Ni–YSZ cermet anodes prepared by citrate/nitrate combustion synthesis

Abstract

The synthesis of Ni–YSZ cermets with tailored particle package, shape and microstructural characteristics is essential when preparing anodes for solid oxide fuel cells (SOFC). These materials are generally prepared by sintering and subsequent reduction of the mixture of metal oxides. In order to obtain cermets with an adequate contact area between electrocatalyst (Ni) and ionic conductor (YSZ), an alternative route was used based on mixed gel combustion with the material synthesis, calcination and partial sintering achieved in one step. The precursor for the combustion synthesis was a mixed citrate/nitrate gel prepared from nickel, zirconium and yttrium nitrates and citric acid by vacuum evaporation of the solution. The combustion reaction of this gel produces submicrometer crystalline NiO–YSZ composite. The influence of the fuel/oxidant molar ratio of the precursor on the combustion rate and end product characteristics was investigated. The reaction period, phase composition, morphology and agglomerate formation were studied in detail. It was shown that the initial fuel/oxidant ratio strongly influences the characteristics of the powder mixtures thus obtained. The morphological properties of the prepared mixed oxides after the combustion synthesis reveal that the particle size distribution and the agglomerate formation in the voluminous intermediate mixed oxide product (green body) differ with the initial fuel/oxidant molar ratio. Narrower agglomerate and pore size distribution has a great influence on the subsequent sintering and reduction of the mixed material. If the particle and pore size distribution in the green body are narrow, the coarsening of the YSZ and NiO grains, and subsequently, YSZ and Ni grains are less pronounced.