Oxide Ion Conductivity Research Articles

Interplay between A-site and oxygen-vacancy ordering, and mixed electron/oxide-ion conductivity in n = 1 Ruddlesden-Popper perovskite Sr2Nd2Zn2O7.

Oxygen vacancies in Ruddlesden-Popper (RP) perovskites (PV) [AO][ABO3] n play a pivotal role in engineering functional properties and thus understanding the relationship between oxygen-vacancy distribution and physical properties can open up new strategies for fine manipulation of structure-driven functionalities. However, the structural origin of preferential distribution for oxygen vacancies in RP structures is not well understood, notably in the single-layer (n = 1) RP-structure. Herein, the n = 1 RP phase Sr2Nd2Zn2O7 was rationally designed and structurally characterized by combining three-dimensional (3D) electron diffraction and neutron powder diffraction. Sr2Nd2Zn2O7 adopts a novel 2-fold n = 1 RP-type Pmmn-superstructure due to the concurrence of A-site column ordering and oxygen-vacancy array ordering. These two ordering models are inextricably linked, and disrupting one would thus destroy the other. Oxygen vacancies are structurally confined to occupy the equatorial sites of "BO6"-octahedra, in stark contrast to the preferential occupation of the inner apical sites in n ≥ 2 structures. Such a layer-dependent oxygen-vacancy distribution in RP structures is in fact dictated by the reduction of the cationic A-A/B repulsion. Moreover, the intrinsic oxygen vacancies can capture atmospheric O2, consequently resulting in a mixed oxide ion and p-type electrical conductivity of 1.0 × 10-4 S cm-1 at temperatures > 800 °C. This value could be further enhanced to > 1.0 × 10-3 S cm-1 by creating additional oxygen vacancies on the equatorial sites through acceptor doping. Bond valence site energy analysis indicates that the oxide ion conduction in Sr2Nd2Zn2O7 is predominated by the one-dimensional pathways along the [Zn2O7] ladders and is triggered by the gate-control-like migration of the equatorial bridging oxygens to the oxygen-vacant sites. Our results demonstrate that control of anion and cation ordering in RP perovskites opens a new path toward innovative structure-driven property design.

Tuning of the Ionic Conductivity of Ba7Nb4MoO20 by Pressure: A Neutron Diffraction and Atomistic Modelling Study

Ba7Nb4MoO20 is a hexagonal perovskite derivative that exhibits both high oxide-ion and proton conductivity. The high oxide-ion conductivity results from the presence of disordered flexible MOx (x = 3, 4,...

High‐Performing Perovskite/Ruddlesden‐Popper Fuel Electrode for High‐Temperature Steam Electrolysis

AbstractRuddlesden‐Popper (RP) oxides have emerged as a promising alternative to Ni cermet electrodes for high‐temperature steam electrolysis due to their superior oxide ion mobility and conductivity. Combining RP with perovskite (P) can provide superior electrocatalytic activity toward hydroxide oxidation and reduction reaction, driving higher efficiency in solid oxide cells (SOC). This work provides a novel approach to enhancing SOC performance by employing A‐site Ce‐substituted Sr0.6Pr0.4‐xCexMnO3 (x = 0.1‐0.3) electrodes, investigating their phase evolution, crystal properties, and cation oxidation states under oxidizing and reducing atmospheres. X‐ray diffraction analysis of heat‐treated powder in a reducing atmosphere revealed forming mixed P and RP structures at 600–800 °C for x = 0.1 and 0.2, which provides excellent conductivity and electrocatalytic activity. Consequently, outstanding cell performance is achieved, with low polarization resistances of 0.053 ± 0.004 Ω cm2 at 800 °C. The voltage response at different current densities in an electrolyte‐supported cell revealed a high power density of 1.084 W cm−2 in fuel cell operation and a current density of 1.00 A cm−2 at the thermoneutral voltage at 850 °C in steam electrolysis. Moreover, a low overpotential degradation rate of 45 mV kh−1 demonstrated the remarkable potential of the SPCM electrode as a promising Ni‐free candidate for SOC application.

Enhanced Phase Stability and Oxide-Ion Conductivity in V- and Sr/Ca-Codoped Bi2O3 Ceramics

Enhanced Phase Stability and Oxide-Ion Conductivity in V- and Sr/Ca-Codoped Bi₂O₃ Ceramics

Solid-State Oxide-Ion Synaptic Transistor for Neuromorphic Computing.

Neuromorphic hardware facilitates rapid and energy-efficient training and operation of neural network models for artificial intelligence. However, existing analog in-memory computing devices, like memristors, continue to face significant challenges that impede their commercialization. These challenges include high variability due to their stochastic nature. Microfabricated electrochemical synapses offer a promising approach by functioning as an analog programmable resistor based on deterministic ion-insertion mechanisms. Here, an all-solid-state oxide-ion synaptic transistor is developed, employing Bi2V0.9Cu0.1O5.35 as a superior oxide-ion conductor electrolyte and La0.5Sr0.5FeO3-δ as a variable-resistance channel able to efficiently operate at temperatures compatible with conventional electronics. This transistor exhibits essential synaptic behaviors such as long- and short-term potentiation, paired-pulse facilitation, and post-tetanic potentiation, mimicking fundamental properties of biological neural networks. Key criteria for efficient neuromorphic computing are satisfied, including excellent linear and symmetric synaptic plasticity, low energy consumption per programming pulse, and high endurance with minimal cycle-to-cycle variation. Integrated into an artificial neural network (ANN) simulation for handwritten digit recognition, the presented synaptic transistor achieved a 96% accuracy on the Modified National Institute of Standards and Technology(MNIST) dataset, illustrating the effective implementation of the device in ANNs. These findings demonstrate the potential of oxide-ion based synaptic transistors for effective implementation in analog neuromorphic computing based oniontronics.

