Zirconia-based Solid Electrolytes Research Articles

Electrochemical Impedance Spectroscopy Study of Ceria- and Zirconia-Based Solid Electrolytes for Application Purposes in Fuel Cells and Gas Sensors.

In this study, the structural and electrochemical properties of commercial powders of the nominal compositions Ce0.8Gd0.2O1.9, Sc0.1Ce0.01Zr0.89O1.95, and Sc0.09Yb0.01Zr0.9O1.95 were investigated. The materials are prospective candidates to be used in electrochemical devices, i.e., gas sensors and fuel cells. Based on a comparison of the EIS spectra in different atmospheres (synthetic air, 3000 ppm NH3 in argon, 10% H2 in argon), the reactions on the three-phase boundaries were proposed, as well as the conduction mechanisms of the electrolytes were described. The Ce0.8Gd0.2O1.9 material is a mixed ionic-electronic conductor, which makes it suitable for anode material in fuel cells. Moreover, it exhibits an apparent and reversible response for ammonia, indicating the possibility of usage as an NH3 gas-sensing element. In zirconia-based materials, electrical conduction is realized by oxygen ion carriers. Among them, the most promising from an applicative point of view seems to be Sc0.09Yb0.01Zr0.9O1.95, showing a high, reversible reaction with hydrogen.

Highly Conducting Sc and Y co-doped Zirconia Solid Electrolyte Thin Films Prepared via Pulsed Laser Deposition for Solid Oxide Electrochemical Cells Applications

Scandia and yttria-stabilized zirconia (ScYSZ) have been widely used as electrolyte materials for solid oxide electrochemical cells (SOEC) applications due to its high ionic conductivity and stable mechanical and thermal properties. In this study, ScYSZ thin films were fabricated via pulsed laser deposition (PLD) method at substrate deposition temperature (Ts ) of 800 °C with varying oxygen oxygen partial pressures (). The as-deposited ScYSZ thin films were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and electrochemical impedance spectroscopy (EIS). XRD analysis revealed that all fabricated films, even with varying , have a polycrystalline cubic-fluorite structure. Morphological FESEM images revealed a good-quality dense ScYSZ electrolyte thin film with no visible cracks. Notably, a high total conductivity thin film of about 2.8x10-1 S/cm at 700 °C and low activation energy (Ea ) of 0.78 eV, from 500°Cto 700 °C, were achieved. Interestingly, the PLD-prepared ScYSZ dense solid electrolyte showed a higher total conductivity than the reported conductivities of zirconia-based solid oxide electrolytes.

Highly Conductive Cerium- and Neodymium-Doped Barium Zirconate Perovskites for Protonic Ceramic Fuel Cells

The rare-earth-doped zirconia-based solid electrolytes have gained significant interest in protonic ceramic fuel cell (PCFC) applications due to their high ionic conductivity. However, these solid electrolytes are susceptible to low conductivity and chemical stability at low operating temperatures, which are of interest in commercializing ceramic fuel cells. Thus, tailoring the structural properties of these electrolytes towards gaining high ionic conductivity at low/intermediate temperatures is crucial. In this study, Ce (cerium) and Nd (neodymium) co-doped barium zirconate perovskites, BaZr(0.80-x-y)CexNdyY0.10Yb0.10O3-δ (BZCNYYO) of various doping fractions (x, y: 0, 0.5, 0.10, 0.15), were synthesized (by the Pechini method) to systematically analyze their structural and conductivity properties. The X-ray diffraction patterns showed a significant lattice strain, and the stress inferences for each co-doped BZCNYYO sample were compared with Nd-cation-free reference samples, BaZrO3 and BaZr(0.80-x-y-z)CexYyYbzO3-δ (x: 0, 0.70; y: 0.20, 0.10; z: 0, 0.10). The comparative impedance investigation at low-to-intermediate temperatures (300–700 °C) showed that BaZr0.50Ce0.15Nd0.15Y0.10Yb0.10O3-δ offers the highest lattice strain and stress characteristics with an ionic conductivity (σ) of 0.381 mScm−1 at 500 °C and activation energy (Ea) of 0.47 eV. In addition, this σ value was comparable to the best reference sample BaZr0.10Ce0.70Y0.10Yb0.10O3-δ (0.404 mScm−1) at 500 °C, and it outperformed all the reference samples when the set temperature condition was ≥600 °C. The result of this study suggests that Ce- and Nd-doped BZCNYYO solid electrolytes will be a specific choice of interest for developing intermediate-temperature PCFC applications with high ionic conductivity.

