Creep behavior of porous La0.6Sr0.4Co0.2Fe0.8O3-δ substrate material for oxygen separation application

Advanced oxygen transport membrane designs consist of a thin functional layer supported by a porous substrate material that carries mechanical loads. Creep deformation behavior is to be assessed to warrant a long-term reliable operation at elevated temperatures. Aiming towards an asymmetric composite, the current study reports and compares the creep behavior of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) perovskite porous substrate material with different porosity and pore structures in air for a temperature range of 800–1000?°C. A porosity and pore structure independent average stress exponent and activation energy are derived from the deformation data, both being representative for the LSCF material. To investigate the structural stability of the dense layer in an asymmetric membrane, sandwich samples of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) with porous substrate and dense layers on both side were tested by three-point bending with respect to creep rupture behavior of the dense layer. Creep rupture cracks were observed in the tensile surface of BSCF, but not in the case of LSCF.     

The effect of an oxygen partial pressure gradient on the mechanical behavior of perovskite membrane materials

In application of perovskite as oxygen conducting materials the membrane is operated at elevated temperatures under an oxygen gradient. The effect of the partial pressure difference on the mechanical properties is reported in the current work. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) samples were annealed under an oxygen gradient. The mechanical properties of cross-sections were characterized using indentation testing. Chemical strains for BSCF and LSCF were too small to detect them after cooling to RT by XRD; however, the results suggest that the indentation crack length is affected by chemical strains for LSCF, but not for BSCF. An anisotropy of the indentation crack length and corresponding apparent fracture toughness is related with the interaction of domain switching and residual strain that is probably also associated with the observation that vacuum (10⁻⁵ mbar) annealed LSCF showed surface cracking on heating in air, whereas for BSCF such fracture features were not observed.     

The Rheology of Stabilised Lanthanum Strontium Cobaltite Ferrite Nanopowders in Organic Medium Applicable as Screen Printed SOFC Cathode Layers

The production and optimisation of screen printing (SP) pastes containing La_0.58Sr_0.42Co_0.21Fe_0.79O_{3 – δ} and La_0.61Sr_0.41Co_0.19Fe_0.79O_{3 – δ} (LSCF) were investigated. The application of these nanopowders is supposed to improve the cathode's microstructure and increase its mechanical strength. Thirty seven pastes containing LSCF were carried out with variation in binders, dispersants and different particle size distribution. The rheological behaviour of these pastes was investigated. It was found that commercially available dispersant Solsperse 3000 resulted in the best suspension stability, achieving almost 55 times lower viscosity value for pastes containing 20 vol.‐%, than pastes without any dispersants. The shear thinning behaviour was found to be favourable for the LSCF cathode deposition. A cathode made from the mixture of nano and submicron powder exhibits a polarisation resistance as low as 0.76 Ω cm² at 592 °C.     

Enhanced LSCF oxygen deficiency through hydrothermal synthesis

High-performance perovskites are promising materials for diverse renewable energy technologies. Besides design characteristics, the devices performance depends on the material synthesis, since the processes occurring during synthesis may produce different structures and properties. In this work, the perovskite La_0.1Sr_0.9Co_0.9Fe_0.1O_3-δ (LSCF1991) was synthesized by different methods, and the phase composition and oxygen deficiency were assessed and discussed. We show that it is possible to increase oxygen deficiency by promoting oxygen release during crystallization within hydrothermal synthesis, producing a remarkable improvement of ?δ ~0.2 in comparison with the citrate method. The single phase LSCF1991 powder was characterized, shaped by different routes and sintered to demonstrate the stability and suitability to manufacture electrochemical devices.     

Characterization and fabrication of LSCF tapes

In this work the manufacture of commercial La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) by aqueous colloidal processing is presented. The surface behavior of LSCF as a function of pH and the effect of a polyelectrolyte (Duramax D3005) on the stability are studied using measurements of zeta potential. Concentrated suspensions were prepared to solid content as high as 35 vol.%. The best dispersing conditions were determined by means of rheological measurements for obtaining stable and fluid slurry for tape casting technique. Different relative densities of the tapes were obtained at different temperatures. The LSCF tapes are good candidates for using as gas separation membrane or cathode for SOFC.     

