Preparation and Oxygen Permeation Properties of BaCrOx Coated Ba0.5Sr0.5Co0.8Fe0.2O3- Tubular Membrane

Barium-chromium oxide (BaCrO_x) coated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) tubular membranes were successfully prepared and evaluated for oxygen separation applications under high pressure–temperature conditions. The oxygen permeation flux was measured in accordance with the temperature, air pressure, and retentate flow rate, ranging from 750–950°C, 3–9 atm, and 200–1000 mL/min, respectively. The permeation testing on the BaCrO_x coated BSCF tubular membranes showed that the oxygen flux increased as the temperature, pressure, and retentate flow rate increased. The oxygen permeation flux was 5.7 mL/(min cm²) with temperature, pressure, and retentate flow rate of 900°C, 9 atm, and 1000 mL/min, respectively. The temperature dependence of the oxygen permeation process is further investigated, and the Arrhenius pre-exponential factor, as well as the apparent activation energy, is determined.     

Preparation and oxygen permeation of U‐shaped perovskite hollow‐fiber membranes

The oxygen permeation of dense U‐shaped perovskite hollow‐fiber membranes based on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ prepared by a phase inversion spinning process is reported. The perovskite hollow fibers with totally dense wall were obtained with the outer diameter of 1.147 mm and the inner diameter of 0.691 mm. The dependences of the oxygen permeation on the air flow rate on the shell side, the helium flow rate on the core side, the oxygen partial pressures, and the operating temperatures were experimentally investigated. According to the Wagner theory, it follows that the oxygen transport through the U‐shaped hollow‐fiber membrane is controlled by both surface reaction and bulk diffusion at the temperature ranges of 750–950°C. High oxygen permeation flux of 3.0 ml/(min cm²) was kept for about 250 h at 950°C under the conditions of the air feed flow rate of 150 ml/min and the helium flow rate of 50 ml/min. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Flame Spray Synthesis of Nanoscale La0.6Sr0.4Co0.2Fe0.8O3–δ and Ba0.5Sr0.5Co0.8Fe0.2O3–δ as Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells

Electrochemical performance and degradation was analysed by conductivity measurements as well as thermogravimetric analysis (TGA) under different atmospheres. CO₂ was identified as a critical parameter in terms of carbonate formation from Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ and causes a strong increase in the material resistivity, whereas La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ is unaffected. The oxygen exchange kinetic of both compositions is affected by CO₂ containing atmospheres.     

Subcritical crack growth behavior of a perovskite‐structured Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen transport membrane

This paper herein studies subcritical crack growth (SCG) behavior of a perovskite‐structured Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) as an oxygen transport membrane material. The SCG behavior of BSCF is evaluated by a constant load method and constant stress rate method at room temperature (RT) and 800°C in air, respectively. The low crack velocity measurements are carried out by ring‐on‐ring bending tests while the high crack velocity measurements by compact tension tests. The stress rates vary approximately from 0.1012 to 101.2 MPa/min. The fracture stress increases with increasing stress rate at 800°C. The SCG parameter, n, of BSCF is determined to be 24.32 and 13.83 at RT and 800°C in air, respectively. This indicates that BSCF is more susceptible to SCG at 800°C. The strength‐probability‐time (SPT) diagram is constructed for design proposes. The stress for a lifetime of 40 years should not exceed 27.21 MPa at RT or 4.53 MPa at 800°C to assure a failure probability below 1%.     

Characterization of Ba0.5Sr0.5Co0.7In0.1Fe0.2O3−δ as the Cathode Material for Proton‐conducting SOFCs

Ba_0.5Sr_0.5Co_0.7In_0.1Fe_0.2O_3−δ powders are successfully synthesized as the cathode materials for proton‐conducting solid oxide fuel cells (SOFCs). The prepared cells consisting of the structure of a BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7)‐NiO anode substrate, a BZCY7 electrolyte membrane and a cathode layer, are measured from 600 to 700 °C with humidified hydrogen (ca. 3% H₂O) as the fuel. The electrochemical results show that the cell exhibits a high power density which could obtain an open‐circuit potential of 0.986 V and a maximum power density of 400.84 mW cm⁻² at 700 °C. The polarization resistance measured at the open‐circuit condition is only 0.15 Ω cm² at 700 °C.     