Ceria-based solid electrolyte exhibits superior mechanical and electric properties compared to zirconia-based solid electrolyte

The mechanical strength and electrical characteristics of 10 mol% gadolinium (Gd)-doped ceria (10GDC) and gadolinium-samarium (Gd–Sm) co-doped ceria ceramics treated by contact reduction were investigated. The observed increased strength of 10GDC was similar to the strength previously reported for ceria with and without Sm doping. X-ray photoelectron spectroscopy revealed that only the surface was reduced in the strengthened material. In addition, the strengthened ceramic displayed improved hardness at low test loads, which suggested the formation of a surface compression layer. The reducing conditions did not affect the electrical conductivity of the 10GDC material. Co-doping with 10 mol% each of Gd and Sm improved strength and conductivity compared with 10GDC and 20GDC. When the co-doped material was reduced, the electric conductivity remained identical; however, the mechanical strength improved, exceeding the strength of zirconia.

Surface structures of ZrO2 films on Rh(111): From two layers to bulk termination

We have studied zirconia films on a Rh(111) substrate with thicknesses in the range of 2–10 monolayers (ML) using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Zirconia was deposited using a UHV-compatible sputter source, resulting in layer-by-layer growth and good uniformity of the films. For thicknesses of 2–4 ML, a layer-dependent influence of the substrate on the structure of the thin films is observed. Above this thickness, films show a (2 × 1) or a distorted (2 × 2) surface structure with respect to cubic ZrO2(111); these structures correspond to tetragonal and monoclinic zirconia, respectively. The tetragonal phase occurs for annealing temperatures of up to 730 °C; transformation to the thermodynamically stable monoclinic phase occurs after annealing at 850 °C or above. High-temperature annealing also breaks up the films and exposes the Rh(111) substrate. We argue that the tetragonal films are stabilized by the interface to the substrate and possibly oxygen deficiency, while the monoclinic films are only weakly defective and show band bending at defects and grain boundaries. This observation is in agreement with positive charge being responsible for the grain-boundary blocking effect in zirconia-based solid electrolytes. Our work introduces the tetragonal and monoclinic 5 ML-thick ZrO2 films on Rh(111) as a well-suited model system for surface-science studies on ZrO2, as they do not exhibit the charging problems of thicker films or the bulk material and show better homogeneity and stability than the previously-studied ZrO2/Pt(111) system.

Stabilized Zirconia-Based Nanostructured Powders for Solid-Oxide Fuel Cells

High-purity uniform powders of zirconia-based solid electrolytes stabilized with yttria (4 and 8 mol %) are synthesized by co-precipitation with subsequent annealing at different temperatures. The obtained powders were studied using X-ray analysis and transmission electron microscopy; the specific surface area was measured by nitrogen adsorption. The stabilized zirconia powder sintering was studied over temperature range from 1000 to 1600°C. The ionic conductivity of samples containing 8 mol % of yttria was 0.06–0.07 S/сm, that is comparable with that obtained with commercial solid electrolytes.

A review of zirconia-based solid electrolytes

Zirconia-based electrolyte is considered to be the most reliable candidate as oxide-ion electrolyte for oxygen sensor, oxygen pump, and solid-oxide fuel cell. The electrical property and stability of zirconia-based electrolyte depend strongly on dopant type and concentration. In this review, phase diagrams, electrical properties, and the latest developments of zirconia-based electrolyte with different dopant are discussed. The methods used to increase oxide-ion conductivity and decrease the electronic conductivity of stabilized zirconia are also discussed.

