Scandia-stabilized Zirconia Research Articles

Structure and electrical properties of cold sintered 8mol% scandia stabilized zirconia ceramics

Scandia stabilized zirconia (ScSZ) has been considered as a promising electrolyte for solid oxide fuel cell (SOFC), but high sintering temperature was always required to achieve high density and electrical conductivity. Herein, high-performance ScSZ ceramics were prepared at low temperature by using the cold sintering process (CSP) associated with post-heat-treatment (PHT). Firstly, 8 mol% ScSZ nano powder was synthesized by an ethanol-assisted two step hydrothermal method. Then, ScSZ ceramics were prepared by using CSP and PHT (CSP-PHT). It was found that the relative density of the green body made by CSP at 180 ○C could exceed 70%, which rose to 96.8% after post-heat-treatment at 1200 ○C, higher than that prepared by conventional sintering method at elevated temperature of 1400 ○C with the relative density of 95.4%. Impedance spectra measured at 800 ○C showed the electrical conductivity of ScSZ ceramics prepared by CSP-PHT at 1200 ○C was 0.115 S/cm, prior to that prepared by the CS at 1400 ○C with electrical conductivity of 0.111S/cm. Our results demonstrate that the CSP is a promising method for preparing high performance ScSZ ceramics at reduced temperature.

Ultra-Thin GDC Buffer Layer Fabricated By Vacuum Slurry Coating Method for High Performance Intermediate-Temperature SOFC

The Vacuum Slurry Coating (VSC) method has the advantage of being able to produce thin films with a simple device and is cost effective over other thin film coating methods. In solid oxide fuel cells (SOFCs), gadolinium doped ceria (GDC) buffer layer should be thin and dense to minimize ohmic resistance as well as prevent the undesirable reactions between scandia-stabilized zirconia (ScSZ) electrolyte and cobaltite perovskite cathodes. In this study, GDC buffer layer was coated on pre-sintered anode-supported half-cell comprising an ScSZ electrolyte layer on the NiO-YSZ anode using vacuum slurry coating process. Then GDC coated cells were sintered at 1400oC. The GDC buffer layer coated on the pre-sintered cell at 1200oC was thin and dense regardless of the immersion time. The maximum power density of the SOFC using VSC was 400 mW/cm2 (@ 700oC), which is 38 % higher than that using screen printing method (290 mW/cm2). The microstructure of the coated GDC buffer layer using the VSC method was much denser than using the screen printing method.

Electrochemical Characteristics of Ni-Co Alloy Cermet Anodes for SOFCs

Introduction Nickel (Ni) is widely used as the anode material for SOFCs. However, metallic Ni is oxidized to nickel oxide (NiO) in an oxidizing atmosphere at SOFC operational temperatures. Ni anode degradation occurs when the hydrogen-containing fuel supply is reduced in high fuel utilization operation or interrupted during the shutdown [1]. Due to the redox cycles, Ni volume expansion and shrinkage cause irreversible degradation, by breaking electrically-conducting pathways. Highly durable SOFC anodes using e.g. Ni-based alloy are therefore under development [2]. Here, in this study, we focus on the Ni-Co alloy, by preparing a new Ni-Co-GDC cermet anode using the Ni-Co alloy, and evaluate the effects of Cobalt addition on their electrode performance and durability. Experimental In this study, we prepared the Ni-alloy cermet by mixing the powder and the binder, preparing and printing electrode paste on the electrolyte plate, followed by heat treatment. Scandia-Stabilized Zirconia (ScSZ, 10mol% Sc2O3-1mol% CeO2-89mol% ZrO2) was used as the solid electrolyte. The powder prepared by mixing Ni-Co-based oxide powder prepared by ammonia co-precipitation with Ce0.9Gd0.1O2 (GDC) in a specific ratio by weight, was used as the anode material. La1-xSrxMnO3 (LSM) powder was used as the cathode material. The electrochemical characteristics of the cells, with the Ni-alloy cermet anodes, were evaluated by I-V measurements and the current interruption method under the condition at 800oC, and 3%-humidified hydrogen fuel was supplied to the anode. Furthermore, the redox stability was evaluated using the durability test protocol simulating the fuel interruption. Anode microstructural changes before and after the redox cycling tests were observed by a focused-ion beam system coupled with a scanning electron microscope (FIB-SEM). Results and discussion Ni-alloy GDC anode with x mol% of Co added to Ni, is denoted as Ni(100 – x)Cox -GDC anode. The anodes of x = 0, 5, 10, 20, and 30 were fabricated. The I-V characteristics and overvoltages, under the condition where the operating temperature was 800oC and 3%-humidified hydrogen fuel was supplied, are shown in Fig. 1 for different Co contents in the alloy. With increasing the Co content, the electrochemical performance decreases due to a gradual increase in overvoltages. However, when the Co content is as low as 5 mol%, performance decrease is negligible. The microstructure of the Ni95Co5-GDC anode shown in Fig. 2 also shows that the distribution of Ni and Co is overlapped, confirming that a Ni-Co alloy is actually formed. Electrochemical properties and durability of the cells with the Ni-Co alloy anodes with various Co concentrations will be reported and discussed.

