Doped Strontium Titanate Research Articles

Comparative Study of Metal Impregnated Lanthanum Doped Strontium Titanate Anodes and Conventional Nickel Oxide Anode for Solid Oxide Fuel Cell Application

In conventional SOFC, composite anode made up of nickel oxide (NiO) and yttria stabilized zirconia (YSZ) are used [1]. NiO is an excellent anode for SOFC application because of its high electrochemical activity towards H2 oxidation. But the amount of NiO used is around 30- 40 wt% to reach a percolation threshold [2]. Agglomeration of nickel particles due to sintering is a serious concern during SOFC operation at elevated temperatures (800 ˚C). In order to address the above mentioned issue, we have developed doped strontium titanate (SrTiO3) based perovskite anodes. Perovskite anodes are mixed ionic and electronic conductors (MIEC) providing enhanced triple phase boundary (TPB) which lowers the over-potential losses compared to pure conductors [3]. To increase the electrical conductivity, lanthanum (La) has been doped at A-site of SrTiO3 using solid state route (SSR), to form La0.3Sr0.7TiO3+δ (LST). Physical characterizations like field emission scanning electron microscope (FESEM), x-ray diffraction (XRD) and image analysis (ImageJ) were carried out before and after reduction of anodes in H2 environment to analyze their microstructure, phase stability and particle sizes as shown in Figure 1 and 2. Conductivity measurements were carried out using two probe conductivity measurement system to determine the activation energy (Ea) required in reducing the anodes in H2 environment. Along with physical characterizations, electro-chemical characterizations were carried out as described. The halfcell (LST/ YSZ) gave a high polarization resistance (Rp) of 62 Ω cm2, low current density of 20 mA cm-2 and power density of 5 mW cm-2 at 800 ˚C. We impregnate the porous anode with varying NiO content using wet impregnation technique and observed that LST impregnated with 4 wt% NiO forms a percolated network and provided enhanced performance. A decrease in Rp to 4 Ω cm2 and increase in current density to 156 mA cm-2 in 4 wt% NiO impregnated LST halfcells is seen at 800 ˚C. The performance increased sharply upto 900 mA cm-2 in case of halfcell with LST anode when impregnated with both nickel(4%) and ceria(6%) with low Rp of 0.5 Ω cm2, giving a maximum power density of 200 mW cm-2 at 800 ˚C as shown in Figure 3(a, b). The above obtained power density is better in performance compared to the one given by half cells of NiO-YSZ anodes [2]. Thus, metal impregnated A-site doped perovskite anodes can be potential substitute for NiO-YSZ anodes for SOFC application. Key words:A-site doped perovskites, Mixed ionic and electronic conductor, Activation energy and half-cell study.

Theoretical Investigation of the Electronic, Structural, Optical and Thermodynamic Properties of LaxSr1-xTiO3 (x=0, 0.125, 0.25)

Lanthanum doped strontium titanate (La1-xSrxTiO­3, La-STO) materials are essential for solid oxide fuel cell (SOFC) electrodes and high-temperature gas sensors.[1] In the literature, their electronic structures and optical properties have been investigated intensively by density functional theory (DFT). However, the obtained information is theoretically limited to low temperature because the traditional DFT approach doesn’t include temperature effects. Since many of the applications of these materials for SOFCs and sensors are 400-800°C range, it is very important to know how their properties change when operating at high-temperatures. To better understand the thermodynamic behavior of La-STO with different La-doping levels at high temperatures, the ab initio thermodynamics calculations which combine first-principles DFT with lattice phonon dynamics have been employed in this study to calculate their electronic structures and thermodynamic evolutions versus temperature.[2-5] The obtained results show that with increasing La-doping level, the corresponding structures of La-STO become unstable with an increasing number of soft modes, especially when the La content over Sr is higher than 25%.[6] The La-STOs are conductors even at T=0K due to the electron of La filling in the conduction band of STO, which is consistent with experimental results. The results also reveal that although the GGA+U method could predict the band-gap of STO closer to the experimental measurements, the predicted lattice parameters of La-STO are larger than experimental values and result in more phonon soft modes. These deviations demonstrate the limitations of such U corrections for property extrapolation to high temperatures in comparison with the standard GGA approach. From the calculated optical dielectric tensors of La-STO, one can see that in the optical frequency range, there is a major difference between pure STO and La-doped STO consistent with the dielectric nature of STO and the finite electrical conductivity in La-doped STO associated with electrons in the conduction band, and consistent with expectations from experiments and simple Drude theory arguments. The reason is that STO possesses a band-gap, while La-doped STOs do not have a band-gap, as the extra electrons occupy the conduction band of STO. Due to the reduced symmetry for the La-doped STO systems, the diagonal elements (εXX, εYY, εZZ) of the dielectric matrix are not equal for some of La-STOs. Related to the DFT energy of pure STO, the calculated thermodynamic properties show that with increasing La-doping levels, the ΔH(T) and ΔG(T) decrease at given temperature T. With increasing T, the ΔH(T) of each La-STO increases while its ΔG(T) decreases because of the effects of TΔS(T) term. These results are useful for understanding the thermodynamic behaviors of La-STO materials at high temperatures and guide further studies of SOFC electrodes and sensor materials.

