Scandia-stabilized Zirconia Research Articles

Electrochemical performance of highly active ceramic symmetrical electrode La0.3Sr0.7Ti0.3Fe0.7O3-δ-CeO2 for reversible solid oxide cells

The electrolyte supported cells with the La0.3Sr0.7Ti0.3Fe0.7O3-δ (LSTF0.7)-CeO2|ScSZ (scandia-stabilized zirconia)|La0.3Sr0.7Ti0.3Fe0.7O3-δ-CeO2 symmetrical configuration are fabricated by infiltration method and investigated as reversible solid oxide cells (RSOCs) at different CO/CO2 ratios. A well-deposited microstructure is observed in the infiltrated electrode. From the I–V curves, the maximum current density of 1.90Acm−2 at 2.0V is obtained for CO: CO2 ratio of 1: 1 for CO2 electrolysis at 850°C, while the corresponding power density under the same conditions is 357mWcm−2 in fuel cell mode. Electrochemical impedance spectroscopy (EIS) is conducted for better understanding the electrode catalytic mechanism. The whole symmetrical cell achieves the polarization resistance (Rp) values of 0.284 and 0.203Ωcm2 at 850°C under open circuit and at 1.4V, respectively. Under reversible operation (cyclic voltammetry) at 800°C, no distinct degradation is found in the cell with the infiltrated LSTF0.7 symmetrical electrodes. The long-term stability at 1.2–2.0V characterized by potentiostatic measurement indicates that the prepared LSTF0.7-CeO2 composite electrode is comparatively stable at low voltages while a degradation behavior is observed at high voltages. These results illustrate that the LSFT0.7-CeO2 composite is a promising active ceramic electrode for symmetrical RSOCs.

Microstructure-scaled active sites imaging of a solid oxide fuel cell composite cathode

Active sites for oxygen reduction reaction in strontium-doped lanthanum manganite (LSM)/scandia-stabilized zirconia (ScSZ) composite cathode of solid oxide fuel cell (SOFC) is visualized in microstructure scale by oxygen isotope labeling. In order to quench a reaction, a SOFC power generation equipment with a nozzle for direct helium gas impinging jet to the cell is prepared. A typical electrolyte-supported cell is operated by supplying 18O2 at 1073 K and abruptly quenched to room temperature. During the quench, the temperature of the cell is decreased from 1073 K to 673 K in 1 s. The 18O concentration distribution in the cross section of the quenched cathode is obtained by secondary ion mass spectrometry (SIMS) with a spatial resolution of 50 nm. The obtained 18O mapping gives the first visualization of highly distributed active sites in the composite cathode both in macroscopic and particle scales.

Role of the gadolinia-doped ceria interlayer in high-performance intermediate-temperature solid oxide fuel cells

In this study, Ba0.5Sr0.5Co0.8Fe0.1Zn0.1O3-δ (BSCFZ) is used as a cathode with anode-supported scandia-stabilized zirconia (ScSZ) as the electrolyte to achieve intermediate-temperature solid oxide fuel cells (SOFCs). A gadolinia-doped ceria (GDC) interlayer is used between the BSCFZ cathode and the ScSZ electrolyte to protect ScSZ against interfacial reaction with the Co-containing cathode. However, an unfavorable reaction occurs at the GDC/ScSZ interface when the sintering temperature of the GDC interlayer is higher than 1200 °C. Thus, the performance of anode-supported SOFCs with GDC interlayers prepared at different sintering temperatures has been evaluated using I–V measurements and AC impedance spectroscopy. The results demonstrate that the sintering temperature of the GDC interlayer should be optimized for both the reaction between the BSCFZ cathode and ScSZ electrolyte and the reaction at the GDC/ScSZ interface. A maximum power density of 0.78 W cm−2 has been achieved at 700 °C with a GDC interlayer sintered at 1200 °C. Furthermore, the cell with the optimized GDC interlayer has been shown to maintain excellent reliability with respect to load cycling and thermal cycling.

