Conductivity Of Ceria Research Articles

Influence of Zinc Oxide on the Conductivity of Ceria

ZnO was studied as dopant or additive for CeO2 in the concentration range x = 0.00 to x = 0.95 referred to (CeO2)1-x(ZnO)x. From XRD, the solubility limit of ZnO in the bulk of CeO2was estimated as 3 ± 0.7 mol% at room temperature. The total conductivity was measured for all compositions in air by impedance spectroscopy. The contributions from bulk and grain boundaries were evaluated separately. Additionally, the electronic conductivities were measured using a Hebb-Wagner technique with microelectrodes. Up to the solubility limit, the total conductivity of the bulk in air increases almost linearly with the ZnO content from 300°C to 800°C, the same holds for the p-type electronic conductivity (in the range 600°C–800°C). Hence, zinc ions act as acceptors on cerium sites, zinc doped ceria is a moderate mixed conductor. In the two-phase domain from 3 mol% up to 20 mol%, the total conductivity increases slightly and the partial electronic conductivity dependence remains nearly constant. Starting at 30 mol% ZnO, the grain boundary conduction rises steeply and the electronic conductivity predominates the total conductivity for this and higher ZnO contents reaching 0.03 S/cm at 800°C in air for a sample with the composition x = 0.95 mol%.

Conductivity of Ceria Based Composite Electrolytes for Intermediate-Temperature-Solid Oxide Fuel Cells

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Effect of Ca2+ or Mg2+ additions on the electrical properties of yttria doped ceria electrolyte system

The effect of Ca2+ or Mg2+ co-doping on the electrical conductivity of ceria–yttria electrolytes of general formulae Ce0.9Y0.1−xMxO2−δ (x=0, 0.05) and Ce0.85Y0.15−xMxO2−δ (x=0, 0.05, 0.1) (M=Ca or Mg) were investigated. Powder samples were synthesized by mechanical milling. The phase identification, microstructures, and ionic conductivities of the samples were studied by X-ray diffraction, scanning electron microscopy and AC impedance spectroscopy. The results showed that in comparison to singly doped ceria, co-doping with appropriate ratio of Y3+ and Ca2+ or Mg2+ showed higher conductivities and lower activation energies. For CaO co-doped system, conductivity of the samples improves as the concentration of the dopant increases and maximum conductivity is obtained for the composition Ce0.85Y0.05Ca0.1O1.875. But for MgO co-doped system, Mg=5mole% show better electrical properties further addition leads to a decreases in the conductivity. The electrical properties of grain boundary have a large influence due to co-doping than that of the bulk.

Effect of co-doping of Mg and La on conductivity of ceria

The aim of present investigation is to study the effect of co-doping of lanthanum and magnesium on the ionic conductivity of CeO2 for its use as a solid electrolyte in the intermediate temperature solid oxide fuel cells (IT SOFC). Attempts have been made to synthesize compositions with x=0.075, 0.07, 0.06, 0.05 and 0.00 and y=0.00, 0.01, 0.03, 0.05 and 0.15 in the system Ce1−x−yMgxLayO2−x−y/2 (CML) by auto-combustion method. X-ray diffraction patterns show that all the samples have fluorite structure similar to undoped ceria. Microstructures of thermally etched samples have been studied by scanning electron microscopy (SEM). In order to evaluate the contributions of grains σg, grainboundaries σgb and electrode–specimen interface to the total conductivity σt, use is made of AC impedance analysis. Impedance measurements are made in the frequency range 1Hz to 1MHz and temperature range 250–500°C. The results show that the conductivity of the system increases with increasing lanthanum content. There is no enhancement in conductivity due to co-doping with La and Mg.

