Scandia-stabilized Zirconia Research Articles

Fabrication and Evaluation of a Micro-Tubular Solid Oxide Fuel Cell with an Inert Support Using Scandia-Stabilized Zirconia Electrolyte

We have proposed a novel micro-tubular solid oxide fuel cell (SOFC) design with an inert support and an integrated current collector for the inner electrode to improve current collection efficiency as well as reduction–oxidation stability of the cell. In this work, a micro-tubular SOFC based on the proposed design was fabricated using scandia-stabilized zirconia (ScSZ) as electrolyte owing to its high ionic conductivity over a wide range of temperatures. Yttria-stabilized zirconia (YSZ), Ni, Ni-ScSZ, strontium-doped lanthanum manganite (LSM)–ScSZ, and LSM were used as the inert support, anode current collector, anode, cathode, and cathode current collector, respectively. The electrochemical performance of the fabricated cell was evaluated at temperatures between 600 and 850°C. Because of the lower ohmic resistance across its components, the cell exhibited good power generation performance at high and intermediate temperatures. Additionally, we confirmed stable operation of the micro-tubular SOFC for over 60 h at 750°C.

Crystal Structure and Electrical Properties of Magnesia Co-Doped Scandia Stabilized Zirconia

Mg-doping effects on the crystal structure and ion conduction properties of Scandia Stabilized Zirconia have been studied in the (ZrO2)0.89(Sc2O3)0.11-x(MgO)x (0 ≤ x ≤ 0.07) system. The materials have been prepared by a sol-gel method. Characterization of the crystal structure has been carried out by means of X-ray diffraction, Raman spectroscopy, synchrotron X-ray diffraction, selected area electron diffraction and high resolution transmission electron microscopy. Cubic phase-stabilization is observed in materials corresponding to x ≥ 0.03 but single cubic-phase formation is achieved only in (ZrO2)0.89(Sc2O3)0.08(MgO)0.03. Ionic conductivity of the oxides has been determined by complex impedance spectroscopy. Stabilization of the cubic phase in the compounds with x ≥ 0.03 values avoids rhombohedral-cubic phase transformation, improving the conductivity bellow 600°C compare to the one of 11ScSZ.

High performance of intermediate temperature solid oxide electrolysis cells using Nd2NiO4+δ impregnated scandia stabilized zirconia oxygen electrode

Nano-structured Nd2NiO4+δ(NNO) impregnated scandia stabilized zirconia (SSZ) is studied as the oxygen electrode of solid oxide electrolysis cells (SOECs). The hydrogen electrode-supported single cell with Ni–Zr0.92Y0.16O2.08 (YSZ) hydrogen electrode, SSZ electrolyte and impregnated NNO-SSZ composite oxygen electrode is prepared by a tape casting, co-firing and impregnation method. The electrochemical properties of the cell are investigated in the electrolysis mode. At the voltage of 1.3 V, current densities of 1.081, 0.677, 0.441 and 0.23 A cm−2 are obtained at 800, 750, 700 and 650 °C respectively. The cell also exhibits comparable performance with variation of the steam-to-hydrogen ratio (50/50 to 80/20) at 750 °C. Additionally, the short-term durability in the electrolysis mode of the cell is also studied.

Equivalent Circuit Model Analysis of Microstructure-Controlled LSM/ScSZ Composite Cathodes by Powder Slurry Impregnation Method

