Thermal stability of the cubic phase in Ba0.5Sr0.5Co0.8Fe0.2O3 - δ (BSCF)1

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 - δ (BSCF) is a material with excellent oxygen ionic and electronic transport properties reported by many research groups. In its cubic phase, this mixed ionic-electronic conducting (MIEC) perovskite is a promising candidate for oxygen permeation membranes. For this application, its long-term stability under operating conditions (especially temperature and oxygen partial pressure) is of crucial importance.The present work is focused on the thermal stability of the BSCF cubic phase in the targeted temperature range for applications (700…900 °C) in light of previous studies in literature reporting a reversible transition to a hexagonal phase somewhere below 900 °C.To this end, single phase cubic BSCF powders were annealed at different temperatures over varying periods of time. Phase composition was subsequently analysed by X-ray diffractometry (XRD) in order to determine both the temperature limit and the time-scale for the formation of the hexagonal phase. Additionally, the long-term behaviour of the electrical conductivity was examined on bulk samples at 700 °C, 800 °C and 900 °C over several hundreds of hours, showing a prolonged decrease at 800 °C. The decrease in electrical conductivity at this temperature was also examined on bulk samples with different grain sizes, showing a more pronounced decrease the smaller the average grain size.Coexistence of both phases (cubic and hexagonal) could also be shown for 700 °C, however with a different phase equilibrium than at 800 °C.     

X-ray photoelectron (XPS) and Diffuse Reflectance Infra Fourier Transformation (DRIFT) study of Ba0.5Sr0.5CoxFe1−xO3−δ (BSCF: x=0-0.8) ceramics

The X-ray photoelectron spectra (XPS) of sintered BSCF ceramics (Ba_0.5Sr_0.5Co_xFe_1-xO_3-δ,0≤x≤0.8) were measured at room temperature (RT). Peak areas of Fe_2p1, Fe_2p3, Fe_3p and Co_3p increased systematically with increasing cobalt concentration, while their binding energies (BEs) remained the same (723.3, 710.0, 55.0 and 60.9 eV, respectively). However, the BEs of lattice oxygen in O_1s (528.1 eV) and Ba_4d for the BaO bond (87.9V and 90.2 eV) increased with increasing cobalt concentration. The shoulder peak of Ba_3d/Co_2p increased from 778.0 to 778.7 eV, which implies that this peak can be attributed to another Ba XPS peak (described as Ba_2nd in this study) due to the overlapping area between barium cations and oxygen anions. The overall peak areas of Ba_4d increased up to x=0.4, and then decreased, which coincides with the behavior of the Diffuse Reflectance Infrared Fourier Transform (DRIFT) bands representing adsorbed CO₃²⁻ (νCO₃²⁻) and structurally bonded CO₃²⁻ (ν₂, ν₃) (800-1200 and 862/1433 cm⁻¹, respectively).     

颗粒弥散Ba0.5Sr0.5Co0.8Fe0.2O3-δ基复合透氧膜材料的制备、结构与性能研究

通过外加等摩尔分数的ZrO2 和过量BaO,原位生成一定比例的BaZrO3 基第二相颗粒复合的制备法,对具有较高氧渗透率的Ba0. 5Sr0. 5Co0. 8Fe0. 2O3-δ进行了原位引入Ba(Sr)Zr(CoFe)O3 第二相颗粒的改性研究发现引入的第二相颗粒不但具有抑制基相晶粒的生长、细化和均化基相晶粒的作用,同时减小了基相高温氧脱量而起到稳定相结构的作用.引入5%ZrO2 和5%过量BaO复合物的抗弯强度比纯相提高68%;引入10%Zr和10%过量BaO、20%ZrO2 和20%过量BaO的复合物(均为摩尔分数),在850℃以下具有更高的氧渗透率;合物的电导率随第二相颗粒含量的增加而减小;氧脱附量较小的复合物的载流子浓度相对稳定,因而升降温过中电导率热回滞峰较小.     

