Conductivity Of Ceria Research Articles

Comprehensive Analysis of Ce1−xSmxO2−δ Solid Electrolytes: Structural, Microstructural, and Electrochemical Characterization for Intermediate Temperature Solid Oxide Fuel Cells

Samarium substituted ceria has been studied as a potential electrolyte material for intermediate temperature range solid oxide fuel cells. The structural, microstructural, morphological and electrochemical properties of the Ce1−xSmxO2−δ solid electrolytes were analyzed, with different substitutions from 0.05 to 0.50. The difference in the ionic radii of Sm3+ and Ce4+ resulted in lattice strain and expansion, which was found to decrease the average size of crystallites. The ionic conductivity of ceria increases as Sm3+ concentration is increased upto a limit of 20%. This is due to the combination of vacancy-vacancy repulsion and vacancy trapping by the substitution at a higher level. However, the conductivity of Ce0.80Sm0.20O2−δ was the highest, at 4.04 × 10−2 S cm−1 at 600 °C. The grain activation energy and grain boundary activation energy were also found to be ∼0.87 eV and ∼0.74 eV respectively. The low values of activation energies indicate that Ce0.80Sm0.20O2−δ could be a suitable electrolyte material for intermediate-temperature solid oxide fuel cells.

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.

Effect of 20 mol % gadolinium doping on oxide ion conductivity of ceria as electrolyte for intermediate temperature solid oxide fuel cells

The gadolinium-doped ceria is investigated as electrolyte materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) due to its oxide ion conductivity. The doping of Gd3+ to Ce4+ can introduce a small strain in the lattice thereby improved conductivity with low activation energy is expected. The 20 mol. % gadolinium doped ceria (Ce0.8Gd0.2O2−δ) nanocrystalline powder is prepared here by citrate - complexation method. The XRD, Rietveld refinement, FTIR, UV-Visible, FESEM/EDX, and a c - impedance techniques are used to characterize this sample. The oxide ion conductivity is determined between 523 − 1023 K. The Ce0.8Gd0.2O2−δ shows highest oxide ion conductivity of 6.79 × 10−3 S cm−1 and 1.11 × 10−2 S cm−1 at 973 K and 1023 K, respectively. The Ce0.8Gd0.2O2−δ showed lower activation energies of 1.09, 0.70, and 0.88 eV for grain, grain boundary, and total conduction, respectively. Thus, the nano crystalline Ce0.8Gd0.2O2−δ is proposed as potential electrolyte for IT-SOFCs.

Effect of Doping Concentration on Grain Boundary Conductivity of Samaria Doped Ceria Composites

This work aims to study the effect of Sm3+ doping concentration on the grain boundary ionic conductivity of ceria. The materials were prepared by a modified co-precipitation method, where molecular water associated with the precursor has been utilized to facilitate the hydroxylation process. The synthesized hydroxide/hydrated oxide materials were calcined and the green body (pellet) has been sintered at high temperature in order to achieve highly dense (∼ 96%) pellet. The structural analyses were done using XRD and Raman spectroscopy, which confirm the single phase cubic structure of samaria doped ceria (SDC) nanoparticles and the surface morphology of sintered samples was studied using FESEM. The ionic conductivity was measured by AC impedance spectroscopy of the sintered pellets in the temperature range of 400 °C–700 °C, which shows superior grain boundary conductivity. The grain boundary ionic conductivity of around 0.111 S cm−1 has been obtained for 15SDC composition at 600 °C.

