Sc2O3-stabilized ZrO2 Research Articles

Effect of B-site cationic substitution on the structural, spectroscopic, and conductivity behaviour of Ho2(Hf1-xZrx)2O7 (x=0 and 1)

Ideally, a solid electrolyte which is a central component of SOFC should exhibit high anionic or cationic ionic conductivity at the proposed operating temperatures. In most cases, the performance is compromised when operating above 1000 °C due to poor mechanical, thermal and chemical stability of selected functional and non-functional materials. In this context, pyrochlores are one of the potential candidates due to their high conductivity, flexibility to accommodate large cations, and high mechanical, thermal and chemical stability. In this study, we report the synthesis of nano-powders of Ho2Hf2O7 (HH) and Ho2Zr2O7 (HZ) pyrochlore ceramics by eco-friendly alginate mediated ion-exchange process also known as Leeds Alginate Process (LAP) and provide further insight into the structure - conductivity relationship of HH and HZ compounds by EXAFS studies. Both the compositions were sintered at temperatures, ranging from 1100 °C-1500 °C at 2 h dwell time to achieve the desired high-density ceramic with stable pyrochlore structure. X-ray diffraction and Extended X-ray absorption fine structural (EXAFS) analysis showed superlattice reflections and complex coordination geometry of these oxides. The coordination number and disorder factor in the case of HZ were found to be more stable than the HH sample, as evident from the EXAFS. Impedance spectroscopy and dc-conductivity analysis showed a better charge transport behavior in HZ ceramics than in HH making HZ as a preferred solid electrolyte for SOFC. The conductivity of Ho2Zr2O7 is comparable with the best-known fluorite oxide-ion conductor such as Sc2O3-stabilized ZrO2 at 500 °C.

Self-Sealing Metal-Supported SOFC Fabricated by Plasma Spraying and Its Performance under Unbalanced Gas Pressure

A novel self-sealing structure for metal-supported solid oxide fuel cells (MS-SOFCs) is designed by applying brazing technology between the metal support and interconnector to solve the sealing problem on the anode side of planar SOFCs. A high-reliability self-sealing effect is thus realized at the anode side of the MS-SOFC. Plasma spraying technology is used to prepare cell functional layers including the anode, cathode, and electrolyte. A single cell is assembled with a 50–60-μm plasma-sprayed Sc2O3-stabilized ZrO2 (ScSZ) electrolyte layer. The gas permeability of the self-sealed MS-SOFC without a cathode layer is 0.42 × 10−17 m2. The open-circuit voltage of the cell is ~ 1.1 V in the operating temperature range from 550 to 750 °C. The power density of the cell reaches 1109 mW cm−2 at 750 °C under standard atmosphere. In addition, the use of a fuel gas pressure that is 20 kPa higher than the cathodic gas pressure results in a significant increase in the power density to 1782 mW cm−2 at 750 °C. The novel cell structure and gas tightness of the ScSZ electrolyte prepared by plasma spraying indicates that it can meet the requirements of SOFC applications.

Nanoscale architecture of (La0.6Sr1.4)0.95Mn0.9B0.1O4 (B[dbnd]Co, Ni, Cu) Ruddlesden–Popper oxides as efficient and durable catalysts for symmetrical solid oxide fuel cells

A-site deficient (La0.6Sr1.4)0.95Mn1-xBxO4 (x = 0, 0.1, B= Co, Ni, Cu) (LSMBO4) Ruddlesden–Popper oxides were demonstrated as promising symmetrical electrodes for Sc2O3 stabilized ZrO2 (SSZ) electrolyte supported solid oxide fuel cells (SOFCs). The formation of oxygen vacancies is facilitated with the B-site transition metal doping. A-site defect promotes the exsolution of catalytic Co, Ni or Cu nanoparticles on the surface of materials in reducing atmosphere. The electrochemical performances in air and reducing atmosphere are significantly optimized via the substitution of Mn by Co, Ni or Cu and exsolved metallic nanoparticle catalysts. Especially, the materials doped by B-site with Cu shows the highest electrical conductivity of 37.54 S cm−1 and 6.82 S cm−1 and lowest polarization resistance of 0.12 Ω cm2 and 0.32 Ω cm2 at 750 °C in air and 5% H2/N2, respectively. The maximum power density of 623.1 mW cm−2 at 750 °C is achieved for an electrolyte-supported symmetrical single cell with the LSMBO4-SSZ composite electrode operating with pure H2. All these results indicate that LSMBO4 can be promising candidates for symmetric electrodes of SOFCs.

