Lanthanum Deficiency Research Articles

How defects in lanthanum iron manganite perovskite structures promote the catalytic reduction of NO by CO

Adjusting the defect level during synthesis of A- and B-site deficient lanthanum iron manganite (LFM) perovskites shows that non-stoichiometry can beneficially influence the catalytic reactivity to N2 in the reduction of NO by CO on noble metal-free LFM-based perovskites. Optimal steering of La deficiency and the associated redox chemistry to reduce the near-surface regions during catalytic operation at low temperatures is the key factor. Surface enrichment by reducible B site cations and a proper design of structural defects resulting from the optimum introduction of La defects exclusively cause in-situ reduction of surface-near regions by CO oxidation, as well as formation of oxygen vacancies for enhanced NO and N2O reactivity. Excess doping with defects causes structural instability and continuous supply of oxygen from the catalyst bulk to the surface at elevated temperatures. Introduction of B site vacancies leads to surface enrichment by non-reducible lanthanum cations, causing suppressed catalyst activity undercutting even stoichiometric LFM.

Enhanced N2 Activation on a Composite Co3Mo3N Nitride and La0.6Sr0.4Co0.2Fe0.8O3 Perovskite Cathode for High-Temperature Electrochemical Ammonia Synthesis

Electrochemical routes for ammonia synthesis could offer improved conversion efficiency, compatible integration with renewable energy sources, and a solution to distributed chemical production. In a conventional Haber–Bosch process, ammonia, NH3, is produced by reacting N2 and H2 at high temperatures and pressures. In an electrochemical pathway, the H2 production and pressurization steps can be bypassed by using N2 and H2O in an ambient-pressure solid-oxide electrolysis cell (SOEC). In this study, a SOEC with a composite cathode of A-site deficient lanthanum ferrite perovskite oxide and transition metal nitride Co3Mo3N was fabricated, and its activity for the nitrogen reduction reaction (NRR) was studied. The composite cathode produced ammonia at a rate of 4.0 × 10–11 mol s–1 cm–2 at 550 °C and 0.65 mA/cm2, which was an 8-fold enhancement compared to either of the pure phase electrodes. Relevant properties of Co3Mo3N, such as thermochemical stability, adsorption behavior, and mobility of nitrogen ions, were characterized by various techniques including in situ XRD, XAFS/XANES, NAP-XPS, temperature-programmed experiments, and in situ DRIFTS.

Electrical Properties of Novel La0. 2Sr0.7-xCaxTi0.95Fe0.05O3-δ Based Fuel Electrode for Solid Oxide Cell

As the fuel electrode is a central component of solid oxide fuel cell (SOFCs), evaluation of novel fuel electrode materials for SOFCs has become a favorite research subject. Perovskite structure offers an excellent framework for altering the chemical and physical properties of a material due to its ability to accommodate many different cations. In the present research novel La0.2Sr0.7-xCaxTi0.3Fe0.7O3-δ (LSCFT) fuel electrode compositions have been synthesized by glycine-nitrate reaction for high-temperature solid oxide fuel cell application. For analysis of the influence of Sr/Ca ratio on the electrical conductivities of different compositions of LSCFT, the DC four-probe conductivity measurement method has been performed. Conductivities have been measured at three different atmospheres: synthetic air; 97% H2 + 3% H2O and in 1%H2 + 3% H2O + 96% Ar. The crystal structure and microstructure have been studied using X-ray diffraction and SEM, respectively to confirm the purity of phase and percentage of porosity.In previous studies by Irvine et al. [1] has been demonstrated the effect of calcium doping on A-site deficient lanthanum strontium titanate, which displayed enhancement of its electrical conductivity and maximum conductivity was observed for La0.20Sr0.25Ca0.45TiO3 (LSCT) composition. Moreover, the same composition was tested and it confirmed as an active fuel electrode in fuel cell conditions, where good performance was achieved [2].Keywords: Fuel electrode, Electrical conductivity, Layered Perovskite, MIEC material, high temperature SOFC.[1] Aljaberi AD, Irvine JTS. Ca-substituted, A-site deficient perovskite La0.2Sr0.7TiO3 as a potential anode material for SOFCs. J Mater Chem A 1 (2013) 5868-74.[2] Verbraeken MC, Iwanschitz B, Mai A, Irvine JTS. Evaluation of Ca doped La0.2Sr0.7TiO3 as an alternative material for use in SOFC anodes. J Electrochem Soc 159 (2012) F757-62.