Advanced modification of scandia‐stabilized zirconia electrolytes for solid oxide fuel cells application—A review

SummarySolid oxide fuel cells (SOFCs) are recognized as an energy generation device with high power density and energy conversion efficiency, low noise, low greenhouse gas emissions, positive environmental effect, and high fuel consumption versatility. Nevertheless, the high operating temperature of SOFCs has restricted the development in various applications due to challenges in material compatibility and working process. To address these issues, scandia‐stabilized zirconia (ScSZ) electrolytes are commonly utilized to explore its potentials as a solid electrolyte toward lowering the operating temperature due to high conductivity properties at low‐to‐intermediate temperature conditions. Thus, this paper provides an advanced modification of ScSZ electrolytes in SOFCs toward lowering the operating temperature. This review presents the potential properties and the progress modification of ScSZ electrolyte including challenges, and advances, summary, and future perspectives in the low‐to‐intermediate operating temperature of SOFCs. This review can serve as a first reference for identifying specific parameters and prospective studies to enhance the properties of ScSZ electrolytes for the application of SOFCs. Based on the literature review, there are no previous review works that have been published on the modification of ScSZ electrolytes.

E-Beam Deposition of Scandia-Stabilized Zirconia (ScSZ) Thin Films Co-Doped with Al

Scandia alumina stabilized zirconia (ScAlSZ) thin films were deposited using e-beam evaporation, and the effects of deposition parameters on the structure and chemical composition were investigated. The analysis of thin films was carried out using Energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-Ray Diffraction Analysis (XRD) and Raman spectroscopy methods. It was found that the chemical composition of ScAlSZ thin films was different from the chemical composition of the initial powder. Moreover, the Al concentration in thin films depends on the deposition rate, resulting in a lower concentration using a higher deposition rate. XPS analysis revealed that ZrOx, oxygen vacancies, high concentrations of Al2O3 and metallic Al exist in thin films and influence their structural properties. The crystallinity is higher when the concentration of Al is lower (higher deposition rate) and at higher substrate temperatures. Further, the amount of cubic phase is higher and the amount of tetragonal phase lower when using a higher deposition rate.

DLP 3D printing of scandia-stabilized zirconia ceramics

The present article aims to explore the printability of scandia-stabilized zirconia ceramic parts using desktop and low-cost DLP 3D printer. The acrylate-based homogeneous slurries with zirconia powder stabilized by 6 mol.% of Sc2O3 (6ScSZ) and 10 mol.% of Sc2O3 and 1 mol.% of Y2O3 (10Sc1YSZ) were prepared with appropriate rheological and UV-curing properties. In comparison with yttria-stabilized zirconia, slurries filled with 6ScSZ and 10Sc1YSZ powders reviled lower viscosity at the same solid content. The cure depth of the suspensions was suitable to print the objects with 50 μm of layer thickness, good interlayers connection, and surface finishing. No critical defects in ceramics such as cracks or delamination were observed. Both ceramics have the Vickers microhardness value of 11 GPa and the high ionic conductivity up to 0.2 S/сm at 900 °C demonstrating that the DLP is a promising method of fabricating scandia-stabilized zirconia parts as electrolyte material for SOFC application.

