Transport and electrocatalytic properties of La0.3Sr0.7Co0.8Ga0.2O3−δ membranes

Incorporation of gallium into the perovskite lattice of La_0.3Sr_0.7CoO_3– leads to increasing unit cell volume and to decreasing thermal expansion, total conductivity and oxygen permeability. At 973–1223 K, the oxygen permeation fluxes through La_0.3Sr_0.7Co_0.8Ga_0.2O_3– ceramics with 96.5% density are determined by the bulk ionic conduction and surface exchange rates. The total conductivity of La_0.3Sr_0.7Co_0.8Ga_0.2O_3–, predominantly p-type electronic, exhibits an apparent pseudometallic behavior due to oxygen losses on heating, whereas the p(O₂) dependencies of the conductivity and Seebeck coefficient suggest a small-polaron mechanism of hole transport. The average thermal expansion coefficients in air are 15.9×10^–6 K^–1 at 360–710 K and 27.9×10^–6 K^–1 at 710–1030 K. On decreasing oxygen pressure down to 4–30 Pa at 973–1223 K, perovskite-type La_0.3Sr_0.7Co_0.8Ga_0.2O_3– transforms into a brownmillerite-like modification, whose electrical properties are essentially p(O₂) independent. Further reduction results in the decomposition of the brownmillerite into a multiphase oxide mixture at p(O₂)=8×10^–10–3×10^–4 Pa, and then in the segregation of metallic cobalt. Due to surface-limited oxygen transport, La_0.3Sr_0.7Co_0.8Ga_0.2O_3– membranes are, however, kinetically stable under an air/CH₄ gradient up to 1223 K. The conversion of dry methane in model membrane reactors increases with oxygen permeation flux and temperature, but yields high CO₂ concentrations (>90%), indicating a dominant role of complete CH₄ oxidation on the membrane surface.     

Surface modification of La0.3Sr0.7CoO3−δ ceramic membranes

The dependence of oxygen permeability of dense La_0.3Sr_0.7CoO_3−δ ceramics on membrane thickness indicates significant surface exchange limitations to the permeation fluxes, which suggests a possibility to increase membrane performance by surface activation. The cobaltite membranes with various porous layers applied onto the permeate-side surface were tested at 850–1120 K. Silver-modified La_0.3Sr_0.7CoO_3−δ membranes showed enhanced permeation at temperatures above 950 K; deposition of porous layers of PrO_x and Pr_0.7Sr_0.3CoO_3−δ had no positive effect. The maximum oxygen permeability at 850–1120 K was observed in the case of porous La_0.3Sr_0.7CoO_3−δ layers with surface density about 10 mg cm⁻². These results suggest that the surface exchange of lanthanum–strontium cobaltite membranes under an oxygen chemical potential gradient is limited by both oxygen sorption at the surface and ion diffusion through the surface oxide layers. Oxygen permeability of La_0.3Sr_0.7CoO_3−δ ceramics was found to increase with increasing grain size due to decreasing grain-boundary resistance to ionic transport.     

Evaluation of manganese and oxygen content in La0.7Sr0.3MnO3−δ and correlation with the thermodynamic data

In order to understand how the thermodynamic properties are related to the oxygen and manganese content in the Sr-doped lanthanum manganites, nonstoichiometric perovskite phases La_0.7Sr_0.3MnO_3−δ have been investigated by using solid state electrochemical techniques, as well as wet chemical methods. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with solid state electromotive force measurements (EMF). The results were correlated with the average Mn valence values as determined by a chemical method based on two independent iodometric titrations, with amperometric dead-stop end point detection. New features related to the relationship between the average Mn valence, the oxygen nonstoichiometry variation, and the thermodynamic behavior were evidenced.     

