Strontium-doped Lanthanum Manganite Research Articles

Functionally graded composite cathodes for solid oxide fuel cells

Functionally graded solid oxide fuel cell (SOFC) cathodes have been prepared from mixtures of strontium-doped lanthanum manganite (LSM) and gadolinia-doped ceria (CGO) using slurry spraying techniques. Similar samples were also prepared from mixtures of LSM and ytrria-stabilised zirconia (YSZ). A current collector comprising a mixture of LSM and strontium-doped lanthanum cobaltite (LSCO) was then applied to both cathode types. Samples were characterised using scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). Characterisation using EIS techniques showed that cathodes incorporating CGO into the structure gave improved performance over those fabricated using YSZ. These performance gains were most noticeable as the temperature was decreased towards 700 °C, and were maintained during the testing of three cell membrane electrode assemblies fabricated to the Rolls-Royce design.

Sintering and mechanical characteristics of lanthanum strontium manganite

The sintering and mechanical characteristics of strontium-doped lanthanum manganites are increasingly important for both processing and device integration. A-site deficiency and reduced strontium fraction were shown to improve sinterability. High-strontium compositions displayed unusual mechanical properties, including unexpectedly low Young's modulus and fracture strength. In addition, plastic deformation of a fully dense LSM plate under modest loading was observed. This surprising and novel result may result from oxygen exchange with the LSM surface at elevated temperatures.

Functionally graded cathodes for solid oxide fuel cells

Functionally graded Solid Oxide Fuel Cell cathodes have been prepared from mixtures of strontium doped lanthanum manganite (LSM) and yttria stabilised zirconia (YSZ) using screen printing techniques. Samples were characterised using scanning electron microscopy, elemental dot mapping, and electrochemical impedance spectroscopy. Characterisation using AC impedance techniques showed that each cathode could be analysed in terms of a low frequency, mid frequency and high frequency response. Results showed that as the level of YSZ-LSM grading within the cathode increased, the polarisation resistance decreased. No region of the graded cathode should contain less than 20 wt% LSM to prevent an accompanying increase in series resistance.

Residual strain and texture in strontium-doped lanthanum manganite thin films

Thin films of La0.67Sr0.33MnO3 (LSMO) have been deposited using liquid-delivery metal-organic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) 2θ/θ scans showed that all the films has a cubic structure. Studies of the in-plane crystallographic orientations (pole figure) revealed an (0 0 1) preferred growth structure on LAO, a weak (1 1 0) texture on Y-ZrO2 (YSZ), a random texture on sapphire (SAP) and silicon (Si). Our attention is focused on residual strain and its deviations from linearity for ɛϕψ vs. sin2ϕ plots. The strain evolution from 0<sin2ϕ<0.8 showed a curvature and a “snake-like” pattern. These anomalies are attributed to texture and strain gradients. In-plane strain decreased as the lattice mismatch increased and varied from 0.05 to 3.03% depending on the substrate. An attempt is made to establish a relationship between lattice mismatch, growth process, and residual strain.

00/03405 Optimisation of flow-field in polymer electrolyte membrane fuel cells using computational fluid dynamics techniques

The application of micro-solid oxide fuels cells in small systems is discussed. Two types of application are examined, namely, leisure CHP systems and micro-hybrid vehicles. A unique triple layer catalyst–SOFC–catalyst system has been designed utilising propane/butane fuel. The system consists of a co-generating gas burner with a pre-reforming catalyst, a micro-SOFC stack and an oxidation catalyst. The pre-reforming catalyst comprising of Ru metal on Saffil® ceramic wool, was used to partially reform the propane/butane gas prior to entering the fuel cell, preventing carbon formation. The micro-SOFCs were YSZ tubes (Adelan, UK) with nickel/YSZ cermet anodes on the outside and strontium-doped lanthanum manganite cathodes on the inside. Final oxidation was provided by a cordierite honeycomb coated with platinum combustion catalyst producing most of the heat for the fuel cell operation. Initial performance results were obtained and it was shown that a co-generating system could be achieved using a propane/butane fuel supply, piezoelectric ignition system and air supply for the triple catalyst system. The application of this micro-SOFC system for leisure and micro-hybrid vehicles, such as golf trolleys and power-assisted bicycles, is described.

