Review of Ni migration in SOC electrodes

Several kinds of Ni migration in Ni-YSZ (yttria stabilized zirconia) electrodes of solid oxide cells (SOCs) have been reported in the literature, which is briefly reviewed in this extended abstract. Emphasis is put onto the migration of Ni away from the YSZ electrolyte in solid oxide electrolysis cells (SOECs) as this is today seen as an important obstacle to the commercialization of SOC systems.

The characteristics of the various migration types are apparently of different nature varying from: (i) Ni-particle migration on top of the YSZ electrolyte in a model electrode to (ii) long distance Ni migration out of porous Ni-YSZ composite cermet electrode in solid oxide fuel cells (SOFCs), and (iii) migration of Ni towards the YSZ electrolyte at 950 C, but away from the electrolyte at temperatures below 875 C under otherwise similar conditions in Ni-YSZ cermet electrodes in SOECs.

Apart from temperature, degradation of Ni-YSZ electrodes in SOCs is related to overpotential and partial pressure of steam (pH2O), hydrogen (pH2), carbon dioxide (pCO2), carbon monoxide (pCO) and oxygen (pO2) through the influence of these parameters on the mobility of Ni. Actually, the local partial pressure values are most important. Furthermore, impurities from cell materials and reactants may increase mobility of Ni and in general highly affect the long-term durability of SOECs negatively.

Possible Ni migration mechanisms and methods of mitigating the degradation associated with Ni migration are discussed. It is concluded that a lot of further work is necessary in order to describe this Ni migration properly, and the type of work is listed.