Microstructural and Electrochemical Characterisation of Degradation in Nickel Impregnated Scandia-Stabilised Zirconia Electrode during Isothermal Annealing

The impregnation of nickel into porous ceramic scaffolds has been introduced as a promising alternative fabrication technique for solid oxide fuel cell (SOFC) anode as compared to conventional powder mixing [1]. Nevertheless, the stability of impregnated electrodes has not been extensively studied and understood [2]. SOFCs operate at a temperature range from 500 oC to 1000 oC and are expected to have a life time of 40,000 hours [3]. In this study, we examine the degradation of impregnated electrodes in a wet hydrogen atmosphere at a working temperature of SOFCs. Nickel was impregnated into a porous scandia-stabilised zirconia scaffold through 12 impregnation cycles. The impregnated electrodes contained approximately 40 wt% of nickel. Continuous van der Pauw measurement was carried out when the electrode was aged at 650 oC, 800 oC and 950 oC in an atmosphere with 5 vol% hydrogen and 95 vol% nitrogen, at open circuit. A fall in sheet conductivity was found in the electrode during ageing within 1000 min. The conductivity fall was attributed to nickel microstructural evolution during ageing. Electrochemical impedance spectra were collected at a constant time intervals during isothermal annealing at 650 oC, 800 oC and 950 oC for the impregnated electrode. Four resistance contributions were decoupled by equivalent circuit fitting, i.e. the ohmic resistance, the oxygen transfer resistance and the anodic reaction resistance, and the gas diffusion resistance. The increase in both the ohmic resistance and the polarisation resistances with time revealed the isothermal degradation brought about a diminishing active anode area. A more pronounced decrease in anodic polarisation resistance suggested a reduction in triple phase boundary (TPB) density. Secondary electron images of electrodes before and after ageing showed an increase in nickel particle size and a decrease in the number of particles, providing microstructural evidence of nickel coarsening. Nickel coarsening was identified as the main mechanism of degradation for the electrode in wet hydrogen as it caused a reduction in both TPB sites and percolating nickel paths. Combining time-dependent electrochemical performance with post-mortem electrode microstructure clarifies the physical processes behind the degradation quantified by increasing area specific resistances. [1] M. Lomberg, E. Ruiz-Trejo, G. Offer, and N. P. Brandon, “Characterization of Ni-infiltrated GDC electrodes for solid oxide cell applications,” J. Electrochem. Soc., vol. 161, no. 9, pp. F899–F905, 2014. [2] S. P. Jiang, “Nanoscale and nano-structured electrodes of solid oxide fuel cells by infiltration: Advances and challenges,” Int. J. Hydrogen Energy, vol. 37, no. 1, pp. 449–470, Jan. 2012. [3] H. Tu and U. Stimming, “Advances, aging mechanisms and lifetime in solid-oxide fuel cells,” J. Power Sources, vol. 127, no. 1–2, pp. 284–293, Mar. 2004.

A Novel Method to Fabricate Inert Substrate-Supported Microtubular Solid Oxide Fuel Cells by Selective Leaching

Microtubular solid oxide fuel cells (MT-SOFCs) are promising electrochemical energy conversion devices due to their high volumetric power density, capability of bearing high thermal stresses, and rapid start-up characteristics. In this study, a novel method is suggested to fabricate inert substrate-supported MT-SOFCs by leaching the nickel from a microtubular anode support. The anode-supported MT-SOFCs were prepared by sequential dip coating and leaching. The manufactured inert substrate-supported MT-SOFCs possess the configuration of yttria-stabilized zirconia (YSZ) support/Ni–scandia-stabilized zirconia (ScSZ) anode/ScSZ electrolyte/strontium-doped lanthanum manganite (LSM)-ScSZ cathode/LSM cathode current collector. The selective leaching of nickel from the Ni-YSZ composite was used in conjunction with burning out of the microcrystalline cellulose pore-former for proper tuning of the porosity and facilitating gas diffusion through the inert support. The electrochemical performance of the manufactured inert substrate-supported cell was characterized in hydrogen at 700 °C.

Particle-Scaled Visualization of Active Sites in LSM/ScSZ Composite Cathode of SOFC through Oxygen Isotope Labeling

Active sites for oxygen reduction reaction in strontium-doped lanthanum manganite (LSM)/scandia-stabilized zirconia (ScSZ) composite cathode of solid oxide fuel cell (SOFC) was visualized in particle scale by oxygen isotope labeling. In order to quench a reaction, a SOFC power generation equipment with a nozzle for direct helium impinging jet to the cell was prepared. A typical electrolyte-supported cell was operated by supplying 18O2 at 1073 K and abruptly quenched to room temperature. During the quench, the temperature of the cell was decreased from 1073 K to 673 K in 1 sec. The 18O concentration distribution in the cross section of the quenched cathode was obtained by secondary ion mass spectrometry (SIMS) with a spatial resolution of 50 nm. The obtained 18O concentration mapping gives the inhomogeneous distribution of active sites in the composite cathode.

