Study on electrochemical re-oxidation of anode-supported solid oxide fuel cells using oxide ion current

The effect of the re-oxidation of anodes in anode-supported planar solid oxide fuel cells was investigated. Scanning electron microscopy (SEM) was used to observe the microstructure of the anode cross section. X-ray diffraction was used to measure the residual stress in the electrolyte.The re-oxidation of the anode by ionic current was conducted at 800 °C by applying the current to a cell by supplying air for the cathode and pure N2 for the anode.From the cross-sectional SEM images of the anodes, it was confirmed that the morphology of Ni particles in a particular area of the anode—an active area for electrochemical reactions—was modified to a sponge-like structure. It was reported that the spongy microstructure was generated when Ni was re-oxidized by O2 in air. On the other hand, the morphology change in the Ni particles outside of the active area was not observed. This suggests that this change in the microstructure resulting from re-oxidation was caused not by O2 in air but by the oxide ion current. The larger degree of morphology change in the Ni particles was observed in close proximity of the electrolyte, which also suggests that the re-oxidation was caused by the ionic current. These results indicate that the effect of the anode structure on the electrolyte re-oxidized by ionic current was successfully investigated without the influence of the gas phase.Then, the influence of anode expansion caused by the re-oxidation on the electrolyte is studied by measuring the residual stress in the electrolyte.A large compressive stress in the electrolyte was observed as that before the re-oxidation. The residual stress in the electrolyte under the cathode was found to be relaxed after passing the ionic current. On the other hand, the relaxation of the residual stress in the electrolyte outside the area of cathode was not measured. This result is consistent with the microstructure observation results.