Microstructure and Electrical Conductivity of Atmospheric Plasma-Sprayed LSM/YSZ Composite Cathode Materials

Yttria stabilized zirconia and lanthanum strontium manganate (YSZ/LSM) have been employed to fabricate the composite cathode layer for solid oxide fuel cells (SOFCs). In the present study, the YSZ/LSM composite coating was deposited by atmospheric plasma spray (APS) using the mechanical blending LSM and YSZ with ratios of 50:50, 40:60, and 20:80 wt.%. The electrical conductivity of the composite coating was measured by the means of direct current (DC) measurement in the temperature range of 500-900 °C. The electrical conductivity of the YSZ-50%LSM coating ranged from 2.17 to 3.60 S/cm along the direction parallel to the coating surface at the temperature range. For the same specimen, the electrical conductivity perpendicular to the plane is less than one-tenth of that in the plane. The anisotropy of the electrical conductivity is attributed to the phases of different properties in the composite coating and the APS coating structure characteristics. The results also showed that the electrical conduction of the composite was strongly influenced by the YSZ content. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Influence of characteristics of stabilized zirconia electrolyte on performance of cermet supported tubular SOFCs

Ni-Al2O3 cermet supported tubular SOFC was fabricated by thermal spraying. Flame-sprayed Al2O3-Ni cermet coating plays dual roles of a support tube and an anode current collector. 4.5mol.% yttria-stabilized zirconia (YSZ) and 10mol.% scandia-stabilized zirconia (ScSZ) coatings were deposited by atmospheric plasma spraying (APS) as the electrolyte in present study. The electrical conductivity of electrolyte was measured using DC method. The post treatment was employed using nitrate solution infiltration to densify APS electrolyte layer for improvement of gas permeability. The electrical conductivity of electrolyte and the performance of single cell were investigated to optimize SOFC performance. The electrical conductivity of the as-sprayed YSZ and ScSZ coating is about 0.03 and 0.07 S·cm-1 at 1000 ℃, respectively. The ohmic polarization significantly influences the performance of SOFC. The maximum output power density at 1000 ℃ increases from 0.47 to 0.76 W·cm-2 as the YSZ electrolyte thickness reduces from 100 μm to 40 μm. Using APS ScSZ coating of about 40 μm as the electrolyte, the test cell presents a maximum power output density of over 0.89 W·m-2 at 1000 ℃.     

Microstructure and Electrochemical Behavior of a Structured Electrolyte/LSM-Cathode Interface Modified by Flame Spraying for Solid Oxide Fuel Cell Application

Plasma-sprayed yttria-stabilized zirconia (YSZ) can be potentially employed as electrolyte layers in solid oxide fuel cells (SOFCs). The formation of a structured electrolyte surface characterized by convex micro deposits generated by only partially molten particles at spraying will increase the specific surface area and subsequently improve the output performance of SOFCs. However, using completely molten YSZ particles during plasma spraying leads to the formation of locally flat surface. In this study, flame spraying was employed to deposit YSZ particles on YSZ substrate using surface-melted particles. The deposition was carried out at different spray distances on YSZ substrate preheated to 650 °C. The surface and cross-section morphology of YSZ particles were characterized by SEM. The electrochemical behavior of single cell with the structured cathode was characterized by the electrochemical impedance spectroscopy. The results show that spray distance exhibits significant influence on the morphology of deposited YSZ particles. The cathode polarization of a structured cathode was decreased by about 30-43% compared to a flat cathode at different temperatures.     

Manufacturing of high performance solid oxide fuel cells (SOFCs) with atmospheric plasma spraying (APS)

The potential of atmospheric plasma spraying (APS) technology has been investigated for the manufacture of anode, electrolyte and cathode of a solid oxide fuel cell. As the substrate a tape-casted FeCr alloy was used. It turned out that all layers can be applied by this technique, however, the APS cathode layer, although applied by suspension plasma spraying led to cells with rather low performance. Much better cell characteristics could be obtained by using screen-printed LSCF cathodes, which do not need any additional thermal treatment.Anode layers with high electrochemical activity were produced by separate injection of NiO and YSZ powders. The manufacturing of gastight electrolyte layers was a key-issue of the present development. As APS ceramic coatings typically contain microcracks and pores their leakage rate is not sufficiently low for SOFC applications.Based on the understanding of the formation of defects during spraying an optimized spraying process was developed which led to highly dense coatings with the appearance of a bulk, sintered ceramic. Open cell voltages above 1 V proofed the low leakage rates of the rather thin (< 50 μm) coatings. With these cells having a screen-printed cathode an output power of 500 mW/cm² could be achieved at 800 °C.It turned out that the long-term stability of the metal substrate based APS SOFCs was rather poor. The aging of the cells was probably due to interdiffusion of anode and substrate material. Hence, diffusion barrier was applied by APS between substrate and anode. These layers were very effective in reducing the degradation rate. For these cells the output power reached 800 mW/cm².     

