Yttria-stabilized Zirconia Membrane Research Articles

Integrated Smart Gas Tracking Device with Artificially Tailored Selectivity for Real-Time Monitoring Food Freshness.

The real-time monitoring of food freshness in refrigerators is of significant importance in detecting potential food spoiling and preventing serious health issues. One method that is commonly reported and has received substantial attention is the discrimination of food freshness via the tracking of volatile molecules. Nevertheless, the ambient environment of low temperature (normally below 4 °C) and high humidity (90% R.H.), as well as poor selectivity in sensing gas species remain the challenge. In this research, an integrated smart gas-tracking device is designed and fabricated. By applying pump voltage on the yttria-stabilized zirconia (YSZ) membrane, the oxygen concentration in the testing chamber can be manually tailored. Due to the working principle of the sensor following the mixed potential behavior, distinct differences in sensitivity and selectivity are observed for the sensor that operated at different oxygen concentrations. Typically, the sensor gives satisfactory selectivity to H2S, NH3, and C2H5OH at the oxygen concentrations of 10%, 30%, and 40%, respectively. In addition, an acceptable response/recovery rate (within 24 s) is also confirmed. Finally, a refrigerator prototype that includes the smart gas sensor is built, and satisfactory performance in discriminating food freshness status of fresh or semi-fresh is verified for the proposed refrigerator prototype. In conclusion, these aforementioned promising results suggest that the proposed integrated smart gas sensor could be a potential candidate for alarming food spoilage.

(Invited) Solid Oxide Membrane (SOM)-Based Electrolytic Production of Metals

Metal producers, such as primary aluminum, ferroalloy, iron and steel, lead, magnesium, and zinc, employ carbothermic, metallothermic, halide reduction, and electrometallurgical methods, to produce the metals and in the process generate significant amounts of pollutants including greenhouse gases (GHG). In the US alone, these industries produce more than 100 million metric tons of CO2e. Because of its impact on the environment there is an urgent need to decarbonize this industrial sector. Electrolysis offers the opportunity to substitute clean electrical energy for fossil fuel energy sources. But electrolytic oxide reduction typically requires carbon anodes and emits CO2 and perfluorocarbons with very high global warming potential. As of this writing, no oxygen-producing inert anode technology has yet scaled commercially beyond a single pilot electrolysis cell.For metal oxide reduction a cost-effective, energy-efficient and low-emissions approach to the inert anode uses a solid oxide membrane (SOM) as a selective electrolyte between the molten salt and anode for the generic production of metals and alloys. The SOM-based electrolysis technology produces metals (Me) and/or their alloys from their respective oxides (MeOx) through direct electrolysis of the oxides: MeOx → Me (cathode) + x/2 O₂ (anode), where x is the stoichiometric amount of oxygen in the metal oxide.The SOM-based electrolytic process brings various advantages such as simplified design, lower cost, lower energy use, lower use of consumables, higher efficiency, zero emissions, and higher purity of metal and oxygen produced. The SOM process features the utilization of an oxygen-ion-conducting membrane-based anode, typically made of stabilized zirconia such as yttria-stabilized zirconia (YSZ) with an oxygen producing electrode on one side of the membrane, a supporting ionic molten salt electrolyte bath, typically an alkali fluoride salt mixture, and a cathode to collect the reduced metal. The oxygen-ion conducting membrane is placed inside the salt and it separates the cathode, also placed in the salt, from the oxygen electrode. The metal oxide is electrolyzed inside the salt medium by applying an electric potential between the cathode and the anode. The metal ions are reduced at the cathode and the oxygen ions migrate through the salt and the membrane and are oxidized at the oxygen electrode. The oxygen produced is pure due to the membrane selectivity, a valuable byproduct, and the membrane completely isolates the anode from the salt bath and metal produced at the cathode. This isolation simplifies anode materials selection, as the anode does not need to be robust to molten salt corrosion. Also, because of the oxygen-ion conducting membrane, the applied potential does not oxidize the halide ions nor the multivalent cations.Since the late 1960s, researchers have recognized and investigated the use of zirconia solid electrolyte for metals production and refining; however, limited progress was achieved due to the poor stability of zirconia membrane in the salt medium. Our work on YSZ membrane stability during SOM electrolysis using eutectic molten salt composition in CaF2 –MgF2 system demonstrated that membrane stability can be greatly enhanced by matching component activity and optical basicity in the salt with that of the membrane. It was shown that the oxide solubility in the salt can be controlled through tailored additions of more stable alkali oxides and monitoring the heat of mixing; higher heat of mixing promotes oxide solubility and vice-versa. Following our initial work, the SOM process has been applied to reduce various metal oxides or oxide compounds to the respective metals or alloys.This talk will provide generic guidelines with examples for the selection of the cell components, including salt electrolyte, cathode, anode configuration, and current collectors. Examples will include both lab and industrial scale applications.

