Sr MnO Research Articles

Effect of Volatile Boron Species on the Electrocatalytic Activity of Cathodes of Solid Oxide Fuel Cells: I. (La,Sr)MnO3 Based Electrodes

The effect of volatile boron species on the microstructure and electrocatalytic activity of conventional (La,Sr)MnO3 (LSM) and nano-structured LSM infiltrated Y2O3-ZrO2 (LSM-YSZ) cathodes of solid oxide fuel cell (SOFC) is studied. The cathodes were heat-treated at 700–800°C in air for 7–30 days in the presence of borosilicate glass powder. Compared to freshly-prepared cathodes, heat-treatment in the presence of glass improves the electrocatalytic activity of LSM cathodes, but has a significant detrimental effect on the microstructure and electrochemical activity of nano-structured LSM-YSZ cathodes. Present study shows that boron is not chemically compatible with LSM and poisoning effect of boron is related to the size of LSM particles. Interaction between volatile boron species and nano-sized LSM leads to the decomposition of LSM perovskite structure and the significant degradation of the electrocatalytic activity of the electrodes. In the case of conventional LSM, the reaction between boron and LSM occurs primarily on the surface. The agglomeration of LaBO3 and formation of nonstoichiometry thin surface may be beneficial to the removal of the segregated species on the LSM surface and thus improves the performance. A reaction mechanism between volatile boron species and LSM electrodes is proposed.

Oxide cathodes for electrochemical devices made with the use of a nanostructured composition material

The possibility of using the LnOx mischmetal (Ln = Ce, La, Nd, Pr, Sm) for preparation of cathodes for solid-oxide fuel cells with the supported YSZ electrolyte is studied. The electrical and electrochemical characteristics of Ln-Mn-O electrodes with the ratio of all lanthanides contained in the mischmetal except for cerium to manganese Ln: Mn = 1: 1 and also of a material comprised of Ln-Mn-O and La0.8Sr0.2MnO3 are studied. The latter electrode material that contains 35–40 wt % of Ln-Mn-O and was sintered at 1200°C has the specific ohmic resistance of 0.1 Ω cm at 800°C. The polarization conductivity is compared for electrodes made of 100% Ln-Mn-O, 40 wt % Ln-Mn-O + 60 wt % La0.8Sr0.2MnO3, and 100% La0.8Sr0.2MnO3 in the initial state and after their modification through the introduction of an electrocatalyst (PrO2 − x). The highest polarization conductivity is typical of Ln-Mn-O + (La, Sr)MnO3 electrodes containing 40 wt % Ln-Mn-O and PrO2 − x. The polarization conductivity of these electrodes is found to be 25 S/cm2 at 800°C.

The formation of the complex manganites LnSr2Mn2O7 (Ln = La, Nd, Gd)

The process of formation of the Ruddlesden–Popper phases LnSr2Mn2O7 (Ln=La, Nd, Gd) was investigated by means of X-ray diffraction, thermal analysis and microprobe analysis. Two parallel pathways were found for the reaction. The first one includes the formation of the intermediate K2NiF4-type compound by interaction of Sr7Mn4O15 and perovskite-type solid solution (Ln,Sr)MnO3, while in the second one the K2NiF4-type intermediate is formed during the reaction of the same perovskite-type solid solution with the corresponding rare earth oxide. This result gives an explanation for the formation of two phases with slightly different cationic compositions. The kinetics of the reaction was investigated for both mechanisms resulting in determination of their relative contribution to the final composition. Pure NdSr2Mn2O7 was obtained using a step-by-step process according to the first mechanism, and the magnetic properties of this phase were investigated.

Temperature-dependent infrared properties of Ca doped (La,Sr)MnO3 compositions with potential thermal control application

The synthesized compositions La0.7Ca0.3−xSrxMnO3 show their potential in thermal control application. The crystal structure, surface morphology, and the temperature dependence of resistivity, infrared spectra and emissivities for the compositions are reported. All of the synthesized compositions exhibit the characteristics of perovskite structure and of metal–insulator transition. Crystal structure of the compositions changes from the orthorhombic to the rhombohedral structure with increasing doping level (x). The infrared properties of compositions can be automatically adjusted basing on variation in temperature. In the temperature range 97–373 K, the variation amplitude of infrared emissivity exceeds 0.5. The potential application of the compositions in thermal management is discussed.