Strain-Enhanced Low-Temperature High Ionic Conductivity in Perovskite Nanopillar-Array Films.

Solid oxide ionic conductors with high ionic conductivity are highly desired for oxide-based electrochemical and energy devices, such as solid oxide fuel cells. However, achieving high ionic conductivity at low temperatures, particularly for practical out-of-plane transport applications, remains a challenge. In this study, leveraging the emergent interphase strain methodology, we achieve an exceptional low-temperature out-of-plane ionic conductivity in Na0.5Bi0.5TiO3 (NBT)-MgO nanopillar-array films. This ionic conductivity (0.003 S cm-1 at 400 °C) is over one order of magnitude higher than that of the pure NBT films and surpasses all conventional intermediate-temperature ionic conductors. Combining atomic-scale electron microscopy studies and first-principles calculations, we attribute this enhanced conductivity to the well-defined periodic alignment of NBT and MgO nanopillars, where the interphase tensile strain reaches as large as +2%. This strain expands the c-lattice and weakens the oxygen bonding, reducing oxygen vacancy formation and migration energy. Moreover, the interphase strain greatly enhances the stability of NBT up to 600 °C, well above the bulk transition temperature of 320 °C. On this basis, we clarify the oxygen migration path and establish an unambiguous strain-structure-ionic conductivity relationship. Our results demonstrate new possibilities for designing applicable high-performance ionic conductors through strain engineering.

(High Temperature Materials Division Outstanding Achievement Award) Development of Novel Ion Conducting Materials for Use as Electrolytes and Electrodes in Intermediate Temperature Solid Oxide Cells

Oxide ion conductor is an important functional material in energy conversion and sensing area. In this presentation, development of new oxide ion conducting materials and their application in high temperature electrochemical cells such as the intermediate temperature solid oxide fuel cells (it-SOFCs) as well as electrolysis cell will be presented. In particular, in 1994, the high oxide ion conductivity in the perovskite structured material La1-xSrxGa1-yMgyO3 (LSGM) was fist time found in our group. LSGM is the first case of a pure oxide ion conducting perovskite oxide and now considered as a viable alternative to Y2O3 stabilized ZrO2 as an electrolyte for the intermediate temperature-SOFC. Not only high ion conductivity but also making thin film are important for achieving the superior electrochemical performance of solid oxide cells. In order to obtain a thin film of LSGM, several processing methods including pulsed laser deposition and conventional wet processing were tried. This resulted in an SOFC with an extremely high power density (3.3W/cm2 at 973K) opening the possibility of intermediate temperature operation at 873K. In addition, a solid oxide cell using an LSGM thin film electrolyte can be used for intermediate temperature (ca.873K) electrolysis and that a reasonable current density (>1A/cm2 at 1.6V) can be achieved at 873K. Preparation of microtubular cell using LSGM film with slurry coat method. In addition to the development of new oxide ion conducting electrolyte materials, highly active cathode and anode materials are also critical for the development of high performance it-SOFCs. In this talk, the development of new electrode materials was also introduced. Examples of the new cathode materials, the perovskite related oxides Pr2NiO4 and Ba(La)CoO3 and, for the anode, Ni-Fe based alloys will be introduced. The surface chemistry of these materials was also analyzed by using the advanced ion beam techniques, i.e., Secondary Ion Mass Spectroscopy (SIMS) and Low Energy Ion Scattering (LEIS). These unique studies showed that three dimensional tensile lattice strain can be successfully introduced by a dispersion of nano size metal particles, mainly, Au, onto the grains of ceramic Pr2NiO4. The resulting tensile strain induces increased oxygen diffusivity and surface activity to oxygen dissociation, providing a new paradigm for the design of the oxygen electrode for solid oxide cells. Using this knowledge, SOFCs with several different designs such as planar, tubular, and metal support cells, which always exhibited the high power densities suitable for commercialization. In particular, cycle stability of SOFC/SOEC was much increased on LSGM/Ni-YSZ tubular cells by infiltration of CeO2 nanoparticles. Figure 1 shows I-V curves of CeO2 infiltrated Ni-YSZ substrate in SOFC mode using LSGM film. It was found that the infiltration of higher concentration of Ce solution increased the maximum power density, because both IR loss and overpotential were significantly decreased. The maximum power density of the cell was 0.95 and 0.42 W cm-2 at 873 and 773 K, respectively at 3 M Ce nitrate infiltrated. The long-term stability of the cell was also measured by using the cell infiltrated with 1.5 M Ce, the stable power generation performance was demonstrated. The steam electrolysis performance of the cell using Ce infiltration was further studied and it was found that Ce also contributes to higher current density in SOEC operation and 1.07 A cm-2 at 1.6 V was achieved at 873 K using 2 M Ce infiltration. In this talk, development of solid oxide cells using LSGM will be introduced from materials to application.Figure 1 I-V, I-P curves of the SOFC cell using LSGM film deposited on Ce infiltrated Ni-YSZ tubular substrate. Figure 1