Performance of Metal Supported SOFCs Operated in Hydrocarbon Fuels and at Low (

Solid oxide cells (SOCs) are a key enabling technology for the required future renewable energy systems by providing an efficient link between the power, gas, and heat sectors. Specifically, solid oxide fuel cells (SOFCs) convert the chemical energy of a fuel into electricity and heat with high electrical efficiencies. Hydrogen is the obvious choice of fuel due to the only emissions being steam, however hydrocarbon fuels are an interesting and safe alternative to hydrogen, especially for mobile applications, facilitating easy integration into existing infrastructure. The state-of-the-art (SoA) Ni:YSZ (yttria-stabilized zirconia) composite fuel electrode is capable of direct oxidation of hydrocarbons or of reforming of hydrocarbon gas mixtures. Unfortunately, the material that facilitates the above reactions, nickel, is also an excellent carbon deposition catalyst. This is problematic, due to the associated volumetric changes and reduction in electrochemically active sites. Ultimately, the fuel electrode can delaminate from the electrolyte, causing a catastrophic failure of the cell.Alternative fuel electrode materials are the key to avoid this. A-site deficient lanthanum doped strontium titanate (La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3, LSFNT) based anodes, coated with Ni:CGO (gadolinium-doped ceria) electrocatalyst are a promising alternative - while avoiding Ni as a major part of the backbone, the infiltrated Ni ensures electrocatalytic activity. The co-infiltrated CGO serves to improve the ionic conductivity. Integrating these anodes into a metal supported (MS) design ensures lower cost and mechanical robustness. Full cells are manufactured through scalable tape casting and screen printing technology, including the metal support. Possible pitfalls for the MS-cell include vulnerability to corrosion of the metal support and decreased reforming activity due to less electrocatalyst (Ni) compared to the SoA-cell.In this study, the reforming activity of methane in 3 different SOFCs is investigated: Cell 1 is with a SoA Ni:YSZ anode, cell 2 is with a LSFNT anode and cell 3 is with a LSFNT-FeCr anode. The investigation involves electrochemical characterization by impedance spectroscopy (EIS) and performance under applied current (I/V-curves) at temperatures 750˚C, 700˚C, 650˚C and 620˚C, so-called fingerprints, with special emphasis on the anode activity for reforming and water gas shift reaction. Furthermore, galvanostatic durability tests are conducted for the MS-cells (cell 2 and cell 3) with focus on the durability at low temperature (650˚C) and corrosion resistance at medium to high fuel utilization (ca. 50%). The degradation of the tested cells is examined by comparing the electrochemical performance before and after durability testing.

Performance of Metal Supported SOFCs Operated in Hydrocarbon Fuels and at Low (

The Ni/YSZ composite electrode is conventionally used for solid oxide cells, in electrolysis (SOEC) as well as fuel cell (SOFC) operation. For enhanced durability and C-tolerance, alternative fuel electrode materials are needed. In this paper, we compare the performance of two distinct Ni:CGO electrocatalyst coated A-site deficient lanthanum doped strontium titanate (La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3, LSFNT) based anodes, integrated into metal supported cells (MSCs), to the SoA Ni/YSZ anode supported cell in fuel cell mode for the first time. The three cells were investigated electrochemically by impedance spectroscopy (EIS) and performance under applied current (iV-curves) at temperatures 750˚C, 700˚C, 650˚C, and at novel 620˚C in steam/hydrogen and methane/steam. Additionally, one of the MSCs was investigated in CO/steam. Furthermore, galvanostatic durability tests were conducted in 4% steam/hydrogen for the MS-cells at low temperature (650˚C) at medium-high fuel utilization (50%).

Studies of exsolution and catalytic activity of metal nanocatalysts from parent perovskite

The synthesis of metal nanocatalysts exsolved from A-site deficient lanthanum strontium titanate (La0.4Sr0.4)(NixTi1−x)O3−γ (LST, x = 0, LSTN, x = 0.06, 0.25, 0.5) perovskites and their catalytic properties were presented.

