Hydrogen Plateau Pressure Research Articles

Optimized hydrogen storage properties of Ti–Zr–Mn–VFe-based alloys by orthogonal experiment for upscaled production

Ti–Mn-based hydrogen storage alloys have been widely developed for hydrogen compressors and storage because of their excellent hydrogen storage properties. However, there is still a need to develop a high-capacity hydrogen storage based on low-cost and large-scale preparation. Herein, cheap VFe80 intermetallic alloy is employed to replace the expensive V, and Ti–Zr–Mn–VFe-based alloys ((Ti1-yZry)1+xMn1.88-z(VFe)z, x = 0, 0.05, 0.1, y = 0.05, 0.1, 0.15, z = 0.34, 0.44, 0.54) with mainly C14 Laves phase structure were designed and prepared to optimize the composition by an orthogonal experiment. According to the orthogonal analysis, it is found that the maximum hydrogen storage capacity and hysteresis factor mainly are controlled by the content of VFe80 in the Ti–Zr–Mn–VFe-based alloys. The over-stoichiometry of A-side governs the plateau slope, and the content of Zr decreases hydrogen plateau pressure. The optimal Ti0.9Zr0.1Mn1.44(VFe)0.44 alloy achieves a revisable hydrogen storage capacity of 1.73 wt% with low plateau hysteresis factor (0.49) and plateau slope factor (0.73) at room temperature, the dehydrogenation enthalpy and entropy values respectively are 33.92 kJ/mol and 120.64 J/(mol∙K), which exhibits the best overall hydrogen storage properties. Furthermore, the optimal alloy was successfully produced on a large scale, which provides empirical value for upscaled production of AB2 Laves-phase hydrogen alloys.

An integrated computational and experimental method for predicting hydrogen plateau pressures of TiFe1-xMx-based room temperature hydrides

First-principles density functional theory (DFT) calculations were performed to investigate the effect of ternary alloying on the hydrogenation properties of the TiFe system. Al, Be, Co, Cr, Cu, Mn and Ni were selected as substitutional elements for Fe sites, in the light of their reported enhancement of activation, kinetic and thermodynamic properties. The use of special quasi-random structures to account for disordering of solute elements in the sub-lattice allowed a quantitative assessment of substitutional effects on the hydrogenation behaviour of single-phase TiFe1-xMx alloys, up to a solute concentration as high as 40 at.%. The energy of monohydride formation obtained by DFT calculations, approximated to the enthalpy of formation, was discussed in terms of changes in lattice parameter and hence plateau pressure. Based on the consistency between DFT calculations and earlier experimental results, a linear relationship between monohydride formation energy/enthalpy and plateau pressure was proposed as a simple method to predict the value of one of these physical properties from the other. The obtained correlation could therefore turn out to be a helpful tool to predict the ab/desorption plateau pressure of unexplored vast multi-component systems from DFT calculation, or, the other way around, could allow to estimate the formation enthalpy from only one pressure-composition-isotherm (PCI) measurement hence without the need of Van't Hoff plot.

Studies on Ti-Zr-Cr-Mn-Fe-V based alloys for hydrogen compression under mild thermal conditions of water bath

Moderate operating temperatures and inert heat transfer medium are necessary for improving the safety of thermal hydrogen compression applications. In this work, the structural characteristics and hydrogen storage properties of Ti0.85Zr0.17Cr1.7-xMn0.2FexV0.1 (x = 0.7, 0.8, 0.9) and Ti0.82+yZr0.20-yCr0.9Mn0.2Fe0.8V0.1 (y = 0, 0.03, 0.06) alloys for hydrogen compression under moderate thermal conditions of water bath were systematically investigated. The results show that all the alloys have a single C14 Laves phase. With increasing the Fe content in Ti0.85Zr0.17Cr1.7-xMn0.2FexV0.1 (x = 0.7–0.9) or Ti content in Ti0.82+yZr0.20-yCr0.9Mn0.2Fe0.8V0.1 (y = 0–0.06) alloys, the hydrogen plateau pressure is obviously raised, however, the hydrogen storage capacity declines slightly. Flattened hydrogen plateau is also achieved when the Ti content increases. Therein, Ti0.88Zr0.14Cr0.9Mn0.2Fe0.8V0.1 alloy, with achievement of hydrogen compression from 6.13 MPa at 293 K up to 23.73 MPa at 353 K and hydrogen capacity of 1.64 wt%, outperforms the others for the proposed thermal hydrogen compressor.

