Temporal stability of oxygen-ion conductivity in 1Nb2O5-10Sc2O3-89ZrO2 (original) (raw)
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Phase stability and ionic conductivity of cubic xNb2O5-(11-x)Sc2O3-ZrO2 (0 ≤ x ≤4)
Journal of Alloys and Compounds, 2017
The present work looks at the effect of Nb 2 O 5 co-doping on the structure and conductivity of 11 mol.% Sc 2 O 3 stabilized ZrO 2 solid electrolyte for intermediate temperature solid oxide fuel cells. XPS analysis confirmed that niobium exists in 5+ valence state in the sintered stabilized ZrO 2 samples. SEM study performed on sintered samples revealed that the addition of Nb 2 O 5 not only assists in the densification of Sc 2 O 3-ZrO 2 but also leads to exaggerated grain growth. Both XRD and Raman analysis confirmed that addition of up to 1 mol.% of Nb 2 O 5 suppresses the formation of low-conductivity rhombohedral β-phase and leads to the stabilization of cubic phase. For compositions with > 1 mol.% Nb 2 O 5 , a mixture of tetragonal and monoclinic phases was identified. Impedance spectroscopy showed that the total ionic conductivity increases significantly on co-doping with 1 mol.% Nb 2 O 5. This increase is attributed to the enhanced sinterability and decrease in space-charge potential of Nb 2 O 5 co-doped samples. While > 1 mol.% Nb 2 O 5 compositions have low conductivity due to the formation of low conductivity secondary phases.
Scientific Reports
In this study, the Ho-substituted BaZrO 3 electrolyte ceramics (BaZr 1-x Ho x o 3-δ , 0.05 ≤ x ≤ 0.20) were synthesized through a low-cost flash pyrolysis process followed by conventional sintering. The effects of Ho-substitution in BaZrO 3 studied in terms of the structural phase relationship, microstructure and electrical conductivity to substantiate augmented total electrical conductivity for intermediate temperature solid oxide fuel cells (IT-SOFCs). The Rietveld refined X-ray diffraction (XRD) patterns revealed that pure phase with Pm m 3 space group symmetry of cubic crystal system as originated in all samples sintered at 1600 °C for 8 h. The Raman spectroscopic investigations also approved that Ho incorporation in BaZro 3 ceramics. Field Emission Scanning Microscopic (FESEM) study informed a mixture of fine and coarse grains in the fracture surface of Ho-substituted BaZrO 3 sintered samples. The relative density and average grain size of samples were observed to decrease as per the addition of Hosubstitution in BaZro 3 ceramics. The electrical conductivity study was accomplished by Electrical Impedance Spectroscopy (EIS) under 3% humidified O 2 atmosphere from 300 to 800 °C. Furthermore, the total electrical conductivity of BaZr 0.8 Ho 0.2 o 3-δ ceramic was found to be 5.8 × 10 −3 S-cm −1 at 600 °C under 3% humidified atmosphere, which may be a promising electrolyte for IT-SOFCs. Recently, the proton conductive oxide ceramics have fascinated worldwide attention due to widespread applications in intermediate temperature solid oxide fuel cells (IT-SOFCs), hydrogen separation and electrolysis of steam, etc. In this context, the rare-earth cerates and zirconates with the perovskite-type A(II)B(IV)O 3 crystallographic structure are the two foremost families of proton-conducting oxides for electrochemical applications 1-4. Generally, in these categories of oxide materials, oxygen vacancies are increased by replacement of tetravalent cation B(IV) by trivalent cation M(III) as given in the Eq. (1) using Kröger-Vink notation.
International Journal of Hydrogen Energy, 2018
Development of high proton conducting, chemically stable electrolyte for solid oxide fuel cell application still remains as a major challenge. In this work, yttrium (0, 5, 10, 15 and 20 mol%) doped barium zirconate synthesised by hydrothermal assisted coprecipitation exhibited highly crystalline cubic perovskite. The results demonstrate that the proton conductivity is higher than oxygen ion conductivity measured in the temperature range of 200e600 C. The 20 mol% Y doped BaZrO 3 exhibited higher protonic conductivity (6.1 mScm À1) with an activation energy 0.64 eV under the reducing atmosphere. The Mott eSchottky analysis carried out in hydrogen atmosphere at 200 C revealed that the barrier height of doped BaZrO 3 reduced from 0.6 to 0.2 V. The Schottky depletion layer width also decreased from 4 to 2 nm with the increase in yttrium concentration and the boiling water test showed good phase stability. Our study highlights the critical role of space charge in the grain boundary and its suppression with the increase in dopant concentration. The results demonstrate that Y doped BaZrO 3 sintered at low temperature is a promising candidate as the electrolyte material for the intermediate temperature proton conducting solid oxide fuel cells.