Effect of graphite pore former on oxygen electrodes prepared with La0.6Sr0.4CoO3−δ nanoparticles

This study aimed at fabricating porous crack-free and delamination-free La_0.6Sr_0.4CoO_3?δ electrodes using nanopowders and investigating oxygen reduction (occurring at solid oxide fuel cell cathodes) and oxygen evolution (occurring at solid oxide electrolysis cell anodes) at 600 °C in air. The electrodes were deposited by screen-printing on Ce_0.8Gd_0.2O_1.9 substrates. The pastes were prepared with nanoparticles synthesised by flame spray synthesis and graphite pore former. Without graphite, the electrodes sintered at 1000 °C exhibit relatively low porosity and significant densification which led to partial delamination and large overpotentials. The addition of graphite, which was removed by combustion at ca. 650 °C during sintering, markedly improves electrode performance by increasing porosity and reducing densification. A minimal overpotential for both the oxygen reduction and oxygen evolution was reached for a layer porosity of ca. 50–60 vol.%.     

Magnetotransport properties of Mo substituted La0.7Ca0.3Mn1−xMoxO3 perovskites

We studied the effects of Mo substitution on the structural, transport, and magnetic properties of the La_0.7Ca_0.3Mn_1−xMo_xO₃ (x ≤ 0.1) samples. Powder X-ray diffraction analysis reveals that the samples studied crystallize in the orthorhombic structure with space group Pbnm. Both particle size and morphology change significantly as the Mo content x varies. The metal-insulator transition temperature (T_MI) and Curie temperature (T_C) decrease monotonically as x increases. Magnetization data reveal that long-range FM ordering persists in all samples and the saturation moment decreases linearly as x increases. The smaller depression rate of dT_C/dx observed is mainly ascribed to the increased amount of Mn²⁺ ions with Mo doping, which opens the FM coupling between Mn²⁺–O–Mn³⁺ in the samples.     

Atomic-Scale Insight into Exsolution of CoFe Alloy Nanoparticles in La0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ with Efficient CO2 Electrolysis

In situ exsolution of metal nanoparticles in perovskite under reducing atmosphere is employed to generate a highly active metal–oxide interface for CO₂ electrolysis in a solid oxide electrolysis cell. Atomic-scale insight is provided into the exsolution of CoFe alloy nanoparticles in La_0.4Sr_0.6Co_0.2Fe_0.7Mo_0.1O_3−δ (LSCFM) by in situ scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy and DFT calculations. The doped Mo atoms occupy B sites of LSCFM, which increases the segregation energy of Co and Fe ions at B sites and improves the structural stability of LSCFM under a reducing atmosphere. In situ STEM measurements visualized sequential exsolution of Co and Fe ions, formation of CoFe alloy nanoparticles, and reversible exsolution and dissolution of CoFe alloy nanoparticles in LSCFM. The metal–oxide interface improves CO₂ adsorption and activation, showing a higher CO₂ electrolysis performance than the LSCFM counterparts.     

Electrochemical reduction of NO and O2 on La2−x Sr x CuO4-based electrodes

A series of La_2 − x Sr _x CuO₄ (x = 0.0, 0.05, 0.15, 0.25 and 0.35) compounds was investigated for the use of direct electrochemical reduction of NO in an all-solid-state electrochemical cell. The materials were investigated using cyclic voltammetry in 1% NO in Ar and 10% O₂ in Ar. The most selective electrode material was La₂CuO₄, which had an activity of NO reduction that was 6.8 times higher than that of O₂ at 400 °C. With increasing temperature, activity increased while selectivity decreased. Additionally, conductivity measurements were carried out, and the materials show metallic conductivity behavior which follows an adiabatic small polaron hopping mechanism.     

