of the glass-ceramic at SOFC operation temperature. Specimens representative for the seal situation in an SOFC stack were prepared, using the glass-ceramic to join a center piece of a NiO-YSZ anode covered by yttria-stabilized zirconia (YSZ) electrolyte layers on both surfaces between two Crofer22APU interconnect steel blocks. Shear stress and based on a rheological model, shear modulus and viscosity of the sealant were determined. The investigations showed that the sealant exhibits viscous shear deformation at 800 °C, a temperature typical for SOFC operation. The influence of increasing crystallization on the shear deformation is demonstrated.     

Investigations of metallic alloys for use as interconnects in solid oxide fuel cell stacks

Some high-temperature alloys have been investigated in order to determine whether they are suitable as metallic interconnect materials in solid oxide fuel cell stacks. The requirements for such alloys are formulated. Thermal dilatometry and oxidation tests, as well as theoretical calculations of the stresses that are induced by differences in thermal expansion of the individual materials, have been performed. The results show that a chromium-rich alloy, with dispersions of fine Y₂O₃ particles, perform best among the samples investigated. Improvements are still needed in order to make the alloy fully applicable in a solid oxide fuel cell stack. Some suggestions for improvements are put forward.     

Study of ceria-carbonate nanocomposite electrolytes for low-temperature solid oxide fuel cells

Composite and nanocomposite samarium doped ceria-carbonates powders were prepared by solid-state reaction, citric acid-nitrate combustion and modified nanocomposite approaches and used as electrolytes for low temperature solid oxide fuel cells. X-ray Diffraction, Scanning Electron Microscope, low-temperature Nitrogen Adsorption/desorption Experiments, Electrochemical Impedance Spectroscopy and fuel cell performance test were employed in characterization of these materials. All powders are nano-size particles with slight aggregation and carbonates are amorphous in composites. Nanocomposite electrolyte exhibits much lower impedance resistance and higher ionic conductivity than those of the other electrolytes at lower temperature. Fuel cell using the electrolyte prepared by modified nanocomposite approach exhibits the best performance in the whole operation temperature range and achieves a maximum power density of 839 mW cm(-2) at 600 degrees C with H2 as fuel. The excellent physical and electrochemical performances of nanocomposite electrolyte make it a promising candidate for low-temperature solid oxide fuel cells.     

玻璃和云母复合封接SOFC电池堆性能

封接技术是影响平板式固体氧化物燃料电池(SOFC)发展的关键技术之一。实验中用云母和Bi2O3-BaO-SiO2-RxOy(R=K,Zn,Al2O3,etc.)玻璃复合,将电解质(氧化钇稳定氧化锆,YSZ)支撑的电池和金属连接体(SUS430不锈钢)封接在一起,对封接后电池堆的封接性能和开路电压以及各组元热膨胀性能进行测试。结果表明:云母在室温到720℃的平均热膨胀系数为8.5×10-6 K-1,Bi2O3-BaO-SiO2-RxOy玻璃20℃到520℃的平均热膨胀系数为11.0×10-6/K,与YSZ和金属连接体匹配。云母的层状结构可以缓解因热膨胀系数不同而产生的应力,在高温状态下云母还能起到固定软化玻璃的作用。通过气密性和电性能测试,在电池堆工作状态下气密性良好,在操作温度为800～900℃下运行28小时,电池堆的开路电压(OCV)维持在1.0V以上,复合封料及其两边材料中的元素没有明显扩散。因此,云母和玻璃Bi2O3-BaO-SiO2-RxOy复合封接技术可适用于高温SOFC的封接。     

质子陶瓷膜燃料电池电解质材料的研究进展

质子陶瓷膜燃料电池作为固体氧化物燃料电池低温工作的一种有效途径而受到了广泛的关注.本文介绍了以高温质子导体为电解质的质子陶瓷膜燃料电池的进展,指出传统质子陶瓷膜燃料电池较差的化学稳定性是阻碍其发展的重要因素.重点评述了近期化学稳定性好的高温质子导体电解质材料的发展以及新的掺杂体系对于经典BaCeO3基质子导体在化学稳定性、电导率和烧结活性等方面的作用,分析了高温质子导体作为电解质材料在质子陶瓷膜燃料电池发展中存在的问题和发展的方向.     

基于热应力分析的固体氧化物燃料电池阳极功能层优化设计

为了降低固体氧化物燃料电池在制备和工作过程中产生的热应力, 提高电池的电化学性能, 在电池中引入功能梯度层可以有效减小电池各层之间材料参数的差异, 从而缓解各层之间的热失配应力。本研究将阳极功能层引入燃料电池中, 通过阳极功能层子层数目和非线性梯度成分指数n控制各子层材料属性的变化情况, 研究了燃料电池在800℃下的热应力分布。结果表明: 选取适当的指数n和阳极功能层的分层数目可以明显降低阳极层的最大拉应力和电解质层的最大压应力。     