Investigations on charcoal as fuel for a refillable scandia-stabilised zirconia electrolyte-based tubular carbon fuel cell

A carbon fuel cell (CFC) is an emerging technology for conversion of solid carbonaceous material into electricity at high theoretical efficiencies (100 %). As carbonaceous material like coal will remain an important energy source for the next few decades, the development of technology that can use it more efficiently is critical to reduce emissions. For practical operation of a CFC, a continual supply or consecutive refuelling of solid carbon fuel is required. Here, we investigated the electrochemical performance and durability of a CFC using a scandia-zirconia tubular electrolyte with Ce0.9Gd0.1O2–Ag electrodes with activated charcoal fuel. We demonstrated that CFCs can be operated in a batch-type process with refuelling and utilisation of fuel. Peak power densities up to 280 mW cm−2 were obtained without major materials degradation. We expect that advances to engineering of fuel delivery will improve the long-term stability and performance of the cells.

Phase stability and conductivity of rare earth co-doped nanocrystalline zirconia electrolytes for solid oxide fuel cells

Abstract Solid oxide fuel cell (SOFC) is a green energy technology that directly coverts chemical energy into electricity. Scandia stabilized zirconia (SSZ) shows the highest conductivity among zirconia based electrolytes for SOFCs. However, the stability of the cubic phase, which is the desirable phase for high conductivity, can be an issue in SSZ electrolytes due to its transformation to other low-conducting phases at higher temperatures. In the present investigation, SSZ electrolyte was co-doped with ytterbia, gadolinia and ceria with an objective of improving the high-temperature phase stability. Both the doped and co-doped compositions exhibited a single cubic phase in the as-processed condition. The phase stability at high temperature was studied by aging the sintered pellets at 900 °C for 500 h in air. X-ray diffraction and transmission electron microscopy analysis revealed formation of small amount of the low-conducting tetragonal phase in 1 mol % ytterbia and gadolinia co-doped compositions on ageing which resulted in conductivity degradation. Increasing the doping level to 2 mol% prevented the formation of the tetragonal phase. The ceria co-doped composition (1 mol%), on the other hand, was clean without any sign of the secondary phases even after high-temperature ageing. The rhombohedral ‘β’ phase formed in the binary composition (SSZ) after sintering but was absent in all the co-doped compositions. The conductivity of the co-doped samples was higher than the binary SSZ. Thus, it can be said that rare earth co-doping is an effective way of improving the phase stability and conductivity of SSZ electrolytes.

Changes in properties of scandia-stabilised ceria-doped zirconia ceramics caused by silver migration in the electric field

Silver is one of the most promising cathode materials for low temperature (300–500 °C) solid oxide fuel cells. The most important disadvantage of silver is its migration in the electric field. For better understanding of this phenomenon, an in situ observation of the migration mechanism was undertaken with the use of high-temperature microscopes. Scandia stabilised ceria doped zirconia CeScSZ electrolyte prepared from commercial powder was examined before and after silver migration experiments using scanning electron microscope. X-ray diffraction, broadband electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. The silver electrodes for solid oxide fuel cells were prepared using magnetron sputtering. The described cells under polarisation were examined using a high-temperature low energy electron microscope. Reference cells and post-mortem cells were observed using a scanning electron microscope equipped with high temperature stage. Under polarisation, silver moved inside the electrolyte and along the surface towards the region between electrodes. The structures thus formed were similar to those previously described in the literature; however, direct observation of the deposit growth was unsuccessful. In situ scanning electron microscopy observations of the silver electrode at 650 °C revealed neither melting of the smallest silver particles nor movement of silver structures. Silver migration through the electrolyte caused a reduction in grain interior conductivity of the electrolyte, whereas its grain boundary conductivity remained unaffected.

