Influence of Gd3+ and Dy3+ co-doping and sintering regime on enhancement of electrical conductivity of ceria-based solid electrolyte

In order to improve the conductivity of ceria-based solid electrolytes, effect of co-doped Gd³⁺ and Dy³⁺ was evaluated. For this purpose, nano-crystalline Gd_0.2???x Dy _x Ce_0.8O_1.9 powders with various composition ranges (x?=?0.05, 0.1, 0.15, 0.2) were initially synthesized by high-energy milling method. The effect of micro-structural evolution and co-doping on electrical properties of the dense sintered samples fabricated by two-step sintering and conventional sintering of the synthesized powders were investigated. Electrical conductivity of the samples was discussed based on the results obtained by AC impedance spectroscopy at temperatures in the range of 300–700 °C. The co-doping and sintering regime were found to significantly influence the conductivity of the electrolytes. The electrical conductivity of the co-doped samples depends on Dy³⁺ content and the maximum conductivity obtained by 0.15 mol% Dy and 0.05 mol% Gd. The conductivity of Gd_0.2???x Dy _x Ce_0.8O_1.9 (x?=?0.15) was 0.03 S/cm at 700 °C. A thorough discussion was made, based on the present experimental data.     

Anomalous conductivity and microstructure in gadolinium doped ceria prepared from nano-sized powder

The temperature and the oxygen partial pressure dependences of the electron and hole conductivities were measured by the dc polarization method using a Hebb–Wagner's ion blocking cell for Gd_0.2Ce_0.8O_1.9 polycrystalline bodies with grain size of 0.5 μm prepared by sintering of nano-sized powder. A significant enrichment of gadolinium was observed in the vicinity of the grain boundary by TEM/EDS analyses. The electron conductivity were comparable with those of conventional Gd_0.2Ce_0.8O_1.9 polycrystalline body with grain size of 2 μm, and it followed p(O₂)^− 1/4 dependence at temperatures T = 973–1273 K. However, the observed hole conductivity was higher than that of conventional Gd_0.2Ce_0.8O_1.9, and it did not follow p(O₂)^1/4 dependence. This anomalous p(O₂) dependence disappeared after the sample was treated at T = 1773 K for 38 h and grain size was enlarged to 2–10 μm.     

Preparation,characterization and electrical property of Mn-doped ceria-based oxides

Manganese-doped ceria-based oxides, Ce_1−xMn_xO_2−δ (0.05 ≤ x ≤ 0.3) and Ce_1−x−yGd_xMn_yO_2−δ˙ (0.05 ≤ x≤ 0.2, 0.05 ≤ y ≤ 0.25) were synthesized, and crystal phase analysis by XRD and measurements of electrical properties were performed. Solubility limit of Mn in Ce_1−xMn_xO_2−δ˙ seemed to be between 5 mol% and 10 mol% and Mn₃O₄ was the main by-product above the solubility limit in the case of heat treatment at 1300 °C. Judging from the oxygen partial pressure dependence of total conductivity and emf measurements, Ce_1−xMn_xO_2−δ˙ is a single-phase mixed conductor within the composition below the solubility limit, and when the composition of Mn exceeds the solubility limit, it becomes the dual-phase mixed conductor of Ce_1−xMn_xO_2−δ˙ and Mn₃O₄. The doing of Mn in gadlia-doped ceria, Ce_1−x−yGd_xMn_yO_2−δ˙ (0.05 ≤ x ≤ 0.2, 0.05 ≤ y ≤ 0.25), was more difficult than that in CeO₂ presumably due to the preferential reaction between Gd and Mn to give GdMnO₃ to the GDC solid solution formation, and the Mn doping seems not to be so effective in preparing the mixed ionic–electronic conductor based on GDC.     

Spray pyrolysis of electrolyte interlayers for vacuum plasma-sprayed SOFC

The effects of gadolinia-doped ceria (CGO, Ce_0.8Gd_0.2O_1.9−x) and yttria-doped zirconia (8YSZ, Zr_0.92Y_0.08O_2−x) interlayers prepared by spray pyrolysis between vacuum plasma-sprayed 8YSZ electrolytes (8YSZ–VPS) and screen-printed (La_0.8Sr_0.2)_0.98MnO₃ cathodes (LSM) on the power output of solid oxide fuel cells (SOFC) are investigated. Amorphous thin films are deposited and then converted to nanocrystalline electrolyte–cathode interlayers during the first heat-up cycle of a SOFC to the operating temperature. CGO thin films between the YSZ plasma-sprayed electrolyte and the LSM cathode increased the power output by more than 20% compared to cells without interlayers, whereas YSZ films degraded the power output of cells. It is assumed that CGO improves the charge transfer at the electrolyte–cathode interface and that the CGO layer prevents the formation of undesirable insulation of La-zirconate at the interface 8YSZ/LSM.     