Preparation and characterization of co-doped (Ce0.80La0.15Al0.05O1.90) and multiple-doped (Ce0.80Sm0.10Gd0.05Al0.05O1.90 and Ce0.80Gd0.10Sm0.05Al0.05O1.90) ceria

Attempts have been made to synthesize a few compositions Ce0.80La0.15Al0.05O1.90, Ce0.80Sm0.10Gd0.05Al0.05O1.90, and Ce0.80Gd0.10Sm0.05Al0.05O1.90 by citrate–nitrate auto-combustion method. The aim of the present investigation was to study the effect of co-doping and multiple doping on the ionic conductivity of CeO2 for its use as solid electrolyte in intermediate temperature solid oxide fuel cells. XRD patterns showed that all the samples have fluorite-type crystal structure similar to undoped ceria. Microstructures of thermally etched samples have been studied by scanning electron microscopy. Contributions of grains σg and grain boundaries σgb to the total conductivity σT, have been determined using impedance analysis. Impedance measurements were made in the frequency range 1 Hz–1 MHz and temperature range 250–500 °C. Our experimental results show that multiple doping is more effective than co-doping for improving the oxide ion conductivity of ceria in the materials investigated in the present study.

Electrical conductivity of undoped, singly doped, and co-doped ceria

In order to investigate the effect of single doping and co-doping on the enhancement of ionic conductivity in ceria (CeO2), CeO2 and a few compositions in the system Ce0.85Sm0.15 − xGdxO1.925 (x = 0.00, 0.06, 0.09, and 0.15) were prepared using citrate–nitrate auto-combustion method. Gels were characterized by simultaneous differential thermal analysis and thermogravimetric analysis to confirm the formation of end product from the precursor. All the compositions were found to be single-phase solid solution from their X-ray diffraction pattern. Complex plane impedance analysis clearly revealed the contribution of grains, grain boundaries, and electrode specimen interface to the total value of the resistance. The value of activation energy, Ea, of conduction shows that the conduction process is mainly due to diffusion of O2− ions through oxygen vacancies. Effect of doping has been analyzed using the concept of radius mismatch and effective index reported earlier in the literature.

Analysis of the regenerative H 2S poisoning mechanism in Ce 0.8Sm 0.2O 2-coated Ni/YSZ anodes for intermediate temperature solid oxide fuel cells

Ceria is used as a sulfur sorbent due to its high affinity for sulfide at high temperatures. In addition, the ionic conductivity of ceria can be dramatically increased by doping with rare metals, including lanthanum, samarium, and gadolinium. Therefore, to enhance sulfur tolerance and improve anode performance, we modified an Ni-based anode with a thin layer coating of Sm 0.2Ce 0.8O 2− δ (SDC) on the pore wall surface of an Ni/YSZ anode. The anode-supported cells were tested with varying H 2S concentrations (0–100 ppm) at 600 and 700 °C. The cell performance was improved in the ceria- (by 20%) and in the SDC- (by 50%) modified anode by extending the additional TPB area in the anode. Under varying H 2S exposure, the polarization resistance was reduced by ceria and the SDC coating on the anode pore wall surface, which led to improved cell performance. A porous SDC layer on the Ni/YSZ anode pore wall acted as a sulfur sorbent as well as an additional TPB area. Otherwise, ceria mainly acted as a sulfur sorbent at high concentrations of H 2S (>60 ppm).

In Situ Characterization of Ceria Oxidation States in High-Temperature Electrochemical Cells with Ambient Pressure XPS

Ambient pressure X-ray photoelectron spectroscopy (XPS) is used to measure near-surface oxidation states and local electric potentials of thin-film ceria electrodes operating in solid oxide electrochemical cells for H2O electrolysis and H2 oxidation. Ceria electrodes which are 300 nm thick are deposited on YSZ electrolyte supports with porous Pt counter electrodes for single-chamber tests in H2/H2O mixtures. Between 635 and 740 °C, equilibrium (zero-bias) near-surface oxidation states between 70 and 85% Ce3+ confirm increased surface reducibility relative to bulk ceria. Positive cell biases drive H2O electrolysis on ceria and further increase the percentage of Ce3+ on the surface over 100 μm from an Au current collector, signifying broad regions of electrochemical activity due to mixed ionic-electronic conductivity of ceria. Negative biases to drive H2 oxidation decrease the percentage of Ce3+ from equilibrium values but with higher electrode impedances relative to H2O electrolysis. Additional tests indicate that increasing H2-to-H2O ratios enhances ceria activity for electrolysis.