Effect of Reduced Atmosphere Sintering on Blocking Grain Boundaries in Rare-Earth Doped Ceria

Rare-earth doped ceria materials are amongst the top choices for use in electrolytes and composite electrodes in intermediate temperature solid oxide fuel cells. Trivalent acceptor dopants such as gadolinium, which mediate the ionic conductivity in ceria by creating oxygen vacancies, have a tendency to segregate at grain boundaries and triple points. This leads to formation of ionically resistive blocking grain boundaries and necessitates high operating temperatures to overcome this barrier. In an effort to improve the grain boundary conductivity, we studied the effect of a modified sintering cycle, where 10 mol% gadolinia doped ceria was sintered under a reducing atmosphere and subsequently reoxidized. A detailed analysis of the complex impedance, conductivity, and activation energy values was performed. The analysis shows that for samples processed thus, the ionic conductivity improves when compared with conventionally processed samples sintered in air. Equivalent circuit fitting shows that this improvement in conductivity is mainly due to a drop in the grain boundary resistance. Based on comparison of activation energy values for the conventionally processed vs. reduced-reoxidized samples, this drop can be attributed to a diminished blocking effect of defect-associates at the grain boundaries.

Local Multimodal Electro‐Chemical‐Structural Characterization of Solid‐Electrolyte Grain Boundaries

AbstractTypical models of polycrystalline ionic materials treat the grain boundary properties as single valued, without consideration of the full range of values that define the macroscopically measured average. Here a unique experimental platform suitable for local multimodal characterization of individual grain boundaries in bicrystal fibers is reported. A variation of three orders of magnitude in the grain boundary conductivity of ceria is observed, as measured across six individual bicrystals by both alternating current impedance spectroscopy and direct current (D.C.) linear sweep voltammetry. Nonlinear behavior of the D.C. measurements is consistent with resistance due to a space charge effect. Time‐of‐flight secondary ion beam spectroscopy reveals a correlation between grain boundary resistance and the concentration of impurities Si, Al, and Ca segregated at the grain boundaries, although the bulk concentrations of these impurities are negligible. Electron backscatter diffraction analysis of the crystal orientations suggests a correlation between the misorientation across the grain boundaries and grain boundary resistance. These correlations point towards a grain boundary resistance that arises from impurity‐generated space charge effects and variations in impurity concentration and hence resistivity driven by the energetics of impurity segregation to grain boundaries of differing surface energies.

Using anion photoelectron spectroscopy of cluster models to gain insights into mechanisms of catalyst-mediated H2 production from water.

Metal oxide cluster models of catalyst materials offer a powerful platform for probing the molecular-scale features and interactions that govern catalysis. This perspective gives an overview of studies implementing the combination of anion photoelectron (PE) spectroscopy and density functional theory calculations toward exploring cluster models of metal oxides and metal-oxide supported Pt that catalytically drive the hydrogen evolution reaction (HER) or the water-gas shift reaction. The utility in the combination of these experimental and computational techniques lies in our ability to unambiguously determine electronic and molecular structures, which can then connect to results of reactivity studies. In particular, we focus on the activity of oxygen vacancies modeled by suboxide clusters, the critical mechanistic step of forming proximal metal hydride and hydroxide groups as a prerequisite for H2 production, and the structural features that lead to trapped dihydroxide groups. The pronounced asymmetric oxidation found in heterometallic group 6 oxides and near-neighbor group 5/group 6 results in higher activity toward water, while group 7/group 6 oxides form very specific stoichiometries that suggest facile regeneration. Studies on the trans-periodic combination of cerium oxide and platinum as a model for ceria supported Pt atoms and nanoparticles reveal striking negative charge accumulation by Pt, which, combined with the ionic conductivity of ceria, suggests a mechanism for the exceptionally high activity of this system towards the water-gas shift reaction.

