Metal hydrides of AB5 compositions have been investigated over the years as they offer extraordinary volumetric hydrogen densities with high cycling stability and purity of released hydrogen. ...Moreover, by doping with different elements, the sorption properties of alloys can be significantly changed according to their foreseen applications. In this contribution, we report the synthesis routes and hydrogenation characteristics of La1−xYxNi4.5Cu0.5. The synthesized alloys exhibit excellent structural purity with all reflections indexed by the hexagonal P6/mmm structure. It was found that the Y content can easily tune (raise) the equilibrium pressures of the pressure–composition isotherms, while overall gravimetric density remains at a level exceeding 1.5 wt.% up to x = 0.3 then strongly decreases. The pressure range for desorption can be tuned from 1.5 to 5 bars at room temperature. Some alloys (x = 0; 0.2) exhibit very good stability during 1000 cycles of hydrogen loading and unloading. Furthermore, activation of the alloys is prompt, making them good candidates for stationary hydrogen storage, non-mechanical hydrogen compressors, or soft actuators.
Abstract
Ca
2
Fe
2
O
5
(CFO) is a potentially viable material for alternate energy applications. Incorporation of nitrogen in Ca
2
Fe
2
O
5
(CFO-N) lattice modifies the optical and electronic ...properties to its advantage. Here, the electronic band structures of CFO and CFO-N were probed using Ultraviolet photoelectron spectroscopy (UPS) and UV-Visible spectroscopy. The optical bandgap of CFO reduces from 2.21 eV to 2.07 eV on post N incorporation along with a clear shift in the valence band of CFO indicating the occupation of N 2p levels over O 2p in the valence band. Similar effect is also observed in the bandgap of CFO, which is tailored upto 1.43 eV by N
+
ion implantation. The theoretical bandgaps of CFO and CFO-N were also determined by using the Density functional theory (DFT) calculations. The photoactivity of these CFO and CFO-N was explored by organic effluent degradation under sunlight. The feasibility of utilizing CFO and CFO-N samples for energy storage applications were also investigated through specific capacitance measurements. The specific capacitance of CFO is found to increase to 224.67 Fg
−1
upon N incorporation. CFO-N is thus found to exhibit superior optical, catalytic as well as supercapacitor properties over CFO expanding the scope of brownmillerites in energy and environmental applications.
Hexagonal Y
1−
x
R
x
MnO
3+
δ
(R: other than Y rare earth elements) oxides have been recently introduced as promising oxygen storage materials that can be utilized in the temperature swing processes ...for the oxygen separation and air enrichment. In the present work, the average and local structures of Tb- and Ce-substituted Y
0.7
Tb
0.15
Ce
0.15
MnO
3+
δ
and Y
0.6
Tb
0.2
Ce
0.2
MnO
3+
δ
materials were studied, and their oxygen storage-related properties have been evaluated. The fully oxidized samples show the presence of a significant amount of the highly oxygen-loaded the so-called Hex3 phase, attaining an average oxygen content of
δ
≈ 0.41 for both compositions. Extensive studies of the temperature swing process conducted in air and N
2
over the temperature range of 180–360 °C revealed large and reversible oxygen content changes taking place with only a small temperature differences and the high dependence on the oxygen partial pressure. Significant for practical performance, the highest reported for this class of compounds, oxygen storage capacity of 1900 μmol O g
−1
in air was obtained for the optimized materials and swing process. In the combined temperature–oxygen partial pressure swing process, the oxygen storage capacity of 1200 μmol O g
−1
was achieved.
In this paper, we report on a synthesis procedure, structural and electrical properties of BaErMn2O5 and BaErMn2O6, A-site double perovskites having layered arrangement of Ba and Er cations. These ...materials belong to a family of BaLnMn2O5+δ oxides, which up to now were successfully synthesized for Ln=Y and La–Ho lanthanides. Up to our knowledge, this is the first report on the successful synthesis of BaErMn2O5 and BaErMn2O6, yielding>95wt% of the considered compounds. Structural characterization of the materials is given at room temperature, together with in situ XRD studies, performed during oxidation of BaErMn2O5 in air, at elevated temperatures up to 500°C. A complex structural behavior was observed, with oxidation process of BaErMn2O5 occurring at around 300°C. The oxidized BaErMn2O6 shows a structural phase transition at about 225°C. Results of structural studies are supported by thermogravimetric measurements of the oxidation process, performed in air, as well as reduction process, preformed in 5vol% of H2 in Ar. Additionally, isothermal oxidation/reduction cycles were measured at 500°C, showing interesting properties of BaErMn2O5+δ, from a point of view of oxygen storage technology. Electrical conductivity of BaErMn2O5 is of the order of 10−4Scm−1 at room temperature and shows activated character on temperature with activation energy Ea=0.30(1) eV. Positive sign of Seebeck coefficient for this material indicates holes as dominant charge carriers. Oxidized BaErMn2O6 possesses much higher electrical conductivity, almost 0.2Scm−1 at room temperature. Additional, about 10-fold increase of electrical conductivity, occurring in the vicinity of 225°C for this material, can be associated with phase transition from charge/orbital-ordered insulator COI(CE) to paramagnetic metal PM phase. The highest conductivity for BaErMn2O6 was measured near 500°C and is almost equal to 40Scm−1, while negative sign of Seebeck coefficient can be associated with electrons being dominant charge carriers.
