Impact of Silica Additions on the Phase Composition and Electrical Transport Properties of Ruddlesden-Popper La2NiO4+δ Mixed Conducting Ceramics

The present work explores the possibility of incorporation of silicon into the crystal structure of Ruddlesden-Popper La2NiO4+δ mixed conducting ceramics with the aim to improve the chemical compatibility with lanthanum silicate-based solid electrolytes. Ceramics with the nominal composition La2Ni1−ySiyO4+δ (y = 0, 0.02 and 0.05) were prepared by the glycine nitrate combustion technique and sintered at 1450 °C. While minor changes in the lattice parameters of the tetragonal K2NiF4-type lattice may suggest incorporation of a small fraction of Si into the Ni sublattice, combined XRD and SEM/EDS studies indicate that this fraction is very limited (≪2 at.%, if any). Instead, additions of silica result in segregation of apatite-type La10−xSi6O26+δ and La2O3 secondary phases as confirmed experimentally and supported by the static lattice simulations. Both total electrical conductivity and oxygen-ionic transport in La2NiO4+δ ceramics are suppressed by silica additions. The preferential reactivity of silica with lanthanum oxide opens a possibility to improve the compatibility between lanthanum silicate-based solid electrolytes and La2NiO4+δ-based electrodes by appropriate surface modifications. The promising potential of this approach is supported by preliminary tests of electrodes infiltrated with lanthanum oxide.