Comparative study of the magnetism of SrTcO3 and Ca(Sr)MnO3

By the first-principles calculations, we studied the electronic structures and the magnetic properties of SrTcO3 and Ca(Sr)MnO3. We found the strikingly high Néel temperature of SrTcO3 is mostly due to the strong Tc(4d)–O(2p) hybridizations, since the Tc-4d states are more extended than the Mn-3d states. Such Tc(4d)–O(2p) hybridizations increase the super-exchange constants, hence increased the Néel temperatures.

Impact of volatile boron species on the microstructure and performance of nano-structured (Gd,Ce)O2 infiltrated (La,Sr)MnO3 cathodes of solid oxide fuel cells

The impact of volatile boron species on the microstructure and performance of nano-structured (Gd,Ce)O2-infiltrated (La,Sr)MnO3 (GDC-LSM) cathodes of solid oxide fuel cells is studied for the first time. The results indicate that after the heat treatment of the cathodes at 800°C in air for 30days in the presence of borosilicate glass significant grain growth and agglomeration of infiltrated GDC nanoparticles are observed. The electrode polarization resistance of the GDC–LSM cathode after the heat treatment in the presence of glass is 3.15Ωcm2 at 800°C, substantially higher than 0.17Ωcm2 of the cathode heat-treated in the absence of glass under identical conditions. ICP-OES analysis shows the deposition of boron species in the cathodes after sintering in the presence of glass powder. The results demonstrate the significant detrimental effect of volatile boron species on the microstructure and activity of nano-structured GDC–LSM cathode.

La0.7Sr0.3MnO3 Nanoparticles Based Ultra-High Sensitive Ammonia Chemical Sensor

A facile, reliable, reproducible and ultra-high sensitive aqueous ammonia chemical sensor has been fabricated based on the utilization of La(0.7)Sr(0.3)MnO3 nanoparticles (LSMO NPs), as efficient electron mediators, and reported in this paper. The LSMO NPs were prepared by hydrothermal protocol followed by the annealing process and characterized in detail in terms of their mophological, structural and compositional properties. The I-V technique based aqueous ammonia sensor exhibits an ultra-high sensitivity of 494.68 +/- 0.01 microA cm(-2)mM(-1) and very low-detection limit of 0.2 microM with a response time less than 10 s. To the best of our knowledge, this is the first report in which LSMO is used as an efficient electron mediator for the fabrication of aqueous ammonia chemical sensor. Moreover, by comparing the literature, it is confirmed that the fabricated sensor exhibits highest sensitivity towards the detection of aqueous ammonia. This LSMO nanomaterial based research broadens the range of efficient electron mediators utilized for the fabrication of ultra-high sensitive chemical sensors.

Oxygen exchange kinetics on solid oxide fuel cell cathode materials—general trends and their mechanistic interpretation

Abstract

Structure and Relative Thermal Stability of Mesoporous (La,Sr)MnO3 Powders Prepared Using Evaporation‐Induced Self‐Assembly Methods

LaMnO3 and (La,Sr)MnO3 were prepared as mesoporous powders using sol‐gel methods that incorporated evaporation‐induced self‐assembly of surfactants as structure‐directing agents. The precursor and surfactant concentrations, surfactant character (mainly molecular weight), aging humidity, and annealing temperature were all found to influence the specific surface area of the powders, which were maximized at 50 and 46 m2/g for LaMnO3 and (La,Sr)MnO3, respectively. Increased metal nitrate contents in the initial sol composition and increased relative humidity during aging led to products having increased specific surface areas owing to increased mesoporosity. The lower molecular weight surfactant CTAB was found to generate higher specific surface area powders than those generated using the larger molecular weight nonionic surfactants P‐123 and F‐127. On the other hand, the relative thermal stability of the mesoporous powders, annealed at 750°C for 100 h, increased with the molecular weight of surfactant. Mesoporous powders having promising thermal stability and surface areas in the 20–25 m2/g range were easily obtainable using P‐123 and F‐127, indicating that high surface area electrocatalysts with mesoporous microstructures can be prepared for high temperature applications.