In Situ Transmission Electron Microscopy for Ultrahigh Temperature Mechanical Testing of ZrO2.

This work demonstrates a novel approach to ultrahigh-temperature mechanical testing using a combination of in situ nanomechanical testing and localized laser heating. The methodology is applied to characterizing and testing initially nanograined 10 mol % Sc2O3-stabilized ZrO2 up to its melting temperature. The results suggest that the low-temperature strength of nanograined, d < 50 nm, oxides is not influenced by creep. Tensile fracture of ZrO2 bicrystals produce a weak-temperature dependence suggesting that grain boundary energy dominates brittle fracture of grain boundaries even at high homologous temperatures; for example, T = 2050 °C or T ≈ 77% Tmelt. The maximum temperature for mechanical testing in this work is primarily limited by the instability of the sample, due to evaporation or melting, enabling a host of new opportunities for testing materials in the ultrahigh-temperature regime.

In situ transmission electron microscopic observations of redox cycling of a Ni-ScSZ cermet fuel cell anode.

In situ transmission electron microscopy (TEM) observations of a Ni(O)-Sc2O3-stabilized ZrO2 (ScSZ; 10 mol% Sc2O3, 1 mol% CeO2, 89 mol% ZrO2) anode in a solid oxide fuel cell (SOFC) have been performed at high temperatures under a hydrogen/oxygen gas atmosphere using an environmental transmission electron microscope (ETEM); the specimens were removed from cross-sections of the real SOFC by focused ion beam milling and lifting. When heating the NiO-ScSZ anode under a hydrogen atmosphere of 3 mbar in ETEM, nano-pores were formed at the grain boundaries and on the surface of NiO particles at around 400°C due to the volume shrinkage accompanying the reduction of NiO to Ni. Moreover, densification of Ni occurred when increasing the temperature from 600 to 700°C. High-magnification TEM images obtained in the early stages of NiO reduction revealed that the (111) planes of Ni grew almost parallel to the (111) planes of NiO. In the case of heating Ni-ScSZ under an oxygen atmosphere of 3 mbar in ETEM, oxidation of Ni starting from the surface of the particles occurred above 300°C. All Ni particles became polycrystalline NiO after the temperature was increased to 800°C. Volume expansion/contraction by mass transfer to the outside/inside of the Ni particles in the anode during repeated oxidation/reduction seems to result in the agglomeration of Ni catalysts during long-term SOFC operation. We emphasize that our in situ TEM observations will be applied to observe electrochemical reactions in SOFCs under applied electric fields.

Performance of Pulse Jet Solid Oxide Fuel Cells Using Liquid Fuel at Steep Output Power Variation