Scattered and linked microcracks in solid oxide fuel cell electrolyte

Scattered and linked microcracks in the electrolyte layer have been analysed in detail in anode supported planar solid oxide fuel cells. The empirical model established allowed critical characteristics of scattered and localised microcracking to be determined using electrolyte microstructural parameters. The model was verified with experimental data obtained for electron beam deposited scandia-stabilised zirconia electrolytes. Suitable annealing and deposition temperatures during fuel cell fabrication could be predicted from the critical temperature difference below which microcracking does not occur.

High-performance solid oxide fuel cells with fiber-based cathodes for low-temperature operation

Low-temperature operation of solid oxide fuel cells (SOFCs) results in deterioration in electrochemical performance due to sluggish oxygen reduction reaction (ORR) at the cathode. To enhance the reaction pathway for ORR, La0.8Sr0.2MnO3 (LSM) nanofibers were fabricated by electrospinning and used for low-temperature solid oxide fuel cells operated at 600–700 °C. The morphological and structural characteristics show that the electrospun LSM nanofiber has a highly crystallized perovskite structure with a uniform elemental distribution. The average diameter of the LSM nanofiber after sintering is 380 nm. A symmetric cell of nanofiber-based LSM cathode on scandia-stabilized zirconia (SSZ) electrolyte pellet exhibits much lower area specific resistances compared to commercial LSM powder-based cathode. A single cell based on the nanofiber LSM cathode on yttrium-doped barium cerate-zirconia (BCZY) electrolyte exhibits a power density of 0.35 Wcm−2 at 600 °C, which increases to 0.85 Wcm−2 at 700 °C. The cell has an area specific resistance (ASR) of 0.46 Ωcm2 at 600 °C, which decreases to 0.07 Ωcm2 at 700 °C. The results indicate that the LSM electrode fabricated by the electrospinning process produces a nanostructured porous electrode which optimizes the microstructure and significantly enhances the ORR at the cathode of SOFCs.

Electrochemical performance of La0.3Sr0.7Ti0.3Fe0.7O3-δ/CeO2 composite cathode for CO2 reduction in solid oxide electrolysis cells

Solid Oxide Electrolysis Cells offer great promise for efficient recycle and cost-effective conversion of carbon dioxide. However, the conventional Ni/YSZ and some other fuel electrodes suffer some technological limitations such as the use of protecting gas CO/H2. Here we demonstrate the efficient electrochemical reduction of carbon dioxide without flowing any protecting gases based on the porous cathode with La0.3Sr0.7Ti0.3Fe0.7O3-δ(LSTF) nanostructured composite which is infiltrated into a scandia-stabilized zirconia scaffold together with ceria. The infiltrated porous electrode provides excellent performance for CO2 electrolysis. By I–V measurement and electrochemical impedance spectroscopic characterization, it shows that the current densities during electrolysis below a cell voltage of 2 V is between 1.20 and 4.44 A cm−2, increasing with temperature increase over the temperature range of 700–850 °C, and the cell with configuration of LSTF/CeO2|ScSZ|LSM/ScSZ possesses low electrode polarization resistances (Rp) within different voltage ranges, for example, Rp = 0.18 Ω cm2 at an applied voltage of 2.0 V, which is one order of magnitude lower than other reported electrode materials such as La0.2Sr0.8TiO3+δ. Moreover, a combination of short-term stability test for 48 h and cyclic stability test of LSTF/CeO2 are also estimated, and the results show that the nanostructured LSTF/CeO2 is highly effective for CO2 electrolysis.

Phase compositions, structures and properties of scandia-stabilized zirconia solid solution crystals co-doped with yttria or ytterbia and grown by directional melt crystallization

