Redox behaviour of Gd-doped ceria–nickel oxide composites

Reduction kinetics of NiO–gadolinium-doped ceria (GDC) composites was studied. NiO–GDC ceramic rods were fabricated by cold isostatic pressing of powders of nanometer size obtained by chemical synthesis. The rods were sintered in air at the maximum contraction temperature, 1350 °C, and treated in reducing atmosphere at different temperatures and reduction times. Progress of the reduction process was followed by the gravimetric method. By adjusting the data obtained from weight loss during the isothermal reduction at temperatures between 500 and 700 °C to standard diffusion models for a cylinder, it was possible to obtain effective diffusion coefficients for the material. The process activation energy was 0.9 ± 0.2 eV indicating that, in the whole temperature range studied, the reduction kinetics is controlled by the diffusion of O2− throughout the ceramic matrix of GDC. SEM studies in reduced, partially reduced and completely reduced samples reveal a submicrometric microstructure with a uniform distribution of Ni phase surrounded by pores within ceramic GDC matrix. This microstructure is suitable for IT-SOFC anodes.