(Invited) Design of Three-Dimensional Reaction Sites of Solid-Electrolyte Gas Sensors for Sensitive Detection of VOCs

The exhaled breath of patients contains a higher concentration of specific volatile organic compounds (VOCs) than that of healthy peoples, and thus the development of highly sensitive gas sensors for these gases enables the early detection of these diseases. Among the various gas sensors, we focused on solid-electrolyte gas sensors consisting of yttria-stabilized zirconia (YSZ) as an oxide-ion conductor, and we previously reported that the toluene response of YSZ-based gas sensors improved by the addition of CeO2 to the Au sensing electrode (SE) [1].The toluene-response mechanism of the YSZ-based gas sensors is explained on the basis of the mixed potential theory. The sensor output voltage (E) is the potential difference between the SE and the counter electrode (CE). The SE potential of the sensor negatively shifted by the concurrent electrochemical reactions of toluene oxidation (eq. 1) and oxygen reduction (eq. 2) at the interface between YSZ and SE (triple-phase boundaries, TPBs) with the same rate upon exposure to toluene balanced with air.C7H8 + 18O2− → 7CO2 + 4H2O + 36e− (1)2O2− → O2 + 4e− (2)On the other hand, a given amount of toluene is oxidized during gas diffusion in the SE, which decreases the concentration of toluene reaching the TPBs. Since this reduces the toluene response of the sensor, thin-film SEs were fabricated in the thickness range of 30–250 nm by using spin-coating method employing a solution containing Au and Ce components in order to increase toluene concentration at the TPBs. However, the toluene response of the fabricated sensors largely increased with an increase in the SE thickness when the additive amount of CeO2 was relatively high [2–3]. The obtained results suggested that the electrochemical reactions of the toluene proceeded not only at the interface (SE/YSZ/gas) but also at the interface between SE and gas (SE(Au/CeO2)/gas) in the SE, because CeO2 worked as an oxide-ion conductor at elevated temperatures.In this study, we focused on the design of three-dimensional reaction sites of YSZ-based gas sensors for sensitive detection of VOCs. We fabricated Au SEs containing a sufficient amount of CeO2 (n) by spin-coating method. The obtained SEs were denoted as Au-nCeO2(x) SEs (n: an additive amount of CeO2, 24, 32 (wt%), x: the number of spin-coating cycles, 5–15).Figure 1(a) shows the response transient to various concentrations of toluene of the Au-24CeO2(5) sensor in dry air. The E value decreased upon exposure to toluene, and the magnitude of the response (ΔE) negatively increased with an increase in the toluene concentration. Figure 1(b) shows the thickness (spin-coating cycle) dependences of ΔE to 50 ppm toluene of the fabricated sensors in dry air. The ΔE value of the Au-24CeO2(x) sensor increased with a decrease in the SE thickness, and the Au-24CeO2(5) sensor showed the largest response (ca. 256 mV). In contrast, the ΔE of the Au-32CeO2(x) sensor increased with an increase in the SE thickness. In addition, we performed electrochemical impedance measurements in order to quantify the number of TPBs, and we discussed the effects of the number of TPBs on the toluene response. The details of this will be explained in the presentation.

(Invited) Effect of Electrode Polarization on Oxide Ion-Conducting and Proton-Conducting Electrolytes