Electrochemical properties of dense (La, Sr)MnO3 − δ films produced by pulsed laser deposition

The electrical conductivity and the electrochemical properties of dense La0.85Sr0.15MnO3−δ (LSM) films, prepared using Pulsed Laser Deposition (PLD) have been investigated. The nano-structure of the LSM films has been characterized using scanning electron microscopy (SEM). The oxygen transport properties of the films were studied using electrochemical impedance spectroscopy (EIS) on symmetrical cells. Electrical conductivity measurement were carried out in an oxygen partial pressure range from 1 to 0.0032atm, and a temperature range from 950 to 700°C. The electrical conductivity at 800°C was 116Scm−1 and did not vary with the pO2. Impedance spectra were recorded in the temperature range from 600 to 950°C in the oxygen partial pressure range from 0.06 to 1.00atm. At 900°C and 1.00atm the area specific resistance was 30.6Ωcm2 and an activation energy of 2.7eV was found. The ASR increased with decreasing pO2. Based on the impedance spectra an oxide ion conductivity of 5.0×10−1Scm−1 was found at 900°C and pO2=1.00atm. The characteristic length, at which the oxide transport reaction switch from being limited by bulk diffusion to that of surface exchange, was estimated to be approximately 220 and 130nm at 700 and 900°C, respectively.

Performance and stability of (La,Sr)MnO3–Y2O3–ZrO2 composite oxygen electrodes under solid oxide electrolysis cell operation conditions

The electrochemical performance and stability of (La,Sr)MnO3–Y2O3–ZrO2 (LSM-YSZ) composite oxygen electrodes is studied in detail under solid oxide electrolysis cells (SOECs) operation conditions. The introduction of YSZ electrolyte phase to form an LSM-YSZ composite oxygen electrode substantially enhances the electrocatalytic activity for oxygen oxidation reaction. However, the composite electrode degrades significantly under SOEC mode tested at 500 mA cm−2 and 800 °C. The electrode degradation is characterized by deteriorated surface diffusion and oxygen ion exchange and migration processes. The degradation in electrode performance and stability is most likely associated with the breakup of LSM grains and formation of LSM nanoparticles at the electrode/electrolyte interface, and the formation of nano-patterns on YSZ electrolyte surface under the electrolysis polarization conditions. The results indicate that it is important to minimize the direct contact of LSM particles and YSZ electrolyte at the interface in order to prevent the detrimental effect of the LSM nanoparticle formation on the performance and stability of LSM-based composite oxygen electrodes.

Reasons for the high stability of nano-structured (La,Sr)MnO3 infiltrated Y2O3–ZrO2 composite oxygen electrodes of solid oxide electrolysis cells

Nano-structured (La,Sr)MnO3 infiltrated Y2O3–ZrO2 (LSM–YSZ) oxygen electrodes show excellent activity and performance stability under solid oxide electrolysis cells (SOECs) operation conditions. LSM–YSZ composite electrodes are prepared by infiltrating pre-sintered YSZ scaffold with LSM nitrate solution, followed by heat-treatment at 900 or 1100°C. The electrodes heat-treated at 900°C exhibit an electrode polarization resistance as low as 0.21Ωcm2 at 800°C and are relatively stable under electrolysis operation at 500mAcm−2 for 100h, while the electrodes heat-treated at 1100°C show the increased stability with the electrolysis polarization. The results clearly indicate that LSM lattice shrinkage under anodic electrolysis conditions inhibits the agglomeration and grain growth of infiltrated LSM nanoparticles, leading to the highly stable nano-structured LSM–YSZ electrode structure.