Microstructural and Electrochemical Characterisation of Degradation in Nickel Impregnated Scandia-Stabilised Zirconia Electrode during Isothermal Annealing

In this study, we examine the degradation of nickel impregnated scandia-stabilised zirconia (ScSZ) electrode in wet hydrogen at a working temperature of SOFCs. Continuous van der Pauw measurement was carried out when the electrode was aged at 650 oC in an atmosphere with 5 vol% hydrogen and 95 vol% nitrogen. A fall in sheet conductivity was found in the electrode during ageing within 1000 min. Electrochemical impedance spectra were collected at a constant time intervals during isothermal annealing at 650 oC, 800 oC and 950 oC for the impregnated electrode, at open circuit. Three resistance contributions were decoupled by equivalent circuit fitting, i.e. the ohmic resistance, the anodic reaction resistance, and the gas diffusion resistance. Secondary electron images of electrodes before and after ageing showed an increase in nickel particle size and a decrease in the number of particles, providing microstructural evidence of coarsening. Nickel coarsening was identified as the main mechanism of degradation for the electrode in wet hydrogen.

Catalytic activity of infiltrated La0.3Sr0.7Ti0.3Fe0.7O3−δ–CeO2 as a composite SOFC anode material for H2 and CO oxidation

Finding cost-effective and efficient anode materials for solid oxide fuel cells (SOFCs) is of prime importance to develop renewable energy technologies. In this paper, La and Fe co-doped SrTiO3 perovskite oxide, La0.3Sr0.7Ti0.3Fe0.7O3−δ (LSTF0.7) composited with CeO2 is prepared as a composite anode by solution infiltration method. The H2 and CO oxidation behavior and the electrochemical performance (electrochemical impedance spectra, I–V and I–P curves) of the scandia-stabilized zirconia (ScSZ) electrolyte supported cells fabricated by tape casting with the LSTF0.7–CeO2 composite anode are subsequently measured at various temperatures (700–850 °C). Electrochemical impedance spectra (EIS) of the prepared cells with the LSTF0.7–CeO2|ScSZ|La0.8Sr0.2MnO3 (LSM)–ScSZ configuration illustrate that the anode polarization resistance distinguished from the whole cell is 0.072 Ω cm2 in H2, whereas 0.151 Ω cm2 in CO at 850 °C. The maximal power densities (MPDs) of the cell at 700, 750, 800 and 850 °C are 217, 462, 612, 815 mW cm−2 in H2 and 145, 349, 508, 721 mW cm−2 in CO, respectively. Moreover, a significant decrease of anode activation energy towards H2 oxidation is clearly demonstrated, indicating a better electrochemical performance in H2 than in CO. These results demonstrate an alternative composite anode with high electrocatalytic activity for SOFC practical applications.

Catalytic activity improvement for efficient hydrogen oxidation of infiltrated La0.3Sr0.7Ti0.3Fe0.7O3-δ anode for solid oxide fuel cell

Abstract In this paper, a ceramic perovskite anode for solid oxide fuel cells (SOFCs) is prepared by infiltrating La and Fe co-doped strontium titanium, La 0.3 Sr 0.7 Ti 0.3 Fe 0.7 O 3-δ (LSTF0.7) into porous backbone of scandia-stabilized zirconia (ScSZ) and tested in pure H 2 at 700–850 °C. LSTF0.7 crystal exhibits high reduction stability and good compatibility with ScSZ electrolyte under reducing atmosphere. In order to improve the electrocatalytic activity, 15 wt% of CeO 2 and 7 wt% of Ni are infiltrated into the backbone pores respectively, thus forming LSTF0.7-CeO 2 and Ni-CeO 2 -LSTF0.7 composite anodes. The cell with LSTF0.7 single anode shows a relatively lower maximal power density (MPD) of 401 mW cm −2 in H 2 at 800 °C. While the maximal power densities of the cells with LSTF0.7-CeO 2 and Ni-CeO 2 -LSTF0.7 composite anodes are 612 mW cm −2 and 698 mW cm −2 operated at the same conditions, respectively. The three anode polarization resistances (Rp,a) distinguished from the corresponding full cells are 0.176, 0.086 and 0.076 Ω cm 2 at 800 °C, respectively. The values of the activation energy (Ea) towards H 2 oxidation for the three anodes can be calculated to be 52.2, 46.0, 43.9 kJ mol −1 based on their respective Rp,a. Therefore, the LSTF0.7-based anodes are considered to be promising alternatives for solid oxide fuel cell applications.