The Effect of Plasma Spraying Power on La0.58Sr0.4Co0.2Fe0.8O3–δ–La0.8Sr0.2Ga0.8Mg0.2O3–δ Composite Cathode Interlayer Microstructure and Cell Performance

The metal‐supported intermediate temperature solid oxide fuel cells with a porous nickel substrate, a nano‐structured LDC (Ce_0.55La_0.45O_2–δ)–Ni composite anode, an LDC diffusion barrier layer, an LSGM (La_0.8Sr_0.2Ga_0.8Mg_0.2O_3–δ) electrolyte, an LSCF (La_0.58Sr_0.4Co_0.2Fe_0.8O_3–δ)–LSGM composite cathode interlayer and an LSCF cathode current collector are fabricated by atmospheric plasma spraying. Four different plasma spraying powers of 26, 28, 30, and 34 kW are used to fabricate the LSCF–LSGM composite cathode interlayers. Each cell with a prepared LSCF–LSGM composite cathode interlayer has been post‐heat treated at 960 °C for 2 h in air with an applied pressure of 450 g cm^–2. The current‐voltage‐power and AC impedance measurements indicate that the LSCF–LSGM composite cathode interlayer formed at 28 kW plasma spraying power has the best power performance and the smallest polarization resistance at temperatures from 600 to 800 °C. The microstructure of the LSCF–LSGM composite cathode interlayer shows to be less dense and composed of smaller dense regions as the plasma spraying power decreases to 28 kW. The durability test of the cell with an optimized LSCF–LSGM composite cathode interlayer gives a degradation rate of 1.1% kh^–1 at the 0.3 A cm^–2 constant current density and 750 °C test temperature.     

Oxygen permeability and structural stability of a novel tantalum‐doped perovskite BaCo0.7Fe0.2Ta0.1O3−δ

Dense BaCo_0.7Fe_0.2Ta_0.1O_3?δ (BCFT) perovskite membranes were successfully synthesized by a simple solid state reaction. In situ high‐temperature X‐ray diffraction indicated the good structure stability and phase reversibility of BCFT at high temperatures. The thermal expansion coefficient (TEC) of BCFT was determined to amount 1.02 × 10^?5 K^?1, which is smaller than those of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) (1.15 × 10^?5 K^?1), SrCo_0.8Fe_0.2O_3?δ (SCF) (1.79 × 10^?5 K^?1), and BaCo_0.4Fe_0.4Zr_0.2O_3?δ (BCFZ) (1.03 × 10^?5 K^?1). It can be seen that the introduction of Ta ions into the perovskite framework could effectively lower the TEC. Thickness dependence studies of oxygen permeation through the BCFT membrane indicated that the oxygen permeation process was controlled by bulk diffusion. A membrane reactor made from BCFT was successfully operated for the partial oxidation of methane to syngas at 900°C for 400 h without failure and with the relatively high, stable oxygen permeation flux of about 16.8 ml/min cm². © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Degradation of La0.6Sr0.4Co0.2Fe0.8O3–δ–Ce0.8Sm0.2O1.9 Cathodes on Coated and Uncoated Porous Metal Supports

The degradation of composite LSCF‐SDC cathodes on porous 430 stainless steel supports was investigated. Two degradation mechanisms were observed: a multi‐layer oxide scale, believed to consist of Cr₂O₃ and SrCrO₄, formed at the support‐cathode interface, and small amounts of chromium were detected within the cathodes. To reduce degradation, La₂O₃ and Y₂O₃ reactive element oxide coatings were deposited on the internal pore surfaces of the metal supports. The reactive element oxide coatings reduced the amount of volatile chromium that deposited in the cathodes. As a result, the degradation rates of the cathodes on coated supports were significantly lower than the degradation rates of cathodes made on uncoated metal supports. In cathode symmetrical cells, polarization resistance degradation rates as low as 2.56 × 10⁻⁶ Ω cm² h⁻¹ were observed over 100 hours on coated metal supports, compared to an average of 1.23 × 10⁻⁴ Ω cm² h⁻¹ on uncoated supports.     

Mechanism and kinetic modeling for steam reforming of toluene on La0.8Sr0.2Ni0.8Fe0.2O3 catalyst

Reaction mechanism for steam reforming of toluene is proposed for La_0.8Sr_0.2Ni_0.8Fe_0.2O₃ perovskite catalyst. The proposed mechanism was derived from various characterization results such as temperature‐programmed desorption (TPD) and temperature‐programmed surface reaction (TPSR) water, TPSR toluene, TPD O₂ and in situ DRIFT of toluene decomposition, and steam reforming of toluene. Five kinetic models were developed based on the proposed dual‐site reaction mechanism using Langmuir–Hinshelwood approach. Subsequently, the parameters of the kinetic models were estimated by nonlinear least square regression. A good agreement was obtained between experimental and model predicted results for the rate determining step based on reaction between adsorbed aldehyde and adsorbed oxygen. The adsorbed aldehyde species is produced from the reaction between adsorbed C₂H₂ or CH₂ and adsorbed oxygen while the adsorbed oxygen species can come from the oxygen from water activation, lattice oxygen species, and/or the redox property of some metals such as Fe. This shows that the adsorbed oxygen species plays important role in this reaction. © 2014 American Institute of Chemical Engineers AIChE J 60: 4190–4198, 2014     