还原剂对La0.7Sr0.3Co0.8Fe0.2O3钙钛矿催化剂NOx储存性能的影响

采用溶胶-凝胶法制备了La0.7Sr0.3Co0.8Fe0.2O3钙钛矿催化剂,考察了还原剂种类(CO,C3H6,H2)对催化剂在氮氧化物储存还原(NSR)循环前后的氮氧化物储存量(NSC)和NO-to-NO2转化率的影响.O2程序升温脱附(O2-TPD)实验结果表明,CO还原后的钙钛矿催化剂上形成了较多的氧空位,而氧空位则是一种有效的NOx储存活性中心.活性测试和傅里叶红外变换(FTIR)光谱表征结果显示:在NSR循环中,以CO为还原剂时催化剂显示了最佳的氮氧化物(NOx)储存效果.进一步的研究结果显示,当采用CO作为还原剂时,经过三次NSR循环后,催化剂中出现了Sr3Fe2O7新物相,而该物相可能具有比La0.7Sr0.3Co0.8Fe0.2O3钙钛矿更佳的NOx储存性能.综上所述,CO作为还原剂时可能使钙钛矿催化剂产生更多的氧空位以及更易于储存NOx的Sr3Fe2O7物相,这些原因使其NOx储存性能得到了大幅度改善.     

Mixed valence states of Cr and Fe in La1−xSrxFe0.8Cr0.2O3−y

The La_1?xSr_xFe_0.8Cr_0.2O_3?y (x = 0.2, 0.4, 0.6 and 0.8) phases were studied by X-ray photoelectron spectroscopy at room temperature and ⁵⁷Fe Mössbauer spectroscopy at different temperatures. Mixed valence states were observed both for chromium and iron ions, justifying the complex magnetic behaviour exhibited by these compounds. The Mössbauer results indicate the simultaneous presence of Fe³⁺, Fe⁴⁺ and Fe⁵⁺ at 4.2 K and the co-existence of Fe³⁺ and Fe⁽³⁺ⁿ⁾⁺ at T = 293 K, with the latter fraction increasing with increasing strontium content. The presence of Cr^3+/4+ is interpreted as being mainly responsible for the incomplete charge disproportionation reaction of iron at low temperature, as deduced from the Mössbauer results.     

Co（Ⅱ）-salen complex encapsulated into MIL-100（Cr） for electrocatalytic reduction of oxygen

Co(II)-salen was encapsulated in MIL-100(Cr) metal organic framework by "ship in a bottle" to synthesize a new electrocatalyst, Cosalen@MIL-100(Cr). The material was characterized by XRD, FT-IR, UV-Vis and N2-adsorption. The Cosalen@MIL-100(Cr) modified glassy carbon electrode exhibits a well-defined reduction peak at the potential of –0.21 V toward the oxygen reduction reaction(ORR) by cyclic voltammetry(CV) in pH = 6.84 phosphate buffer. Almost 400 mV positive shift of potential at Cosalen@MIL-100(Cr) modified electrode for ORR compared with that at bare glassy carbon, indicates that Cosalen@MIL-100(Cr) possesses excellent electrocatalytic activity. The transferred number of electrons for ORR was determined by chronocoulometry. The result suggests that the introduction of Co(II)-salen complex into MOF increases the electrocatalytic activity via a four-electron reduction pathway. Furthermore, this electrocatalyst exhibits good stability and reproducibility.     

Properties and performance of BaxSr1−xCo0.8Fe0.2O3−δ materials for oxygen transport membranes

The present paper discusses the oxygen transport properties, oxygen stoichiometry, phase stability, and chemical and mechanical stability of the perovskites (BSCF) and (SCF) for air separation applications. The low oxygen conductive brownmillerite phase in SCF is characterized using in-situ neutron diffraction, thermographic analysis and temperature programmed desorption but this phase is not present for BSCF under the conditions studied. Although both materials show oxygen fluxes well above 10 ml/cm²·min at T=1,273 K and pO₂=1 bar for self-supporting, 200 μm-thick membranes, BSCF is preferred as a membrane material due to its phase stability. However, BSCF’s long-term stable performance remains to be confirmed. The deviation from ideal oxygen stoichiometry for both materials is high: δ>0.6. The thermal expansion coefficients of BSCF and SCF are 24×10⁻⁶ and 30×10⁻⁶ K⁻¹, respectively, as determined from neutron diffraction data. The phenomenon of kinetic demixing has been observed at pO₂<10⁻⁵ bar, resulting in roughening of the surface and enrichment with alkaline earth metals. Stress–strain curves were determined and indicated creep behavior that induces undesired ductility at T=1,073 K for SCF. Remedies for mechanical and chemical instabilities are discussed.     