Polarization at the electrode/electrolyte interface has been a central issue in electrochemistry, which is also true for high-temperature ion conducting devices. Charge transfer limitation does not simply apply to such interfaces where electrons make direct exchange. Thus, overpotentials have been discussed in terms of a shift of local oxygen activity from equilibrium with the gas phase [1]. This approach has been successful in explaining phenomena at high temperature electrodes, such as large chemical capacitance of mixed-conducting film electrodes and appearance of Gerischer-type impedance with porous electrodes [2,3]. As well as the effect on the electrode properties, the interface potential shift can also affect the electrolyte. The Hebb-Wagner polarization technique utilizes the polarization at ion-blocking electrode to control the chemical potential within the electrolyte. The enhanced n-type or p-type conduction of ceria-based or proton conducting oxide electrolyte under water electrolysis operations is recognized as a practical problem of reduced energy conversion efficiency. However, the effect of overpotential at practical semi-blocking electrodes (or incompletely reversible electrodes) on the electrolyte is not well understood. The aims of this study is to elucidate whether the overpotential directly defines the chemical potential shift in the electrolyte near the interface.Two kinds of approaches were taken: i.e. direct measurement of the chemical potential in the electrolyte and non-linear equivalent circuit analysis of electrochemical responses. The direct measurements were made by using embedded probes in an oxide ion conductor. Yttria stabilized zirconia powder was pressed and sintered with five platinum wires of 0.1~0.2 mm thick. Porous platinum electrodes were pasted on the both ends of the sample and direct or alternating current was applied. Electric fields between the probes/electrodes were estimated by high frequency response of ac impedance, and subtracted from the electrical potential of the probes under dc. The distribution of chemical potential of electron, and thus, of oxygen was obtained assuming constant chemical potential of oxide ion. When oxygen exchange on the cathode was blocked using a glass paste, the oxygen potential at each probe shifted to the negative direction, and the change propagated slowly from the cathode side to the anode side. The result was in close agreement with the estimation from the mobility and carrier concentration of electrons in YSZ, suggesting that the probes successfully detected the local oxygen potential. Evaluation of chemical potential shifts with half-blocking electrode is in progress.For the equivalent circuit approach, a model circuit was developed to simulate the linear and nonlinear responses of electrode/electrolyte systems. The circuit consisted of charged carrier transport lines connected by capacitors exhibiting the local defect equilibrium, as proposed by Jamnik and Maier[4]. Unlike an equivalent circuit for a regular impedance analysis, the circuit elements were defined using parameters that varied with local chemical potentials. The responses of the circuit were calculated by using a general-purpose circuit analyzer LT-SPICE that enabled analyses of transient response as well as linear impedance. It also enabled higher harmonic analyses which is useful for detecting electrolyte polarization. The developed model was tested with a simple symmetric cell with mixed conducting electrode. Qualitative correspondence was obtained regarding the oxygen potential dependence in the linear impedance and the second harmonic responses. In addition to the oxide ion conductors, proton conductor with hole and oxide ion as minor carriers were modeled. Calculations with different assumptions of the oxygen (and hydrogen) potential shift will be compared with the ongoing experiments to find the effect of polarization of a half-blocking electrode.

(Invited) Impact of Sr-Containing Secondary Phases on Oxide Conductivity in Solid-Oxide Electrolyzer Cells

Solid oxide electrolyzer cells (SOECs) are among the most promising devices for producing hydrogen from the electrolysis of water, as they can be operated using excess heat. However, due to their high operating temperatures, they can suffer from materials degradation phenomena, including the segregation of cation species across the cells. Notably, in SOECs based on yttria-stabilized zirconia (YSZ) as the oxide-conducting electrolyte, strontium segregation from the air electrode through the barrier layer toward the electrolyte can lead to the formation of certain unwanted secondary phases, including SrO and SrZrO3.Here, use density-functional theory (DFT) calculations based on a hybrid functional in conjunction with energy dispersive x-ray spectroscopy (EDS) to characterize these Sr-containing secondary phases and to quantify their impact on oxide conductivity in SOECs. Using our DFT calculations, we calculate the mobility and concentration of oxygen vacancies in each material, which we combine to estimate their oxide ion conductivity. We find that SrO has a low oxide ionic conductivity that will dramatically reduce SOEC performance if it is present in thick, continuous layers. Similarly, we find that SrZrO3 does not have a high conductivity due to low intrinsic vacancy concentrations; however, doping SrZrO3 with yttrium will raise oxygen vacancy concentrations and thereby oxide conductivity, even to the point of parity with YSZ. Yttrium-doping of SrZrO3 is likely, considering that it is present in YSZ alongside zirconium, which reacts with strontium to produce SrZrO3.Our EDS analysis of post-operando SOECs confirms that Y is present as a majority impurity in SrZrO3 precipitates, while other dopants, which we find will generally reduce oxide conductivity, have significantly lower concentrations. Furthermore, EDS does not reveal large concentrations of SrO, implying that it will not form in sufficiently thick layers to block oxide conductivity, at least until the duration of testing considered here.Our work provides a comprehensive analysis into the role that Sr-containing secondary phases play in YSZ-based SOECs, along with insights that can be used to engineer better performing and more resilient devices.Part of this work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Part of this work was performed by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308.

Dissociative Oxygen Adsorption and Incorporation in Co3O4-Dispersed BaZr0.9Sc0.1O2.95 for a PCFC Cathode

Proton-conducting fuel cells (PCFCs), which operate at lower temperatures than conventional solid oxide fuel cells (SOFCs) using an oxide-ion conductor, have attracted considerable attention owing to their high energy-conversion efficiency. Even with mobile protons, oxygen reduction reactions at the cathode limit the performance of PCFCs. To improve their cathode properties, a variety of functional oxides, including the so-called triple conductors, which have mobile oxide ions, protons, and holes, have been developed. Our group has focused on the excellent catalytic activity of Co3O4 for the dissociative adsorption of oxygen at elevated temperatures, which is comparable to that of Co-containing mixed conductors with a perovskite-type structure [1,2]. In this study, Co3O4-dispersed proton-conducting oxides, BaZr0.9Sc0.1O2.95, were prepared for use as composite-type cathodes, and their surface oxygen exchange behavior was investigated using the pulse isotopic exchange (PIE) technique.BaZr0.9Sc0.1O2.95 (BZS10) was prepared using a solid-state reaction method by sintering at 1300 °C for 10 h. To prepare the composite powders, BZS10 powders were ball-milled with x vol%Co3O4 (x = 1, 3, 5, 10, and 50). Prior to PIE, their specific surface area was determined by a BET method for the sample powder heat-treated at 880 °C, which is slightly higher than the highest PIE temperature to avoid morphological and compositional change during PIE. The PIE was performed at 800–200 °C under a 10%O2 atmosphere.Based on the almost constant lattice constants, regardless of the Co3O4 volume fractions, the composites were successfully prepared as a dual-phase state, unlike Co-doped BZS10 prepared at 1225 °C, which showed a decrease in the lattice constant [3]. At 500 °C, the surface oxygen exchange rate R0 of BZS10 without Co3O4 particles was 4.0×10-6 mol/m2·s. R0 was surprisingly enhanced by an order of magnitude with the addition of only 1 vol%Co3O4. Further addition of Co3O4 exceeding 10 vol% reduced R0 . This trend indicates that the optimum amount of Co3O4 provided sufficient dissociated oxygen on BZS10, which was readily incorporated into BZS10. Their electrochemical properties and detailed microstructural analyses are also discussed.[1] Y. Tomura et al., J. Mater. Chem. A, 8 (2022) 21634–21641.[2] A. Ishii et al., ACS Appl. Mater. Interfaces, 15 (2023) 34809−34817.[3] H. Uehara et al., Int. J. Hydrogen Energy, 47 (2022) 5577−5584.

