Oxygen Reduction, Transport and Separation in Low Silver Content Scandia-Stabilized Zirconia Composites

Abstract

Dense composites of silver and Sc-stabilized ZrO 2 (Ag-ScSZ) are manufactured from ScSZ sub-micrometric particles coated with silver using Tollens' reagent. A composite with 8.6 vol % of silver exhibits metallic conductivity of 186 S cm −1 and oxygen flux of 0.014 μmol cm −2 s −1 at 600 • C for a 1-mm thick membrane when used as a pressure-driven separation membrane between air and argon. To gain insight into the role of oxygen transport in Ag and ScSZ, a dense non-percolating sample (Ag 4.7 vol%) is analyzed by impedance spectroscopy and the transport of oxygen through both phases is modelled. Oxygen transport takes place in both silver and ScSZ but it is still dominated by transport in the ionic conductor and therefore a large volume fraction of the ion conductor is beneficial for the separation. The oxygen transport in the silver clusters inside the composite is dominated by diffusion of neutral species and not by the charge transfer reaction at the interface between ScSZ and Ag, yet small silver particles on the surface improve the reduction of oxygen. Oxygen reduction is highly promoted by silver on the surface and there are no limitations of charge transfer at the interface between silver and ScSZ. The capability of metallic silver to reduce oxygen has been long known and recorded since Lucas reported it to Dalton as early as 1818 1 but better understood and quantified over the last 50 years. 2,3This particular capability of silver can be exploited to incorporate oxygen into a solid system, be it a solid oxide fuel cell cathode 4 or an oxygen separation membrane. 5 Oxygen can be separated from air using a double phase membrane with an oxygen-ion conductor, e.g. scandia stabilized zirconia, and an electronic conductor, such as silver, that acts as a very efficient oxygen reducing agent. Scandia stabilized zirconia is known to have higher conductivity than yttria stabilized zirconia since it was first reported in 1900 by Nernst 6 but identified as an oxygen ion conductor by Wagner 7 several decades later. The use of double-phase metal ceramic composites to separate oxygen from air was first described by Mazanec, 8 who mixed Pd or Pt with YSZ and used the composite to drive chemical reactions. Although feasible, this approach normally requires large amounts of expensive metal to achieve percolation, ranging from 30 to 50 vol %, so that composite membranes cannot compete with the high oxygen flux reported in single phase perovskites, such as those reported by Teraoka 9 in 1985. To date, most of the research in high temperature oxygen separation is still performed in mixed ionic electronic conductive perovskites ABO 3 , where A is one or more lanthanides and/or alkaline earths and B one or more transition metal oxides, for example Nonetheless, these materials have two persistent problems: the high operating temperatures (>800 • C) and the unsatisfactory chemical and mechanical stability in operating conditions. Furthermore, the chemical composition of the material affects both the stability and the oxygen sepa...