Synthesis and characterization of dense proton-conducting La1-xSrxScO3-α ceramics

Research into the synthesis of oxide materials exhibiting high ionic conductivity values allows these materials to be used for the fabrication of such high-temperature electrochemical devices as fuel cells, gas sensors, electrolyzers, etc. A focused interest in the physical and chemical properties of oxide proton conductors is driven by the phenomenon of proton transfer in those solids, where hydrogen is not a structural unit. LaScO3-based materials are considered to be promising for high-temperature engineering due to their bulk conductivity at low temperatures, chemical resistance and mechanical strength as compared to widely-used proton-conducting cerate- and zirconate-based electrolytes. A series of experiments was performed to compare the properties of La1-xSrxScO3-alpha (x = 0.05; 0.10) solid proton-conducting electrolytes synthesized using different methods. An alternative combustion method that does not apply nitrates as precursor materials is proposed. This method allowed ceramics with a density greater than 98% of theoretical to be obtained. A detailed qualitative and quantitative evaluation was performed at different synthesis stages using the methods of X-ray diffraction, scanning electron microscopy, X-ray fluorescent and atomic emission spectroscopy. The structure parameters of La1-xSrxScO3-alpha were determined by full-profile Rietveld X-ray diffraction analysis. The thermal expansion and electrical conductivity of La1-xSrxScO3-alpha (x = 0.05, 0.10) materials having various densities were studied in oxidizing and reducing atmospheres under changes in the temperature and humidity of the gas phase. The contribution of bulk and grain boundary conductivities was assessed using the impedance method. Both conductivities are established to exhibit the same activation energy for the materials with a density of 94-98% of the theoretical value. The high porosity of the materials (30%) is shown to adversely affect the total conductivity, with the bulk conductivity remaining almost at the same level. A bridge model based on semi-coherent boundaries is proposed for explaining a low grain boundary conductivity in proton electrolytes having a low-symmetry crystal lattice. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.