Niobium-doped Strontium Titanate Research Articles

Stabilizing High-Voltage Performance of Nickel-Rich Cathodes via Facile Solvothermally Synthesized Niobium-Doped Strontium Titanate.

Ni-rich layered ternary cathodes are promising candidates thanks to their low toxic Co-content and high energy density (∼800 Wh/kg). However, a critical challenge in developing Ni-rich cathodes is to improve cyclic stability, especially under high voltage (>4.3 V), which directly affects the performance and lifespan of the battery. In this study, niobium-doped strontium titanate (Nb-STO) is successfully synthesized via a facile solvothermal method and used as a surface modification layer onto the LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode. The results exhibited that the Nb-STO modification significantly improved the cycling stability of the cathode material even under high-voltage (4.5 V) operational conditions. In particular, the best sample in our work could provide a high discharge capacity of ∼190 mAh/g after 100 cycles under 1 C with capacity retention over 84% in the voltage range of 3.0-4.5 V, superior to the pristine NCM811 (∼61%) and pure STO modified STO-811-600 (∼76%) samples under the same conditions. The improved electrochemical performance and stability of NCM811 under high voltage should be attributed to not only preventing the dissolution of the transition metals, further reducing the electrolyte's degradation by the end of charge, but also alleviating the internal resistance growth from uncontrollable cathode-electrolyte interface (CEI) evolution. These findings suggest that the as-synthesized STO with an optimized Nb-doping ratio could be a promising candidate for stabilizing Ni-rich cathode materials to facilitate the widespread commercialization of Ni-rich cathodes in modern LIBs.

Spontaneous pattern of orthogonal ferroelectric domains in epitaxial KNN films

Lead-free piezoelectric (K, Na)NbO3 (KNN) is considered one of the promising candidates for the replacement of Pb(ZrxTi1−x)O3. Several studies underlined the issue of K and Na volatility with increasing deposition temperatures, leading to high leakage currents in thin films, which still represents a major drawback for applications. This paper shows how epitaxial growth with concomitant preferred orientation of KNN films on niobium-doped strontium titanate (Nb:STO) depends on growth temperature and substrate strain. A preferred out-of-plane polar (001) orientation of KNN is obtained at high temperatures (>600 °C), while (100) orientation is dominant for lower ones. The (001) orientation is forced out-of-plane due to the sizeable in-plane stress derived from a negative lattice mismatch of pseudo-cubic KNN with respect to the underlying cubic (001) Nb:STO substrate. Moreover, we show that K-Na deficiency and high leakage of epitaxial KNN films deposited at high temperatures are accompanied by the appearance of a pattern of orthogonal spontaneous ferroelectric domains aligned to the [100] and [010] directions of Nb:STO. This pattern, visible in secondary electron microscopy, piezoforce response microscopy, and conductive atomic force microscopy images, is uncorrelated to the surface morphology. Supported by reciprocal space mapping by x-ray diffraction, this phenomenon is interpreted as the result of strain relaxation via ferroelectric domain formation related to K-Na deficient films displaying a sizable and increasing compressive strain when grown on Nb:SrTiO3. Our findings suggest that strain engineering strategies in thin films could be used to stabilize specific configurations of piezo- and ferroelectric domains.

T-Square Dependence of the Electronic Thermal Resistivity of Metallic Strontium Titanate.

The temperature dependence of the phase space for electron-electron (e-e) collisions leads to a T-square contribution to electrical resistivity of metals. Umklapp scattering is identified as the origin of momentum loss due to e-e scattering in dense metals. However, in dilute metals like lightly doped strontium titanate, the origin of T-square electrical resistivity in the absence of umklapp events is yet to be pinned down. Here, by separating electron and phonon contributions to heat transport, we extract the electronic thermal resistivity in niobium-doped strontium titanate and show that it also displays a T-square temperature dependence. Its amplitude correlates with the T-square electrical resistivity. The Wiedemann-Franz law strictly holds in the zero-temperature limit, but not at finite temperature, because the two T-square prefactors are different by a factor of ≈3, like in other Fermi liquids. Recalling the case of ^{3}He, we argue that T-square thermal resistivity does not require umklapp events. The approximate recovery of the Wiedemann-Franz law in the presence of disorder would account for a T-square electrical resistivity without umklapp.

Investigation and characterization of the annealing effects on the ferroelectric behavior of PLD BaTiO3

This work investigates the ferroelectric behavior of barium titanate (BaTiO3) with and without annealing of the deposited film. 105.7 nm of BaTiO3 was deposited via pulsed laser deposition on a niobium-doped strontium titanate substrate. 50 nm of tungsten was sputtered and patterned into contact pads, and the film was subjected to electrical and material characterization. Annealing times greater than 30 min decreased the leakage current by 2–3 orders of magnitude. We demonstrate multiple, distinct, repeatable polarization states of BaTiO3 as a function of the film annealing time, and the forming and hysteresis sweep pulse lengths and amplitudes.

