Novel layered perovskite GdBaCuFeO5+x as a potential cathode for proton-conducting solid oxide fuel cells

A layered perovskite GdBaCuFeO_5+x (GBCuF) was developed as a cathode material for intermediate-temperature solid oxide fuel cells based on a proton-conducting electrolyte of stable BaZr_0.1Ce_0.7Y_0.2O_3?δ (BZCY). The X-ray diffraction results showed that GBCuF was chemically compatible with BZCY after co-fired at 1,000 °C for 10 h. The thermal expansion coefficient of GBCuF, which showed a reasonably reduced value (15.1?×?10^?6 K^?1), was much closer to that of BZCY than the cobalt-containing conductor. The button cells of Ni–BZCY/BZCY/GBCuF were fabricated and tested from 500 to 700 °C with humidified H₂ (～3 % H₂O) as a fuel and ambient oxygen as the oxidant. A high open-circuit potential of 1.04 V, maximum power density of 414 mW cm^?2, and a low electrode polarization resistance of 0.21 Ω cm² were achieved at 700 °C, with calculated activation energy (E _a) of 128 kJ mol^?1 for the GBCuF cathode. The experimental results indicated that the layered perovskite GBCuF is a good candidate for cathode material.     

Reaction mechanisms of the copper-manganese spinel cathode in a solid oxide fuel cell

Cu_1.25Mn_1.75O₄ spinel (CMO) was studied as a potential solid oxide fuel cell (SOFC) cathode material at intermediate temperatures. The reaction mechanism of a composite cathode consisting of Cu_1.25Mn_1.75O₄ and yttria-stabilized zirconia (YSZ) was investigated by impedance spectroscopy. The influence of the CMO/YSZ ratio, time exposed to current passage and temperature on the impedance spectra was examined. Activation energy of the corresponding processes was calculated to be near 1 eV and between 1.32 and 1.96 eV for the high and low frequency arcs in the impedance spectra. Comparison between CMO-YSZ and Sr-doped LaMnO₃ (LSM)-YSZ composite cathodes showed they had similar reaction mechanisms. The transport or transfer of oxygen intermediates or oxide ions between the catalyst and electrolyte was suggested to be the rate determining steps between 700 and 800 °C, whereas dissociative adsorption, mass transfer and surface diffusion were rate controlling between 600 and 700 °C.     

高温固体氧化物燃料电池实验演示

介绍一种高效低污染的新型能源－高温固体氧化物燃料电池的工作原理及其演示实验装置。     

An intermediate temperature solid oxide fuel cell utilizing superior oxide ion conducting electrolyte, doubly doped LaGaO3 perovskite

LaGaO₃-based perovskite oxide doped with Sr and Mg exhibits high ionic conduction over a wide oxygen partial pressure. In this study, the stability of the LaGaO₃ based oxide was investigated. It became clear that LaGaO₃ based oxide is very stable for reduction and oxidation. SOFCs utilizing LaGaO₃-based perovskite type oxide for electrolyte were further studied for the decreased temperature solid oxide fuel cells. The power generation characteristics of cells were strongly affected by the electrode, both anode and cathode. It became clear that Ni and LnCoO₃ (Ln: rare earth) are suitable for anode and cathode, respectively. Rare earth cations in the Ln-site of Co-based perovskite cathode also have a great effect on the power generation characteristics. In particular, high power density could be attained in the temperature range from 973 to 1273 K by using doped SmCoO₃ for the cathode. The electrical conductivity of SmCoO₃ increases with increasing Sr amount doped for the Sm site and attained the maximum at Sm_0.5Sr_0.5CoO₃. The cathodic overpotential and the internal cell resistance exhibit almost opposite dependence on the amount of doped Sr. Consequently, the power density of the cell reaches a maximum when Sm_0.5Sr_0.5CoO₃ is used for cathode. On this cell, the maximum power density is as high as 0.58 W/cm² at 1073 K, although a 0.5 mm thick electrolyte is used. Therefore, this study reveals that the LaGaO₃ based oxide for electrolyte and the SmCoO₃ based oxide for cathode are promising for solid oxide fuel cells at intermediate temperature. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June, 14–17, 1998.     

