Enhancing conductivity of apatite-type lanthanum silicate by Li and Cu co-doping

Abstract

Solid oxide fuel cells (SOFC) are all-solid power generation devices that directly convert chemical energy stored in fuel gases and oxidizers to electrical energy efficiently and without pollution at medium to high temperatures. Recognized as the green energy technology of the 21st century, it is of great significance to alleviating the energy crisis, meeting electricity demand, and reducing environmental pollution. Using the urea-nitrate combustion method, the solid electrolyte powder (LLSCO) of lanthanum silicate of apatite type La9.33LixSi5.5Cu0.5O25.5 + 0.5x (x = 0.1, 0.2, 0.3, 0.4) was successfully prepared by igniting 6-8 mins at 600 °C. According to XRD analysis, LLSCO has a typical P63/m apatite structure and high purity. The volume of LLSCO lattice increases further than that of LSO and LSCO and increases with the increase of Li+ doping. This is because when Li and Cu are doped simultaneously, Li+ enters the empty space in the LSO lattice that occupies the LaII position, which causes the lattice to expand. At the same time, Cu2+ enters the LSO lattice to replace the partial Si4+, which will also cause lattice expansion. The IR analysis shows that Cu2+ replaces Si4+ and enters the [SiO4] tetrahedron to form the [Si(Cu)O4] tetrahedron, while Li+ occupies the vacancy of the LaII position. The optimal sintering temperature of Li and Cu co-doped LLSCO was determined to be 1575 °C based on shrinkage rate, relative density with temperature, and microscopic morphology analysis. The conductivity of La9.33LixSi5.5Cu0.5O25.5+0.5x (x=0.1, 0.2, 0.3, 0.4) was significantly higher than that of LSO and LSCO. With the increase of Li doping, the impedance of the doping sample was lowest when x = 0.3, and the conductivity reached the highest of 9.27 × 10−4 S · cm ·−1 at 600°C.