Microstructure-property relations in composite yttria-substituted zirconia solid electrolytes

A study of composite 8 mol% yttria stabilized zirconia (8YSZ) and 3 mol% yttria tetragonal zirconia polycrystal (3YTZP) solid electrolytes sintered under isothermal and two-step sintering cycles is reported. The nominal phase composition is retained for composites with up to 25 wt.% 3YTZP. These composites show a combination of beneficial effects with respect to pure 8YSZ, including slight improvement in sinterability, gains in bulk and grain boundary conductivity and also enhanced fracture toughness. Impedance spectroscopy revealed an enhancement of the specific grain boundary conductivity for samples with finer grain sizes, attained by increasing the fraction of 3YTZP or by hindering grain growth under two-step sintering cycles. This effect is rationalized in terms of a decrease of the grain boundary space-charge potential. The conductivity gains decrease with increasing temperature, but even at 700 °C the total ionic conductivity of ceramics with 25 wt.% 3YTZP is still higher than that of pure 8YSZ, whereas at 900 °C there is a performance loss of less than 10%. The improved mechanical and electrical performance in the intermediate temperature range represents an important advantage of the heterostructured electrolytes for low/intermediate temperature SOFC operation.     

Structural and electrical properties of ZnO-doped 8 mol% yttria-stabilized zirconia

8 mol% Yttria-stabilized zirconia (8YSZ) powder was prepared by coprecipitation. ZnO (0.5, 1.0, 2.0, 5.0, 10.0 wt.%) was added to the YSZ powder through a mechanical mixing method. The densification , microstructure and electrical properties of the YSZ ceramics sintered at 1300 °C for 2 h, were investigated. It was found that the small addition of ZnO was effective in reducing the sintering temperature and promoting the densification rate of the ceramics. The 5.0 wt.% ZnO-doped YSZ has ∼ 96% relative density, as compared to ∼ 89% relative density for the undoped sample. The total conductivity of 8YSZ was evidently increased by doping small amount of ZnO. For the 0.5 wt.% doped sample, the total conductivity of 2.89 × 10^− 2 Ω^− 1 cm^− 1 and an increase of 120% in conductivity were observed at 800 °C, as compared to that of the undoped one. We also found that the grain boundary (GB) conductivity could be improved by small addition of ZnO. At intermediate temperature (∼ 300 °C), the maximum enhancement of GB conductivity was observed with 5.0 wt% ZnO dopant. Finally, the volume percentage of GB in the ceramics was estimated by the brick layer model. The possible mechanism related to the improved GB conduction of the YSZ due to the ZnO additions was discussed.     

Electrical conductivity of thick film YSZ

When yttria-stabilized zirconia (YSZ) electrolyte is coated and co-sintered on top of Ni–YSZ anode support, the measured conductivities of YSZ thick films (10–30 μm thick) are often lower than that of bulk YSZ. In this study, we found the observation by fabricating free-standing YSZ thick films and measuring and comparing in-plain and across-plain conductivities. The in-plane conductivity of free-standing YSZ film matched very well with the conductivity of mm-thick bulk sample. It was further shown that the conductivity decrease can be minimized by using better electrode morphology.Another factor that decreases the film conductivity was identified when the thick film was reduced. The conductivity decrease, ∼26% after reduction for 1h in humidified hydrogen gas, was due to Ni-doping into YSZ during sintering process.In order to minimize the conductivity drop of thick film YSZ during SOFC (solid oxide fuel cell) operation, an intermediate layer may be used between YSZ and anode support to prevent Ni-doping during co-sintering process in addition to the well-designed electrode morphology.     

Effect of sintering temperature on the elemental diffusion and electrical conductivity of SrTiO<Subscript>3</Subscript>/YSZ composite ceramic

The 50 vol% SrTiO₃/yttria-stabilized zirconia (YSZ) composite ceramic was prepared through powder sintering route in 1400～1500 °C. Only the cubic YSZ and SrTiO₃ phases are detected in all the sintered ceramics, and the typical XRD peak positions of both phases have varied dramatically. The grain sizes and relative densities of all specimens increase evidently with the sintering temperature. The width of the SrTiO₃/YSZ interfacial region increases from 100.4 to 468.8 nm as the sintering temperature rises from 1400 to 1500 °C. The total electrical conductivities of the sample sintered at 1500 °C are remarkably higher than those at 1400 and 1450 °C, while the ion transference numbers drop from 0.837 to 0.731 with sintering temperature from 1400 to 1500 °C. The variations in the electrical properties above can be interpreted based on the effects of sintering temperature on the elemental diffusions during the sintering process.     