Improved electrochemical performance of strontium-doped lanthanum manganite (LSM)/scandia-stabilized zirconia (ScSZ) composite cathodes for solid oxide fuel cells was accomplished by using the powder slurry impregnation (PSI) method where a prefabricated thin porous ScSZ electrolyte layer is impregnated with LSM/ScSZ powder mixture in slurry. The PSI method enabled control of the electrolyte surface roughness, resulting in a decreased electrode polarization resistance despite a tendency of the ohmic resistance to increase due to the added porous ScSZ electrolyte layer. Resistance separation by electrochemical impedance spectroscopy with an equivalent circuit model (ECM) revealed that the proposed composite cathodes exhibited a significant reduction in the oxygen reduction reaction (ORR) resistance at the triple-phase boundary (TPB). Optimization of the electrolyte surface structure is therefore critical so as not to offset the improvement in the ORR resistance. Furthermore, besides the ECM used to separate the resistances, two ECMs that represent the microstructure of actual composite electrodes were evaluated to clarify the electrode structural factors determining the measured impedance spectra. The analysis revealed that impedance response is influenced by the area ratio of the double-phase boundary to the TPB, although cathode performance is mainly determined by the TPB area and the ionic conduction resistance within composite electrodes.

Ionic and electronic conductivities of yttria- and scandia-stabilized zirconia

We report a study and comparative analysis of the medium-temperature (850–1000°C) ionic and total conductivities of zirconia stabilized by yttriaand scandia-based mixed oxides. Zirconia stabilized by combined yttria- and scandia-based dopants is shown to have low electronic conductivity in a wide range of oxygen activities. Our data suggest the possibility of using the synthesized materials as membranes in intermediate temperature ceramic fuel cells.

Infiltrated SmBa0.5Sr0.5Co2O5+δ cathodes for metal–supported solid oxide fuel cells

This paper reports the fabrication of the SmBa0.5Sr0.5Co2O5+δ (SBSCO) infiltrated scandia–stabilized–zirconia (SSZ) composite cathode for metal–supported solid oxide fuel cells (MS–SOFCs). The effects of the calcination temperature on the structure, morphology and polarization resistance of the composite cathode were studied. Cathode calcinated at 700°C showed the finest microstructure and the lowest polarization resistance, e.g., 0.054 and 0.172Ω cm2 at 700 and 600°C, respectively. Thermal shock resistance of the composite cathode was studied and no degradation was found after the 35 thermal cycles conducted between 100 and 600°C with a rate of 10°Cmin−1. MS–SOFC with the SBSCO infiltrated SSZ cathode was prepared and the maximum power density (MPD) can be as high as 1.25, 0.92, 0.61 and 0.39Wcm−2 when measured at 700, 650, 600 and 550°C, respectively.

Characteristics of electrolyte supported micro-tubular solid oxide fuel cells with GDC-ScSZ bilayer electrolyte

In this study, a gadolinia-doped ceria (GDC)-supported micro tubular SOFC (T-SOFC) was fabricated using extrusion and dip-coating techniques (Cell A). The effects of inserting a scandium-stabilized zirconia (ScSZ) layer as an electron blocking layer between the GDC layer and the GDC-Ni anode layer were also explored (Cell B). The microstructures and electrochemical performances of Cell A and Cell B were investigated and compared. The layer thicknesses of the GDC and ScSZ bi-layer electrolytes were approximately 285 μm and 8 μm respectively. With the inserted ScSZ layer, both the ohmic resistance and the polarization resistance significantly increased at all the operating temperatures. The increase in the ohmic resistance of Cell B was predominantly due to the interfacial resistance, while the substantial escalation in the polarization resistance was mainly because of the low bulk oxygen diffusion process in the ScSZ layer and the smaller charge transfer processes occurring at the interfaces. The OCV of Cell B showed a slight decrease from 1.06 to 0.98 V and that of Cell A experienced an obvious decline from 0.92 to 0.76 V as the temperature rose from 650 to 800 °C. The ScSZ layer of Cell B successfully inhibited the OCV loss caused by the electronic conduction in GDC. The maximum power densities (MPDs) of Cell A at 650, 700, 750, and 800 °C were 0.20, 0.27, 0.33, and 0.36 Wcm−2, and those of Cell B 0.16, 0.23, 0.32, and 0.42 Wcm−2. The MPD of Cell B was improved at temperatures above 750 °C but remained inferior to that of Cell A below 750 °C. This is due to the fact that, as operating temperature increased above 750 °C, the benefit of the higher OCV in Cell B surpassed the deficiency of the higher cell resistance, thereby leading to a higher MPD.