Fabrication of lanthanum-based perovskites membranes on porous alumina hollow fibre (AHF) substrates for oxygen enrichment

In this work, two types of lanthanum-based MIEC perovskite oxides, namely La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and La_0.6Sr_0.4Co_0.2Ni_0.8O_3-δ (LSCNi), were deposited onto porous alumina hollow fibre (AHF) substrates and used for oxygen enrichment. Such structure was developed to shorten oxygen ion diffusion distances in dense membranes and simultaneously leading to higher oxygen flux. The perovskite oxides were prepared using Pechini sol-gel method and deposited via a vacuum-assisted technique. The deposition of lanthanum-based membranes onto the outer and inner sides of the porous AHF has been facilitated through numerous microchannels in the AHF substrates. The effects of operating temperature and argon sweep gas flowrate on oxygen permeation flux of lanthanum-based AHF membrane were investigated. The results revealed that the oxygen permeation flux of LSCF-AHF and LSCNi-AHF increased with operating temperatures due to the improvement of bulk diffusion and surface exchange properties after the lanthanum-based perovskite deposition. Higher oxygen flux was observed for LSCNi-AHF as LSCNi possessed balanced oxygen ionic and electronic conductivities as compared to LSCF membranes. Benefitting from improved oxygen activation and vacancy generation properties after Ni substitution into the B-site ion of LSC perovskite, a dramatic increased oxygen fluxes up to 4.5 mL/min·cm² was observed at 950 °C. The present work demonstrated a feasible method for fabricating oxygen transport membrane (OTM) using porous AHF substrates     

Characterization of La0.58Sr0.4Co0.2Fe0.8O3−δ–Ce0.8Gd0.2O2 composite cathode for intermediate temperature solid oxide fuel cells

La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Gd_0.2O₂ (LSCF–GDC) composite cathodes with various weight ratios 90%, 70% and 50% of LSCF were prepared. Mechanical properties, thermal expansion properties and electrical properties were measured for potential applications in solid oxide fuel cells (SOFCs) with graded cathodes. LSCF and GDC as pure cathode and electrolyte materials were characterized as reference. The absence of new phases as confirmed by X-ray diffraction (XRD) analysis demonstrated the excellent compatibility between the cathode and electrolyte materials. Mechanical properties such as hardness and fracture toughness were measured by the micro-indentation technique, while hardness and elastic modulus were measured by the nano-indentation technique. Thermal expansion behavior was recorded by a dilatometer. Electrical conductivity was measured by the four probe DC method. The 50% LSCF–GDC composite has the lowest relative density among all the samples. Thermal expansion coefficients (TECs) and electrical conductivity increased with addition of LSCF contents in the composite, while mechanical properties depended more on the density than the LSCF content.     

A high performance solid oxide fuel cells operating at intermediate temperature with a modified interface between cathode and electrolyte

By adding 1% Bi₂O₃ (mol%) into LSCF (La_0.54Sr_0.44Co_0.2Fe_0.8O_3?δ), a layer of dense LSCF film is introduced to the upside of yttria stabilized zirconia (YSZ) electrolyte. The dense film increases the interface contact area and reduces the interface ion transfer resistance between cathode and electrolyte remarkably. As a result, the cell performance is greatly elevated from 492 to 901 mW cm^?2 at 650 °C. Besides, on the basis of careful observation of the cathode surface by FE-SEM, the function of Bi₂O₃ to promote the cathode sintering is speculated. The Bi₂O₃ and the LSCF come into being a kind of eutectic liquid. The eutectic liquid flows down from the cathode bulk to the interface between the cathode and the electrolyte where it accumulates to form a dense layer. This dense layer illuminates the function of adding Bi₂O₃ into LSCF cathode.     

Degradation behaviours of Fe-doped La0.8Sr0.2CoO3-δ by thermal stress

ABSTRACT

The improvement in durability by Fe doping to La_0.8Sr_0.2CoO_3-δ (LSC) and its degradation mechanism were investigated in terms of structural and morphological analyses. Polarisation resistance of LSC was increased approximately by 65.7% after exposure to thermal stress at 900°C for 100?h because the oxygen reduction reaction was declined by particle coarsening and aggregation of LSC powders. The polarisation resistance of La_0.8Sr_0.2Co_0.8Fe_0.2O_3-δ (LSCF) was increased from 0.269 to 0.328?Ωcm² after the heat treatment. Increasing rate of polarisation resistance of LSCF was 21.9%, which was three times lower than that of LSC due to reduced particle coarsening and aggregation by Fe doping. Particle size of LSCF powders was maintained about 0.5–3?μm, and morphologies were also similar before and after the heat treatment. Chemical reaction between the LSCF and Sm-doped Ceria electrolyte powders did not occur despite the thermal stress, indicating the interface between the cathode and electrolyte is chemically stable.     