Energy integration strategies for solid oxide fuel cell systems

Solid oxide fuel cells (SOFCs) have operating temperatures ranging from as low as 600 °C for intermediate temperature operation to above 900 °C for higher temperature operation. These high temperatures are often viewed as a considerable disadvantage from a materials point of view because of the occurrence of unwanted interfacial reactions, stresses as a result of thermal expansivity mismatches, etc. However, higher temperatures are also an advantage of SOFC systems. Fuel pretreatment that may involve such processes as reforming is very often highly endothermic in nature. The high operating temperature of an SOFC allows for efficient system energy integration with the waste heat from the fuel cell being used to drive fuel pretreatment processes. Here, we demonstrate this propensity for energy integration by looking at the use of a novel hydrogen-carrier system working with an SOFC.     

Reaction between solid oxide fuel cell materials

In fabrication processes of solid oxide fuel cells, high-temperature heat treatments cannot be avoided. It will give rise to mutual reaction and interdiffusion of the cell component materials: yttria-stabilized zirconia (YSZ, electrolyte), (La, Sr)MnO₃ (cathode), Ni-YSZ cermet (anode) and (La, Ca)CrO₃ (separator). Reactivity of LaMnO₃ and YSZ was estimated by thermodynamic calculations, and it was found that the nonstoichiometry at La site in LaMnO₃ plays an important role on the reaction. Diffusion of Mn into YSZ leads to increase of La activity at the interface and promotes the reaction. Electrical conductivity of YSZ decreases when Mn dissolves in the cubic phase of YSZ. Oxidation state of the dissolving Mn varies with partial pressure of oxygen and affects the electrical properties of YSZ. Migration of Ca from (La, Ca)CrO₃ separator to other cell components is one of the largest problems in the co-firing cell fabrication process because it prohibits the sintering of the separator.     

Characterisation of novel anodes for solid oxide fuel cells based on oxygen-excess perovskite related structures

Mixed oxides belonging to the La₂Sr_n–2Ti_nO_3n+1 family have been targeted as potential materials for high performance anodes in fuel cells. The most important feature of these ompounds is the existence of excess oxygen along preferential directions. Substitution by lower valence cations might lead to a removal of this excess oxygen along preferential directions and therefore possibly increase the potential for ionic conductivity. High total conductivity (>1 S·cm⁻¹) was found after reduction for some compounds within this family of compounds. The activation energy varied with the structure and the more disordered the structure (thicker perovskite blocks), the lower the activation energy was found. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

中温固体氧化物燃料电池的基本性质测量

介绍了固体氧化物燃料电池的工作原理,测量了中温薄膜固体氧化物燃料电池的开路电压、放电曲线及功率曲线,并分析了电池内阻随电流密度的变化.     

Spectroelectrochemical cell for in situ studies of solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are able to produce electricity and heat from hydrogen‐ or carbon‐containing fuels with high efficiencies and are considered important cornerstones for future sustainable energy systems. Performance, activation and degradation processes are crucial parameters to control before the technology can achieve breakthrough. They have been widely studied, predominately by electrochemical testing with subsequent micro‐structural analysis. In order to be able to develop better SOFCs, it is important to understand how the measured electrochemical performance depends on materials and structural properties, preferably at the atomic level. A characterization of these properties under operation is desired. As SOFCs operate at temperatures around 1073 K, this is a challenge. A spectroelectrochemical cell was designed that is able to study SOFCs at operating temperatures and in the presence of relevant gases. Simultaneous spectroscopic and electrochemical evaluation by using X‐ray absorption spectroscopy and electrochemical impedance spectroscopy is possible.     

A practice of single layer solid oxide fuel cell

It has been found that the electrical conduction behavior of La_0.9Sr_0.1InO_3−δ varies with oxygen partial pressure. P-type and n-type conduction at high and low oxygen partial pressure have been observed respectively. While at intermediate oxygen partial pressures, the electrical conductivity changes slightly with the oxygen partial pressure. Thus, La_0.9Sr_0.1InO_3−δ may be a possible material for making single layer solid oxide fuel cell (SLFC). The concept of SLFC has been tested using a piece of thick ceramic pellet of La_0.9Sr_0.1InO_3−δ. The maximum current density and power density is 12 mA/cm² and 3 mW/cm² at 800 °C when dilute H₂ and air were used as fuel and oxidizing agent, respectively. The phase stability of La_0.9Sr_0.1InO_3−δ has been studied by Raman spectra and XRD. It is confirmed that secondary phase may appear in La_0.9Sr_0.1InO_3−δ after long term testing in low oxygen partial pressure, and finally it may be decomposed into La₂O₃ and metal Indium. Much attention should be paid to stabilize La_0.9Sr_0.1InO_3−δ and to improve the performance of SLFC.     