Flexural strength and elastic modulus of gradient structured YSZ membranes with multi-scale pores

3 mol% yttria-stabilized zirconia membranes prepared by a non-solvent induced phase separation technique can balance the performance between robust mechanics and high porosity, because of having a multi-scale porous microstructure. However, the fracture mechanism of membranes with multi-scale pores is incomplete due to the nonlinear structure, which limits its performance optimization. In this study, effects of the various pores on stress distributions and related mechanical properties were experimentally studied and simulated. The gradient diameter of the macro-channels led to a loading direction dependent flexural strength. Meanwhile, no strength degradation was found when penetration depths of these macro-channels increased from 70% to 80% of the membrane thickness, because the channel tails hardly caused interference to the maximum bending stress. The flexural strength was determined by the cellular structure surrounding the macro-channels, and its bimodal porosity distribution reduced the elastic modulus, which might favor an improvement of strain tolerance.

Fracture of Porous Ceramics: Application to the Mechanical Degradation of Solid Oxide Cell During Redox Cycling

Solid Oxide Cell (SOC) is a high-temperature electrochemical device that can be operated in both fuel and electrolysis modes. It is constituted of a dense electrolyte in Yttria Stabilized Zirconia (YSZ) sandwiched between two porous electrodes. The hydrogen electrode is classically made of a cermet composed of Nickel and YSZ (Ni-YSZ). Because of system failures in operation (e.g. a fuel shortage or air re-introduction during the system shutdown), the Nickel is liable to be re-oxidized inducing its volume expansion. The Ni swelling within the cermet leads to a mechanical damage in the YSZ backbone [1] resulting in a degradation of the overall cell performances [2]. Therefore, it is still required to enhance the hydrogen electrode robustness for a higher redox tolerance. For this purpose, it is essential to predict accurately the formation and distribution of micro-cracks in the YSZ network during the re-oxidation. Nevertheless, the modeling of the initiation and propagation of cracks in complex microstructures of porous ceramics is still a subject of investigation. Moreover, there is also a lack of data on fracture properties for the porous YSZ material. In this frame, the present study was focused to propose a relevant model for the initiation and the growth of cracks in porous ceramics in order to simulate the fracture induced by Ni re-oxidation. For this purpose, a coupled experimental and modeling approach has been adopted (Fig.1). For the experimental part, the mechanical characterizations have been performed on thin YSZ membranes of different porosities representative of SOC microstructures. The compression test has been chosen as it generates a quasi-uniform stress field in the tested samples. To this end, micro-pillars of few tens of micro-meters in section have been milled in the YSZ membrane using a xenon plasma focused ion beam. For each investigated porosity, the compressive fracture strength has been measured on several pillars. The tested samples have been carefully characterized to assess the damage in the microstructure. As expected, the compressive strength was found to decrease with increasing the porosity. Moreover, a transition has been observed from a brittle behaviour at low porosity towards a local damage at high porosity [3]. All these experimental data have been used to validate a numerical tool to simulate the fracture in porous microstructure. In this study, a model based on the Phase Field Method (PFM) has been developed. In this approach, the crack is modeled implicitly through a smooth scalar damage variable and its width is controlled by a length scale parameter. Therefore, this method is well adapted to solve the fracture problem in complex and heterogeneous materials since it is mesh independent. The implemented model corresponds to the one proposed by C. Miehe et al. [4] who proposed a staggered scheme for the numerical resolution. A special attention has been paid in this work to evaluate the model capability to accurately predict the crack initiation for porous electrode material. For this purpose, the results of the PFM simulations have been compared to the mixed criterion proposed by D. Leguillon [5]. The study has been conducted on notched samples with different opening angles and for macro-cracks blunted by different cavity sizes. It has been highlighted that the two approaches provide consistent predictions for the crack initiation. Moreover, it has been checked that the PFM model is able to reproduce accurately the expected decrease of the fracture toughness when increasing the material porosity. Once these studies completed, the PFM model has been used to simulate the micro-compression test using a multiscale approach. It has been found that the model is able to reproduce correctly the decrease of the measured compressive fracture strength with the sample porosity. Moreover, the distribution of cracks in the simulations has enabled analysing the transition from a brittle to a local damage detected during the experiments. The validated model has been applied to simulate the fracture induced by the Ni re-oxidation. The repartition and density of micro-cracks in the YSZ backbone has been computed during the Ni reoxidation and the results have been confronted with an available tomographic data set. Finally, the impact on the cermet redox tolerance of the Yttrium content in YSZ has been discussed thanks to the modelling. [1] Faes, A. et al. J. Power Sources 193, 55–64. [2] Laurencin, J. et al. J. Power Sources 192, 344–352. [3] Meille, S. Et al . J. Eur. Ceram. Soc. 32, 3959–3967. [4] Miehe, C. et al. Comput. Methods Appl. Mech. Eng. 199, 2765–2778. [5] Leguillon, D. J. Mech. – A/Solids 21, 61–72. Figure 1