Processing and microstructure characterization of ceria-doped yttria-stabilized zirconia powder and ceramics

Cubic stabilized zirconia is a promising material as target for the transmutation of actinides in nuclear reactors. In this concept, actinides are incorporated into an inert matrix (zirconia) to form a solid solution. The present work is focused on the synthesis of 8 mol% yttria-stabilized zirconia doped with 10 mol% ceria (10Ce–8YSZ) in which Ce is used to simulate the incorporation of tetravalent actinides. A wet chemical route powder synthesis method was applied to make homogeneous single-phase ceria-doped yttria-stabilized zirconia ceramics. The synthesis as well as the characterization of samples by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray emission Spectrometry (EDXS) and Rutherford Backscattering Spectroscopy (RBS) is presented.     

Different conduction behaviors of grain boundaries in SiO2-containing 8YSZ and CGO20 electrolytes

Both doped zirconia and ceria have been widely recognized as promising electrolytes in solid oxide fuel cells (SOFC). Total conductivity is an important parameter to evaluate solid electrolytes. It is well know that the contribution to the total conductivity by grain boundaries is especially pronounced for SiO₂-contaminated electrolytes. In this study, we report on the different conduction behaviors of grain boundaries (GB) found in SiO₂-containing (impure) 8YSZ (8 mol% Y₂O₃-doped ZrO₂) and CGO20 (10 mol% Gd₂O₃-doped CeO₂) ceramics. In the grain size range (∼ 0.5–10 μm) studied, the GB conductivity of impure CGO20 ceramics constantly decreases with increasing grain size, in contrast to that observed in impure 8YSZ electrolytes whose GB conductivity increases almost linearly with grain size. It is also found that the variation in GB conductivity versus grain size is different from case to case, depending on the sintering/annealing conditions used to fabricate the ceramics. Two mechanisms were proposed to explain the GB behaviors of the impure 8YSZ and CGO20 ceramics. For doped ceria, the GB phases are supposed to be inert, which do not react with or dissolve into the matrix. Increasing sintering temperature leads to not only grain growth but also change in viscosity and wetting nature of the GB phases. These two factors promote further propagation of the GB phases along the grain boundaries, leading to an increased GB coverage fraction. For doped zirconia, however, the major factor dominating the GB conduction is the further dissolution of SiO₂ into zirconia lattice as a result of increase in sintering temperature or/and time. In addition, we will also evaluate and discuss the validities of the three models that are widely used to analyze the GB conduction in solid electrolytes.     

Influence of sintering conditions on the electrical properties of 10% ZrO2–10% Y2O3–CeO2 (mol%)

Yttria–zirconia doped ceria, 10% ZrO₂–10% Y₂O₃–CeO₂ (mol%) (CZY) and 0.5 mol% alumina-doped CZY (CZYA), prepared through oxide mixture process, were sintered by isothermal sintering (IS) and two-step sintering (TSS) having as variable the temperature and soaking time. The electrical conductivity of sintered samples was investigated in the 250 to 600 °C temperature range by impedance spectroscopy in air atmosphere. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Alumina, as additive, improves the grain boundary conductivity of samples sintered at temperatures lower than 1500 °C. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (> 95%) at relatively low sintering temperature and long soaking time.     

Improvement of plasma-sprayed YSZ electrolytes for solid oxide fuel cells by alumina addition

Atmospheric plasma spray is a fast and economical process for deposition of yttria-stabilized zirconia (YSZ) electrolyte for solid oxide fuel cells. YSZ powders have been used to prepare plasma-sprayed thin ceramic films on the metallic substrate employing plasma spray technology at atmospheric pressure. Alumina doping was employed to improve the structural characteristics and electrical properties of YSZ. The effect of alumina addition from 1 to 5 wt.% on the properties of plasma-sprayed YSZ films was investigated. It was found that the gas permeability of the Al-doped YSZ electrolyte layer reached a level of 8.6 × 10⁻⁷ cm⁴ gf⁻¹ s⁻¹, which is a necessary value for the practical operation of solid oxide fuel cells. Alumina doping considerably increased the ionic conductivity of plasma-sprayed YSZ. The open circuit voltage of the alumina-doped YSZ coating was approximately equal to the theoretical value for dense YSZ material.     