Analysis of Crystal Structure and Phase Transition of Partially Cation Substituted SrFeO3- δ

Transformation of ordered arrangement of oxide ion vacancies in solid oxides to random one is one of the probable methods for development of new oxide ion or mixed conductor. The crystal structures of SrFeO2.875 and SrFeO2.75 at room temperature are tetragonal and orthorhombic distorted perovskite, respectively, with ordered oxide ion vacancies. In this work, partial cation substitution to SrFeO3-δ was examined as a method to transform the crystal structure to cubic perovskite with random distribution of oxide ion vacancies.Two kinds of cations were examined as substituents. One was trivalent ions such as Y and lanthanoid (Ln). The other was Ca2+ or Ba2+ for controlling size of A-site ion resulting in tolerance factor. The samples were prepared by Pechini method. SrCO3 and CaCO3 were dissolved in dilute HNO3. BaCO3 was dissolved in citric acid. Y2O3 and Ln2O3 were dissolved in HNO3 or mixture of HNO3 and H2O2 with heating. As an Fe source, Fe(NO3)3•9H2O, whose purity was verified with mass of Fe2O3 after heating at 800 ºC, was employed and dissolved in distilled H2O. The dissolved raw materials were mixed with nominal composition. After addition of citric acid and ethylene glycol, the solution was heated, resulting in precursor. The precursor was calcined at 750 ºC for more than 17 h in air. The obtained powder was pressed into pellet after pulverization and sintered at 1200 ºC for more than 17 h in air. The phase of the specimens was identified by X-ray diffraction measurements. Some specimens were subjected to Mössbauer spectroscopy and electron diffraction measurement for minute analysis of crystal structure. Thermal analyses such as TG-DTA and DSC were performed to clarify phase transition behavior.X-ray diffraction measurements of the samples with nominal composition of Sr1-x Ln x FeO3-δ and SrFe1-x Ln x O3-δ revealed that La, Nd, Ho, Y, Tm, Yb, and Lu could be substituted for Sr site with x=0.1-0.2 resulting in cubic perovskite structure, whereas only Tm, Yb, and Lu could be substituted for Fe site. This tendency was successfully explained assuming that Ln3+ substituted site with smaller difference of ionic radius. Although spots assigned as long-range order was observed in electron diffraction pattern of Sr0.9Y0.1FeO2.80, they were diffuse ones with small intensity, indicating large randomness of oxide ion distribution. Broad Mössbauer spectrum of Sr0.9Y0.1FeO2.80 and Sr0.9Yb0.1FeO2.80 identified as summation of Fe3.5+ with main intensity and smaller Fe3+ and Fe4+ also indicated random distribution of oxide ion vacancy. No DTA signal was observed between 25-1000 ºC, indicating arrangement of oxide ion vacancies was maintained.X-ray diffraction patterns of Sr1-x Ca x FeO3-δ were not assigned as cubic; whereas those of Sr1-x Ba x FeO3-δ with x=0.1, 0.15, and 0.2 were apparently identified as cubic perovskite. Iodometric titration revealed that Fe valence in Sr1-x Ba x FeO3-δ with x=0.1 and 0.2 was 3.5+. Because tolerance factor approaches 1.0 by partial substitution of Ba if Fe valence is 3.5+, the observed variation of crystal structure to apparently cubic could be explained by using tolerance factor. However, clear spots assigned as long-range order observed in electron diffraction patterns and Mössbauer spectrum showing both Fe3+ in octahedron and Fe4+ in pyramid indicated that crystal structure of Sr0.8Ba0.2FeO2.75 was not cubic but orthorhombic with ordered oxide ion vacancies. The X-ray diffraction pattern identified as apparent cubic could be attributed to small distortion of octahedron, pyramid, and their linkage from cubic perovskite. The endothermic peaks without mass variation were observed in TG-DTA and DSC curves in Sr1-x Ba x FeO2.75, indicating the structural phase transition. The phase transition temperature observed at 420 ºC for SrFeO2.75 decreased to 200 ºC for Sr0.8Ba0.2FeO2.75. The crystal structure at higher temperature than the phase transition temperature was identified as cubic with random oxide ion arrangement because the clear spots assigned as long-range order in electron diffraction patterns at room temperature disappeared or became diffused ones at 250 ºC.The investigation of electrical conductivity of cation substituted SrFeO3-δ is in progress at present.