Influence of compatibility between sol and intermediate layer on the performance of yttria-stabilized zirconia nanofiltration membrane

This paper reports the synergistic effect of the sol and intermediate layer on the performance of yttria-stabilized zirconia (YSZ) nanofiltration (NF) membranes. We have focused on the characterization of the microstructure, pure water permeance, and molecular weight cut-off (MWCO) of the NF membranes derived from zirconia sols of different precursor concentrations on two types of supported ZrO2 ultrafiltration (UF) membranes. We found that the performance of YSZ membranes strongly depends on the sol concentration and the pore size of the intermediate layer. In addition, YSZ gel membrane formation was found to follow the filtration process. Therefore, it is essential to maintain the compatibility between the sol and intermediate layer to fabricate high-performance NF membranes. A crack-free thin YSZ layer with an MWCO of 816 Da (pore size: 1.4 nm) and a water permeance of 25 L m-2 h-1 bar-1 was fabricated using a precursor concentration of 0.03 mol/L, on ZrO2 UF membrane with a pore size of 5.5 nm. The YSZ NF membrane exhibited a relatively high retention rate towards MgCl2 (71%), whereas a lower retention rate was observed for NaCl (35%).

Effects of pyrolytic carbon on carburization strengthening and corrosion of 3YSZ hollow fiber membranes

Carbon species incorporation and carbon deposits are common in the manufacture process and application of yttria-stabilized zirconia (YSZ) ceramics, while the potential corrosion induced by the resultant carburization is unclear. The 3YSZ hollow fiber membranes were prepared by wet spinning method. A sintering procedure at 1300–1450 °C under 0.02 MPa oxygen partial pressure, and thereafter an oxidation treatment at 800 °C were applied to investigate the carbothermal reaction and carburization corrosion mechanism. It was found that the trace carbon species of 1.0–2.2 mol% derived from polymeric precursors triggered the carburizing strengthening of porous YSZ membranes without sacrificing porosity and deformability. However, the carburization corrosion also occurred at the mild oxidation treatment, resulting in deterioration in bending strength and integrated structure converting into fine grains. Raman spectra, XPS bands and HR-TEM images confirmed that the in-situ synthesized ZrCxOy oxidized into monoclinic ZrO2, and micro-cracking formed along grain boundaries, which led to substantial mechanical strength loss.

