In order to investigate the relevant factors affecting the transport properties of solid oxide fuel cell cathodes upon coating with Ce0.9Gd0.1O1.95 (GDC), the surface oxygen exchange coefficient, k*, ...and oxide ion diffusivity, D*, of La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) were measured by a combination of isotope exchange technique and secondary ion mass spectrometry (SIMS). The k* of LSCF/GDC enhanced compared to a bare LSCF for all measured temperatures. In contrast, the k* of LSC/GDC only showed an enhancement compared to the bare LSC at relatively low temperatures (below 873 K) but not at higher temperatures. From these results, GDC has a less significant effect on the k* of LSC compared to LSCF. This suggests that the enhancement of k* is possibly induced only when the oxygen vacancy concentration, δ, of the cathode is lower than that of GDC, although another factor may affect the enhancement. The enhancement of k* is attributed to the formation of triple-phase boundary where a spill-over mechanism controlled the oxygen reduction reaction. The change of rate-determining step of LSCF due to the GDC coating suggests that the enhancement of the k* is not only with respect to the incorporation process, but it is possibly enhancing the reaction steps that involve δ as well.
•The influence of GDC coating on transport properties of LSC/LSCF was studied.•GDC has a less significant effect on the ORR rate of LSC compared to LSCF.•The δ of cathode determines the enhancement degree of k* upon GDC coating.•The inter-diffusion region was observed at LSC/GDC and LSCF/GDC.
In order to perform stress analyses of a solid oxide fuel cell (SOFC) under operation, we propose a characterization method of its time-varying macroscopic electro-chemo-mechanical behavior of ...electrodes by considering the time-varying geometries of anode microstructures due to Ni-sintering. The phase-field method is employed to simulate the micro-scale morphology change with time, from which the time-variation of the amount of triple-phase boundaries is directly predicted. Then, to evaluate the time-variation of the macroscopic oxygen ionic and electronic conductivities and the inelastic properties of the anode electrode, numerical material tests based on the homogenization method are conducted for each state of sintered microstructures. In these homogenization analyses, we also have to consider the dependencies of the properties of constituent materials on the temperature and/or the oxygen potential that is supposed to change within an operation period. To predict the oxygen potential distribution in an overall SOFC structure under long-period operation, which determines reduction-induced expansive/contractive deformation of oxide materials, an unsteady problem of macroscopic oxygen ionic and electronic conductions is solved. Using the calculated stress-free strains and the homogenized mechanical properties, both of which depend on the operational environment, we carry out the macroscopic stress analysis of the SOFC.
Local lattice instability analysis based on the atomic elastic stiffness (AES), Bijα=Δσiα/Δεj (i,j=1–6 in Voigt notation), is applied to mode I crack tip in covalent binary crystal β-SiC of Tersoff ...interatomic potential. The application limit of the AES is disclosed at the discontinuous local energy surface of disordered broken bonds in the discrete binary system; however, we also captured interesting results with the 1st eigenvalue ηα(1) of Bijα as follows, (1) the atomic stress of C atoms coincides with the stress singularity in the linear fracture mechanics, when the minimum eigenvalue of C atom shows the first sudden drop or the first instability, (2) but the crack doesn’t propagate nor there is no remarkable change in the stress–strain curve, (3) bond breaking and reconstruction occurs in the vicinity of crack tip at the second instability in the eigenvalue, (4) C atoms in the disordered configuration show extraordinary large negative eigenvalue (possibly due to discontinuity of energy surface), while Si atoms show third drop in the eigenvalue just before unstable crack propagation, (5) linear fracture mechanics fails to predict the stress singularity at the crack propagation, (6) the principal axes of the eigenvector of ηα(1)<0 atoms reveal the deformation mode for local slip and cracking on the multiple (111) planes in the unstable domain at the forefront of the propagating crack.
High durability against carbon deposition on Ni anodes of internal‐reforming solid oxide fuel cells (SOFCs) is one of the most significant challenges in the utilization of SOFCs. Carbon deposition is ...strongly affected by the type of solid oxide electrolyte, although it occurs on Ni surfaces which are a few hundred nanometers away from the solid oxide electrolyte. Carbon deposition under CH4 flow at 1,073 K on Ni particles on yttria‐stabilized ZrO2 was determined to occur more readily than on Ni particles on Gd‐doped CeO2. We have examined how the solid oxides interact with the Ni surfaces in anodes using Auger electron microanalysis and infrared spectroscopy with adsorbed molecules. The results suggest that the CeO2 species migrate on the Ni surfaces under reducing conditions. The chemical influence of CeO2 migration on the Ni surface was confirmed by infrared spectra of adsorbed CO on Ni sites.
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