Mechanistic analysis of the oxygen reduction reaction at (La,Sr)MnO3 cathodes in solid oxide fuel cells

The primary aim of this work was to establish the mechanism of the oxygen reduction reaction (ORR) at (La(0.8)Sr(0.2))0.98MnO3 (LSM)-based cathodes in solid oxide fuel cells. Rate equations, based on the Butler-Volmer equation and employing either Langmuir or Temkin adsorption conditions for reactant and intermediate species, were derived, yielding predicted reaction orders and transfer coefficients. Experimental data were collected using half-cell cyclic voltammetry in a variable pO2 atmosphere (0.03 to 1 atm) at 600 to 900 degrees C, using both dense and porous LSM-based cathodes, employed to establish the impact of the accessibility of the active site on cathode activity. The rate of the ORR at dense LSM has been found to be limited by the dissociation of O(2ads)- at low currents and by the first electron-transfer step, reducing O(2ads) to O(2ads)-, at high currents. However, at porous LSM cathodes, the reaction mechanism is more difficult to deduce because the electrode morphology impacts significantly on the measured kinetic and mechanistic parameters, giving anomalous transfer coefficients of <0.5.     

Scandia-stabilized zirconia-impregnated (La,Sr)MnO3 cathode for tubular solid oxide fuel cells

We have studied the properties of a LSM-ScSZ composite cathode fabricated by a two-step process including dip-coating LSM framework and ion-impregnating ScSZ, for using with anode-supported tubular solid oxide fuel cells. A preliminary examination of the single tubular cell, consisting of a Ni-YSZ anode support tube, a Ni-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode fabricated by ion-impregnating, has been carried out, and an improved performance was obtained. The polarization resistance of the cathode side clearly decreased for impregnating the electronic conducting phase (LSM) with the ionic conducting phase (ScSZ). And the single cell with the impregnated cathode generated a maximum power density of 433 mW cm⁻² at 850 °C, when operating with humidified hydrogen.     

(La0.8Sr0.2)0.9MnO3–Gd0.2Ce0.8O1.9 composite cathodes prepared from (Gd, Ce)(NO3) x -modified (La0.8Sr0.2)0.9MnO3 for intermediate-temperature solid oxide fuel cells

Development of high performance cathodes with low polarization resistance is critical to the success of solid oxide fuel cell (SOFC) development and commercialization. In this paper, (La_0.8Sr_0.2)_0.9MnO₃ (LSM)–Gd_0.2Ce_0.8O_1.9(GDC) composite powder (LSM ~70 wt%, GDC ~30 wt%) was prepared through modification of LSM powder by Gd_0.2Ce_0.8(NO₃) _x solution impregnation, followed by calcination. The electrode polarization resistance of the LSM–GDC cathode prepared from the composite powder was ~0.60 Ω cm² at 750 °C, which is ~13 times lower than that of pure LSM cathode (~8.19 Ω cm² at 750 °C) on YSZ electrolyte substrates. The electrode polarization resistance of the LSM–GDC composite cathode at 700 °C under 500 mA/cm² was ~0.42 Ω cm², which is close to that of pure LSM cathode at 850 °C. Gd_0.2Ce_0.8(NO₃) _x solution impregnation modification not only inhibits the growth of LSM grains during sintering but also increases the triple-phase-boundary (TPB) area through introducing ionic conducting phase (Gd,Ce)O_2-δ, leading to the significant reduction of electrode polarization resistance of LSM cathode.     

Sims characterization of La0.7Sr0.3MnO3 films for solid oxide fuel cell applications

Among solid oxides exploited to prepare efficient fuel cells, La(1-x)SrxMnO3 manganites have been widely studied and used as cathodes, because of their high conductivity at the working temperatures, good thermal stability and compatibility with other cell components. A fundamental goal in solid oxide fuel cells technology consists in lowering the normal operating temperatures, e.g. increasing the surface/volume ratio of electrodic materials, so as to enhance their catalytic performances. In this work, the preparation of high surface area La(1-x)SrxMnO3 (x approximately 0.3) films on silicon wafers by the nitrate-citrate Pechini process is described. The films were characterized by X-ray diffraction, Atomic Force Microscopy and Secondary Ion Mass Spectrometry. Good quality nanostructured perovskite-type films were obtained. SIMS methodology enabled to show the surface and in-depth coatings composition and residual contaminants. Moreover, it allowed defining the best synthesis conditions for complete in-depth decomposition of precursors and obtaining homogeneously thick coatings.     