Development of a Li2O Sensor Based on a Yttria Stabilized Zirconia Membrane for Oxide Reduction in a Molten LiCl-Li2O Electrolyte at 650°C

LiCl-Li2O salt is a widely used electrolyte for the electrochemical reduction of spent oxide (mainly uranium oxide) fuels, and the Li2O concentration is usually controlled at about 1.41 mol % for optimum operation and corrosion mitigation of the platinum anode material. Due to the small difference in reduction potential between UO2 and Li2O, some Li2O will be reduced during the oxide reduction process, leading to a Li2O deviation from the desired 1.41 mol %. Monitoring of the Li2O concentration is desired for proper control of the electrochemical oxide reduction process. In this paper, a Li2O sensor based on a yttria stabilized zirconia (YSZ) membrane was developed and tested to evaluate the feasibility of using the Li2O sensor to monitor the Li2O concentrations in the range of 0.57 to 1.69 mol % Li2O in LiCl-Li2O salt systems—one without dissolved Li metal and another with 0.24 mol % dissolved Li metal. The experimental results show that the open circuit potential of the Li2O sensor logarithmically responded to the concentration of Li2O in LiCl-Li2O salts with or without the presence of Li metal, suggesting that YSZ appears promising for real-time monitoring of the Li2O concentration in LiCl-Li2O salt for an oxide reduction process. The Li2O sensor developed herein is based on a potentiometry measurement that requires a stable, reliable reference electrode (RE), particularly for long-term Li2O concentration monitoring. To this end, a novel Ag/AgCl RE that is contained in a high-density MgO tube with a closed end was developed and exhibited stable electrode potential, chemical compatibility with LiCl-Li2O, and good mechanical strength. The performance of the newly developed Ag/AgCl RE was demonstrated in Li2O monitoring by comparing it with a traditional Ni/NiO RE.

Electrochemical Studies on Na-β”-Alumina + Yttria-Stabilized Zirconia (YSZ) Composite Mixed Na+-Ion-O2−-Ion Conductors

Composite Na-β”-alumina + yttria-stabilized zirconia (YSZ) samples were fabricated by vapor phase conversion of sintered α-alumina + YSZ two-phase samples. In the converted samples Na-β”-alumina and YSZ were both contiguous. The samples thus are mixed Na+-ion-O2−-ion conductors. Discs were tested separately under an oxygen chemical potential differential (H2-H2O-N2 gas mixture on one side and air on the other – fuel cell mode) and under a sodium chemical potential differential (Na-β-alumina + Na-β”-alumina on one side and α-alumina + Na-β-alumina on the other side). In both cases finite open circuit voltages were measured. In testing in fuel cell mode, the initially observed open circuit voltage was lower than expected for a single phase YSZ membrane, which was attributed to the transient flow of Na+ ions. After prolonged testing, the surface exposed to hydrogen containing gas mixture degraded forming a thin layer of α-alumina. The ionic conductivity of the samples was measured using both AC and DC methods.

Effect of bimodal pores on sintering and residual stresses of YSZ membranes prepared by non-solvent induced phase separation method

Densification mismatch and residual stresses of tri-layered yttria-stabilized zirconia (YSZ) membranes prepared by a non-solvent induced phase separation (NIPS) method were investigated. The tri-layered membrane consisted of sponge-like structures and finger-like voids in macroscale. The densification of the two structures were characterized to elaborate their contribution to the densification mismatch, which led to residual stresses of hundreds of megapascal retained in the sintered membranes. The profile of residual stress suggested that it was related to the strain rate mismatch within the NIPS membranes, which was further quantified with an in-situ monitored camber evolution.

