Structural Stability of Nickel Electrodes in Solid Oxide Cells

Abstract

1. IntroductionLong-term durability is an issue of solid oxide cells (SOCs). Degradation of SOCs is most pronounced in a fuel electrode, where length of three-phase boundary, that is active reaction site, decreases. The major cause of the decrease in the three-phase boundary is nickel aggregation. The mechanism of Ni aggregation has been proposed to be Ni transport through the gas phase by nickel hydroxide. However, experimental facts did not support the hypothesis, suggesting that Ni transport through the gas phase should not be major factor [2].Because Ni-YSZ fuel electrode has a complex three-dimensional structure, it has been difficult to elucidate the mechanism of Ni transport. Therefore, it is natural to use nickel model electrode to simplify the evaluation of microstructure change of nickel, which will lead to the elucidation of the degradation behavior. Jiao et al. observed the microstructure change of Ni using Ni grid electrodes [3]. The structural change of Ni was attributed to change in wettability of Ni by applied voltage, where the wettability increased under anodic polarization and decreased under cathodic polarization.In this study, we aim to establish a method to observe the structure change of Ni inside the electrodes by modeling the complex structure of Ni-YSZ porous electrodes using patterned electrodes.2. ExperimentalFig. 1 (a) shows the simulated cross section of a symmetrical Ni-YSZ porous electrode cell. To model the cross section of porous Ni-YSZ fuel electrode, dense nickel-film patterned electrodes were fabricated on the YSZ substrate as shown in Fig. 1 (b) and (c). In this model cell, YSZ locating between two opposed nickel electrodes simulates the electrolyte, while the comb-shaped Ni pattern electrode simulates the porous Ni; YSZ surface between Ni fingers represents porous YSZ. Using this cell, transport of Ni in the direction of the electrode cross-section can be observed from top. After certain period of polarization to the cell, microstructure change of nickel was observed by microscope.3. Results and discussionAfter SOFC operation, the cross-sectional area of nickel electrode was the largest at the tip of the model electrode, suggesting that Ni migrated toward the tip. On the other hand, after SOEC operation, the cross-sectional area was larger toward the root, suggesting that Ni migrated toward the root. These migration directions were consistent with those assumed by wettability. The results confirmed that wettability contributed to Ni migration in the model electrode and that wettability-induced Ni migration also occurred in the porous electrode.