A novel high performance composite anode with in situ growth of Fe-Ni alloy nanoparticles for intermediate solid oxide fuel cells

A redox reversible composite anode with Fe-Ni alloy nanoparticles in situ growth on SrLaFeO4-type and LaFeO3-type oxide substrates has been prepared for intermediate temperature solid oxide fuel cell (IT-SOFC) by reducing perovskite precursor La0.4Sr0.6Fe0.75Ni0.1Nb0.15O3-δ (LSFNNb) in wet H2 at 900°C for 1h. The anode has shown an excellent electrochemical catalytic activity for oxidation of hydrogen with much smaller Ea (25.1∼68.9kJmol−1) than the value (>160kJmol−1) at Ni-YSZ anode. A scandium stabilized zirconia (ScSZ) electrolyte supported SOFC with the anode achieves maximum power densities of 0.71, 0.52, 0.35, and 0.21Wcm−2 at 800, 750, 700 and 650°C, respectively in wet H2 (3% H2O), and the corresponding Rp of 0.16, 0.21, 0.35, and 0.60Ωcm2 under OCV. Moreover, the single cell shows stable power output during ∼105h operation at 800°C under 0.7V in wet H2 after a initial degradation, indicating that R-LSFNNb is an excellent candidate as anode of IT-SOFC.

Nanostructured CuCo2O4 cathode for intermediate temperature solid oxide fuel cells via an impregnation technique

Abstract Spinel structure CuCo 2 O 4 nanoparticles are coated onto porous 10mol% scandia stabilized zirconia (SSZ) framework via a solution impregnation process. X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscopy and current–voltage performance measurements have been used to characterize impregnated CuCo 2 O 4 cathodes. The CuCo 2 O 4 nano-particles are uniformly distributed on the surface of the porous SSZ backbones, thus increasing the length of the triple phase boundaries (TPBs). As expected, the polarization resistance of impregnated nanostructured CuCo 2 O 4 is as low as 0.087 Ωcm 2 in air at 800 °C, and delivers a high peak power density of 1136 mW cm −2 .

Toward Selective Lignin Liquefaction: Synergistic Effect of Hetero- and Homogeneous Catalysis in Sub- and Supercritical Fluids

Selective liquefaction of lignin is important for synthesis of value-added phenolic monomers contributing to green chemistry and sustainable energy applications. In the present study, a synergistic effect of a supercritical carbon dioxide (scCO2) acidic catalyst in combination with a heterogeneous metal oxide catalyst, specifically nickel oxide (NiO) or ceria-doped scandia-stabilized zirconia (CeScSZ), in sub- or supercritical water (sbcr/scH2O) for selective liquefaction of alkali lignin is demonstrated for the first time. The scCO2-assisted hydrothermal process in the temperature range of 100–400 °C resulted in highly selective synthesis of the phenolic monomers. On the basis of the total organic carbon (TOC) and gas chromatography–mass spectroscopy (GC–MS) analysis, it is evident that the scCO2 catalyst is essential for enhancing the reaction selectivity in the presence of a heterogeneous catalyst. A combination of homogeneous scCO2 and heterogeneous NiO catalysts resulted in the highest total relative...