Performance of a double-layer CGO/YSZ electrolyte for solid oxide fuel cells

The use of a double-layer ceria-gadolinia (CGO) - yttria-stabilized zirconia (YSZ) electrolyte has been suggested as an alternative for efficient intermediate temperature operation of Solid Oxide Fuel Cells (SOFC). CGO offers the advantage of high ionic conductivity and good chemical compatibility with Co-containing cathode perovskite materials, while YSZ serves as an electron blocking layer. The main problem for the applicability of such a composite film still remains the formation of a poorly conductive solid solution phase at the CGO/YSZ interface. The microstructure and the elemental distribution of this solid solution phase were examined with the aid of electronic probe microanalysis. Powders with the same composition were synthesized in order to examine their crystal structure and electrical properties, with the objective to propose a suitable gradation at the interface in order to improve the feasibility of CGO/YSZ two- layer composite electrolyte films. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998     

Characterization of yttrium-doped ceria with various yttrium concentrations as cathode interlayers of SOFCs

This study focuses on enhancing the efficiency of solid oxide fuel cells (SOFCs) by modulating the thickness of the highly resistive solid solution layer of (Ce,Zr)O₂ formed between the yttria-stabilized zirconia (YSZ) electrolyte and the CeO₂-based interlayer on the cathode side. The effects of the concentration of dopant in CeO₂ on the thickness of the solid solution were analyzed. Yttrium-doped CeO₂ (YDC) interlayers were studied, with dopant concentrations in the range of 5–40 mol%. The results revealed that the thickness of the solid solution decreased with increasing dopant concentration up to 20 mol% and then saturated at higher dopant concentrations. In addition, the electrical conductivities of yttrium-doped ceria (YDC) and the solid solution of YSZ and YDC were measured. YDC with a dopant concentration of 20 mol% exhibited the highest conductivity. The conductivities of the YSZ/YDC solid solution decreased compared to those of YDC and YSZ for each dopant concentration, and the extent of the reductions was approximately the same for all dopant concentrations. These results indicate that a dopant concentration of 20 mol% is optimal to minimize the internal resistance of SOFCs when YDC is used as the interlayer material.     

Electrical properties of thin yttria-stabilized zirconia overlayers produced by atomic layer deposition for solid oxide fuel cell applications

Thin films of yttria-stabilized zirconia (YSZ) electrolyte were prepared by atomic layer deposition at 300 °C for solid oxide fuel cell (SOFC) applications. YSZ samples of 300-1000 nm thickness were deposited onto La_0.8Sr_0.2MnO₃ (LSM) cathodes. A microstructural study was performed on these samples and their electrical properties were characterised between 100 and 390 °C by impedance spectroscopy. A remarkable feature is that the as-deposited layers were already crystalline without any annealing treatment. Their resistance decreased when reducing the layer thickness; nevertheless, their conductivity and activation energy were significantly lower than those reported in the literature for bulk YSZ.     

Nanostructure and conductivity study of yttria doped zirconia films deposited on samaria doped ceria

Yttrium doped zirconia (YSZ) film was deposited on poly-crystalline 10 at.% samaria doped ceria (SDC) and YSZ plate (doped with 8 at.% yttria) by electron beam evaporation deposition. For electrolyte application in solid oxide fuel cells, YSZ can be used with SDC and act as an electron barrier. The conductivity of YSZ and SDC was measured after sintering at 1000 °C. Results indicated that YSZ film became columnar structure, and a new layer formed between the YSZ film and SDC, due to the inter-diffusion between zirconium ions and cerium ions.     

Transitional metal-doped 8 mol% yttria-stabilized zirconia electrolytes

The sintering, grain growth and ionic conductivities (especially the grain-boundary (GB) conductivity), of 8YSZ electrolytes with various silica levels (~ 30 ppm, ~ 500 ppm and ~ 3000 ppm), doped with 1 at% transitional metal oxides (TMOs), have been systematically investigated by means of dilatometer, electron microscopy and impedance analyzer. It is confirmed that small additions of TMOs (i.e., Fe, Mn, Co or Ni) promote the densification and grain growth of both the pure and Si-containing 8YSZ. The effect of TMOs on the ionic conductivities could be negative or positive, relying on the type of TMOs, sintered density and impurity level. For the dense and pure 8YSZ (with ~ 30 ppm SiO₂), the addition of 1 at% TMOs led to a reduction in grain interior (GI) conductivity by ~ 25–33% with little effect on the GB conduction. For the impure 8YSZ (with ~ 500 ppm or 3000 ppm SiO₂), except for FeO_1.5, the other TMOs (i.e., Mn, Co or Ni) are extremely detrimental to the total conductivity by significantly reducing the GB conduction. Moreover, it is also found that the GB conductivity of the impure 8YSZ doped with Co or Ni is less sensitive to sintering temperature. FeO_1.5 showed a scavenging effect on SiO₂ in the impure 8YSZ, which is specially beneficial to the total conductivity of samples with higher silica levels and/or sintered at relatively low temperatures.     