Atmospheric Plasma Spraying Low-Temperature Cathode Materials for Solid Oxide Fuel Cells

Atmospheric plasma spraying (APS) is attractive for manufacturing solid oxide fuel cells (SOFCs) because it allows functional layers to be built rapidly with controlled microstructures. The technique allows SOFCs that operate at low temperatures (500-700 °C) to be fabricated by spraying directly onto robust and inexpensive metallic supports. However, standard cathode materials used in commercial SOFCs exhibit high polarization resistances at low operating temperatures. Therefore, alternative cathode materials with high performance at low temperatures are essential to facilitate the use of metallic supports. Coatings of lanthanum strontium cobalt ferrite (LSCF) were fabricated on steel substrates using axial-injection APS. The thickness and microstructure of the coating layers were evaluated, and x-ray diffraction analysis was performed on the coatings to detect material decomposition and the formation of undesired phases in the plasma. These results determined the envelope of plasma spray parameters in which coatings of LSCF can be manufactured, and the range of conditions in which composite cathode coatings could potentially be manufactured.     

Characterization of planer cathode-supported SOFC prepared by a dual dry pressing method

Cathode-supported solid oxide fuel cells (SOFCs), comprising porous (La_0.75Sr_0.25)_0.95MnO_3−δ (LSM) + Sm_0.2Ce_0.8O_1.9 (SDC) composite cathode substrate and 11 mol%Sc₂O₃-doped ZrO₂ (ScSZ) electrolyte membranes layer, were successfully fabricated via dual dry pressing method. NiO-SDC anode was prepared by slurry coating method. Phase characterizations and microstructures of electrolyte and cathode were studied by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). No interface reaction took place between LSM/SDC cathode substrate and ScSZ electrolyte layer after sintered at 1300 °C. The cell performances were measured at 800 and 750 °C, respectively, by changing the external load. The peak power densities were 0.228 and 0.133 W cm⁻², and the corresponding open-circuit voltages of the cell were 1.092 and 1.027 V at 800 and 750 °C, respectively. Impedance analysis indicated that the performances of the SOFCs were determined essentially by the composition and microstructure of the electrode.     

SOEC多层复合阳极的制备及性能研究

采用共沉淀-共沸蒸馏法合成锶掺杂的锰酸镧(LSM)粉体,在此基础上制备了LSM与钇稳定的氧化锆(YSZ)的复合材料,并研究了该材料应用于固体氧化物电解池(SOEC)阳极的性能.通过XRD、TEM、SEM等手段分析了该材料的化学稳定性及微观结构.通过动电位扫描以及电化学阻抗谱(EIS)研究了该阳极材料的电化学性能.TEM分析显示共沉淀-共沸蒸馏法在减小粉体粒径方面要优于传统的共沉淀方法.SEM结果显示经过1200 ℃,2 h的烧结后,复合阳极与电解质结合紧密,阳极材料内部孔隙均匀,YSZ与LSM两相各自形成连通的网络结构.对不同组成和不同结构的阳极复合材料的电化学性能进行了测试,结果显示多层的阳极结构增加了三相界面(TPB)的长度.     

Microstructure and polarization resistance of thermally sprayed composite cathodes for solid oxide fuel cell use

Porous composite cathode coatings containing (La_0.8 Sr_0.2)_0.98MnO₃ (LSM) and ZrO₂-12% Y₂O₃ (YSZ) were prepared by vacuum plasma spraying (VPS) and flame spraying (FS) on prefabricated substrate-based planar solid oxide fuel cells (SOFC) with 60 mm in diameter. Microstructural observations reveal the open porosity of the cathode coatings and prove qualitatively the compositional gradient from YSZ-LSM composite to pure LSM. The electrochemical behavior was investigated by impedance spectroscopy. The results of graded cathodes compared with nongradient and bilayered ones are discussed with respect to the cathodic polarization resistance between 750 and 950°C. Bilayered cathodes indicate the lowest cathodic losses followed by the graded ones and the conventional composite. Flame spraying as a rarely used processing tool for SOFC components can provide cathodes of high electrochemical performance.     