Mutual impact of crystal volume and heat of alloy formation on hydride stabilities in Zr(PdxCr1−x)2 and similar intermetallics

Hydrides of Zr(PdxCr1−x)2 pseudobinary compounds (x = 0.05, 0.1, 0.175) exhibit slight destabilization and more sloped hydrogen plateau pressures with the increase of the palladium content. The former feature conforms to the rule of reverse stability but violates the trend of increased hydride stability upon increasing the unit cell intermetallic volume. A combination of the two effects, the heat of alloy formation and the crystal volume, shows better correlation with the hydride stabilities than considering separately each property. Hydrogen absorption is found in ZrPd2 at ∼60 atm and associated with its moderate shear modulus.

Enhancement of Destabilization and Reactivity of Mg Hydride Embedded in Immiscible Ti Matrix by Addition of Cr: Pd-Free Destabilized Mg Hydride

Nanometer-sized Mg hydride clusters may form in a self-organized way by the hydrogenation of a nonequilibrium Mg–Ti alloy. Here the Mg hydride is destabilized by the interface energy between the two metal hydrides. To obtain an even more destabilized Mg hydride, we increased the interface energy by the addition of Cr, which is immiscible with Mg as Ti. Indeed, Mg layers surrounded by Ti–Cr layers show hydrogen plateau pressures higher than when Mg is surrounded by Ti. Destabilization of Mg hydride is also observed in hydrogenated Mg–Ti–Cr thin-film alloys, resulting in hydrogenation plateaus flatter and at higher pressures than in hydrogenated Mg–Ti thin film alloys. Our results suggest that by screening alloys on the basis of their immiscibility with Mg, we can tune the thermodynamics and kinetics of hydrogen absorption and desorption in Mg–H. This concept paves the way for the development of light-weight and cheap Mg-based functional materials in the metal–hydrogen system.

Destabilization of the Mg-H System through Elastic Constraints

We tune the thermodynamics of hydrogen absorption in Mg by means of elastic clamping. The loading isotherms measured by hydrogenography show that Mg films covered with Mg-alloy-forming elements, such as Pd and Ni, have hydrogen plateau pressures more than 2 orders of magnitude higher than bulk Mg at the same temperature. An elastic model allows us to interpret the Mg thickness dependence of the hydrogen plateau pressure. Our results suggest an alternative route for the development of new hydrogen storage materials with optimized thermodynamic properties.

Effect on structure and hydrogen storage characteristics of composite alloys Ti 0.32Cr 0.43V 0.25 with LaNi 5 and rare-earth elements La, Ce, Y

Ti 0.32Cr 0.43V 0.25 and LaNi 5 were prepared independently by arc melting. These two alloys were made into composite as Ti 0.32Cr 0.43V 0.25 + x wt% LaNi 5 (with x = 5, 10 and 20). In addition, an alloy of Ti–Cr–V with rare-earth elements La, Ce and Y were also prepared. These composite alloys were characterized for their structure and hydrogen storage characteristics. Phase analysis and microstructural studies were carried out using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. A Rietveld refinement for the alloy 80 wt% Ti 0.32Cr 0.43V 0.25 + 20 wt% LaNi 5 shows five phases assigned as two BCC, La 2O 3, VO 2, and TiCr 2. Hydrogen storage capacity and hydrogen plateau pressure were likely to be affected by the lattice parameter of BCC phase and the formation of secondary phases. The hydrogenation characteristics for these mixed compositions were investigated.