Mechanochemical synthesis of La 0.7Sr 0.3MnO 3 by grinding constituent oxides

Perovskite-type strontium manganite (SrMnO 3) and strontium-doped lanthanum manganite (La 0.7Sr 0.3MnO 3, LSM3) powders were synthesized by grinding the constituent oxides of SrO–MnO 2 and La 2O 3–SrO–Mn 2O 3–MnO 2, respectively, using a planetary mill in air. These synthesis reactions proceed with an increase in grinding time. The product powder of LSM3 has the morphology of a strong agglomeration of fine grain in nanosize.

Novel applications for micro-SOFCs

The application of micro-solid oxide fuels cells in small systems is discussed. Two types of application are examined, namely, leisure CHP systems and micro-hybrid vehicles. A unique triple layer catalyst–SOFC–catalyst system has been designed utilising propane/butane fuel. The system consists of a co-generating gas burner with a pre-reforming catalyst, a micro-SOFC stack and an oxidation catalyst. The pre-reforming catalyst comprising of Ru metal on Saffil® ceramic wool, was used to partially reform the propane/butane gas prior to entering the fuel cell, preventing carbon formation. The micro-SOFCs were YSZ tubes (Adelan, UK) with nickel/YSZ cermet anodes on the outside and strontium-doped lanthanum manganite cathodes on the inside. Final oxidation was provided by a cordierite honeycomb coated with platinum combustion catalyst producing most of the heat for the fuel cell operation. Initial performance results were obtained and it was shown that a co-generating system could be achieved using a propane/butane fuel supply, piezoelectric ignition system and air supply for the triple catalyst system. The application of this micro-SOFC system for leisure and micro-hybrid vehicles, such as golf trolleys and power-assisted bicycles, is described.

High-temperature interfacial reactivity of LaMnO3�? nanometric films on (100)YSZ

This paper aims to study the properties of the triple phase boundaries (TPBs) in the vicinity of the strontium-doped lanthanum manganite (cathode)/yttria-stabilized zirconia (electrolyte YSZ) interface in a solid oxide fuel cell (SOFC). The LaMnO3±δ (LM) nanometric films deposited on (100)YSZ can be considered as a realistic and simple model of this half-cell. Low pressure metal–organic chemical vapour deposition was used to elaborate these films and to control the La/Mn ratio in the range 0.8–1.3 by using La(tmhd)3 and Mn(acac)3 solid precursors at restricted temperature, pressure and time conditions. The film characteristics have been studied: morphology by SEM, chemical composition by energy-dispersive x-ray emission analysis and AES/scanning Auger microscopy and crystallography by x-ray diffraction and reflection high-energy electron diffraction. The half-cell performances at high temperature can be affected both by cation diffusion into YSZ and by the formation of foreign phases. Data are presented on the interfacial reactivity as analysed by the above techniques: important changes in the YSZ morphology in the TPB regions and the formation of epitaxial lanthanum zirconate are analysed for two La/Mn ratios (0.80 and 1.30). Copyright © 2000 John Wiley & Sons, Ltd.

Mechanical Properties of Strontium‐Doped Lanthanum Manganite

Solid oxide fuel cells that incorporate stabilized‐zirconia electrolytes commonly use the transition‐metal oxide perovskite La0.875Sr0.125MnO3+δ as the cathode. While in operation, the cathode can be subjected to significant stresses, because of thermal expansion mismatch between mating components. The mechanical behavior of La0.875Sr0.125MnO3+δ has been studied using three‐point bend strength measurements at ambient, 400°C, 800°C and 1000°C. The results show that phase transformations have an important role in the mechanical‐strength behavior. The results also indicate some unique strengthening effects that are associated with the evolution of the unit cell with temperature and the increasing symmetry of the crystal lattice.

