Abstract
The oxygen permeation flux of Ce0.9Gd0.1O1.95-δ (CGO)-based oxygen transport membranes under oxidizing conditions is limited by the electronic conductivity of the material. This work aims to enhance the bulk ambipolar conductivity of CGO by partial substitution of Ce with the redox active element Pr. A series of compositions of PrxGd0.1Ce0.9-xO1.95-δ (x = 0, 0.02, 0.05, 0.08, 0.15, 0.25, 0.3 and 0.4) was prepared by solid state reaction. X-ray powder diffraction (XPD) indicates that Pr is completely dissolved in the fluorite structure up to 40 at.%. Pronounced nonlinear thermal expansion behavior was observed as a function of temperature, due to the simultaneous contributions of both thermal and chemical expansion. The electronic and ionic conductivities were measured as a function of temperature and oxygen partial pressure. Within the range from 10 to 15 at.% Pr, a drastic drop of the activation energy of the hole mobility and an abrupt increase of the hole conductivity at low temperature was observed. The behavior could be rationalized by a simple percolation model. Oxygen permeation fluxes through disk shaped samples fed with air on one side and N2 on the other side were also measured. The oxygen flux through Pr0.05Gd0.1Ce0.85O1.95-δ was higher than that for CGO by one order of magnitude owing to the enhanced electronic conductivity albeit the flux is still limited by the electronic conductivity. In terms of the electronic and ionic conductivity, the estimated maximum oxygen permeation flux of a 10 μm Pr0.4Gd0.1Ce0.9O1.95-δ -based membrane exceeds 10 Nml cm−2 min−1 at 900°C under a small oxygen potential gradient (0.21/10−3 bar) which is promising for use in oxygen production and in oxy-fuel combustion. Also the material may be well applicable to SOFC/SOEC composite electrodes where mixed conductivity is also desirable.
Highlights
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Besides being applicable for oxygen transport membranes (OTMs),[2] acceptor doped-ceria has been intensively studied for use in a number of other applications e.g. solid oxide fuel cells (SOFCs),[3] solid oxide electrolysis cells (SOECs)[4] and for electrocatalysis.[5]
No indication of secondary phases originating from side reactions or precipitation of insolvable dopants can be detected within the resolution limit of the X-ray powder diffraction (XRD), indicating that the solubility limit of Pr in CGO is more than 40 at.%, which is in accordance with the solubility limit of 70 at.% reported by Taksu et al.[28] for partial Pr-substitution for cerium in PrxCe1-xO2-0.5x at 1400◦C
Summary
General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Besides being applicable for OTMs,[2] acceptor doped-ceria has been intensively studied for use in a number of other applications e.g. solid oxide fuel cells (SOFCs),[3] solid oxide electrolysis cells (SOECs)[4] and for electrocatalysis.[5] In particular, acceptor doped ceria (e.g. CGO) is interesting owing to high oxide ion conductivity (0.12 Scm−1 for Gd0.1Ce0.9O1.95-δ at 900◦C6), appreciable electrocatalytic activity, high electronic conductivity under reducing conditions, and excellent chemical stability under harsh reducing and even corrosive gaseous conditions.[3,7] Kaiser et al.[8] reported that the oxygen permeation flux of a 27 μm asymmetric 10 at.% Gd-doped ceria-based membrane exceeds 10 ml cm−2 min−1 under a gradient of air/H2 at 850◦C,9 which is promising for applications provided that sufficient lifetime can be achieved. To provide technologically relevant oxygen fluxes for high pO2 applications, such as production of pure oxygen and in oxy-coal combustion conditions, the electronic conductivity of CGO needs to be enhanced preferably to a value close to that of the ionic conductivity so that ambipolar conductivity can be maximized
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