Effect of inhomogeneous re-oxidation on Ni-based SOFC oxidation resistance

Abstract

Inhomogeneous re-oxidation, which causes graded NiO content along anode thickness, has been confirmed to be a key reason for Ni-based cell cracking during redox progress. In this paper, an analytical model is developed to estimate the impact of inhomogeneous re-oxidation on Ni-based solid oxide fuel cell (SOFC) oxidation resistance. And experiments, in which the SOFC was partially re-oxidized, were implemented for model trial. Model results show that electrolyte internal stress can be significantly reduced (from 367 MPa to 135 MPa, when the oxidation degree is 60%), and the electrolyte can remain intact even when the oxidation degree reaches about 70%, if the anode was re-oxidized uniformly. This impact of inhomogeneous re-oxidation on stress in the electrolyte decreases as the anode thickness increases. Scanning electron microscopic (SEM) images of partially oxidized anode cross-sections confirmed that Ni oxidation was inhomogeneous, in which the outer regions of the anode became almost fully oxidized, while the inner regions remained metallic. And the inhomogeneity increases with the redox times. Consequently, it is important to avoid gradients in NiO content during oxidation progress to prevent cell cracking.