Introduction Because solid oxide fuel cells (SOFCs) operate at high temperature, there is a possibility that various fuels can be used directly with high efficiency. In the future, along with spreading renewable energy, the power sources which can compensate for unstable power supply is needed. However, typical SOFCs need a certain residence time of fuel to change the power with keeping fuel utilization and thus it is difficult to have a quick response to electric demand. Electrode degradation by deposited carbon is also a challenging issue when hydrocarbons are used as fuel directly. We have been developed Pulse jet-solid oxide fuel cell (PJ-SOFC) to achieve the efficient operation at steep changes of the power and improve the tolerance for carbon deposition[1], however, the performance at steep output power variation have not been investigated yet. In PJ-SOFCs, as shown in Fig.1, a small amount of liquid fuel is supplied via pulse jet injection. PJ-SOFC can use not only liquid fuel which is injected to the anode, but also hydrogen, lower hydrocarbons, and deposited carbon generated by the decomposition of liquid fuel. PJ-SOFCs can be a promising device which can use various energy carriers of hydrocarbon originated by renewable energy. PJ-SOFC will realize the steep output power variation keeping high fuel utilization by the quick control of gas composition above fuel electrode because several micro-litter of liquid fuel is supplied at controlled frequency using a via pulse jet injector. Thus PJ-SOFC can also be expected as a high efficient mobile power source for transportation. The high power density, high fuel utilization, and the high tolerance for carbon deposition will be realized by the quick control of operating conditions, such as the current density (j), the supplied amount in a single pulse (PJ vol), and the fuel supply interval time (T int). Anode degradation can be prevented by the electrochemical and chemical removing reactions of deposited carbon which can be enhanced by adjusting the anode potential connecting the lower terminal voltage. However, anode degradation can be caused by Ni oxidation which can be also induced by the lower terminal voltage[2]. Therefore, the degradation characteristics of PJ-SOFCs should be clarified under steep change of the power to keep the efficiency. In this study, the degradation rates of PJ-SOFCs operated at the steep output power variation with keeping the fuel utilization were investigated. Furthermore, the performance of PJ-SOFCs with Ni/YSZ and Ni/GDC was compared evaluating the degradation rate quantitatively. Finally, we demonstrated PJ-SOFCs under steep output power variation with high power density and fuel utilization. Experimental In PJ-SOFC, YSZ disk (8 mol% Y2O3-stabilized ZrO2, 0.25 mm thickness, 20 mm diameter), Ni / GDC (Gd0.10Ce0.90O2- δ) composite and Ni / YSZ composite, La0.8Sr0.2MnO2 / ScSZ (10 mol% Sc2O3-stabilized ZrO2) composite were used as an electrolyte, an anode, a cathode respectively. The operating temperature was 900℃. Pure oxygen was introduced to the cathode at 100 ccm. In the power generation with H2 fuel, H2 was introduced to the anode at 200 ccm. In the power generation of PJ-SOFC, isooctane was used as a model fuel of gasoline. After injecting 3 pulses of fuel (3.6 µL) and waiting for 1 min, the power generation at constant j was started. The separation of voltage loss into the overpotential and the ohmic loss was carried out by current interruption method. The degradation rate of PJ-SOFC was also investigated at some conditions. The fuel utilization (U fuel) was calculated as the ratio of the actual number of generated electrons to theoretical shifting electrons assuming the perfect utilization of supplied fuel. The characteristics of PJ-SOFCs under steep output power variation with Ni/YSZ anode and Ni/GDC anode were investigated at a constant (U fuel) by controlling the amount of a pulse injection (PJ vol) and interval time between the injections (T int). Conclusion In the power generation of PJ-SOFC using Ni/GDC anode, the maximum power density of 239 mW cm-2 was achieved at the current density of 300 mA cm-2 and the fuel utilization of 60%. The fuel utilization could be kept at 60% when the power density was steeply changed from 125 mW cm-2 at 150 mW cm-2 to 70 mW cm-2 at 75 mA cm-2. REFERENCES [1] A. Yabuki, M. Ihara et al., ECS Trans., 42(12) 57-67 (2012) [2] S. Sugiyama, M. Ihara et al., ECS Trans., 58(45) 21-33 (2014) Figure 1

Evaluation of isotope diffusion coefficient and surface exchange coefficient of ScSZ series oxide by oxygen isotope exchange method

In this work, the Isotope diffusion coefficient (D*) and surface exchange coefficient (k*) of Sc2O3 stabilized ZrO2 (ScSZ) series oxide, 6mol% Sc2O3-ZrO2 (6ScSZ), 10mol% Sc2O3-ZrO2 (10ScSZ) and 10mol% Sc2O3- 1mol% CeO2-ZrO2 (10Sc1CeSZ) were evaluated by using oxygen isotope exchange method. It was found that these ScSZ oxides showed a relatively higher D* value; 10Sc1CeSZ showed 1.1×10−7cm2s−1, 10ScSZ showed 9.3×10−8cm2s−1 and 6ScSZ showed 6.1×10−8cm2s−1 at 711°C, respectively. In addition, its activation energy was 1.0eV for 10Sc1CeSZ, 1.1eV for 10ScSZ and 1.0eV for 6ScSZ, respectively. The rapid change of D* due to the phase transition was also confirmed at around 600°C in the case of 10ScSZ. The surface exchange coefficient (k*) of these ScSZ oxides showed a relatively high value at lower temperature. 10Sc1CeSZ showed 1.3×10−8cms−1 at 498°C under lower vapor pressure of less than 20Pa.

