Proton-conducting Perovskite Oxides Research Articles

Co-doping effect on the microstructural and electrical properties of barium stannate materials

Proton-conducting perovskite oxides are of considerable interest to researchers as promising electrolytes for low- and intermediate solid oxide electrochemical cells. Therefore, designing new potential proton-conducting phases and improving the functional properties of known materials are of great importance from both fundamental and applied viewpoints. In the present work, BaSnO3 was selected as a reference proton-conducting system and then a co-doping strategy was employed to analyze ‘composition – structure – microstructure – transport properties’ relationships. To perform such an analysis, the properties of previously studied BaSn0.7M0.3O3–δ (M = In, Sc, Y) compounds were compared here to their co-doped derivatives, BaSn0.7In0.15Sc0.15O3–δ, BaSn0.7Y0.15Sc0.15O3–δ, and BaSn0.7In0.15Y0.15O3–δ. It is found that the type of dopant affects the materials sinterability, when more coarse-crystalline ceramics are formed with increasing the average ionic radii at the Sn-position. The introduction of Y3+-cations reduces both ionic and hole conductivities compared to single-doped with In3+ or Sc3+ barium stannate materials. However, simultaneous doping with In3+/Sc3+ cations minimizes the contribution of hole conductivity compared to that of Sc-doped barium stannate with the same acceptor dopant concentration.

Machine Learning-Assisted Discovery of High-Performance Perovskite Electrolytes for Intermediate-Temperature Protonic Ceramic Electrochemical Cells

Solid oxide electrochemical cells (SOECs) have attracted intensive attention because of their high efficiency and low environmental impact. Much effort has been made to decrease the operating temperature of SOECs down to the intermediate-temperature range. The electrolyte materials with high ionic conductivity are the crucial factor in deciding the operating temperature of SOECs among all components. Proton-conducting perovskite oxides are one of the most promising electrolyte materials for intermediate-temperature SOECs due to their excellent ionic conductivity, usually called protonic ceramic electrochemical cells (PCECs). The most common method to discover the new potential proton-conducting perovskite electrolytes is that the introduced aliovalent dopants facilitate the creation of oxygen vacancy to maintain electroneutrality and give rise to protonic defects when exposed to a reduced atmosphere. In addition to the dopant elements and the dopant content, the total conductivities of doped perovskite electrolytes also depend on the synthesis conditions, the measurement condition and so on. Therefore, the investigation of potential perovskite electrolytes for the intermediate-temperature range should not only be based on the search for the most promising compositions but also requires a detailed study of the influences of its microstructure and processing conditions on the total conductivities. The traditional models of trial-and-error experimental processes limit the development of promising electrolyte materials. Machine learning has emerged as a powerful tool to gain hidden rules from extensive data. By focusing on the investigation of the potential perovskite electrolytes, we collected 400 training data and defined 34 desirable features. Features related to basic elemental properties, crystal structure, microstructure, synthesis conditions, and measurement conditions have been included. Based on these data from the published literature, machine learning models are utilized to predict the total conductivity of perovskite compositions at targeted conditions. Thereafter, feature importance is evaluated to establish a guideline for the rapid and accurate screening of potential perovskite electrolytes in the intermediate-temperature range. Consequently, the screened compositions with high total conductivity from the trained model are prepared, and the experimental results are consistent with the predicted results. This study establishes a new avenue to discover perovskite electrolytes with high total conductivity for intermediate-temperature PCECs. Keywords: Perovskite electrolyte, Proton conducting, Protonic ceramic cells, Machine learning. Figure 1

Accelerated Discovery of Proton-Conducting Perovskites through Density Functional Theory and Machine Learning

