Effects of niobium doping site and concentration on the phase structure and oxygen permeability of Nb-substituted SrCoO_x oxides

Niobium doping effect on phase structure, phase stability and electrical conductivity of SrCoO_x oxides and oxygen permeability of the corresponding membranes were systematically investigated. Niobium was successfully incorporated into A-site, B-site or simultaneously A and B double sites of SrCoO_x oxide and stabilized the perovskite structure with a cubic symmetry in air down to room temperature at a proper doping amount. However, the A-site doping could not stabilize the cubic structure under a more reducing atmosphere of nitrogen, as to SrCo_1-yNb_yO_3-δ, y ≥ 0.1 is necessary to sustain the cubic perovskite structure. Simultaneous doping of Nb at A and B sites is the most effective way to stabilize the perovskite structure under nitrogen atmosphere. Irrespective of doping site, the electrical conductivity decreased monotonously with Nb-doping amount. Both Nb_xSr_1-xCoO_3-δ and Nb_zSr_1-zCo_1-zNb_zO_3-δ envisaged a decrease in oxygen permeation flux with Nb-doping amount while SrCo_1-yNb_yO_3-δ reached the maximum flux at y = 0.1. Among all the membranes, SrCo_0.9Nb_0.1O_3-δ and Nb_0.05Sr_0.95Co_0.95Nb_0.05O_3-δ show the highest oxygen fluxes of 3.5 and 2.7 ml cm^-2 min^-1 at 900 °C under an air/helium gradient, respectively.