Zirconia-based solid electrolyte obtained by tape casting

The possibility of using tape casting to produce solid electrolyte films with a thickness of 100–300 μm from zirconia stabilized with oxides of scandium and cerium is considered. A method of slip casting onto a moving tape is used to produce rather thin ceramic sheets with a thickness of less than 1 mm. An organic slip based on azeotropic mixture of isopropyl alcohol and methyl ethyl ketone is used in the present work. The effect of solvents on the properties of the obtained film and sintered ceramic is analyzed. Dibutylphthalate, polyethylene glycol, and benzyl butylphthalate are used as plasticizers. The effectiveness of these plasticizers is based on the possibility of producing a slip with a viscosity of 2000–4000 mPa s by using them. Polyvinyl butyral appears to be the most optimal temporary binder, because it is satisfactorily removed in a continuous mode of annealing of plated ceramic blanks. The maximum density of ceramic plates is achieved at the annealing temperature of 1500°C. It is shown that the working range of the obtained solid electrolyte is 800–850°C, as the maximum ionic conductivity (more than 0.15 Ω−1 cm−1) is registered in it.

Codoping of scandium-containing zirconia-based solid electrolytes with iron, cerium, and copper oxides

Codoping with iron, cerium, and copper oxides has been proposed as a means of improving the performance of scandium-containing zirconia-based solid electrolytes. We have examined three procedures for the synthesis of the (ZrO2)0.825(CeO2)0.07(Sc2O3)0.07(Fe2O3)0.035 solid solution through precipitation from solution. It has been shown that the highest oxygen ion conductivity is ensured by the synthesis procedure that includes the mechanochemical activation of presynthesized scandium ferrate (Sc1.33Fe0.67O3) with cerium and zirconium hydroxides and a solution containing 0.5 mol % Cu.

Effect of Basicity on Electroreduction with Controlled Oxygen Flow of Molten Slag Containing FeO

This paper reports on electroreduction with controlled oxygen flow (COF) for extraction of iron along with oxygen by-product from molten slags containing iron oxide at 1723 K. An electrolytic cell with COF was constructed by an iridium wire cathode and a porous platinum anode sintered on a one-end-closed magnesia-stabilized zirconia-based solid electrolyte tube. Effect of basicity of SiO2–CaO–Al2O3–MgO–FeO slag on electroreduction behavior of FeO was investigated by means of the linear sweep and the potentiostatic electrolysis. The possibility of the zirconia membrane as conductive noncorrosive anode material was also discussed. The results indicate that both cathode alloying and increasing slag basicity are helpful to the FeO electrolytic reduction in the molten slag. With the iridium wire as a cathode and an applied voltage at 2.5 V, higher slag basicity results in higher electrolysis rate and higher reduction ratio of the FeO; however, silicon is also precipitated during electrolysis. The applied voltage can cause the increased porosity in the zirconia membrane. The corrosion of the molten slag to the zirconia membrane during electrolysis is evidently aggravated when the slag basicity reaches 0.8. In order to minimize the corrosion of the molten slag to the zirconia membrane during electrolysis, the slag basicity of 0.6 is relatively advisable for extraction of iron under conditions of experimentation.

Light emission during electric field-assisted sintering of electroceramics

Electric field-assisted sintering has been shown to occur in ion, electron conducting as well as in insulating polycrystalline ceramics. Considerable linear shrinkage with microstructure control at temperatures lower than the conventionally utilized has been attained mainly in oxide ion conducting zirconia-based solid electrolytes. Some mechanisms, based on localized Joule heating and enhancement of defects diffusion, have been set forth to explain the sudden volume reduction in those ceramics. The results here reported contribute to the understanding of what happens during electric field-assisted sintering, by evidencing the occurrence of light emission simultaneously to the quasi instantaneous specimen shrinkage. An experimental setup with an optical fiber inserted in a dilatometer furnace allowed for light emission detection simultaneously to the electric current pulse in zirconia-yttria and also in tin dioxide specimens upon electric field-assisted sintering.