Preparation of proton conducting BaCe0.8Gd0.2O3 thin films

Thin films of BaCe_0.8Gd_0.2O₃ were prepared by solid state reaction of two screen-printed layers over porous substrates. The first layer consists of the oxygen ion conductor Ce_0.8Gd_0.2O₂ with a fluorite structure, whereas the top layer consists of BaCO₃. After decomposition of the carbonate, BaO reacts with Ce_0.8Gd_0.2O₂ forming the perovskite oxide BaCe_0.8Gd_0.2O_3−δ with protonic conductivity. The in-situ reaction and densification on the porous substrates results in gastight thin layers of 10 to 50 μm and allows overcoming the problems due to the poor sinterability of the proton conductor. Two different porous substrates prepared by warm-pressing were studied as membrane supports, i.e., (i) porous composite NiO–Zr_0.85Y_0.15O₂, commonly employed as solid oxide fuel cell anode and (ii) porous Ce_0.8Gd_0.2O₂ oxide. The structural properties of the layer, compositional gradients and occurring phases are described, as well as water uptake, gastightness (He leaking rate) and emf measurement. Protonic conducting membranes are particularly suited not only for hydrogen separation combined with reforming and water–gas-shift converters but also as a protonic fuel cell electrolyte.     

A co-doping approach towards enhanced ionic conductivity in fluorite-based electrolytes

A co-dopant strategy is used to investigate the effect that the elastic strain in the lattice has on the grain ionic conductivity of doped ceria electrolytes. Based on critical dopant ionic radius (r_c), different compositions in the Lu_xNd_yCe_1−x−yO_2−δ (x + y = 0.05, 0.10, 0.15, and 0.20) system are studied. Dopants are added such that the weighted average dopant ionic radius matches r_c for all the compositions. Dense ceramic discs are prepared using conventional solid oxide route and sintering methods. Precise lattice parameter measurements are used to calculate the lattice strain. The ionic conductivity of the samples is measured in the temperature range of 250 °C to 700 °C using two-probe electrochemical impedance spectroscopy technique. The elastic strain present in Lu_xNd_yCe_1−x−yO_2−δ system is found to be negligible when compared to Lu_xCe_1−xO_2−δ (negative) and Nd_xCe_1−xO_2−δ (positive) systems. Grain ionic conductivity of Lu_xNd_yCe_1−x−yO_2−δ (where x + y = 0.05) at 500 °C is observed to be 1.9 × 10^− 3 S/cm which is twice as high as that of Lu_0.05Ce_0.95O_2−δ. These results extend the validity of the r_c concept as a strategy for co-doping ceria electrolytes and open new designing avenues for solid oxide electrolytes with enhanced ionic conductivity.     

Ceria: Relation among thermodynamic,electronic hole and proton properties

The proton solubility and the hole conductivity of the rare earth doped ceria have been examined in their relations to the thermodynamic properties of doped ceria under the assumption that the hypothetical species, LnOOH and LnOO (Ln = Rare earth), can be regarded as constituents for representing protons and holes in the fluorite lattice. Focus is made on the dopant dependence, the host dependence and the temperature dependence in the rare earth doped zirconia(or ceria) fluorite lattice. The chemical potentials of the rare earth dopant are less stabilized in the ceria-based oxides than in the zirconia-based ones. The proton solubility in the ceria-based, zirconia-based, and ceria–zirconia solid solutions has been well interpreted in terms mainly of the hydroxidation energy and the stabilization energy of LnO_1.5 in the fluorite lattice. Since the dopant dependence of the stabilization energy of LnO_1.5 is stronger than the hydroxidation energy, the proton solubility becomes high in the smaller dopants. To account for less dopant-dependent behavior in the hole conduction, the peroxidation energy is assumed to have about the same dopant dependence as the stabilization energy. The calculated temperature dependences of proton solubility and hole concentration were compared with available experimental data; it has been suggested that holes and protons in ceria reach to saturation levels with lowering temperature. Some discussions are made on the possible explanation on recently observed anomalous hole conductivity in nano-size Ce_0.8Gd_0.2O_1.9 in terms of plausible effects of miscibility gap, associated Gd enrichment, and simultaneous formation of Ce³⁺ and holes.     