Computer simulation of defects and oxygen transport in yttria-stabilized zirconia

We have used molecular dynamics simulations and energy minimization calculations to examine defect energetics and oxygen diffusion in yttria-stabilized zirconia (YSZ). Oxygen vacancies prefer to be second nearest neighbors to yttrium dopants. The oxygen diffusion coefficient shows a peak at 8 mol% yttria consistent with experimental findings. The activation energy for oxygen diffusion varies from 0.6 to 1.0 eV depending on the yttria content. The Y_Zr′–V_O^··–Y_Zr′ complex with a binding energy of − 0.85 eV may play an important role in any conductivity degradation of YSZ.     

Reaction between solid oxide fuel cell materials

In fabrication processes of solid oxide fuel cells, high-temperature heat treatments cannot be avoided. It will give rise to mutual reaction and interdiffusion of the cell component materials: yttria-stabilized zirconia (YSZ, electrolyte), (La, Sr)MnO₃ (cathode), Ni-YSZ cermet (anode) and (La, Ca)CrO₃ (separator). Reactivity of LaMnO₃ and YSZ was estimated by thermodynamic calculations, and it was found that the nonstoichiometry at La site in LaMnO₃ plays an important role on the reaction. Diffusion of Mn into YSZ leads to increase of La activity at the interface and promotes the reaction. Electrical conductivity of YSZ decreases when Mn dissolves in the cubic phase of YSZ. Oxidation state of the dissolving Mn varies with partial pressure of oxygen and affects the electrical properties of YSZ. Migration of Ca from (La, Ca)CrO₃ separator to other cell components is one of the largest problems in the co-firing cell fabrication process because it prohibits the sintering of the separator.     

Processing and characterization of ultra-thin yttria-stabilized zirconia (YSZ) electrolytic films for SOFC

Sub-micron yttria-stabilized zirconia (YSZ) electrolyte layer was prepared by a liquid state deposition method and with an average thickness of 0.5 μm to improve the performance of the anode-supported solid oxide fuel cell (SOFC). The YSZ precursors, containing yttrium and zirconium species and an additive, poly-vinyl-pyrrolidone (PVP), were spin-coated on a Ni/YSZ anode substrate. Several properties, including crystalline phases, microstructures, and current–voltage (I–V) characteristics, were investigated. The thin film of 4 mol% Y₂O₃-doped ZrO₂ (4YSZ) consisted of cubic, tetragonal, and a trace of monoclinic phases, and showed a crack-free layer after sintering at 1300 °C. The anode supported SOFC, which consists of the Ni–YSZ anode, 4YSZ electrolyte, and Pt/Pd cathode, showed power densities of 477 mW/cm² at 600 °C, and 684 mW/cm² at 800 °C. Otherwise, the surface cracks of the other YSZ-coated samples (e.g. 8YSZ) can be repaired by a multi-coating method.     

(Ce0.8Sm0.2O2-δ/Y2O3:ZrO2)N超晶格电解质薄膜的制备及表征

采用脉冲激光沉积技术(PLD), 在MgO单晶基底上, 依次沉积氧化钐掺杂的氧化铈(Ce_0.8Sm_0.2O_2-δ, SDC)和钇稳定氧化锆(8 mol%Y₂O₃:ZrO₂, YSZ)制备了五种(SDC/YSZ)_N (N=3, 5, 10, 20, 30) 超晶格电解质薄膜. 利用X射线衍射(XRD)、高分辨透射电子显微镜(HR-TEM)和交流阻抗对其形貌、相结构和电学性能进行了表征. 结果显示, (SDC/YSZ)_N超晶格电解质薄膜之间形成了明显的界面和较好的超晶格结构; 薄膜表面颗粒生长均匀、致密、平滑, 在薄膜的界面处没有元素相互扩散也未出现裂纹, 外延生长良好; 电导率随着(SDC/YSZ)_N超晶格电解质界面数的增加而增加, 而活化能则随之减少, 是较为理想的低温固体氧化物燃料电池电解质.     