Hydrogen is an important energy carrier resource in response to limiting greenhouse gas emissions. Proton-conducting perovskite oxide is one of the key materials for highly efficient carbon-neutral hydrogen technologies, such as hydrogen production, CO2 hydrogenation, and ammonia synthesis. Many attempts have been made based on doped perovskites made of well-tested materials, such as BaZrO3, BaCeO3, BaHfO3, BaTiO3, and SrZrO3. However, the resulting perovskites have often suffered stability and conductivity problems. Furthermore, complex phenomena occurring during hydration present challenges for expanding the materials library. Herein, we demonstrate accelerated discovery of proton-conducting perovskites with high conductivity using machine learning (ML) predictions. We constructed consistent training data using density functional theory (DFT) which enable high accuracy of ML model. DFT computations were performed on > 1000 doped perovskite compositions to get their properties of lattice parameters, point defects (e.g., O vacancies, H interstitials), density of states, hydration energy, and proton migration energy. Several ML algorithms including Linear Regression, Bayesian Ridge Regression, Random Forest Regression, Neural networks, and k-Nearest Neighbor were tested for minimum errors and coefficient of determination. The multidimensional relationships between a set of >50 features and conductivity were mapped out using the optimized ML model. We screened a large material space of A-site and B-site doped perovskites to predict potential proton-conducting materials for various energy applications. The outcomes are promising for accelerating the design and applications of proton-conducting perovskite oxides in hydrogen technologies.

Development and application of lanthanum tungstate series proton conductors

<p indent=0mm>As one of the most important clean energy and commodity chemicals, hydrogen is widely used in human life and industrial production. In the present decade, three main technologies for hydrogen recovery have been developed, namely pressure swing adsorption (PSA), cryogenic separation, and membrane separation. Among these three different hydrogen recovery technologies, the membrane separation method is favored for its high efficiency. Benefiting from its absolute selectivity of hydrogen and the low cost, mixed protonic-electronic conductors (MPEC) have become an important research object in the field of hydrogen separation membrane materials, which enable a significant approach for the purification and utilization of hydrogen energy. For the traditional proton-conducting perovskite oxides doped with Ce and Zr, although they have high proton conductivity, their poor chemical stability under the atmosphere in the presence of CO₂ and H₂O hinders the commercial application. Rare-earth metal compound lanthanum tungstate material (LWO) is a kind of crucial proton conductor, which has good chemical stability in CO₂ atmosphere. However, the hydrogen permeability of LWO is not satisfactory because of the low electronic conductivity. A variety of effective methods to improve the hydrogen permeability of lanthanum tungstate film are summarized in this review. This review introduces the basic electronic conductivity of LWO-based single-phase materials, with the emphasis on improving the electronic conductivity of LWO by adjusting La/W ratio, cation doping strategies, and preparation methods. Furthermore, it reviews breakthroughs in their application in proton-conducting solid oxide fuel cells (SOFC) as well as hydrogen separation membranes. The problems now faced by LWO oxides are analyzed and their future development is predicted. Firstly, the proton conductivity of LWO is positively correlated with the La/W ratio. Adjusting the La/W ratio improves proton conductivity of LWO. For cation doping, the substitution of W rather than La more effectively improves the proton conductivity of LWO. Secondly, compositing LWO and the electronic conductors effectively enhance the performance of the hydrogen permeation membranes. Thirdly, in terms of the SOFC applications, an additional oxide layer between the LWO electrolyte layer and the electrode layer avoids the phase reaction on the interface, which prevents the chemical incompatibility between the LWO and electrode materials. Many studies show that the maximum utilization of LWO can be realized by optimizing crystal structure, phase composition, and the SOFC structure. In view of the problems existing in the research of LWO materials, future research can be focused on the following aspects: (1) Further exploring the electronic-ionic conducting pathway and catalytic mechanism of LWO series materials, and then optimizing the electrochemical properties under the guidance. (2) Developing the electrode materials with good catalytic performance and good chemical compatibility with LWO which is significant to the stable operation of SOFC. Also, it is necessary to improve the cell preparation process in which the unbeneficial interface reaction between the electrode and the LWO can be avoided. (3) For hydrogen permeation membrane, future research can be developed towards optimization with cation doping, surface engineering, and composite materials preparation.

Non-stoichiometry, structure and properties of proton-conducting perovskite oxides

The demand for clean and sustainable energy has garnered great interest in new energy materials. Among them, high temperature proton-conducting perovskite oxides are, or can be widely used in clean energy applications (including fuel cells, electrochemical reactors, solid-state separators and supports of catalytic components via various reduction and oxidation reactions) in the intermediate temperature range. The control of defect chemistry is the main strategy to fine tune properties for these applications. This review provides a critical discussion about non-stoichiometry-structure-property relation in terms of structure distortions by intrinsic octahedral titling and extrinsic acceptor doping, chemical stability, hydration behavior, transport properties, and catalytic effects in some typical classes of proton conducting perovskites and perovskite-related derivatives. A good understanding of A-site and B-site non-stoichiometry is also given to allow the perovskite structure with desired properties.

