Layered Perovskite Oxides Research Articles

Realization of new n = 3 and n = 4 Dion-Jacobson layered perovskite series with interlayer K+ ions and perovskite blocks stabilized by Pr3+ ions

The structural flexibility of the perovskite blocks in the Dion-Jacobson family of layered perovskite oxides has successfully resulted in the generation of KPrNan-2NbnO3n+1 and KPrCan-2Nb2Tin-2O3n+1 (n = 3, 4). The stabilization of K+ ions preferring a six coordination in the interlayer region is significant compared to the facile formation of layered perovskite oxides possessing bigger Rb+ or Cs+ ions with an eight coordination in the interlayer. The structure of the n = 3 members, KPrNaNb3O10 (S. G. Cmcm; a = 3.8858(1) Å, b = 29.6553(9) Å, c = 7.7635(3) Å) and KPrCaNb2TiO10 (S. G. Cmcm; a = 3.8634(2) Å, b = 29.4808(2) Å, c = 7.6945(5) Å) were confirmed respectively using the Rietveld and Le Bail methods utilizing the powder X-ray diffraction data. The n = 4 members, KPrNa2Nb4O13 and KPrCa2Nb2Ti2O13 are synthesized through high-temperature ceramic methods. The compounds are phase pure as confirmed from the X-ray diffraction data and were characterized by UV–visible diffuse reflectance measurements. These two-dimensional oxides also undergo facile topochemical ion-exchange and intercalation reactions and opens up several interesting possibilities. We have replaced the K+ ions in the interlayer by H+, Li+, Na+ ions and [CuCl]+ groups through low temperature reactions. The usefulness of various Dion-Jacobson layered perovskite oxides for potential photocatalytic applications has been explored by determining the optical band gaps, the valence band, and the conduction band energy levels based on the UV–visible diffuse reflectance measurements.

Tuning the Oxygen Reduction Reactivity of Layered Perovskites Using the Jahn-Teller Effect.

Compositional tuning of layered perovskite oxides provides a means of systematically studying how local distortions affect fundamental aspects of electrochemical reaction pathways. Structural analysis of a family of samples La1.2Sr0.8Ni1-yCoyO4 shows that Ni-rich compositions have an expanded crystalline c axis, which is anisotropically compressed by systematic Co incorporation. Raman spectra reveal the strong growth of a symmetry forbidden mode, which suggests that Co acts through localized distortions. Crystallographic and spectroscopic parameters describing this structural distortion correlate to the measured Tafel slopes for the oxygen reduction reaction for all Ni-containing samples, which is attributed to the distortion of potential energy surfaces by the Jahn-Teller expansion of d7 Ni(III) cations. Incorporation of Co not only minimizes the distortion but alters the apparent selectivity of the oxygen reduction reaction away from H2O2 and toward H2O. Rotating ring-disk electrochemical measurements, however, indicate that the apparent change in selectivity is due to activation of a first-order chemical disproportionation of H2O2 that is activated by Co in the lattice. These outcomes will support efforts to design electrocatalysts and reactors for the electrochemical synthesis of H2O2.

Boosting the electrocatalytic activity of NdBaCo2O5+δ via calcium co-doping as bifunctional oxygen electrodes for reversible solid oxide cells

The oxygen electrode of the solid oxide cells (SOCs) operates under an oxidizing atmosphere. Lattice shrinkage in a high oxygen partial pressure environment results in cation misalignment, leading to a decline in performance. Addressing the drawbacks associated with A-site cation mismatch in LnBaCo2O5+δ double perovskite oxides for reversible solid oxide cells (RSOCs) oxygen electrodes, a novel calcium ion co-doping strategy is proposed. This approach effectively mitigates A-site cation segregation and enhances stability. The glycine-nitrate method was employed to synthesize Nd0.8Ca0.2Ba1−xCaxCo2O5+δ (x = 0–0.2) layered double perovskites oxides co-doped with calcium ions, denoted as NCBCC. NCBCC exhibits excellent compatibility with commonly used electrolytes. The doping of calcium co-doped in NdBaCo2O5+δ decreases the thermal expansion coefficient and improves the electron transfer characteristics. The sample with x = 0.1 exhibited an area-specific resistance (ASR) of 0.024 Ω cm2 when operated at a temperature of 800 °C in air. In the solid oxide fuel cell mode, employing x = 0.1 as an oxygen electrode, the maximum power density achieved was 766 mW cm−2 at 800 °C. In the solid oxide electrolysis cell mode, operating with CO2 and steam at an electrolysis voltage of 1.5 V and a temperature of 800 °C, the corresponding current densities were −1.51 and −1.85 A cm−2, respectively. The incorporation of calcium ions into layered perovskite oxides offers a promising strategy to enhance the performance of oxygen electrodes in RSOCs.

