High Oxygen Ion Conductivity Research Articles

New apatite‐type oxide ion conductors Ce9.33+xSi6O26+δ: Structures, phase stabilities, electrical properties, and conducting mechanisms

AbstractApatite‐type rare earth silicate with high oxygen ion conductivity are attractive materials for their wide potential applications, such as being used in solid oxide fuel cells as electrolytes. Herein, a series of new apatite‐type oxide ion conductors Ce9.33+xSi6O26+δ (x = 0.43, 0.67, 1.03) have been prepared, with their phases, stabilities, electrical properties, and conducting mechanisms been thoroughly studied. The results reveal that these materials are stable under inert or reducing atmospheres, but decomposed under an oxidizing environment at elevated temperatures. The combined alternative current and directive current techniques reveal dominating oxide ion conductions and minor electronic conductions in them. The bond‐valence‐based method is applied to investigate the possible oxide ion migration pathways and disclose the main contribution from interstitial oxygens, which locate between two neighboring tetrahedral SiO4 units and enable the oxide ion transporting among them along the c‐axis direction, whereas the intrinsic channel oxygens show an inferior responsibility for the oxide ion conductivities.

Study of oxygen-ion conductivity and luminescence in the ZrO2 – Nd2O3 system: Impact of local heterogeneity

Stabilized fluorites and pyrochlores with different types of substitution in the (Nd2O3 − ZrO2 system (NdZrO)) have been studied by Raman spectroscopy, X-ray diffraction, and impedance spectroscopy methods. Ionic and proton conductivity maps for 9 compositions in the NdZrO system are presented. Oxygen partial pressure measurements show a typical ionic conductor behavior, with a significantly increase of conductivity for (Nd2−xZrx)Zr2O7+x/2 (x = 1.27) fluorite and (Nd2−xZrx)Zr2O7+x/2 (x = 0.4, 0.2) pyrochlores. For the same compositions the strong luminescence was observed. Strong luminescence and high oxygen-ion conductivity of these solid solutions can be associated with the presence of phases with the different degree of structural disorder (tetragonal phase, fluorite) in local nanodomains in fluorite or pyrochlore matrix in the ZrO2 − Nd2Zr2O7 region. Really, Raman spectra of (Nd2−xZrx)Zr2O7+x/2 (x = 0.5 − 0.2) solid solutions demonstrated the fluorite + pyrochlore structural type in the short-range order. Fluorite nanodomains in pyrochlore matrix (ZrO2 – Nd2Zr2O7 region) resulting from an order–disorder transition can only be detected at a radiation wavelength comparable to the fluorite nanodomain size. Thus, using Raman spectroscopy the broad isomorphism range in the ZrO2 – Nd2Zr2O7 system has been shown to be nonuniform.

Sub-second sintering process for La6Sr4Co2Fe8O3-δ-gadolinium doped ceria composite cathode via a flash light irradiation method for intermediate temperature-solid oxide fuel cells

A solid oxide fuel cell (SOFC) requires an elevated operating temperature to achieve high oxygen ion conductivity and the oxygen reduction reaction (ORR). However, at high operating temperatures above 800 °C, performance deteriorates due to the chemical reaction of the electrode and electrolyte materials at the interface. Therefore, it is necessary to lower the operating temperature to the intermediate temperature regime while maintaining the high oxygen reduction reaction kinetics. One approach is adopting the composite cathode material such as La6Sr4Co2Fe8O3-δ-gadolinium doped ceria (LSCF-GDC). However, the high temperature in the fabrication process results in non-conduction material of strontium zirconate (SrZrO3, SZO) layers at the cathode/electrolyte interface, which blocks oxygen ion conduction and degrades the fuel cell performance. By utilizing the novel flash light sintering process, which can be completed in a few seconds, high temperature fabrication issues are solved. The surface microstructure and thickness of the composite electrode are inspected using a scanning electron microscope (FE-SEM). The secondary phase generation and chemical reaction are analyzed using XRD (X-ray diffraction), SEM-EDS, and TEM-EDS. The anode supported LSCF-GDC cathode cells are measured from 600 °C to 750 °C. The maximum power density at 750 °C of the novel flash light sintered LSCF-GDC composite cathode cell is 727.4 mW/cm2. In addition to the utilization of high performing cathode structure, the flash light sintering process dramatically reduces the process time and suppresses the side reaction at the cathode/electrolyte interface.