Meso-scale stress response of thin ceramic membranes with honeycomb support

Planar solid oxide fuel cells are made up of repeating sequences of electrolytes, electrodes, seals, and current collectors. For electrochemical reasons it is best to keep the electrolyte as thin as possible. However, for electrolyte-supported cells, the thin electrolytes are susceptible to damage during production, assembly, and operation. One of the latest generation electrolytes employs a meso-scale honeycomb layer to support thin, electrochemically efficient membranes. Using finite element analysis, a two-scale model computes distributions of first principal stresses throughout a representative unit cell of the meso-scale structure. Displacement at the macro-scale is informed by meso-scale geometry via a homogenized equivalent stiffness, while the stresses at the two scales are related via a scalar magnification factor. The magnification factor is computed for a variety of geometries and loading conditions. Physical specimens are measured in tension to obtain an experimental magnification factor which agrees well with the simulations. When both the stiffness and magnification factor for a given meso-scale pattern are known, the macro-scale geometry can be analyzed without revisiting the meso-scale model, thus reducing computational time and costs.     

Solid oxide fuel cell with corrugated thin film electrolyte

A low temperature micro solid oxide fuel cell with corrugated electrolyte membrane was developed and tested. To increase the electrochemically active surface area, yttria-stabilized zirconia membranes with thickness of 70 nm were deposited onto prepatterned silicon substrates. Fuel cell performance of the corrugated electrolyte membranes released from silicon substrate showed an increase of power density relative to membranes with planar electrolytes. Maximum power densities of the corrugated fuel cells of 677 mW/cm2 and 861 mW/cm2 were obtained at 400 and 450 degrees C, respectively.     

Finite element analysis of the effects of comers on residual stresses in protective oxide scales

Finite element simulations are used to examine residual thermal stresses and strains in corner regions of protective Al₂O₃ scales on Fe₃Al specimens, both during cooling from oxide formation temperatures and during subsequent thermal cycling. The effects of a corner's radius of curvature and oxide thickness, as well as the impact of aluminide plasticity, are considered. Localized plasticity is found to have a major influence on net deformation and on the magnitude and location of maximum stress. As the ratio of corner curvature to oxide thickness (r_s/t) is reduced, stresses within the oxide corner shift from highly compressive to tensile and the location of the maximum principal stress moves from the substrate to the oxide scale. Based on these stress distributions prior to the development of any flaws, key implications about the tendencies for damage are addressed. The stress evolution during cooling and thermal cycling is presented; these results demonstrate the effects of temperature-dependent material properties. For the material behavior assumed in this study, thermal cycling does not cause significant stress relaxation.     

Material science and engineering: The enabling technology for the commercialisation of fuel cell systems

The critical role of materials science and engineering in the development of fuel cell technology is surveyed. The inability to fabricate reliable triple-phase-boundary (tbp) structures involving electrolytes, electronic conductors, and gaseous reactants, severely restricted the progress of fuel cells until about four decades ago (1960). However at the start of the new millennium, commercialisation of four fuel cell types: polymeric electrolyte membrane (PEMFC), phosphoric acid (PAFC), molten carbonate (MCFC), and solid oxide (SOFC), is now being very energetically pursued. Materials selection for each type of fuel cell is briefly examined, and the predominant engineering issues related to the development of commercial products are summarised. The fabrication, reliability, and cost of the relevant materials is of paramount importance to ensure rapid market penetration. The choice of fuel and relevant infrastructure is also considered, and the crucial role of materials for energy storage (particularly hydrogen) and fuel processing, is emphasised.     

Influence of microstructure on nano-mechanical properties of single planar solid oxide fuel cell in pre- and post-reduced conditions

The present work investigates, both the macro- and nano-mechanical properties of all the three component layers e.g., anode, cathode and electrolyte of a planar single solid oxide fuel cell (SOFC). The flexural fracture strength experiments in three point bending mode are employed in both pre- and post-reduced conditions to study the macro-mechanical failure behavior of the single cell. Further, the nanoindentation technique is utilized in both pre- and post-reduced conditions to evaluate the nanomechanical properties e.g. nanohardness, Young’s modulus, mean contact pressure, relative stiffness and relative spring back at scale in both pre- and post-reduced conditions. The nanohardness and Young’s modulus of the pre-reduced anode are considerably degraded after reduction as NiO gets converted to Ni. However, as expected; those of the pre-reduced electrolyte and cathode are only slightly decreased after reduction because there are no chemical conversions involved. Further, the experimentally obtained data of nanomechanical properties, is explained with the application of the well established Weibull statistics as the microstructures with characteristically present pores and defects are highly heterogeneous in nature. The characteristic values of the various nanomechanical properties are analyzed using Weibull distribution for the anode, electrolyte and cathode layers of the SOFC in both pre- and post-reduced conditions.     