Synthesis of nano-crystalline K2NiF4-type phases La1+xSr1-xCoO4 (x = 0.0, 0.2, and 0.5): Effect of mixed valence state of Co on structural, magnetic and photocatalytic properties

A lot of work has already been reported on bulk K2NiF4-type layered perovskite oxides but hardly is known about their nano-crystalline state as they are generally prepared at high temperatures. In this connection, we have synthesized nanocrystalline La1+xSr1-xCoO4 layered perovskites oxides via sol-gel Pechini method. Efforts were also done to synthesize phases with x > 0.5 but remained unsuccessful. The Rietveld refinement investigation has confirmed the successful formation of single-phase compounds that crystallize in the I4/mmm space group with tetragonal symmetry. The X-ray photoelectron spectroscopy (XPS) analysis revealed that Co in La1+xSr1-xCoO4 has mixed valence states of +2 and + 3. Dominant anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions were observed in all the synthesized nanophases and the existence of weak ferromagnetic (FM) interactions due to the generation of double-exchange Co2+―O―Co3+ interaction because of mixed valent state of Co. The increase in Néel temperature (TN) values with La doping has been attributed by the increased prevalence of anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions. The band gap energy values (Eg) for all the prepared nano samples vary from 1.57 to 1.64 eV, rendering them highly competent catalysts for degradation of dyes. Additionally, the photocatalytic performance of the prepared nano photocatalysts was evaluated using cationic (MB & RhB) and anionic (MO) dye contaminants as well as their ternary mixture. LaSrCoO4 photocatalyst has been found to efficiently and swiftly degrade all three dye pollutants separately as well as in mixtures, with outstanding recyclability and durability. It has been observed that the x = 0.0 photocatalyst showed best degradation efficacy for RhB dye, i.e., ∼99 % in 100 min, compared to ∼91 % and ∼84 % for the x = 0.2 and 0.5 photocatalysts in the same time interval, respectively. These results suggest that LaSrCoO4 nano powder can be considered a promising photocatalyst in the dyes degradation.

Morphological design of LaTiO2N particles by topotactic growth mechanisms for photocatalytic applications

Solar water-splitting using particle photocatalysts is a promising approach to sustainably produce hydrogen. LaTiO2N is an auspicious visible light absorbing photocatalyst regarding the oxygen evolution reaction. In this work, the topotactic growth mechanism of LaTiO2N particles is investigated by varying the precursor material and the synthesis conditions during thermal ammonolysis. Their influence is discussed in regard to structure, composition, morphology, optical, and functional properties. Using the conventional, layered perovskite oxide, La2Ti2O7, as precursor resulted in brick-shaped porous LaTiO2N particles with a high degree of crystallinity and a high surface area. When adding flux, the increased mobility during thermal ammonolysis leads to larger morphology changes resulting in non-porous, perforated particles with skeletal features. In a novel, alternative approach, LaTiO2N is prepared via the topotactic conversion of a double-layered Sillén-Aurivillius type oxyhalide material, La2·1Bi2·9Ti2O11Cl. The facile formation of LaTiO2N results in a perforated porous structure exhibiting skeletal features whilst maintaining a high surface area due to the presence of pores. By alternating the morphology of the material in this matter the oxygen evolution under one sun illumination is improved by around 10% or 30% depending on whether thermal ammonolysis of La2Ti2O7 is performed with or without flux, respectively.

Double-Layered Perovskite Oxyfluoride Cathodes with High Capacity Involving O-O Bond Formation for Fluoride-Ion Batteries.