Hydrogen induced site depopulation in the LaMgNi4-hydrogen system

Abstract The LaMgNi4-hydrogen system was investigated by in-situ neutron powder diffraction and pressure-composition isotherm measurements at 100 °C and hydrogen (deuterium) pressures of up 50 bar. The system displays three hydride phases that have distinctly different hydrogen plateau pressures and H atom distributions. The cubic α-LaMgNi4H0.75 phase forms below 0.01 bar hydrogen pressure and H atoms fill one type of tetrahedral Ni4 interstices. The orthorhombic distorted β-LaMgNiH3.7 phase forms at about 3 bar hydrogen pressure and H atoms fill both tetrahedral LaNi3 and triangular bi-pyramidal La2MgNi2 interstices. Interestingly, tetrahedral Ni4 interstices are no longer occupied. Finally, the most hydrogen rich γ-LaMgNi4H4.85 phase forms above 20 bar. It has again cubic symmetry and H atoms continue to occupy triangular bi-pyramidal La2MgNi2 interstices while filling a new type of tetrahedral Ni4 interstices that are neither occupied in the α- nor in the β-phase. The tetrahedral LaNi3 interstices occupied in the β-phase are empty. Hydrogen induced depopulations of interstitial sites in metal hydrides are relatively rare and consistent with, but not entirely due to, the onset of repulsive H–H interactions at increasing hydrogen concentrations.

Effects of B addition on the hydrogen absorption–desorption property of Ti 0.32Cr 0.43V 0.25 alloy

The effects of boron addition on the hydrogen absorption–desorption properties of the Ti 0.32 Cr 0.43 V 0.25 alloy were studied. Boron was added either directly or indirectly through a mother alloy Ti 0.75 B 0.25 . Direct boron addition caused the decrease in the titanium content of the BCC matrix through formation of Ti–B phases, resulting in the decrease in the lattice constant. Conversely, mother alloy addition increased the titanium content and the lattice constant of the matrix, for it contained enough titanium to contribute to the matrix even after forming the second phase TiB. Such lattice constant changes caused by boron addition resulted in drastic changes in hydrogen plateau pressure and great decrease in effective hydrogen storage capacity.

Synthesis, Crystal Structure and Hydrogenation Properties of the Novel Metal Compound LaNi2Mn3.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis, crystal structure and hydrogenation properties of the novel metal compound LaNi 2Mn 3

LaNi 2Mn 3 has been synthesised from the elements and investigated with respect to crystal structure and hydrogenation properties. Single crystal X-ray diffraction reveals a YNi 2Al 3 type structure (space group P6/mmm, a=9.2099(9), c=4.1892(4) Å, c/ a=0.4548) that is distinctly different from the CaCu 5 type structure of LaNi 5. The compound absorbs reversibly up to six hydrogen atoms per formula unit near ambient conditions and shows a hydrogen plateau pressure of ∼1 bar near 60 °C. The hydrogen atoms in the expanded metal atom structure were localised by neutron powder diffraction on deuterides of composition LaNi 2Mn 3D 4.6 ( a=9.7125(2), c=4.3057(1) Å, c/ a=0.4433) and LaNi 2Mn 3D 5.6 ( a=9.8379(2), c=4.3038(1), c/ a=0.4375). They occupy one octahedral coordinated site [La 2Mn 4] with full occupancy and three tetrahedral coordinated sites [La 2Mn 2], [La 2NiMn] and [LaMn 2Ni], and a deformed square pyramidal coordinated site [LaMn 2Ni 2] with partial occupancies.

Development of catalytically enhanced sodium aluminum hydride as a hydrogen-storage material

The dehydriding of sodium aluminum hydride, NaAlH4, is kinetically enhanced and rendered reversible in the solid state upon doping with selected titanium compounds. Following the initial reports of this catalytic effect, further kinetic improvement and stabilization of the cyclable hydrogen capacity have been achieved upon variation in the method of the introduction of titanium and particle-size reduction. Rapid evolution of 4.0-wt % hydrogen at 100 °C has been consistently achieved for several dehydriding/rehydriding cycles. An improved, 4.8-wt % cyclable capacity has been observed in the material doped with a combination of Ti and Zr alkoxide complexes. Doping the hydride with Ti(OBun)4 and Fe(OEt)2 also produces a synergistic effect, resulting in materials that can be rehydrided to 4 wt % at 104 °C and 87 atm of hydrogen within 17 h. The improved kinetics allowed us to carry out constant-temperature, equilibrium-pressure studies of NaAlH4 that extended to temperatures well below the melting point of the hydride. The 37-kJ/mol value determined for enthalpy of the dehydriding of NaAlH4(s) to Na3AlH6 and Al and the hydrogen plateau pressure of 7 atm at 80 °C are in line with the predictions of earlier studies. The nature of the active catalyst and the mechanism of catalytic action are unknown. The catalytically enhanced hydrides appear to be strong candidates for development as hydrogen carriers for onboard proton exchange membran (PEM) fuel cells. However, further research and development in the areas of rehydriding catalysts, large-scale, long-term cycling, safety and adjustment of the plateau hydrogen pressure associated with dehydriding of AlH6- are required before these materials can be utilized in commercial onboard hydrogen-storage systems.