Structural and mechanical properties of La0.6Sr0.4M0.1Fe0.9O3-δ (M: Co,Ni and Cu) perovskites

La_0.6Sr_0.4M_0.1Fe_0.9O_3-δ (M: Co, Ni and Cu) perovskite nanostructures were synthesized using low frequency ultrasound assisted synthesis technique and effect of substitution of Fe by Co, Ni and Cu on crystal structure and mechanical properties in La_0.6Sr_0.4FeO_3-δ perovskite was studied. The HRTEM and Rietveld refinement analyses revealed the uniform equi-axial shape of the obtained nanostructures with the existence of La_0.6Sr_0.4M_0.1Fe_0.9O_3−δ with mixed rhombohedral and orthorhombic structures. Substitution by Cu decreases the melting point of La_0.6Sr_0.4FeO_3-δ. The results of mechanical characterizations show that La_0.6Sr_0.4Co_0.1Fe_0.9O_3−δ and La_0.6Sr_0.4Ni_0.1Fe_0.9O_3−δ have ferroelastic behavior and comparable elastic moduli, however, substitution by Ni shows higher hardness and lower fracture toughness than Co in B-site doping.     

An architectural approach to the oxygen permeability of a La0.6Sr0.4Fe0.9Ga0.1O3−δ perovskite membrane

Multilayer membranes based on La_0.6Sr_0.4Fe_0.9Ga_0.1O_3−δ (LSFG) and La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) perovskite materials were fabricated to study the impact of membrane architecture on the oxygen permeability. Thick dense membrane and asymmetric membranes were shaped by tape casting and stacked to reach the desired architecture. Asymmetric membranes composed of a thin dense LSFG layer (120 μm) and a thick porous support layer (820 μm) of the same material were co-sintered to obtain crack-free and flat membranes. The use of large corn-starch particles (14 μm) as pore forming agent to the tape-casting slurries resulted in a connected porosity in the sintered support layer with low gas diffusion resistance. Oxygen permeation measurements in an air/argon gradient between 800 and 925 °C showed that the thickness of self-supported LSFG membranes was not the determining factor in the membrane performance for our testing conditions. A catalytic layer of La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF), deposited on the membrane surfaces to catalyze the oxygen exchange reactions, leads to a significant increase of oxygen permeation rates. As the membrane thickness had no effect even if a catalyst coating was used, surface-exchange reactions were thought to be still limiting for the oxygen permeation fluxes. Thus, the improvement of surface activity of LSFG membrane was found to be a key point to reach higher oxygen permeation fluxes.     

Electrophoretic deposition of ceramic materials for fuel cell applications

La_0.8Sr_0.2Ga_0.875Mg_0.125O_3-x (LSGM), La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ (LSCF), yttria stabilized zirconia (YSZ) and (Ce_0.8Gd_0.2)O_1.9 (CGO) were electrophoretically deposited on Ni foils and Ni-yttria stabilized zirconia substrates prepared by tape casting. It was demonstrated that the ethyl alcohol–phosphate ester–polyvinyl butyral system is an effective solvent–dispersant–binder system for electrophoretic deposition of these materials. The influence of dispersant, binder and current density on deposition efficiency and deposit morphology was studied. The microstructure of the deposits was examined by electron microscopy. The proposed solvent–dispersant–binder medium for electrophoretic deposition of LSGM, LSCF, YSZ and CGO has important advantages and implications in fuel cell design.     

