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.

Structural and Electrochemical Properties of Lanthanum Silicate Apatites La10Si6−x−0.2AlxZn0.2O27−δ for Solid Oxide Fuel Cells (SOFCs)

An excellent oxide ion conductivity with high oxygen transportation of lanthanum silicate apatite at the solid oxide fuel cell (SOFC) can be achieved through the solid-state reaction method. The doped La10Si6−x−0.2AlxZn0.2O27−δ (x = 0.2 and 0.4) materials sintered at 1600°C accomplished crystallinity and crystal structure of apatite-type. The structural and electrochemical characterizations of La10Si6−x−0.2AlxZn0.2O27−δ (x = 0.2 and 0.4) were executed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and electrochemical impedance spectroscopy (EIS) measurements. The total oxide ion conductivities of La10Si6−x−0.2AlxZn0.2O27−δ (x = 0.2 and 0.4) were measured from low to intermediate operating temperature range (450 to 800°C) using electrochemical impedance spectroscopy. Room temperature XRD patterns of La10Si6−x−0.2AlxZn0.2O27−δ (x = 0.2 and 0.4) exhibited La10Si6O27 apatite phase with space group P63/m as the main phase with the minor appearance of La2SiO5 as an impurity phase. The highest total oxide ion conductivity of 3.24 × 10−3 Scm−1 and corresponding activation energy of 0.30 eV at 800°C were obtained for La10Si5.6Al0.2Zn0.2O26.7 which contains a low concentration of Al3+ dopant.

Highly enhanced electrical properties of lanthanum-silicate-oxide-based SOFC electrolytes with co-doped tin and bismuth in La9.33−xBixSi6−ySnyO26

Structure modification of La9.33Si6O26 (LSO) as SOFC electrolyte via a bi-doping mechanism provides enhanced electrical properties of La7.83Bi1.5Si5.7Sn0.3O26 at 873 K (1.84 × 10−2 S cm−1) with low activation energy of 0.80 eV compared to pristine LSO (0.08 × 10−2 S cm−1).