La0.6Sr0.4Co0.8Ga0.2O3‐δ (LSCG) hollow fiber membrane reactor: Partial oxidation of methane at medium temperature

In this study, La_0.6Sr_0.4Co_0.8Ga_0.2O_3‐δ (LSCG) hollow fiber membrane reactor was integrated with Ni/LaAlO₃‐Al₂O₃ catalyst for the catalytic partial oxidation of methane (POM) reaction. The process was successfully carried out in the medium temperature range (600–800°C) for reaction of blank POM with bare membrane, catalytic POM reaction and swept with H₂:CO gas mixture. For the catalytic POM reaction, enhancement in selectivity to H₂ and CO is obtained between 650–750°C when O₂:CH₄ <1. High CH₄ conversion of 97% is achieved at 750°C with corresponding H₂ and CO selectivity of about 74 and 91%. The oxygen flux of the membranes also increased with the increase in oxygen partial pressure gradient across the membrane. The postreacted membranes were tested via XRD and FESEM‐EDX for their crystallinity and surface morphology. XPS analysis was further used to investigate the O1s, Co 2p and Sr 3d binding energies of the segregated elements from the reducing reaction environment. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3874–3885, 2013     

Electrochemical Properties of La0.5Sr0.5Co0.8M0.2O3–δ (M=Mn,Fe, Ni,Cu) Perovskite Cathodes for IT‐SOFCs

The electrochemical properties of La_0.5Sr_0.5Co_0.8M_0.2O_3–δ (M=Mn, Fe, Ni, Cu) cathodes are investigated with chemical bulk diffusion coefficients (D_chem) and polarization resistances. The electrochemical performance of long‐term testing for La_0.5Sr_0.5Co_0.8Cu_0.2O_3–δ cathode was carried out to investigate its electrochemical stability. In this work, an anode‐supported single cell with a thick‐film SDC electrolyte (30 μm), a Ni‐SDC cermet anode (1 mm), and a La_0.5Sr_0.5Co_0.8Cu_0.2O_3–δ cathode (10 μm) reaches a maximum peak power density of 983 mW/cm² at 700°C. Obviously, Cu substitution for B‐site of La_0.5Sr_0.5CoO_3–δ cathode reduced thermal expansion coefficient (TEC) value and enhanced oxygen bulk diffusion and electrochemical properties. La_0.5Sr_0.5Co_0.8Cu_0.2O_3–δ is a promising cathode material for intermediate temperature solid oxide fuel cells (IT‐SOFC).     

Investigation of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ based cathode SOFC: II. The effect of CO2 on the chemical stability

The effect of carbon dioxide on the chemical stability of a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ cathode in the real reaction environment at 450 °C was investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), temperature programmed desorption (TPD), X-ray diffraction (XRD) and electrochemical impedance spectra (EIS) techniques. It was found that the presence even of very small quantities of CO₂ seriously deteriorates the fuel cell performance at 450 °C. XPS, TPD and XRD results strongly evidenced the formation of carbonates involving strontium and possibly barium after the BSCF cathode was operated in 1% CO₂/O₂ gas mixture at 450 °C for 24 h. SEM-EDX analysis of the BSCF cathode surface, after treatment in CO₂/O₂ environment at 450 °C, showed small particles on the surface probably associated with a carbonate phase and a segregated phase of the perovskite. The corresponding EDX spectra confirmed the presence of a carbonate layer and also revealed the surface enrichment of strontium and barium elements. EIS results indicated that both ohmic and polarization resistances increased gradually with the introduction of carbon dioxide in the oxidant stream, which could be interpreted by the decreased oxygen reduction kinetics and the formation of carbonate insulating layer.     