Half-Cell Study of La and Ca Doped Strontium Titanates Anode for Direct Methane Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) has been providing high conversion efficiency of chemical energy to electricity without any pollution. One of the major advantages of SOFC over other fuel cell is use of direct natural gas at high temperature without any external reformer. Conventional Ni-yttria stabilized zirconia (YSZ) composite anode provides excellent catalytic property, current collection and stability for H2 oxidation but it is not tolerant towards sulphur poisoning and also accelerates coke deposition in presence of hydrocarbon fuels. It necessitates the use of alternate anode for direct hydrocarbon SOFC. In the present work, attempts have been made to apply A-site deficient La and Ca doped SrTiO3 (LSCTA-) as potential anode for direct methane SOFC. Electrolyte supported half-cells with LSCTA- anode backbone have been tested in H2 and humidified CH4 fuel. Low catalytic activity of LSCTA- towards electro oxidation of fuel is significantly improves in presence of metallic catalyst. 6% CeO2 loading in LSCTA- backbone improves the performance of the cell in H2 as well as in humidified CH4 at 800 °C. It has been observed that coke deposition did not degrade the anode active surface even after 6 h of exposure of CH4 at the elevated temperature of 800 °C. 4% Ni infiltration in 6%CeO2-LSCTA- enhances the performance from 131 mW cm-2 to 200 mw cm-2 at 800 °C in H2. In humidified CH4 environment, cell having 4% Ni-6% CeO2-LSCTA- anode shows lesser performance (140 mw cm-2 Power density) due to sluggish methane oxidation reaction but provided enhanced power density (216 mw cm-2) once H2 is again fed after 6 h cell operation in humidified CH4. i-V/ i-P analysis of half-cell shown in fig. 1 explains the performance in H2 as well as in CH4 at 800 °C. Maximum power density of cell is 200 mW cm-2 (554 mA cm-2) at 1 h of operation in H2, which increases up to 216 mW cm-2 (578 mA cm-2) in H2 after 6h of cell operation in humidified CH4. In humidified CH4 exposure, maximum power density of cell is 89 mW cm-2 (285 mA cm-2) at 1 h of operation, which increases up to 140 mW cm-2 (409 mA cm-2) at 6 h of operation. The performance of the cell is reasonably good as results presented here correspond to a cell having thick electrolyte (300 μm). The cells are found to be structurally robust even after 12 h of accelerated testing. LSCTA-is a potential anode backbone material for direct methane SOFC and could be an excellent anode for other hydrocarbons. Key Words: A-site deficient SrTiO3, Direct Methane SOFC, LSCTA- anode, half-cell study. Figure 1

Comparative Study of Metal Impregnated Lanthanum Doped Strontium Titanate Anodes and Conventional Nickel Oxide Anode for Solid Oxide Fuel Cell Application

Nickel sintering studies are carried out on La0.3Sr 0.7TiO3+δ (LST) anode backbone impregnated with different quantities of nickel oxide (NiO). It is observed that 4 wt % NiO is sufficient to form a percolated network with low sintering at 800 ˚C using particle migration and coalescence process. Two different mechanisms contributing to sintering process are studied for particle size distribution (PSD). To enhance catalytic activity, cerium oxide (CeO2) is impregnated along with NiO in LST. The performance of bilayer, 4 wt % NiO-6 wt % CeO2-LST/YSZ provided less polarization resistance (Rp) of 0.5 Ω cm2and a maximum power density of 200 mWcm-2 (900 mA cm-2) compared to that without NiO and CeO2 impregnation.