Ionic conductivity of Ce1−xNdxO2−x / 2

The electrical conductivity of sintered samples of Ce_1−xNd_xO_2−x / 2 (0.01 ≤ x ≤ 0.2) was investigated in air as a function of temperature between 150 and 600 °C using AC impedance spectroscopy. The individual contribution of the bulk and grain boundary conductivities has been discussed in detail. In the low temperature range (< 350 °C), the activation enthalpy for bulk conductivity exhibited a shallow minimum at 3 mol% Nd, with a value of 0.68 eV. The activation enthalpy also produced a shallow minimum at 5 mol% Nd in the high temperature range (> 350 °C), with a value of 0.56 eV. It was shown that Ce_1−xNd_xO_2−x / 2 is an electrolyte that obeys the Meyer Neldel rule. The bulk conductivity data measured by others for the same system has also been recalculated and re-evaluated to facilitate easier comparison with our own data.     

LFN and LSCFN perovskites — structure and transport properties

Structural, electronic and transport properties of LFN (LaFe_1−zNi_zO₃) and LSCFN (La_1−xSr_xCo_1−y −zFe_yNi_zO₃) perovskites synthesized by a modified citric acid method were studied. Structure of the samples was characterized by X-ray studies with Rietveld method analysis. Magnetic properties and valence states of iron ions were characterized by ⁵⁷Fe Moessbauer spectroscopy performed at RT, which were found to be greatly dependent on the chemical composition of the samples. Electrical conductivity was measured in the 20–800 °C temperature range and for some compositions relatively high values (exceeding 100 S cm^− 1) were observed in the 600–800 °C range. Chemical stability studies in relation to Ce_0.8Gd_0.2O_1.9 electrolyte, performed for selected perovskite samples, revealed decreasing stability with increasing Ni concentration and formation of solid solutions in CGO/perovskite composites. The coefficient of thermal expansion (CTE) of LFN perovskites was found to match that of CGO electrolyte (CTE in the 10–13 · 10^− 6 K^− 1 range).     

PVA-assisted synthesis and characterization of nano-crystalline La3+ and Mg2+ co-doped CeO2 electrolyte for intermediate-temperature solid oxide fuel cells

A series of nano-crystalline ceria-based solid solution electrolyte, Ce_0.8La_0.2?x Mg_xO_2?δ (x?=?0.0, 0.05, 0.10, 0.15, and 0.2), were synthesized via the polyvinyl alcohol (PVA) assisted combustion method, and then characterized to the crystalline structure, powder morphology, sintering micro-structure, and electrical properties. Present study showed that Ce_0.8La_0.2?x Mg _x O_2?δ was exceedingly stable as a cubic phase in all temperature range and exhibited fine crystals ranging from 15 to 20 nm. After sintering at 1,400 °C, the as-prepared pellets exhibited a dense micro-structure with 96 % of theoretical density. The electrical conductivity was studied using AC impedance spectroscopy and it was observed that the composition Ce_0.8La_0.1?Mg_0.1O_2?δ showed higher electrical conductivity of 0.020 S?cm^?1 at 700 °C. The thermal expansion was measured using dilatometer technique in the temperature range 30–1,000 °C. The average thermal expansion coefficient of Ce_0.8La_0.1?Mg_0.1O_2?δ was 12.37?×?10^?6 K^?1, which was higher than that of the commonly used SOFC electrolyte YSZ (~10.8?×?10^?6 K^?1).     

Electrode reaction of Sr1−xLaxCo0.8Fe0.2O3−δ with x = 0.1 and 0.6 on Ce0.9Gd0.1O1.95 at 600 ≤ T ≤ 800 °C

The electrode reaction of the perovskite phases Sr_1−xLa_xCo_0.8Fe_0.2O_3−δ (x = 0.1 and 0.6) on Ce_0.9Gd_0.1O_1.95 has been investigated by impedance spectroscopy in the temperature range 600 ≤ T ≤ 800 °C. Thick porous electrodes (t ∼ 20 μm) were sprayed on Ce_0.9Gd_0.1O_1.95 and ac impedance spectra were recorded on symmetrical cells at the equilibrium. The analysis of the complex impedance diagrams clearly indicates the presence of two contributions. The low frequency one was assigned to the gas phase oxygen diffusion through the porous electrode and a finite length diffusion (Warburg) impedance was used to describe the high frequency (HF) data. The polarization resistance of the HF impedance contribution (R_w) is higher for x = 0.1 while the activation energy of R_w is higher for x = 0.6. The variations of R_w versus the La content, temperature and thickness indicate that the Warburg-type impedance contains information of both bulk oxygen diffusion and surface processes.     