Characterization of La0.6Sr0.4Co0.2Fe0.8O3–δ + La2NiO4+δ Composite Cathode Materials for Solid Oxide Fuel Cells

The structure, electrical conduction, thermal expansion and electrochemical properties of the La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ + La₂NiO_4+δ (LSCF‐LNO) composite cathodes were investigated with regard to the volume fraction of the LNO composition. No chemical reaction product between the two constituent phases was found for the composite cathodes sintered at 1,400 °C for 10 h within the sensitivity of the XRD. Compared to the performance of the LSCF cathode, the LNO composition in the composite cathode plays a role in deteriorating both electrical conductivity and electrochemical properties, however, improving the thermal expansion properties. The trade‐off between electrical conducting and thermal expansion classifies the composite cathode containing 30 volume percent (vol.%) LNO as the optimum composition. For characterizing cathode performance in a single cell, a slurry spin coating technique was employed to prepare a porous cathode layer as well as a YSZ/Ce_0.8Sm_0.2O_3–δ (SDC) electrolyte. The optimum conditions for fabricating the YSZ/SDC electrolyte were investigated. The resulting single cell with 70 vol.% LSCF‐30 vol.%LNO (LSCF‐LNO30) cathode shows a power density of 497 mW cm^–2 at 800 °C, which is lower than that of the cell with a LSCF cathode, but still within the limits acceptable for practical applications.     

Improvement of the Electrochemical Performance of Ln0.58Sr0.4Fe0.8Co0.2O3 – δ IT‐SOFC Cathodes by Ternary Lanthanide Combinations (La‐Pr‐Sm)

Active perovskite‐based SOFC cathodes have been developed through lanthanide combination in the (La_{1 – x – y}Pr_xSm_y)_0.58Sr_0.4Fe_0.8Co_0.2O_{3 – δ} system following a ternary mixture experimental design. These compositions were prepared through a sol–gel method and characterised by electrochemical impedance spectroscopy (EIS) as symmetrical cells on GDC‐electrolyte samples in the 450–650 °C temperature range. The electrochemical properties of the single lanthanide‐based Ln_0.58Sr_0.4Fe_0.8Co_0.2O_{3 – δ} compounds were enhanced when different lanthanides were combined together in the same crystalline structure. The observed improvement does not follow a mere additional effect of the performance from the parent Ln_0.58Sr_0.4Fe_0.8Co_0.2O_{3 – δ} compounds, i.e. it does not follow a linear behaviour, and the better performance is ascribed to synergetic catalytic effects among lanthanide cations. A reduction in electrode polarisation resistance with respect to non‐substituted compositions is stated for most Ln_0.58Sr_0.4Fe_0.8Co_0.2O_{3 – δ} electrode compositions combining two or three lanthanides. Samarium addition to the electrode material involves a substantial reduction in the activation energy and the reduction degree is directly dependant on the samarium amount incorporated in the lattice. The best performing composition comprises a praseodymium‐rich lanthanum‐based electrode material. The experimental data derived from the ternary mixture design were modelled using nonlinear functions and this modelling allowed finding an electrode composition minimising the polarisation resistance while maintaining the activation energy at reduced values. Selected cathode compositions were tested in fully assembled anode‐supported cells and electrochemical characterisation supports the cooperative effect of lanthanide combination.     

Decreasing the Polarization Resistance of BaCo0.7Fe0.2Nb0.1O3–δ Cathodes by Infiltration of Ce0.8Y0.2O2–δ

Ce_0.8Y_0.2O_2–δ (YDC) was infiltrated into a BaCo_0.7Fe_0.2Nb_0.1O_3–δ (BCFN) cathode of intermediate temperature sold oxide full cells (IT‐SOFCs) in order to decrease its cathodic polarization resistance. BCFN and YDC infiltrated BCFN electrodes were fabricated on dense Ce_0.8Gd_0.2O_2–δ (GDC) thin pellets to form symmetrical cells. The electrochemical impedance spectra of the symmetrical cells were investigated in this present study. Firstly, the thickness of BCFN electrodes was optimized, and controlled at 30 µm for further study. The effects of infiltrated YDC amount and firing temperature on electrode polarization resistance were studied. The symmetrical cells infiltrated with 30 μL YDC solution and fired at 900 °C exhibited the lowest electrode polarization resistance in all samples. It was suggested that infiltration of YDC resulted in more active sites and prolonged TPBs in electrodes, improving the surface oxygen exchange, and finally improved the electrode performance.     

