Influence of reduction conditions on electrical properties of NiO-zirconia composites for solid oxide fuel cell electrode

Abstract

The electrical properties of nickel-zirconia cermets as the anode material for solid oxide fuel cells (SOFCs) were studied for Sc2O3–Al2O3-stabilized ZrO2 (SASZ), together with conventional Y2O3-stabilized ZrO2 (YSZ). The reduction behavior of the cermets in terms of achieving better electrical conductivity was examined under constant temperature ramp rate (CTR) and constant temperature (CT) conditions. The reduction process and electrical conduction thus obtained were affected by the NiO particle size and porosity, and the zirconia composition of the starting NiO-zirconia composite material. All the NiO-zirconia anodes examined in this study exhibited high conductivity after reduction at a CT of 800 °C. The CTR condition resulted in lower electrical conductivity for the Ni-zirconia cermet. In particular, NiO-SASZ using coarse NiO powder exhibited only ionic conduction with a low electronic contribution, which was due to the absence of nickel–nickel percolation. Thermogravimetric analysis of the NiO reduction indicated that NiO-SASZ and NiO-YSZ had different reduction mechanism which might correspond to the interaction between NiO and zirconia and the long-term stability of these cermets. A kinetic analysis of the NiO reduction process revealed that fast and constant nucleation during the initial stage of reduction is important for the construction of nickel–nickel connections in the cermets.