The effect of co-doping an impurity on the transport parameters and cubic phase stabilization of ZrO2–Sc2O3-based solid solutions has been compared for Y2O3 and Yb2O3 dopants. Solid solution crystals of (ZrO2)1-x-y(Sc2O3)x(Yb2O3)y and (ZrO2)1-x-y(Sc2O3)x(Y2O3)y (x = 0.08–0.10, y = 0.01, 0.02) were grown by directional melt crystallization in a cold crucible. Co-doping of the (ZrO2)1-x(Sc2O3)x crystals (x = 0.08–0.10) with 2 mol% Y2O3 or Yb2O3 for all experimental compositions produced transparent homogeneous single crystals of the t” phase or the cubic phase. For co-doping of the scandia-stabilized zirconia crystals with 1 mol% Y2O3 or Yb2O3, the stabilization of the high-temperature phases depended on the Sc2O3 content in the solid solutions. For co-doping of the solid solutions with 1 mol% Y2O3 or Yb2O3, the conductivity of the crystals increased with the Sc2O3 concentration. For co-doping of the crystals with 2 mol% Y2O3 or Yb2O3, the conductivity decreased with the Sc2O3 concentration. At scandia concentrations of 9–10 mol%, the high-temperature conductivity of the crystals co-doped with ytterbia were higher compared to those with yttria co-doping. The (ZrO2)0.9(Sc2O3)0.09(Yb2O3)0.01 crystals had the highest conductivity over the entire experimental temperature range.

Fabrication of Scandia-Stabilized Zirconia Thin Films by Instant Flash Light Irradiation

In this study, scandia-stabilized zirconia (ScSZ) electrolyte thin-film layers were deposited via chemical solution deposition (CSD). We selected 10ScSZ (10% Sc 2 O 3 , 90% ZrO 2 molar ratio) as the target material, and the precursor solution was prepared by precise calculations. The 10ScSZ solution was deposited on Al2O3 substrate using a spin-coating method. Then, the substrate was sintered using two methods: flash light irradiation and thermal. The characteristics of the thin films were compared, including ionic conductivity, surface morphology, and chemical composition. Pulsed light sintering was applied in the sintering step under a variety of energy density conditions from 80 to 130 J/ cm 2 , irradiation on/off times of 10 ms and 10 ms/500 ms, number of pulses, and bottom heat from 300 to 600 °C. The ionic conductivity of the ScSZ electrolyte layers fabricated by thermal or flash light irradiation methods was tested and compared. The results show that the ScSZ electrolyte layer sintered by flash light irradiation within a few seconds of process time had similar ionic conductivity to the electrolyte layer that was thermal sintered for about 10 h including cooling process.

Development of ScSZ Electrolyte by Very Low Pressure Plasma Spraying for High-Performance Metal-Supported SOFCs

Very low pressure plasma spraying (VLPPS) is an attractive method for metal-supported solid oxide fuel cells, wherein it can significantly avoid the oxidation of metal substrate. In this study, scandia-stabilized zirconia electrolyte was fabricated by VLPPS. To investigate the microstructure of coatings, the spraying distances were set to 150, 250 and 350 mm. The fractured morphology suggests that, at a relatively low deposition temperature (< 350 °C), typical lamellar structure was replaced by trans-granular structure. The apparent porosity of coatings ranges from 4.75 to 6.83%, as per the image analysis method. The ionic conductivity of coating at 250 mm was 0.068 S cm−1, which was ~ 68% of bulk. The ratio of surface area to projected area of coatings surface could reach 6.53-8.30 measured by a 3D confocal laser microscope. Finally, testing cells showed a maximum output power density of 1112 mW cm−2 and an OCV of 1.07 V at 750 °C. The ohmic resistance of whole cell was 0.1 Ω cm2, and the total resistance was 0.15 Ω cm2. The results suggest that very low pressure plasma spraying is a promising approach for the manufacturing of high-performance MS-SOFCs.

Comparison of solid oxide fuel cell (SOFC) electrolyte materials for operation at 500 °C

Solid oxide fuel cells (SOFCs) operating at low temperature (~500 °C) enable new fields of application, such as auxiliary power units (APUs) or power generation for mobile applications. However, the state-of-the-art electrolyte material currently used in intermediate-temperature SOFCs (yttria-stabilized zirconia (YSZ)) does not provide sufficiently high ionic conductivity for low temperature applications. When looking for alternatives, the conductivity values for each material found in widely cited literature can be confusing, as the reported values are sometimes in conflict with each other. Therefore, we present a systematic comparison of the conductivity of the three most popular, commercially available electrolyte materials, i.e., YSZ, scandia-stabilized zirconia (ScSZ), and gadolinium-doped ceria (GDC). By using electrochemical impedance spectroscopy (EIS) to characterize the ionic conductivities, we find that at 500 °C, GDC has a higher ionic conductivity (5.8 × 10−3 S cm−1) than ScSZ (2.5 × 10−3 S cm−1) and YSZ (1.1 × 10−3 S cm−1). The properties of the starting powders, powder processing and the microstructure after sintering were considered. This conductivity comparison can be used as a guide when deciding on electrolyte materials for different SOFC applications, especially when the fabrication of different thickness of the electrolyte layer has to be considered and rectify misleading information in the literature.

