Conductivity Of Doped Ceria Research Articles

Processing, Phase Stability, and Conductivity of Multication-Doped Ceria

Multicomponent doping of ceria with four cations is used as a preliminary investigation into the ionic conductivity of high-entropy-doped ceria systems. Different compositions of Ce1-x(Ndx/4Prx/4Smx/4Gdx/4)O2-δ (x = 0.05, 0.10, 0.15, and 0.20) are synthesized using the oxalate co-precipitation method yielding single-phase oxalate precursors. X-ray diffraction, Raman spectroscopy, and Fourier-transform infrared spectroscopy are used to characterize the precipitated oxalates. Simultaneous thermal gravimetric analysis and differential scanning calorimetry reveal a two-step decomposition of the oxalates into the doped oxide. The ionic conductivity of the samples is measured from 250 °C to 600 °C using electrochemical impedance spectroscopy. All samples exhibit similar grain conductivity values at 600 °C, comparable to singly doped samples. However, an increase in total conductivity is observed with an increase in doping concentration up to 15% followed by a decrease beyond this concentration. These findings suggest that multicomponent doping may not significantly enhance the grain conductivity of doped ceria beyond conventional single and co-doped compositions but can modulate the grain boundary conductivity and thus the total conductivity of ceria ceramics.

Thermodiffusion and ion transport in doped ceria by molecular dynamics simulations

We simulated thermodiffusion and ionic conductivity in doped ceria by molecular dynamics. Four approaches to estimate the ionic conductivity were applied using the auto-correlation function and collective displacement of oxygen ions, the tracer diffusion coefficient from the mean squared displacement, and the displacement in an electric field. The Green-Kubo method was applied to determine the mass-heat coupling cross-coefficients LOq and LqO of the Onsager transport matrix. This is the first calculation of these coefficients in an ionic compound. The simulated parameters are negative throughout the applied temperature range suggesting net transport of anions from a cold to a hot region in a temperature gradient. The results emphasize that the coupling between mass and heat transport in an oxygen ion electrolyte could have an impact for practical applications.

The Effect of Jump Attempt Frequencies on the Ionic Conductivity of Doped Ceria

The macroscopic oxygen ion conductivity in doped ceria is determined by the microscopic activation energy barriers and jump attempt frequencies of oxygen ion jumps. While the influence of the local jump environment on the migration energy is widely investigated, its influence on the attempt frequency is rarely investigated. In this work, attempt frequencies in Sm, Yb, and Gd doped ceria are calculated using density functional theory. Moreover, ionic conductivities for varying local jump attempt frequencies in different jump environments are investigated using Kinetic Monte Carlo simulations. For doping along the migration pathway, where the migrating oxygen ion passes between two adjacent cations, large dopants lead to an increase and small dopants to a decrease in the attempt frequency. Sm doping in nearest neighborhood to the start position of the migrating oxygen vacancy also leads to an increase in attempt frequency. Kinetic Monte Carlo simulations show that at intermediate Sm dopant fractions oxygen vacancies frequently jump toward and away from dopants explaining why for Sm doped ceria one of the highest conductivities for a ternary cerium oxide was measured due to its low dopant-oxygen vacancy association in both nearest and next-nearest neighborhood.

Review of experimental and modelling developments for ceria-based solid oxide fuel cells free from internal short circuits

Ceria-based solid oxide fuel cells (SOFCs) are the promising candidates for the low- and intermediate-temperature SOFCs. However, the Ce4+ in the ceria-based electrolyte materials is likely reduced to Ce3+ under low oxygen partial pressure, leading to high electronic conductivity. Therefore, ceria-based SOFCs commonly show low open-circuit voltage and low efficiency due to the leakage current. While extensive studies of the electron transport mechanism and the methods for the prevention of leakage current in ceria-based electrolytes have been conducted, systematic reviews of the relevant literature have been notably rare. In this review, the formation mechanism and factors affecting electronic conductivity in doped ceria are clearly described. Additionally, two kinds of methods including the optimization of the single ceria-doped electrolyte and the use of the various electron blocking layers (e.g., ZrO2-based, doped BaCeO3/SrCeO3 and doped Bi2O3 materials) are systematically summarized. Finally, mathematical models describing the electrochemical characteristics of ceria-based SOFCs based on different assumptions are reviewed. This review can provide useful guidance for the further development and application of internal short-circuit-free ceria-based SOFCs.

Performance Comparison between Ni–SDC and Ni–YSZ Cermet Electrodes for Carbon Dioxide Electrolysis on Solid Oxide Electrolysis Cell

Solid oxide electrolysis cells (SOECs) have already proven their ability to electrolyze steam (H2O). Recently, the attention has been drawn towards the electrolysis of carbon dioxide (CO2). In this paper, the electrochemical reduction of CO2 over Ni–Ce0.8Sm0.2O1.9 (SDC) and Ni–8 mol% Y2O3–ZrO2(YSZ) electrodes was studied on SOEC device.Cells used in this work consist of Ni–SDC or Ni–YSZ cermet electrodes, YSZ electrolyte, and (La0.8Sr0.2)0.97MnO3 (LSM) electrode. Electrochemical properties of SOEC were evaluated by impedance spectroscopy and current–voltage (I–V) curves with a supply of CO2/H2 and pure O2 gases to the cathode and anode sides, respectively, at 1000 oC.The results of preliminary experiments indicated that in CO2/H2 mixture, CO2 reduction proceeds indirectly via kinetically fast reverse water-gas shift reaction (RWGSR, CO2 + H2 ⇌ CO + H2O). The impact of cathode material on CO2 reduction was studied as depicted in Fig. 1 (a) and (b), which shows I–V curves of single button cells employing Ni–SDC and Ni–YSZ electrodes, respectively. The cell with Ni–SDC electrode exhibited relatively high current density compared with Ni–YSZ electrode. The better performance of Ni–SDC electrode depends on the conductivity of doped ceria; it exhibits higher ionic conductivity than YSZ and shows electronic conductivity under a reducing atmosphere. These two factors are most likely extended the effective triple phase boundary (TPB) region and improved the electrochemical reactions at electrode/electrolyte interface.