LSM-infiltrated LSCF cathodes for solid oxide fuel cells

Mixed ionic-electronic conductors in the family of La_xSr_1–xCo_yFe_1–yO_3–δ have been widely studied as cathode materials for solid oxide fuel cells (SOFCs). However, the long-term stability was a concern. Here we report our findings on the effect of a thin film coating of La_0.85Sr_0.15MnO_3–δ (LSM) on the performance of a porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ (LSCF) cathode. When the thicknesses of the LSM coatings are appropriate, an LSM-coated LSCF electrode showed better stability and lower polarization (or higher activity) than the blank LSCF cathode without LSM infiltration. An anode-supported cell with an LSM-infiltrated LSCF cathode demonstrated at 825 °C a peak power density of ～1.07 W/cm², about 24% higher than that of the same cell without LSM infiltration (～0.86 W/cm²). Further, the LSM coating enhanced the stability of the electrode; there was little degradation in performance for the cell with an LSM-infiltrated LSCF cathode during 100 h operation.     

Electrochemical properties of cathodes made of (La,Sr)(Fe,Co)O3 containing admixtures of nanoparticles of cupric oxide and intended for fuel cells with a solid electrolyte based on ceric oxide

The electric and electrochemical characteristics of cathodes made of La_0.6Sr_0.4Fe_0.8Co_0.2O_3?δ (LSFC) and intended for fuel cells with electrolytes based on ceric oxide are studied. Adding cupric oxide into the LSFC cathode is shown to exert a favorable effect of the properties of the LSFC-CuO/SDC electrode system, where SDC stands for the CeO₂-Sm₂O₃ electrolyte. The effect produced by cupric oxide when added in the form of nanopowder is perceptibly greater than in the case of micropowdered CuO. Adding a mere 0.5 wt % of nanopowdered CuO reduces the LSFC cathode resistance nearly tenfold. The cathode’s adhesion to the electrolyte substantially improves as well, which makes it possible to lower the cathode’s firing temperature by 100°C. The maximum of electrochemical activity is intrinsic to cathodes containing 2 wt % CuO, with the caking temperature of 1000°C. According to a 2011-h life test of the LSFC-SDC composite cathodes containing nanopowdered CuO, temporal stability of their electrochemical characteristics improves with the SDC content. The time dependences of the polarization resistance of cathodes containing 40–50 wt % SDC look like decaying exponential curves. The steady-state polarization resistance, calculated on the basis of this, is equal to 0.1–0.2 ohm cm². At an overvoltage of less than 100 mV, the cathodes may provide for a current density of 0.5–1.0 A cm^?2.     

Synthesis and electrocatalytic performance of La0.3Ce0.1Sr0.5Ba0.1TiO3 anode catalyst for solid oxide fuel cells

Ce-doped La_0.4Sr_0.5Ba_0.1TiO_3–δ (LCSBT) perovskite anode catalysts in solid oxide fuel cells were successfully synthesized by a modified rheological phase reaction for the first time. Pure LCSBT could be obtained under a reducing atmosphere and nano-CeO₂ particles could be exsoluted from LCSBT after being sintered in air. The catalytic activity and electrochemical performance of LCSBT anodes for the H₂ oxidation were obviously improved comparing with the pure La_0.4Sr_0.5Ba_0.1TiO_3–δ (LSBT) and LSBT&CeO₂ admixture anodes. The improved performance could be attributed to the nanostructure of LCSBT and the exsoluted nano-CeO₂ particles.     

Deposition of chromium species on Sr-doped LaMnO3 cathodes in solid oxide fuel cells

The system of chromia-forming alloy/Sr-doped LaMnO₃ (LSM) electrode/3 mol%Y₂O₃–ZrO₂ (TZ3Y) electrolyte has been investigated at 900°C in air under cathodic polarization conditions. Deposition of chromium species was found to occur preferentially at the TZ3Y electrolyte surface, forming a deposit ring around the edge of the LSM electrode coating. The width of the ring was about 60 μm for an LSM electrode polarized for about 50 h. Overpotential (η) increases with polarization time. In contrast to η, electrode interface resistance (R_E) measured under open circuit conditions decreases initially after passing the current and remains almost constant with polarization. The results indicate that the deposition process of chromium species may not be dominated by electrochemical reduction processes in competition with O₂ reduction at an early stage of polarization.     