Developing electrochemical high-energy storage systems is of crucial importance toward a green and sustainable energy supply. A promising candidate is fluoride-ion batteries (FIBs), which can deliver a much higher volumetric energy density than lithium-ion batteries. However, typical metal fluoride cathodes with conversion-type reactions cause a low-rate capability. Recently, layered perovskite oxides and oxyfluorides, such as LaSrMnO4 and Sr3Fe2O5F2, have been reported to exhibit relatively high rate performance and cycle stability compared to typical metal fluoride cathodes with conversion-type reactions, but their discharge capacities (∼118 mA h/g) are lower than those of typical cathodes used in lithium-ion batteries. Here, we show that double-layered perovskite oxyfluoride La1.2Sr1.8Mn2O7-δF2 exhibits (de) intercalation of two fluoride ions to rock-salt slabs and further (de) intercalation of excess fluoride ions to the perovskite layer, leading to a reversible capacity of 200 mA h/g. The additional fluoride-ion intercalation leads to the formation of O-O bond in the structure for charge compensation (i.e., anion redox). These results highlight the layered perovskite oxyfluorides as a new class of active materials for the construction of high-performance FIBs.

Exploring Key Factors for Oxygen Evolution Reaction on La2-X Sr x NiO4+Δ by Oxygen Defect Engineering

IntroductionDevelopment of Oxygen Evolution Reaction (OER) catalyst is an important research issue for green hydrogen production by water splitting. Perovskite oxides have attracted attention as OER catalysts which can achieve both low cost and high catalytic activity. So far, some hypotheses have been proposed to understand key factors of catalytic activity of perovskite catalyst. Suntivich et al. reported volcano-like relationship between the OER activity and the number of the e g-electron of the B-site transition metal and the highest catalytic activity is achieved when the number of the e g-electron is close to 1.21. Besides, oxygen vacancies act as electrochemically active sites by incorporating reaction intermediates directly2,3. However, it is difficult to distinguish the contribution of electric state and oxygen defects on OER activity because both electronic state and oxygen defect concentration change simultaneously. For instance, the oxygen vacancy formation creates two electrons to maintain charge neutrality (2TMTM ×+OO ×↔2TMTM '+V O ∙∙+1/2 O2(g)).The aim of this study is to clarify the key factors improving OER catalytic activity. For the aim, we controlled the Ni valence state and oxygen defect concentration in layered perovskite oxide La2-x Sr x NiO4+δ by tuning Sr content (x) and the amount of oxygen defect (δ) independently (Fig. 1-a), to reveal the influence of transition metal electronic state and oxygen defects on OER activity. La2-x Sr x NiO4+δ has two types of oxygen defects: oxygen vacancy (V O ∙∙) and unoccupied interstitial site (V i ×). Comparing OER activity of oxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) allowed us to evaluate the influence of Ni valence state on OER activity while no oxygen vacancy and fully unoccupied interstitial sites ([V O ∙∙]=0,[V i ×]=2.0). Additionally, comparing OER activity of La2NiO4.1 (Ni2.2+,[V O ∙∙]=0,[V i ×]=1.9), La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) and La1.6Sr0.4NiO3.9 (Ni2.2+,[V O ∙∙]=0.1,[V i ×]=2.0) allowed us to evaluate the influence of oxygen defects on OER activity with negligible effect of Ni valence state.ExperimentalLa2-x Sr x NiO4+δ (x=0, 0.2, 0.4) were synthesized by Pechini method. The oxygen content of La2-x Sr x NiO4+δ was controlled by annealing them under suitable oxygen partial pressure and temperature and then quenched based on oxygen nonstoichiometric data4. Obtained samples were characterized by XRD, XAS and SEM.For the OER test, the catalyst ink was prepared by mixing the catalyst powder, acetylene black, K+-exchanged Nafion and tetrahydrofuran. 6.4 μl of the catalyst ink was sonicated and dropped onto the glassy-carbon rotating disk electrode. The electrochemical measurements were performed in O2-saturated 0.1 M KOH. The disk electrode potential was controlled between 0.3 and 0.9 V versus Hg/HgO reference electrode filled with 0.1 M KOH at a scan rate of 10 mVs-1.Result and DiscussionOxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) showed similar catalytic activity regardless of Ni valence (Fig. 1-b). This implies that the electronic state of Ni has insignificant influence on catalytic activity when the sample has almost no oxygen vacancy. The observed tendency is inconsistent with the Suntivich’s result that the relationship between OER activity and e g filling1.La2NiO4.1 (Ni2.2+,[V O ∙∙]=0,[V i ×]=1.9) and La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) which have different amount of unoccupied interstitial sites and almost no oxygen vacancy showed similar OER activity (Fig. 1-c). In contrast, La1.6Sr0.4NiO3.9 (Ni2.2+,[V O ∙∙]=0.1,[V i ×]=2.0) which has oxygen vacancy and fully unoccupied interstitial sites showed much better OER activity than La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) which has almost no oxygen vacancy and fully unoccupied interstitial sites (Fig. 1-c). These results indicate that oxygen vacancy has high OER activity as suggested in earlier works2.3, while unoccupied interstitial site which has available space for oxygen intercalation has negligibly low catalytic activity.The difference between oxygen vacancy and interstitial site is the presence of direct bonding with B-site cation. Reaction intermediates in oxygen vacancies can form direct bonding with Ni, while that in the interstitial site cannot. As suggested in the previous studies, a higher hybridization promotes charge transfer between the transition metal and reaction intermediate and effective charge transfer improves OER activity1,5.SummeryIn this work, to reveal the influence of transition metal electronic state and oxygen defects on OER activity, we have synthesized La2-x Sr x NiO4+δ , and controlled Ni valence state and oxygen defect concentration independently by tuning x and δ. It was revealed that oxygen vacancy boosts OER activity while influence of Ni valence state and unoccupied interstitial sites is insignificant.