Doping effect and vacancy formation on ionic conductivity of zirconia ceramics
Journal of Physics and Chemistry of Solids, 2008
Doping effect and vacancy formation on ionic conductivity of ZrO 2 ceramics doped with RENbO 4 (RE ¼ Yb, Er, Y, Dy) were investigated using X-ray diffractometry, scanning electron microscope and corresponding ionic conductivity were evaluated using impedance spectroscopy in this work. The results show that defect distribution can be correlated with the phase transformation behavior modified by ionic radius of dopants. The total conductivity of 5 mol% RENbO 4 -doped ZrO 2 (3Y) comprises the intragrain and grain boundary (GB) conductivity. The intragrain conductivity of 5 mol% RENbO 4 -doped ZrO 2 (3Y) are lower than 3 mol% Y 2 O 3 -doped ZrO 2 (3Y-TZP) and 8 mol% Y 2 O 3 -doped ZrO 2 (8YSZ). The additions of Nb 2 O 5 to ZrO 2 (3Y) increase average lattice binding energy and activation energy, and the amount of oxygen vacancies was decreased. The average radius of oxygen vacancies of 5 mol% RENbO 4doped zirconia (3Y) were smaller than that of 8YSZ identified using hard-sphere model. The results imply that a specific doping content in zirconia which contributes a maximum content of non-interfering oxygen vacancies, the average radius of doping ions close to that of Zr 4+ and average binding energy as smaller as possible help obtain the highest conductivity of zirconia. To acquire an appropriate operation condition in the application of solid oxide fuel cell, electrical properties, phase transformation behavior and related mechanical properties need to be compromised. r
Solid State Ionics, 1998
In search for better ionically conducting ceramics for oxygen separators, fuel cells and sensors, the electrical conductivity and microstructure of the 9 mol% (Sc O-Y O)-ZrO system with varying Sc O / Y O ratios has been investigated in 2 3 2 3 2 2 3 2 3 detail with XRD, SEM, TEM and conductivity measurements as a function of temperature. The stability of electrolyte compositions was studied by continuously monitoring conductivity as a function of time at 850 and 10008C. Impedance spectroscopy was employed for determining the contribution of the grain boundary resistivity. The role of alumina additions to selected Sc O-Y O-ZrO compositions was studied as alumina is known to reduce the grain boundary resistivity by 2 3 2 3 2 scavenging silica impurities and enhance mechanical properties in zirconia-based systems. Al O-containing compositions 2 3 show much higher conductivity degradation compared with alumina-free materials. This behaviour has been investigated in more detail with XRD and TEM analysis.
2010
The system Y 2 O 3 -Sc 2 O 3 -ZrO 2 has been extensively studied in the last decade, with the purpose of using it as an electrolyte for solid oxide fuel cells (SOFCs). Scandia stabilized zirconia (SSZ) materials have good ionic conductivity, and yttria, when introduced, stabilizes the SSZ against phase transformation. As cerium is a large ion, it is expected to stabilize the cubic fluorite phase and enhance the conductivity of the scandia zirconia system. The electrical properties of system (Ce x Y 0.2 − x Sc 0.6 )Zr 3.2 O 8 − δ were studied, observing how conductivity changes when cerium is introduced and the amount of yttrium is decreased. Compositions were produced by an innovative sol-gel combustion method in an attempt to obtain a compositionally homogeneous, dense material. This material has the potential to be used as an electrolyte, at intermediate temperatures, for SOFCs. Conductivity in the system (Ce x Y 0.2 − x Sc 0.6 )Zr 3.2 O 8 − δ , has been investigated as a function of partial pressure of oxygen down to 10 − 24 bars, at 700°C. Samples with Ceria content have better conductivity at higher pO 2 values. When they are subjected to lower partial pressures of oxygen, there is a clear drop in the conductivity. This drop in the conductivity could be a consequence of the clustering of vacancies, resulting in a decrease of ionic conductivity, as the number of vacancies increases with ceria content. For the samples tested in this system, there is no evidence of significant electronic conductivity. The temperature dependence of conductivity was determined at ambient pressure from 300 to 800°C. From the Arrhenius plot a phase transition is observed to occur between 550 and 600°C. Composition does not have a very significant effect on conductivity values for each temperature. At low temperatures, the dominant contribution to conductivity is due to the poor grain boundary conductivity.
Characteristics of cathode materials for solid oxide fuel cells with mixed conductivity
IOP Conference Series: Materials Science and Engineering
In this article, we define promising materials for creating the structure of a nonporous cathode-electrolyte. The synthesis of initial powders by the method of self-propagating high-temperature synthesis is studied and described. The specific surface area, morphology and phase composition of powders were investigated.