Evaluation of micro flat-tube solid-oxide fuel cell modules using simple gas heating apparatus

Micro flat-tube solid-oxide fuel cell (SOFC) modules consisting of 1 mm thick, 1.2 cm wide micro flat-tube SOFCs, gas manifold, and insulator have been fabricated and evaluated using simple gas heating apparatus. The cell consists of NiO – yttria stabilized zirconia (YSZ) as an anode (flat-tube support), scandia stabilized zirconia (ScSZ) as an electrolyte, gadolinia doped ceria (GDC) for an interlayer, and (La, Sr)(Fe, Co)O3 (LSCF) – GDC as a cathode, which has been fabricated using cost effective extrusion technique and dip-coating technique. The cell has been investigated between 600 and 650 °C operating temperature and showed the power density at 0.75 V of 0.19 and 0.385 W cm−2, respectively. Using the cell, a five and ten-series modules were assembled and stored in insulator with small gas heaters powered by a 24 V power source for start-up. The module successfully operated using hydrogen and methane fuel.

Electrochemical performance and long-term durability of a 200 W-class solid oxide regenerative fuel cell stack

The electrochemical performance and durability of a 200 W-class solid oxide regenerative fuel cell (SORFC) stack are investigated for cyclic mode-changing and long-term operation. Three unit cells (10 cm × 10 cm), each based on a Ni – yttria-stabilized zirconia (YSZ) fuel electrode, a scandia-stabilized zirconia (ScSZ) electrolyte and a Sr-doped LaCoO3 (LSC) – gadolinia-doped ceria (GDC) air electrode, are used for the stack development. Delamination of the air electrode is suppressed by using a mixed ionic- and electronic-conducting air electrode with no oxygen excess non-stoichiometry, and gas leakage is minimized by using novel glass-ceramic composite sealants. Excellent electrochemical performance is achieved in a single cell level by minimizing the ohmic and electrode polarizations, and the three-cell stack is successfully configured without major performance loss associated with electrical contacts, gas supply or sealing. Stable operation is confirmed at a thermal neutral voltage for 1000 h in the solid oxide electrolysis cell (SOEC) mode, and the periodic change of the operation mode between the solid oxide fuel cell (SOFC) and SOEC is found to accelerate the performance degradation. The effect of cyclic mode-changing on the stability of the SORFC stacks is discussed in detail based on the observations from the post-mortem microstructural analysis.

Effect of Manganese Dioxide Addition on the Cubic Phase Stability, Densification and Electrical Conductivity of Scandia-Stabilized Zirconia

Zirconia stabilized with 8-12 mol% scandia shows the highest value of oxide ion conductivity among zirconia-based oxides. However, the cubic-rhombohedral phase transition at approximately 600 °C prevents its use as high temperature solid electrolyte. To circumvent this problem, additives have been proposed. In this work, ZrO2:10 mol% Sc2O3 was mixed to MnO2 and sintered at 1200 oC/2 h + 1450 °C/2 h with addition of 1, 1.5, 2 and 2.5 mol% MnO2. All specimens were analyzed by X-ray diffraction for structural phase evaluation and dilatometry for densification. Polished and thermally etched surfaces were observed in scanning electron microscope and scanning probe microscope. Impedance spectroscopy measurements were carried out in the frequency range 5 Hz-13 MHz in the 300-500 oC temperature range. The main results show that the manganese dioxide addition prior to sintering leads to the formation of single cubic phase, to the reduction of the maximum shrinkage temperature, and to an increase in the value of the total oxide ion conductivity.