Electrochemical reduction of oxygen on a strontium doped lanthanum manganite electrode

The electrochemical reduction of oxygen on a strontium doped lanthanum manganite (LSM) electrode has been studied by both low field and high field polarization at various partial pressures of oxygen and temperature. The reciprocal of the electrode resistance ( T/ R e) was found to be 1/4 power dependent on the partial pressure of oxygen at high temperatures, and became P O 2 independent at low temperatures. Two temperature dependence regions with an activation energy of ∼160 kJ mol −1 in the high temperature region and ∼120 kJ mol −1 in the low temperature region were determined. Both the anodic and cathodic charge transfer coefficients were found to be unity at temperatures above 923 K. Based on the results for the temperature and oxygen partial pressure dependence, an electrochemical mechanism for oxygen reduction on the LSM has been proposed.

La 1−xSr xMnO 3–YSZ composite film electrodes prepared by metal-organic decomposition for solid oxide fuel cells

A composite film composed of strontium doped lanthanum manganite (SLM), La 0.9Sr 0.1MnO 3 and yttria stabilized zirconia (YSZ), (ZrO 2) 0.96(Y 2O 3) 0.04, has been successfully prepared by metal-organic decomposition (MOD) at temperatures of 800–1000°C using a single coating solution containing all the constituent elements. The addition of YSZ has been found to suppress the growth of SLM particles, therefore, increasing the porosity of the film. The composite film electrode has exhibited much lower overpotentials toward oxygen reduction in solid oxide fuel cell (SOFC) application than the pure La 0.9Sr 0.1MnO 3.

Preparation of cathodes for thin film SOFCs

SOFCs are expected to become competitive devices for electrical power generation, but successful application is dependent on decreasing working temperature from 1000 to 800 °C, without detrimental effects on resistance and on electrode processes. This requires a reduction of the stabilized zirconia electrolyte thickness and an optimization of the electrodes, especially the cathode, where losses are higher. Strontium doped lanthanum manganites are the most common materials tested as cathodes for SOFCs working at high temperature (1000 °C). This cathode material presents high electronic and oxygen-ion conductivities, a thermal expansion coefficient compatible with stabilized zirconia and good catalytic activity. For thin film SOFC devices working at intermediate temperatures (less than 800°C), we have studied the optimization of this type of cathode. Strontium doped lanthanum manganite has been deposited on yttria stabilized zirconia electrolyte substrates by spray-pyrolysis and by RF sputtering. The electrode performances depend strongly on cathode microstructure, influenced by processing conditions. With spray-pyrolysis processes, large porosity is expected. This is important for the supply of oxygen, via O2 molecules through the pores to the triple phase boundaries, where the gas, the cathode and the electrolyte are in contact and where oxygen reduction may occur. However, large porosity can have a nefaste effect on electronic conductivity. With RF sputtering, denser films with higher electronic conductivity are obtained. But, in that case, the supply of oxygen occurs via adsorbed O-atoms in a diffusion process through the cathode to the electrolyte. Spraypyrolysis and RF sputtering have been compared relative to electrode properties.

Thermoelectric power and electrical conductivity of strontium-doped lanthanum manganite

Thermoelectric power and electrical conductivity of pure and 5, 10 and 20% strontium-doped lanthanum manganite are determined as function of temperature in air and of P o 2 at 1000 °C. At high temperatures the thermoelectric power is negative. Both thermoelectric power and conductivity are found to be nearly independent of doping at high temperatures, but not at low. This can be explained if it is assumed that Mn 3+ disproportionates into Mn 2+ and Mn 4+. In reducing atmospheres the thermoelectric power is found to be only slightly dependent on P o 2 , which also might be explained by the disproportionation reaction.

Oxygen tracer diffusion coefficient of (La, Sr)MnO 3 ± δ

To understand the mechanism of oxygen diffusion, the oxygen tracer diffusion coefficient, D o ∗ , of strontium-doped lanthanum manganites, La 1 − x Sr xMnO 3 ± δ ( x = 0.05, 0.10, 0.15 and 0.20) was measured as a function of composition, temperature and oxygen partial pressure. The measured D o ∗ was as low as 10 −12 to 10 −11 cm 2 s −1 at 1000 °C, from which the ionic conductivity of 10 −7 to 10 −6 S/cm was estimated. Because of such a poor ionic conductivity, the possibility for the bulk diffusion of oxide ions to contribute to the cathode reactions in solid oxide fuel cells is considered to be sparse. The negative dependence of D o ∗ on the oxygen partial pressure suggests the vacancy mechanism for oxygen diffusion.