Effect of structural parameters of Ni-ScSZ cermet components on the SOFC anodes characteristics

The effect of the degree of dispersion and the ratio of initial components of metalloceramic composites based on Ni and Sc2O3-stabilized ZrO2 (Ni/ScSZ) on the kinetics of sintering, conductivity, and polarization resistance of the corresponding anodes in solid-oxide fuel cells (SOFC) is studied. The composites are prepared from nano- and submicrosized powders of NiO and ScSZ (10.5 mol % Sc2O3) containing particles with the average size of 0.02–0.33 μm. Anode composites of three types differing in the ratio of initial components (NiO-ScSZ) with different degrees of dispersion: micro-micro, nano-micro, and nano-nano are studied. Due to the ratio of particle sizes, the anodic composites of the nano-nano type demonstrate the preferential electronic conduction (the percolation threshold) starting from the Ni content of about 35 vol %, in contrast to the other two types of anodic composites for which this threshold is achieved at 30 vol %. The lowest polarization resistance is typical of anode composites with the Ni content of about 40 vol %. The use of one or both components in the nanosized state makes it possible to decrease the anodic polarization up to two times. It is demonstrated that an active cermet anode for SOFC can be fabricated in the form of a planar three-layer structure Ni/ScSZ-ScSZ-Ni/ScSZ prepared from nanosized powders by the tape casting technique and cosintering.

High performance cobalt-free Cu1.4Mn1.6O4 spinel oxide as an intermediate temperature solid oxide fuel cell cathode

In this work Cu1.4Mn1.6O4 (CMO) spinel oxide is prepared and evaluated as a novel cobalt-free cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). Single phase CMO powder with cubic structure is identified using XRD. XPS results confirm that mixed Cu+/Cu2+ and Mn3+/Mn4+ couples exist in the CMO sample, and a maximum conductivity of 78 S cm−1 is achieved at 800 °C. Meanwhile, CMO oxide shows good thermal and chemical compatibility with a 10 mol% Sc2O3 stabilized ZrO2 (ScSZ) electrolyte material. Impedance spectroscopy measurements reveals that CMO exhibits a low polarization resistance of 0.143 Ω cm2 at 800 °C. Furthermore, a Ni-ScSZ/ScSZ/CMO single cell demonstrates a maximum power density of 1076 mW cm−2 at 800 °C under H2 (3% H2O) as the fuel and ambient air as the oxidant. These results indicate that Cu1.4Mn1.6O4 is a superior and promising cathode material for IT-SOFCs.

Fabrication and characterization of Ni-SSZ/SSZ/LSM-SSZ anode-supported SOFCs by tape casting and single-step co-sintering techniques

In this work, Ni-10 % Sc2O3-stabilized ZrO2 (SSZ)/SSZ/La0.8Sr0.2MnO3-δ (LSM)-SSZ anode-supported solid oxide fuel cells (SOFCs) have been successfully prepared by tape casting and single-step co-sintering procedures. The structure contains Ni-SSZ anode substrate and Ni-SSZ anode functional, dense SSZ electrolyte, LSM-SSZ cathode functional, and LSM-SSZ cathode layers were successfully prepared at 1250, 1300, and 1350 °C, respectively. The microstructures of the single cells were examined by SEM. There were some close pores in electrolyte of Cell-1250, and the cathode particle size obviously increased in Cell-1350. Therefore, Cell-1300 showed the optimal cell performance, the maximum power density attained 920 mW cm−2 at 800 °C. The impedance analysis demonstrated that the co-sintered temperatures have effects on not only the polarization resistance R P of a single cell but also its overall ohmic resistance R S . The results indicate that the tape casting and single-step co-sintering methods are both time saving and feasible for the development of anode-supported SOFCs.