Effect of optical basicity on the stability of yttria‐stabilized zirconia in contact with molten oxy‐fluoride flux

AbstractSolid oxide membrane (SOM) electrolysis process can produce high‐purity silicon from SiO2 dissolved in molten oxy‐fluoride flux at elevated temperatures. Yttria‐stabilized zirconia (YSZ), the preferred material for the oxygen‐conducting membrane for this application, is found to degrade over time upon exposure to the silica‐containing molten oxy‐fluoride flux. This YSZ degradation is caused by the acidity of the dissolved silica, especially when the optical basicity of the molten flux is lower than that of the yttria present in the YSZ membrane. To counteract this mismatch, the addition of CaO, a basic oxide, to the flux can adjust the optical basicity of the flux and successfully mitigate the YSZ membrane degradation. The detailed correlation between the rate of YSZ membrane degradation and the optical basicity of the flux is investigated by systematically testing a series of flux compositions. It is found that as the oxide optical basicity in the flux approaches that of the yttria in the YSZ, the degradation of the YSZ membrane is mitigated and essentially vanishes when the flux acidity with respected to the yttria is neutralized. This approach provides a guideline for eliminating membrane degradation during the production of silicon using the SOM electrolysis process.

High Performance Anode-Supported Solid Oxide Fuel Cells with Thin Film Yttria-Stabilized Zirconia Membrane Prepared by Aerosol-Assisted Chemical Vapor Deposition

In this study, we have successfully fabricated yttria-stabilized zirconia (YSZ) electrolyte thin films by aerosol-assisted chemical vapor deposition (AACVD) for solid oxide fuel cells (SOFCs) working at an intermediate temperature range under 600°C. The mix ratio of precursor sources and the deposition temperature of the AACVD process are optimized to obtain target composition and crystal structure of the YSZ film. The optimized AACVD YSZ membrane is incorporated onto Ni-YSZ anodes supported without interlayers for evaluation of SOFC performance. Fully dense 1-μm-thick AACVD YSZ electrolyte successfully produces a stable open circuit voltage (OCV) greater than 1 V at all test temperatures in the range of 450–600°C. Power output of the test cell is measured as about 600 mW cm−2 at 600°C. Performance of our cell is compared to that of a reference cell with the same structure but with an 8-μm-thick membrane produced by screen-printing; our cell produces 1.4 to 4 times as much power as the reference cell at all test temperatures. Electrochemical impedance analysis has confirmed that the power enhancement with the thin AACVD YSZ membrane is due to effective reduction of both ohmic and polarization resistances.

Enhanced performance of Fe2O3 doped yttria stabilized zirconia hollow fiber membranes for water treatment

Fe2O3 powders were introduced as sintering aid to fabricate yttria-stabilized zirconia (YSZ) hollow fiber membranes using a combined wet-spinning and post-sintering method. The obtained Fe2O3-YSZ hollow fiber membranes show enhanced performance for water treatment with fine crystal structure in terms of bending strength and pure water permeability. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA) along with mechanical tests were employed to investigate the structural evolution in the sintering process and the effect of Fe2O3. It is suggested that the Fe2O3 dopants dissolve into YSZ at elevated temperatures, providing defect sites and vacancies for fast ion migration, favoring for densification and grain growth of the YSZ, which yields dense microstructures of fine crystallites at relatively low sintering temperature. The Fe2O3-YSZ hollow fiber membranes sintered at 1150°C show a 3-fold increase of the permeate flux of pure water (F) (743Lm−2h−1) along with comparable bending strength (152MPa) compared to pure YSZ membranes. This modified method can reduce sintering costs and therefore fabrication costs which should pave the way for scale-up production for ceramic hollow fiber membranes.

Nano-Structured Porous Yttria-Stabilized Zirconia Membrane for High-Temperature $${{\rm {CO}}_{2}}$$ CO 2 Capture from $${{\rm{H}}_{2}/{\rm {CO}}_{2}}$$ H 2 / CO 2 Mixture