A dynamic infiltration technique to synthesize nanolayered cathodes for high performance and robust solid oxide fuel cells

Solution infiltration is a popular technique for the surface modification of solid oxide fuel cell(SOFC)cathodes. However, the synthesis of nanostructured SOFC cathodes by infiltration is a tedious process that often requires several infiltration and high temperature(≥500 °C) calcination cycles. Moreover, fabricating large-area nanostructured cathodes via infiltration still requires serious attention. Here, we propose a facile and scalable urea assisted ultrasonic spray infiltration technique fo...     

高温合成锰酸锶镧的缺陷

严格按照化学式拟定配方,采用固相合成方法合成的锰酸锶镧材料中含有杂相La2O3.借助XRD,DSC,TG等分析手段分析了高温合成锰酸锶镧的缺陷,尝试对锰酸锶镧进行高温再处理,结果获得的纯锰酸锶镧不含La2O3杂相.     

Activation,microstructure, and polarization of solid oxide fuel cell cathodes

Activation effect can be defined as the enhancement of the electrochemical performance or activity of the solid oxide fuel cell cathodes such as Sr-doped LaMnO₃ (LSM) with the polarization/current passage treatment under fuel cell operation conditions. In this paper, the activation effect of the cathodic polarization/current passage on the O₂ reduction reaction of the LSM-based cathodes is reviewed. In addition to the activation effect, cathodic polarization/current passage also has a significant effect on the microstructure of the LSM electrodes and the morphology between the LSM electrode and Y₂O₃-ZrO₂ electrolyte interface. A mechanism involving the incorporation of SrO species into the LSM lattice and the formation of oxygen vacancies is proposed for the activation effect of the polarization.     

Silver incorporation into cathodes for solid oxide fuel cells operating at intermediate temperature

Silver (Ag) at 0.1–2.0 wt% was incorporated into cathodes for solid oxide fuel cells as a catalyst for oxygen reduction. A novel processing route for Ag incorporation ensuring a very homogeneous Ag ion distribution is presented. From the results of X-ray powder diffraction it can be concluded that the La_0.65Sr_0.3MnO_3– perovskite phase is already formed at 900 °C. The solubility of Ag in the crystal lattice in this type of perovskite was below 1 wt%. The electrochemical tests of these materials show that there is only a slight catalytic effect of Ag. Scanning electron microscopy reveals a low mechanical contact of the cathode grains to the electrolyte due to the low cathode sintering temperature that was chosen.     

Effect of polarization on the electrode behavior and microstructure of (La,Sr)MnO<Subscript>3</Subscript> electrodes of solid oxide fuel cells

The electrode behavior and microstructure of freshly prepared (La_0.8Sr_0.2)_0.9MnO₃ (LSM) electrodes were investigated under various polarization conditions. The original, large agglomerates in freshly prepared LSM electrodes were broken down into sphere-shaped grains when exposed to cathodic or anodic current passage of 200 mA cm^–2 at 800 °C in air for 3 h. Microstructural changes under cathodic polarization could be related to the pronounced diffusion and migration of oxygen vacancies and Mn ions on the LSM surface and lattice expansion, while lattice shrinkage under oxidation conditions most likely contributes to the structural changes under anodic polarization. Such morphological changes were irreversible and were found to be beneficial to the performance of freshly prepared LSM electrodes. Freshly prepared LSM electrodes behaved very differently with respect to the cathodic and anodic current passage treatment.     