The microstructural evolution in high sodium epitaxial sodium potassium niobate films deposited by low-temperature hydro-thermal method

Sodium potassium niobate [(K x Na1−x )NbO3] films with high sodium composition (x = 0.06 and 0.24) were fabricated using a low-temperature (240 °C) hydro-thermal method on (001) niobium-doped strontium titanate (Nb-STO) substrate. The films were annealed subsequently at 600 °C. Thicknesses of the films were maintained in the range of ~800–1000 nm. Transmission electron microscopy-selected area electron diffraction studies revealed the appearance of super-spots, thereby confirming the tilting of the oxygen sub-lattice at both high Na compositions. The bright-field imaging and scanning transmission electron microscopy-energy dispersive spectroscopy elemental mapping revealed KNN film with K-rich interfacial regions and Na-rich top region of the film at both the high Na composition, whereas the potassium niobate (KNbO3, x = 1) film showed no such oxygen sub-lattice tilting, and a sharp substrate-film interface was observed. The observed tilting of oxygen sub-lattice is a direct consequence of the reduced structural stability in high Na compositions in the KNN solid solutions.

Electric field and temperature dependence of dielectric permittivity in strontium titanate investigated by a photoemission study on Pt/SrTiO3:Nb junctions

Schottky junctions made from platinum and niobium-doped strontium titanate (SrTiO3:Nb) were investigated by hard X-ray photoemission (HXPES) and through a band bending behavior simulation using a phenomenological model, which assumes a decrease in dielectric constant due to an electric field. Thus, we confirmed that the observed HXPES spectra at relatively high temperatures, e.g., >250 K, were well simulated using this phenomenological model. In contrast, it was inferred that the model was not appropriate for junction behavior at lower temperatures, e.g., <150 K. Therefore, a reconstruction of the phenomenological model is necessary to adequately explain the dielectric properties of SrTiO3.

High thermoelectric performance of niobium-doped strontium titanate bulk material affected by all-scale grain boundary and inclusions

The large thermal conductivity of SrTiO3 bulk material limits its potential application for high-temperature thermoelectricity. The effects of all-scale grain boundaries and inclusions on the thermoelectric performance of Nb-doped bulk SrTiO3 materials are investigated in this study. Nano- to microscale grain boundaries and inclusions reduce the thermal conductivity by 30%. As a result, the ZT value is enhanced 2.6 times by a combination of all-sized crystals, energy filtering effect, multilevel scattering behaviors of nano/microscale grain boundaries and inclusions.

Mechanical and Electrical Properties of Ceramic Electrodes for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) operate efficiently on both hydrogen and hydrocarbon fuels. Their fuel flexibility makes them lucrative energy conversion devices; they can integrate immediately into the current hydrocarbon infrastructure and can utilize renewable fuels in the future once that infrastructure exists. Carbon deposition, or coking, due to the CO disproportionation reaction (Eq. 1) or CH4cracking (Eq. 2) can occur when using hydrocarbon fuels, which blocks the anode gas channels.2CO -> CO2+ C(s) (1)CH4 -> 2H2+ C(s) (2)Typical nickel-based cermets used for anode-supported cells exacerbate coking problems because Ni surfaces catalyze solid carbon deposition. Furthermore the ~60% expansion of Ni upon reoxidation complicates its use as supporting material. Many researchers have suggested and demonstrated conductive ceramic anodes as supports for SOFCs to avoid the coking and mechanical problems of conventional cermet anode supports. The anodes must be >30% porous to allow for effective gas transfer to the electrochemically active triple phase boundary (TPB). Unfortunately, the properties of porous materials for this purpose are not as well characterized as the dense materials used for electrolytes or cermets for supporting anodes. To employ these materials in anode-supported cells, it is imperative to better understand the relationship between porosity and the electronic and mechanical properties of anode materials. General relationships between the elastic modulus and flexural strength of ceramics are described by Eq. 3 and 4, respectively, where n is an empirical fitting parameter and P is porosity.E=Eo(1-1.9P+0.9P2) (3)σfs = σoe-nP(4)Original work in preparing and characterizing porous ceramics of gadolinia doped ceria (GDC), yttria-stabilized zirconia (YSZ), and niobium-doped strontium titanate (SNT) for electrodes is presented. The empirical parameters that relate flexural strength of the materials to their porosity are determined as shown in Fig. 1 for GDC10 and YSZ3. The effect of pore former on pore geometry and strength is discussed. Finally, the electrical properties of the anodes are measured, both in their native state and with the addition of conductive and catalytic materials to the scaffolds.