Electronic conductivity of Ce0.8Gd0.2−xPrxO2−δ and influence of added CoO

AbstractDoped ceria and ceria based solid oxide solutions show a unique combination of oxygen ion mobility, electronic conductivity, and high catalytic activity for redox reactions. In this work, the minority conductivity of electrons has been measured directly as a function of the composition of ceria–praseodymia based solid solutions in order to maximize the electronic conductivity without depressing the oxygen ion mobility. The influence of Co as well as the Gd/Pr dopant ratio on the electronic conductivity of ceria–praseodymia pellets was studied for the compositions Ce0.8Gd0.2−xPrxO2−δ (0.05 ≤ x ≤ 0.15) with and without an additional Co content of 0.02 with respect to the formula. The Hebb–Wagner polarization technique was used with ion‐blocking microcontacts. In the temperature range 700–800 °C, the presence of high amounts of praseodymium increases the p‐type conductivity by a factor of more than 10 for oxygen partial pressures higher than 10−10 bar. Co‐doped ceria–gadolinia–praseodymia solid solutions showed a further increase of the electronic conductivities in a partial pressure range where the Co‐free materials showed the minimum of the electronic conductivities. It is assumed that the effect of the additional cobalt doping is due to electronic short circuits along the grain boundaries via segregated CoO.

Experimental Characterization of Thin-film Ceria Solid Oxide Fuel Cell Anodes

This study presents an experimental effort to elucidate the role of CeO2-x in composite SOFC anodes, by characterizing electrochemical oxidation of small fuel molecules (H2, CO) on thin-film ceria anodes. Thin ceria films were sputter-deposited onto thick YSZ-electrolyte-supported button cells with high-surface-area, porous LSM/YSZ cathodes. The mixed ionic-electronic conductivity of ceria provides multiple reaction pathways for electrochemical oxidation and electrochemical characterization of the ceria films over a range of gas compositions at temperatures from 650 to 850 {degree sign}C provide a basis for elucidating the importance of competing mechanisms for fuel oxidation on the ceria. Analysis of the experiment results provides a basis for formulating a micro-kinetic mechanism for the oxidation of small fuel species on ceria anodes which can be the implemented in SOFC modeling efforts for optimizing ceria-based composite anodes.

Dopant-concentration dependence of grain-boundary conductivity in ceria: A space-charge analysis

Here we elucidate the origin of the characteristic development of grain-boundary conductivity and of its activation energy measured in Gd-doped ceria (GDC) ceramics upon change in the dopant concentration in the context of the space-charge effect. The grain-boundary conductivity in GDC was measured as a function of the dopant concentration (1–20 cat.%). The total conductivity of the sample with 1 cat.% dopant is lower than the bulk conductivity by several orders of magnitude largely due to exceedingly high grain-boundary resistance in the sample, consistent with the results previously reported in literature. The specific grain-boundary conductivity of GDC however rapidly increases with increasing dopant content to approach a plateau at concentrations above 15 cat.%, leading to the total conductivity being close to its bulk conductivity at the relatively high dopant level. On the other hand, the grain-boundary width estimated experimentally in GDC is found to decrease substantially as the dopant concentration increases, implying that the observed increase in the grain-boundary conductivity is correlated with such reduction in the grain-boundary width. In addition, the values of the grain-boundary width determined at each dopant concentration are in quantitative agreement with those calculated based on a space-charge model, in which the grain-boundary resistance is attributed to the Schottky-type potential barrier present at the grain boundaries. Furthermore, the values of activation energy of the grain-boundary conductivity measured at different dopant concentrations can also be reproduced quantitatively using the space-charge model. It is therefore verifed unambiguously that the characteristic increase in the grain-boundary conductivity measured in GDC upon increasing dopant concentration results from concurrent reduction in the height of the potential barrier formed inherently at the grain boundaries.