Boosting Proton-Conducting Ceramic Electrochemical Performance Through Mixed Ionic and Electronic Conducting Cathode Functional Layers

Proton conducting ceramic fuel cells (PCFCs) are a promising new technology for clean energy generation and green fuels production. PCFCs show favorable performance, but still lag behind their higher-temperature solid-oxide fuel cell predecessors. This work sets out to tailor the cathode-electrolyte interface in PCFCs by increasing the oxygen reduction reaction (ORR) activity and oxygen transference to bolster electrochemical performance and durability.The cathode-electrolyte interface is a major source of voltage loss during PCFC operation and is also suspected to be a primary contributor to cell-level degradation. Both oxygen reduction (ORR) and H2O formation must be simultaneously facilitated in PCFC cathodes. This contrasts with SOFC cathodes, where only oxygen reduction is required since water formation occurs separately at the anode. The dual reaction processes occurring in PCFC cathodes can lead to competitive interference between water adsorption/desorption and ORR activity. We hypothesize that placing materials with high ORR activity and oxygen-ion conduction at the electrolyte-cathode interface may help alleviate polarization loss.To explore this hypothesis, Er0.4Bi1.6O3 (ESB), Dy0.08W0.04Bi0.88O1.56 (DWSB), Gd0.1Ce0.9O1.95 (GDC10) and SmCoO3 were fabricated into the cathode functional layers (CFLs) in this study. As the addition of an interlayer to the cathode-electrolyte interface can inadvertently effect the morphology (e.g. roughness, porosity) and mechanical integrity of the interface, control cells with a porous BaCe0.4Zr0.4Y0.1Yb0.1O3- δ BCZYYb cathode functional layer were also fabricated and tested. Button cells with the CFLs were fabricated through ultrasonic spray coating of thin layers of the CFL material onto a precursor BCZYYb electrolyte and co-fired. The cathode is then brush painted onto the resulting half-cell and sintered. Preliminary results, shown in Figure 1a, demonstrate that adding an ESB CFL can increase performance by over 40%. While GDC-based CFLs have led to significant increases in PCFC durability, displayed in Figure 1b, underscoring the promise of this technique. Figure 1

Effects of Strontium Content on the Microstructure and Ionic Conductivity of Samarium-Doped Ceria

Due to its high oxygen ion conductivity at elevated temperatures, samarium-doped ceria (SDC) is a very promising material for application in solid state electrochemical devices and especially in the electrolytes of solid oxide fuel cells. Several prior studies have reported a further improvement in the ionic conductivity of SDC on doping with small amounts of strontium. It is suggested that strontium acts as a sintering aid—improving the microstructure of SDC—and as a scavenger of silicon impurities, decreasing its tendency to form resistive phases at grain boundaries. However, because of the range of preparation methods and the resulting differences in microstructure and silicon levels, some inconsistencies exist in the literature. Furthermore, the effect of strontium on the intrinsic (bulk) conductivity of SDC is not often discussed. To address these issues, a systematic, combined microstructural and conductivity study has been performed on a compositional series with a range of strontium contents, Ce0.8−xSm0.2SrxO2−δ (x = 0, 0.002, 0.005, 0.01, 0.02, 0.03, 0.04). A low temperature synthesis affording products with low silicon was employed. Total bulk and grain boundary conductivity data were obtained over a wide temperature range. Increasing strontium content caused a general decrease in total and intrinsic conductivity, but there was an improvement in grain boundary conductivity at the lowest strontium levels. These results were interpreted by reference to the microstructures using, among other parameters, the blocking, and normalised blocking, factors.