Redox-reversible niobium-doped strontium titanate decorated with in situ grown nickel nanocatalyst for high-temperature direct steam electrolysis.

Composite cathodes based on Sr0.94Ti0.9Nb0.1O3 (STNO) can be utilized for direct steam electrolysis; however, the insufficient electrocatalytic activity limits electrode performance and current efficiency. In this work, redox-reversible (Sr0.94)0.9(Ti0.9Nb0.1)0.9Ni0.1O3 (STNNO) with A-site deficiency and B-site excess has been designed as a cathode material in an oxide-ion-conducting solid oxide electrolyzer for direct steam electrolysis. The XRD, TEM, SEM, EDS, TGA and XPS results together confirm that the exsolution or dissolution of Ni nanoparticles anchored on the STNO surface is completely reversible in the redox cycles. The electrical properties of STNO and STNNO are investigated and correlated to electrode performances. The current efficiency with an STNNO cathode is enhanced by about 20% compared to the values with a bare STNO cathode in 5% H2O/H2/Ar or 5% H2O/Ar at 800 °C. The synergetic effect of catalytically active nickel nanoparticles and the redox-stable STNO skeleton contributes to the improved performance and excellent stability of the cathode for steam electrolysis.

Conversion of toluene and water to methylcyclohexane and oxygen using niobium-doped strontium titanate photoelectrodes.

Methylcyclohexane (MCH) is regarded as a promising hydrogen carrier that enables hydrogen to be harnessed as an alternate fuel source, which paves the way to a clean-energy future. A photoelectrochemical (PEC) system with a Nb:SrTiO3 photoelectrode for oxygen evolution from an aqueous electrolyte and a Pt/C electrode for toluene (TL) hydrogenation to MCH was investigated under UV irradiation. A Nb:SrTiO3 single-crystal electrode and an ionomer/Pt/C membrane-electrode assembly (MEA) were used as the photoanode and cathode, respectively. A steady-state current density of 0.12 mA cm(-2) was observed for the two-electrode system without any bias voltage for >2 h, and a Faradaic efficiency of 97% was obtained for MCH production from TL. This is the first demonstration of the production of MCH from TL and water using only light energy. This means that light energy was converted directly into MCH from TL and water without any electricity. The PEC properties of the devices are discussed.

Impact of Electrode Oxidation on the Current Transport Properties at Platinum/(Niobium-Doped Strontium-Titanate) Schottky Junctions

The current transport properties and resistance-switching (RS) behavior at platinum/niobium-doped strontium-titanate (Pt/NSTO) Schottky junctions were investigated for junctions with different degrees of oxidation (PtOx) in the Schottky electrode. It was found that the rectifying current-voltage (I–V) profile strongly depends on the heat-treatment of the NSTO crystal and oxidation of the Pt electrode. The hysteresis in the I–V relationships was larger for the PtOx/NSTO junction than for the pure Pt/NSTO junction, and heat-treatment of the NSTO crystal suppresses the hysteresis in I-V and increases the leakage current. The change in the rectifying I–V profiles can be reasonably explained by the increase in the built-in potential caused by partial oxidation of the electrode interface, as determined by soft X-ray photoemission spectroscopy. The experimental results indicate that control of the built-in potential and the partial oxidation of the electrode, which affects the homogeneity of the potential profile, is the key to improving and controlling the rectification and RS properties in Pt/NSTO junctions.

Synthesis by spark plasma sintering of a novel protonic/electronic conductor composite: BaCe0.2Zr0.7Y0.1O3−δ /Sr0.95Ti0.9Nb0.1O3−δ (BCZY27/STN95)

A novel two-phase ceramic composite (cercer) material consisting of a solid solution of barium cerate and -zirconate doped with yttrium (BaCe0.2Zr0.7Y0.1O3−δ: BCZY27), together with niobium-doped strontium titanate (Sr0.95Ti0.9Nb0.1O3−δ: STN95), has been synthesized by solid-state reaction and sintered conventionally (CS) at 1350–1500 °C, as well as by spark plasma sintering (SPS) at 1300–1350 °C. CS samples were porous and exhibited high degrees of inter-phase reaction. Nickel oxide sintering aids did not improve CS sample density. In contrast, samples made by SPS were significantly denser (>95 %) and showed less reaction between phases. A pseudo-optimum SPS profile was developed, accounting for the effects of thermal expansion mismatch between BCZY27 and STN95. X-ray diffraction indicated secondary phases exist, but there was no indication of their presence at grain boundaries based on thorough study of these regions with high-resolution transmission electron microscopy and selective area electron diffraction. We thus suggest that these phases are present as independent grains in the bulk. It is believed these secondary phases inhibit electronic conductivity in the composite.