Surface modulated B-site doping of PrBa0.5Sr0.5Co2-xFexO5+δ as highly efficient cathode for intermediate-temperature solid oxide fuel cells

Layered perovskite oxides (PrBaCo2O5+δ) have gained widespread interest as promising electrodes in intermediate-temperature solid oxide fuel cells (IT-SOFCs) because of their excellent oxygen diffusion and transport performance. However, it remains a great challenge to simultaneously realize thermal match and sufficient operational stability in existing IT-SOFCs. Herein, the layered perovskite PrBa0.5Sr0.5Co2-xFexO5+δ (PBSCFx) with strong oxygen reduction reaction (ORR) activity in half-cell and high power density in single cell was designed by partial substitution of Co with Fe. A systematic and comprehensive elaboration on the composition-structure-property relationships of PBSCFx was proposed. Furthermore, the complex electrochemical behavior of the single cell was investigated using equivalent circuit method (ECM) and distribution of relaxation time (DRT). Impressively, the polarization impedance of PBSCF05 (x = 0.5) is only 0.037 Ω·cm2 (T = 750 °C). The single cell with PBSCF05-GDC as the cathode can reach the maximum power density of 0.996 W/cm2 (T = 800 °C), and exhibits good stability in 216 h of long-term operation (T = 750 °C). The superior electrochemical performance of PBSCF05 is mainly attributed to the reduction of high frequency and intermediate frequency polarization resistance, which corresponds to the enhanced transfer process of oxygen ions, oxygen surface exchange and charge transfer process. This work highlights a simple B-site doping strategy to achieve the thermal match in IT-SOFCs, and furthermore, it expands the opportunities for effective utilization of ECM and DRT technology in the high-performance of IT-SOFCs.

Coupling of surface oxygen vacancy and 2D structure of Aurivillius-phase CaBi2Ta2O9 nanoplates for enhanced photocatalytic H2 production

Solar-driven water splitting over semiconductor-based photocatalyst is considered as one of the most promising approaches to address the looming energy shortage and environmental pollution. However, most of photocatalysts still suffer from low hydrogen production activity owing to the poor photo-absorption ability, rapid recombination of photo-generated charge carriers and insufficient surface active sites. Herein, we tackle these challenges by introducing surface oxygen vacancies (OVs) on the layered Aurivillius-phase perovskite CaBi2Ta2O9 nanoplates via a combined high-temperature molten salt and post-reduction process. Theoretical and experimental results demonstrate that the rich OVs anchored on the surface of CaBi2Ta2O9 nanoplates not only extend the photo-responsive range to visible region, improve the separation efficiency of photo-induced charge carriers, but also enrich the surface reactive sites. As a result, under a 300 W xenon lamp irradiation, the OVs-abundant CaBi2Ta2O9 nanoplates exhibit an outstanding H2 production rate, which is 7.4 times higher than that of the bulk CaBi2Ta2O9 in the absence of OVs. Our study provides a general and efficient coupling strategy to advance the H2 production performance of layered perovskite oxide for solar energy conversion.