Fabrication and characterization of Ni-supported solid oxide fuel cell

Metal-supported solid oxide fuel cells (SOFCs) are promising due to their good mechanical and thermal properties. Stainless steels are often used as a supporting metal. In this study, the possible use of Ni as a supporting metal was tested. Ni is also a good model of supporting metal due to its lack of Cr which poisons Ni anode.YSZ electrolyte and Ni-YSZ anode were coated on a porous Ni support to fabricate Ni-supported SOFC. A porous Ni support in thick-film form (t ~ 150 µm) was prepared by the mold casting of Ni powders. An anode and an electrolyte layer were sequentially coated on the Ni support using screen printing and tape casting methods, respectively. The Ni-supported cell (t ~ 200 µm) was sintered at 1400 °C in a reducing atmosphere and the performance was evaluated at 800 °C with a La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) cathode. The unit cell showed an open circuit voltage (OCV) of 0.96 V and a peak power density of 470 mW/cm² at 800 °C.     

固体线膨胀系数测定实验的改进

针对固体线膨胀系数测定实验中样品所处温度梯度场过大、升温和降温速率控制精度低的问题,提出了实验改进的措施.阐述了减小实验样品的温度场梯度,用设定程序控制实验样品的升温、恒温和降温的实施过程,并对金属样品线膨胀系数测定实验结果进行了分析,取得了固体线膨胀系数测定实验的改进设计的预期效果.     

Oxy-combustion of coal in liquid-antimony-anode solid oxide fuel cell system

A novel system based on the indirect oxy-combustion of coal in a liquid Sb anode solid oxide fuel cell (SOFC) has been used to produce electricity for over 48?h. Pulverized anthracite was fed to the liquid-antimony-anode of the fuel cell, and a peak power density of 47?mW cm^?2 was reached at 1023?K and 35?mW cm^?2 at 973?K. The fuel cell was prepared using a porous stainless-steel tube as a support for an LSM cathode, antimony oxide (Sb₂O₃)/yittria stabilized zirconia (YSZ, Y_0.08Z_0.92O_1.96) composite electrolyte (membrane), while liquid antimony acted as the anode. Liquid antimony/antimony oxide served as the intermediate medium for coal oxidation producing mainly carbon dioxide, which evolved as a separate gas stream. The fuel cell will facilitate carbon capture process, and simultaneously convert the chemical energy of coal directly to electricity. The experiment showed that while the fabricated electrolyte was porous, it became dense during the actual operation, preventing nitrogen leakage into the Sb/C side and producing reasonable open circuit voltage. Analysis of the experimental EIS data illustrates that the Ohmic resistance was the primary loss mechanism in the system. It further suggests approaches to improve the design. Continuous operation of this coal fueled oxy-combustion/fuel cell system achieved an overall efficiency of 28.2% despite of its tiny scale. Simple technologies can be employed to scale up this system at relatively low cost of fabrication and materials.     

Isotope effect for the thermal expansion coefficient of germanium

The first experimental and theoretical investigation of the difference in the temperature behavior of the linear expansion coefficients of single crystals grown from isotopically highly enriched and natural germanium is reported. A comparison of the data for ⁷⁰Ge and ^natGe crystals reveals the significant influence of isotopic composition over a wide range of temperatures 30–230 K. Zh. éksp. Teor. Fiz. 115, 243–248 (January 1999)     