Ionic Conductivity of Zirconia-Scandia-Dysprosia Solid Electrolyte

Zirconia-based solid electrolytes are the preferred materials for high-temperature applications due to the combination of their electrical and mechanical properties. Zirconia-8-11 mol% scandia possess higher values of ionic conductivity at temperatures above 600 °C than other zirconia-based electrolytes [1]. At low temperatures the ionic conductivity of this system decreases due to the transition from the cubic fluorite-type phase to the ordered Sc2Zr7O17(β) rhombohedral phase [2].It has been reported that the cubic phase can be stabilized at room temperature by adding minor amounts of e.g. Y2O3, CeO2 and Gd2O3[3].In this work, small amounts of Dy2O3 was added to zirconia-10 mol% scandia to evaluate the effect of the additive on the phase composition and the ionic conductivity.Zirconia-10 mol% scandia (DKKK) and Dy2O3(Alfa Aesar) were the starting materials. Dysprosia was added in the 1-2 mol% contents to the zirconia-scandia solid electrolyte. Cylindrical pellets were prepared by pressing followed by sintering at 1500 °C. Sintered pellets were characterized by X-ray diffraction, apparent density measurements, scanning electron microscopy observations and ionic conductivity evaluation by impedance spectroscopy.Table 1 summarizes the relative densities, ρR, of sintered samples with different additive contents, x. Table 1. Dy2O3 content (x, in mol%) and relative density (ρR) of sintered samples.x (mol%)ρR (%)195.6 ± 0.51.595.5 ± 0.1292.3 ± 0.4 The sintered density decreases with increasing Dy2O3 content. The relative density is higher than 92% for sintering experiments at 1500 °C for 5 h for all compositions.The X-ray diffraction patterns of sintered pellets show that for the additive content of 1 mol% the cubic phase was partially stabilized at room temperature. A small amount of the rhombohedral phase coexists for this additive content. For dysprosia additions of at least 2 mol% the rhombohedral phase was suppressed, as shown in Fig. 1. The X-ray diffraction pattern of Fig. 1 corresponds to that of cubic zirconia-based solid electrolytes.The ionic conductivity was determined by impedance spectroscopy in the 500-800 °C range. The Arrhenius plot of the total ionic conductivity is depicted in Fig. 2 for the sample containing 2 mol% dysprosia.The calculated apparent activation energy is 1.2 eV, similar to that of other zirconia-scandia systems.Stabilization of the cubic phase was successfully accomplished by small additions of dysprosia to zirconia-10 mol% scandia. The magnitude of the ionic conductivity value decreases with increasing additive content. The activation energy for oxide ion conduction is similar to that of other zirconia-scandia systems containing a second additive.

Structural parameters and oxygen ion conductivity of Y2O3–ZrO2 and MgO–ZrO2 at high temperature

Abstract The oxygen ion conductivity of zirconia-based solid electrolytes doped with 8 mol% Y2O3–ZrO2 (YSZ) and 9 mol% MgO–ZrO2 (Mg-PSZ) at high temperature was investigated in terms of their thermal behavior and structural changes. At room temperature, YSZ showed a single phase with a fluorite cubic structure, whereas Mg-PSZ had a mixture of cubic, tetragonal and some monoclinic phases. YSZ exhibited higher ionic conductivity than Mg-PSZ at temperatures from 600 °C to 1250 °C because of the existence of the single cubic structure and low activation energy. A considerable increase in the conductivity with increasing temperature was observed in Mg-PSZ, which showed higher ionic conductivity than YSZ within the higher temperature range of 1300–1500 °C. A monoclinic-to-tetragonal phase transformation was found in Mg-PSZ and the lattice parameter of the cubic phase increased at 1200 °C. The phase transformation and the large lattice free volume contributed to the significant enhancement of the ionic conductivity of Mg-PSZ at high temperatures.