Multiscale Assessment of Solid Oxide Electrolysis with a Combination of Modeling and Material Property Optimization

Hydrogen has emerged as a versatile energy carrier poised to play a pivotal role in the future energy landscape. Electrolysis, particularly when powered by renewable energy sources, stands out as a promising method for hydrogen production. However, the cost of hydrogen from renewables remains relatively high, hindering widespread adoption across key markets and industries. Overcoming this barrier requires establishing market mechanisms and policies that promote cheaper renewable electricity costs and technology penetration. Additionally, radical innovation across multidisciplinary aspects, including materials, design, scaling up, operation, and digitalization, is crucial to reducing capital expenditure (CAPEX) and accelerating the transition to widespread sustainable energy systems.Our study addresses these challenges by providing a connected multiscale assessment of Solid Oxide Electrolysis (SOE) systems for hydrogen production. This assessment spans from the exploration of novel materials to the development of digital modeling tools for evaluating various scales, including the investigation on the material level, cell and stack assembly level, system integration, and grid performance. Given the significant duration it takes for a new product to transition from material development in the laboratory to the market, this approach of bridging the gap between scales, from fundamental material properties to system performance and grid integration holds great importance in understanding the interplay between these different scales and allows for rapid decision making along the design chain.Figure 1a) provides a comprehensive overview of all the scales involved in an electrolysis system, encompassing material, cell/stack assembly, system integration with auxiliary components, and the connection to the electric grid. At the material scale, we focus on the transformative effects of enhancing ionic conductivity, a fundamental property influencing multiple scales of electrolysis technology. Values for ionic conductivity are derived from materials simulation of state-of-the-art solid oxide ion conductors, which are further improved by application of a conductivity-enhancement concept. For this approach, we extended a method for the synthesis of Yttria-stabilized zirconia and Strontium-magnesium-doped lanthanum gallium electrolytes incorporating internal nanoparticles ('endoparticles’). At the cell level, a comprehensive mathematical model allows access to cell voltage data at set current density. At the system level, we delve into electric-to-hydrogen conversion efficiency and CAPEX. Finally, at the grid scale, we explore the levelized cost of hydrogen (LCOH), and the potential energy and cost savings if materials with improved conductivity were implemented at a large scale to meet early hydrogen production targets.The primary findings of our study are illustrated in a parallel axis plot shown in figure 1b). We found that increasing the ion conductivity of the electrolyte by 1-2 orders of magnitude results in a noticeable decrease in cell voltage, and therefore boosts the electric to hydrogen conversion efficiency. The CAPEX is projected to range, depending on several factors such as cell operating conditions (for instance, utilizing affordable steel supports at lower temperatures if the ionic conductivity allows), advancements in cell manufacturing, and scaling benefits. When we consider these factors along with the enhanced conductivity, the hydrogen production cost with this technology could significantly vary. However, the cost reduction linked to electrolyte conductivity enhancement is minor, contributing to less than a tenth of this range. Essentially, our findings suggest that major alterations at the material level have a marginal direct effect on the final hydrogen cost LCOH. Significant material improvements therefore play a more crucial role in reducing the CAPEX, as they could allow for the use of cheaper materials, which in turn could lower the cost of hydrogen significantly.In conclusion, our study highlights the nuanced interplay between material properties and system performance in electrolysis technology, emphasizing the importance of multidisciplinary innovation and scaling up efforts to realize the ambitious hydrogen production targets. By bridging the gaps between different scales and leveraging advancements in materials and digital modeling, we can allow for quicker decision making and thus accelerate the transition towards a sustainable hydrogen economy. Figure 1

Surface Oxide Ion Conduction of a - Axis Oriented Ce0.75Sm0.25O2- δ Thin Film Prepared on (200) YSZ Substrate