Solid solution formation, densification and ionic conductivity of Gd- and Sm-doped ceria

The processes of solid solution formation, densification and electrical conductivity in samaria and gadolinia-doped ceria solid electrolytes were studied by Raman spectroscopy, density and impedance spectroscopy measurements. Bulk specimens of Ce_0.9Gd_0.1O_1.95 and Ce_0.8Sm_0.2O_1.9 were prepared by solid state reactions at several dwell temperatures and holding times. Hydrostatic density results show a fast increase in sintered density up to 1 h holding time. Raman spectra of specimens sintered for 1 h show a prominent band at 463 cm^− 1 assigned to the cubic fluorite-type lattice of cerium oxide, and low-intensity bands at 344 and 363 cm^− 1 attributed to free samarium and gadolinium sesquioxides, respectively. Solid solution completion was achieved only at temperatures above 1400 °C. Electrical conductivity measurements were used to study mass transport. Analysis of impedance data allowed for determining the activation energy for cation diffusion in Ce_0.9Gd_0.1O_1.95 and Ce_0.8Sm_0.2O_1.9 sintered specimens.     

Bridging between structure and optimum luminescence for nearly monodispersed Ce1−xGdxF3:Eu3+ nanoparticles

This work reports a systematic study on bridging between structure and optimum luminescence for Ce_1?xGd_xF₃:Eu³⁺ nanoparticles. It is found that all Ce_1?xGd_xF₃:Eu³⁺ nanoparticles were nearly monodispersed, showing average grain diameter of 30–35 nm. Regardless of the dopant level, all nanocrystals crystallized in a single hexagonal phase. With increasing Gd³⁺ content, the lattice dimension for Ce_1?xGd_xF₃:Eu³⁺ linearly decreased, which was followed by the highly distorted lattice symmetry surrounding Eu³⁺. The consequence of the structural modification is that the color purity was significantly improved. Furthermore, the excitation energy of Ce³⁺ in the ultraviolet range was efficiently transferred to Eu³⁺ ions via the sensitizer Gd³⁺, which significantly enhanced the red emission and showed a maximized quantum efficiency of 59.7%.     

Electrical conductivity and thermal expansion of La0.8Sr0.2(Mn,Fe,Co)O3-δ perovskites

La_1−xSr_xMeO₃ (Me = Mn, Co, Fe) perovskites are used as cathodes and are also attractive materials for application as the contact layer between cathode and interconnect in solid oxide fuel cells. In this contribution, three perovskite series, La_0.8Sr_0.2Mn_1−xCo_xO_3-δ (series 1), La_0.8Sr_0.2Fe_1−xCo_xO_3-δ (series 2) and La_0.8Sr_0.2Mn_1−x/2Fe_(1−x)/2Co_xO_3-δ (series 3) with x = 0, 0.25, 0.5, 0.75 and 1 were re-investigated under identical synthesis and measurement conditions with the aim of obtaining a full overview of the quasi-ternary system La_0.8Sr_0.2MnO_3-δ–La_0.8Sr_0.2FeO_3-δ–La_0.8Sr_0.2CoO_3-δ. The distribution of the different crystallographic phases in the selected series, the DC electrical conductivity and the thermal expansion coefficients are presented.     

Effect of Eu dopant concentration in SrCe1 − xEuxO3 − δ on ambipolar conductivity

The europium dopant concentration in strontium cerate was studied to achieve maximum hydrogen permeation. In order to determine high ambipolar conductivity, total conductivity and open circuit potential measurements were performed. Among the three different compositions of Eu-doped SrCe_1 ? xEu_xO_3 ? δ (x = 0.1, 0.15 and 0.2) studied, SrCe_0.9Eu_0.1O_3 ? δ showed highest total conductivity between 600 °C and 900 °C. However, transference number measurements showed increasing electronic conductivity with increasing dopant concentration and a stronger temperature dependence for electronic conduction. Therefore, the highest ambipolar conductivity was obtained over the compositional range from SrCe_0.85Eu_0.15O_3 ? δ to SrCe_0.8Eu_0.2O_3 ? δ depending on temperature. Finally, the hydrogen permeation flux was calculated based on the ambipolar conductivity and compared with experimental results.     

