Structural interpretation of optical properties and ion transport mechanism in mixed valent Pr containing nanoceria (original) (raw)
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Chemistry of Materials, 2009
Dense Ce 0.8 Pr 0.2 O 2-δ ceramic membranes with submicron grain size can be formed at 1000°C by minor additions of cobalt oxide. X-ray energy-dispersive spectroscopy shows the additive to be located in the grain boundary. Although a very fine grain interface is obtained for the composition containing 2 mol % cobalt oxide, pronounced grain boundary layers and Co-rich fringes are noted at 5 mol %. The cobalt oxide additions enhance electronic conductivity by around 2-3 times. For 2 mol % additions, no change to the level or nature of ionic conductivity is observed, whereas at 5 mol %, a depleted ionic conductivity is noted at lower temperatures. Coulombic titration studies show the bulk Pr oxidation state to be unaffected by the additions. Jointly, these results prove that the Co additions are not accommodated in the bulk material but instead form additive-rich grain boundary networks that are electronically conductive. Materials essentially free from oxygen surface exchange limitations are produced on addition of 2 mol % cobalt oxide, in contrast to that noted for 5 mol %. The combination of enhanced ambipolar conductivity and enhanced oxygen surface exchange kinetics boosts oxygen permeability in 2 mol % cobalt oxide doped Ce 0.8 Pr 0.2 O 2-δ to offer one of the highest levels of oxygen permeability reported to date for a single component mixed conducting fluorite material.
Analysis of Charge Transport in Ce 0.8 Gd 0.2-x Pr x O 2-δ at T ≤ 600°C
Journal of The Electrochemical Society
Doped ceria pellets with the composition Ce 0.8 Gd 0.2-x Pr x O 2-δ with x = 0.15, 0.1, 0.05, and 0.03 were investigated with a special focus on the partial conductivities in the temperature range of 200-600 °C. Temperature dependent conductivity provided by impedance spectroscopy in air was compared to measurements of the oxygen partial pressure dependent electronic conductivity. The electronic conductivity was analyzed down to 200 °C by using a modified Hebb-Wagner setup with encapsulated Pt microcontacts. A small polaron hopping process introduced by reduction of praseodymium was found to have a pronounced influence on the electronic conductivity at low temperatures. A splitting of the maximum introduced by praseodymium small polaron hopping was observed. Especially for the compositions x ≤ 0.1, a strong deviation of the electronic conductivity curves from the standard acceptor doping case was measured due to superimposed electronic conductivity from the Pr 3+/4+ redox reaction.
Mixed conductivity, thermal expansion, and oxygen permeability of Ce(Pr,Zr)O
Solid State Ionics, 2005
The use of Pr x Ce 1 À x O 2 À d fluorites in electrochemical devices is hindered by several fundamental problems, one of which is the high thermal expansion coefficients (TECs), which have been shown to vary in the range (10 -40) Â 10 À 6 K À 1 between the temperatures 0 and 1000 -C. Thermogravimetric study shows that such a fluctuation of TECs, and non-linear lattice expansion on heating, are related to oxygen losses and can be controlled, to a substantial extent, by compositional selection. The influence of composition upon resultant mixed conductivity and oxygen permeability values is analysed for the compositions Zr 0.1 Ce 0.9 À x Pr x O 2 À d and
Chemistry of Materials, 2012
We combined first-principles calculations with several experimental studies to investigate the complex role for high oxygen storage capacity (OSC) in multivalent Pr-doped ceria. TPR and Raman spectra were measured for confirming oxygen vacancy concentration and oxygen mobility. The coordination number was fitted via EXAFS spectra, and it was the correlated DFT calculation that has been corrected as effective U (5.3 eV) to well express the reducing state (4+ → 3+) for both Ce and Pr elements. In our study, when Pr is incorporated into pure ceria, Pr 3+ and Pr 4+ ions are incorporated as majority and minority ions, respectively. Pr 3+ ions play a key role to create oxygen vacancies and induce a local distortion, which improves oxygen mobility, and Pr 4+ can contribute to diminishing reduction energy and a respectable OSC via the formation of an additional redox couple.
