Reduction and Re-Oxidation Kinetics of Nickel-Based Solid Oxide Fuel Cell Anodes

Reduction and re-oxidation kinetics of Ni-based anodes were investigated over a range of temperatures between 650 and 800°C. The experimental part of the investigation consisted of first fabricating NiO + YSZ two- phase composites of essentially full density. The fully dense samples were subsequently reduced in a hydrogen-containing environment. The reduced layer thickness was measured as a function of time at temperature. Some of the samples were reduced for a time long enough to ensure that they were fully reduced. Subsequently, the reduced samples were re-oxidized in air at various temperatures, ranging between 650 and 800°C. In this set of experiments, the re-oxidized layer thickness was measured as a function of time at temperature. These two types of experiments were used for determining the kinetics of reduction as well as that of re-oxidation. It was observed that the reduction kinetics was linear (interface-controlled), while the re-oxidation kinetics was parabolic (diffusion-controlled). A simple theoretical analysis was conducted to describe the two processes. The interface control of the reduction process was attributed to the rapid gaseous transport through the pores formed upon reduction of NiO to Ni. By contrast, diffusion control for the re-oxidation process was attributed to the existence of a very small amount of porosity that forms when Ni re-oxidizes to NiO, which occurs due presumably to a slight shape change of Ni particles.