Ionic/Electronic Conductivity, Thermal/Chemical Expansion and Oxygen Permeation in Pr and Gd Co-Doped Ceria PrxGd0.1Ce0.9-xO1.95-δ

8 The oxygen permeation flux of Ce0.9Gd0.1O1.95-δ (CGO)-based oxygen transport membranes 9 under oxidizing conditions is limited by the electronic conductivity of the material. This work 10 aims to enhance the bulk ambipolar conductivity of CGO by partial substitution of Ce with the 11 redox active element Pr. A series of compositions of PrxGd0.1Ce0.9-xO1.95-δ (x=0, 0.02, 0.05, 0.08, 12 0.15, 0.25, 0.3 and 0.4) was prepared by solid state reaction. X-ray powder diffraction (XPD) 13 indicates that Pr is completely dissolved in the fluorite structure up to 40 at.%. Pronounced 14 nonlinear thermal expansion behaviour was observed as a function of temperature, due to the 15 simultaneous contributions of both thermal and chemical expansion. The electronic and ionic 16 conductivities were measured as a function of temperature and oxygen partial pressure. Within 17 the range from 10 to 15 at.% Pr, a drastic drop of the activation energy of the hole mobility and 18 an abrupt increase of the hole conductivity at low temperature was observed. The behaviour 19 could be rationalized by a simple percolation model. Oxygen permeation fluxes through disk 20 shaped samples fed with air on one side and N2 on the other side were also measured. The 21 oxygen flux through Pr0.05Gd0.1Ce0.85O1.95-δ was higher than that for CGO by one order of 22 magnitude owing to the enhanced electronic conductivity albeit the flux is still limited by the 23 electronic conductivity. In terms of the electronic and ionic conductivity, the estimated 24 maximum oxygen permeation flux of a 10 μm Pr0.4Gd0.1Ce0.9O1.95-δ -based membrane exceeds 25 10 Nml cm-2 min-1 at 900 oC under a small oxygen potential gradient (0.21/10-3 bar) which is 26 promising for use in oxygen production and in oxy-fuel combustion. Also the material may be 27 well applicable to SOFC/SOEC composite electrodes where mixed conductivity is also 28 desirable. 29