Development of Low-temperature SOECs with Excellent Performance for CO2 and H2O Co-Electrolysis

Because of the advantages of high efficiency, safety, pollution-free and the waste gas can be converted into clean fuel (such as methane) in situ [1], using the solid oxide electrolysis cells (SOECs) to co-electrolyze water and carbon dioxide to produce oxygen has great application potential in the aerospace field. As early as the 1960s, NASA has begun to research on the use of SOECs to supply oxygen to astronauts [2]. However, the existing SOECs can only operate in high temperature range (750~850 ºC) with short lifetime and high energy consumption [3]. Therefore, it is urgent to reduce the operating temperature of SOECs in order to extend the operating life and shorten its start-up time, then providing a feasibility guarantee for its service in the aerospace industry.However, lowering the cells’ operating temperature will bring about the problem of low oxygen ion conductivity. There are two strategies to solve this problem. One is to select materials with high oxygen ion conductivity in the low temperature range; the other is to reduce the thickness of the electrolyte layer to shorten the mass transfer distance. Herein, GDC was selected as the thin electrolyte material. And by adjusting the paste ratio of screen-printing method and sintering process parameters, the thickness of the dense electrolyte layer can be reduced to <5 μm. Besides, Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and GDC are mechanically mixed as the anode materials. The measured ohmic resistance and polarization resistance are only 0.079 and 0.019 Ω·cm2 at 650 ºC, and the maximum output power density reaches 1.229 W/cm2 in SOFC mode. In SOEC mode, the steam and carbon dioxide are injected into the cell at a volume ratio of 3:1. The output current density is as high as 1.2 A/cm2 at 650 ºC under an applied voltage of 1.1 V. All these results confirm that the cells are promising for the applications in low temperature SOECs.

DC Aging Mechanism of Co2O3-Doped ZnO Varistors

Lowered power loss and asymmetrically electrical parameters are reported in the DC aging of Co2O3-doped ZnO varistors in this paper. Based on the frequency domain dielectric responses of the pristine and degraded samples, the present study explores the roles of point defects in the aging process via dielectric relaxations and their parameters. It is found that breakdown field increased more in the positive direction than the negative direction. Nonlinearity increased in the positive direction, whereas it decreased in the negative direction, and leakage current density increased more in the negative direction than the positive direction. Given the lowest migration energy of Zinc interstitial (Zni, 0.57 eV) and high oxygen ion conductivity in Bi2O3-rich phase, it is speculated that Zni and adsorbed oxygen (Oad) migrate under DC bias, and then change the defect structure and the double Schottky barrier (DSB) at grain boundaries. As a result, the forward-biased barrier height gradually decreases more than the reverse-biased one.

Promoted electrocatalytic activity and ionic transport simultaneously in dual functional Ba0.5Sr0.5Fe0.8Sb0.2O3-δ-Sm0.2Ce0.8O2-δ heterostructure

Structural doping is often used to prepare materials with high oxygen-ion conductivity and electrocatalytic function, but its wider application in solid oxide fuel cells (SOFCs) is still a major challenge. Here, a novel approach to developing materials with fast ionic conduction and high electrocatalytic activity is reported. A semiconductor-ionic heterostructure of perovskite Ba0.5Sr0.5Fe0.8Sb0.2O3-δ (BSFSb) and fluorite structure Sm0.2Ce0.8O2-δ (SDC) is developed. The BSFSb-SDC heterostructure exhibits a high ionic conductivity >0.1 S cm−1 (vs 0.01 S cm−1 of SDC) and achieves a remarkable fuel cell performance (>1000 mWcm−2) at 550 °C. It was found that the BSFSb-SDC has both electrolyte and electrode (cathode) functions with enhanced ionic transport and electrocatalytic activity simultaneously. When using BSFSb-SDC as an electrolyte, the interface energy-band reconstruction and charge transfer at particle level forming a built-in electric field (BIEF) and it make electronic confinement. The BIEF originates from the potential gradient due to differences in the electron density of BSFSb and SDC particles/grains facilitates ionic conduction at the interface of the BSFSb and SDC particles. This work provides a new insight in designing functional materials with high ionic conductivity and electrocatalytic function, which can be used both for energy conversion and storage device.