A yttria-stabilized zirconia (YSZ, 8 mol % $${{Y}_{2} {O}_{3}}$$ ) membrane was prepared employing zirconium n-propoxide as a precursor via sol–gel method. Unlike conventional methods, hydrolyzing water was slowly released in this method through esterification of 1-propanol and glacial acetic acid. X-ray diffraction verified that YSZ membrane was partially transformed to the tetragonal phase. Ten times dip-coated membrane thickness was about 742 nm, determined by field emission scanning electron microscopy (FE-SEM) micrograph. Brunauer–Emmet–Teller measured average pore size of unsupported YSZ from gas adsorption and desorption indicating two modes centered at 2.66 and 2.38 nm. Crystallite size (D) as calculated from Scherrer’s equation was 6.8, 12.23, and 13.61 nm for unsupported YSZ powder before and after calcination, respectively, at 25, 500, and $${600\,^{\circ} {C}}$$ . Thermogravimetric analysis showed that the mass loss of the samples calcined at temperatures higher than $${600\,^{\circ} {C}}$$ had no significant differences indicating the completion of loss of organics. Supported YSZ membrane was tested by a gas permeation apparatus. The single gas permeance of YSZ membrane including $${\alpha-{Al}_{2} {O}_{3}}$$ was measured as $${14.89\times10^{-6}}$$ and $${1.75\times10^{-6} \,{ mol m}^{-2} { s}^{-1} { Pa}^{-1}}$$ for $${{\rm{H}}_{2}}$$ and $${{\rm {CO}}_{2}}$$ at $${300\,^{\circ} {C}}$$ , respectively. Permeance of $${{\rm{H}}_{2}}$$ and $${{\rm {CO}}_{2}}$$ through YSZ in 75:25 $${{\rm{H}}_{2}}$$ / $${{\rm {CO}}_{2}}$$ binary feed mixture was $${9.696\times10^{-6}}$$ and $${0.97\times10^{-6} \,{ mol\,m}^{-2} { s}^{-1} \,{ Pa}^{-1}}$$ , respectively, at $${300\,^{\circ} {C}}$$ . The selectivity of hydrogen in YSZ was 10. It was concluded that the membrane was highly promising for the separation of $${{\rm{H}}_{2}/{\rm {CO}}_{2}}$$ mixture from various $${{\rm{H}}_{2}}$$ -containing process streams under steam environment for high-temperature separation.

Preparation and characterization of Pr2NiO4+δ infiltrated into Gd-doped ceria as SOFC cathode

Composite electrodes of Pr2NiO4+δ and Gd-doped ceria (Ce0.8Gd0.2O1.90 so-called GDC) have been prepared starting from a Pr-Ni nitrate solution (2:1 stoichiometry ratio) that was infiltrated into a porous GDC backbone made by screen printing onto 3 % yttria-stabilized zirconia membrane (90 μm thick). The crystallization of the Pr2NiO4+δ powder was first studied by X-ray diffraction: it highly depends on the annealing atmosphere (air or N2) and the formation of this phase as infiltrate is more difficult than for the powder. The electrochemical study performed on symmetrical cells shows that the polarization resistance decreases when the annealing temperature of the infiltrate electrode decreases. The lowest R p value is obtained for an annealing at 900 °C, for 2 h in N2, the value being 0.075 Ω cm2 at 600 °C in air.

Electrocatalyticaly-Assisted Oxidative Dehydrogenation of Lower Alkanes to Olefins