NiO+YSZ anode substrate for screen-printing fabrication of YSZ electrolyte film in solid oxide fuel cell

Anode substrate has a great effect on screen-printing fabrication of yttria-stabilized zirconia (YSZ) electrolyte film and cell performance. In this work, NiO+YSZ anode substrate was prepared by a conventional ceramic sintering method, on which dense YSZ electrolyte film was successfully fabricated by screen-printing method. Microstructure of the anode substrate and cell performance were investigated. The optimal amount of addition of starch to the anode substrate was 20 wt%. The optimal temperature for pre-sintering of NiO powder was 800 °C. A single cell with the NiO powder pre-sintered at 800 °C exhibited the highest power density of 0.95 W cm⁻² at 700 °C.     

Microstructure and performance of novel Ni anode for hollow fibre solid oxide fuel cells

Nickel anodes were deposited on hollow fibre yttria-stabilised zirconia (YSZ) electrolyte substrates for use in solid oxide fuel cells (SOFCs). The hollow fibres are characterised by porous external and internal surfaces supported by a central gas-tight layer (300 μm total wall thickness and 1.6 mm external diameter). The YSZ hollow fibres were prepared by a phase inversion technique followed by high temperature sintering in the range 1200 to 1400 °C. Ni anodes were deposited on the internal surface by electroless plating involving an initial catalyst deposition step with PdCl₂ followed by Ni plating (with a NiSO₄, NaH₂PO₂ and sodium succinate based solution at 70 °C). Fabrication and nickel deposition parameters (nature of solvents, air gap, temperature, electroless bath composition) and heat treatments in oxidising/reducing environments were investigated in order to improve anode and electrolyte microstructure and fuel cell performance. A parallel study of the effect of YSZ sintering temperature, which influences electrolyte porosity, on electrolyte/anode microstructure was performed on mainly dense discs (2.3 mm thick and 15 mm diameter). Complete cells were tested with both disc and hollow fibre design after a La_0.2Sr_0.8Co_0.8Fe_0.2O_3?δ (LSCF) cathode was deposited by slurry coating and co-fired at 1200 °C. Anodes prepared by Ni electroless plating on YSZ electrolytes (discs and hollow fibres) sintered at lower temperature (1000–1200 °C) benefited from a greater Ni penetration compared to electrolytes sintered at 1400 °C. Further increases in anode porosity and performance were achieved by anode oxidation in air at 1200–1400 °C, followed by reduction in H₂ at 800 °C.     

Characterization of NiO–yttria stabilised zirconia (YSZ) hollow fibres for use as SOFC anodes

NiO–yttria stabilised zirconia (YSZ) hollow fibres with varying NiO content and a desired microstructure were prepared using a phase inversion technique and sintering. By controlling the fabrication parameters, microstructures with predominately finger-like pores near the inner and outer surfaces and a denser central layer with sponge-like pores were produced, for use as substrates for anode-supported hollow fibre solid oxide fuel cells (HF-SOFC). The NiO–YSZ fibres were reduced to Ni–YSZ at 250–700 °C in hydrogen flowing at 20 cm³ min^? 1 to produce Ni–YSZ hollow fibres, the mechanical and electrical properties of which were determined subsequently, reduction to Ni being verified by X-ray diffraction. The effects of NiO concentration and sintering temperature of the fibre precursors on the conductivity, strength and porosity of the reduced hollow fibres were investigated to assess their suitability for use as anode substrates. As expected, increasing Ni concentration increased electrical conductivities and decreased mechanical strength. Sintering temperature had a critical effect in producing axially conductive hollow fibres of sufficient mechanical strength for use as SOFC anodes. The hollow fibres retained their initial microstructure through the reduction process, though ca. 41% volume contraction is predicted on reduction of NiO to Ni, producing increased porosity in the reduced fibres. The mean porosity of the Ni–YSZ hollow fibres was ca. 60% and ca. 40% after sintered at 1250 °C and 1400 °C, respectively. The mean pore sizes for all the fibres after reduction varied between ca. 0.3 and 1 µm. The hollow fibres produced with 60% NiO, of length ca. 300 mm, electrical conductivities of ca. (1–2.25) × 10⁵ S m^? 1 and a porosity of ca. 43% are being used currently to construct and test the electrical behaviour of an anode-supported HF-SOFC.