Oxygen mobility and surface reactivity of PrNi1−xCoxO3+δ–Ce0.9Y0.1O2−δ cathode nanocomposites
Solid State Ionics, 2014
Cobalt-doped praseodimium nickelates PrNi 1 − x Co x O 3 − δ and Y/(Pr, Y)-doped ceria oxides were synthesized via Pechini route. Nanocomposites were prepared via ultrasonic dispersion of the mixture of perovskite and fluorite nanopowders in isopropanol with addition of polyvinyl butyral followed by drying, pressing and sintering at 1000-1100°C. The oxygen mobility and reactivity of perovskites, fluorites and powdered composites obtained by crushing and milling of pellets were estimated by oxygen isotope heteroexchange with 18 O 2 and C 18 O 2 using both closed and flow (SSITKA) reactors in the temperature-programmed (TPIE) mode. Coexistence of two routes of oxygen bulk diffusion-the fast one and the slow one in perovskites and composites was revealed. In perovskites, the fast oxygen diffusion is a minor channel related to some defects in their structure. In composites, the fast oxygen diffusion is the main route being related to both phases disordered due to cation redistribution between them, first of all, to fluorite-like domains of Pr,Y-doped ceria and perovskite/fluorite interfaces.
P-type electronic transport in Ce0.8Gd0.2O2-δ: The effect of transition metal oxide sintering aids
Journal of Electroceramics, 2002
Small (2 mol%) additions of cobalt, iron and copper oxides into Ce 0.8 Gd 0.2 O 2−δ considerably improve sinterability of ceria-gadolinia (CGO) solid electrolyte, making it possible to obtain ceramics with 95-99% density and sub-micron grain sizes at 1170-1370 K. The minor dopant additions have no essential effect on the total and ionic conductivity, whilst the p-type conduction in the transition metal-containing materials at 900-1200 K is 8-30 times higher than that in pure CGO. The oxygen ion transference numbers of the Co-, Fe-and Cu-doped ceramics, determined by the modified e.m.f. technique under oxygen/air gradient, are in the range 0.89-0.99. The electron-hole contribution to the total conductivity increases with temperature, as the activation energy for ionic conduction, 78 to 82 kJ/mol, is significantly lower than that for the p-type electronic transport (139-146 kJ/mol). The results show that CGO sintered with such additions can still be used as solid electrolytes for intermediatetemperature electrochemical applications, including solid oxide fuel cells (SOFCs) operating at 770-970 K, but that increasing operation temperature is undesirable due to performance loss.
Ionic Transport Properties in Nanocrystalline Ce0.8A0.2O2-δ (with A = Eu, Gd, Dy, and Ho) Materials
Nanoscale Research Letters, 2010
The ionic transport properties of nanocrystalline 20 mol% Eu, Gd, Dy, and Ho doped cerias, with average grain size of around 14 nm were studied by correlating electrical, dielectric properties, and various dynamic parameters. Gd-doped nanocrystalline ceria shows higher value of conductivity (i.e., 1.8 9 10-4 S cm-1 at 550°C) and a lower value of association energy of oxygen vacancies with trivalent dopants Gd 3? (i.e., 0.1 eV), compared to others. Mainly the lattice parameters and dielectric constants (e ?) are found to control the association energy of oxygen vacancies in these nanomaterials, which in turn resulted in the presence of grain and grain boundary conductivity in Gd-and Eu-doped cerias and only significant grain interior conductivity in Dy-and Ho-doped cerias.