INFLUENCE OF DEFECTS FORMATION WAY ON PROPERTIES OF PROTON CONDUCTORS BASED ON LANTHANUM SCANDATE

The structure and properties of proton conductors based on LaScO3 synthesized by acceptor doping and the creation of a deficiency in La cation were studied. It has been shown that the number of intercalated protons in the oxide lattice increases with an increase in the degree of cationic doping and is higher when doped with strontium than calcium. It has been noted that in lanthanum scandate samples with cationic non-stoichiometry, the number of intercalated protons decreases with increasing lanthanum deficiency, which is probably due to the possibility of binding oxygen vacancies VO•• to (LaLa''' - VO••)' complexes and blocking their participation in the process of water dissolution. The activation energies of the conductivity of the volume and grain boundaries of lanthanum scandate synthesized by various methods are determined. It was shown that the conductivity and activation energy of conductivity of the volume of grains of samples doped with strontium are higher than when doped with calcium. It is noted that the activation energy of conductivity of samples with cationic non-stoichiometry is practically independent of the lanthanum deficiency.

Structural and magnetic properties of La1-x⎕xMnO3 (x = 0.1; 0.2 and 0.3) manganites

The intrinsic objective of this work is to investigate the structural (RX, MEB, TGA and IR) and magnetic properties of La1 La1-x⎕xMnO3 (x = 0.1; 0.2 and 0.3) series prepared through the sol–gel method. All these systems crystallize in the rhombohedral structure, $$ {{\rm R}\bar{3}{\rm c}} $$ space group, with the presence of a secondary phase for x = 0.3. We recorded the increase of a unit cell volume when the La deficiency amount increases, which refers to the fact that the vacancy is characterized by a radius other than zero. Another point that can be quoted is the increase of Mn–O–Mn bond angle and the decrease of Mn–O length with lanthanum deficiency (x), confirming the increase of the unit cell volume. The evaporation of water rose by the weight loss as a function of temperature in all samples. The FT-IR measurement revealed one peak corresponding to the Mn–O bond; this can be explained by the Jahn–Teller effect where the Mn ion makes an internal movement against the MnO6 octahedron. The thermal variation of the magnetization indicates the existence of a paramagnetic–ferromagnetic transition at Curie temperature with the increase of lanthanum deficiency. Saturation magnetization and Curie temperature increased with x (0.1; 0.2 and 0.3) deficiency. Hysteresis cycles confirm the ferromagnetic character at low temperatures and paramagnetic state with temperature increase. The increase of saturation magnetization can be accounted for in terms of the change of high spin Mn3+ to low spin Mn4+. Departing from the study of magnetic properties in terms of Arrott plots, the nature of magnetic transitions proves to be of second order.

Investigation on physical properties in lanthanum vacancy of La0.65Ca0.35MnO3 elaborated at high temperature

The lanthanum deficient La0.65−x□xCa0.35MnO3 (0 ≤ x ≤ 0.12) have been elaborated by the solid-state reaction at high temperatures. All samples have been studied by several techniques including X-ray diffraction, scanning electron microscopy (SEM), spectrum two FT-IR Spectrometer and vibrating sample magnetometer. Rietveld refinement shows that all samples crystallize in the orthorhombic structure with Pbnm space group and that lanthanum deficiency leads to a decrease of the unit cell volume. Magnetization measurements versus temperature exhibit a large ferromagnetic-paramagnetic transition with decreasing temperature. The TC decreases from 270 K for x = 0 to 254 K for x = 0.12. A large magnetic entropy change associated with the ferromagnetic-paramagnetic transition has been observed. The maximum of the magnetic entropy change ΔSMMax decreases from 5 Jkg−1K−1 for x = 0 2.1 Jkg−1K−1 for x = 0.12. Moreover, the maximum magnetic entropy change exhibits a linear dependence with applied magnetic field. The relative cooling power (RCP) decreases from 214 J/Kg for x = 0 to 106 J/Kg for x = 0.12 under an applied magnetic field of 5 T.