Microstructure and Electrochemical Behavior of La0.8Sr0.2MnO3 Deposited by Solution Precursor Plasma Spraying

采用液料等离子喷涂方法(SPPS)制备固体氧化物燃料电池多孔La0.8Sr0.2MnO3(LSM)阴极。用SEM观察LSM的微结构,用XRD研究其相结构。考察了喷涂距离和热处理温度对LSM微结构的影响规律。结果表明,SPPSLSM在1050℃热处理2h后形成连续的具有微纳介孔结构的涂层,且LSM具有单一的钙钛矿结构。利用电化学交流阻抗谱方法研究了LSM极化行为。微结构对极化性能有显著影响,1000℃时,LSM在喷涂距离为60mm时具有最佳的电化学性能,阴极极化电阻约为0.3Ω·cm2。通过工艺的控制,SPPS可以实现SOFC阴极相和微结构的优化。     

Development of a Ni/Al2O3 Cermet-Supported Tubular Solid Oxide Fuel Cell Assembled with Different Functional Layers by Atmospheric Plasma-Spraying

A cermet-supported tubular configuration amenable to preparation by a relatively low-cost thermal spraying process is proposed. An Al₂O₃-Ni cermet thick deposit prepared by flame spraying is employed as both support tube and anode current collector. Atmospheric plasma spraying (APS) has been employed to prepare the anode, cathode, and stabilized ZrO₂-based electrolyte with the aim of reducing manufacturing costs. Gas-tightness of the APS electrolyte has been achieved by a postdensification process. The effects of the densification process on the gas-tightness of the plasma-sprayed YSZ electrolyte and the open-circuit voltage of the SOFC have been investigated. The effects of the microstructures of the plasma-sprayed anode, electrolyte, and cathode on the performance of the SOFC test cell have been investigated.     

A facile and environment-friendly method to fabricate thin electrolyte films for solid oxide fuel cells

A facile and environment-friendly method, the so-called vertical deposition (abbreviated as VD) method, is used to prepare thin yttria-stabilized zirconia (YSZ) films (≤5 μm) for solid oxide fuel cells (SOFCs). The YSZ films are self-assembled by VD process based on capillary force. The influence of experimental conditions (e.g. concentration of YSZ dispersion, deposition times, and sintering procedure) on the morphology of the films produced and thereby on the performance of SOFC devices is investigated. The single cell utilizing a 5 μm dense YSZ film as solid electrolyte achieves a high open circuit voltage of 1.05 V which remains stable at 700 °C for 4 h. The peak power density is 0.4 W cm⁻² at 800 °C for the phase inversion anode-supported fuel cell composed of an YSZ electrolyte film of 5 μm thick. The VD method developed herein is promising for preparing ultra-thin electrolyte films for SOFCs.     

Advanced thermal spray technologies for applications in energy systems

Atmospheric plasma spraying technologies have been used to produce coatings for different applications in energy systems as gas turbines and solid oxide fuel cells (SOFCs). Thermal barrier coatings (TBCs) are widely used in gas turbines to protect structural parts from the combustion gases. Although they are in use since several decades there is still a large amount of research focused on a further improvement going on. Especially increased temperature capability, improved microstructures or optimized optical properties of the coatings will be described in the present paper. Atmospheric plasma spraying (APS) technology has also been used to manufacture different coatings for solid oxide fuel cell systems. These coatings include NiO/YSZ coatings for anodes, YSZ coatings for electrolytes and different functional coatings as Cr-evaporation layer on interconnects. Recent results on these different kinds of coatings will be shown. Performance data of SOFCs on metallic porous substrates will also be given.     