Effect of Ru/CGO versus Ni/CGO Co‐Infiltration on the Performance and Stability of STN‐Based SOFCs

AbstractElectrolyte supported cells (ESC), with Sc2O3‐stabilized ZrO2 (ScSZ) electrolytes, Gd‐doped ceria (CGO) or M/CGO (M = Ni, Ru) infiltrated Sr0.94Ti0.9Nb0.1O3 (STN94) anodes and LSM/YSZ cathodes, were evaluated for their initial performance and long‐term stability. Power density for the Ru/CGO infiltrated cell reached ∼0.7 W cm–2 at 850 °C, 4% H2O/H2, whereas the Ni/CGO infiltrated cell reached ∼0.3 W cm–2, with the current morphologies and loadings. Operation at 0.125 A cm–2, 850 °C, feeding 50% H2O/H2 to the anode and air to the cathode, for a period &gt;300 h, showed superior stability for the Ru/CGO infiltrated cell, with ∼0.04 mV h–1 degradation rate, when compared to the Ni/CGO infiltrated cell (∼0.5 mV h–1). For the Ni/CGO case, the observed degradation has been tentatively linked to initial changes in the electrochemical active area and long‐term detrimental interactions between components.

Electrochemical performance and stability of electrolyte-supported solid oxide fuel cells based on Y-substituted SrTiO3 ceramic anodes

Electrolyte-supported solid oxide fuel cells (SOFCs) based on 6% Sc2O3-stabilized ZrO2 (6ScSz) electrolyte and Y0.07Sr0.895TiO3 (YST) anode materials were prepared. Ni and CGO particles were infiltrated on the pore walls within the ceramic anode framework as catalysts for the anode reaction. The performance, redox stability, and time-dependent degradation of the cells were tested. Electrochemical impedance spectroscopy was used to determine the non-Ohmic losses of the cells. Possible reasons for the observed performance degradation were investigated. The mechanism of the anode reaction is also discussed based on the electrochemical results.

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.

High performance intermediate temperature micro-tubular SOFCs with Ba0.9Co0.7Fe0.2Nb0.1O3−δ as cathode

In this study, anode supported intermediate temperature micro-tubular solid oxide fuel cells (MT-SOFCs) have been fabricated by combination of phase-inversion, dip-coating, co-sintering and printing method. The MT-SOFC consists of a ∼300 μm wall-thickness Ni–Sc2O3 stabilized ZrO2 (ScSZ) anode tube, ∼10 μm ScSZ dense electrolyte layer, ∼10 μm Ce0.9Gd0.1O2−δ (GDC) membrane buffer layer and ∼50 μm Ba0.9Co0.7Fe0.2Nb0.1O3−δ (BCFN) cathode layer. SEM and electrochemical impedance spectroscopy (EIS) analysis suggested that the novel structured anode can remarkably diminish the porous anode geometrical tortuosity and improve the fuel gas diffusivity. High peak power densities of 0.34, 0.51 and 0.72 W cm−2 have been achieved with humidified hydrogen as the fuel and ambient air as oxidant at 550, 600 and 650 °C, respectively. Further, the cell has demonstrated a very stable performance with no significant cell voltage degradation under a constant current of 0.6 A cm−2 for over 213 h test at 650 °C.

Equivalent Circuit Model Analysis of LSM/ScSZ Composite Cathodes Prepared by Impregnating LSM/ScSZ Powder Slurry into a Prefabricated Porous ScSZ Layer

Relationship between microstructure and performance of Sr-doped LaMnO3 (LSM)/Sc2O3-stabilized ZrO2 (ScSZ) composite cathodes was investigated using equivalent circuit models (ECMs) analysis. The cathodes were prepared by impregnating prefabricated porous ScSZ layers with a LSM/ScSZ powder slurry. This method enabled control of the electrolyte surface roughness. Three different ECMs were used for analysis: (i) commonly used ECM in which resistances were connected in series; (ii) ECM consisting of the double-phase boundary (DPB) and the triple-phase boundary (TPB) at the LSM/ScSZ interface; and (iii) ECM representing a three-dimensional electrode. The analysis revealed that impedance response is influenced by the area ratio of the DPB to the TPB although cathode performance is mainly determined by the TPB.