Infiltrated La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3 based anodes for all ceramic and metal supported solid oxide fuel cells

For improved robustness, durability and to avoid severe processing challenges alternatives to the Ni:YSZ composite electrode is highly desirable. The Ni:YSZ composite electrode is conventionally used for solid oxide fuel cell and solid oxide electrolysis cell. In the present study we report on high performing nanostructured Ni:CGO electrocatalyst coated A site deficient Lanthanum doped Strontium Titanate (La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3) based anodes. The anodes were incorporated into the co-sintered DTU metal supported solid oxide fuel cell design and large sized 12 cm × 12 cm cells were fabricated. The titanate material showed good processing characteristics and surface wetting properties towards the Ni:CGO electrocatalyst coating. The cell performances were evaluated on single cell level (active area 16 cm2) and a power density at 0.7 V and 700 °C of 0.650 Wcm−2 with a fuel utilization of 31% was achieved. Taking the temperature into account the performances of the studied anodes are among the best reported for redox stable and corrosion resistant alternatives to the conventional Ni:YSZ composite solid oxide cell electrode.

Impact of Annealing Temperature on the Physical Properties of the Lanthanum Deficiency Manganites

The lanthanum deficiency manganites La0.8-x□xCa0.2MnO3 (x = 0, 0.1 and 0.2), where □ is a lanthanum vacancy, were prepared using the classic ceramic methods with different thermal treatments (1373 K and 973 K). The structural, magnetic, and magnetocaloric properties of these compounds were studied as a function of annealing temperature. It was noted that the annealing temperature did not affect the crystal structure of our samples (orthorhombic structure with Pnma space group). Nevertheless, a change in the variation of the unit cell volume V, the average bond length dMn–O, and the average bond angles θMn–O–Mn were observed. Magnetization versus temperature study has shown that all samples exhibited a magnetic transition from ferromagnetic (FM) to paramagnetic (PM) phase with increasing temperature. However, it can be clearly seen that the annealing at 973 K induced an increase of the magnetization. In addition, the magnetocaloric effect (MCE) as well as the relative cooling power (RCP) were estimated. As an important result, the values of MCE and RCP in our Lanthanum-deficiency manganites are reported to be near to those found in gadolinium, considered as magnetocaloric reference material.

A-Site Deficient La2-XNiO4+δ Infiltrated LSCF Cathode with Improved Performance and Stability

In this study, to avoid the surface segregation of La2O3 and LaNiO3 phases on the lanthanum nickelate (La2NiO4+δ) infiltrated LSCF cathode, A-site deficient lanthanum nickelate (La2-xNiO4±δ) are investigated as the infiltrant. La2-xNiO4±δ (x=0-0.2) are synthesized and characterized first. The maximum deficient concentration of lanthanum is less than 0.2. La2-xNiO4±δ with higher deficient exhibits the enhanced electrical conductivity below 800oC. Through the electrochemical impedance spectroscopy on the symmetrical cells, it is found that A-site deficiency lead to an increase in the polarization resistance. The surface charge transfer process gradually dominates the impedance with lanthanum deficiency increasing. In the infiltrated LSCF cathode, it is observed that about one order of magnitude decrease in polarization resistance has been achieved after La2-xNiO4±δ ­infiltration, with La2-xNiO4±δ)-infiltration being the smallest. In a short-term measurement, after 960 min cathodic polarization at -0.2V, the degradation of La2-xNiO4+δ (x=0-0.2) coated LSCF reduces with increasing A-site deficient concentration. The enhanced stability of La2-xNiO4+δ (x=0-0.2) coated LSCF demonstrates that A-site deficient La2-xNiO4+δ is one potential infiltration materials for LSCF cathode to improve the surface stability.