Electrical Conductivity of Ceria Co-Doped with Zr and Re (Re=La,Y)

The series of powders with the general formula of Ce0.8-xZrxLa0.2O1.9 (x=0, 0.05, 0.10, 0.15) and Ce0.8-xZrxY0.2O1.9 (x=0, 0.05, 0.10, 0.15) were synthesized by the sol-gel method. The samples sintered at 1500°C all possess the single phase with cubic fluorite structure. The lattice parameter and the ionic conductivity decrease with increasing the content of Zr. However, the ionic conductivity of ceria co-doped with Zr and La reduces with decreasing the lattice strain.

Ionic conductivity of Sm, Gd, Dy and Er-doped ceria

Ceria-based materials are prospective electrolytes for intermediate-temperature solid oxide fuel cells. The ionic conductivities of ceria-doped with Sm, Gd, Dy and Er are investigated as a function of temperature by using a.c. impedance. The results show that conductivity depends on the rare earth dopant, its amount, an appearance of second phase, and the microstructure. With 10 mol% dopant, Sm exhibits higher conductivity than Gd, Dy and Er, respectively. With an increase in Dy content, the total conductivity increases, which is attributed to an increase in grain boundary conductivity. By contrast, an increasing amount of Er from 10 to 20 mol% reduces the conductivity of ceria and results in a separated phase of Er 2O 3 as detected by X-ray diffraction and scanning electron microscopy. In addition, the grain size corresponding to grain boundary density affects the conductivity due to the contributions from the grain interior and grain boundary conductivities.

RETRACTED ARTICLE: Determination of dopant of ceria system by density functional theory

Oxides with the cubic fluorite structure, e.g., ceria (CeO2), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy–dopant interactions, represented by association (binding) energies of vacancy–dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately.

Development of Higher Ionic Conductivity Ceria Electrolyte

A co-dopant strategy was used to further investigate the doping effects on the ionic conductivity of ceria based electrolyte. Experiments were conducted using Lu3+ and Nd3+ as co-dopants, added in a proportion such that the effective dopant ionic radius matches critical dopant ionic radius for ceria electrolyte. Ionic conductivity measurements were performed on LuxNdyCe1-x-yO2-d (where x/y = 1.16) electrolytes with different total dopant concentration (x+ y) using two-point probes electrochemical impedance spectroscopy technique from 150oC to 700oC in air. In addition, to understand the effect of polarizibility alone, Lu3+ doped CeO2 was also investigated as the ionic radius of Lu3+ is approximately the same as Ce4+. LuxNdyCe1-x-yO2-d electrolytes exhibit higher ionic conductivity than LuxCe1-xO2-d, which indicates that co-doping strategy works. Around 700oC, its grain ionic conductivity surpasses that of the GdxCe1-xO2-d, which is the highest ionic conductive ceria based electrolyte.

Optimization of ionic conductivity in doped ceria

Oxides with the cubic fluorite structure, e.g., ceria (CeO2), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present a quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy-dopant interactions, represented by association (binding) energies of vacancy-dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately.

Oxygen transport studies in nanocrystalline ceria films

Oxygen uptake and conductivity were measured by nuclear-reaction analysis and alternating current impedance technique at the intermediate temperature range on sol-gel grown nanocrystalline ceria films with average grain-sizes 7 nm and 38 nm synthesized at 723 and 1173 K, respectively. Higher oxygen uptake and lower ionic conductivity were observed in ceria films with ∼7-nm grain size. High permeation-assisted oxygen diffusion in nanocrystallites combined with oxygen trapping in the disordered region contributed to higher oxygen uptake. However, the lower ionic conductivity in the film resulted from the absence of long-range lattice ordering and inactive grain-boundary/surface oxygen vacancy sites due to oxygenation. The relationship between oxygen uptake and conductivity in ceria is discussed in details by considering grain-size dependent defect density, related surface area, and enhanced oxygen mobility.

