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Cheng, Shiyang
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article
Ionic/Electronic Conductivity, Thermal/Chemical Expansion and Oxygen Permeation in Pr and Gd Co-Doped Ceria PrxGd0.1Ce0.9-xO1.95-δ
Abstract
The oxygen permeation flux of Ce<sub>0.9</sub>Gd<sub>0.1</sub>O<sub>1.95-δ</sub>(CGO)-based oxygen transport membranes under oxidizing conditions islimited by the electronic conductivity of the material.This work aims to enhance the bulk ambipolarconductivity of CGO by partial substitution of Ce with the redox activeelementPr. A series of compositions of Pr<sub>x</sub>Gd<sub>0.1</sub>Ce<sub>0.9-x</sub>O<sub>1.95-δ</sub>(x = 0, 0.02, 0.05, 0.08, 0.15, 0.25, 0.3 and 0.4) was prepared bysolid state reaction. X-ray powder diffraction (XPD) indicatesthat Pr is completely dissolved in the fluoritestructure up to 40 at.%. Pronounced nonlinear thermal expansion behaviorwasobserved as a function of temperature, due to thesimultaneous contributions of both thermal and chemical expansion. Theelectronicand ionic conductivities were measured as afunction of temperature and oxygen partial pressure. Within the rangefrom 10to 15 at.% Pr, a drastic drop of the activationenergy of the hole mobility and an abrupt increase of the holeconductivityat low temperature was observed. The behavior couldbe rationalized by a simple percolation model. Oxygen permeation fluxesthrough disk shaped samples fed with air on oneside and N<sub>2</sub> on the other side were also measured. The oxygen flux through Pr<sub>0.05</sub>Gd<sub>0.1</sub>Ce<sub>0.85</sub>O<sub>1.95-δ</sub>was higher than that for CGO by one order of magnitude owing to theenhanced electronic conductivity albeit the flux is stilllimited by the electronic conductivity. In terms ofthe electronic and ionic conductivity, the estimated maximum oxygenpermeationflux of a 10 μm Pr<sub>0.4</sub>Gd<sub>0.1</sub>Ce<sub>0.9</sub>O<sub>1.95-δ</sub> -based membrane exceeds 10 Nml cm<sup>−2</sup> min<sup>−1</sup> at 900°C under a small oxygen potential gradient (0.21/10<sup>−3</sup> bar) which is promising for use in oxygen production and in oxy-fuel combustion. Also the material may be well applicableto SOFC/SOEC composite electrodes where mixed conductivity is also desirable.