Theoretical Investigation of the Electronic, Structural, Optical and Thermodynamic Properties of LaxSr1-xTiO3 (x=0, 0.125, 0.25)

To better understand the electronic, optical, and thermodynamic behaviors of lanthanum doped strontium titanate (STO) LaxSr1-xTiO3 (x = 0, 0.125, 0.25) at high temperatures for possible fuel cell and sensor applications, ab initio thermodynamics have been employed by combining the first-principles density functional theory with lattice phonon dynamics to investigate their electronic structures and thermodynamic evolutions versus temperatures. Different from pure STO, our results show that after La-doping into STO, the corresponding LaxSr1-xTiO3 become metallic without a band-gap. Doping La into STO dramatically alters the optical properties of pure STO which have wide applications for sensor technology development. With increasing La-doping level x, the calculated free energy ΔG(T) and enthalpy ΔH(T) of LaxSr1-xTiO3 decrease at given temperature T, while with increasing T, the ΔG(T) decrease, but the ΔH(T) increase for all LaxSr1-xTiO3 studied.

Half-Cell Study of La and Ca Doped Strontium Titanates Anode for Direct Methane Solid Oxide Fuel Cell

One of the major advantages of Solid oxide fuel cell (SOFC) over other fuel cell is use of direct natural gas at high temperature without any external reformer. Conventional nickel-yttria stabilized zirconia (Ni-YSZ) composite anode provides excellent catalytic property, current collection and stability for H2 oxidation but it is not tolerant towards sulphur poisoning and also accelerates coke deposition in presence of methane fuel. It necessitates the use of alternate anode for direct hydrocarbon fuel. In the present work, attempts have been made to apply La and Ca doped A-site deficient SrTiO3 (LSCTA-) as potential anode for direct methane SOFC. Low catalytic activity of LSCTA- is improved by infiltration of Ni and CeO2 catalyst. Half cell (YSZ/4%Ni-6%CeO2-LSCTA-) provided 200 mW cm-2 maximum power density and regain its initial performance in H2 even after 6 h exposure to humidified CH4 at 800 °C.

Deposition of NiO on 3 mol% yttria-stabilized zirconia and Sr0.96Y0.04TiO3 materials by impregnation method

The main purpose of this work was the synthesis of composite materials by deposition of nickel oxide on 3 mol% yttria-stabilized zirconia (3 mol% Y2O3–97 mol% ZrO2–YSZ) and Sr0.96Y0.04TiO3 (4 mol% Y-doped strontium titanate—SYTO) supports. Yttria-stabilized zirconia and yttrium-doped strontium titanate were obtained by modified citrate method. The deposition of nickel oxide was performed by impregnation method. All materials were impregnated with proper amount of nickel(II) nitrate aqueous solution to receive the 1 and 5 mass% of nickel in the final composite. The temperature-programmed reduction (TPR) was applied to analyze the reduction mechanism of deposited NiO. Three forms of nickel oxide: non-stoichiometric NiO1+x, hexagonal and cubic are postulated on the base of the TPR profiles and analysis of bonds length in nickel oxides structures. Moreover, TPR analysis shows that support strongly affects the reduction mechanism as a consequence of various interactions between NiO and YSZ/SYTO. In the case of YSZ, the nickel oxide reveals the tendency to deposit on the support surface, while using SYTO as the support, nickel oxide exhibits significant ability to react with doped strontium titanate and what is more important nickel incorporates into Sr0.96Y0.04TiO3 structure.