Fuel cells are attracting attention as a clean energy source because their only waste product is water. Solid oxide fuel cells (SOFC), which use oxide with oxygen ion conductivity as the electrolyte, can simplify the configuration of the power generation system and facilitate management, but they require a high operating temperature of 800 ℃ or higher [1]. Yttria-stabilized zirconia, which is currently in practical use as an electrolyte, exhibits high oxygen ion conductivity at temperatures above 800 ºC. Therefore, an electrolyte that exhibits high ion conductivity at low temperatures is required.Fluorite-type CeO2-δ oxides are good solid electrolyte candidates for solid oxide fuel cells due to their high oxygen ion conductivity in high-temperature regions above 800 ℃ because they take on a mixed valence state of Ce3+ and Ce4+ in the high temperature. Furthermore, it is known that doping CeO2 with trivalent dopants increases the amount of oxygen defects and improves ionic conductivity [2-4]. While we have reported the surface ionic conduction Sm-doped CeO2 thin films, the conductivity was not sufficiently high for practical applications [5].In this study, in order to realize high surface oxide ion conductivity, we prepared a - Axis Oriented Sm-doped CeO2 (Ce0.75Sm0.25O2- δ : SDC) Thin Film without lattice distortion and investigated its surface oxide ion conduction.The SDC thin films were prepared on YSZ (200) substrates by RF magnetron sputtering using ceramic targets. The flow rate of Ar gas controlled by a mass-flow controller was kept at approximately 2.81 sccm. The pressure of Ar gas was fixed at 3.5 × 10−3 Torr during the deposition. The prepared thin films were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and A.C impedance method using a frequency response analyzer.The prepared SDC thin films showed a 200 peak and a 400 peak on the YSZ substrates, indicating no change in lattice constants with the ceramics. Figure shows an Arrhenius plot of the conductivity of the As-deposited and Wet-annealed SDC thin films. As a reference, the Arrhenius plot of the conductivity of the YSZ substrate and ceramics is shown. The conductivity of the SDC thin film is much higher than that of the YSZ substrate and ceramics because the amount of oxygen vacancies increased more than those. The conductivity does not much change by wet annealing and depends on the film thickness. The conducting carrier above 200 °C is considered to be oxide ion. In this presentation, the author will show the details of the conductivity and discuss including XPS results as well as details of the conductivity.

Catalytic Combustion Type H2 Gas Sensor Based on Cerium Oxide

Hydrogen gas is the most promising clean energy source, but its use carries an explosion hazard (LEL: 4 vol%). Therefore, the real-time and accurate detection of H2 gas leakage at every emitting site is required with the compact sensor to prevent a critical accident. Among the compact H2 gas sensors proposed, the catalytic combustion type gas sensor using precious metal-loaded γ-Al2O3 such as Pt/Al2O3 as the catalyst is one of the sensors in practical use because of its long life and low-cost features. However, platinum is an expensive and scarce resource, and its use is desired to be reduced. Therefore, for the realization of the coming hydrogen society, the development of novel catalytic combustion type sensor using a noble-metal-free catalysts is important.In this study, we focused on CeO2, Fe2O3, Co3O4, NiO as H2 oxidation catalyst and investigated the sensing performance of these metal oxides. In addition, we applied CeNbO4 +δ [1], having high oxide ion conductivity and redox property, as a promoter for CeO2 catalyst, and the H2 gas sensing performance of the sensor with 4 wt% CeO2/CeNbO4 +δcatalyst was investigated.Among the sensors applied metal oxide (CeO2, Fe2O3, Co3O4, NiO), the sensor with CeO2 showed highest sensor output which is over 2 times higher than the sensors with other metal oxides. This is due to high specific surface area and low heat capacity of CeO2. Although the sensor using CeO2 showed good sensing performance, quantitative detection of H2 required a temperature of 105 °C or higher, which is still high for practical use.To improve the H2 oxidation activity of CeO2, we applied CeNbO4 +δ as a promoter and the sensor with 4 wt% CeO2/CeNbO4 +δcatalyst. Figure 1(a) displays the response curves to H2 gas concentration change for the sensor applied 4 wt% CeO2/CeNbO4 +δ and CeO2. The sensor with 4 wt% CeO2/CeNbO4 +δcatalyst showed about 2.6 times higher sensor output compared to the sensor with only CeO2, indicating that the combination of CeNbO4 +δ improves H2 oxidation activity. Furthermore, the sensor with 4 wt% CeO2/CeNbO4 +δcatalyst was able to detect H2 gas concentration quantitatively even at 75 °C (Fig. 1(b)) with a 90% response time of about 14 seconds. The sensitivity to other gases such as carbon monoxide, methane, ethanol, and acetaldehyde was examined, and the sensor showed excellent selectivity for H2 at 75 °C.[1] Y. Jin, Y. Quan, J. Liu, C. Qui, P. Pan, B. Shan, H. Luo, and P. Yang, J. Solid State Chem., 313, 123318 (2022). Figure 1

Topotactic Reduction-Induced Stabilization of β-La2Mo2O8.68 Phase: Structure, Static Oxygen Disorder, and Electrical Properties.

La2Mo2O9 is acknowledged as an exceptional oxide ion conductor. It undergoes a reversible phase transition around 580 °C from the nonconductive low-temperature monoclinic α-La2Mo2O9 phase to the highly conductive high-temperature cubic β-La2Mo2O9 phase. In addition, La2Mo2O9 demonstrates complex chemistry under reducing conditions. This study reports, for the first time, the stabilization at ambient temperature of a novel cubic phase through a topotactic reduction of α-La2Mo2O9 employing CaH2. This phase contains approximately ∼3 atom % oxygen vacancies relative to the nominal composition (La2Mo2O8.68(1)). The cubic symmetry is associated with a static distribution of these vacancies, in contrast to the dynamic distribution observed in the high-temperature cubic β-La2Mo2O9 phase reported previously. Additionally, the material exhibits mixed-ion-electronic conduction, which expands its potential use in applications requiring both ionic and electronic transport.

Investigation of structural and electrical properties of electrolyte LaGaO3 for solid oxide fuel cell.