Effect of Bi0.75Y0.25O1.5 electrolyte additive in collector layer to properties of bilayer composite cathodes of solid oxide fuel cells based on La(Sr)MnO3 and La(Sr)Fe(Co)O3 compounds

The sintering features, electroconductivity, and electrochemical characteristics of bilayer electrodes with functional composite layers based on La(Sr)MnO₃ (LSM) and La(Sr)Fe(Co)O₃ with LSM collector layer and Bi(Y)O_1.5 (YDB) electrolyte additive in contact with Ce (Sm)O₂(SDC), La(Sr)Ga(Mg)O₃, and Zr(Sc)O₂ electrolytes were studied. YDB additive to the electrode collector layer was shown to produce a positive effect to the properties of the studied electrode systems. The maximum electrochemical activity and electroconductivity was observed for the electrodes with 5 wt % of YDB electrolyte additive in the collector layer. Thus, electroconductivity of electrodes is almost doubled and 100 mV cathode overvoltage current density is increased by 30% at the temperatures of 800 to 900°C and up to 10-fold at 650 to 700°C. The collector layer sintering temperature of bilayer electrodes can be reduced from 1150 to 1000°C without loss of electrochemical activity. The service life tests (about 1200 h) of composite electrodes with LSM2-SDC functional layer and 90% LSM2 + 10% YDB collector layer in contact with SDC electrolyte showed the time dependences of polarization resistance tending to saturation and described with damped exponent. Original Russian Text ? N.M. Bogdanovich, D.I. Bronin, G.K. Vdovin, I.Yu. Yaroslavtsev, B.L. Kuzin, 2009, published in Elektrokhimiya, 2009, Vol. 45, No. 4, pp. 486–494.     

不同含钴阴极在YSZ薄膜电解质固体氧化物燃料电池中的性能

固体氧化物燃料电池阴极材料的阻抗对固体氧化物燃料电池的性能有较大影响.我们通过XRD、对称电池以及单电池性能测试等方法比较系统地研究了4种最为常用的含钴阴极材料直接用于钇稳定化氧化锆（YSZ）电解质薄膜与通过引入SDC夹层后用于YSZ电解质薄膜后的性能.我们发现,不同的含钴阴极材料与YSZ材料之间都不同程度地发生相反应,在应用于YSZ电解质薄膜上时,相反应大大降低了含钴阴极材料的性能,在使用了SDC夹层后,单电池的功率输出显著提高.     

钙钛矿锰氧化物(La_(1-x)Gd_x)_(0.5)Sr_(0.5)MnO_3的磁电阻效应

采用传统的高温固相法制备了多晶样品(La1-xGdx)0.5Sr0.5MnO3(x=0,0.1,0.2,0.3,0.4),利用X射线衍射仪(XRD)、超导量子磁强计(SQUID)、标准四端引线法分别对样品结构、磁性、电性以及磁电阻效应进行了研究。研究表明:Gd的少量替代并没有引起结构变化;随着Gd含量的增加,所有样品的居里温度TC和金属-绝缘体转变温度Tp都降低了;在TC附近发现了磁电阻效应,同时在低温下发现了更大的磁电阻;并且Gd的少量替代可使磁电阻MR增大。     

A new anode for solid oxide fuel cells with enhanced OCV under methane operation

A new SOFC anode material based upon oxygen excess perovskite related phases has been synthesised. The material shows better electrochemical performance than other alternative new anodes and comparable performance to the state-of-art of the electrodes, Ni-YSZ cermets, under pure hydrogen. Furthermore, this material shows an enhanced performance under methane operation with high open circuit voltages, i.e. 1.2-1.4 V at 950 degrees C, without using steam excess. The effect of the anode configuration was tested in one and four layer configurations. The optimised electrode polarisation resistances were just 0.12 ohm cm(2) and 0.36 ohm cm(2), at 950 degrees C, in humidified H(2) and humidified CH(4), respectively. Power densities of 0.5 W cm(-2) and 0.35 W cm(-2) were obtained in the same conditions. A very low anodic overpotential of 100 mV at 1 A cm(-2) was obtained under humidified H(2) at 950 degrees C. Samples were tested for two days in reducing and oxidising conditions, alternating heating and cooling processes from 850 degrees C to 950 degrees C, showing stable electrode performance and open circuit voltages. The results show that the substituted strontium titanates are very promising anode materials for SOFC.     

Cathode infiltration with enhanced catalytic activity and durability for intermediate-temperature solid oxide fuel cells

To lower the operation temperature and increase the durability of solid oxide fuel cells(SOFCs), increasing attentions have been paid on developing cathode materials with good oxygen reduction reaction(ORR)activity at intermediate-temperature(IT, 500–750 °C) range. However, most cathode materials exhibit poor catalytic activity, or they thermally mismatch with SOFC electrolytes and undergo severe degeneration. Infiltrating catalysts on existing backbone materials has been proved to be an efficie...