Ionic Conductivity and Crystallographic Index of Ceria Doped with Yttria and Dysprosia

Ce 1-x (Y 0.5 Dy 0.5 ) x O 2-8 solid solutions with 0.030 ≤ x ≤ 0.150 were prepared by the precipitation of the corresponding cation hydroxides. The main purposes of this work were to verify the effect of combined addition of yttrium and dysprosium oxides on the ionic conductivity of ceria, and to evaluate the application of the crystallographic index model in the studied dopant content range. X-ray diffraction and scanning electron microscopy results evidence the homogeneous nature of the synthesized materials. The grain conductivity of the sintered pellets at a fixed temperature increases with increasing dopant content up to x=0.045, and then decreases for higher concentrations. These results show that the addition of a codopant influences the composition at which the maximum value of conductivity is attained. Moreover, the crystallographic index model seems to be unsuitable to describe the behavior of the ionic conductivity of either grain or grain boundaries when the total dopant content is relatively low.

Electric conductivity and oxygen permeability of modified cerium oxides

Electrical conductivities of samarium (Sm), terbium (Tb), praseodymium (Pr) and zirconium (Zr) doped ceria membranes were measured in T = 600–900°C and pO2 = 10−22–0.21 atm. Doping Sm and Pr in CeO2 respectively enhances the ionic conductivity and electron-hole conductivity of ceria. Sm-Pr doped ceria exhibits both n-type (at lower pO2) and p-type (at high pO2) electronic and ionic conductivity in the temperature range studied. Adding Tb in the Sm doped ceria causes a reduction in ionic conductivity. Zr-Pr doped ceria is an n-type electronic conductor at low \(P_{{\text{O}}_{\text{2}} }\) and p-type electronic conductor at high pO2. The ionic conductivity of Zr-Pr doped ceria is lower than Sm doped ceria but higher than pure ceria. Oxygen permeation flux through the Zr-Pr doped ceria membrane, dominated by the slow oxygen ionic conduction, is similar to the ytttria doped bismuth oxide membrane, and smaller, but close to that of perovskite-type lanthanum cobaltite membrane.

Stable High Conductivity Bilayered Electrolytes for Low Temperature Solid Oxide Fuel Cells

A ceria/bismuth oxide bilayer electrolyte with high ionic conductivity is being developed to reduce solid oxide fuel cell (SOFC) operating temperatures. The bilayer structure overcomes the limited thermodynamic stability of bismuth oxides and the electronic conductivity of ceria based oxides in reducing atmosphere. Measurements of the conductivity, open-circuit potential, and transference number of (Sm2O3)0.1(CeO2)0.9 (SDC) and SDC coated with (Er2O3)0.2(Bi2O3)0.8 (ESB) are presented. Bilayer electrolytes were formed by depositing a thin layer of ESB of varying thickness via pulsed laser deposition (PLD) and dip-coating on a SDC substrate. The conductivity of bilayer ESB/SDC electrolytes was measured in air and compared with that of SDC. Bilayer samples exhibited slightly higher total conductance than SDC. The OCP of ESB/SDC electrolytes was tested in a fuel cell arrangement as a function of temperature with H2/H2O on the anode side and air on the cathode side. These OCP measurements were compared with the open circuit potential (OCP) of uncoated SDC samples under the same conditions. The results showed a significant increase in OCP and ti with the bilayer structure, as compared to cells with a single SDC electrolyte layer. Further, the OCP and ti increased with increasing relative thickness of the ESB layers.