Structure and electrochemical properties of Zr(V 0.2Mn 0.2Ni 0.6 − xCo x) 2.4 hydrogen storage alloys

The structure and hydrogen absorbed properties of a series of the multicomponent Zr(V 0.2Mn 0.2Ni 0.6 − xCo x) 2.4 alloys have been investigated as a negative electrode in the metal hydride batteries. The crystal structure turned out to change as a function of substituted cobalt from a cubic C15 type structure to a hexagonal C14 type structure. There were anomalies in the relation among the hydrogen plateau pressure from PCT, electronic factor of the alloy and A 2B 2 tetrahedral interstice for hydrogen occupation. The Zr(V 0.2Mn 0.2Ni 0.5Co 0.1) 2.4 electrode was used to evaluate the electrochemical characteristics including activation, cycle life, electrocatalytic activity and hydrogen adsorption. It was found that both the hydrogen adsorption process and hydrogen diffusion process took place in the electrochemical impedance spectra of the electrode used here.

Characteristics of the stoichiometric and non-stoichiometric Laves phase alloys and their hydride electrodes

The pressure-composition isotherms, lattice parameters and electrode properties of the stoichiometric and non-stoichiometric C14 Laves phase alloys with various compositions were studied in this work. The flatness of the plateau pressure of the stoichiometric alloys with vanadium was inferior; however the flatness of the non-stoichiometric alloys on adding manganese improved for the Ti 1− x Zr x Fe 0.15Mn 0.21+ y V 0.64Ni 1.0 ( x = 0.2–0.4, y = 0–0.4) system. The changes in the hydrogen plateau pressure and flatness of the plateau pressure were attributed to the geometric factor and affinity of environmental elements of the tetrahedral interstices hydrogen occupation. The electrode properties of all alloys studied were dependent on the hydrogen plateau pressure and flatness of the plateau pressure of the alloy.

Application of the hydrogen titration method to a thermodynamic investigation of solid Al-Ca alloys

The hydrogen titration method (HTM) was employed to investigate the condensed phases in the Al-Ca system. The method is based on the reaction Ca(in alloy) + H 2(g) ⇄ CaH 2(c) in which hydrogen acts as the titrant and CaH 2 acts as the titration product. The calcium activity in the alloy is calculated from the known equilibrium constant K for this reaction and the experimental hydrogen pressure P via a simple relationship, a Ca = 1 KP . Hydrogen plateau pressures, determined for the Al 4Ca + Al and Al 2Ca + Al 4Ca phase fields in the ranges 400–630 °C and 0.02–1.1 MPa, were used to derive temperature-dependent expressions for the corresponding calcium activities and electromotive forces in Ca-Ca 2+-(Al-Ca)-type galvanic cells. Integration of these expressions has produced 1. (1) the standard free energies of formation for Al 4Ca(c) ΔG f o (kJ (g atom) −1) = −(20.18 ± 0.38) + (4.29 ± 0.78) × 10 −3 T and for Al 2Ca(c) ΔG f o (kJ (g atom) −1) = −(31.28 ± 0.46) + (5.67 ± 0.93) × 10 −3 T and 2. (2) the standard free energies accompanying the reaction of Al 4Ca(c) or Al 2Ca(c) with H 2(g) to produce Al(c) and CaH 2(c). Favorable log P versus 1 T relationships and no significant alloy degradation during cycling suggest a potential utility for Al 2Ca as a “high temperature” hydrogen storage medium having an absorption capacity of 2.13 wt.% H and an operation range of 450–650 °C. The potential for hydrogen storage of other calcium-containing binary systems undergoing HTM reactions is discussed.