Disentangling small-polaron and Anderson-localization effects in ceria: Combined experimental and first-principles study

By comparison of the electrical conductivity of ceria doped with penta- and hexavalent ions, we separate the total electron localization energy into the two contributions originating from the small polaron effects and the Coulomb interaction with the donor ions. The upper bound of the itinerant small polaron hopping energy is estimated at $66\ifmmode\pm\else\textpm\fi{}20$ meV. The binding energy of the ${\mathrm{Ce}}^{3+}--{M}^{5+/6+}$ defect complex increases from 121 meV for $M={\mathrm{Nb}}^{5+}/{\mathrm{Ta}}^{5+}$ to 243 meV for $M={\mathrm{W}}^{6+}/{\mathrm{U}}^{6+}$. The first-principles simulations are in qualitative agreement with the experimental findings. At low temperatures the $f$ electrons bound to the donor defects show dielectric relaxation with the lowest activation energy of 2.7 and 17 meV for Nb(Ta)- and W-doped ceria, respectively. Remarkably, these energies are significantly smaller than the hopping energy of the itinerant small polarons. While both the electron-lattice and the electron-defect interactions cause the $f$ electron localization in real-case ceria, the latter effects seem to be the dominant.

Exotic electronic structures of SmxCe3-xOy (x = 0-3; y = 2-4) clusters and the effect of high neutral density of low-lying states on photodetachment transition intensities.

Lanthanide (Ln) oxide clusters have complex electronic structures arising from the partially occupied Ln 4f subshell. New anion photoelectron (PE) spectra of SmxCe3-xOy- (x = 0-3; y = 2-4) along with supporting results of density functional theory (DFT) calculations suggest interesting x and y-dependent Sm 4f subshell occupancy with implications for Sm-doped ionic conductivity of ceria, as well as the overall electronic structure of the heterometallic oxides. Specifically, the Sm centers in the heterometallic species have higher 4f subshell occupancy than the homonuclear Sm3Oy-/Sm3Oy clusters. The higher 4f subshell occupancy both weakens Sm-O bonds and destabilizes the 4f subshell relative to the predominantly O 2p bonding orbitals in the clusters. Parallels between the electronic structures of these small cluster systems with bulk oxides are explored. In addition, unusual changes in the excited state transition intensities, similar to those observed previously in the PE spectra of Sm2O- and Sm2O2- [J. O. Kafader et al., J. Chem. Phys. 146, 194310 (2017)], are also observed in the relative intensities of electronic transitions to excited neutral state bands in the PE spectra of SmxCe3-xOy- (x = 1-3; y = 2, 4). The new spectra suggest that the effect is enhanced with lower oxidation states and with an increasing number of Sm atoms, implying that the prevalence of electrons in the diffuse Sm 6s-based molecular orbitals and a more populated 4f subshell both contribute to this phenomenon. Finally, this work identifies challenges associated with affordable DFT calculations in treating the complex electronic structures exhibited by these systems, including the need for a more explicit treatment of strong coupling between the neutral and PE.

Effect of sintering aid MoO3 on the microstructure and ionic conductivity of ceria- and lanthanum gallate-based electrolytes

ABSTRACTThe effect of a small addition of MoO3 on the microstructure and ionic conductivity of Nd0.2Ce0.8O1.9 (NDC), La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM) and Nd0.2Ce0.8O1.9-La0.8Sr0.2Ga0.8Mg0.2O2.8 (NDC-LSGM) has been investigated. The microstructure and electrical properties of the samples were characterized by X-ray diffraction, field-emission scanning electron microscopy and electrochemical impedance spectroscopy. The results show that MoO3 doping can obviously increase the densification and grain sizes, and decrease the grain and grain boundary resistances of the NDC, LSGM and NDC-LSGM electrolytes. It expands the oxygen ion channels and reduces the total conductance activation energy of the system. The total conductivities of MoO3-doped NDC and NDC-LSGM samples are 1.56 and 2.10 times higher than that of the undoped NDC system at 450°C. The total conductivity of LSGM-Mo is 1.46 times higher than that of LSGM at 450°C. These finding suggest that MoO3 is considered to be an effective sintering aid that optimizes the electrical properties of NDC, LSGM and NDC-LSGM electrolytes.

