Yttria-stabilized Zirconia Solid Electrolyte Research Articles

Production and characteristics of composite nanopowders using a fiber ytterbium laser

A fiber ytterbium laser is used to obtain weakly aggregated nanopowders for yttria-stabilized zirconia solid electrolytes, ZnO and ZnS phosphors, and YAG- and Y2O3-based optical ceramics. The characteristics of the nanopowders are reported. The productivity and energy consumptions of the process with the use of fiber and CO2 lasers are compared. The influence of the buffer gas pressure in the evaporation chamber on the specific surface area of the nanopowder and particle size distribution is studied. The elemental composition of nanoparticles is found to change relative to the composition of the target when yttrium aluminum garnet is evaporated. In the pulsed mode of operation, the energy needed for nanopowder production is minimal when the radiation pulse is about 100 μs long.

Electrochemical Investigation of the O2 ( g ) , Ni / YSZ System Using Cyclic Voltammetry

Cyclic voltammetry was used for the electrochemical study of a Ni thin film deposited on an yttria-stabilized zirconia (YSZ) solid electrolyte pellet at temperatures between 350 and at 20 kPa and atmospheric pressure. NiO formed electrochemically by the species supplied from the electrolyte upon anodic polarization, while it grew according to the parabolic growth law, in agreement with Wagner’s oxidation theory of metals at high temperatures. A model for the NiO formation was proposed, where NiO is formed at the Ni/YSZ interface and grows by the outward diffusion of species through NiO, which is determined as the rate-limiting step. This implies the autoinhibition of NiO formation by its continuous growth. The above result agrees with Wagner’s theory; however, a difference between the latter and the current model is the means of supplying oxygen for NiO formation. Also, evolution reaction is not firmly affected by NiO formation because it is controlled by the electronic current. An apparent activation energy of the limiting diffusion step was calculated to be 35 kJ/mol under 400 mV and decreased with potential increase.

Influence of various lanthanides on the properties of Sr 0.8Ce 0.1Ln 0.1MnO 3 − δ and Sr 0.9Ce 0.05Ln 0.05CoO 3 − δ ceramics and thick film electrodes

In this work nonstoichiometric strontium cerium manganites and cobaltites Sr 0.8Ce 0.1Ln 0.1MnO 3 − δ and Sr 0.9Ce 0.05Ln 0.05CoO 3 − δ with low content of various lanthanides (Ln = La, Ce, Nd, Sm, Gd, Dy, Yb) were investigated as potential electrode materials for solid state fuel cells and gas sensors. Synthesis and sintering conditions as well as dc resistivities and thermal expansion coefficients of the developed ceramics in the range 20–900 °C were studied. On the basis of complex impedance spectroscopic studies of yttria stabilized zirconia solid electrolyte samples with thick film perovskite electrodes four contributions to the dielectric response were found, attributed to the grains and the grain boundaries of the solid electrolyte, to the electrode–electrolyte interface and to the electrodes, in the descending order of frequency. Dc conductivity of the ceramic samples and thick films exhibited semiconducting, semimetallic or metallic character, depending on the composition and the examined temperature range. In the temperature range of 600–900 °C, a very good agreement was found between the experimental and the theoretical values of electromotive force of concentration oxygen cells based on zirconia solid electrolyte with perovskite electrodes.

Zirconia-based electrochemical gas sensors using nano-structured sensing materials aiming at detection of automotive exhausts

The mixed-potential-type yttria-stabilized zirconia (YSZ)-based planar sensors using nano-structured sensing-electrodes (SE) were fabricated and evaluated in the wide range of operating temperatures under the wet (5 vol.% water vapor) condition aiming at detection of exhaust gases and ammonia slip in automobiles. The sensors using nano-structured NiO-, laminated (NiO–Au)- or pure Au-SEs fabricated by r.f. sputtering gave the sensitive and selective responses to NO 2 at 600 °C. Furthermore, it was found that the fabrication method of Au-SE as well as the roughness of surface of YSZ solid electrolyte influenced to a large extent on the sensing characteristics of the fabricated sensors. The higher catalytic activity to electrochemical reaction of oxygen for the Au nano-particles fabricated by impregnation method resulted in the excellent sensitivity and selectivity to propene. The implementation of smooth YSZ substrates led to achieve the highly selective response to ammonia in the concentration range of 20–200 ppm. The detailed sensing characteristics and the proposed sensing mechanisms for the obtained sensors are discussed here.

