Synergetic effect of NiO and SiO2 on the sintering and properties of 8 mol% yttria-stabilized zirconia electrolytes

Yttria-doped zirconia electrolytes (e.g., 8 mol% yttria-stabilized ZrO₂, 8YSZ) have been considered to be the most promising candidates for applications in solid oxide fuel cells (SOFC). Due to the ubiquitous presence of SiO₂ impurities and wide use of Ni-containing anodes, it is therefore of great technical importance to understand the synergetic effect of NiO and SiO₂ on densification, grain growth and ionic conductivities (especially the grain boundary (GB) conduction) of zirconia electrolytes. In this study, three groups of 8YSZ ceramics, with Si contents of ∼30, ∼500 and ∼3000 ppm, have been designed. 1 at% NiO was added into these materials by a wet chemical method. The addition of SiO₂ has a negative effect on the sintering and densification, while the introduction of 1 at% NiO reduced the sintering temperature and promotes grain growth of the zirconia ceramics. However, the presence of small amount of NiO prevented full densification of 8YSZ ceramics. NiO also led to a decrease by ∼33% in grain interior (GI) conductivity, with little effect on the GB conduction of high-purity 8YSZ (∼30 ppm SiO₂). However, the coexistence of NiO and SiO₂ is extremely detrimental to total conductivity by significantly reducing the GB conduction. Moreover, it is observed that, unlike the 8YSZ-doped SiO₂ with only, whose GB conduction increases greatly with increasing sintering temperature, the GB conduction of the NiO and SiO₂ codoped samples is less sensitive to sintering temperature.