Oxygen Reduction, Transport and Separation in Low Silver Content Scandia-Stabilized Zirconia Composites

Dense composites of silver and Sc-stabilized ZrO2 (Ag-ScSZ) are manufactured from ScSZ sub-micrometric particles coated with silver using Tollens’ reagent. A composite with 8.6 vol % of silver exhibits metallic conductivity of 186 S cm−1 and oxygen flux of 0.014 μmol cm−2 s−1 at 600°C for a 1-mm thick membrane when used as a pressure-driven separation membrane between air and argon. To gain insight into the role of oxygen transport in Ag and ScSZ, a dense non-percolating sample (Ag 4.7 vol%) is analyzed by impedance spectroscopy and the transport of oxygen through both phases is modelled. Oxygen transport takes place in both silver and ScSZ but it is still dominated by transport in the ionic conductor and therefore a large volume fraction of the ion conductor is beneficial for the separation. The oxygen transport in the silver clusters inside the composite is dominated by diffusion of neutral species and not by the charge transfer reaction at the interface between ScSZ and Ag, yet small silver particles on the surface improve the reduction of oxygen. Oxygen reduction is highly promoted by silver on the surface and there are no limitations of charge transfer at the interface between silver and ScSZ.

Influence of cerium doping on mechanical properties of tetragonal scandium-stabilized zirconia

Mechanical properties of tetragonal scandium-stabilized zirconium oxide (ScSZ) co-doped with cerium oxide are studied before and after a complete reduction of Ce4+ to Ce3+. Samples of 6Sc1CeSZ (i.e., 6mol% Sc2O3, 1mol% CeO2), 6Sc2CeSZ, and 4Sc1CeSZ are produced by pressure-less sintering in air at 1450°C. Test pieces are annealed at 1200°C in an atmosphere of forming gas (5% H2, 95 % N2). Vickers hardness, Young’s modulus, 4-point bending strength, and fracture toughness are measured at room temperature. Values of hardness and Young’s modulus are similar for different compositions of ScSZ:Ce and stay practically unaffected by the reduction. Both the fracture toughness and strength significantly decrease and the largest drop is measured for the 6Sc2CeSZ, having the highest concentration of cerium. The observed degradation of mechanical properties suggests that cerium-free electrolytes should be preferred for the fabrication of the electrolyte-supported planar SOFCs.

Structural Characteristics and Electrical Conductivity of Spark Plasma Sintered Ytterbia Co‐doped Scandia Stabilized Zirconia

The effect of replacing Sc2O3 with Yb2O3 on the structural and electrical properties of xYb2O3–(12–x)Sc2O3–88ZrO2 has been investigated. Spark plasma sintering technique is employed to fabricate dense bulk samples from the nano‐sized powders. X‐ray diffraction and transmission electron microscopy performed on pellets indicate the existence of cubic and rhombohedral phases in 12ScSZ, and a single cubic phase in all the co‐doped compositions. However, Raman spectroscopic studies suggest the presence of a metastable tetragonal t″‐phase along with rhombohedral phases in 12ScSZ, whereas a single cubic phase in all the co‐doped compositions. Significant enhancement in the conductivity of grain and grain boundary is observed on replacing Sc2O3 with Yb2O3. In the intermediate temperature range, 1Yb11ScSZ exhibits the highest, while 12ScSZ shows the lowest conductivity values, which is attributed to corresponding phases present in that range. Through co‐doping with >1 mol% Yb2O3 leads to conductivity decrease, but the value remains higher than that of 12ScSZ. A sharp conductivity change is observed in 12ScSZ and 1Yb11ScSZ samples, which is attributed to partial phase transition as well to the formation of cation‐vacancy complexes. In this work, the beneficial effect of Yb2O3 co‐doping in 12ScSZ on the phase and conductivity has been highlighted.

Validation of a physically-based solid oxide fuel cell anode model combining 3D tomography and impedance spectroscopy

This study presents a physically-based model for the simulation of impedance spectra in solid oxide fuel cell (SOFC) composite anodes. The model takes into account the charge transport and the charge-transfer reaction at the three-phase boundary distributed along the anode thickness, as well as the phenomena at the electrode/electrolyte interface and the multicomponent gas diffusion in the test rig. The model is calibrated with experimental impedance spectra of cermet anodes made of nickel and scandia-stabilized zirconia and satisfactorily validated in electrodes with different microstructural properties, quantified through focused ion beam SEM tomography. Besides providing the material-specific kinetic parameters of the electrochemical hydrogen oxidation, this study shows that the correlation between electrode microstructure and electrochemical performance can be successfully addressed by combining physically-based modelling, impedance spectroscopy and 3D tomography. This approach overcomes the limits of phenomenological equivalent circuits and is suitable for the interpretation of experimental data and for the optimisation of the electrode microstructure.