Degradation and recovery of iron doped barium titanate single crystals via modulus spectroscopy and thermally stimulated depolarization current

Understanding resistance degradation during the application of DC bias and recovery after removing the DC bias provides insight into failure mechanisms and defects in dielectric materials. In this experiment, modulus spectroscopy and thermally stimulated depolarization current (TSDC) techniques were used to characterize the degradation and recovery of iron-doped barium titanate single crystals. Modulus spectroscopy is a very powerful analytical tool applied during degradation and recovery to observe changes in the local conductivity distribution. During degradation, oxygen vacancies migrate to the cathode region, and a counter flow of oxygen anions migrates towards the anode. With increasing time during degradation, the distribution of conductivity broadens only slightly exhibiting crucial differences to iron doped strontium titanate. After removing the DC bias, the recovery shows that a second previously unobserved and distinct conductivity maximum arises in the modulus data. This characteristic with two maxima related to different conductivities in the anode and cathode region is what can be expected from the published defect chemistry. It will be concluded that only the absence of an external electric field during recovery measurements permits the observation of local conductivity measurements without the presence of non-equilibrium conditions such as charge injection. Equilibrium conductivity as a function of oxygen vacancy concentration is described schematically. Oxygen vacancy migration during degradation and recovery is verified by TSDC analysis. We establish a self-consistent rationale of the transient changes in the modulus and TSDC for the iron doped barium titanate single crystal system including electron, hole and oxygen vacancy conductivity. During degradation, the sample fractured.

Large polaron hopping phenomenon in lanthanum doped strontium titanate

Abstract A few compositions of La doped strontium titanate, LaxSr1−xTiO3−δ (x = 0.0, 0.1, 0.2, 0.3, and 0.4) were synthesized via solid state reaction route. The XRD patterns of the compositions with x ≤ 0.2 showed single phase whereas for x ≥ 0.3 compositions, secondary phases were observed. The Rietveld refinement of the XRD data for all compositions indicated cubic structure with space group Pm 3 ¯ m at room temperature. The SEM micrographs for x = 0.0 showed large grains, whereas the compositions with 0.0

Tunable resistance switching in solution processed chromium-doped strontium titanate nanoparticles films

In this work, resistance switching behaviours in solution processed chromium (Cr)-doped strontium titanate (SrTiO3) films have been investigated. Undoped SrTiO3 film shows I-V characteristics of typical nonlinear resistor and no resistance hysteresis loops are observed. On the contrary, Cr-doped SrTiO3 films show stable and reversible hysteresis loops, which can be controlled by applying different voltage bias. Based on a series of characterization results, including X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS), we infer that Ti4+ is substituted by Cr3+, giving rise to increased concentration of oxygen vacancies. Therefore, the observed resistance switching phenomenon is attributed to voltage driven oxygen vacancy migration. Furthermore, gradually decreased overall resistance is also realized under repeated sweeping cycles.

Nano-structured rhodium doped SrTiO3–Visible light activated photocatalyst for water decontamination

A modified hydrothermal synthesis, avoiding high temperature calcination, is used to produce nano-particulate rhodium doped strontium titanate in a single-step, maintaining the rhodium in the photocatalytically active +3 oxidation state as shown by X-ray spectroscopy. The photoactivity of the material is demonstrated through the decomposition of aqueous methyl orange and the killing of Escherichia coli in aqueous suspension, both under visible light activation. A sample of SrTiO3 containing 5 at% Rh completely decomposed a solution of methyl orange in less than 40min and E. coli is deactivated within 6h under visible light irradiation.

Superconductivity at very low density: The case of strontium titanate

Doped strontium titanate becomes superconducting at a density as low as n = 5 x 10^17 cm^-3, where the Fermi energy is orders of magnitude smaller than the longitudinal-optical-phonon frequencies. In this limit the only optical mode with a frequency which is smaller than the Fermi energy is the plasmon. In contrast to metals, the interaction strength is weak due to screening by the crystal, which allows the construction of a controllable theory of plasmon superconductivity. We show that plasma mediated pairing alone can account for the observed transition temperatures if the screening by the crystal is reduced in the slightly doped samples compared with the insulating ones. This mechanism can also explain the pairing in the two-dimensional superconducting states observed at surfaces and interfaces with other oxides. We also discuss unique features of the plasmon mechanism, which appear in the tunneling density of states above the gap.