Influence of yttria and ytterbia doping on phase stability and ionic conductivity of ScSZ solid electrolytes

In this paper the effect of scandia-stabilized zirconia solid electrolytes doping with yttria and ytterbia on phase stability and ionic conductivity of the material is studied. The novelty of the research lies in replacing part of scandia with an unequal amount of yttria or ytterbia, unlike most of previously reported studies, where amount of introduced dopant was equal to amount of removed scandia. It is shown that in composition with 8 mol% Sc2O3 + 1.6 mol% M2O3 and 6 mol% Sc2O3 + 3.2 mol% M2O3 the phase transformation of cubic zirconia to rhombohedral phase is completely suppressed. However, decrease in ionic conductivity in 500 °C–900 °C range in comparison with 10 mol% scandia composition is observed. The drop is attributed to decrease in concentration of oxygen vacancies and to increased migration enthalpy as calculated from the Arrhenius plots. It was also discovered that such doping reduces association enthalpy for oxygen vacancy—dopant cation complexes.

Fabrication and Characterization of YSZ/ScSZ Bilayer Electrolyte via Cold-Isostatic Pressing (CIP) Method for Intermediate Temperature-Solid Oxide Fuel Cell (IT-SOFC) Application

Solid oxide fuel cell (SOFC) technology has advanced significantly in the recent years, and now is an interest of many renewable energy-related industries to invest. However, the main issue of SOFC is the high operating temperature that negatively influence its performance. To reduce the temperature, a method of using bilayer electrolyte is proposed. In this study, a bilayer electrolyte of Yttria-stabilised Zirconia (YSZ) and Scandia-stabilized Zirconia (ScSZ) is used with the objective to reduce the temperature of SOFC to intermediate temperature ranges. ScSZ has been chosen as an alternative to YSZ due to its superior ionic conductivity and good mechanical properties. To achieve the objective, bilayer YSZ/ScSZ electrolyte has been fabricated at different compositions using cold-pressed method. This paper presents the fabrication and characterization results of the work.

Conductivity of ScSZ Ceramics in Vicinity of Polymorphic Phase Transitions

The effect of phase composition on ionic conductivity behavior in scandia-stabilized zirconia ceramics with various scandia concentrations during heating was studied. It was found that the homogeneous single-phase (cubic) ceramics reveals no hysteresis or kinks in Arrhenius plots (log σT versus 1/T) indicating on polymorphic transitions in course of heating within temperature range under study (523 to 823 K). Unlike the homogeneous material, two-phase ceramics demonstrates a conductivity jump in the low temperature range (600-630 K) of Arrhenius plot, which grows with increase in scandia concentration or decrease in grain size and corresponding rise in rhombohedral phase β content at room temperature. It is shown that this effect can be caused by intragrain crystal structure ordering under mechanical stresses arising in rhombohedral phase containing ceramics.

Sintering of translucent and single-phase nanostructured scandia-stabilized zirconia

Fully-dense and single-phase nanostructured scandia-stabilized zirconia specimens were produced by high-pressure spark plasma sintering technique. Nanocrystalline powders were prepared by the coprecipitation method. Green pellets were sintered at temperatures varying from 700 to 900 °C and pressures from 1.4 to 2 GPa, resulting in dense microstructures with single-phase fluorite-type cubic structure within a wide range of Sc2O3 content (6–15 mol%). The average grain size of sintered specimens ranged from 8 to 20 nm. Transmittance spectra confirm translucence in sintered specimens, which is consistent with full density. The results reported here reveal that the polymorphism challenge in the zirconia-scandia system can be successfully suppressed by this consolidation technique, which allows for controlling the grain size of bulk specimens.