A first-principles study on defect association and oxygen ion migration of Sm3+ and Gd3+ co-doped ceria

First-principles calculations based on density functional theory were performed to investigate the co-doping effects of Sm and Gd in ceria on its oxygen ion conduction. The focus of this study is on the interactions between the cation dopants and an oxygen vacancy within the two adjacent tetrahedral sites of fluorite structure surrounding the oxygen migration path. Vacancy formation energies, dopant–vacancy association energies, and migration energies were calculated to elucidate the doping effects on oxygen ion conduction. The migration energies show remarkable dependences on the ionic radii of the cations located at the edges of the migration path. A simple relation between migration energy and vacancy formation energy is proposed. This work provides an informative insight into vacancy diffusion that could be useful in optimizing doping materials for improving oxygen ion conductivity in doped ceria.

A density functional study of defect migration in gadolinium doped ceria

Oxygen ion conductivity of doped ceria is observed to be two-three orders of magnitude higher than yttria stabilized zirconia, the most widely used solid electrolyte material at temperatures below 600 degrees C. Gadolinium doped ceria (GDC) is known to be one of the most promising solid electrolyte materials for operation of solid oxide fuel cells below 600 degrees C. To understand the atomic defect migration in GDC, we have used total energy calculations within the framework of density functional theory to follow oxygen vacancy migration in GDC. We report activation energies for various oxygen vacancy migration pathways in GDC. Oxygen vacancy formation and migration were evaluated for first, second, and third nearest neighbor positions to a Gd(3+) ion. Due to the comparable ionic radii of Gd(3+) and host Ce(4+) ions, the first nearest neighbor site with respect to the dopant cation is found to be the most favorable oxygen vacancy formation site. The migration pathway where the vacancy migrates from a second to first nearest neighbor is found to be most favorable. The calculated activation energies for oxygen vacancy migration in GDC are compared against the reported measured and calculated values from the literature. This work will provide a foundation for the development of a kinetic lattice Monte Carlo model for vacancy diffusion in GDC, which will improve the understanding of oxygen ion conductivity in doped ceria.

First-principles study on defect chemistry and migration of oxide ions in ceria doped with rare-earth cations

Oxygen transport in rare-earth oxide (RE(2)O(3)) doped CeO(2) with fluorite structure has attracted considerable attention owing to both the range of practical usage (e.g., fuel cells, sensors, etc.) and the fundamental fascination of fast oxide ion transport in crystalline solids. Using density-functional theory, we have calculated the formation energies of point defects and their migration properties in RE(2)O(3) doped CeO(2)(RE = Sc, Y, La, Nd, Sm, Gd, Dy, and Lu). The calculated results show that oxygen vacancies are the dominant defect species obtained by RE(3+) doping. They form associates with the RE(3+) ions, and the corresponding defect association energy is a strong function of the ionic radii of the RE(3+) dopants. The migration of an oxygen vacancy was investigated using the nudged elastic band method. The lowest activation energy for oxygen vacancy hopping is obtained for a straightforward migration path between two adjacent oxygen sites. The migration energy of an oxygen vacancy also strongly depends on the ionic radii of the neighbouring dopant cations. Accordingly, we have identified two factors that affect the oxygen vacancy migration; (1) trapping (or repelling) of an oxygen vacancy at the NN site of the RE(3+) dopant, and (2) reduction (or enlargement) of the migration barrier by RE(3+) doping. These findings provide insight for atomistic level understanding of ionic conductivity in doped ceria and would be beneficial for optimizing ionic conductivity.

Atomistic calculation of association energy in doped ceria

Atomistic simulation based on energy minimization techniques was carried out to study the association energies of more than 300 defect structures of ceria doped with Lu 2O 3, Yb 2O 3, Er 2O 3, Y 2O 3, Gd 2O 3, Eu 2O 3, Sm 2O 3, Nd 2O 3 and La 2O 3. The calculation ensemble includes all the possible defect structures in 6.25 mol% doped ceria. It is revealed that a clear preference for vacancy–dopant association occurs energetically for small dopant cations and vacancy–Ce 4+ association for large dopant cations, with the crossover at Gd 3+. The defect structures with the maximum association effect for different trivalent cations were determined. For Gd 3+ doped ceria which presents the lowest association effect in the trivalent doped ceria, the association energies of four different defect structures are extremely close, which agrees well with a recent EXAFS investigation, that is, Gd 3+ is distributed randomly in the ceria lattice. This work provides insight for the understanding of ionic conductivity in doped ceria.

Oxygen vacancy ordering in heavily rare-earth-doped ceria

25 at. % Rare-earth (RE)-doped ceria samples (RE=Sm, Dy, Y, and Yb) were examined using transmission electron microscopy and electron energy loss spectroscopy, from which the oxygen vacancy ordering in nanosized domains was confirmed. The relationships of the dopant type, oxygen vacancy ordering, and ionic conductivity of doped ceria were established. It is found that the ordering of oxygen vacancies depends strongly on the dopant type, and the development of nanosized domains with a higher degree of ordering can lead to a more dramatic decrease of ionic conductivity in doped ceria.

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.