Assessment of the Effect of Transition Metal Oxide Addition on the Conductivity of Commercial Gd-Doped Ceria

The effect of addition of transition metal oxides on the charge transport of commercially available Gd-doped ceria (Ce0.9Gd0.1O2-δ) was assessed with special emphasis on the electronic partial conductivity. The samples were doped by adding 2, 4 or 6 cat% of CoO3/4 or FeO2/3, or 2 mol% of the transition metal oxides V2O5, MnO2, and CuO, respectively. It was found that even small amounts of transition metal oxides severely change the partial electronic conductivity of ceria while the majority oxygen ion conductivity was only mildly affected: for high oxygen ion conductivity, Fe or Mn oxide addition as well as small amounts of Co oxide are beneficial, as especially the grain boundary conductivity is slightly increased. Addition of V or Cu oxide in contrast increases the minority charge carrier conductivity of electrons and thus enhances the mixed ionic-electronic conductivity. For Co oxide addition, an increasing electronic conductivity with decreasing oxygen partial pressure from 10−8 to 10−4 bar at temperatures below 600°C indicates the changing redox state of Co from divalent at low oxygen partial pressures to trivalent under ambient oxygen partial pressure.

Influence of microstructural characteristics on ionic conductivity of ceria based ceramic solid electrolytes

Solid electrolyte powders based on gadolium-doped and samaria-codoped ceria of compositions Ce0.8Gd0.2−xSmxO1.9 (x = 0.00; 0.01; 0.03; 0.05) were sintered using the polymeric precursor method (Pechini). The X-ray diffractometry results confirmed the formation of a single crystalline phase, that is, a solid solution. Test specimens were compacted using uniaxial cold pressing followed by two-stage sintering. Relative densities were higher than 96% of theoretical density in all the samples. Ionic conductivity was determined using impedance spectroscopy, obtaining values of 10−2Scm−1 at 700°C, with the best results for codoped electrolytes, primarily the Ce0.8Gd0.15Sm0.05O1.9. system. This study aimed at correlating ionic conductivity variations of codoped electrolytes with microstructural alterations (on a nanometric scale) resulting from the addition of codopants. Energy dispersive spectroscopy (EDS) was used to analyze the grain boundary compositions of the electrolytes produced and assess possible dopant segregation in these regions. Interplanar spacings corresponding to the crystallographic planes of the cubic (fluorite-type) phase were identified using high-resolution transmission electron microscopy (HRTEM) images. Extra diffraction spots and diffuse stains were observed in the doped and codoped electrolytes, using selected area electron diffraction (SAED), confirming the presence of amorphous nanodomains at the atomic level. However, the codoped system exhibited diffraction patterns with no diffuse stains, indicating that codoping favored obtaining nanodomain-free regions. These regions displayed better structural homogeneity, which may explain the enhanced ionic conductivity in codoped systems.

Development and assessment of ceria–propylene glycol nanofluid as an alternative to propylene glycol for cooling applications

Spherical, crystalline ceria nanoparticles of 18–25nm were synthesized from cerium nitrate precursor. The dispersion of as-synthesized ceria nanoparticles in propylene glycol was achieved through extended probe ultrasonication for 14h, leading to ceria–propylene glycol nanofluids. The influence of nanoparticle concentration (0–1vol.%) and temperature on viscosity and thermal conductivity of ceria–propylene glycol nanofluids were investigated. Our data indicate that the higher thermal conductivity enhancement at elevated temperatures (18.8% at 80°C for 1vol.% nanofluid) can be attributed to the particle clustering and Brownian-motion induced microconvection. Ceria nanoparticles interact with propylene glycol leading to disturbance in hydrogen bonding network prevalent in propylene glycol. This resulted in lower viscosity of 0.5vol.% and 1vol.% ceria–propylene glycol nanofluids than propylene glycol over a wide range of temperatures. The heat absorption by ceria–propylene glycol nanofluids under transient, natural convective heat transfer conditions increased with ceria nanoparticle concentration. Hence ceria–propylene glycol nanofluids are suitable for cooling applications.