Hydration-induced chemical expansion of BaCa(1+y)/3Nb(2−y)/3O3−δ∙xH2O (BCN) and other proton-conducting perovskite oxides

Thermal expansion and chemical strain caused by water uptake were measured for BaCa(1+y)/3Nb(2−y)/3O3−δ∙xH2O (BCN) perovskite-type oxides by dilatometric and X-ray diffraction methods. To describe the total and chemical expansion, we proposed a simple geometric chemical strain model based on the relative change in ionic radii upon hydration. The model calculations agree well with the experimental data for BCN oxides, for which the values of the linear chemical expansion coefficient, β, were found to be around 0.038 per mole of water-filled oxygen vacancies. Next, the model was applied to different perovskite-type proton-conducting oxides with existing literature data on chemical expansion. For example, the values of β, estimated using the model, are around 0.036 and 0.031 for BaZr1−yMyO3−δ and BaCe1−yMyO3−δ, respectively (M = rare-earth metal, Y or Sc). These values are close to those obtained by others using either density functional theory (DFT) calculations or direct experimental measurements, especially given the rather large scatter in the data reported in the literature. The latter is most probably related to the kinetic limitations since, unlike the most oxygen exchange phenomena, humidification of oxides happens at lower temperatures where the slow kinetics of water uptake plays an important role.

Multiple-doped barium cerate proton-conducting electrolytes for chemical-energy cogeneration in solid oxide fuel cells

Proton-conducting perovskite oxides such as doped barium cerate and barium zirconate are promising electrolytes for solid oxide fuel cells (SOFCs). Multiple-doped barium cerate perovskite oxide proton conductors of BaCe0·7Zr0·1Y0.2-xNdxO3−δ (0 ≤ x ≤ 0.1) are prepared by solid state reaction and the properties of the fabricated material are characterized by various technologies. Nd doping improves the sinterability and Zr doping enhances the chemical stability in CO2 atmosphere. The electrical conductivity order of the as-prepared electrolytes at elevated temperature in hydrogen atmosphere is: BaCe0·7Y0·17Zr0.1Nd0.03O3−δ > BaCe0·7Y0·2Zr0·1O3−δ > BaCe0·7Y0·1Zr0.1Nd0.1O3−δ. Ethane is selectively dehydrogenated to ethylene with cogeneration of electrical power in the SOFC operated at 650–700 °C. The maximum power density of the SOFC is 123 mW cm−2 for ethane fuel and 138 mW cm−2 for hydrogen fuel at the 700 °C. These unique features make the multiple-doped barium cerate perovskite oxide a promising electrolyte for chemical-energy cogeneration in SOFCs.

A-Site Cation Substitutions in Strained Y-Doped BaZrO3 Multilayer Films Leading to Fast Proton Transport Pathways

Proton-conducting perovskite oxides form a class of solid electrolytes for novel electrochemical devices operating at moderate temperatures. Here, we use hard X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and density functional theory calculations to investigate the structure and elucidate the origin of the fast proton transport properties of strained ultrathin films of Y-doped BaZrO3 grown by pulsed lased deposition on NdGaO3. Our study shows that our BaZr0.8Y0.2O3 films incorporate a significant amount of Y dopants, and to a lesser extent also Zr ions, substituting for Ba2+, and that these substitutional defects agglomerate forming columnar regions crossing vertically from the surface to the interface the entire film. Our calculations also show that, in regions rich in Y substitutions for both Zr and Ba, the proton transfer process involves nearly zero-energy barriers, indicating that A-site cation substitutions by Y lead to fast transport pathways and hence are responsibl...