Composite electrodes for proton conducting electrolyte of CaZr0.95Sc0.05O3 – δ

A new method of obtaining gastight ceramic based on CaZr0.95Sc0.05O3 – δ is presented. The microstructure and electric properties of the obtained samples, same as the behavior of composite electrodes in contact with this electrolyte, are studied for application of the obtained results in the technology of formation of electrochemical devices. The design of bilayer electrodes is suggested, in which the materials tested as the functional layer were layered lanthanum nickelate La2NiO4 + δ and substituted lanthanum nickelate La1.7Ca(Sr,Ba)0.3NiO4 + δ in combination with the electrolyte components of Ce0.8Sm0.2O2 – δ and BaCe0.89Gd0.1Cu0.01O3 – δ. The collector layer used was lanthanum nickelate–ferrite LaNi0.6Fe0.4O3 – δ and manganite La0.6Sr0.4MnO3 – δ that are characterized by high electron conductivity, low layer resistance and are close by their values of coefficient of linear thermal expansion to the materials of functional layers. Electrochemical activity of the obtained electrodes are compared with the characteristics of composite electrodes based on lanthanum ferrite–cobaltite La0.6Sr0.4Fe0.8Co0.2O3 – δ and deficient lanthanum manganite La0.75Sr0.2MnO3 – δ.

A-Site Deficient La2-XNiO4+δ Infiltrated LSCF Cathode with Improved Performance and Stability

In this study, A-site deficient lanthanum nickelate (La2-xNiO4±δ) are explored as infiltrants. La2-xNiO4±δ (x=0-0.2) samples are synthesized and characterized first. La2-xNiO4±δ with high deficient exhibits enhanced electrical conductivity. Through the electrochemical impedance spectroscopy on the symmetrical cells, it is found that A-site deficiency lead to an increase in the polarization resistance. The surface charge transfer process gradually dominates the polarization resistance as lanthanum deficiency increases. In the infiltrated LSCF cathode, it is observed that about one order of magnitude improvement in the polarization resistance has been achieved after La2-xNiO4±δ infiltration, with La2NiO4±δ-infiltration being the best. In a short-term measurement of 16 h under cathodic polarization of -0.2V, the degradation rate of La2-xNiO4+δ (x=0-0.2) coated LSCF reduces with the increasing A-site deficiency. The enhanced stability of La2-xNiO4+δ (x=0-0.2) coated LSCF demonstrates that A-site deficient La2-xNiO4+δ are promising infiltration materials for LSCF cathode for the improvement of the electrode stability.

Role of lanthanum vacancy on the structural, magnetic and magnetocaloric properties in the lacunar perovskite manganites La0.8−x□xNa0.2MnO3(0 ≤ x ≤ 0.15)

Lacunar La0.8−x□xNa0.2MnO3(0 ≤x≤ 0.15) ceramics where □ is a lanthanum deficiency were synthesizedviaa sol–gel method.

Crystal structure of Ba2(La0.727Ba0.182M0.091)MO6 (M = Nb, Sb, Bi): symmetry nuance identified in photoluminescence and IR spectroscopy studies.

A one-third lanthanum deficiency was created in Ba2LaM5+O6 compounds (LaM compounds) to form Ba2La2/3M5+O5.5 compounds (La2/3M compounds) for M = Nb, Sb, and Bi. The compounds were prepared by a gel-combustion method using citric acid as a fuel. All the compounds were characterized by powder X-ray diffraction (XRD). The XRD analysis showed that the space group of the La2/3M compounds remains the same for the Bi and Sb samples when compared to the reported LaM compounds, except for the Nb sample. La2/3Nb and La2/3Sb adopt a rhombohedral structure with the space group R3[combining macron], whereas La2/3Bi adopts a monoclinic structure with the space group I2/m. As the positron annihilation spectroscopy (PALS) technique is sensitive to cation deficiency, it was used to detect the presence of cation vacancies in the samples, which are formed due to the decrease in the lanthanum concentration. The PALS analyses indicated that the absence of cation deficiency in the La2/3M compounds is similar to that observed in the LaM compound. Thus, the crystal structure of the La2/3M compound was modeled, such that the cation deficiency at the La site is filled by Ba2+ and M5+ ions, and the crystal structure formula is given as Ba2(La0.727Ba0.182M0.091)MO6. This model was confirmed by Rietveld refinement of the XRD data. The emission spectra of Eu3+ showed a strong dependence on its local site symmetry in the host material, in which it is being doped and this can be used as a spectroscopic probe for detecting any differences in the symmetry. Comparison of the local symmetry around La3+ cation was studied using photoluminescence (PL) by doping 2 atom% Eu3+ in LaM and La2/3M compounds. Infrared spectroscopy (IRS) analyses were also carried out for LaM and La2/3M compounds. There was complete agreement between the PL and IRS results and they were also in concordance with the predicted crystal structure model. Interestingly in these La2/3M compounds, the equilibrium structure prefers large Ba2+ ion to occupy the octahedral B-site rather than forming an octahedral vacancy at that site, making these perovskite compounds rare and novel in their class.