Effect of synthesis method on oxygen adsorption/desorption properties of La–Sr–Co–Fe–O perovskite-type oxide

There has been a growing interest in utilizing La–Sr–Co–Fe–O perovskite-type oxide for efficient high temperature oxygen adsorption applications and oxygen removal process. In this paper, we focus our attention on the analysis of the determinants of the synthesis methods of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) powders for the oxygen adsorption/desorption applications. To this aim, LSCF powders were successfully synthesized by different synthesis routes using polymerized complex and citrate methods. The effects of synthesis methods on the structure, particle size, specific surface area, oxygen adsorption/desorption kinetics, and oxygen uptake capacities of LSCF perovskite-type oxides were investigated. The oxygen adsorption/desorption capacities and kinetics of the LSCF oxides increase with increasing (1) the temperature from 700 to 900 °C and (2) the surface area observed at a given temperature. Collectively, the experimental observations suggest that particle sizes may play an important role in oxygen uptake capacities and adsorption/desorption kinetics.     

Electronic transport and magnetic properties of the perovskites La0.8Sr0.2Co1−xFexO3; x ≤ 0.3

Transport and magnetic properties of LaCoO₃-based compounds, doped with 20% Sr and 2.5, 5, 10, 15, 20 and 30% Fe, were investigated by means of magnetization, resistivity and magnetoresistance measurements as well as by ⁵⁷Fe Mössbauer spectroscopy. While the temperature dependence of the dc and ac magnetic susceptibilities reveals the presence of magnetic phase separation accompanied by spin-glass and cluster-glass behavior, the electrical resistivity and magnetoresistance characteristics indicate that the mesoscopic structure of the present compounds is rather well described as consisting of ferromagnetic, metallic grains embedded in an insulating matrix. The effect of the partial Co → Fe substitution on the bulk magnetic and transport properties, as well as on the local state of Co and Fe ions is discussed.     

Stability and functional properties of Sr0.7Ce0.3MnO3 − δ as cathode material for solid oxide fuel cells

Studies of oxygen diffusion, interphase exchange, specific electric conductivity, and thermal expansion showed that perovskite-like Sr_0.7Ce_0.3MnO_{3 ? δ} (SCMO) as a potential cathode material for solid oxide fuel cells (SOFCs) has considerable advantages over the conventional materials based on lanthanum-strontium manganites. To prevent the interactions of SCMO with solid electrolyte membranes of stabilized zirconia and lanthanum gallate, it is necessary to deposit protective layers of solid solutions based on cerium oxide, which do not form new phases in contact with SCMO and electrolytes. The trials of model SOFCs with porous SCMO-based cathodes demonstrated satisfactory electrochemical and endurance characteristics of these electrodes.     

Characteristics of the La0.6Sr0.4Fe0.8Co0.2O3 − δ electrode in contact with the lanthanum gallate-based electrolyte

Lowering the working temperature of solid oxide fuel cells (SOFCs) is the main trend in their development, which requires selection of materials for electrolyte and electrodes. A highly conducting lanthanum gallate-based electrolyte is a promising material for creating medium-temperature SOFCs. The electrochemical characteristics of the La_0.6Sr_0.4Fe_0.8Co_0.2O_{3 ? δ} cathode that contacted with the La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 electrolyte subject to electrode formation temperatures have been investigated. It was found that at optimum bake-on temperatures of 1200–1250°C, the cathode polarization resistance at 800°C was ～0.08 Ohm cm², which is comparable to the world’s best achievements.     

Synthesis of efficient Vulcan–LaMnO3 perovskite nanocomposite for the oxygen reduction reaction

In situ autocombustion has been developed as a novel and efficient route for the synthesis of perovskite–carbon nanocomposites for the oxygen reduction reaction (ORR) in alkaline media. We demonstrate the synthesis of crystalline LaMnO_3 + δ perovskite–Vulcan composite with a high accessibility of active sites and high electronic conductivity required for efficient electrocatalysis. The rotating disc electrode measurements evidenced an excellent activity of the composite for the ORR.