High ionic conductivity of Lu2O3–TiO2 co-doped Bi2O3 ceramics

In this study, a promising electrolyte material Bi0.76Lu0.16Ti0.08O1.5+δ (abbreviated as ‘8T16LSB’) with high oxygen ion conductivity was obtained by co-doping with Lu2O3 and TiO2 into Bi2O3. The phase composition, microstructure, and conductivity of TLSB ceramics were respectively investigated by x-ray diffraction, scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). XRD results exhibited that the sintered TLSB ceramics with an unchanged total doping concentration of 24 mol.% existed as a cubic fluorite structure. SEM results showed that the TLSB pellets were relatively dense. The relationship between temperature and conductivity showed that the 8T16LSB ceramic exhibited the highest conductivity of 0.56 S cm−1 at 700 °C, which was higher than the value of 0.37 S cm−1 for the representative single-doping Er0.4Bi1.6O3 (abbreviated as ‘20ESB’). The results certified that the electrochemical performance of Bi2O3 was promoted by the co-dopant of Lu3+and Ti4+.

Plasma-Sprayed High-Performance (Bi2O3)0.75(Y2O3)0.25 Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFCs).

Rare earth element-doped bismuth oxides with the fluorite structure (δ-Bi2O3) exhibit high oxygen ion conductivity at low temperature, which is promising electrolyte materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, traditional co-sintering process is not applicable to the manufacturing of IT-SOFCs using low melting point Bi2O3-based electrolyte, while further high-temperature processing is not required for deposition Bi2O3-based electrolytes. In this study, plasma spraying was used to examine the possibility to deposit high-performance Bi2O3-based electrolytes without the following high-temperature process. (Bi2O3)0.75 (Y2O3)0.25 (YSB) spray powders were prepared by the sinter-crushing method. The YSB electrolytes were fabricated by plasma spraying at different deposition temperatures. The effects of deposition temperature on the coating microstructure, crystalline stability, and ion conductivity were investigated. Results showed that the as-sprayed YSB electrolytes present a dense microstructure with well-bonded lamellar interfaces. The pure δ-phase YSB electrolyte was deposited with 37.5-75 μm powders at a deposition temperature of 350 °C. The deposited YSB electrolyte presented the excellent ionic conductivity of 0.19 S cm−1 at 700 °C in comparison with 0.21 S cm−1 for sintered bulk.

A Compatibility in the Single Cell of the NiO/LSGM/LSCF

The compatibility between anode, electrolyte, and cathode in a solid fuel cell determines its performance. Research on the compatibility between fuel cell components is challenging, especially for SOFCs that operate at high temperatures. Therefore, efforts to reduce the operating temperature to become intermediate temperature SOFC (IT-SOFC) are essential to facilitate compatibility between its components. La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) has been recognized as one of the most promising cathode materials for (IT-SOFC) due to its high electronic conductivity and excellent electrical performance. While La0.8Sr0.2Ga0.8Mg0.2O3–δ (LSGM) has a high oxygen ion conductivity at low temperatures, its chemical stability is still not good. LSGM is known to have interface reactivity with other components such as NiO and LSCF in fuel cells. This study looked at the compatibility of NiO/LSGM/LSCF cells prepared by the solid chemical synthesis method. Compatibility evaluation is determined by the Thermal Expansion Coefficient (TEC) parameter using the dilatometric method, Area Specific Resistance (ASR), and TBF area morphology by Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS). While the conductivity of the cells is determined by Electrochemical Impedance Spectroscopy (EIS). NiO/LSGM/LSCF cells have good compatibility with a value of 78.05 kg-1.K.A.s3.µ2 at a temperature of 600°C. The ASR values of cells tend to decrease with increasing temperature and conductivity values at small TEC values. Based on these parameter values, delamination in NiO/LSGM/LSCF cells did not occur.

Tuning the Microstructure and Thickness of Ceramic Layers with Advanced Coating Technologies Using Zirconia as an Example

The properties of ceramic layers are not only related to the coating material but also—to a very high degree—the processing technology used. In particular, microstructure and thickness are key to the successful implementation of functional layers in application. This will be shown using yttria‐stabilized zirconia (YSZ) as an example, a highly versatile compound with high fracture toughness, high chemical and thermal stability, high biological compatibility, and high oxygen ion conductivity. For each application, specific microstructures are required, which can only be obtained by suitable processing. Herein, coating technologies for layers with thicknesses spanning the nanometer range up to several hundred micrometers, and from full density to tailored open porosity are focused. Wet processing routes, thin‐film deposition from the gas phase as well as thermal and plasma spraying are presented along with the resulting YSZ layers.