Performance of oxygen sensor in lead–bismuth at high temperature

Performance of yttria-stabilized zirconia solid electrolyte oxygen sensor with a reference electrode of Bi/Bi 2O 3 was investigated. The oxygen sensor was tested in alumina vessel in order to prevent generating of impurities. The oxygen potential in the melt was controlled by injecting steam–hydrogen gas mixture ( P H 2 / P H 2 O ) into stagnant LBE. The electromotive force (EMF) of the sensor was compared with the theoretical EMF derived from the Nernst equation at various LBE temperatures (550–700 °C). The influences of various injection gas temperatures (200–500 °C) on the sensor output were also investigated. It was found that the sensor signals of the oxygen potential in LBE have not been affected by the injection gas temperature. The results also showed that the measured EMFs were in good agreement with the theoretical values of the EMF. The material aspects were investigated as well. The SEM (Scanning Electron Microscope) devices were used to analyze the cross-section of oxygen sensors after the exposition to LBE at 700 °C for 1000 h. The SEM micrograph showed that the yttria-stabilized zirconia solid electrolyte had an excellent corrosion resistance to the high temperature LBE as the working fluid and high temperature bismuth as the reference fluid.

Ru-C Nano-Composite Films Prepared by PECVD as an Electrode for an Oxygen Sensor

Ruthenium-Carbon (Ru-C) nano-composite films were prepared by microwave-induced plasma-enhanced chemical vapor deposition (PECVD) and the effects of deposition conditions on the microstructure and electrical properties were investigated. The films consisted of agglomerated grains of 10 to 20 nm in diameter, in which Ru particles of 2.5 to 3.5 nm in diameter were dispersed in an amorphous C matrix. The C contents of the films ranged from 86 to 94 vol%. The electrical properties of Ru-C nano-composite films as a catalytic electrode for an yttria-stabilized zirconia (YSZ) solid electrolyte were evaluated by AC impedance spectroscopy. The electrical conductivity at the Ru-C/YSZ interface (σi) was 0.2 × 10-3 Sm-1 at 500 K and increased with increasing temperature. The activation energy of the σi was 70 kJ/mol. Electro-motive-force (EMF) values of an oxygen concentration cell constructed from YSZ electrolyte and Ru-C nano-composite electrodes responded to the change of oxygen partial pressure above 473 K. The response time of the concentration cell was less than 10 s above 573 K.

Microstructures and Electrical Properties of Ru-C Nano-Composite Films by PECVD

Ruthenium-Carbon (Ru-C) nano-composite films were prepared by plasma-enhanced chemical vapor deposition (PECVD) and the effects of deposition conditions on the microstructure and electrical properies were investigated. The films consisted of agglomerated grains of 10 to 20 nm in diameter, in which Ru particles of 2.5 to 3.5 nm in diameter were dispersed in a C matrix. The C content of the films was about 90 vol%. The electrical properties of Ru-C nano-composite films as a catalytic electrode for an yttria-stabilized zirconia (YSZ) solid electrolyte were evaluated by AC impedance spectroscopy. The interfacial electrical conductivity at the Ru-C/YSZ interface was 0.2 x 10 -3 Sm -1 at 500 K and increased with increasing temperature. The activation energy of the interfacial electrical conductivity was about 70kJ/mol, implying an oxygen diffusion limited process at the interface.

Methane oxidation over nanocrystalline Ce0.45Zr0.45La0.10O2-δ/Pt and Ce0.9Sm0.1O2-δ/Pt anodes

The electrocatalytic activity of composite anodes, comprising metallic platinum and nanocrystalline Pt-modified Ce0.45Zr0.45La0.10O2-δ and Ce0.9Sm0.1O2-δ , was evaluated for the oxidation of dry methane in a solid oxide fuel cell (SOFC) – type reactor with yttria-stabilized zirconia solid electrolyte. At 923–1073 K, the total combustion is dominant, whilst above 1100 K the composite anodes exhibit significant catalytic activity towards the partial oxidation of methane (POM). At 1223 K and for a O2:CH4 ratio equal to 0.5, stoichiometric for the POM reaction, CO selectivity and CH4 conversion over Ce0.9Sm0.1O2-δ /Pt achieves 55% and 41%, respectively. Under these conditions, Ce0.45Zr0.45La0.10O2-δ /Pt anodes possess higher catalytic activity for partial oxidation, associated with the lower lattice oxygen mobility and instability of the Ce0.45Zr0.45La0.10O2-δ cubic fluorite structure, and provide 54% conversion efficiency with 73% selectivity to carbon monoxide. Further increase in CH4 conversion and CO selectivity can be achieved by packing the reactor with an additional catalyst, such as Pt-promoted Ce0.45Zr0.45La0.10O2-δ or LaNiO3-δ / Al2O3 .

