Investigation of multivalent double perovskites as electrodes for high temperature energy conversion

Abstract

Solid Oxide Fuel Cells (SOFCs) are direct energy conversion devices that have demonstrated viability due to the associated high efficiencies, utilization of transition metal catalyst, and their unique fuel flexibility, which allows the use of dirty hydrocarbons. These high temperature systems typically utilize fuel electrodes composed of a ceramic/metal (cermet) composite that is comprised of nickel and yittria-stabilized zirconia (YSZ). While these systems have demonstrated performance potential due to the catalysis and electrical conductivity of nickel metal, a key shortcoming is the poor thermal stability of nickel metal at operating temperatures of 750-1000°C, for which increased temperature enhances performance. Nickel metal particle networks as well as other transition metal catalysts operating at high temperatures coarsen or agglomerate resulting in the loss of continuous electronic pathways. To address these challenges, new materials have been sought after to replace the mixed metal and ceramic two-phase Ni/YSZ fuel electrode. One proposed solution is to utilize a single phase Mixed Ionic Electronic Conductor (MIEC) to replace the traditional cermet structure. In this study, the analysis and characterization of the processing and sintering of Sr 2-xVMoO 6-delta perovskites, where x=0.0, 0.1 and 0.2, was investigated. Sr 2-xVMoO 6-delta substrates were sintered in a reducing atmosphere (5%H 2 95%N 2) and the x-ray diffraction patterns indicate that the double perovskite is the primary phase for Sr 2-xVMoO 6-delta pellets sintered at 1200°C and 1300°C for 20 hours. However, these pellets show a secondary phase of SrMoO 4-delta. X-ray photoelectron spectroscopy revealed a deficiency of vanadium on the pellet surfaces in which samples yielded surface vanadium concentrations of less than 5%. The vanadium inhomogeneity can be explained by the formation of the SrMoO 4-delta scheelite phase due to oxygen exposure on the surface of the pellets, which indicates inward vanadium migration to the bulk. Sr 2-xVMoO 6-delta pellets sintered at 1300°C showed very high conductivity, with Sr 1.9VMoO 6-delta exhibiting conductivity over 100,000S/cm at room temperature. The conductivity tests also indicate a semiconductor to metallic transition for all double perovskites related to the reduction of Mo6+ to Mo4+. Utilizing the double perovskites as fuel electrodes proved to be difficult, due to anion transport leading to secondary phases and thus delamination.