Conductivity study of porous yttria-doped zirconia and strontia-doped lanthanum manganite bilayer film by glancing angle deposition

Porous bilayer films of yttria-doped zirconia and strontia-doped lanthanum manganite are deposited by using electron beam evaporation. For practical use in solid oxide fuel cells, fully-stabilized zirconia is a candidate for the solid electrolyte due to its high oxygen diffusion rate. Longer triple phase boundary, which consists of catalyst, reacting gas and solid electrolyte in the cathode, is helpful for the exchange of oxygen gas and ions. By glancing angle deposition, higher density of triple phase boundary is achieved in the form of catalyst and electrolyte bilayer structure. This kind of triple phase boundary can be estimated from SEM images and it is rather easier than from conventional composite cathode which can only be analyzed with assistance of mathematic simulation. The resistance of this bilayer at 600 °C is ten times lower than porous catalyst single layer.     

Fabrication of porous lanthanum strontium manganite films by electrostatic spray deposition

In this work, electrostatic deposition was used to prepare porous lanthanum strontium manganite (LSM) films on a silicon substrate for cathode application in solid oxide fuel cells (SOFCs). The precursor solution was prepared by dissolving lanthanum nitrate hydrate, manganese nitrate hexahydrate and strontium chloride hexahydrate into a mixture of methanol and water. The morphology of the LSM film depended on process parameters such as substrate temperature, precursor flow rate, nozzle-substrate distance, and deposition time. The effect of heating temperature on the film’s crystal structure was investigated with X-ray diffraction in the heating temperature range of 700 °C to 900 °C. Porous LSM film was successfully prepared in the solution flow rate range of 2 l/min to 4.5 l/min, substrate temperatures of 127 °C to 330 °C, the nozzle-substrate distance range of 3 cm to 8 cm, and deposition times of 1 min to 16 min.     

等离子喷涂制备固体氧化物燃料电池电解质涂层研究进展EI北大核心CSCD

等离子喷涂作为一种高性价比的涂层沉积工艺,在固体氧化物燃料电池(SOFC)电解质制备方面比传统方式更灵活、高效,尤其在大面积电解质快速成形上,表现出良好的发展潜力。介绍SOFC的工作原理和研究趋势,综述电解质材料及等离子喷涂制备工艺的研究进展,指明等离子喷涂制备SOFC电解质涂层的发展方向。研究表明:氧化钇、氧化钪稳定的氧化锆是目前商业化应用最广泛的电解质材料,其他如氧化铈基及氧化铋基电解质还须解决还原气氛下价态变化问题,而镧锶镓镁氧化物和硅酸盐电解质则需解决成分和结构稳定性问题。在制备方面,传统湿化学法的高温烧结过程难以制备金属支撑型SOFC,磁控溅射和气相沉积等镀膜技术成本高、效率低,不适合电解质大规模生产。而等离子喷涂技术具有沉积效率高,对基体热输入小,可灵活调控涂层微观结构等优势。等离子喷涂SOFC电解质还存在较大探索空间,基于前期相关工作为后续中低温电解质制备及优化提供思路,随着电解质粉末成本下降及喷涂设备迭代升级,等离子喷涂技术有望在未来成为大规模高效制备SOFC电解质涂层的重要手段。     

Fabrication and study on Ni1−xFexO-YSZ anodes for intermediate temperature anode-supported solid oxide fuel cells

Bimetal oxides Ni_1−xFe_xO (x = 0.01, 0.04, 0.08, 0.1, 0.15, 0.2, 0.4, 0.5) were synthesized and studied as anodes for intermediate temperature solid oxide fuel cells (SOFCs) based on yttria-stabilized zirconia (YSZ) film electrolyte. A single cell consisted of Ni_1−xFe_xO-YSZ anode, YSZ electrolyte film, LSM–YSZ composite cathode was prepared and tested at the temperature from 600 °C to 850 °C with humidified hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant. It was found that the cell with Ni_0.9Fe_0.1O-YSZ anode showed the highest power density, 1.238 W cm⁻² at 850 °C, among the cells with different anode composition. The promising performance of Ni_1−xFe_xO as anode suggests that bimetal anodes are worth studied for SOFCs in future.     