The phase transition and phase stability of magnetoelectric BiFeO 3

A study of phase stability of BiFeO 3 has been carried out employing a solid-state ionic titration technique, which involved yttria-stabilized zirconia (YSZ) solid electrolyte. As oxygen was intermittently titrated out of the sample, the oxygen activity monotonically decreased in the system until the oxygen nonstoichiometry in the BiFeO 3 phase reached its limiting value. We have observed the appearance of a plateau at this point where the BiFeO 3 phase started to undergo a decomposition reaction. X-ray diffraction data show that BiFeO 3 phase is transforming to a new Bi 25FeO 40 phase. Especially, an interesting result is that Bi 2Fe 4O 9 was determined in the decomposed components. Therefore, further research is needed about the decomposition reaction of BiFeO 3. This work is useful for the production of novel devices.

Transport properties of sealants for high-temperature electrochemical applications: RO–BaO–SiO 2 (R = Mg, Zn) glass–ceramics

The electrical properties and oxygen permeability of glass–ceramics 55SiO 2–27BaO–18MgO, 55SiO 2–27BaO–18ZnO and 50SiO 2–30BaO–20ZnO (%mol), which possess thermal expansion compatible with that of yttria-stabilized zirconia (YSZ) solid electrolytes, were studied between 600 and 950 °C in various atmospheres. The ion transference numbers, determined by the modified electromotive force (e.m.f.) technique under oxygen partial pressure gradients of 21 kPa/(1–8) × 10 2 Pa and 21 kPa/(1 × 10 −18–2 × 10 −12) Pa, are close to unity both under oxidizing and reducing conditions. The electronic contribution to the total conductivity increases slightly on increasing temperature, but is lower than 2% and 7% for the Zn- and Mg-containing compositions, respectively. The conductivity values measured by impedance spectroscopy vary in the range (1.4–7.8) × 10 −6 S/cm at 950 °C under both oxidizing and reducing conditions, with activation energies of 122–154 kJ/mol and a minor increase in H 2-containing atmospheres, indicating possible proton intercalation. In agreement with the electrical measurements which indicate rather insulating properties of the glass–ceramics, the oxygen permeation fluxes through sintered sealants and through sealed YSZ/glass–ceramics/YSZ cells are very low, in spite of an increase of 15–40% during 200–230 h under a gradient of air/H 2–H 2O–N 2 due to slow microstructural changes.

Strong Performance Improvement of La0.6Sr0.4Co0.8Fe0.2O3 − δ SOFC Cathodes by Electrochemical Activation

It is shown that the electrochemical resistance of mixed conducting solid oxide fuel cell (SOFC) model cathodes can be reduced drastically by a short but strong dc polarization of the cell. The samples investigated are dense thin-film microelectrodes of on a yttria-stabilized zirconia solid electrolyte. Relative performance improvements of more than two orders of magnitude can be achieved with a cathodic dc bias of the order of 1 V, applied for a few minutes at fuel cell operating temperature. The positive effect on the electrode performance corresponds to an acceleration of the oxygen surface exchange reaction, initially the resistance determining process. For this surface-related resistance, absolute values as low as at 700°C have been obtained. After such an activation, the resistance slowly increases again on a much larger time scale, indicating the possibility of a steady performance enhancement by a periodic activation with short dc pulses. X-ray photoelectron spectra show that a strong cathodic polarization severely changes the cation concentrations within the outermost surface layer of the electrode, and these field-induced surface compositional changes are assumed to be the main cause of the performance improvement.

Oxygen measurements in stagnant lead–bismuth eutectic using electrochemical sensors

Sensors are the major part of an active oxygen control system (OCS) to be used in ADS reactors employing lead bismuth eutectic (LBE). We tested Pt/air and Bi/Bi 2O 3 probes based on yttria-stabilized zirconia (YSZ) solid electrolytes. The sensors were calibrated by evaluating the electromotive force (EMF) – temperature dependencies in oxygen un-/saturated stagnant LBE compared to the van’t-Hoff’s isotherm. Also, probe kinetics while changing the H 2/H 2O ratio was studied. Typical, reproducible curves are presented confirming attainment of oxygen equilibrium between the fluids. The sensor outputs are deterministic, predictable. Exceptional small drifts were due to interfacial kinetics, not to the sensors behavior. Simultaneous testing of several probes in one melt was performed. The sensors seemed to be qualified for large scale use.

