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The electrical conductivity of nickel (2 wt %)-doped BaZr0.1Ce0.7Y0.1Yb0.1O3−δ(BZCYYb) acceptor-doped perovskite oxide was evaluated under air and a 1% H2atmosphere. The partial conductivity was calculated from the total conductivity and the transport number of each carrier (tH+, tO2–, and th+) obtained using the concentration cell method. Its correlation with the dissolution state of the protons in the oxide as studied by in situ diffuse reflection Fourier transform infrared spectroscopy is discussed. When the concentration of protons that dissolved in BZCYYb-Ni was high, the proton partial conductivity was also high. An increase in hole conductivity in the high-temperature region in an air atmosphere was observed, suggesting that dissociation of protons strongly correlates with such a dominant carrier change. The dissociation of protons should be determined by the stability of protons in the oxide by the interaction with the lattice oxygen, and it was suggested that the dissolution state of protons can be controlled by modifying such stability in the oxide.

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 (La0.6Sr0.4)CoO3-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 10−5cm/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.

In this study, we examined the properties of model heterointerfaces created with yttrium-doped SrZrO3(SZY) thin films, a type of secondary phase that most frequently occurs at interfaces of solid oxide fuel cells utilizing yttria-stabilized zirconia (YSZ) electrolytes. Electrochemical impedance results of symmetrical cells prepared with LSCF cathodes were evaluated using DRT (distribution of relaxation times) analysis, and 18O isotope exchange depth profiling with SIMS (secondary ion mass spectrometry) analysis was performed on post-tested samples to examine the ionic transport across heterointerfaces. Results showed high polarization resistance contributed by interfacial resistance associated with interfaces with SZY, i.e., LSCF/SZY, SZY/YSZ or SZY/GDC (gadolinia-doped ceria). The careful selection of heterostructure configurations lead to the conclusion that the interfacial contact of SZY with LSCF, YSZ and GDC results into high interfacial resistances, possibly due to an ion blocking effect.

The effect of temperature on the deuteron solubilities in YSZ and GDC polycrystalline samples was examined by changing the rate of sample cooling following the equilibrium annealing to incorporate deuterons into the samples. From the deuteron concentration profiling by using secondary ion mass spectrometry, the concentration increment of deuterons near the surface of the samples was observed by decreasing cooling rate. It was attributable to the deuteron incorporation into the samples during gradual cooling, indicating the higher solubility at lower temperature region and fast deuteron diffusion along the grain boundaries in YSZ and GDC.

The electrochemically induced interaction between Ni and GDC was investigated using a network-shaped Ni electrode on a dense interlayer of gadolinia-doped ceria (GDC) with yttria-stabilized zirconia (YSZ) electrolyte. The sample with a platinum counter electrode and a reference electrode was treated at 1173 K for 5 h with flowing 1% H2O - H2gas mixture under different applied potentials. The morphology of Ni electrode changed when potential was applied. A large number of ceria particles in several ten-nanometer size was precipitated on the Ni surface. The Ni-GDC interaction became more significant with increasing applied potential. The relationship between electrochemically induced morphological change and overpotential-induced oxygen potential change is discussed.

Understanding degradation mechanism is essential in improving performance and long-term stability of solid oxide fuel cells. Model cells consisting of porous La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) cathodes were implemented on yttria-stabilized zirconia (YSZ) electrolytes with dense Gd-doped ceria (GDC) interlayers and tested in an open-circuit voltage condition at 800°C for 300 h in air to elucidate the degradation at the cathode-interlayer interfaces. The GDC films were prepared on poly-YSZ and single-crystal YSZ(100) electrolytes by pulsed laser deposition to generate dense and well-defined barrier microstructures namely granular polycrystalline (poly-GDC) and grain boundary-free single crystalline (sc-GDC) interlayers. Distinct features of the reaction phases at the cathode-interlayer interfaces were observed depending on the type of GDC interlayers. The severe SrZrO3accumulation at the LSCF/poly-GDC interface caused the rapid cell performance degradation with the poly-GDC interlayers. Whereas, the SrZrO3formation was suppressed successfully with the sc-GDC interlayers resulting in improved cell performance. However, the active reaction between cathode and interlayer results in the possible formation of GdFeO3at the LSCF/sc-GDC interface and the CoO dissolution in sc-GDC interlayer appear to affect cell stability. Based on these results, plausible thermodynamic and kinetic considerations on interface chemistry and associated electrochemical behaviors were discussed.

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, SrZrO3formed at the LSCF/GDC interface in consequence of the fast Zr migration in the PLD-grown GDC interlayers. However, SrZrO3formation and GDC microstructural feature differ between OCV and cathodic polarization. Under OCV, SrZrO3laterally 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 SrZrO3was 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.

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 SrZrO3at the GDC/YSZ interface. Instead, SrZrO3formed 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 SrZrO3formation; by contrast the dense cathode only showed localized SrZrO3grains. 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.

Performance degradation of SOFC cathodes has been partly attributed to sulfur dioxide (SO2), which is one of the major impurities naturally present in air. In this work, the sulfur poisoning behavior of (LaSr)(CoFe)O3(LSCF) cathodes exposed to 1 ppm of SO2in air was investigated as a function of time under various operation temperatures of 923 K, 973 K, and 1073 K. Severe performance degradation was observed at low temperatures of 923 K and 973 K. For these cases, only the polarization resistance (Rp) was increased, additionally no sulfur component was detected in the LSCF cathode. At 1073 K, both Rpand ohmic resistance (Rohmic) increased during the progress of degradation and SrSO4was observed in cathode, especially near the LSCF/GDC interface. Plausible sulfur poisoning mechanisms were proposed based on the observed experimental phenomena.