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Generalized Ellingham diagram for the P-O-H and the Ni-P-OH systems have been constructed to investigate thermodynamically the chemical stability of nickel anode against the gaseous impurities containing phosphorous compounds. In the same way as the original Ellingham diagram, the oxygen potential is used as the vertical axis, while the temperature is adopted as horizontal axis. For the P-O-H system which contains many gaseous species, the dominant areas of gaseous species are displayed with a parameter of their partial pressure in an analogous way to the aqueous species in the Pourbaix diagram. The multicomponent Ellingham diagram for the Ni-P-O-H system was constructed in a similar manner to the multicomponent Pourbaix diagram. The obtained diagrams have been discussed to examine the reactivity of nickel anodes with phosphorus compounds in SOFCs in terms of operational variables such as temperature, oxygen potential, overpotential under the anode polarization and so on.

The effect of low concentrations of SO2 (0.01 ppm) in air on the degradation of a porous La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) electrode was investigated as a function of operating temperature. At 1073 K, most of the introduced SO2 became trapped as SrSO4 in the vicinity of the air inlet, and consequently the degradation of cathode performance was not observed. At 973 K and 923 K, the SO2 reached directly the electrochemically active region. Thus, the formed SrSO4 caused a gradual decrease in cathode performance. These results suggest that sulfur poisoning behavior at 0.01 ppm SO2 is mainly governed by two processes, the chemical reactions in the vicinity of air inlet and the electrochemically enhanced reactions in the active region; the former is strongly enhanced by temperature, whereas the latter additionally by the applied overpotential. SEM-EDX analysis confirmed the sulfur distribution was correlated to the degradation in the electrode performance. The concentration of SrSO4 found near the triple-phase boundaries corresponded to the case where both the polarization and the ohmic resistances increased. Typical values for the effective reaction length essentially correlated with the k* and D* values obtained using the transmission line model. Results are in order-ofmagnitude agreement with the features obtained using SEM-EDX. [ABSTRACT FROM AUTHOR]

Cathode materials with highly reactive surfaces and long-term stability are required to achieve high-performance solid oxide fuel cells (SOFCs). In this study, a promising cathode material, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF), was prepared as a nanostructured thin film using pulsed laser deposition (PLD) on gadolinia-doped ceria (GDC)-buffered YSZ single crystal substrates having (100) and (111) orientations. Characterization revealed intrinsic differences among the as-grown LSCF thin films in terms of dominant crystalline orientation and nanostructure depending on GDC preparation as well as the YSZ substrate orientation. Evaluation of the oxygen exchange properties using the isotope exchange depth profile method revealed that LSCF thin films grown on (111) GDC/YSZ exhibited higher values of the apparent surface exchange coefficient compared to LSCF thin films grown on (100) GDC/YSZ. However, when subjected to long-term annealing at high temperatures, the former exhibited a stronger tendency to surface segregation as compared to the latter. These behaviors are correlated with the intrinsic properties of LSCF thin films, including the nanostructure, the possible effects attributed to SrO activity, and the stability of perovskite surfaces which would drive surface segregation. These results have implications for tailoring the performance of cathode thin films by understanding the dependence of oxygen exchange properties and surface segregation on driving forces such as surface chemistry and nanostructure. [ABSTRACT FROM AUTHOR]

Factors governing the change of the surface composition and the degradation of transport properties of La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) under low concentration sulfur-contained air (0.01 ppm) were examined as functions of water vapor pressure (p(H2O)) and oxygen partial pressure (p(O2)). Bulk LSCF samples were heat-treated at 973 K in dry or wet air containing SO2 and more enhanced SrSO4 formation/growth was observed under wet compared to dry air. When the p(O2) was changed from 0.21 to 0.01 bar, the SrSO4 precipitates became smaller in size and higher in number, indicating that the p(H2O) and the p(O2) have different effects on secondary phase formation. Detailed analysis of the surface composition indicates the presence of CoFe2O4 in the vicinity of the SrSO4 particle, suggesting the Sr and Co depletion in the subsurface region. Based on the analysis, the water vapor affects the secondary phase growth due to the possibility of additional reaction pathways that may enhance the growth rate. This behavior is correlated with the logarithmic surface oxygen exchange coefficient, log k*, that was observed to decrease monotonically within 20 h of annealing time but its degradation was found to be faster under wet air, in accordance with Sr and Co depletion rate. [ABSTRACT FROM AUTHOR]

In order to investigate the long-term degradation of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathodes due to chromium (Cr) poisoning, the transport properties were investigated by two different approaches: one is electrochemical impedance spectroscopy (EIS) on porous LSCF electrodes, the other is the oxygen isotope exchange and SIMS analyses on LSCF pellets. The oxygen surface exchange coefficient (k*) determined from EIS decreased with increasing poisoning time. For LSCF pellets, the k* values decreased linearly with poisoning time (the first stage) and then became nearly constant (the second stage). The first degradation of k* was attributed to the formation of SrCrO4 and CoFe2O4 within the initial 7~8 h poisoning time, whereas in the second stage, LaCrO3 formation was detected on the surface of LSCF. Those chemical behaviors were compared with thermodynamic calculations where the k* was observed to decrease in the first stage corresponding to the precipitation of SrCrO4 and CoFe2O4, and LaCrO3 is formed in the second stage. Those considerations lead to the conclusions that the k* decrease is dominated by the decrease in available sites such as Sr, Co or oxygen vacancies on the LSCF surface, whereas the constant k* values may be attributed to the remaining/formed LaCrO3 component of the reacted LSCF. [ABSTRACT FROM AUTHOR]