Sub-nA spatially resolved conductivity profiling of surface and interface defects in ceria films

Spatial variability of conductivity in ceria is explored using scanning probe microscopy with galvanostatic control. Ionically blocking electrodes are used to probe the conductivity under opposite polarities to reveal possible differences in the defect structure across a thin film of CeO2. Data suggest the existence of a large spatial inhomogeneity that could give rise to constant phase elements during standard electrochemical characterization, potentially affecting the overall conductivity of films on the macroscale. The approach discussed here can also be utilized for other mixed ionic electronic conductor systems including memristors and electroresistors, as well as physical systems such as ferroelectric tunneling barriers.

Reduced-temperature firing of solid oxide fuel cells with zirconia/ceria bi-layer electrolytes

Solid oxide fuel cells (SOFCs) with bi-layer Zirconia/Ceria electrolytes have been studied extensively because of their great potential for producing high power density at reduced operating temperature, important for reducing cost and thereby allowing broader SOFC commercialization. The bi-layer electrolytes are designed to take advantage of the high oxygen ion conductivity of Ceria, the low electronic conductivity of Zirconia, and the low reactivity of Ceria with high-performance cathodes. However, zirconia/ceria processing has proven problematic due to interdiffusion during high temperature co-firing, or ceria layer porosity after two-step firing. Here we first show a new method for bi-layer co-firing at a reduced temperature of 1250 °C, ∼150 °C lower than the usual sintering temperature, achieved using Fe2O3 as a sintering aid. This novel process enables high power density SOFCs by producing: (1) low-resistance Y0.16Zr0.92O2−δ (YSZ)/Gd0.1Ce0.9O1.95 (GDC) electrolytes that also yield high open-circuit voltage, (2) dense GDC layers that prevent reactions between highly-active La0.6Sr0.4Fe0.8Co0.2O3 (LSFC) cathode materials and YSZ, and (3) Ni–YSZ anodes with high electrochemical activity due to fine-scale microstructure with high TPB densities.

Scalable Oxygen‐Ion Transport Kinetics in Metal‐Oxide Films: Impact of Thermally Induced Lattice Compaction in Acceptor Doped Ceria Films

In this paper, we focus on the effect of processing‐dependent lattice strain on oxygen ion conductivity in ceria based solid electrolyte thin films. This is of importance for technological applications, such as micro‐SOFCs, microbatteries, and resistive RAM memories. The oxygen ion conductivity can be significantly modified by control of lattice strain, to an extent comparable to the effect of doping bulk ceria with cations of different diameters. The interplay of dopant radii, lattice strain, microstrain, anion‐cation near order and oxygen ion transport is analyzed experimentally and interpreted with computational results. Key findings include that films annealed at 600 °C exhibit lattice parameters close to those of their bulk counterparts. With increasing anneal temperature, however, the films exhibited substantial compaction with lattice parameters decreasing by as much as nearly 2% (viz, Δd600–1100 °C: –1.7% (Sc+3) > –1.5% (Gd+3) > –1.2% (La+3)) for the annealing temperature range of 600–1100 °C. Remarkably 2/3rd of the lattice parameter change obtained in bulk ceria upon changing the acceptor diameter from the smaller Sc to larger La, can be reproduced by post annealing a film with fixed dopant diameter. While the impact of lattice compaction on defect association/ordering cannot be entirely excluded, DFT computation revealed that the main effect appears to result in an increase in migration energy and consequent drop in ionic conductivity. As a consequence, it is clear that annealing procedures should be held to a minimum to maintain the optimum level of oxygen ion conductivity for energy‐related applications. Results reveal also the importance to understand the role of electro‐chemo‐mechanical coupling that is active in thin film materials.