Discovery and Understanding of the Ambient-Condition Degradation of Doped Barium Cerate Proton-Conducting Perovskite Oxide in Solid Oxide Fuel Cells

Proton-conducting perovskite oxides such as doped barium cerate and barium zirconate are promising electrolytes for solid oxide fuel cells (SOFCs). Here we report that the typical high performance proton conductor, BaZr0.1Ce0.7Y0.2O3±δ (BZCY), is prone to physical, chemical and thereby electrochemical degradations as it ages under ambient conditions. This aging effect destroys the electrolyte disk and breaks down the perovskite lattice. Electrochemical measurements indicate degraded BZCY based SOFC suffers from significant performance losses, and various advanced materials characterization results confirm that this degradation of BZCY is exclusively caused by water and CO2 from the surrounding environment. The water molecules catalyze the degradation, initiating and promoting the carbonatation of BZCY. The subsequent reaction with CO2 creates two major phases: BaCO3 nano-rod single crystals and amorphous oxide mixtures. The entire process is visualized via TEM observations, based on which we propose that this occurs via the microcrucible mechanism. As this mechanism does not incur significant segregations of decomposition products, the aged BZCY powder is easily regenerated by re-calcination. We then study the effects of doping on the stability of BZCY, showing that a higher Zr content obviously improves the aging resistance.

Significant performance enhancement of yttrium-doped barium cerate proton conductor as electrolyte for solid oxide fuel cells through a Pd ingress–egress approach

Proton-conducting perovskite oxides are excellent electrolyte materials for SOFCs that may improve power density at reduced temperatures and increase fuel efficiency, thus encouraging the widespread implementation of this attractive technology. The main challenges in the application of these oxides in SOFCs are difficult sintering and insufficient conductivity in real cells. In this study, we propose a novel method to significantly enhance the performance of a yttrium-doped barium cerate proton conductor as an electrolyte for SOFCs through a Pd ingress–egress approach to the development of BaCe0.8Y0.1Pd0.1O3−δ (BCYP10). The capability of the Pd egress from the BCYP10 perovskite lattice is demonstrated by H2-TPR, XRD, EDX mapping of STEM and XPS. Significant improvement in the sinterability is observed after the introduction of Pd due to the increased ionic conductivity and the sintering aid effect of egressed Pd. The formation of a B-site cation defect structure after Pd egress and the consequent modification of perovskite grain boundaries with Pd nanoparticles leads to a proton conductivity of BCYP10 that is approximately 3 times higher than that of BCY under a reducing atmosphere. A single cell with a thin film BCYP10 electrolyte reaches a peak power density as high as 645 mA cm−2 at 700 °C.

Perovskite-Structured BaScO2(OH) as a Novel Proton Conductor: Heavily Hydrated Phase Obtained via Low-Temperature Synthesis

A novel proton-conducting material, BaScO2(OH) has been successfully fabricated. The known high-temperature proton conductors are typically perovskite-type oxides, in which the proton concentration is determined by hydration reaction of oxygen vacancies introduced by a small amount of acceptor dopant. On the other hand, the novel material BaScO2(OH) is still associated with the A2+B4+O3 perovskite structure but with the B-site cation fully consisting of an acceptor cation Sc3+, which facilitates to retain an appreciable amount of protonic defects. While it is difficult to obtain the material by simply hydrating the unhydrated form (Ba2Sc2O5), a combination of a new low-temperature sol–gel synthesis and ultrahigh-pressure (4 GPa) compaction at room-temperature enables us to obtain the heavily hydrated phase BaScO2(OH) due to on-synthesis hydration. The BaScO2(OH) synthesized has been proved to be a pseudocubic perovskite phase with XRD and Raman analyses. The thermal dehydration analyses have verified the composition BaScO2(OH) in terms of proton concentration, and their mobile nature has been observed with in situ FT-IR analysis. The protonic conductivity of the material is as high as 1.7 × 10–2 S·cm–1 at 500 °C, which is well higher than the total conductivity of the best proton-conducting perovskite oxides at intermediate temperature range.

Partial Conductivities and Chemical Diffusivities of Mixed Protonic–Electronic Conducting CaZr0.9Y0.1O3 − δ

The electrical properties of (CZY) were studied as a function of both oxygen partial pressure and water vapor partial pressure in the temperature range of and the partial conductivities of protons, holes, and oxygen vacancies were calculated from the defect model. In a wet atmosphere, CZY was a mixed conductor of protons, holes, and oxygen ions. No conduction transition from protons to holes and/or oxygen ions occurred even as temperature was increased up to . The calculated activation energy of proton transport was . The standard solution enthalpy for water dissolution was best fitted with a slope of . The twofold conductivity relaxation behavior that was observed at during hydration/dehydration was interpreted in terms of decoupled ambipolar diffusion of oxygen vacancies and protons by holes. The best-estimated chemical diffusivities of oxygen and hydrogen were several orders of magnitude lower than that reported for other proton-conducting perovskite oxides due to the relatively low ion conductivity of CZY.