Synthesis, sintering and conductivity behavior of ceria-doped Scandia-stabilized zirconia

Abstract In this work, the effect of ceria addition on the phase formations, sinterability and electrical conductivity of 10 mol.% scandia stabilized zirconia (SSZ) is investigated. Ceria co-doped SSZ (SSZ–xCeO2 (x = 0.5, 1.0, 5.0 mol.%)) powders were successfully synthesized by glycine-nitrate process and the electrical conductivity was measured in the temperature range of 600–900 °C. Fourier-transform infrared spectroscopy (FT-IR) analysis showed that the samples sintered at 1300 °C/3 h are free from impurities such as un-decomposed nitrates, residual water and nitrile intermediates. From the X-ray diffraction (XRD) analysis, the sintered samples results revealed that the presence of cubic and rhombohedral β-phases and Raman spectroscopy analysis further revealed the presence and monoclinic and tetragonal phases. From the linear shrinkage spectra, a linear shrinkage of 27% was observed at 1300 °C for SSZ and was decreased with the increase of cerium oxide content. The electrical conductivity has been decreased with the increase of ceria content and the SSZ–0.5CeO2 sample displayed a higher electrical conductivity compared to others indicating that as small as 0.5 mol.% CeO2 doping in SSZ is enough to ensure good conductivity results for its potential application as an electrolyte material in intermediate-temperature solid oxide fuel cells (IT-SOFCs).

Development of mesoporosity in scandia-stabilized zirconia: particle size, solvent, and calcination effects.

We present the mechanisms of formation of mesoporous scandia-stabilized zirconia using a surfactant-assisted process and the effects of solvent and thermal treatments on the resulting particle size of the powders. We determined that cleaning the powders with water resulted in better formation of a mesoporous structure because higher amounts of surfactant were preserved on the powders after washing. Nonetheless, this resulted in agglomerate sizes that were larger. The water-washed powders had particle sizes of >5 μm in the as-synthesized state. Calcination at 450 and 600 °C reduced the particle size to ∼1-2 and 0.5 μm, respectively. Cleaning with ethanol resulted in a mesoporous morphology that was less well-defined compared to the water-washed powders, but the agglomerate size was smaller and had an average size of ∼250 nm that did not vary with calcination temperature. Our analysis showed that surfactant-assisted formation of mesoporous structures can be a compromise between achieving a stable mesoporous architecture and material purity. We contend that removal of the surfactant in many mesoporous materials presented in the literature is not completely achieved, and the presence of these organics has to be considered during subsequent processing of the powders and/or for their use in industrial applications. The issue of material purity in mesoporous materials is one that has not been fully explored. In addition, knowledge of the particle (agglomerate) size is essential for powder handling during a variety of manufacturing techniques. Thus, the use of dynamic light scattering or any other technique that can elucidate particle size is essential if a full characterization of the powders is needed for achieving postprocessing effectiveness.