A combined DFT + U and Monte Carlo study on rare earth doped ceria

We investigate the dopant distribution and its influence on the oxygen ion conductivity of ceria doped with rare earth oxides by combining density functional theory and Monte Carlo simulations. We calculate the association energies of dopant pairs, oxygen vacancy pairs and between dopant ions and oxygen vacancies by means of DFT + U including finite size corrections. The cation coordination numbers from ensuing Metropolis Monte Carlo simulations show remarkable agreement with experimental data. Combining Metropolis and Kinetic Monte Carlo simulations we find a distinct dependence of the ionic conductivity on the dopant distribution and predict long term degradation of electrolytes based on doped ceria.

Size Effects on the Electrical Conductivity of Ceria: Achieving Low Space Charge Potentials in Nanocrystalline Thin Films

Thin films of nominally pure and 10 mol % Gd-doped ceria were grown on Al2O3⟨1102⟩ (r-cut) and MgO⟨100⟩ substrates with pulsed laser deposition (PLD). Their electrical conductivity properties were measured using impedance spectroscopy. Oxygen partial pressure and temperature dependence indicate that the nominally pure films are contaminated with acceptor impurities whose concentration is found to vary perceptibly among the samples. Quite remarkably, the nanocrystalline 10 mol % Gd-doped thin films show conductivities that are, as expected, lower than those for epitaxial films but surprisingly much larger than those obtained previously from other comparable nanocrystalline films (e.g., grown on SiO2), indicating that the oxygen vacancies are much less depleted at the grain boundaries. Correspondingly, the space charge potential was found to be unusually small with a value of 0.19 ± 0.05 V.

Pattern Electrodes for Studying SOFC Electrochemistry

Pattern anodes can be used to localize reactions and study individual processes like charge transfer, adsorption, diffusion etc. Ceria and Nickel (Ni) pattern anodes were fabricated with the same dimensions with Triple phase boundary (TPB) lengths of 0.2707 m/cm2. Electrochemical Impedance Spectroscopy (EIS) of Ni anodes yielded results comparable with the results in literature (0.0001 Scm-2 at p H2O = 2.4 kPa and 700oC). Ceria shows a different value for conductivity compared to the nickel, possibly because of the mixed electronic ionic conductivity of ceria and the spreading of the TPB over the surface of ceria. Identifying rate limiting mechanisms on ceria pattern anodes and comparing them with those of nickel gives a more complete picture about elementary processes at the cermet anode.

Dynamical response and instability in ceria under lattice expansion

We present results of density functional theory calculations on the phonon dispersion and elastic constants of bulk ceria (CeO${}_{2}$) as a function of positive and negative isotropic strain, which could be induced thermally or by cationic doping. We find that, as the lattice is expanded, there is a significant softening of the ${B}_{1u}$ mode at the $X$ point. This mode consists of motions of oxygens in the $[001]$ direction. At a strain of $1.6$$%$, corresponding to a temperature of 1600 K, the ${B}_{1u}$ and ${E}_{u}$ modes at the $X$ point cross, with an associated high, narrow peak in the phonon density of states appearing. We infer that this crossing indicates a coupling of the modes, leading to a transition to a superionic phase, where conductivity occurs in the $[001]$ direction, mediated by anion interstitial site occupation. As the lattice is expanded further, the ${B}_{1u}$ mode continues to soften, becoming imaginary at a strain of $3.4$$%$, corresponding to a temperature of 2500 K. Following the imaginary mode would result in a cubic to tetragonal phase transition, similar to those known to occur with reducing temperature in zirconia (ZrO${}_{2}$) and hafnia (HfO${}_{2}$). Our calculated elastic constants, however, indicate that the structure remains mechanically stable, even at this level of expansion. As confirmed by our semiclassical free energy calculations, the cubic phase of ceria remains the most stable, while the imaginary mode indicates a change to a thermally disordered cubic phase, with the majority of disorder occurring on the anion sublattice. Our results explain the high temperature ionic conductivity in ceria and other fluorite-structured materials in terms of the intrinsic lattice dynamics, and give insight to the stability and anionic disorder at elevated temperatures.