Electrochemical properties of La0.8Sr0.2FeO3-δbased composite cathode for intermediate temperature SOFC

La0.8Sr0.2FeO3-δ is a new kind of cathode material for intermediate SOFC, but its electrochemical activity is relative poor for the lanthanum gallate based solid oxide fuel cell. In this paper, a novel composite cathode of La0.8Sr0.2FeO3-δ/La0.9 Sr0.1Ga0.8Mg0.2O3-δ was prepared on the LSGM electrolyte substrate by screen-printing method. The results of cathodic polarization measurements show that the overpotential decreases significantly when the composite cathode is used instead of the La0.8Sr0.2FeO3-δ single layer cathode. The cathodic overpotential of the composite La0.8Sr0.2FeO3-δ/La0.9Sr0.1Ga0.8 Mg0.2O3-δ cathode is 150 mV at the current density of 0.2 A·m-2 at 800 ℃, while the cathodic overpotential of the La0.8 Sr0.2 FeO3-δ single layer cathode is higher thaN260 mV at the same condition. The electrochemical impedance spectroscopy was employed to investigate the polarization resistance of the cathode. The polarization resistance of the composite cathode is 1.20 Ω·m2 in open circuit condition, while the value of the single La0.8 Sr0.2 FeO3-δ cathode is 1.235 Ω·m2.     

Performance and stability of La0.8Sr0.2MnO3 cathode promoted with palladium based catalysts in solid oxide fuel cells

The effect of catalyst loading, operation temperature and co-infiltration of the palladium-based catalysts on the performance and stability of La_0.8Sr_0.2MnO₃ (LSM) cathode of solid oxide fuel cells (SOFCs) is investigated. The result shows that adding a small amount of Pd catalyst (0.08 mg cm⁻²) has a remarkable effect on the reduction of overpotential of LSM cathodes and high palladium loading is detrimental to the electrochemical activity of LSM cathodes. The performance and stability of the Pd-impregnated LSM cathodes can be enhanced significantly by co-infiltration of palladium with either 20 mol% of silver or 5 mol% of cobalt. Increased stability of the co-infiltrated catalyst materials is probably related to the enhanced resistance of the co-impregnated Pd_0.95Co_0.05 and Pd_0.8Ag_0.2 nanoparticles against agglomeration and sintering at SOFC operation temperatures. The results indicate the co-impregnation is effective not only to enhance the electrochemical activity but also to improve the stability of LSM cathodes for the O₂ reduction reaction of SOFCs.     

Fabrication and performance of La0.8Sr0.2MnO3/YSZ graded composite cathodes for SOFC

The performance of multi-layer (1-x)La0.8Sr0.2MnO3/xYSZ graded composite cathodes was studied as electrode materials for intermediate solid oxide fuel cells (SOFC). The thermal expansion coefficient, electrical conductivity, and electrochemical performance of multi-layer composite cathodes were investigated. The thermal expansion coefficient and electrical conductivity decreased with the increase in YSZ content. The (1-x)La0.8Sr0.2MnO3/xYSZ composite cathode greatly increased the length of the active triple phase boundary line (TPBL) among electrode, electrolyte, and gas phase, leading to a decrease in polarization resistance and an increase in polarization current density. The polarization current density of the triple-layer graded composite cathode (0.77 A/cm2) was the highest and that of the monolayer cathode (0.13 A/cm2) was the lowest. The polarization resistance (Rp) of the triple-layer graded composite cathode was only 0.182Ω·cm2 and that of the monolayer composite cathode was 0.323Ω·cm2. The power density of the triple-layer graded composite cathode was the highest and that of the monolayer composite cathode was the lowest. The triple-layer graded composite cathode had superior performance.     

Gadolinia-doped ceria and yttria stabilized zirconia interfaces: regarding their application for SOFC technology

For solid oxide fuel cells (SOFCs) operating at intermediate temperatures the adjacency of the state-of-the-art yttria-stabilized zirconia (YSZ) electrolyte with ceria-based materials to both anodic and cathodic sides is regarded as crucial for the effectiveness of the cell. Solid-state reaction, however, and interdiffusion phenomena between YSZ and ceria-based materials can cause degradation of the electrolyte. When a gadolinia-doped-ceria (GDC) layer is used to protect YSZ against interaction with Co-containing cathodes, an unfavorable solid state reaction at the YSZ–GDC interface can be efficiently suppressed when a thin (≤1 μm thick) interlayer with nominal composition of Ce_0.43Zr_0.43Gd_0.10Y_0.04O_1.93 is incorporated at the interface. When ceria is to be employed at the electrolyte–anode interface to reduce polarization losses, use of a ceria–40% vol Ni cermet is recommended, since suppression of the reactivity between YSZ and ceria can also be achieved in the presence of Ni.