Chemical Compatibility between Strontium‐Doped Lanthanum Manganite and Yttria‐Stabilized Zirconia

Equimolar powder mixtures and multilayer pellets of single‐phase Sr‐doped lanthanum manganite perovskite materials Lay‐xSrxMnO3 with La content y = 1 and 0.95 and Sr content 0 ≤ x ≤ 0.5 were annealed in air with 8 mol% Y2O3‐ZrO2 at 1470 K, up to 400 h and at 1670 K. up to 200 h. X‐ray diffraction and electron probe microanalysis confirmed the formation of La2Zr2O7 or SrZrO3 depending on the composition of the perovskites. No reaction products could be detected for La0.95‐xSr xMnO3 with 0.2 ≤ x ≤ 0.4 after annealing for 400 h at 1470 K, and for the perovskite La0.65Sr0.3MnO3 even after annealing for 200 h at 1670 K. The results demonstrate the improved chemical compatibility of La‐deficient perovskites against reaction with zirconia and can provide a basis for the selection of a sufficiently chemically stable material for the air electrode of solid oxide fuel cells.

Structure, stoichiometry and phase purity of strontium-doped lanthanum manganite powders

Strontium-doped lanthanum manganite La1, xSrxMnO3 (LSM) is characterized by hexagonal-rhombohedral distorted perovskite-type structure, deriving from the ideal cubic symmetry through rotation of the [MnO6] octahedra around the ternary axis. The extent of distortion and unit cell volume depend on Sr content. The linear variation of the unit cell volume, observed in the compositional range 0.1 ⩽ x ⩽ 0.5, can be effectively used for determining the stoichiometry of LSM. A procedure for the quantitative analysis of segregated impurities (La2O3 and Mn3O4) based on X-ray powder diffraction analysis, has also been developed.

Electrochemical vapor deposition of stabilized zirconia and interconnection materials for solid oxide fuel cells

Due to the high operating temperatures (900–1000°C) the material demands upon Solid Oxide Fuel Cell (SOFC) components are quite stringent. Preferably lower operating temperatures (700–800°C) are desired so that gas feed lines, heat exchanges, and structure components can be fabricated from relatively cheap stainless steel components. Typically, the materials used in a SOFC are yttria-stabilized zirconia (YSZ) as the solid electrolyte, nickel-zirconia cermet as the anode, strontium-doped lanthanum manganite as the cathode, and magnesium-doped lanthanum chromite as the interconnection material. The electrolyte and interconnect are difficult to fabricate,, because they need to be gas tight, yet thin (30–50 microns) and mechanically stable. Due to the high volatility of CrO 3 the densfication of LaCrO 3 into thin layers is a more demanding challenge than the fabrication of the electrolyte. Electrochemical Vapor Deposition is the key technology for making thin layers of the solid electrolyte as well as the interconnection material LaCrO 3. In the simplest case the oxide growth can be modeled with the Wagner oxidation theory for metals. In this paper theory and experiment of the growth of ionically conducting YSZ and electronically conducting LaCrO 3 will be discussed.

Solid State Ionics in Solid Oxide Fuel Cells

AbstractDue to the high operating temperatures (900-1000 °C) the material demands upon Solid Oxide Fuel Cell (SOFC) components are quite stringent. Preferably lower operating temperatures (700-800 °C) are desired so that gas feed lines, heat exchangers, and structure components can be fabricated from relatively cheap stainless steel components.Typically, the materials used in a SOFC are yttria stabilized zirconia (YSZ) as the solid electrolyte, nickel-zirconia cermet as the anode, strontium doped lanthanum manganite as the cathode, and magnesium doped lanthanum chromite as the interconnection material. The electrolyte and interconnect are difficult to fabricate, because they need to be gas tight, yet thin (30-50 microns) and mechanically stable. Due to the high volatility of CrO3the densification of LaCrO3into thin layers is a more demanding challenge than the fabrication of the electrolyte.Electr°Chemical Vapor Deposition is the key technology for making thin layers of the solid electrolyte as well as the interconnection material LaCrO3. In the simplest case the oxide growth can be modeled with the Wagner oxidation theory for metals. In this paper theory and experiment of the growth of ionically conducting YSZ and electronically conducting LaCrO3will be discussed.