Microstructure and electrical transport in nano-grain sized Ce 0.9Gd 0.1O 2− δ ceramics
Journal of Solid State Chemistry, 2007
An enhancement of the electrical conductivity has been found in nano-grain sized Ce 0.9 Gd 0.1 O 2Àd ceramics when measured in N 2 (p O2 ¼ 3.5 Â 10 À6 atm) in comparison with the most commonly accepted values of bulk ionic conductivity. We first present the synthesis and characterisation of the nanoparticles later used for the preparation of dense nanoceramics of Gd-doped CeO 2 . The nanoparticles were characterised by X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The good sintering properties of these nanopowders allowed us to obtain very dense ceramics (490% theoretical density) while keeping the grain size close to 100 nm. The microstructure of these nanoceramics was analysed by AFM and scanning electron microscopy (SEM) while the electrical characterisation was performed by the 4-point dc technique between 500 and 950 1C in air or N 2 and ac impedance between 150 and 400 1C in air and or argon. We briefly discuss the possibilities of electron vs. oxygen ion conduction and grain boundary vs. bulk conductivity. The features exhibited by these ceramics represent an increased potential to process solid electroceramics materials with specific levels of electronic and/or ionic conductivities for a variety of electrochemical devices. r 2007 Elsevier Inc. All rights reserved.
Electrical transport properties of In-doped Ce1−xInxO2−δ (x = 0.1; 0.2)
Dalton Transactions, 2009
We report the synthesis and electrical conductivity of fluorite-type Ce 1-x In x O 2-d (x = 0.1; 0.2) in air, dry Ar, N 2 and H 2 in the temperature range of 300-800 • C. We have employed a new CO 2 capture technique to prepare the "metastable" fluorite-related structure Ce 1-x In x O 2-d from the corresponding In-doped Ba-containing perovskite of the nominal chemical formula BaCe 1-x In x O 3-d at 800 • C. The amount of CO 2 gained per ceramic gram was found to be consistent with the formation of BaCO 3 , confirming the complete leaching of Ba in BaCe 1-x In x O 3-d. The CO 2 capture efficiency was found to be in the range of 90-99% at 800 • C, which is significantly higher than those of well-known low-temperature CO 2 absorbing materials, including Li 2 O, Li 6 Zr 2 O 7 , Li 1.8 Na 0.2 ZrO 3 and LiNaZrO 3. Powder X-ray diffraction (PXRD) and energy dispersive X-ray analysis (EDAX) confirmed the perovskite into fluorite structural transformation reaction. The AC impedance study showed a clear intercept to the real axis at the low-frequency over the investigated temperatures in all the atmospheres, indicating a non-blocking nature of electrode (Pt) and electrolyte interface. The constant phase element (CPE) value was found to be in the order of 10-10 F in air, N 2 and Ar for high-frequency part of the semicircle due to bulk contribution, and about two orders of magnitude lower values were observed for the low-frequency semicircle which may correspond to grain-boundary effect. The 20 mol% In-doped CeO 2 exhibits a total electrical conductivity of about 4 ¥ 10-6 S/cm at 600 • C in Ar, while in H 2 an about four orders of magnitude higher electrical conductivity of 3 ¥ 10-2 S/cm was observed. The activation energy for electrical conductivity was found to be 1.36 eV in Ar, 1.43 eV in N 2 , 1.34 eV in H 2 , and 1.1 eV in air for Ce 0.8 In 0.
Microstructure and electrical transport in nano-grain sized Ce{sub 0.9}Gd{sub 0.1}O{sub 2-} ceramics
Journal of Solid State Chemistry, 2007
An enhancement of the electrical conductivity has been found in nano-grain sized Ce 0.9 Gd 0.1 O 2Àd ceramics when measured in N 2 (p O2 ¼ 3.5 Â 10 À6 atm) in comparison with the most commonly accepted values of bulk ionic conductivity. We first present the synthesis and characterisation of the nanoparticles later used for the preparation of dense nanoceramics of Gd-doped CeO 2 . The nanoparticles were characterised by X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The good sintering properties of these nanopowders allowed us to obtain very dense ceramics (490% theoretical density) while keeping the grain size close to 100 nm. The microstructure of these nanoceramics was analysed by AFM and scanning electron microscopy (SEM) while the electrical characterisation was performed by the 4-point dc technique between 500 and 950 1C in air or N 2 and ac impedance between 150 and 400 1C in air and or argon. We briefly discuss the possibilities of electron vs. oxygen ion conduction and grain boundary vs. bulk conductivity. The features exhibited by these ceramics represent an increased potential to process solid electroceramics materials with specific levels of electronic and/or ionic conductivities for a variety of electrochemical devices. r 2007 Elsevier Inc. All rights reserved.