Search

The early-stage degradation behaviour of a porous lanthanum strontium cobalt ferrite (LSCF) cathode in a solid oxide fuel cell is investigated at a low temperature of 600 °C by operating four cells under different conditions: in dry air and at open circuit voltage (Dry-OCV), in dry air and under bias (Dry-bias), in wet air and at OCV (Wet-OCV), and in wet air and under bias (Wet-bias). Compared with Dry-OCV, either H2O or bias shows a negative effect on the electrochemical and transport characteristics, resulting in the most severe degradation in Wet-bias. The mechanism is explained in terms of LSCF deterioration, which is intensively studied on a micro- to atomic scale. No surface segregation is observed in the as-prepared cathode; however, sulfur is found to be incorporated into the lattice of the LSCF(110) surface. After operation, nano-segregation occurs in all the cathodes. SrSO4 particles form in all the cathodes while Sr(OH)2 flakes precipitate under wet conditions. The size and distribution of segregations vary with the conditions. For example, some of the SrSO4 particles in Dry-bias grow into bar-like ones (up to 280 nm) compared with smaller ones (36 nm) in Dry-OCV indicating that the kinetics of Sr diffusion and O vacancy formation is accelerated by bias. Their distribution is limited in the cathode surface layer (CSL) in Dry-OCV, Dry-bias and Wet-OCV, while it extends to the cathode/electrolyte interface in Wet-bias, which is caused by the decrease of SO2 adsorption/dissociation kinetics in the CSL in the presence of competitive H2O species (for SrSO4) and the enhancement of H2O mass transport driven by the O2 concentration gradient under bias (for Sr(OH)2). With the formation of segregation, other deteriorations such as sulfur incorporation into the surface, Sr-deficiency in the subsurface, and La–Co-rich regions near the surface occur and evolve in LSCF grains. All these changes lead to the deactivation of the surface O exchange, which is believed to be the dominant reason for the performance degradation. [ABSTRACT FROM AUTHOR]

The study explains the surface morphology and microstructure of thick Y[Ba.sub.2][Cu.sub.3][O.sub.7-[delta]] (YBCO) films prepared on vicinal r-cut sapphire buffered with Ce[O.sub2]. These sapphire substrates can be easily used for practical applications of cuprate superconductors.

Catalysts are widely utilized to promote reactions in liquid and gaseous phases but are rarely encountered in solid state synthesis. Here we use catalytically active (001) ceria buffers to modify the structure of the epitaxial high temperature superconductor YBa2Cu3O7. The modification is achieved by catalytically-assisted synthesis of a previously unknown metastable phase. The new phase, a long-period (3.5 nm) perovskite, intercalates into the YBa2Cu3O7 matrix without negatively affecting the critical temperature of the film. Analysis of electron microscopy and synchrotron x-ray diffraction data allow identification of the phase as a long-period YBa2Cu3O7 derivative formed through short-range cation displacement. The 0.8 μm thick films exhibit strong enhancement of the critical current density, reaching a maximum of 4.2 MA/cm2 at 77 K. The result emphasizes the critical role of catalysis for synthesis of novel complex materials. [ABSTRACT FROM AUTHOR]

Catalytically active (001) ceria buffers are used for modifying the structure of the epitaxial high temperature superconductor Y[Ba.sub.2][Cu.sub.3][O.sub.7]. The analysis of electron microscopy and synchrotron x-ray diffraction data has helped in the identification of the phase as a long-period Y[Ba.sub.2][Cu.sub.3][O.sub.7] derivative formed through short-range cation displacement and has highlighted the role of catalysis for preparing novel complex materials.

To systematically investigate the influence of electrolyte substrates on Sr-segregation and SrSO4 formation in (LaSr)(CoFe)O3 (LSCF) cathodes in solid oxide fuel cells, model thin films were grown on Gd-doped ceria (GDC) and on Y-doped BaZrO3 (BZY) electrolytes by pulsed laser deposition and heat treated at 800–1000 °C in synthetic air with a trace amount of SO2. A severe SrSO4 formation was observed in LSCF on GDC as compared with the BZY, especially at low temperature. The difference in Sr-segregation and SrSO4 formation on the LSCF was discussed in relation to Sr diffusion and related elemental redistribution across the interfaces. [ABSTRACT FROM AUTHOR]