Model composite microelectrodes as a pathfinder for fully oxidic SOFC anodes

All-oxide model-composite electrodes were established consisting of a thin, micropatterned, electronically conducting oxide, which acts as a current collector, and a thin film of gadolinia-doped ceria, which is an electrochemically highly active mixed conductor under reducing atmospheres. The choice of the current collecting oxides was based on their electronic conductivity assessed by measurements of thin films using the van der Pauw method. Lanthanum and niobium doped strontium titanate as well as alumina doped zinc oxide, were investigated this way in a humid hydrogen atmosphere. Promising materials were incorporated as a current collector into model-composite microelectrodes and tested for their stability and efficiency in electrochemically activating the microelectrode. Alumina doped zinc oxide, while being an excellent electron conductor, showed severe stability problems at temperatures above 600°C. However, a microelectrode with a current collector of niobium doped strontium titanate (Sr0·9Ti0.8Nb0·2O3) performed comparable to an electrode with a Pt current collector and, additionally, showed an improved tolerance to sulphur poisoning in a humid hydrogen atmosphere with 10ppm of hydrogen sulphide.

The Effect of Mn Co-doping on the Electrochemical Properties of Gd0.2Ce0.8O1.9- d/Pt Model-composite Electrodes

For solid oxide fuel cells (SOFCs) to reach widespread commercial distribution several obstacles still have to be overcome. Material-wise one of the main problems remains the optimization of the anode for use in harsh environments or operation conditions. The still prevalent anodes based on Ni/YSZ cermets offer excellent catalytic properties for hydrogen oxidation and reformation of hydrocarbon fuels. Nevertheless, when feeding a fuel containing hydrogen sulphide impurities to an SOFC, a dramatic drop in the anode performance is the consequence. Therefore, work is in progress to identify the causes of this sulphur triggered polarization resistance increase, which is partially explained by sulphur adsorption. The hope is that a more complete understanding in this degradation will help to formulate mitigation strategies as a way to prolong the life of an SOFC. In this contribution alternative SOFC anode materials are being surveyed and compared to the classic Ni/YSZ cermet. By exchanging the ionically conductive YSZ phase with e.g. gadolinium doped ceria (GDC) and/or the electronically conducting nickel with e.g. donor doped strontium titanate, formulations may be found, which increase the sulphur tolerance of an SOFC anode. Moreover, a fully oxidic anode may offer a significantly higher redox cycling stability than the conventional Ni/YSZ cermet. By using impedance spectroscopy on geometrically well-defined model-composite electrodes of alternative SOFC anode materials, we are able to separate elementary processes such as ionic and electronic conductivity as well as the chemical capacitance and the surface reaction resistance. By introducing H2S in the feed gas system the effects of sulphur poisoning on these parameters can be studied. This way a fair comparison of alternative SOFC anode materials to Ni/YSZ is possible.

Preparation and Dielectric Properties of Ce-Doped Barium Strontium Titanate Nano-Powders

Citric acid and ethanol are attempted to prepare cerium doped barium strontium titanate (Ce-BST) nano-powders, and the structure and dielectric properties of the powders were studied. X-ray diffraction (XRD) shows that the powders are ABO3 cubic perovskite structure almost with no second phase. Scanning electronic microscope (SEM) reveals that the powders calcined at 850°C contain many spherical and loose soft conglobations composed of uniform grains with average size less than 100 nm. The powders display excellent dielectric properties with particularly low dielectric loss tangent of 0.002 due to Ce doping. The optimum dielectric properties were achieved at 900°C with dielectric constant of 63.2 and dielectric loss of 0.003 at 1GHz. The powders can be fully satisfied the requirements in many applications like microwave devices. The effect of the calcination temperature and related mechanisms are discussed.

ChemInform Abstract: Synthesis and Characterization of Ho3+‐Doped Strontium Titanate Downconversion Nanocrystals and Its Application in Dye‐Sensitized Solar Cells.

AbstractSrTiO3:Ho3+ downconversion nanocrystals (Ho levels of 0, 0.5, 1, 3, 5, and 8 wt%) are prepared by solid state reaction of appropriate amounts of TiO2 Sr(NO3)2, Ho2O3, and NaCl as flux (Al2O3 crucible, 950 °C, 1 d) The obtained material is tested as a photoanode in dye‐sensitized solar cel (DSSC).