Effects of Bi-doping on structure and properties of YBaCo2O5+δ layered perovskite cathode for intermediate-temperature solid oxide fuel cells

Layered perovskite oxides, Y1–xBixBaCo2O5+δ (YBBCx, x = 0.0–0.2) were investigated as cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). XRD results indicate the YBBCx could be indexed to a tetragonal phase with trace of 3 × 3 × 1 superstructure configuration. No significant change in thermal expansion coefficients (TECs) was detected with the Bi-doping, and the TECs were 15.7–16.8 × 10−6 K−1 at 100–935 °C in air. Although the Bi-doping results in a decrease in conductivity of the YBBCx, the x = 0.2 sample still shows higher conductivity than that of a standard SOFC cathode (∼100 S cm−1). Additionally, the Bi-doping leads to an enhancement in the electrochemical performance mainly due to improved ion and electron charge transfers. Among the YBBCx compositions, the YBBC0.2 cathode shows the lowest polarization resistance (Rp) of 0.034 Ω cm2 at 700 °C. For a single cell with 300 µm thick La0.9Sr0.1Ga0.8Mg0.2O3–δ (LSGM) and YBBC0.2 cathode, a peak power density (Pmax) of 537 mW cm−2 at 700 °C was achieved with no significant deterioration in performance after a 100–h testing.

Tuning exsolution of nanoparticles in defect engineered layered perovskite oxides for efficient CO2 electrolysis

Solid oxide electrolysis cell (SOEC) could be a potential technology to afford chemical storage of renewable electricity by converting water and carbon dioxide. In this work, we present the Ni-doped layered perovskite oxides, (La4Srn−4)0.9Ti0.9nNi0.1nO3n+2 with n = 5, 8, and 12 (LSTNn) for application as catalysts of CO2 electrolysis with the exsolution of Ni nanoparticles through a simple in-situ growth method. It is found that the density, size, and distribution of exsolved Ni nanoparticles are determined by the number of n in LSTNn due to the different stack structures of TiO6 octahedra along the c axis. The Ni doping in LSTNn significantly improved the electrochemical activity by increasing oxygen vacancies, and the Ni metallic nanoparticles afford much more active sites. The results show that LSTNn cathodes can successfully be manipulated the activity by controlling both the n number and Ni exsolution. Among these LSTNn (n = 5, 8, and 12), LSTN8 renders a higher activity for electrolysis of CO2 with a current density of 1.50A cm−2@2.0 V at 800 °C It is clear from these results that the number of n in (La4Srn−4)0.9Ti0.9nNi0.1nO3n+2 with Ni-doping is a key factor in controlling the electrochemical performance and catalytic activity in SOEC.

Shifting Oxygen Evolution Reaction Pathway via Activating Lattice Oxygen in Layered Perovskite Oxide

AbstractDeveloping high‐performance oxygen evolution reaction (OER) catalysts are critical for the practical application of many electrochemical energy devices. In this study, taking layered perovskite oxide thin films as the model system, it is demonstrated that the OER pathway can be effectively shifted by activating lattice oxygen, leading to strongly enhanced intrinsic activity. The OER performance of Ruddlesden‐Popper (RP)‐phase cobaltite is significantly enhanced as Sr doping at the A site increases, which is attributed to the shift of the reaction pathway from adsorbate evolution mechanism (AEM) to lattice oxygen‐mediated mechanism (LOM). Advanced spectroscopic techniques and density functional theory calculations reveal that the Sr dopant effectively facilitates oxygen ligand hole formation, charge transfer from the oxygen sites, and the formation and migration of oxygen vacancy, hence promoting lattice oxygen to participate in surface reactions. The results provide critical insight into the role of oxygen activity and offer a potential way for constructing highly active electrocatalysts.