Electrochemical devices such as solid oxide fuel cells (SOFCs) allow the direct transformation of fuel's chemical energy into electrical power. Even though YSZ electrolyte-based conventional SOFCs are widely used in both laboratories and on a commercial scale, developing alternative ion-conducting electrolytes is crucial for enhancing SOFC performance at lower operating temperatures. In this work, we conducted a thorough computational analysis on the characteristics of Sr- and Mg-doped superior oxide ion conductors. We have used the DFT technique to examine the system's electrical and structural characteristics and the impact of doping. The GII value and LaGaO3 formation energy are used to investigate thermodynamical and structural stability, respectively. Theoretical investigations are validated against data to ensure the accuracy of the computational model. The research shows that the properties of Sr- and Mg-doped LaGaO3 have changed in a desirable way. This DFT study sheds light on the underlying mechanisms that affect the structural and electronic properties of LaGaO3 electrolytes and offers a thorough investigation of the synergistic effects of strontium and magnesium co-doping. The knowledge acquired is critical for the logical design and development of more stable and efficient solid oxide fuel cells.

Electrical properties and redox stability of series novel high-entropy Bi2VO5.5-based oxides

Exceptional high ionic conductivities have been well documented in the Bi2VO5.5-based materials. However, the poor phase and redox stabilities under a reducing atmosphere hindered their wide applications. Herein, with the aim to improve the phase and redox stabilities under a reducing atmosphere, a high-entropy strategy of multiple-metal-doping was applied to prepare series Bi2V1–4x(GaSiTaZr)xO5.5-δ (0 ≤ x ≤ 0.1) materials by a traditional solid state reaction method. The results revealed that multiple-metal-doping could stabilize the tetragonal phase of Bi2VO5.5 to room temperature and shown good phase and structural stabilities under inert or high oxygen partial pressure atmospheres, as well as pure oxide ion conduction which was about two orders of magnitude higher than that of the parent material, e.g. ∼2.0×10−2 S/cm at 400 ºC for the x = 0.05 sample while ∼1.5×10−4 S/cm for the x = 0 sample. Although the Bi2V1–4x(CuNiNbTi)xO5.5-δ materials would still be readily reduced under a reducing environment and introduce strong electronic conduction, the phase stability was apparently improved. Thus, improving the redox stability and suppressing the electronic conduction of the stabilized Bi2VO5.5-based materials under a reducing atmosphere is still the direction we should strive for.

Ba3Ti0.4W1.6O8.6: An Oxygen- and B-Site-Deficient 9R Hexagonal Perovskite Exhibiting Triple Conduction.

The hexagonal perovskite derivatives Ba3M'M″O8.5 featuring a hybrid structure composed of 9R hexagonal perovskite and palmierite structure motifs exhibit significant oxide ionic conductivity due to the highly disordered oxide-ion and M-cation sublattices. Herein, we report the structure and electrical properties of the perovskite Ba3Ti0.4W1.6O8.6. Three-dimensional (3D) electron diffraction (ED), neutron powder diffraction (NPD), and neutron pair distribution functions (nPDF) revealed a 9R hexagonal perovskite structure for Ba3Ti0.4W1.6O8.6 with fully occupied central M2 sites, partially occupied outer M1 sites, and oxygen-deficient cubic c-BaO2.6 sublayers. These cation and oxygen arrangements differ significantly from those in Ba3M'M″O8.5 and enable Ba3Ti0.4W1.6O8.6 to capture atmospheric water and O2, resulting in triple conduction (oxide ion, proton, and hole) under wet air conditions. Proton and oxide ion conductions predominate at temperatures <400 and >650 °C in wet Ar and dry air, respectively. Bond-valence site energy calculations, together with structure analysis, deciphered that the two-dimensional oxide-ion diffusion pathways along the c-BaO2.6 layers are disrupted by the M1 vacancies, thereby resulting in relatively low oxide ionic conductivity. Our findings open up a new strategy of utilizing the cation's propensity of coordination geometry to design new oxygen- and B-site-deficient perovskites and thus achieve desired conductivity.

Mixed Conduction in A-Site Double-Perovskite Na1+xLa1-xZr2O6-δ Proton Conductors.

Perovskite-type proton conductors exist in two structural forms, ABO3 and A2B'B″O6. In this study, novel A-site double-perovskite proton conductors (A'A″B2O6) were proposed. Na1+xLa1-xZr2O6-δ (x = 0, 0.1, 0.2) perovskites were prepared by a solid-state reaction at 1200 °C. However, raising the sintering temperature to 1300 °C resulted in the Na to volatilize, converting the Na1.1La0.9Zr2O6-δ into La0.9Zr2O6-δ. The conductivities of these materials in a humid atmosphere were tested using electrochemical impedance spectroscopy, and their carrier transport numbers were measured using the defect equilibria model and concentration cell method. Na1.1La0.9Zr2O6-δ and Na1.2La0.8Zr2O6-δ are predominantly proton conductors, with Na1.1La0.9Zr2O6-δ exhibiting the highest proton transport number of 0.52 at 800 °C. In contrast, NaLaZr2O6 is predominantly an electronic conductor, while La0.9Zr2O6-δ functions as an oxide ion conductor. Due to their high protonic transport numbers, these Na1+xLa1-xZr2O6-δ A-site double-perovskite oxides present a promising avenue for the development of proton conductors.