Ionic/Electronic Conductivity, Thermal/Chemical Expansion and Oxygen Permeation in Pr and Gd Co-Doped Ceria Pr_xGd_0.1Ce_0.9-xO_1.95-δ

Abstract

The oxygen permeation flux of Ce 0.9 Gd 0.1 O 1.95-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 Pr x Gd 0.1 Ce 0.9-x O 1.95-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 N 2 on the other side were also measured. The oxygen flux through Pr 0.05 Gd 0.1 Ce 0.85 O 1.95-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 Pr 0.4 Gd 0.1 Ce 0.9 O 1.95-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. Dense ceramic oxygen transport membranes (OTMs) could potentially be applied for production of high purity oxygen for medical purposes, supply of oxygen in the steel industry, oxy-fuel combustion schemes, as well as in the cement and glass industries. Also, importantly, OTMs can beneficially be integrated with a biomass gasifier, allowing production of syngas (CO and H 2 ), which is a precursor for a variety of high value chemicals. 1 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 Gd 0.1 Ce 0.9 O 1.95-δ at 900• C 6 ), 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 gradie...

Interdependence of Point Defects and Reaction Kinetics: CO and CH4 Oxidation on Ceria and Zirconia

“…•• ], a smaller exponent m ≈ 1/8 to 1/12 is expected at higher [Pr] and oxidizing conditions. 75 Overall, this yields a p(CO) dependence with an exponent of m′ = −1.4m ≈ −0.35 and a p(O 2 ) dependence of n = 2.1m ≈ 0.26. Insertion into eq 13 results in…”

“…In acceptor-doped oxides with constant [V O •• ] as for GDC, the concentration of [ e ′] depends on the oxygen activity in the oxide particles, p (O 2 ) eff , not the external p (O 2 ), according to [ e ′] ∝ p (O 2 ) eff 1/4 . For PDC with slightly varying [V O •• ], a smaller exponent m ≈ 1/8 to 1/12 is expected at higher [Pr] and oxidizing conditions . Overall, this yields a p (CO) dependence with an exponent of m ′ = −1.4 m ≈ −0.35 and a p (O 2 ) dependence of n = 2.1 m ≈ 0.26.…”

Surface, Subsurface, and Bulk Oxygen Vacancies Quantified by Decoupling and Deconvolution of the Defect Structure of Redox-Active Nanoceria

“…The presence of cerium vacancies is likely to enhance the redox/photocatalytic effects owing to (1) a second such mechanism (in addition to oxygen vacancies) for these processes, (2) the presence of the associated unpaired electrons from Ce 3+ , and (3) the same effect from increased surface roughness and hence surface area. , It is clear that the simultaneous presence of vacancies and electrons suggests a mixed ionic-electronic conductor (MIEC). While this has been established for doped ceria, , it does not appear to have been suggested for the semiconductivity involving the intrinsic defects in ceria. It also is clear that, if the charge deriving from the oxygen vacancies (V Ö ) does not balance with the charge deriving from the cerium vacancies (V Ö , V Ö ), then electronic charge compensation ( e – , h *) must occur.…”

Fast oxide-ion conductors in Bi2O3-V2O5 system: Bi108-xVxO162+x(x=4-9) with 3×3×3 superstructure

“…In general, the oxide ion conducting materials should possess highly symmetric structure with significant number of oxygen vacancies [5] as it is the case with commercially used yttria stabilized zirconia (YSZ), i. e. the ZrO 2 crystallizes in a cubic structure having oxygen vacancies provided by Y 2 O 3 [6] and cubic gadolinium-doped ceria (GDC) [7] in which the substitution of 10-20 % of Ce 4+ by Gd 3+ generates enough oxygen vacancies in the lattice for optimal ionic conductivity [8]. The greatest disadvantage of YSZ is its very high operating temperature (about 1000 °C) [9,10] while GDC is sensitive to reducing conditions and possesses undesirable electronic conductivity [11]. As a solution, Wachsman and Lee [12] have proposed a bilayer electrolyte, which consists of GDC layer on the anode side and erbia stabilized δ-Bi 2 O 3 on the cathode side.…”