(Invited) In Situ Optical and Electrochemical Studies of SOFC Carbon Tolerance

High operational temperatures needed for solid oxide fuel cells (SOFCs) are required by the large activation energy of the oxide diffusion through the electrolyte. These high temperatures, in the range of 800-1000 °C, accelerate cell degradation and severely reduce the number of materials suitable for construction. Employing newly developed, more highly conducting electrolytes such as scandia-stabilized zirconia (ScSZ) enable SOFCs to operate at lower temperatures without sacrificing power (1). Intermediate temperature (IT) solid oxide fuel cells, operating in the 500-750 °C range, permit non-ceramic and non-metallic components to be used in the fuel cell stack, reducing costs and enabling a larger variety of stack geometries (2). However, the high catalytic activity of nickel still causes unwanted carbon deposition on the anode surface (3). This carbon buildup can cause premature cell degradation as well as reduce the power density of the fuel cell. Chronopotentiometry, a technique that couples real time electrochemical monitoring of cell voltage with synchronous in situRaman spectroscopy, provides insight into the mechanisms of carbon deposition and removal as well as the effect deposited carbon has on fuel cell performance (3). This technique can quantify the carbon growth with < 2 second temporal resolution and 1 μm spatial resolution across the anode surface.Figure 1: Raman “G” peak growth at 675 °C observed in situfrom a cell operating under methane with a ScSZ electrolyte supported membrane electrode assembly.In this work, we examine qualitative differences in carbon buildup (Fig. 1) between yittria-stabilized zirconia (YSZ) and ScSZ supported solid oxide fuel cells with a Ni-YSZ cermet anode exposed to methane at 675 °C and 725 °C. We also discuss the differences in the linear sweep voltammetry (LSV) between cells running under methane and hydrogen. (Fig. 2)Figure 2: (Top) Comparison of carbon growth at different polarizations of YSZ and ScSZ SOFCs operating at 725 °C under methane fuel. (a) ScSZ OCV (b) ScSZ 75% IMax (c) YSZ OCV (d) YSZ 75% IMax (Bottom) Comparison of LSV traces running under H2 and CH4. The “swing” in the CH4 trace, indicating a change in carbon removal mechanism can be seen at 150 mA (4).Observed differences between the amounts of carbon formed for YSZ and ScSZ fuel cells at high and low temperatures and different polarizations (Fig. 2) implicate a role played by the electrolyte in the formation of carbon on the anode. Different carbon removal mechanisms have fundamentally different Nernst potentials, and when coupled with Raman spectroscopy, allow us to identify not only the relative amounts, but also the type of carbon formed.

(Invited) In Situ Optical and Electrochemical Studies of SOFC Carbon Tolerance

Correlating real time, in situ spectroscopic observations with electrochemical performance provides insight into the ability of solid oxide fuel cells (SOFCs) to function sustainably despite adverse operating conditions. Experiments described in this work monitored the performance of SOFCs constructed with two different electrolyte materials: 8% yttria stabilized zirconia (YSZ) and 10% scandia stabilized zirconia (ScSZ). Both types of SOFCs were operated with CH4 as a fuel and carbon accumulation was monitored by in situ vibrational Raman scattering. SOFC performance was characterized using both linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Data showed that despite the more favorable conductivity properties of ScSZ, neither the rate nor the type of carbon formed is affected by electrolyte composition.

Reduction in ohmic contact resistance at interface between Gd-doped CeO2 interlayer and Sc2O3-stabilized ZrO2 electrolyte in SOFCs to improve performance

Abstract Influence of the ohmic contact resistance at the interface between Gd-doped CeO2 (GDC) interlayer and scandia-stabilized zirconia (ScSZ) electrolyte on the performance of solid oxide fuel cells (SOFCs) was investigated. When GDC interlayer was used, the ohmic resistance of SOFCs was increased due to insufficient contact conditions between the GDC interlayer and ScSZ electrolyte. To improve the contact conditions, the effect of sintering temperature and sintering aids (Mn, Fe, Bi) on the ohmic contact resistance was evaluated. As a result, using an optimized Mn-modified GDC interlayer (with added 0.3 mol% Mn and sintered at 1300 °C) to decrease the ohmic contact resistance, an anode-supported cell with Sr-doped LaFeO3 cathode and ScSZ electrolyte achieved a power density of 1.5 W cm− 2 at 800 °C.

Enhancing the phase stability and ionic conductivity of scandia stabilized zirconia by rare earth co-doping

Effect of co-doping Yb, Gd and Ce in scandia stabilized zirconia (SSZ) on the phase stability, high temperature aging behavior and ionic conductivity was studied. Both binary (10mol% SSZ) and the ternary (co-doped) compositions were found to be in single cubic phase in the as-processed condition. However, the binary composition exhibited the rhombohedral ‘β’ phase after sintering whereas the ternary compositions remained in the single cubic phase. The sintered pellets were aged at 900°C for 500h in air to study the phase stability at high temperature. Transmission electron microscopy revealed that the aged samples of Yb and Gd co-doped compositions contain small amount of the tetragonal phase which resulted in considerable degradation in conductivity (more than 20%). The Ce co-doped sample, on the other hand, was in single cubic phase after aging and this ensured that conductivity reduction was minimal in this composition. The co-doped samples however, showed higher conductivity before and after aging compared to the binary composition. The rhombohedral ‘β’ phase was absent in all the co-doped ternary compositions even after high temperature aging.