Dielectric properties of B–site charge balanced Dy–doped SrTiO3 ceramics for energy storage

Trivalent rare earth dysprosium doped strontium titanate ceramics with B–site charge balanced formula of DyxSr1–xTi1–x/4O3 (DyST, x = 0.01, 0.02, 0.03, and 0.04) were prepared by solid state reaction method. Single phase with cubic perovskite structure was indexed for all DyST ceramics from their x–ray diffraction patterns. Dense microstructures with grain size lower than 5 μm were observed for DyST ceramics using scanning electron microscope. Both high dielectric constant (er = 3300 at 1 kHz) and strong breakdown strength (Eb > 150 kV/cm) were obtained for DyST ceramics with the composition of x = 0.02. Together with good stability of er vs. bias electric fields and low dielectric loss (tanδ < 0.03 at 1 kHz), the DyST ceramics should be very promising for high voltage energy storage capacitors.

Synthesis and characterization of Ho3+-doped strontium titanate downconversion nanocrystals and its application in dye-sensitized solar cells

Ho3+-doped strontium titanate (SrTiO3:Ho3+) downconversion (DC) nanocrystals are synthesized by the solid state interaction of titanium dioxide, strontium nitrate, holmium oxide and sodium chloride and then used as a photoanode in dye-sensitized solar cells (DSSCs) to investigate the effect of DC nanocrystals in DSSCs. Differential thermal analysis, X-ray diffraction, scanning electronic microscope, energy dispersive spectroscopy and Brunauer–Emmet–Teller analysis confirmed the formation of cubic structured SrTiO3:Ho3+ nanocrystals with diameters of 40–400nm, pore size of ∼45nm, sintering temperature of 950°C. The photofluorescence and UV–Vis absorption spectra of the SrTiO3:Ho3+ nanocrystals revealed strong emission intensity and visible light absorption when doped content of holmium oxide was between 1wt% and 3wt%. Compared with the pure SrTiO3 photoanode, SrTiO3:Ho3+ DC photoanode showed a greater photovoltaic efficiency. The photoelectric conversion efficiency (η) of the DSSCs with a SrTiO3:Ho3+ photoanode doped with 1wt% holmium oxide was 59% higher than that with a pure SrTiO3 photoanode. This phenomenon could be explained by SrTiO3:Ho3+ nanocrystals’ ability to absorb ultraviolet light and downconvert it to visible light, which extends spectral response range of DSSC to the ultraviolet region and increased the short-circuit current density (Jsc) of DSSCs.

High temperature dielectric relaxation anomaly of Y3+ and Mn2+ doped barium strontium titanate ceramics

Relaxation like dielectric anomaly is observed in Y3+ and Mn2+ doped barium strontium titanate ceramics when the temperature is over 450 K. Apart from the conventional dielectric relaxation analysis method with Debye or modified Debye equations, which is hard to give exact temperature dependence of the relaxation process, dielectric response in the form of complex impedance, assisted with Cole-Cole impedance model corrected equivalent circuits, is adopted to solve this problem and chase the polarization mechanism in this paper. Through this method, an excellent description to temperature dependence of the dielectric relaxation anomaly and its dominated factors are achieved. Further analysis reveals that the exponential decay of the Cole distribution parameter n with temperature is confirmed to be induced by the microscopic lattice distortion due to ions doping and the interaction between the defects. At last, a clear sight to polarization mechanism containing both the intrinsic dipolar polarization and extrinsic distributed oxygen vacancies hopping response under different temperature is obtained.

Structural Investigation of Manganese Doped SrTiO3 Single Crystal and Ceramic

We present a detailed investigation of structural properties of manganese doped strontium titanate ceramics and Verneuil grown single crystals, the last prepared with Sr deficiency compensated by proper Mn amount. EXAFS, TEM, ESR and Raman techniques, combining local and lattice probes, allows to inspect the local chemical environment and electronic structure of manganese taking into account possible chemical non-homogeneities and/or phase segregation. On the basis of experimental evidences, structural models for Mn insertion in SrTiO3 are discussed considering possibility for Mn ions to occupy both Ti4+ and Sr2+ sites as well as manganese segregation and Mn incorporation related non-homogeneities.