Crystal structure,thermochemical stability,electrical and magnetic properties of the two-phase composites in the La0.8Sr0.2MnO3 ± δ–CeO2 system

Crystal structure, thermochemical stability, transport and magnetic properties of compositions in the (100-x) La_0.8Sr_0.2MnO_3 ± δ xCeO₂ (LSMC) system were studied. All compositions in the LSMC series containing more than 2 mol% CeO₂ were two phase and consisted of the modified perovskite constituent with rhombohedral structure (R3?c) and ceria as a secondary phase with cubic structure (Fm3?m). The presence of both Ce⁴⁺and Ce³⁺ cations in LSMC compositions was revealed by X-ray Photoelectron Spectroscopy (XPS). CeO₂ and compositions in the LSMC series showed good thermochemical stability in air and argon. However, in H₂–Ar atmosphere all LSMC compositions underwent reduction followed by decomposition. Transport and magnetic properties change in a non-linear way with the increase in the CeO₂ content. The LSMC2 composition showed enhanced electronic conductivity and magnetic characteristics. Metallic type conductivity was observed for LSMC compositions with x ≤ 36 mol% CeO₂ in a narrow temperature range of 770–900 °C. A small degree of substitution of Ce into LSM was found to change structural, magnetic and electrical properties.     

Electrochemical characterization of mixed conducting and composite SOFC cathodes

The electrochemical performance of La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (L58SCF), La_0.9Sr_1.1FeO_4−δ (LS2F) and LSM (La_0.65Sr_0.3MnO_3−δ)/LSM–YSZ (50 wt.% LSM–50 wt.% ZrO₂ (8 mol% Y₂O₃)) cathode electrodes interfaced to a double layer Ce_0.8Gd_0.2O_2−δ (CGO)/YSZ electrolyte was studied in the temperature range of 600 to 850 °C and under flow of 21% O₂/He mixture, using impedance spectroscopy and current density–overpotential measurements. The L58SCF cathode exhibited the highest electrocatalytic activity for oxygen reduction, according to the order: LS2F/CGO/YSZ ≤ LSM/LSM–YSZ/CGO/YSZ < L58SCF/CGO/YSZ.     

Structural characteristics and morphology of SmxCe1−xO2−x/2 thin films

Effect of the deposition temperature (200 and 500 °C) and composition of Sm_xCe_1−xO_2−x/2 (x = 0, 10.9–15.9 mol%) thin films prepared by electron beam physical vapor deposition (EB-PVD) and Ar⁺ ion beam assisted deposition (IBAD) combined with EB-PVD on structural characteristics and morphology/microstructure was investigated. The X-ray photoelectron spectroscopy (XPS) of the surface and electron probe microanalysis (EPMA) of the bulk of the film revealed the dominant occurrence of Ce⁴⁺ oxidation state, suggesting the presence of CeO₂ phase, which was confirmed by X-ray diffraction (XRD). The Ce³⁺ oxidation states corresponding to Ce₂O₃ phase were in minority. The XRD and scanning electron microscopy (SEM) showed the polycrystalline columnar structure and a rooftop morphology of the surface. Effects of the preparation conditions (temperature, composition, IBAD) on the lattice parameter, grain size, perfection of the columnar growth and its impact on the surface morphology are analyzed and discussed.     

Time-dependent performance of mixed-conducting SOFC cathodes

The time-dependent degradation of anode-supported Solid Oxide Fuel Cells (SOFCs) with La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes has been studied. Eight SOFCs have been tested over a period of 1000 h under different operation conditions to investigate the influence of different operation parameters on the degradation of the electrochemical performance. The cells were tested at 700 or 800 °C, at 0.3 or 0.6 A/cm² and with 21% or 5% O₂ at the cathode side and showed performance losses of 2–4% per 1000 h. While an elevated temperature and an elevated oxygen partial pressure had a negative influence on long-term performance, the current density did not have a clear effect. Material analysis of the cells showed a formation of SrZrO₃ at the interface of the Ce_0.8Gd_0.2O_2−δ interlayer and the yttria stabilized zirconia (8YSZ) electrolyte during sintering of the cathode. There are indications of a further formation of this phase during the electrochemical characterization obtained from X-ray diffraction analysis on LSCF–YSZ powder mixtures that were exposed to 800 °C for 200 h.     

Properties of doped ceria solid electrolytes in reducing atmospheres

In this work we present investigations of stability of rare earth-doped ceria electrolytes in reducing atmosphere. The effect of dopant type and dopant concentration on reducibility was studied on the basis of thermogravimetric and impedance spectroscopy measurements on materials grouped into two series: Ce_1 − xGd_xO_2 − x/2 (0 ≤ x ≤ 0.4) and Ce_0.85R_0.15O_1.925 (R = Y, Nd, Sm, Gd, and Dy). Relationship between an initial vacancy concentration introduced by the amount of dopant and the characteristic temperature of reduction was found. Much less pronounced dependence was observed for different dopants with the same concentration, which indicates that it is the dopant and vacancy concentration and not the dopant type, which is responsible for reducibility of ceria electrolytes. Impedance spectroscopy measurements allowed for calculation of changes of oxygen ions transport number during the reduction process.