YSZ Thin Films Deposited on NiO‐CSZ Anodes by Pulsed Injection MOCVD for Intermediate Temperature‐SOFC Applications

Yttria‐stabilized zirconia (YSZ) films are prepared on NiO–CaSZ by PIMOCVD (pulsed injection metal organic chemical vapor deposition). High quality, 5 to 10 μm thick, totally dense YSZ layers (Figure) are prepared by controlling the oxygen partial pressure during the deposition. YSZ solid electrolyte deposition onto Ni–YSZ eutectic substrate is found to be a promising combination with regard to intermediate‐temperature solid‐oxide fuel cell applications.

Cellulose-precursor synthesis of nanocrystalline Ce0.8Gd0.2O2?? for SOFC anodes

Developments of intermediate-temperature solid oxide fuel cells (IT SOFCs) require novel anode materials with a high electrochemical activity at 800–1070 K. The polarization of cermet anodes, made of nickel, ceria and yttria-stabilized zirconia (YSZ) and applied onto a YSZ solid electrolyte, can be significantly reduced by catalytically active ceria additions, the relative role of which increases with decreasing temperature. Further improvement is observed when using Ce0.8Gd0.2O2−δ (CGO) having a high oxygen ionic conductivity instead of undoped ceria, owing to enlargement of the electrochemical reaction zone. Nanocrystalline CGO powders with grain sizes of 8–35 nm were thus synthesized via the cellulose-precursor technique and introduced into Ni–CGO–YSZ cermets, and tested in contact with a (La0.9Sr0.1)0.98Ga0.8Mg0.2O3−δ (LSGM) electrolyte at 873–1073 K. The results showed that the anode performance can be enhanced by additional surface activation, in particular by impregnation with a Ce-containing solution, and also by incorporation of YSZ, which probably acts as a cermet-stabilizing component. The overpotential of the surface-modified Ni–CGO (25 wt%–75 wt%) anode in a 10% H2/90% N2 atmosphere was approximately 110 mV at 1073 K with a current density of 200 mA/cm2.

Relation between potential and catalytic activity of rhodium in propylene combustion

The relation between the catalyst potential and the catalytic performance has been investigated in the gas-phase combustion of propylene with oxygen over rhodium catalysts at 375 °C. The rhodium catalyst, deposited on yttria-stabilized zirconia (YSZ) solid electrolyte, also served as working electrode in the electrochemical cell. Under open-circuit conditions, the measured catalyst potential was found to be a sensitive indicator of the oxidation state of the rhodium catalyst, which influences the catalytic reaction rate dramatically and depends strongly both on the method of catalyst film preparation and on the composition of the reacting gas mixture. In turn, under closed-circuit conditions, the applied catalyst potential is a convenient tool to maintain the catalyst in its more active, reduced form and to control its catalytic performance. The activity of atomic oxygen at the three-phase boundary (tpb) during open-circuit catalytic reaction was estimated from solid electrolyte potentiometric (SEP) measurements, in good agreement with the average surface oxidation state obtained from XRD and XPS analyses. O/Rh atomic ratios higher than stoichiometric were found by XPS at the outer surface of the catalysts suggesting a strong open circuit O2− spillover due to strong metal support interactions (SMSI) and a concomitant extension of the electric double layer to the gas-exposed catalyst surface, similarly to emersed electrodes in aqueous electrochemistry. Applying potentials up to several hundreds of mV, highly nonfaradaic promotion of propylene combustion was achieved. Electrochemical promotion of catalysis (EPOC) was most efficient at stoichiometric gas composition, that is, close to the limit of surface reduction, and with the catalyst exhibiting the smallest O2− spillover population at open-circuit conditions.

Low cost fabrication and characterization of thin-substrate YSZ solid electrolyte

A new technique for the fabrication of yttria-stabilized zirconia (YSZ) substrates using aqueous gel-casting has been developed. This technique has been used to fabricate planar thin-substrate YSZ fuel cells. A thin-substrate YSZ electrolyte with high density and low porosity was prepared by this method with 57 vol.% solid content slurry. The character of the technique was discussed based on the influence of dispersant and pH value on slurry. After sintering, the YSZ electrolyte thickness is between 100 and 200 μm, and the electrolyte area is 100 mm×100 mm. The research shows that aqueous gel-casting allows fabricate thin YSZ substrate with high density and homogenous structure. The method is suitable for preparing thin-substrate electrolyte of YSZ.

