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

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 ex-periments, 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.

Automated summary

This study investigates the mechanism of nickel migration in nickel/yttrium-stabilized zirconia (Ni/YSZ) cermet electrodes through a combination of experimental and theoretical analysis. The results show that NiO is present at the Ni-YSZ interfaces in the active layer of both fuel cells and electrolyzer cells, despite the low nominal oxygen partial pressure at the hydrogen electrode. The volume expansion during Ni oxidation and the outward diffusion nature of NiO growth lead to nickel relocation. Thermodynamic analysis indicates that the oxygen partial pressure near the Ni-electrolyte interface can be higher than at the hydrogen electrode under polarization, promoting Ni oxidation and concentration increase of gaseous Ni(OH)2, thereby facilitating nickel transport. Therefore, a new hypothesis is proposed, suggesting the crucial role of interfacial oxidation of nickel in solid oxide cells.