Thin porous Ni-YSZ films as anodes for a solid oxide fuel cell

Porous Ni-YSZ (YSZ—yttria-stabilized zirconia) films were fabricated by reactive co-sputtering of a Ni and a Zr-Y target, followed by sequentially annealing in air at 900 °C and in vacuum at 800 °C. The Ni-YSZ films comprised small grains and pores that were tens of nanometers in size. The porous Ni-YSZ films were used as an anode on one side of a YSZ electrolyte disc and a La_0.7Sr_0.3MnO₃ thick film was used as a cathode on the other side of the disc to form solid oxide fuel cells (SOFCs). The voltage-current curves of the SOFCs with single- and a triple-layered porous anodes were measured in a single-chamber configuration, in a mixture of CH₄ and air (CH₄:O₂ volume ratio=2:1). The maximum power density of the SOFC using the single-layered porous Ni-YSZ thin films as the anode was 0.38 mW cm⁻², which was lower than that of 0.76 mW cm⁻², obtained using a screen-printed Ni-YSZ thick anode. The maximum power density of the SOFC with a thin anode was increased, but varied between 0.6 and 1.14 mW cm⁻² when a triple-layered porous Ni-YSZ anode was used.     

Simultaneous thermogravimetry - mass spectrometry for a solid oxide fuel cell cathode: (La0.7Sr0.3)0.9MnO3

A SOFC cathode related perovskite material, (La_0.7Sr_0.3)_0.9MnO₃, has been investigated by simultaneous thermogravimetry - mass spectrometry from room temperature to 1770 K. Water, carbon dioxide and oxygen were detected by mass spectrometry. Water and carbon dioxide evolution can be interpreted by assuming that prior to the thermogravimetry-mass spectrometry measurement about 0.5 % of the lanthanum component had reacted with carbon dioxide and water to form La₂(CO₃)₃*8H₂O, which dehydrated and decomposed via La₂O₂CO₃ into La₂O₃ and evolving H₂O and CO₂ during the present experiment. The observation that the lanthanum strontium manganite emitted oxygen in two stages can be ascribed to the two different oxygen sites in the perovskite lattice, that is, the oxygen excess and deficient regions.     

Zirconia based oxide ion conductors for solid oxide fuel cells

The electrical conductivity in the ZrO₂-Ln₂O₃ and ZrO₂-MO₂-Ln₂O₃(M = Hf, Ce, Ln = lanthanides) systems has been examined.The highest conductivity of 0.3 S/cm at 1000 °C was found in the ZrO₂-Sc₂O₃ system. The addition of MO₂ into the ZrO₂-Ln₂O₃(Ln = Sc, Y, Yb) systems showed the conductivity decreasing. The conduction mechanism in the zirconia based oxide ion conductors was discussed in view of the dopant ionic radius. The aging effect of the conductivity in the ZrO₂-Ln₂O₃ systems has been measured in a temperature rang 800–1000 °C. ZrO₂ with a high content of Ln₂O₃ showed no significant conductivity degradation. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June 14–17, 1998.     

Technical and economic evaluation of a methane solid oxide fuel cell

Solid oxide fuel cells offer the possibility of high temperature synthesis of chemical products with cogeneration of electricity, a process known as chemical cogeneration. This research primarily addresses the likelihood of upscaling present bench-scale experimental results of methane fuelled SOFCs. Methane coupling, i.e. production of C₂ hydrocarbons, is one of the possibilities of chemical cogeneration. In evaluating the co-generating SOFC, the methane-coupling design was compared to two other possible competing designs, namely, a regular SOFC plant (complete oxidation of methane to CO₂ and H₂O) and an SOFC plant coproducing synthesis gas. It was found that the rate of return for the regular fuel cell exceeded 25% whereas that for the ethylene plant was about 21%. The synthesis gas plant was well behind at about 17%. The reasons that have so far prohibited large-scale application of such systems are discussed. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995.     

Oxide anode materials for solid oxide fuel cells

A major advantage of solid oxide fuel cells (SOFCs) over polymer electrolyte membrane (PEM) fuel cells is their tolerance for the type and purity of fuel. This fuel flexibility is due in large part to the high operating temperature of SOFCs, but also relies on the selection and development of appropriate materials — particularly for the anode where the fuel reaction occurs. This paper reviews the oxide materials being investigated as alternatives to the most commonly used nickel–YSZ cermet anodes for SOFCs. The majority of these oxides form the perovskite structure, which provides good flexibility in doping for control of the transport properties. However, oxides that form other crystal structures, such as the cubic fluorite structure, have also shown promise for use as SOFC anodes. In this paper, oxides are compared primarily in terms of their transport properties, but other properties relative to SOFC anode performance are also discussed.