Engineering Triple-Phase Interfaces Enabled by Layered Double Perovskite Oxide for Boosting Polysulfide Redox Conversion.

The electrocatalytic conversion of polysulfides is crucial to lithium-sulfur batteries and mainly occurs at triple-phase interfaces (TPIs). However, the poor electrical conductivity of conventional transition metal oxides results in limited TPIs and inferior electrocatalytic performance. Herein, a TPI engineering approach comprising superior electrically conductive layered double perovskite PrBaCo2O5+δ (PBCO) is proposed as an electrocatalyst to boost the conversion of polysulfides. PBCO has superior electrical conductivity and enriched oxygen vacancies, effectively expanding the TPI to its entire surface. DFT calculation and in situ Raman spectroscopy manifest the electrocatalytic effect of PBCO, proving the critical role of enhanced electrical conductivity of this electrocatalyst. PBCO-based Li-S batteries exhibit an impressive reversible capacity of 612 mAh g-1 after 500 cycles at 1.0 C with a capacity fading rate of 0.067% per cycle. This work reveals the mechanism of the enriched TPI approach and provides novel insight into designing new catalysts for high-performance Li-S batteries.

Impact of Layered Perovskite Oxide La0.85Yb0.15AlO3 on Structure and Transport Properties of Polyetherimide.

This study aims to improve properties of Ultem® polyetherimide (PEI) by incorporating up to 2 wt% additives of the perovskite oxide La0.85Yb0.15AlO3 (LYA). The structure of dense PEI/LYA films was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) in combination with an analysis of their elemental composition using energy-dispersive spectroscopy (EDS). The PEI/LYA films exhibit a two-layer structure. Contact angle measurements revealed hydrophilization of the membrane surface enriched with the perovskite. The transport properties were tested via gas separation and pervaporation processes. The separation selectivity of He/N2 and O2/N2 gas pairs increased with the growth of the LYA content in the membranes. Pervaporation of a methanol(MeOH)-cyclohexane(CH) mixture was effective due to the high sorption of MeOH in the PEI/LYA membranes. The maximal pervaporation separation index was found for the PEI/LYA(2%) membrane.

Tuning of photovoltaic response in multiferroic heterojunction photodiodes by a built-in electric field and carrier transport: Effect of magnetization and ferroelectric polarization

Photovoltaic devices consisting of multiferroic thin films have been studied extensively for carrier separation regulated by a built-in electric field and the above bandgap. Here, a novel photodiode with photoelectric conversion efficiency (PCE) of 3.70% by introducing TiO2 and NiOx:5%Cu as the electron and the hole transport layers (ETL/HTL) on both sides of multiferroic bismuth layered perovskite oxide Bi2FeCrO6/Bi6Fe1.6Co0.2Ni0.2Ti3O18 (BFCO/BFCNT) heterojunction with the appropriate band gap were reported based on our recent studies. The results showed that BFCNT displays an Aurivillius structure, but BFCO is of a double perovskite, the bandgaps are ∼1.62 eV and 1.74 eV, respectively. The open-circuit voltage, short-circuit current density, and fill factor of the devices are 0.73 V, 10.5 mA·cm-2, and 48.3% separately under the illumination of 100 mW·cm-2 simulated AM 1.5 G solar light. Investigations of the effects of both external electric and magnetic fields on the photovoltaic responses delivered that the magnetization and ferroelectric polarization can effectively tune the built-in electric field and the carrier transport in multiferroics, thus offering an experimental and theoretical basis for further improving the performance of multiferroic photovoltaic devices.

