Oxidation of nickel in solid oxide cells during electrochemical operation: Experimental evidence, theoretical analysis, and an alternative hypothesis on the nickel migration

Abstract

Solid oxide cells with nickel/yttrium-stabilized zirconia (Ni/YSZ) cermet electrodes exhibit Ni migration which can cause severe cell performance degradation. The experimentally reported migration behavior of Ni is complicated, and the mechanisms remain under debate. This work discusses the possible mechanism of Ni migration related to the oxidation of Ni at the Ni-electrolyte interfaces under polarization via combined experimental study and theoretical analysis. In the experiments, NiO is found at the Ni-YSZ interfaces in the active layer in both tested fuel cells and electrolyzer cells, despite that the nominal oxygen partial pressure at the hydrogen electrode is well below the thermodynamic threshold for Ni oxidation. Due to the volume expansion during Ni oxidation and the outward diffusion nature of NiO growth, Ni oxidation and reduction of NiO back to Ni can cause Ni relocation. Thermodynamic analysis shows that the oxygen partial pressure near the Ni-electrolyte interface can be significantly higher than the hydrogen electrode under polarization, which can cause Ni oxidation and concentration increase of the gaseous Ni(OH)2, and the latter accelerates the transport of Ni. As such, a new hypothesis for Ni migration in solid oxide cells is proposed in which the interfacial oxidation of Ni plays an essential role.