Reduction and Reoxidation Kinetics of Nickel-Based SOFC Anodes

Abstract

Reduction and reoxidation kinetics of Ni-based solid oxide fuel cell (SOFC) anodes were investigated over a range of temperatures between 600 and . Dense (no open porosity) two-phase (yttria-stabilized zirconia) samples, with and without small amounts of oxide additives (CaO, MgO, ), were fabricated and then reduced in a hydrogen-containing environment. The time dependence of the reduced layer thickness at various temperatures was measured. Reoxidation studies were conducted on fully reduced anodes that were subsequently reoxidized in air over a temperature range between 650 and . A simple theoretical model was developed to describe the kinetics of reduction and reoxidation based on two series kinetic steps: diffusion and interface reaction. It was observed that the reduction kinetics was linear (interface-controlled), while the reoxidation kinetics was nearly parabolic (diffusion-controlled). Also, the kinetics of reduction was thermally activated with an activation energy of . By contrast, over the temperature range investigated, the kinetics of reoxidation was essentially independent of temperature. The interface control of the reduction process implies that gas-phase diffusion through porous , formed upon reduction of NiO to Ni, is considerably faster than the kinetics of the actual reduction reaction occurring at the interface separating the pristine and the reduced regions. By contrast, diffusion control of the reoxidation process was attributed to slow, gaseous diffusion on account of the very small amount of porosity that remains when Ni reoxidizes to NiO, developed presumably due to a slight shape change of Ni particles that may occur at high temperatures. Doping the anodes with stable oxides, such as CaO and MgO, significantly reduced both the reduction and reoxidation kinetics of Ni-based anodes.