Cation non-stoichiometry in yttrium-doped barium zirconate: phase behavior, microstructure, and proton conductivity

Recent literature indicates that cation non-stoichiometry in proton-conducting perovskite oxides (ABO3) can strongly influence their transport properties. Here we have investigated A-site non-stoichiometry in Ba1−xZr0.8Y0.2O3−δ, a candidate electrolyte material for fuel cell and other electrochemical applications. Synthesis is performed using a chemical solution approach in which the barium deficiency is precisely controlled. The perovskite phase is tolerant to barium deficiency up to x = 0.06 as revealed by X-ray diffraction analysis, but accommodates the non-stoichiometry by incorporation of yttrium on the A-site. The dopant partitioning can explain the decrease in cell constant with increasing x, the decrease in proton conductivity (the latter as measured by a.c. impedance spectroscopy under humidified atmosphere), and the decrease in grain size in the sintered compacts. Within the single-phase region barium deficiency also has a detrimental impact on grain boundary conductivity, as a result both of the decreased grain size, leading to a higher number density of grain boundaries and of an increased per boundary resistivity. At higher values of x, a two phase system is observed, with yttria appearing as the predominant secondary phase and the barium zirconate reverting to an undoped composition. From the relative concentrations of the observed phases and their lattice constants, the ternary phase behavior at 1600 °C (the sintering temperature) is generated. Both the bulk and grain boundary conductivities are sharply lower in the two-phase system than in the single phase compositions. The control over processing conditions demonstrates that it is possible to reproducibly prepare large-grained, stoichiometric BaZr0.8Y0.2O3−δ with a total conductivity of 1 × 10−2 Scm−1 at 450 °C, while revealing the mechanisms by which barium deficiency degrades properties.

Location of Deuterium Atoms in BaZr0.5In0.5O2.75+α by Neutron Powder Diffraction

The neutron powder diffraction data were collected at 10 K on the proton-conducting perovskite oxide BaZr0.5In0.5O2.75 with and without dissolved D2O. Obtained diffraction data were analyzed by the Rietveld method and the maximum entropy method. It was found that deuterium atoms were located close to the 12h (0, y, 0.5) site of the cubic perovskite structure (space group ). Such deuterium position seems common with many proton-conducting perovskite oxides.

Location of proton in proton-conducting perovskite oxides Ba3Ca1.18Nb1.82O8.73and BaZr0.8Sc0.2O2.9

Location of proton in proton-conducting perovskite oxides Ba₃Ca_1.18Nb_1.82O_8.73and BaZr_0.8Sc_0.2O_2.9

Location of deuterium atoms in BaSn 0.5In 0.5O 2.75+ α at 77–473 K by neutron powder diffraction

This paper describes the location of deuterium atoms in BaSn 0.5In 0.5O 2.75 by neutron powder diffraction at 77 K, room temperature and 473 K. The diffraction data were collected on BaSn 0.5In 0.5O 2.75 with and without dissolved D 2O, and analyzed by the Rietveld method and the maximum entropy method. It was found that deuterium atoms were located at or very close to the 12 h site of the cubic perovskite structure (space group Pm3¯ m) with an O–D distance of 1.0 Å at these temperatures. This deuterium site is basically the same as that at 10 K and similar to those suggested by the previous computational studies for proton-conducting perovskite oxides.

Location of deuterium atoms in BaSn 0.5In 0.5O 2.75 + α by neutron powder diffraction at 10 K

This paper describes a neutron diffraction study that has located hydrogen atoms in a proton-conducting perovskite oxide more definitely than ever. The neutron diffraction data were collected on BaSn 0.5In 0.5O 2.75 with and without dissolved D 2O at 10 K. Data analysis by the Rietveld method and the maximum entropy method revealed that deuterium atoms were located at or very close to the 12 h site of the cubic perovskite structure (space group Pm3¯ m) with an O–D distance of 1.0 Å. This hydrogen position is similar to those suggested by the previous computational studies for proton-conducting perovskite oxides.