High Field Electroformation of Sodium Bismuth Titanate

Lead-free ceramics based on Na1/2Bi1/2TiO3 (NBT) have been shown to exhibit excellent ferroelectric properties putting them amongst the best materials to replace lead-based piezoelectrics. The defect chemistry of NBT is, however, very complex. High oxygen ionic conductivity can be induced by acceptor doping or bismuth evaporation, which is quite detrimental to the ferroelectric properties. Nevertheless, this conductivity is non-linearly dependent on the acceptor concentration, which allows for tuning of NBT-based material from highly ionically conducting to highly resistive. This unique behavior raises the question of whether NBT ceramics also need to be treated differently with respect to high electric fields as they are used for ferroelectric and dielectric applications. To elucidate this, high field electrodegradation experiments have been performed on NBT and a solid solution with BaTiO3 (BT). It could be shown that reversible electroformation can be induced, which is non-linearly dependent on acceptor concentration. Additionally, a fast surface degradation process could be identified, which could be attributed to a reversible field-induced change in composition at the anode due to sodium becoming mobile. These results will be of high importance for reliability investigations of NBT based material and also hint towards possible applications in resistive switching memory applications.

Direct evidence of high oxygen ion conduction of perovskite-type ceramic membrane under hydrogen atmosphere in solid oxide fuel cell

The ionic conduction of perovskite-type oxides remains a fundamental and important issue in the research of solid oxide fuel cells (SOFCs). In this research, a thin perovskite-type ceramic membrane was fabricated in situ at anode side attached to the surface of Gd0.2Ce0·8O1.9 (GDC20) electrolyte membrane. The single cell working between H2 and static air showed good stability (over 50 h), high open circuit voltages (above 1.0 V) as well as high peak power densities (749-264 mW cm−2) from 600 to 500 °C. Detailed analyses of current research demonstrated that the thin perovskite film mainly possessed the oxygen ion conductivity under reducing atmosphere, while the proton conductivity was severely suppressed, showing the high flexibility in ionic conductivity of perovskite oxide. This work also implies that the oxygen ion and proton conduction may be in high correlation with each other, which provides important information to unveil the nature of the ionic conduction of perovskite-type oxides.

Efficient water splitting through solid oxide electrolysis cells with a new hydrogen electrode derived from A-site cation-deficient La0.4Sr0.55Co0.2Fe0.6Nb0.2O3-δ perovskite

Steam splitting using solid oxide electrolysis cells (SOECs) is highly attractive for mass production of high-purity hydrogen, while its economic competitiveness strongly relies on the availability of low-cost, active and stable hydrogen electrodes. Here, we report an A-site cation-deficient La0.4Sr0.55Co0.2Fe0.6Nb0.2O3-δ (LSCFN55) perovskite is such a hydrogen electrode for SOEC, exhibiting a high electrolysis current density of 0.956 A/cm2 with an applied voltage of 1.3 V at 850 °C and good stability in a high humidity and hydrogen partial pressure environment. The introduction of A-site cation deficiency into the perovskite oxide lattice of LSCFN55 promotes the exsolution of partial cobalt and iron from the oxide bulk with the formation of Co2Fe alloy nanoparticles that decorate the surface of perovskite main phase, simultaneously SrO is segregated to form nanocomposites with the alloy. The outstanding electro-catalytic activity of alloy nanoparticles, the favorable water adsorption capability of SrO, the high oxygen-ion conductivity of the perovskite main phase, and the improved electronic conductivity of the material, cooperatively contribute to the superior performance of the electrode. The strong interaction between the alloy nanoparticles and the perovskite substrate effectively suppresses the sintering of the nanoparticles while the SrO phase protects the alloy nanoparticles from oxidation, both then contribute to the high operating stability of the electrode.