Significant attention has been directed towards the enhancement of the efficiency of solid oxide fuel cells (SOFCs) for energy applications. One of the main issues of an industry-applicable cathode material, (La0.6Sr0.4)(Co0.2Fe0.8)O3−δ, used in conjunction with an yttria-stabilized zirconia (YSZ) electrolyte, is the formation of a SrZrO3 (SZ) phase which results from the chemical reaction between these two materials at high operating temperatures, notwithstanding the utilization of a diffusion barrier layer such as gadolinia-doped ceria (GDC). To gain a better understanding of the effect of the SZ phase on the oxide ion transport through SOFC heterostructures, dense GDC/SZ/YSZ and GDC/SZY(Y-doped SrZrO3)/YSZ multilayer films with well-defined interfaces are prepared using pulsed laser deposition (PLD). Isotope exchange depth profiling with secondary ion mass spectrometry (SIMS) is employed to probe oxide ion transport through the multilayer films, and advanced analytical techniques including scanning/transmission electron microscopy (S/TEM) and spatially resolved electron energy loss spectroscopy (EELS) are used to examine the heterointerfaces. A steep drop in the concentration of the 18O tracer SIMS depth profile reveals the existence of an oxide ion blocking effect occurring at the GDC/SZ(Y) and SZ(Y)/YSZ interfaces. By correlating this result with the EELS analysis of the O K-edge at SZ(Y)/YSZ interfaces, it was found that an interface-induced oxygen disorder possibly arising from Y diffusion across the interface could account for this behavior. Thermodynamic considerations have been made on the construction of plausible phase diagrams for the Sr–Y–Zr–O system to analyze the stability of interfaces. The implications of this interfacial disorder on the performance of SOFCs are discussed. [ABSTRACT FROM AUTHOR]

Evaluation of the interfaces is critical for improved long-term stability and performance of solid oxide fuel cells (SOFC) under required operating conditions. In this work, cells consisting of LSCF-cathode|GDC-interlayer|8YSZ-electrolyte|Pt-anode were tested under open-circuit voltage (OCV) and cathodic polarization at a constant current of -0.3 A at 800°C for 300 h in air. Dense GDC films by pulsed laser deposition (PLD) were used as reaction barrier layers. The results showed that after cell tests, SrZrO3 formed at the LSCF/GDC interface in consequence of the fast Zr migration in the PLD-grown GDC interlayers. However, SrZrO3 formation and GDC microstructural feature differ between OCV and cathodic polarization. Under OCV, SrZrO3 laterally formed at the O2/LSCF/GDC triple phase boundary with GDC interlayer showing increased porosity near the YSZ electrolyte. Both of these led to a drastic increase in polarization resistance with time. Under cathodic polarization, localized transverse growth of SrZrO3 was observed, and an apparent pore elimination occurred in GDC. Cells also showed no performance degradation under cathodic polarization. Based on the above observation, a plausible cation diffusion behavior and oxygen transport is discussed. [ABSTRACT FROM AUTHOR]

The degradation of GDC interlayer is one of the key issues in the long-term stability of LSCF cathode-YSZ electrolyte based solid oxide fuel cells (SOFCs). Here we demonstrate that the cathode microstructure has a strong influence on the stability of GDC interlayer and cation diffusion behavior of LSCF/GDC/YSZ triplets. LSCF was prepared into two distinct morphologies: porous layers by screen-printing and dense layers by pulsed laser deposition (PLD). Dense GDC barriers by PLD were applied to both porous and dense LSCF cathodes. Interdiffusion and microstructural evolution were investigated at temperatures of 800°C to 1000°C in air. At 800°C, both cathodes showed no apparent formation of SrZrO3 at the GDC/YSZ interface. Instead, SrZrO3 formed at the LSCF/GDC interface, and was more evident for the porous LSCF annealed at 900°C. At 1000°C, severe cation interdiffusion at the LSCF/GDC interface became remarkable for the porous cathode, which exhibited rapid degradation of the GDC layer accompanied by the severe SrZrO3 formation; by contrast the dense cathode only showed localized SrZrO3 grains. The superior barrier stability of GDC in a dense cathode can be related to the air-tight interface which would minimize microstructural degradation occurring near the oxygen/LSCF/GDC triple phase boundary. [ABSTRACT FROM AUTHOR]