Correlation between degradation of cathode performance and chromium concentration in (La,Sr)MnO3 cathode

The relationship between the Chromium (Cr) concentration in the porous (La,Sr) MnO3 (LSM) cathode and the polarization resistance was examined at accelerated Cr poisoning conditions. By supplying Cr vapor (p(Cr)~10−8atm) to the porous LSM cathode the polarization resistances were increased significantly. Secondary ion mass spectrometry (SIMS) analysis of the LSM cathode suggested that an increase of Cr concentration levels both at the surface and at the triple phase boundary (TPBs) under polarization. The precise Cr concentration in the porous LSM cathode was successfully determined by SIMS to be in the orders of 0.0001–1wt.% levels. The deposited Cr at the TPB effectively reduced the cathode performance under polarization; and the deposited Cr concentration at the TPB was above 1wt.% in the Cr-poisoned cells. The polarization resistance increased with the Cr-concentration at the TPBs as well as at the cathode surface areas.

Synthesis, characterization and CO oxidation of TiO2/(La,Sr)MnO3 composite nanorod array

Ordered TiO2/(La,Sr)MnO3 composite nanorod array catalysts have been successfully fabricated by using a combination of hydrothermal synthesis and radio-frequency (RF) magnetron sputtering method on Si and glass substrates. Electron microscopy, energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) method are used to characterize the structure, morphology, composition and surface area of the as-prepared nanorod arrays. The TiO2/(La,Sr)MnO3 core–shell nanorod arrays exhibit an enhanced CO oxidation activity compared to TiO2 nanorods, which might be due to the lattice oxygen in (La,Sr)MnO3. The TiO2/(La,Sr)MnO3 composite nanorod array may provide a promising class of nanocatalysts for energy and environment applications.

Rectifying Property of the Heterojunction Composed of La<sub>0. 8</sub>Sr<sub>0.2</sub>MnO<sub>3</sub> on Si with and without SrMnO3 Diffusion Barrier Layers

La0.8Sr0.2MnO3 (LSMO) films with SrMnO3 (SMO) diffusion barrier layers were deposited on (100) Si substrates at 600oC by RF magnetron sputtering. From X-ray diffraction patterns (XRD), (110) peak of LSMO has been greatly enhanced in LSMO/SMO/Si, which may result from small lattice mismatch between SMO and LSMO Rutherford backscattering spectrometry spectra (RBS) measurements clearly show that there is a sharp interface between SMO and Si and small diffusion between LSMO and SMO after introducing SMO diffusion barrier layer. Small lattice mismatch is also considered to play an important role in deciding good interface quality. The current-voltage measurement shows a good rectifying property of LSMO/SMO/Si when the thickness of SMO is 50 nm. On further increasing SMO thickness, the junction currents are depressed at the same applied positive voltage. We attribute the results to the bigger junction resistance caused through introducing thicker barrier layer.

Enhanced electrochemical performance and stability of (La,Sr)MnO3–(Gd,Ce)O2 oxygen electrodes of solid oxide electrolysis cells by palladium infiltration

Palladium-impregnated or infiltrated La0.8Sr0.2MnO3–Gd0.2Ce0.8O1.9 (LSM-GDC) composites are studied as the oxygen electrodes (anodes) for the hydrogen production in solid oxide electrolysis cells (SOECs). The incorporation of small amount of Pd nanoparticles leads to a substantial increase in the electrocatalytic activity and stability of the LSM-GDC oxygen electrodes. The electrode polarization resistance (RE) at 800 °C on a 0.2 mg cm−2 Pd-infiltrated LSM-GDC electrode is 0.13 Ω cm2, significantly smaller than 0.42 Ω cm2 for the reaction on the pure LSM-GDC electrodes. The overpotential loss is also substantially reduced after the Pd infiltration; at an anodic overpotential 50 mV and 800 °C, the current increases from 0.15 A cm−2 for the pure LSM-GDC anode to 0.47 A cm−2 on a 0.3 mg cm−2 Pd-infiltrated LSM-GDC. The infiltrated Pd nanoparticles enhance the stability of the LSM-GDC oxygen electrodes and are most effective in the promotion of the diffusion, exchange and combination processes of oxygen species on the surface of LSM-GDC particles, leading to the increase in the oxygen evolution reaction rate.