Electrochemical characteristics of La0.6Sr0.4Co1−yFeyO3 (y=0.2–1.0) fiber cathodes

Electrospun La_xSr_1−xCo_1−yFe_yO₃ (LSCF) fibers with y=0.2 – 1.0 have been investigated as the cathode of intermediate solid oxide fuel cells (IT-SOFC). The electrochemical performances of LSCF (y=0.2–1.0) fibers were studied by impedance spectroscopy in symmetrical cells containing gadolinium doped ceria (CGO) electrolyte and LSCF electrode infiltrated with CGO. Impedance measurements showed that the impedance spectra have two or three semicircles, depending on the measurement temperature. The LSCF electrodes with higher cobalt content exhibit lower polarization resistance (R_p) and the La_0.6Sr_0.4Co_0.8Fe_0.2O₃ electrode displayed the lowest polarization resistance between 500 and 900 °C, classifying this composite cathode as a promising material for intermediate temperature SOFC based on CGO electrolyte.     

Effect of optimal GDC loading on long-term stability of La0.8Sr0.2Co0.2Fe0.8O3-δ/Gd0.2Ce0.8O1.9 composite cathodes

Three types of La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ/Gd_0.2Ce_0.8O_1.9 (LSCF/GDC) composite cathodes with different optimal GDC loading are fabricated through electrospinning, screen printing and solution infiltration method. Constant current polarization with current density of 100 mA cm^?2 at 750°C is applied to test the stability of LSCF/GDC composite cathodes. After constant current polarization for 144 h, the polarization resistance (R_p) of 280 nm-nanofiber skeletal LSCF/GDC composite cathode after pore-forming exhibits the minimum increase, from 0.062 Ω cm² to 0.098 Ω cm². Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) results show that the microstructure and surface chemical composition of the cathode maintain stable during the constant current polarization. Combined with the X-ray diffraction (XRD) result, a relationship among GDC loading, stress, Sr surface segregation and long-term stability is established.     

La0.8Sr0.2Co0.3Fe0.7O3−δ as a novel candidate coating material for the positive current collector in sodium sulfur battery

The corrosion behaviors of perovskite La_0.8Sr_0.2Co_0.3Fe_0.7O_3−δ (LSCF) in the sodium tetrasulfide melt have been studied to demonstrate its possibility to be used as a coating material for the positive current collector in the sodium sulfur battery (Na/S). The electrochemical techniques including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) are applied in the study. The results demonstrate that the corrosion process of the LSCF in sodium tetrasulfide melt is activation controlled. The corrosion current density of the LSCF is approximate 10 times smaller than that of the 316L stainless steel. Two compact corrosion layers which may act as a stable barrier retarding the corrosion process have formed on the surface of LSCF after immersed for 130 days. The promising results suggest that LSCF exhibits high corrosion resistance against molten sodium tetrasulfide and may be an appropriate candidate coating material for the positive current collector in Na/S battery.     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part II: Influence on oxygen transport and phase stability

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) in its cubic perovskite phase has attracted much interest for potential use as oxygen transport membrane (OTM) due to its very high oxygen permeability at high temperatures. However, performance degradation due to a sluggish phase decomposition occurs when BSCF is operated below 840?°C. Partial B-site substitution of the transition metal cations in BSCF by larger and redox-stable cations has emerged as a potential strategy to improve the structural stability of cubic BSCF. In this study, the influence of yttrium doping (0…10?mol-%) on oxygen transport properties and stability of the cubic BSCF phase is assessed by in situ electrical conductivity relaxation (ECR) and electrical conductivity measurements during long-term thermal annealing both at 700?°C and 800?°C. Detailed phase analysis is performed by scanning electron microscopy (SEM) after long-term annealing of the samples in air at different temperatures.     

Influence of Microstructure and Surface Activation of Dual‐Phase Membrane Ce0.8Gd0.2O2−δ–FeCo2O4 on Oxygen Permeation

Dual‐phase oxygen transport membranes are fast‐growing research interest for application in oxyfuel combustion process. One such potential candidate is CGO‐FCO (60 wt% Ce_0.8Gd_0.2O_2?δ–40 wt% FeCo₂O₄) identified to provide good oxygen permeation flux with substantial stability in harsh atmosphere. Dense CGO‐FCO membranes of 1 mm thickness were fabricated by sintering dry pellets pressed from powders synthesized by one‐pot method (modified Pechini process) at 1200°C for 10 h. Microstructure analysis indicates presence of a third orthorhombic perovskite phase in the sintered composite. It was also identified that the spinel phase tends to form an oxygen deficient phase at the grain boundary of spinel and CGO phases. Surface exchange limitation of the membranes was overcome by La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) porous layer coating over the composite. The oxygen permeation flux of the CGO‐FCO screen printed with a porous layer of 10 μm thick LSCF is 0.11 mL/cm² per minute at 850°C with argon as sweep and air as feed gas at the rates of 50 and 250 mL/min.     