Effects of Partial Substitution of CeO2 with M2O3 (M = Yb, Gd, Sm) on Electrical Degradation of Sc2O3 and CeO2 Co-doped ZrO2

Scandia-stabilized zirconia co-doped with CeO₂ is a promising electrolyte for intermediate temperature SOFC, but still shows rapid degradation during a long-term operation. In this study, CeO₂ (1 ㏖%) as a stabilizer is partially substituted with lanthanum oxides (M₂O₃, M=Yb, Gd, Sm) to stabilize a cubic phase and thus durability in reducing atmosphere. 0.5M0.5Ce10ScSZ electrolytes were prepared by solid state reaction and sintered at 1450℃ for 10 h to produce dense ceramic specimens. With addition of the lanthanum oxide, 0.5M0.5Ce10ScSZ showed lower degradation rates than 1Ce10ScSZ. Since Gd₂O₃ showed the highest ionic conductivity among the co-dopants, an electrolyte-supported cell with 0.5Gd0.5Ce10ScSZ was prepared to compare its longterm performance with that of 1Ce10ScSZ-based cell. Maximum power density of 0.5Gd0.5Ce10ScSZ-based cell was degraded by about 2.3% after 250 h, which was much lower than 1Ce10ScSZ-based cell (4.2%).

Isolated hydrogen configurations in zirconia as seen by muon spin spectroscopy andab initiocalculations

We present a systematic study of isolated hydrogen in diverse forms of ${\mathrm{ZrO}}_{2}$ (zirconia), both undoped and stabilized in the cubic phase by additions of transition-metal oxides $({\mathrm{Y}}_{2}{\mathrm{O}}_{3},{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$, MgO, CaO). Hydrogen is modeled by using muonium as a pseudoisotope in muon-spin spectroscopy experiments. The muon study is also supplemented with first-principles calculations of the hydrogen states in scandia-stabilized zirconia by conventional density-functional theory (DFT) as well as a hybrid-functional approach which admixes a portion of exact exchange to the semilocal DFT exchange. The experimentally observable metastable states accessible by means of the muon implantation allowed us to probe two distinct hydrogen configurations predicted theoretically: an oxygen-bound configuration and a quasiatomic interstitial one with a large isotropic hyperfine constant. The neutral-oxygen-bound configuration is characterized by an electron spreading over the neighboring zirconium cations, forming a polaronic state with a vanishingly small hyperfine interaction at the muon. The atom-like interstitial muonium is observed also in all samples but with different fractions. The hyperfine interaction is isotropic in calcia-doped zirconia $[{A}_{\mathrm{iso}}=3.02(8)$ GHz], but slightly anisotropic in the nanograin yttria-doped zirconia $[{A}_{\mathrm{iso}}=2.1(1)$ GHz, $D=0.13(2)$ GHz] probably due to muons stopping close to the interface regions between the nanograins in the latter case.

Oxygen Transport of Lscrf-Scsz Based Mixed Conducting Ceramic Composites

Materials for membrane reactors must possess both high ionic and electronic conductivity as well as good stability over a wide range of oxygen partial pressure. In order to improve the oxygen diffusion, substituting elements is an approach commonly adopted to increase vacancy concentration, but it may also reduce the chemical stability of the membrane [1]. Single phase materials, e.g. La1-x Sr x Co1-y Fe y O3-δ (LSCF), have been shown to possess excellent oxygen permeation properties but suffer from dimensional and chemical instabilities under operation. To improve the long-term stability manganese, chromium etc. can substitute for cobalt but this will sacrifice the permeability of a membrane [2]. Alternatively, focus has been shifted onto dual phase materials which consist of one phase as a good ionic conductor and the other phase as a good electronic conductor. Among all the essential criteria, transport properties of the dense composite layer can profoundly affect the membrane performance. Previous transport measurements work have been carried out on La0.8Sr0.2MnO3-δ (LSM)-Yttria-Stabilised Zirconia (YSZ) [3], LSCF-Ce0.9Gd0.1O2-δ [4] etc. dual phase systems, whilst in this work the La1-x Sr x Cr1-y Fe y O3-δ (LSCrF)-Scandia-Stabilised Zirconia (ScSZ) based dual phase composite system has been investigated. Isotopic Exchange Depth Profiling (IEDP)[5] combined with Secondary Ion Mass Spectrometry (SIMS) was performed in this present work to investigate the mass transport in the composite materials. In order to achieve improved performance optimisation strategies have been applied, which include changing doping level and doping elements in the component LSCrF and ScSZ, and using different volume fractions (LSCrF:ScSZ) in the dual phase composites and optimising the microstructures of the dual phase composites. Additionally, the effects of a humidified environment on the diffusion behaviours of both the single phase and dual phase samples have been studied by using pure labelled H2 18O to reflect operating atmospheres. The diffusion coefficient (D*) at 800 ̊C of LSCrF was found to increase dramatically by 3 orders of magnitude compared to a similar experiment under dry conditions indicating an increase of oxygen vacancy concentration whilst ScSZ remains a good ionic conductor in a pure water atmosphere, and moreover the surface exchange coefficient (k) of ScSZ has been improved to 1.54x10-6 cm/s at 800 ̊C while in dry O2 conditions almost no 18O has been exchanged into the sample. An explanation for this behaviour is that electronic species are not required for the surface exchange process in pure water atmosphere [6] so the rate-limiting steps of surface exchange in ScSZ shifts from dissociation and oxygen reduction in the case of pure oxygen to adsorption steps for water. To obtain further understanding of how the surface has changed during the annealing, X-ray Photoelectron Spectroscopy (XPS) and Low Energy Ion Scattering (LEIS) have been performed in order to observe the surface and sub-surface chemistry and compositional information.