Prediction of Magnetocaloric Effect in Lanthanum Deficiency with Phenomenological Model

In this paper, a profound theoretical work has been conducted on magnetic and magnetocaloric properties of La0.65−x x Ca0.35MnO3 (0 ≤ x ≤ 0.15) perovskite manganites. Our work is based on a phenomenological model. The magnetization data as a function of temperature at several magnetic fields allows to determine the magnetocaloric parameters such as the maximum of the magnetic entropy change (ΔS M), the relative cooling power (RCP), and the magnetic specific heat change (ΔC P). The theoretical predictions nearly match with the experimental measurements. These materials present large magnetocaloric effect, suggesting their possible application in moderate magnetic fields.

Characterization of surface modification in atomic force microscope-induced nanolithography of oxygen deficient La0.67Ba0.33MnO3−δ thin films

We report our studies of the nanolithographic surface modifications induced by an Atomic Force Microscope (AFM) in epitaxial thin films of oxygen deficient Lanthanum Barium Manganese Oxide (La0.67Ba0.33MnO3−δ). The pattern characteristics depend on the tip voltage, tip polarity, voltage duration, tip force, and humidity. We have used Electron Energy Dispersive X-Ray Spectroscopy (EDS) to analyze the chemical changes associated with the surface modifications produced with a negatively biased AFM tip. A significant increase in the oxygen stoichiometry for the patterned regions relative to the pristine film surface is observed. The results also indicate changes in the cation stoichiometry, specifically a decrease in the Lanthanum and Manganese concentrations and an increase in the Barium concentration in the patterned regions.

Deficiency Effect on Magnetic and Magnetocaloric Properties of La0.65−x □ x Ca0.35MnO3 Manganites Synthesized Using Sol–Gel Technique

Magnetic and magnetocaloric properties of La0.65−x □ x Ca0.35MnO3 (≤ x ≤ 0.15) perovskite manganites were investigated. Our polycrystalline samples were elaborated using sol–gel method. The Rietveld refinement shows that all our samples are single phase and crystallize in the orthorhombic structure with Pbnm space group. Lanthanum deficiency leads to a decrease of the unit cell volume. The surface morphology was carried out using scanning electron microscopy (SEM). Magnetization measurements versus temperature in a magnetic applied field of 50 mT showed that all our deficient samples exhibit a paramagnetic–ferromagnetic transition with decreasing temperature. With increasing lanthanum deficiency, the Curie temperature T c decreases from 275 K for x = 0 to 257 K and for x = 0.15. A large magnetic entropy change |ΔS M |, deduced from isothermal magnetization curves, has been observed in our samples with a peak centred around their respective transition temperature. The maximum of the magnetic entropy change \( \vert {\Delta } S_{M}^{\text {Max}}\vert \) under a magnetic field change of 5 T is found to decrease from 5.11 J/kg/ K for x = 0 to 3.98 J/kg K and for x = 0.15. The relative cooling power decreases from 237.5J/kg for x = 0 to 217.4 J/kg −1 and for x = 0.15.

Electrical transport and giant magnetoresistance in La0.8−x□xCa0.2MnO3 (x=0, 0.1 and 0.2) oxides

La0.8−x□xCa0.2MnO3 (x=0, 0.1 and 0.20) samples were prepared by the conventional solid-state reaction and annealed at 1200K. X-ray diffraction and scanning electron microscopy were used to check the crystalline structure and the samples morphology. Resistivity measurements versus temperature under different magnetic fields are performed to investigate the electronic transport as well as the magnetoresistance properties. A classical metal–insulator transition at TMI is observed for all samples. As other results, the increase of the lanthanum deficiency content induces the decrease of the resistivity values. In addition, a resistivity minimum has been observed for all samples in the range of temperatures from 5 to 50K, which is strongly influenced by an applied magnetic field. Therefore, the electrical resistivity is fitted with the phenomenological percolation model which is based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. Magnetoresistance measurements show high values which reach 76% for x=0.1 sample.