Introduction Conversion of short-chain alkanes to olefins is a very important process for the chemical industry. The large availability of lower alkanes from refineries as well as their abundance in natural gas reserves and oil shale makes these reactions economically very relevant. A major challenge in the dehydrogenation of alkanes is that these reactions are highly endothermic. At lower temperatures, thermodynamic equilibrium limits the conversion, making use of high temperatures a necessity. When they are exposed to high temperatures in an oxygen-free environment, however, cracking/decomposition of alkanes becomes dominant, leading to coking, and hence deactivation of the catalyst, as well as feedstock loss through coke formation. Oxidative dehydrogenation (ODH) has been studied as an alternative to direct dehydrogenation. In this reaction, hydrogen abstraction is achieved oxidatively, hence removing the thermodynamic limitation. Also, ODH reactions are exothermic and do not require additional energy input to the reaction. In ODH, the major challenge is controlling the selectivity at high conversion levels. The presence of gas phase oxygen at high temperatures leads to complete oxidation products, mostly through further oxidation of the olefin product, which is often more reactive than the alkane. Electrocatalytic reactors using an oxide ion-conducting membrane offer a potential solution to control the selectivity to the desired olefins by regulating the availability of oxygen to the alkane. In this scheme, the hydrocarbons and gas-phase oxygen never come in contact, and the selectivity can be controlled by tuning the adsorption/desorption characteristics of the anode electro-catalyst and by controlling the oxide ion transfer rate. We have been working with Ti-based perovskites as anode electro-catalysts for this application. Lanthanum strontium titanates, LaxSr1-xTiO3-δ or LSTs, as well as chlorine-doped perovskites of the same form (LST-Cls) have been examined for potential anode catalysts in electrocatalytic ODH reactions. Materials and Methods LST catalysts, with varied La doping from 10 to 50 mol% at the A-site, were prepared via a modified Pechini route. Cl-doped LST catalysts were also synthesized using a similar method. These catalysts were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier Transform spectroscopy, electrical conductivity measurements, and other techniques such as surface area analysis and scanning electron microscopy. Electrocatalytically-asssisted ODH tests were performed in an electrochemical reactor consisting of an yttria-stabilized zirconia (8 mole% doped YSZ) membrane, an LSM (Lanthanum strontium manganite)-YSZ composite cathode and titanate-perovskite type catalyst on the anode. The cell was sealed using SCHOTT GM31107 glass sealant. A Keithley 6220 current source was used to apply current and the reactor effluent was monitored using a Shimadzu GC 2014. Oxygen flux tests were conducted in the 550-700°C temperature range. Results and Discussion XRD confirmed cubic perovskite structure of the LST and LST-Cl catalysts after calcination. At lanthanum-doping levels of 40% or higher at the A-site of the perovskite, other phases begin to precipitate out of the structure. Because of this, lanthanum-dopant of 0, 10, 20, and 30% were of most interest. Although these materials are similar to each other in behavior and surface features, characterization confirmed 20% doping as an optimal value in this particular application. Oxygen flux tests displayed a good match between the actual and predicted oxygen flux values (by Faraday’s Law), for all temperatures. This confirms the reactor’s ability to regulate oxygen amount as a function of applied current. Ethane ODH tests using the same reactor showed the most promising results for LST28-Cl (La0.2Sr0.8TiO3-δClσ). Although activity improvement is needed, these studies showed that the anode electrocatalyst characteristics can be fine tuned by dopant levels, and oxygen flux can be controlled by the current applied. Acknowledgement The financial support provided by the National Science Foundation through the Grant CHE-1213443 and the Ohio Development Services Agency is gratefully acknowledged.

Direct Electrolytic Reduction of Solid Ta2O5 to Ta with SOM Process

A process that uses the solid-oxide–oxygen-ion conducting membrane has been investigated to produce tantalum directly from solid Ta2O5 in molten CaCl2 or a molten mixture of 55.5MgF2-44.5CaF2 (in wt pct). The sintered porous Ta2O5 pellet was employed as the cathode, while the liquid copper alloy, saturated with graphite powder and encased in a one-end-closed yttria-stabilized-zirconia (YSZ) tube, acted as the anode. The electrolysis potential in this method is higher than that of the Fray–Farthing–Chen Cambridge process because the YSZ membrane tube blocks the melts to electrolyze, and only Ta2O5 is will be electrolyzed. The microstructures of reduced pellets and a cyclic voltammogram of solid Ta2O5 in molten CaCl2 were analyzed. In addition, the influence of particle size and porosity of the cathode pellets on metal-oxide-electrolyte, three-phase interlines was also discussed. The results demonstrate that the sintering temperature of cathode pellets and electrolytic temperature play important roles in the electrochemical process. Furthermore, this process can be used to produce Ta metal efficiently without the expensive cost of pre-electrolysis and generation of harmful by-products.