Oxidation-Induced Degradation of SOFC Ni Anodes at High Fuel Utilizations

In the downstream of SOFC systems, higher oxygen partial pressure may cause oxidation-induced Ni anode degradation, associated with the formation of NiO and/or Ni(OH)2. Under experimental conditions for Ni anodes exposing to high oxygen partial pressures, conduction pathways could be destroyed resulting in cell performance degradation. This study is therefore focusing on the changes in microstructure and cell performance at high fuel utilizations. The objective of this study is to derive guidelines and limit for high fuel utilization operation towards higher power generation efficiency. The cells used consist of Ni-based anode with scandia-stabilized zirconia (ScSZ), ScSZ electrolyte, and lanthanum-strontium-manganite(LSM)-ScSZ cathode. Reference electrode made from Pt paste was attached to the cathode side to separate anode and cathode overvoltages. The electrochemical properties of the cells were measured at 800oC by feeding humidified fuel, H2 (20 cc/min)-H2O (80 cc/min)-N2 (100 cc/min), to the anode, and air (150 cc/min) to the cathode. As carrier gas, N2was fed to lower the humidifier temperature for simulating higher humidity conditions. The condition for Ni oxidation was estimated by thermochemical equilibrium calculation software, HSC Chemistry. Performance stability was examined at a constant cell voltage for a constant partial oxygen pressure at the anode for 100h. I-V characteristics before and after the 100h test were measured for confirming performance changes. Focused-ion-beam-scanning electron microscopy (FIB-SEM) observation was conducted for the anode, reconstructed by the brightness differences of BSE images, and microstructures were observed in detail by STEM. Anode voltage threshold was derived from the oxygen partial pressure at the boundary where both Ni and NiO coexist in the phase stability diagram as shown in Fig. 1. The boundary shifted to higher oxygen partial pressure as temperature became higher. The threshold voltage was 0.701 V at 800oC. The performance stability was examined for 100h at the constant cell voltage of 0.6 V, showing stable performance. The resistances did not change before and after the test, and no microstructural change of the anode was observed. The performance stability was also examined at the constant cell voltage of 0.5 V in order to set the experimental condition near the threshold. Anode voltage started to vibrate during the test when anode voltage approached about 0.7 V. The anode-side ohmic and nonohmic resistances increased during the vibration, suggesting Ni oxidation. But the electrochemical measurements revealed no change in anode resistances, indicating that the oxidation of Ni was limited to their surfaces during power generation. The number of isolated Ni particles was still small judging from FIB-SEM reconstruction images, maintaining their conduction pathways. The presence of NiO was also not confirmed by STEM mapping images. The performance stability was also examined at the constant cell voltage of 0.65 V when further humidified fuel, H2 (5 cc/min)-H2O (95 cc/min)-N2 (100 cc/min), was supplied. The performance was stable as shown in Fig. 2 and it was verified that power generation was possible even under such a condition if the anode voltage was kept sufficiently higher than the threshold. Figure 1