固体氧化物燃料电池电化学与热分析耦合的二维模型

计算机模拟是燃料电池设计的一个重要辅助工具。本文在分析了燃料电池的电学和热学性质之后,设计了一套二维平板型团体氧化物燃料电池(SOFC)的模拟软件,软件能回答诸如温度场分布、电流场分布、输出功率等问题,作为应用例子,该软件被用于分析比较不同气流流向(交叉流、并流、对流)设计的燃料电池工作情况,指出了它们的优缺点,为燃料电池设计提供了有益的信息。     

电泳沉积及其在新型陶瓷工艺上的应用

介绍了电泳沉积的特点、悬浮液的稳定机制、电泳沉积的机理及动力学原理,并对该技术在制备固体表面陶瓷涂层、孔状结构陶瓷、多层及复合结构、固体氧化物电池、纳米材料及纳米结构陶瓷上的应用进行了总结。     

Porous Ni/TiO2 substrates for planar solid oxide fuel cell applications

Anode substrates based on Ni/TiO₂ cermets were fabricated for planar solid oxide fuel cells (SOFCs) with the aim of reducing the material costs and preventing thermoelastic bending of the currently used Ni/8YSZ-based cells. Ni/TiO₂ substrates were produced via sintering of NiO/NiTiO₃ powder compacts and subsequent reduction. The sintering behavior in air and the resulting microstructure were studied in detail. Excellent electrical conductivity and gas permeability was achieved before and after reduction due to coarse microstructure. The co-firing behavior of substrates coated with an anode layer and an 8YSZ electrolyte membrane was analyzed with the aim of identifying those sintering schedules which give flat cells with a gastight electrolyte. Thermoelastic bending of cells is negligible since the thermal expansion coefficient is well adjusted to the 8YSZ electrolyte.     

Processing and Quality Control of Planar SOFC Components

The implementation of a PC‐based quality control system for the manufacturing of anode‐supported solid oxide fuel cells (SOFCs) is a prerequisite for achieving reproducibility and high quality of the cells. To reach that goal a broad database for each manufacturing step including materials and processes is needed. This database is generated on the basis of the well developed manufacturing route of the Jülich planar SOFC. This paper focuses on first results obtained through the BREDA database system (Brennstoffzellen‐Datenbank, Database for Fuel Cells).     

Nanocrystals of zirconia- and ceria-based solid electrolytes: Syntheses and properties

Nano-effect on solid electrolytes that appears as a conductivity enhancement is expected as a way to develop or improve practical solid ionic devices, such as solid oxide fuel cells. Interfaces play a major role in the enhanced conductivity. Using nanocrystals (NCs) as the starting material, the nanostructured materials containing many interfaces, i.e., grain boundaries, can be simply made by forming the NCs with successive sintering without grain growth. The past decade has seen significant advances in the syntheses of solid electrolyte NCs and understanding the nano-effects on the ionic conductivity. In the present paper, the syntheses of zirconia- and ceria-based NCs, which are important constituent materials of solid oxide fuel cells, and their grain size-dependent conductivity due to the nano-effect are briefly reviewed.     

Nanocrystals of zirconia- and ceria-based solid electrolytes: Syntheses and properties

Nano-effect on solid electrolytes that appears as a conductivity enhancement is expected as a way to develop or improve practical solid ionic devices, such as solid oxide fuel cells. Interfaces play a major role in the enhanced conductivity. Using nanocrystals (NCs) as the starting material, the nanostructured materials containing many interfaces, i.e., grain boundaries, can be simply made by forming the NCs with successive sintering without grain growth. The past decade has seen significant advances in the syntheses of solid electrolyte NCs and understanding the nano-effects on the ionic conductivity. In the present paper, the syntheses of zirconia- and ceria-based NCs, which are important constituent materials of solid oxide fuel cells, and their grain size-dependent conductivity due to the nano-effect are briefly reviewed.     

全对称构型金属支撑固体氧化物燃料电池

与传统的全陶瓷结构的固体氧化物燃料电池(Solid Oxide Fuel Cell, SOFC)相比, 金属支撑固体氧化物燃料电池(MS-SOFCs)具有材料成本低, 结构稳定性高, 抗热震性高等优点。为了促进SOFC的商业化, 采用流延-烧结-浸渗工艺制备了Ce_0.8Sm_0.2O_2-δ(SDC)-430L阳极/Zr_0.88Sc_0.22Ce_0.01O_2.12(SSZ)电解质/SDC-430L阴极构型的全对称结构金属支撑固体氧化物燃料电池(MS-SOFC)。电池以湿氢气为燃料、空气为氧化气, 在600、650和700℃时的最大功率密度为220、250和280 mW/cm²。电化学阻抗谱的测试表明, 电池的性能由SDC-430L电极的极化阻抗所主导, 在700、650和600℃时, 电池欧姆阻抗分别为0.16、0.21和0.29 Ω•cm², 极化阻抗分别为0.67、0.90 和1.22 Ω•cm²。与阳极相比, 阴极的极化阻抗更为显著。对称SDC-430L电池在3%H₂O-97%H₂和空气气氛中测得的极化阻抗分别为0.23和1.92 Ω•cm² (650℃)。进一步优化电池结构(例如采用更加精细的430L骨架)和催化材料(例如含有Ag、Pt的复合材料)将有助于提升该MS-SOFC的电化学性能。