Influence of Barium Incorporation on the Electrochemical Performance of Ln0.58Sr0.4Fe0.8Co0.2O3–δ (Ln=La,Pr, Sm) Perovskites for Oxygen Activation at Intermediate Temperatures

Highly active perovskite‐based electrodes were obtained through partial substitution of lanthanides (La, Pr and Sm) by barium in the Ln_0.58Sr_0.4Fe_0.8Co_0.2O₃ system. These compositions were obtained as single phase crystalline compounds through a sol–gel synthetic route and tested by electrochemical impedance spectroscopy (EIS) as symmetrical cells on GDC‐electrolyte samples in the temperature range 450–650 °C. Cooperative effects arose through the incorporation of a large divalent active cation (Ba⁺²) in the perovskite lattice of lanthanide‐based strontium ferrites‐cobaltites. Reduction in the electrode polarisation resistance with respect to non‐substituted compositions is obtained irrespective of the amount of barium substitution and lanthanide nature. Barium addition also reduces the activation energy of these compositions, indicating changes in the oxygen activation processes. The specific effect of the barium incorporation on the electrode performance was dependent on the nature of the partially substituted lanthanide. Lanthanum–barium‐based compositions exhibited the lowest electrode polarisation resistance when the amount of the replaced barium was increased up to 50% molar. On the other hand, for praseodymium‐ or samarium–barium‐based compositions, the minimum in electrode polarisation resistance was achieved at barium substitutions close to 25% molar. The study of the effect of oxygen partial pressure on EIS data allowed in improving the understanding of the processes governing the electrode operation.     

Temperature‐stable dielectric and energy storage properties of La(Ti0.5Mg0.5)O3‐doped (Bi0.5Na0.5)TiO3‐(Sr0.7Bi0.2)TiO3 lead‐free ceramics

A new type of (0.7?x)Bi_0.5Na_0.5TiO₃‐0.3Sr_0.7Bi_0.2TiO₃‐xLaTi_0.5Mg_0.5O₃ (LTM1000x, x = 0.0, 0.005, 0.01, 0.03, 0.05 wt%) lead‐free energy storage ceramic material was prepared by a combining ternary perovskite compounds, and the phase transition, dielectric, and energy storage characteristics were analyzed. It was found that the ceramic materials can achieve a stable dielectric property with a large dielectric constant in a wide temperature range with proper doping. The dielectric constant was stable at 2170 ± 15% in the temperature range of 35‐363°C at LTM05. In addition, the storage energy density was greatly improved to 1.32 J/cm³ with a high‐energy storage efficiency of 75% at the composition. More importantly, the energy storage density exhibited good temperature stability in the measurement range, which was maintained within 5% in the temperature range of 30‐110°C. Particularly, LTM05 show excellent fatigue resistance within 10⁶ fatigue cycles. The results show that the ceramic material is a promising material for temperature‐stable energy storage.     

Co‐synthesis of Sm0.5Sr0.5CoO3‐Sm0.2Ce0.8O1.9 Composite Cathode with Enhanced Electrochemical Property for Intermediate Temperature SOFCs

A 70 wt.% Sm_0.5Sr_0.5CoO₃ – 30 wt.% Sm_0.2Ce_0.8O_1.9 (SSC–SDC73) composite cathode was co‐synthesized by a facile one‐step sol–gel method, which showed lower polarization resistance and overpotential than those of physically mixed SSC–SDC73 cathode. The polarization resistance of co‐synthesized SSC–SDC73 cathode at 800 °C was as low as 0.03 Ω cm² in air. Scanning electron microscopy (SEM) images showed that the enhanced electrochemical property was mainly attributed to the smaller grains and good dispersion of SSC and SDC phases within the composite cathode, leading to an increase in three‐phase boundary length. The dependence of polarization resistance with oxygen partial pressure indicated that the rate‐limiting step for oxygen reduction reaction was the dissociation of molecular oxygen to atomic oxygen process. An anode supported fuel cell with a co‐synthesized SSC–SDC73 cathode exhibited a peak power density of 924 mW cm⁻² at 800 °C. Our results suggested that co‐synthesized composite was a promising cathode for intermediate temperature solid oxide fuel cells (IT‐SOFCs).     

Degradation and initiation polymerization mechanism of α‐methylstyrene‐containing macroinitiators

Copolymers obtained from radical copolymerization of α‐methylstyrene (AMS) and glycidyl methacrylate (GMA) behave as macroinitiators, when heated in the presence of a second monomer, giving rise to block copolymers. The relevant degradation and initiation polymerization mechanism of the macroinitiators were studied. Thermal depropagation of the macroinitiators generated monomers, identified by ¹H‐NMR, photoionization mass spectroscopy and FT‐IR. According to the results of structure analysis by GPC, ESR and NMR spectroscopy, the AMS‐GMA (head‐head) and AMS‐AMS (head‐head) bonds in the macroinitiators are easily scissored providing free radicals when the temperature is above 80°C. The radicals lead to subsequent polymerization of the second monomer, and thereby block copolymers are formed. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011