Electrode Performance in a Solid Oxide Electrolyzer Cell (SOEC)

An electrolyte-supported solid oxide electrolysis cell (SOEC) consisting of a ScSZ (Sc2O3-stabilized ZrO2) electrolyte, Ni-YSZ cathode and LSCF (lanthanum strontium cobalt ferrite)-GDC (Gd2O3-doped CeO2) anode was fabricated. Current density-voltage (j-V) curves were obtained for the cell under SOFC and SOEC modes and the cell performance was discussed in combination with corresponding EIS measurements. It was possible to determine the ohmic, cathodic and anodic contributions to the total polarization loss using the three-electrode technique. It was found that the overpotential of the Ni-YSZ electrode contributed largely to the performance of the cell.

Steam Electrolysis Characteristics for Various H2 and O2 Concentrations in Supply Gases

The steam electrolysis performance of a Ni-based electrode-supported tubular cell with a Sc2O3-stabilized ZrO2 electrolyte was evaluated at 650°C for various concentrations of H2 and O2 in the supply gases. It was found that an increase in the concentration of electrolytic products (H2 and O2) around the relevant electrode decreased the electrode polarization, and enhanced the electrolysis performance. This result suggests that the electrolysis can be performed with no carrier gases, and the direct storage of the steam electrolysis products H2 and O2 is feasible.

Effect of grain size on electrical properties of scandia-stabilized zirconia

10 mol % Sc2O3-stabilized ZrO2 (10ScSZ) was sintered by several processes including spark plasma sintering, milliwave sintering and conventional sintering. The electrical conductivity of the sintered 10ScSZ samples was obtained by AC impedance spectra, and compared with grain size and the crystal phase. As average grain size decreased, the electrical conductivities decreased and increased at temperatures higher and lower than the phase transition point, respectively. These trends suggested that the fraction of a low conductivity rhombohedral phase increases with increasing grain size at a temperature lower than the phase transition point, and that the total thickness of the grain boundary affects the electrical conductivity at a temperature higher than the phase transition point.

SOFC-SOEC Characteristics of an Intermediate-temperature Micro-tubular Ceramic Reactor Using Ag for Current Collecting

Ni-based electrode-supported cells with a Sc2O3-stabilized ZrO2 electrolyte have been evaluated as solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) in intermediate-temperature operation. Using a Ag current collector, a power density of 1.68 W cm−2 at 2.87 A cm−2 and an electrolysis voltage of 1.42 V at 0.57 A cm−2 were obtained at 650°C. The current efficiency in steam electrolysis was almost 100%. This suggests that the high performance is related to decreases in current collector resistance and polarization of the air electrode, due to the formation of Ag current collecting paths on the air electrode. During a 450 h durability measurement in SOEC mode, a degradation rate of 0.056 mV h−1 was observed. Impedance measurements showed the polarization of the steam electrode increased, while the polarization of the air electrode and ohmic resistance showed no change, after the long-term SOEC operation.

Improvement of Micro Tubular SOFCs using Multi-Layered Electrolyte

Fabrication technology of multi-layered electrolyte for micro tubular SOFCs has been developed. Sc2O3 stabilized ZrO2(ScSZ) and Gd2O3-doped CeO2 (GDC) was selected for the electrolyte materials and a GDC-ScSZ-GDC multi-layered electrolyte was successfully prepared using multiple dip coating and co-firing technique. The thickness of each layer for the electrolyte was about 3, 3, and 12 micron, respectively. The cell performance test showed the open circuit voltage (OCV) of over 1 V at 600oC or higher, while a SOFC with 10 micron thick GDC electrolyte showed the OCV of 0.78 V at 600oC. The cell performance of 0.35 Wcm-2 @ 0.7 V was obtained at the operating temperature of 650 oC.