Evaluating the thermal stability of Sr 2-x VMoO 6-beta (SVMO) and SVMO's promise as a solid oxide cell electrode material

Abstract

A family of double perovskite materials Sr 2-x VMoO 6-delta (SVMO, x = 0-0.2) shows exceptionally large conductivities under reducing atmospheres, making these materials attractive as electrode candidates for high temperature electrochemical devices. To function as electrodes, SVMO must be stable at elevated temperatures and under both oxidizing and reducing conditions. Work described in this dissertation examines SVMO stability as a function of temperature, P O2, and A-site deficiency. My research uses Raman spectroscopy, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) to examine the behavior of SVMO under different atmospheres at temperatures up to 1000 ?C. Raman spectra show that SVMO is stable under forming gas (5% H 2) at temperatures up to 1000 ?C. Similar behavior is observed with humidified argon and CO 2 atmospheres. However, when heated above 600 ?C in air, Raman spectra show that SVMO phase separates. Secondary phases include scheelite (SrMoO 4) and single perovskite (SrVO 3) phases. Phase separation is reversible under reducing atmospheres at high temperature, restoring the original SVMO structure and demonstrating its redox-sensitive and atmosphere-dependent structural stability. While Raman data suggest that SVMO will oxidize above 600 ?C, TGA measurements show that oxygen uptake starts at temperatures as low as 400 ?C. TGA data show that SVMO is stable under N 2 at temperatures as high as 900 ?C, consistent with Raman data. SVMO oxidation kinetics in air are analyzed using a simple kinetic model consisting of two independent first-order processes with the faster process describing surface decomposition and the latter reaction describing conversion of the bulk material. An Arrhenius analysis results in an activation energy (Ea) between 0.1 and 0.8 eV for these processes depending on A-site deficiency. Activation energies show that SVMO stability is maximized with x = 0.05. These findings imply that SVMO can function as an anode in solid oxide fuel cells and the fuel electrode in solid oxide electrolysis cells.