Glycothermal synthesis of 3 mol% yttria stabilized tetragonal ZrO2 nano powders at low temperature without mineralizers

Tetragonal 3YSZ (3mol% yttria-stabilized zirconia) nanoparticles were synthesized at temperature as low as 200°C through a glycothermal reaction using coprecipitated amorphous hydrous gel of ZrCl4 and YCl3·6H2O as precursors and 1,4-butanediol as solvent. XRD and TEM data support that glycothermal processing method provides a simple low temperature route for producing highly crystallized tetragonal ZrO2 nanoparticles without mineralizers, which could also be extended to other system. Raman analysis also indicated that 3YSZ nanoparticles synthesized in glycothermal condition had tetragonal phase without any trace of monoclinic phase. The as-prepared 3YSZ nanoparticles have spherical morphology with an average crystal size of 7–10nm and agglomerated into spherical shape with a diameter of about 100–200nm. The tetragonal phase remained stable with heat treatment when the calcinations of the as-synthesized powder were carried out from 400 to 1400°C. The combination of uniform glycothemally-synthesized tetragonal 3YSZ nano-powder and two-step sintering can suppress grain boundary migration but not affect grain boundary diffusion without long annealing period. Fine-grained microstructure with 100–500nm was obtained without abnormal growth although the pellet was fully dense and had 98.1% of the theoretical density at 1400°C. The pellets sintered at 1350 and 1400°C exhibit activation energies for conduction in range 0.84eV, which are close to the values usually found for zirconia-based solid electrolytes.

Electrical conductivity of zirconia-based solid electrolyte with submicron grain size

The electrical conductivity of the Zr(Y)O2 electro� lyte ceramics with the highest density and the grain size from 90 to 800 nm was studied by impedance spectroscopy. The ceramics was obtained by magnetic pulse compacting of weakly aggregated nanopowders and subsequent sintering. The grain boundary con� ductivity was shown to depend on the grain size. The grain boundary resistance was found to increase with an increase in the grain size in the range of 90- 800 nm. In the grain size range of 1-18 µm, according to the data obtained previously, the grain boundary resistance in the ceramics of this composition mono� tonically decreased with an increase in the grain size. This indicates that a maximum exists in the size dependence of the grainboundary resistance. One of the factors that influence the efficiency of devices with solid oxide electrolytes are the losses due to the voltage drop in the electrolyte. In this connec� tion, the technological problem of decreasing the thickness of the electrolyte layer becomes increasingly urgent. However, in the framework of the ceramic method, the thickness of membranes cannot be decreased without the corresponding decrease in the grain size. This dictates the practical significance of the present work. Attempts to obtain nanosized oxide materials with a facecentered cubic (fcc) lattice of the fluorite type are known in the literature. For ceria� based materials, it was shown (1) that these microand nanosized ceramic samples differ in both the conduc� tivity values and the character of the temperature and oxygen pressure dependences of the conductivity. The models were developed (2, 3) to describe the depen� dences observed by regarding the grain boundaries in the ceramic as a second phase. This concept is obvious for describing heterogeneous materials, for example, a mixture of a cationconducting solid electrolyte and an insulator, as it was done in the classic works of J. Maier (see, for example, (3)), but should be eluci� dated in the general case of homogeneous ceramics. On the other hand, the presence of the Ce 3+ /Ce 4+ redox system complicates the interpretation of the phenomena observed. In this connection, it is of inter� est to study the zirconiabased ceramics. Dense nano� sized Zr(Y)O2 ceramic samples with fcc lattice have not been previously obtained. It turned out to be a very difficult technical task. To succeed, we used magnetic pulse compaction of weakly agglomerated nanopow� ders obtained by laser evaporation. This method allows one to reach pressures up to 15-17 t/cm 2 .

