Understanding Nickel Oxidation and Reduction Processes in SOFC Systems

In solid oxide fuel cells, Ni-YSZ cermets have many advantages, but degradation can be severe if the anode is exposed to an oxidizing environment. It has been found, in this study and others, that the primary cause of degradation is due to a >60% volume change from Ni to NiO. It is shown here that the Ni is reduced and oxidized in a shorter time period for each redox cycle after the first full cycle. For reduction, this is likely due to the development of a larger surface area, as the cellular NiO surface structure is observed. Also, controlled oxidation of the anode leads to reduced series and reaction resistance values and the Van der Pauw technique indicates that the conductivity of the Ni-YSZ electrode improves after the first redox cycle. Thus, as long as there is enough space for the Ni to expand as it is oxidized, there are actually benefits to cell performance from redox cycling.