Probing La0.7Sr0.3MnO3multilayers via spin wave resonances

La(0.7)Sr(0.3)MnO3/BiFeO3 and La(0.7)Sr(0.3)MnO3/PbZr20Ti80O3 epitaxial heterostructures have been grown on SrTiO3 substrates. Spin wave resonances are used to study interface properties of the ferromagnetic La(0.7)Sr(0.3)MnO3. We find that the addition of the BiFeO3 or PbZr20Ti80O3 causes out-of-plane surface pinning of the La(0.7)Sr(0.3)MnO3. We are able to place new limits on the exchange constant D of La(0.7)Sr(0.3)MnO3 grown on these substrates and confirm the presence of uniaxial and biaxial anisotropies caused by the SrTiO3 substrate.

Transformations of head-to-head domain walls in (La,Sr)MnO3 thin films

Kinetics of the in-plane magnetization reversal in a (La,Sr)MnO3 thin film has been studied at different temperatures using the magneto-optic visualization technique. In addition to changes of the domain wall coercivity and mobility, dramatic transformations of the domain wall structure have been observed. Motion of a single zig-zag domain wall is typical for high temperature magnetization reversal, whereas complicated diffuse transient structure between the two principal domains is observed at low temperature. Generation of secondary bubble domains in front of a moving wall has been observed for the first time in a quasistatic regime, contrary to previous superfast experiments.

Preparation and Characterization of (La0.8Sr0.2)0.95MnO3−δ (LSM) Thin Films and LSM/LSCF Interface for Solid Oxide Fuel Cells

Uniform, dense, and conformal coatings of (La,Sr)MnO3−δ (LSM) have been successfully deposited on a silicon wafer and a dense (La,Sr)(Co,Fe)O3−δ (LSCF) substrate using a stable LSM sol consisting of metal acetate and nitrate precursors dissolved in a mixed organic solvent of 2‐methoxyethanol and acetic acid with enhanced wettability. The processing conditions are optimized for precise control of composition, morphology, microstructure, and thickness of the LSM films to examine the microstructure and chemical stability of an LSM film as a catalytic coating for an LSCF cathode. The thicknesses of the LSM films are controlled within the range of 5–60 nm by spin‐coating of LSM sol with different concentrations. The LSM films grow epitaxially on the LSCF substrate grains after annealing at 800°C for 1 h due to their structural similarity. The LSM coatings show good stability on LSCF substrates and may suppress strontium oxide segregation on LSCF surface during annealing at 850°C for 900 h, implying that LSM‐coated LSCF surfaces have better structural and chemical stability under typical fuel cell operating conditions.

Quantification of Microstructural Change at the Interface between (La,Sr)MnO3+δ Cathode and YSZ Electrolyte Upon Discharge Operation

The microstructural change at the interface between LSM cathode and YSZ electrolyte upon discharge was quantitatively evaluated by FIB-SEM technique to elucidate the activation process of cathode. The performance of the cell was enhanced soon after the current passage up to ca. 5 h at 1000°C due to the reduction in cathodic impedance. In response to this behavior, the LSM/YSZ interfacial structure changed depending on the time duration of discharge; the roughness of YSZ surface both in and out of contact with LSM increased. Furthermore, the closed pores were formed at the well-adhered LSM/YSZ interface. The TPB-length of the as-prepared sample was evaluated to be 1.26 μm/μm2, whereas after the discharge at 200 mA/cm2 for 300 min the total TPB-length was increased to 1.72 μm/μm2. In the latter case, however, the active TPB-length was 1.35 μm/μm2. Then the contribution rate of TPB-length to the performance enhancement of cathode was discussed.