Oxidation of Dry Methane on the Surface of Oxygen Ion-Conducting Membranes

The surface exchange limitations of oxygen permeation through dense mixed-conducting membranes enhance membrane stability, enabling the operation of mixed conductors, such as La0.3Sr0.7Co0.8Ga0.2O3-δ (LSCG) and La2Ni0.9Co0.1O4+δ (LNC), under air/dry CH4 gradient up to temperatures as high as 1173–1223 K. Testing of these materials in a model disk-shaped membrane reactor at 1023–1223 K showed high CO2 yields (>75%). In particular, at 1173 K, the CO selectivity was 17% for LNC and 2% for LSCG ceramics, with methane conversion efficiency of 20 and 37% respectively. Similar tendency was observed for a fuel cell-type reactor with yttria-stabilized zirconia solid electrolyte and cermet Ce0.8Gd0.2O2-δ/Pt anode, where decreasing the molar ratio between methane and electrochemically supplied oxygen from approximately 10 to 2 decreases CO/CO2 ratio at the outlet down to 0.3. This behavior suggests significant role of the complete methane oxidation on the interface between an oxygen ion-conducting membrane and gas phase, thus making it necessary to incorporate reforming catalysts in the reactors.

Catalytic and electrocatalytic oxidation of carbon monoxide on a Fe electrode in a solid electrolyte cell

The catalytic oxidation of carbon monoxide on Fe catalyst was studied at 300–500°C and atmospheric total pressure. The reaction was studied under both open- and closed-circuit operation in an yttria-stabilized zirconia solid electrolyte cell. The technique of Solid Electrolyte Potentiometry (SEP) was used to monitor the thermodynamic activity of oxygen adsorbed on the Fe electrode under open circuit. Kinetic and potentiometric measurements were combined in order to elucidate the reaction mechanism. The results are in agreement with a Langmuir–Hinselwood type of adsorption-reaction with two different adsorption sites for carbon monoxide and oxygen. Under closed circuit, the effect of electrochemical oxygen “pumping” to the catalyst was examined. The operation of the cell was almost Faradaic as the rate enhancement factor (Λ) values measured were close to unity.

High–temperature phase chemistry of the system Gd–Pd–O

Anisothermal sectionof the phase diagram for the system Gd–Pd–O at 1223 K has been established by equilibrationof samples and phase identification after quenching by optical and scanning electron microscopy, X–ray powder diffraction, and energy dispersive spectroscopy. Three ternary oxides Gd4PdO7,Gd2PdO4 and Gd2Pd2O5 were identified. Liquid alloys, the four inter–metallic compounds and Pd–rich solid solutionwere found to be inequilibrium with Gd2O3.Based on the phase relations, four solid–state cells were designed to measure the Gibbs energies of formation of the three ternary oxides in the temperature range from 920 to 1320 K. Although three cells are sufficient to obtain the properties of the three compounds, the fourth cell was deployed to cross check the data. An advanced version of the solid–state cell incorporating a buffer electrode with yttria–stabilized zirconia solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode was used for high–temperature thermodynamic measurements. The standard Gibbs energy of formation of the inter–oxide compounds from their component binary oxides can be represented by the following equations:Gd4PdO7(s) : Δf(ox)G0/J mol–1 = –25,030 + 0.33T (±140), Gd2PdO4(s) : Δf(ox)f(ox)G0/J mol–1 = –25,350 + 0.84T (±135), Gd2Pd2O5(s) : Δf(ox)f(ox)G0/J mol–1 = –48,700 + 0.38T (±270)Based on the thermodynamic information, isothermal chemical potential diagrams and isobaric phase diagrams for the system Gd–Pd–O are developed.

Oxygen Potential in Molten Tin and Gibbs Energy of Formation of SnO2 Employing an Oxygen Sensor

On-line monitoring of the dissolved impurities in molten metals is important for better process control during alloying and metal refining operations. Disposable-type oxygen sensors are commercially available and are used in the iron and steel industry worldwide. However, there is a need for developing low-cost, reliable, long-life oxygen sensors for continuous monitoring of dissolved oxygen in molten metals. In this paper we present the results of the physical and electrical characterization of the solid electrolyte material ZrO2 (8 mol%Y2O3). The experimental results of the application of long-life solid-state electrochemical sensors designed using yttria-stabilized zirconia solid electrolyte for the measurement of oxygen potential in molten tin between 823 to 1273 K and standard Gibbs energy of formation of SnO2 from its elements are also reported.