Laplace’s Pressure Induced Rhombohedral Phase in the Scandia-Stabilized Zirconia Ceramics

Grain-size effect on the conductivity and phase composition ofscandia stabilized zirconia ceramics produced from DKKK(Japan) powder with composition 89 mol.% ZrO2 + 10 mol. %Sc2O3 + 1 mol. % CeO2 was established. It was shown using Xrayphase analysis that decreasing the grain size of ceramics from 16 to 0.6 mkm leads in formation of rhombohedral phase. For finegrained material, anisotropic properties of material are fixed clearly in Arrhenius plots of ionic conductivity.

Composite Anodes with Ni Impregnated LST-SSZ for Direct Methane Solid Oxide Fuel Cells

Solid Oxide Fuel Cells (SOFCs) is one of the most attractive energy conversion devices because of their high efficiency, low pollution, and fuel flexibility. La0.2Sr0.8TiO3 (LST) anode material shows its potential utilization in direct oxidation of methane fuel without carbon formation. In this paper, LST powders were synthesized by traditional solid-state method, and then mixed with scandia-stabilized zirconia (SSZ) in mass ratio of 5: 5 to prepare the composite anode materials. Symmetrical cells with LST-SSZ composite anode were fabricated and measured. Their polarization resistances in hydrogen at 700°C, 750°C and 800°C were 5.3, 3.0 and 2.0 Ω·cm, respectively. Considering the insufficient conductivity of LST, 10wt%Ni was impregnated into the composite anode to improve the anode performance. The polarization resistance of the symmetrical cell with 10wt% Ni impregnation load is obviously reduced. The maximum power density of a cathode supported single cell with 10wt%Ni-LST-SSZ composite anode are 225 mW/cm and 175 mW/cm in hydrogen and in methane at 750°C, respectively, and the single cell shows its stability running in methane fuels.

One-step mechanical processing to prepare LSM/ScSZ composite particles for SOFC cathode

Novel one-step mechanical processing was proposed to prepare LSM/ScSZ composite particles from the starting raw powders of LSM (strontium doped lanthanum manganite) and ScSZ (scandium stabilized zirconia) fine powder. In this paper, the properties of composite particles made by this method and the properties of the resultant cells were evaluated. As a result, LSM/ScSZ composite particles were obtained by only 10min mechanical processing using three kinds of raw powder materials of LSM and ScSZ fine particles without extra heat. The maximum power density of the cell made by the composite particles at 800°C reached 320mW/cm2. It was higher than that made by commercially available LSM nanosized powder. Besides, the polarization resistance of the cathode made by the composite particles at 800°C was 0.5Ωcm2 which was lower than that made by using commercially available LSM powder. It suggests that the proposed method is very promising for producing high quality composite particles used for SOFC cathodes by more simple and energy-saving way.

Fabrication of planar-type SOFC single cells by a novel vacuum dip-coating method and co-firing/infiltration techniques

A novel vacuum dip-coating method has been adopted to form a dense thin yttrium stabilized zirconia (YSZ) layer on the pre-fired Ni/YSZ anode substrate. Scandia-stabilized Zirconia (ScSZ) was screen-printed onto the YSZ layer as the cathode backbone material and co-fired in a single step. Sm0.5Sr0.5CoO3−δ (SSC) nanoparticles were infiltrated into the ScSZ porous layer with a loading of 29.17wt% to gain the composite cathode. Without any optimization, the cell reached a maximum power density of 0.85 W cm−2 with an area-specific resistance of 0.914 Ω cm2 at 750 °C.