Determination of Schottky barrier profile at Pt/SrTiO3:Nb junction by x-ray photoemission

The platinum/[niobium-doped strontium titanate] junction (Pt/SrTiO3:Nb) was investigated by x-ray photoemission (XPE) spectroscopy. Aluminum Kα and synchrotron radiation (6 keV) were used to obtain XPE spectra with different probing depths. The broadening and shift of the XPE peaks for SrTiO3:Nb, which resulted from the formation of a potential barrier at the interface, were quantitatively analyzed by fitting simulations. The barrier height was calculated to be 0.7–0.8 regardless of the Nb concentration. Furthermore, the XPE profile of the junction was reproduced when the permittivity of SrTiO3 was assumed to depend on the electric field.

ChemInform Abstract: Perovskites in Solid Oxide Fuel Cells

AbstractReview: 34 refs.

Solid oxide fuel cells with Ni-infiltrated perovskite anode

Niobium-doped strontium titanate was used as the anode for a solid oxide fuel cell. Electrolyte-supported cells were prepared with pure perovskite and composite perovskite–yttria–stabilized-zirconia (YSZ) anodes. Fuel cell tests demonstrated significant improvement in power density after infiltration with a small amount of Ni. The obtained values of power densities are comparable with conventional Ni/YSZ cermet anode. In addition, stability tests showed initial degradation followed by stable operation.

Perovskites in Solid Oxide Fuel Cells

Perovskite oxides comprise large families among the structures of oxide compounds, and several perovskite-related structures are also known. Because of their diversity in chemical composition, properties and high chemical stability, perovskite oxides are widely used for preparing solid oxide fuel cell (SOFC) components. In this work a few examples of perovskite cathode and anode materials and their necessary modifications were shortly reviewed. In particular, nickel-substituted lanthanum ferrite and iron-substituted strontium titanate as cathode materials as well as niobium-doped strontium titanate, as anode material, are described. Electrodes based on the modified perovskite oxides are very promising SOFC components.

Orientation-dependent work function of in situ annealed strontium titanate

We have used energy-filtered x-ray photoelectron emission microscopy (XPEEM) andsynchrotron radiation to measure the grain orientation dependence of the work function ofa sintered niobium-doped strontium titanate ceramic. A significant spread in work functionvalues is found. Grain orientation and surface reducing/oxidizing conditions are the mainfactors in determining the work function. Energy-filtered XPEEM looks ideallysuited for analysis of other technologically interesting polycrystalline samples.

Electrical and structural properties of Nb-doped SrTiO3 ceramics

Niobium-doped strontium titanate synthesized via conventional solid-state reaction has been studied. Influence of niobium content on the lattice parameters and electrical conductivity has been reported. Various reduction conditions have been investigated. For samples reduced in hydrogen at 1400°C, a transition from thermally activated to metallic behavior has been observed. Maximum electrical conductivity (ca. 55 Scm−1 at 650°C) has been observed for the SrTi0.98Nb0.02O3-δ sample. The relation of electrical conductivity with the porosity of the samples has been shown.

Effect of ruthenium oxide electrode on the resistive switching of Nb-doped strontium titanate

We studied resistance switching characteristics of ruthenium oxide (RuOx)/niobium-doped strontium titanate (Nb:STO) contact. With increasing oxygen content of oxide electrode, the resistance window was improved. The switching speed of RuOx electrode also showed improvement compared to platinum (Pt) electrode. The RuOx film contains amorphous phase and also forms an interface oxide layer at the RuOx/Nb:STO contact, which suggests defect generation near the interface. Additionally, the interface reaction disturbs the crystalline orientation of Nb:STO. These defect sites facilitate switching properties by easy drift of current and oxygen ion and also by modulation of barrier height.

Changes in macroscopic behaviour through segregation in niobium doped strontium titanate

The electrical properties of electroceramics are largely governed by their complex microstructure which must be precisely controlled for applications (as, for example, sensors, actuators and capacitors). In this study, the dependence of niobium segregation in strontium titanate on different sintering parameters is shown. 1.2 mol% Nb doped SrTiO3 was sintered at 1420°C in air for different times and with different post sintering cooling rates. Scanning electron microscopy images provided evidence of morphological differences among the prepared samples. X-ray diffraction revealed a decrease in lattice parameter due to niobium leaving titanium substitutional sites and going to grain boundary zones. X-ray photoemission spectroscopy has shown substitutional or segregated niobium has a 5+ chemical state. Impedance spectra analysis permitted estimation of changes in grain boundary thickness in agreement with other results. The analysis establishes that niobium segregation happens during cooling and is diffusion controlled, depending on grain size.