Heterogeneous Zirconia-based Solid Electrolytes Obtained by Sintering TZP+YSZ Powder Mixtures

Powder mixtures of 3%Y2O3 (3Y) and 8%Y2O3 (8Y) were used to obtain heterogeneous solid electrolytes with good sinterability and mostly fine grain sizes, without significantly undue effects on ionic conductivity. Samples with 8Y:3Y{less than or equal to}1:1 retain submicrometer grain sizes, with very rare grains above 1 {less than or equal to}m. Impedance spectroscopy was used to de-convolute the bulk and grain boundary contributions, and to analyze their dependence on the fraction of 3Y. Bulk conductivity and its activation energy vary gradually with increasing 8Y:3Y ratio. The lower grain boundary conductivity of homogeneous 3Y can be ascribed to a combination of finer grain sizes and also lower specific grain boundary conductivity. However, the grain boundary conductivity differences of heterogeneous samples obtained from 3Y+8Y mixtures can be mainly ascribed to differences in grain size. Specific grain boundary conductivity results of these heterogeneous solid electrolytes are similar to those obtained for homogeneous 8Y.

Molecular dynamics simulation of the complex dopant effect on the super-ionic conduction and microstructure of zirconia-based solid electrolytes

The complex dopant effect on the super-ionic conduction and structural stability of zirconia-based solid electrolyte for solid oxide fuel cell (SOFC) applications was investigated using the molecular dynamics (MD) technique. Various components of Sc 2O 3( x) –Y 2O 3(1− x) were added to the cubic zirconia cell to build a scandia–yttria-stabilized zirconia (Sc–Y–SZ) model. The oxygen ion diffusion mechanism and ionic conductivity obtained by MD simulation were examined to gain insight into how the performance was improved by adding different Sc 2O 3 concentrations. The radial distribution function (RDF) of the O–O pair was determined to analyze the oxygen ion mobility using microstructure analysis. The mean-square displacement (MSD) of the cations and RDF of the Zr–Zr pair were investigated to determine how the structural stability was affected by the concentration of doped Sc 2O 3. The simulated results were in agreement with the experimental data reported in the literature, suggesting that MD simulation is a feasible technique for use in the material design and development of SOFC applications.

Oxygen‐Ion Transport in a Dual‐Phase Scandia–Yttria‐Stabilized Zirconia Solid Electrolyte: A Molecular Dynamics Simulation

A molecular dynamics (MD) simulation is adopted to investigate oxygen-ion diffusion mechanisms in dual-phase (cubic/monoclinic phase) zirconia-based solid electrolytes. XRD analysis is performed to validate the phase structure composition after an MD duration of 500 ps. The radial distribution function of the ion pairs is used to analyze the microstructure and mobility of the oxygen ions inside the electrolyte. The mean-squared displacement, displacement distribution, and moving path of oxygen ions indicate that ion mobility increases with Sc(2)O(3) content. Furthermore, the mobility of oxygen ions is relatively lower in the dual-phase solid-electrolyte models than in the pure cubic models. This is due to their intrinsically unfavorable configuration for ion penetration. Comparison of the diffusion coefficient and ionic conductivity of the oxygen ions in various phase compositions shows an obvious effect of phase composition complexity in the systems with high Sc(2)O(3) concentration. The introduction of the dual-phase composition concept for the MD simulation of oxygen-ion conduction behavior is a promising approach to obtaining a more realistic understanding of ion transport in a solid electrolyte.

Physical properties of alumina/yttria-stabilized zirconia composites with improved microstructure

Cubic stabilized zirconia is the preferred material for application as solid electrolyte in solid oxide fuel cells. However, this material has low fracture toughness, which can lead to formation of cracks during long-term operation. Moreover, increase of mechanical as well as electrical properties would be useful for cost-effectiveness of this type of device. In this context, addition of alumina to zirconia-based solid electrolyte can be an interesting option to accomplish that purpose. In this work, ceramic composites containing various amounts of alumina in a 9 mol% yttria-stabilized zirconia matrix were synthesized by the coprecipitation route using a low-silica zirconium precursor. Characterization techniques included scanning electron microscopy, X-ray diffraction, Vickers hardness and impedance spectroscopy. As a consequence of optimization of the synthesis route a homogeneous dispersion of the additive along with good densification was obtained. Although alumina addition to stabilized zirconia exerts a deleterious effect on the electrical conductivity, it improves the Vickers hardness and fracture toughness of the composite materials.

Heterogeneous Zirconia-based Solid Electrolytes Obtained by Sintering TZP+YSZ Powder Mixtures

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