Tuning Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode to high stability and activity via Ce-doping for ceramic fuel cells

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) fuel-cell cathode stands out because of its ultrahigh ionic conductivity and excellent electrocatalytic activity, but it is still very subject to instability. Here, a new strategy of Ce doping is proposed to boost the stability and activity of the BSCF cathode. A one-pot combustion method is employed to synthesize (Ba_0.5Sr_0.5)_1–xCe_xCo_0.8Fe_0.2O_3-δ (x=0–0.2) cathodes. Both BSCF and (Ba_0.5Sr_0.5)_0.9Ce_0.1Co_0.8Fe_0.2O_3-δ have a cubic perovskite structure. (Ba_0.5Sr_0.5)_0.8Ce_0.2Co_0.8Fe_0.2O_3-δ shows two phases of cubic perovskite and fluorite ceria. Proper Ce doping can boost the electrical conductivity of BSCF, and can dramatically reduce the polarization resistance of BSCF cathode. Ce doping significantly improved BSCF cathode long-term stability by 160 h. Moreover, ten-percent Ce doping in BSCF highly improves single-cell output performance from 516.33 mW cm^?2 to 629.75 mW cm^?2 at 750 °C. The results reveal that Ce doping as a potential strategy for enhancing the stability and activity of BSCF cathode is promising.     

A novel lanthanum strontium cobalt iron composite membrane synthesised through beneficial phase reaction for oxygen separation

Composite ceramic membrane is one of the most attractive concepts which combines the advantages of different phases into a single membrane matrix. Recently, the reported significant increased oxygen surface kinetics on the Perovskite/Ruddlesden-Popper composite system because of the formation of novel and fast oxygen transport paths along the hetero-interface has been implanted into the oxygen permeation membrane system. In this work, a novel La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-(La_0.5Sr_0.5)₂CoO_4+δ (LSCF-LSC) composite hollow fiber membrane is synthesized with oxygen permeation flux of 4.52 mL min⁻¹ cm⁻² at 950 °C. It presents round 4 times and 2.3 times of that of the single LSCF membrane and LSC-coated LSCF membrane at 900 °C. For better comparison, (La_0.576Sr_0.424)_1.136Co_0.3Fe_0.7O_3-δ (LSCF-new) is prepared based on the composition of LSCF-LSC composite. The enhanced oxygen permeability was further investigated through electrochemical impedance spectroscopy (EIS) measurements. We also confirm that LSCF-LSC shows significantly lower area specific resistance (ASRs) for LSCF-LSC|Ce_0.8Sm_0.2O_1.9 (SDC)|LSCF-LSC symmetrical cell relative to other symmetrical cells. This novel LSCF-LSC composite membrane also presents high CO₂ tolerance, with stable oxygen permeation fluxes round 2.6 mL min⁻¹ cm⁻² at 900 °C for 100 h.     

Improvement of catalytic activity of oxide electrode by double layer pore structure using nanofiber polyaniline

Improving the catalytic activity of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) by applying polyaniline nanofiber (PA) as a pore former and the electrochemical property of each pore structure was investigated by structural and morphological analyses. The pore volume and specific surface area of the PA-added LSCF layer increased by about 3.2 and 2.7 times, respectively, relative to the existing LSCF layer. Coarse pores of 2?μm diameter were observed by PA agglomeration. The double-layered LSCF (LSCF_PA1) coated with PA-added LSCF on an LSCF single layer presented the lowest polarisation resistance at all measured temperatures. This was due to increased oxygen reduction reaction by the LSCF?+?PA layer with high porosity that promoted oxygen gas diffusion. Charge transfer was also improved by the wide contact area between the LSCF layer with lower porosity and the Sm-doped ceria powder electrolyte. Micro-porous LSCF showed about 19% lower polarisation resistance than the nano-porous LSCF at 600°C because of mass transfer through oxygen diffusion.