Synthesis and Characterization of Exfoliated Layered K2La2Ti3O10 Perovskite Oxide

Synthesis and Characterization of Exfoliated Layered K2La2Ti3O10 Perovskite Oxide

Hybrid Improper Ferroelectricity: A Theoretical, Computational, and Synthetic Perspective

We review the theoretical, computational, and synthetic literature on hybrid improper ferroelectricity in layered perovskite oxides. Different ferroelectric mechanisms are described and compared, and their elucidation using theory and first-principles calculations is discussed. We also highlight the connections between crystal chemistry and the physical mechanisms of ferroelectricity. The experimental literature on hybrid improper ferroelectrics is surveyed, with a particular emphasis on cation-ordered double perovskites, Ruddlesden–Popper and Dion–Jacobson phases. We discuss preparative routes for synthesizing hybrid improper ferroelectrics in all three families and the conditions under which different phases can be stabilized. Finally, we survey some synthetic opportunities for expanding the family of hybrid improper ferroelectrics.

Constructing highly active alloy-perovskite interfaces for efficient electrochemical CO2 reduction reaction

The electroreduction of CO2 through solid oxide electrolysis cells (SOEC) is considered as a promising technology for mitigating climate changes related to global warming. A SOEC system can effectively reduce polarization losses and best utilize process heat to electrolyze CO2 to produce CO. However, this technology is hindered by the sluggish cathode kinetics, which is mainly due to the poor catalytic activity for the CO2 reduction reaction. Herein, this work develops an A-site ordered layered perovskite oxide, (PrBa)0.95Fe1.6Ni0.2Nb0.2O5+δ (PBFNN), as an efficient cathode catalyst for CO2-SOEC. FeNi3 nanoparticles are in situ generated on the surface of perovskite substrate after reduction, the resulting active alloy-perovskite interfaces between FeNi3 and PBFNN substrate can effectively promote the CO2 reduction reaction (CO2RR). The nanoparticles and abundant oxygen vacancies of FeNi3-PBFNN significantly enhance CO2 adsorption and activation, which is verified by the CO2-temperature programmed desorption measurement and density functional theory calculations. Distribution of relaxation time analysis and electrochemical performance characterization indicate that this interface engineering at nanoscale greatly improves the catalytic activity of the cathode for CO2RR.

Mosaic growth induced magnetic anisotropy in double perovskite PrBaCo2O5+δ thin films

Controlling crystal structure is one of the most crucial steps for tuning the physical properties in layered perovskite oxides. The growth of mosaic-patterned double perovskite PrBaCo2O5+δ (PBCO) thin films was achieved to study the growth dynamics and the effect on the magnetic anisotropy (MA). High-resolution X-ray diffraction and scanning transmission electron microscopy studies indicate the crystal structures of PBCO thin films are highly oriented with three variants, i.e., <100>(013) grains tilted around ±18° away from [00l] direction along a or b axis, <110>(001) domains rotated 45° along c axis, and common cube-on-cube <100>(001) epitaxy. The MA transformation to isotropic and an anomalous anisotropic magnetoresistance were observed in the films with the mosaic-patterned nanostructures. These evidences not only highlight the correlation between nanostructures and physical properties in perovskite systems, but also lay out the new avenues to manipulate the MA through controlling the preferred oriented nanostructures, which paves the way to create future nanoelectronics and spintronic devices via nanostructure design.

Promoted Performance of Layered Perovskite PrBaFe2O5+δ Cathode for Protonic Ceramic Fuel Cells by Zn Doping

Proton-conducting solid–oxide fuel cell (H-SOFC) is an alternative promising low-temperature electrochemical cell for renewable energy, but the performance is insufficient because of the low activity of cathode materials at low temperatures. A layered perovskite oxide PrBaFe1.9Zn0.1O5+δ (PBFZ) was synthesized and investigated as a promising cathode material for low-temperature H-SOFC. Here, the partial substitution of Fe by Zn further enhances the electrical conductivity and thermal compatibility of PrBaFe2O5+δ (PBF). The PBFZ exhibits improved conductivity in the air at intermediate temperatures and good chemical compatibility with electrolytes. The oxygen vacancy formed at the PBFZ lattice due to Zn doping enhances proton defects, resulting in an improved performance by extending the catalytic sites to the whole cathode area. A single cell with a Ni-BZCY anode, PBFZ cathode, and BaZr0.7Ce0.2Y0.1O3-δ (BZCY) electrolyte membrane was successfully fabricated and tested at 550–700 °C. The maximum power density and Rp were enhanced to 513 mW·cm−2 and 0.3 Ω·cm2 at 700 °C, respectively, due to Zn doping.