XRD study of yttria stabilized zirconia irradiated with 7.3 MeV Fe, 10 MeV I, 16 MeV Au, 200 MeV Xe and 2.2 GeV Au ions

To simulate energetic neutron irradiation effects, yttria-stabilized zirconia (YSZ) which is one of the major materials for electrical corrosion potential sensors (ECP sensors) was irradiated with heavy ions at energies ranging from 7.3MeV to 2.2GeV. Ion irradiation effects on the lattice structure were analyzed using the X-ray diffraction (XRD). The increase in lattice constant was induced by the ion irradiation. It was dominated by the elastic collision process and not by the electronic excitation process. The lattice disordering which was observed as a broadening of XRD peaks was also induced by the irradiation especially for 200MeV Xe ion irradiation. The present result suggests that the expansion and/or the disordering of YSZ lattice induced by energetic neutrons may affect the durability of a joint interface between a metal housing and YSZ membrane for the usage of ECP sensors in nuclear power reactors.

Low-temperature performance of yttria-stabilized zirconia prepared by atomic layer deposition

We report on the performance of thin-film yttria-stabilized zirconia (YSZ) synthesized by atomic layer deposition (ALD) at temperatures of 50–525 °C. Electrochemical impedance spectroscopy (EIS) was used for conductivity measurements. Relatively high conductivity values were observed in the low-temperature region when compared to reference values of YSZ synthesized by other methods. To investigate the conduction mechanism, various experimental variables were modified during the electrical measurements, including the ratio of yttria to zirconia in the ALD YSZ films and the atmospheric conditions. To relate the electrical properties to the structural characteristics, the crystallinity and microstructure were investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Finally, the suitability of an ALD YSZ membrane as the electrolyte of micro solid oxide fuel cells was evaluated. An open circuit voltage of almost 1 V and decent power output were successfully measured below 100 °C.

Voltage pulsing for performance recovery of yttria-stabilised zirconia membranes in oxygen/sulfur dioxide separation

The regeneration of yttria-stabilized zirconia (YSZ) membranes exposed to high concentration sulfur dioxide in oxygen at 850 °C using DC voltage pulses was investigated by in-situ impedance spectroscopy. The membranes consisted of a dense YSZ layer as the solid electrolyte coated with two platinum layers as electrodes. On operation in the presence of SO2, the serial resistance and polarization resistance of the Pt/YSZ cell increased. This is most likely due to the formation of sulfide at the interface area of the electrode and electrolyte combined with sulfur adsorption on electrode surface. DC Voltage pulses were found to have an effect on the charge transfer and mass transfer properties of the Pt/YSZ cell, assisting the removal of sulfur on the cathode surface and leading to performance recovery of the cell. In these experiments, the greatest rate of membrane performance recovery is achieved with a cathodic DC bias of 10 V, applied for 0.08 s. Higher or longer voltage pulses may cause the rate controlling step for the oxygen reduction reaction to shift to oxygen supplied in feed from oxygen surface exchange and diffusion processes. A relatively steady membrane performance was achieved during 20 h SO2 exposure tests. It is concluded that DC voltage pulses show promise as a method for reducing the performance degradation effects of poisoning due to sulfur containing gases in the fields of fuel cells and in the sulfur family of thermochemical cycles.

Mitigating Electronic Current in Molten Flux for the Magnesium SOM Process

The solid oxide membrane (SOM) process has been used at 1423 K to 1473 K (1150 °C to 1200 °C) to produce magnesium metal by the direct electrolysis of magnesium oxide. MgO is dissolved in a molten MgF2-CaF2 ionic flux. An oxygen-ion-conducting membrane, made from yttria-stabilized zirconia (YSZ), separates the cathode and the flux from the anode. During electrolysis, magnesium ions are reduced at the cathode, and Mg(g) is bubbled out of the flux into a separate condenser. The flux has a small solubility for magnesium metal which imparts electronic conductivity to the flux. The electronic conductivity decreases the process current efficiency and also degrades the YSZ membrane. Operating the electrolysis cell at low total pressures is shown to be an effective method of reducing the electronic conductivity of the flux. A two steel electrode method for measuring the electronic transference number in the flux was used to quantify the fraction of electronic current in the flux before and after SOM process operation. Potentiodynamic scans, potentiostatic electrolyses, and AC impedance spectroscopy were also used to characterize the SOM process under different operating conditions.