Search

Abstract Cathode materials with significantly enhanced oxygen exchange properties are required to achieve high-performance solid oxide fuel cells (SOFCs). Here, it is shown that in multilayer heteroepitaxial films prepared by alternately stacking conventional mixed ionic and electronic conductors of (La 0.6 Sr 0.4)CoO 3- δ (LSC) and gadolinia-doped ceria (GDC), extremely fast oxide ion incorporation and transport can be achieved at relatively lower temperatures. Multilayer LSC and GDC heteroepitaxial films prepared on YSZ electrolytes exhibit enhanced oxygen surface exchange coefficient (k *) values as compared to single LSC thin films. This is attributed to the existence of Ce3+ induced by the LSC-GDC interface, implying the presence of a relatively high density of oxygen vacancies which facilitate oxide ion incorporation and transport. Accordingly, unprecedented values of k * in the order of 10−5 cm/s were obtained for LSC-GDC multilayer thin film cathodes even at a relatively low temperature of 500 °C. The results here conclusively show that leveraging the effect of heterointerfaces through multilayering can be a viable approach to achieving superior cathode materials with enhanced oxygen exchange properties. Graphical abstract fx1 Highlights • LSC and GDC thin films are prepared in multilayer nanostructures. • Multilayered LSC and GDC exhibit superior oxygen exchange properties. • Oxygen surface exchange coefficient k * values were in the order of 10−5 cm/s even at 500 °C. • Enhanced oxygen incorporation is attributed to interface-induced oxygen vacancies. • Cathodes suitable for intermediate-temperature solid oxide fuel cells are obtained. [ABSTRACT FROM AUTHOR]

Sulfur in the air stream is one of the primary contaminants affecting the stability of cathodes in solid oxide fuel cells (SOFCs). In this work, sulfur poisoning of La 1-x Sr x Co 1-y Fe y O 3-δ (LSCF) thin films with different compositions was investigated. The composition of the cathode was systematically controlled by preparing La 0.75 Sr 0.42 Co 0.15 Fe 0.68 O 3-δ (A-site excess LSCF) and La 0.60 Sr 0.39 Co 0.20 Fe 0.81 O 3-δ (stoichiometric LSCF) thin films on Gd-doped ceria (GDC) substrates by pulsed laser deposition. The LSCF films were then heat-treated at 800 °C for 100 h in synthetic air with a trace amount of SO 2 (ppb level). As an informative complement to the microstructural and compositional analyses, secondary ion mass spectrometry was also performed to effectively determine the extent of sulfur accumulations from LSCF cathode surface to GDC. The results showed that sulfur poisoning behavior of LSCF cathodes depends strongly on composition. The A-site excess LSCF films exhibited significant SrSO 4 formation with severe porosity, whereas, only an isolated SrSO 4 formation was observed in the stoichiometric LSCF films; instead, the partial poisoning by sulfur minimizes excessive porosity while enhancing Co-rich oxide precipitation. Electrochemical impedance spectroscopy was carried out to further elucidate the influence of cathode composition on cell stability. Thermodynamic considerations have been made to clarify the plausible chemical reaction mechanism between LSCF of different compositions with SO 2 . [ABSTRACT FROM AUTHOR]

La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) is a mixed ionic-electronic conducting perovskite (ABO 3 ) material which has attracted significant research interest in recent years as a cathode for solid oxide fuel cell (SOFC) applications at intermediate temperatures (500 °C–750 °C). Fundamental understanding of the oxygen surface exchange kinetics and diffusion in LSCF cathodes is necessary to optimize the cell performance at the required operating temperatures. In this study, we controlled the surface microstructure and crystalline orientation of LSCF thin films on (100) Gd 2 O 3 -doped CeO 2 thin films (GDC) prepared by pulsed laser deposition on yttria-stabilized zirconia (YSZ) single crystal substrates. Tailoring of the LSCF microstructure was achieved by modifying the underlying GDC thin film surface morphology and microstructure. To clarify the effect of the LSCF microstructure on its oxygen surface exchange kinetics, we employed the 18 O/ 16 O oxygen isotope exchange technique in conjunction with secondary ion mass spectroscopy (SIMS) depth profile analysis. Comparison of the qualitative features of the measured concentration of the 18 O profile and evaluation of the oxygen surface exchange coefficient k * were performed and the results correlated to the LSCF microstructure. [ABSTRACT FROM AUTHOR]

Cathode materials with high activity for oxygen surface exchange are required to achieve high-performance solid oxide fuel cells (SOFCs) at intermediate operating temperatures. LSC (La 0.6 Sr 0.4 CoO 3- δ) and gadolinia-doped ceria (GDC, Ce 0.9 Gd 0.1 O 1.95) thin films are alternately stacked to create multilayered cathode structures on yttria-stabilized zirconia (YSZ) electrolytes using pulsed laser deposition. Analysis of the electrochemical impedance spectra of symmetrical cells having varied number of LSC and GDC layers revealed that the existence of additional heterointerfaces leads to catalytic effects on the oxygen exchange properties. This is evidenced by a monotonic enhancement of the oxygen surface exchange coefficient (k *), commensurate with the increasing number of LSC-GDC interfaces. Due to the remarkable enhancement of the oxygen exchange properties, a significant lowering of the polarization resistance with increasing number of LSC-GDC interfaces is achieved. Furthermore, the LSC-GDC multilayers exhibited better long-term stability of the polarization resistance compared to a single-layer LSC in cell tests up to 600 h performed at 600 °C in air. These results highlight the important role of similar heterointerfaces in SOFC structures and provide guidance for the rational design of novel cathode materials with superior electrochemical performance at intermediate temperatures. Image 1 • Multilayered cathode structures comprising LSC and GDC are examined. • Heterointerfaces lead to catalytic effects on the oxygen exchange properties. • Significant lowering of the polarization resistance is achieved for multilayers. • Better long-term stability of the polarization resistance is demonstrated. [ABSTRACT FROM AUTHOR]

The degradation of electrochemical performance of solid oxide fuel cells (SOFCs) with long-term durability testing is generally evaluated in terms of increases in polarization resistances with time. Practical SOFCs normally employ an interlayer between cathode and electrolyte to improve the stability with long-term operation. To elucidate the relationship between the cathode-interlayer interfacial properties and degradation, various types of model interfaces based on La 0.6 Sr 0.4 CoO 3- δ (LSC) thin film cathodes prepared on gadolinia-doped ceria (GDC) interlayers having different microstructures are investigated. Equivalent circuit analysis of the electrochemical impedance spectra reveals an increase of the interfacial resistance associated with degradation of the LSC/GDC interface with operation time. The physical origin of the interfacial resistance is attributed to the formation of yttrium-doped SrZrO 3 (SZY) at the interface of LSC and GDC, a behavior which appears exacerbated for a nanocolumnar GDC interlayer. Investigation of model interfaces created with SZY inserted between LSC and GDC reveals large polarization resistances mostly dominated by the interfacial resistance, consistent with oxide ion blocking effects observed at interfaces. The role of the GDC interlayer microstructure on the long-term stability of LSC and GDC interfaces and cell performance is discussed. Image 1 • Long-term stability of model interfaces of LSC cathodes and GDC interlayers is examined. • Physical origin of interfacial resistance is attributed to secondary phase formation. • Oxide ion blocking effects at interfaces due to yttrium-doped SrZrO 3 are revealed. • GDC interlayer microstructure affects long-term stability of interface. [ABSTRACT FROM AUTHOR]