Modeling Ni redistribution in the hydrogen electrode of solid oxide cells through Ni(OH)2 diffusion and Ni-YSZ wettability change

Ni redistribution in the hydrogen electrodes of solid oxide cells is an important degradation mechanism. Its driving force is still under debate. This work focuses on the interplay between Ni(OH)2 diffusion and Ni-YSZ wettability change. With a Ni(OH)2 diffusion model developed in our previous work, we further employ three models for the Ni-YSZ contact angle to quantify the Ni-YSZ wettability change. The microstructure evolutions in a reconstructed Ni-YSZ electrode are simulated under selected experimental operating conditions. It is shown that the phenomenological model can capture the driving force of Ni spreading/detachment on YSZ surface and the Ni migration reported in experiments can be qualitatively reproduced through the competition between Ni(OH)(2) diffusion and Ni-YSZ wettability change. It is also found that the initial microstructure of the active layer and the microstructure characteristics in the support layer can strongly affect the distribution of steam partial pressure and overpotential, hence the Ni redistribution, in the active layer, which may explain the inconsistencies in experiments. Our results show that there are two missing pieces in current theory of Ni redistribution: a mechanism of fast Ni diffusion under humid atmosphere and the physical reason behind the Ni spreading/detachment.

Automated summary

Ni redistribution in the hydrogen electrodes of solid oxide cells is an important degradation mechanism. This study investigates the interplay between Ni(OH)2 diffusion and Ni-YSZ wettability change, and identifies the driving force of Ni spreading/detachment and the factors affecting Ni redistribution. Two missing pieces in current theory of Ni redistribution are also identified.