Europium-Doped Ceria Nanowires as Anode for Solid Oxide Fuel Cells

CeO2-based materials have been studied intensively as anodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this work, pristine and europium (Eu)-doped CeO2 nanowires were comprehensively investigated as anode materials for IT-SOFCs, by a combination of theoretical predictions and experimental characterizations. The results demonstrate: (1) Oxygen vacancies can be energetically favorably introduced into the CeO2 lattice by Eu doping; (2) The lattice parameter of the ceria increases linearly with the Eu content when it varies from 0 to 35 mol.%, simultaneously resulting in a significant increase in oxygen vacancies. The concentration of oxygen vacancies reaches its maximum at a ca. 10 mol.% Eu doping level and decreases thereafter; (3) The highest oxygen ion conductivity is achieved at a Eu content of 15 mol.%; while the 10 mol.% Eu-doped CeO2 sample displays the highest catalytic activity for H2-TPR and CO oxidization reactions. The conducting and catalytic properties benefit from the expanded lattice, the large amount of oxygen vacancies, the enhanced reactivity of surface oxygen and the promoted mobility of bulk oxygen ions. These results provide an avenue toward designing and optimizing CeO2 as a promising anode for SOFCs.

Oxygen Ion Migration and Conductivity in LaSrGa3O7 Melilites from First Principles

High oxygen ion conductivity of solid electrolytes is a crucial prerequisite for the efficiency of electrochemical devices such as solid oxide fuel cells and rechargeable oxide batteries. Lanthanum...

Layered Solid-Electrolyte Membranes Based on Zirconia: Production Technology

A solid electrolyte membrane is a key part of a solid oxide fuel cell (SOFC). This paper presents the results of our study of the effects of particle size of the starting powders, composition of organic additives in suspension, and process parameters on the quality of three-layer ceramic sheets of solid electrolyte with dimensions of 100 × 100 mm and a thickness of 0.15 mm, made by casting on a moving tape. The inner layer was 10Sc1YSZ (10 mol % Sc2O3, 1 mol % Y2O3, 89 mol % ZrO2)—a material with the highest oxygen ion conductivity among zirconia-based solid solutions. 6ScSZ was chosen for the outer layers (6 mol % Sc2O3, 94 mol % ZrO2). The three-layer architecture of the solid electrolyte membranes allows the improvement of the mechanical characteristics while maintaining the required functional properties (primarily, anion conductivity). This study is devoted to optimization of the production technology of these layered membranes by tape casting.

Design Rules for High Oxygen-Ion Conductivity in Garnet-Type Oxides

We performed a first-principles screening on 90 types of garnet-type oxides (A3B2C3O12) to consider their potential as oxygen ion conductors based on prior knowledge of Ca3Fe2Ge3O12. We identified several oxides that are energetically and dynamically stable and have a low migration barrier energy (Emig) for interstitial oxygen (Oi) on the delocalized migration path. Among them, Cd3Sc2Ge3O12, Ca3Y2Ge3O12, Ca3In2Ge3O12, and Ca3Ga2Ge3O12 may be able to generate Oi by donor dopants such as LaCa+ and InCd+. On the basis of the calculations by first-principles molecular dynamics, we expect that these oxides may have oxygen ion conductivities (σO) of a similar order of magnitude to that of the reference level of ∼10–2 S/cm exhibited by yttria-stabilized zirconia at 1000 K. We provide scientific reasoning and design rules regarding the high σO of garnet-type oxides. For example, we find that the low Emig of Oi is attributed to smaller deviations in the atomic radii of cations and bond lengths.

High-Temperature Corrosion of Copper Induced by TeO2

It was found that copper is susceptible to the accelerated high-temperature corrosion induced by TeO2 at 650°C in air, which occurs at a constant rate. The calculated corrosion rate constant is 4.5 × 10−4 kg·m−2·s−1 and does not depend on the specific mass of tellurium oxide. Based on the results of the analysis of the microstructure (scanning electron microscopy/energy dispersive x-ray spectroscopy) and the phase composition (x-ray diffraction) of two formed corrosion layers, the phase distribution in the corrosion product has been ascertained. It was shown that during the corrosion process at 650°С, the inner corrosion layer containing Cu2O and Cu2Te and the outer corrosion layer mainly containing CuTe2O5 and Cu2O were formed. The inner layer provides a high copper ion conductivity due to Cu2Te, while the outer layer possesses a high oxygen ion conductivity due to the oxide melt. The mechanism of the overall corrosion process has been proposed.