Structure and oxygen storage properties of BaErMn2O5+δ. Display omitted
•Synthesis of BaErMn2O5 and BaErMn2O6.•Structural and transport properties of BaErMn2O5+δ.•Oxygen storage in BaErMn2O5–BaErMn2O6 system.•Phase transitions in BaErMn2O6.
The need for improved UV emitting luminescent materials underscored by applications in optical communications, sterilization and medical technologies is often addressed by wide bandgap semiconducting ...oxides. Among these, the Mg-doped ZnO system is of particular interest as it offers the opportunity to tune the UV emission by engineering its bandgap via doping control. However, both the doped system and its pristine congener, ZnO, suffer from being highly prone to parasitic defect level emissions, compromising their efficiency as light emitters in the ultraviolet region. Here, employing the process of femtosecond pulsed laser ablation in a liquid (fs-PLAL), we demonstrate the systematic control of enhanced UV-only emission in Mg-doped ZnO nanoparticles using both photoluminescence and cathodoluminescence spectroscopies. The ratio of luminescence intensities corresponding to near band edge emission to defect level emission was found to be six-times higher in Mg-doped ZnO nanoparticles as compared to pristine ZnO. Insights from UV-visible absorption and Raman analysis also reaffirm this defect suppression. This work provides a simple and effective single-step methodology to achieve UV-emission and mitigation of defect emissions in the Mg-doped ZnO system. This is a significant step forward in its deployment for UV emitting optoelectronic devices.
Structural investigations showed that the crystal structure influences the hydrogen storage properties of MgH2. The crystal structure and dehydriding temperature of MgH2 activated by high energy ball ...milling under an argon atmosphere were studied. The X-ray diffraction characterization showed that a small amount of tetragonal α-MgH2 phase transforms into crystalline γ-MgH2 of an orthorhombic structure during milling, and γ-MgH2 peak intensity increased with increasing ball milling time within a certain limit, but disappeared early in the hydrogenation cycle at a temperature of 300 °C. We studied the phase structure and the stability of the MgH2 during absorption/desorption cycles as well as thermal stability in a context of sustainable use of MgH2.
•γ-MgH2 phase has a lower temperature of hydrogen desorption than β-MgH2 phase.•The disappearance of the γ-MgH2 phase was found to be temperature dependent.•Working at a temperature of 250 °C can stabilize the γ-MgH2 phase.•Milling decreases particle size but cycling and heating cause agglomeration.
The influences of Nb-containing oxides and ternary compound in hydrogen sorption performance were investigated. As faster desorption kinetic and lower activation energy were reported by addition of a ...ternary compound catalyst such as K2NiF6, the influence of KNbO3 on hydrogen storage properties of MgH2 has been investigated for the first time. The MgH2 - KNbO3 composite desorbed 3.9 wt% of hydrogen within 10 min, while MgH2 and MgH2-Nb₂O₅ composites desorbed 0.66 wt% and 3.2 wt% respectively under similar condition. For MgH2 with other Nb-contained catalysts such as Nb, NbO and Nb₂O3, the desorption rate was almost the same as the one registered for as-milled MgH2. The analysis of differential scanning calorimetry (DSC) showed that MgH2-KNbO3 composite started to release hydrogen at ∼335 °C which is 50 °C lower compared to as-milled MgH2 without any additives. The activation energy for the hydrogen desorption was estimated to be about 104 ± 6.8 kJ mol−1 for this material, while for the as-milled MgH2 was about 165 ± 2.0 kJ mol−1. It is believed that Nb-ternary oxide catalyst (KNbO3) showed a good catalytic effect and enhance the sorption kinetics of MgH2.
•The effect of new Nb-oxide type catalyst on sorption performance of MgH2.•Faster desorption rate measured by a PCT apparatus.•Lower desorption temperature and activation energy.