Performance and stability of niobium-substituted Ba0.5Sr0.5Co0.8Fe0.2O3 − δ membranes

The phase stability, thermal expansion, electrical conductivity, and oxygen permeation of perovskite-type oxides Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_1 − xNb_xO_3 − δ (x = 0 − 0.2) have been investigated. Room-temperature X-ray diffraction of as-prepared powders indicates that in the investigated compositional range solid solutions are formed. Long-term annealing experiments both in flowing air and nitrogen, at 750 °C, demonstrate that the phase instability observed in parent Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ (BSCF) is suppressed already at the minimum substitution of 5 mol% of niobium for (Co, Fe). Both electrical conductivity and thermal expansion are found to decrease with increasing niobium concentration, which behaviors can be explained by defect chemical considerations, taking into account charge compensation mechanisms by doping BSCF with Nb⁵⁺ donor cations. The oxygen permeation flux of 10 mol% Nb-substituted BSCF, in the range 800-900 °C, is reduced by 10% relative to that found for parent BSCF. Switching from helium to a CO₂-containing purge gas results in a severe reduction or cessation of the oxygen flux. Options are discussed to avoid undesired formation of surface carbonates.     

Grain boundaries as barrier for oxygen transport in perovskite-type membranes

Perovskite-type membranes of (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)O_3−δ (BSCF) and (Ba_0.5Sr_0.5)(Fe_0.8Zn_0.2)O_3−δ (BSFZ) were successfully prepared via liquid-phase sintering using BN as sintering aid. The obtained membranes were examined via powder X-ray diffraction pattern (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and oxygen permeation experiments. It has emerged that the use of BN as sintering aid lowers sintering temperatures in order to obtain dense membranes with relative densities in the range of 93–96% as proven by the Archimedes method. It was further shown that the perovskite structure could be maintained after sintering with BN. Additionally, BN was completely removed from the sintered membranes. Investigation of the microstructure revealed that the average grain size of the membranes was influenced by the amount of BN added prior the sintering process. It was found that large amounts of BN effectively lower the average grain size. Oxygen permeation experiments have shown that the lower the average grain size the lower the oxygen permeation performance, particularly in the case of BSCF. Transmission electron microscopy revealed that no evidence for an amorphous layer or any other interfacial phase in the grain boundary is present.     

Synthesis and Characterization of Ba0.5Sr0.5NixCo0.8‐xFe0.2O3‐δ (x=0 and 0.2) Perovskites as Electro‐catalysts for Methanol Oxidation in Alkaline Media

In this study, the perovskite‐type oxides Ba_0.5Sr_0.5Ni_0.2Co_0.6Fe_0.2O_3‐δ (BSNCF) with different B‐site transition metal elements were investigated as a new catalyst for methanol oxidation. They were prepared by the partial substitution of cobalt by nickel in Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) sample. Both materials prepared using a combined citrate/EDTA complexing method and their catalytic activitiesfor methanol oxidation were comparativelystudied using cyclic‐voltammetry (CV) and chronoamperometry (CA) in alkaline media at room temperature. Structural, textural, morphological and redox properties of the synthesized solids were investigated by several physicochemical methods such as XRD, FTIR, LRS, SEM‐EDX, BET and H₂‐TPR. The as‐prepared materials were obtained with high purity and high crystallinity. The partial substitution of cobalt by nickel in BSCF perovskite changes the structure of the solid which pass from cubic structure to a mixture of cubic and hexagonal forms. This change is accompanied by an increase in BET surface area and a decrease in crystalline size. For the electro‐catalysis measurements, results demonstrated that partially substituting of Co by Ni in BSCF structure enhance the electro‐catalytical activity towards the oxidation of methanol under alkaline conditions, making it an electro‐catalyst candidate for practical utilization in DMFCs.     

Oxygen permeation of BSCF membrane with varying thickness and surface coating

The oxygen permeation of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) membranes was measured between 750 and 900 °C as a function of membrane thickness with or without La_0.7Sr_0.3CoO₃ (LSC) coating layer under controlled PO₂-gradient$P_{{\text{O}}_2}\text{-gradient}$ (Air/He). In order to see the relative effects of bulk diffusion and surface-exchange kinetics, the thickness of membrane was varied from 0.5 to 2.0 mm. The oxygen-permeation flux at 900 °C increased with LSC coating from that of uncoated membrane. For example, it increased ∼1.8 times for 1 mm-thick BSCF membrane. The characteristic membrane thickness (L_C) which divides the bulk-diffusion limit and surface-exchange kinetics limit was estimated using the modified Wagner equation. The L_C values were 0.55 and 1.10 mm at 900 °C for the coated and uncoated BSCF membranes, respectively, and decreased with decreasing temperature.