It is documented that the mean radius r¯ of rare-earth cations occupying Y1-xRx sublattice in Y1-xRxMnO3+δ hexagonal oxides plays a decisive role in terms of thermodynamics and kinetics of reactivity ...of the materials with oxygen, and consequently, influences strongly the oxygen storage performance in thermal swing processes conducted in oxygen and air. Y1-xRxMnO3+δ samples with designed r¯ being close to the critical one, at the border of stability between hexagonal- and perovskite-type phases, can reversibly incorporate/release significant amounts of oxygen in pure O2 or air atmospheres, at the moderate temperatures on the order of 200–300 °C. Characteristic temperatures of oxidation and reduction are dependent on r¯, therefore, it is possible to adjust conditions of the temperature swing operation by the chemical doping in Y1-xRxMnO3+δ with larger rare-earth elements. Crucial from a practical point of view, an increase of the oxidation temperature in such compounds greatly enhances the speed of the oxidation process (20 °C increase can reduce half-time of oxidation twice), which is found to be the limiting factor concerning the performance. Based on the comprehensive studies of the physicochemical properties of Y1-xRxMnO3+δ, the optimized Y0.95Pr0.05MnO3+δ composition is proposed, doped only with a small amount of more expensive praseodymium. The material exhibits excellent oxygen storage-related properties and is able for the effective production of oxygen in air by the thermal swing process, utilizing medium-/low-temperature industrial waste heat.
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Hexagonal YMnO3+δ is shown to be an effective temperature-swing oxygen storage material working at low temperatures (150–300°C) in pure oxygen if adequately processed or obtained having ...sub-micrometer primary particles with limited number of big agglomerates. A substantial increase of a practical oxygen storage capacity is observed for a sample synthesized by a solid-state method, which was subjected to a high impact mechanical milling. However, even better properties can be achieved for the sol-gel technique-produced YMnO3+δ. The reversible incorporation and release of the oxygen is associated with a structural transformation between stoichiometric YMnO3 (Hex0) phase and a mixture of oxygen-loaded Hex1 with δ ≈ 0.28 and Hex2 with δ ≈ 0.41 phases, as documented by in situ structural X-ray diffraction studies, supported by thermogravimetric experiments. Contrary to HoMnO3+δ, it was not possible to obtain single phase Hex1 material in oxygen, as well as to oxidize YMnO3 in air. Results confirm crucial role of the ionic size of rare earth element Ln on the oxygen storage-related properties and stability of the oxygen-loaded LnMnO3+δ phases.
Temperature dependence of the oxygen content for YMnO3+δ sol-gel, calculated from data obtained during TG studies. Data was recorded during sequential heating and cooling in pure oxygen atmosphere with rates of 5, 1, and 0.1°min−1. Inset shows YMnO3+δ sol-gel microstructure. Display omitted
•Synthesis of hexagonal YMnO3+δ using different methods.•Temperature-swing process for YMnO3+δ in pure oxygen.•Oxygenated Hex1 and Hex2 phases for YMnO3+δ.•Morphology as a crucial factor for optimizing oxygen storage properties.•Limited range of formation of Hex1, YMnO3.28 phase.
A facile, mechanical milling method resulting in a significant improvement of the oxygen release kinetics from cation-ordered BaLnMn2O5+δ-type (Ln: selected lanthanides) materials is shown for Pr- ...and Sm-containing compounds. Synthesized by a soft chemistry method, BaPrMn2O5 and BaSmMn2O5 oxides were milled in an appropriately selected conditions, which allowed to obtain samples with largely decreased crystallite size, increased specific surface area, and likely, having a partial Ba-Ln disorder. Such materials were characterized in terms of their oxygen storage-related properties and were found to exhibit enhanced oxygen release kinetics, with a reduction rate parameter being increased several times. The milling process did not influence the mechanism of the oxygen release significantly. A decrease of a characteristic temperature of reduction was also noticed, however, while the oxygen storage capacity of BaSmMn2O5+δ was found to increase after milling, it decreased for BaPrMn2O5+δ. It seems that the enhanced reduction rates can be related to a diminished tendency of a formation of the oxygen vacancy-ordered BaLnMn2O5.5. The reported studies are supplemented by scanning electron microscopy and X-ray photoelectron spectroscopy measurements.
Display omitted
•Modification of structure and microstructure of milled BaLnMn2O5+δ•Improvement of oxygen storage properties by mechanical milling•Oxygen release and intake kinetics from BaPrMn2O5+δ and BaSmMn2O5+δ•XPS studies of BaLnMn2O5+δ
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