Search

Magnesium hydride (MgH2) is a hydrogen-rich compound generating significant amounts of hydrogen in the process of hydrolysis, i.e., in the course of its chemical interaction with water or with aqueous solutions. This process is of great interest for the on-site hydrogen generation aimed at application of H2 as a fuel for PEM fuel cells. We propose a review of recent reference publications in the field and also present the results of our own research. The increase of the rates of H2 release and the completeness of transformation of MgH2 are two important goals, which can be attained by optimizing the size of the powders of MgH2 by ball milling, by using catalysts added to MgH2 and to aqueous solutions, and by increasing the interaction temperature. The effect of these parameters on the degree of conversion and the rates of hydrogen evolution are analyzed in detail and the best systems to reach the efficient hydrolysis performance are identified. The mechanism of catalytic hydrolysis is proposed, while further improvements of the process of hydrolysis are required and additional studies of this important topic are needed.

New La1-xNdxMgNi4-yCoy (x = 0, 0.5, 1; y = 0, 0.5, 1) alloys have been synthesized by powder sintering method, and their crystal structure and hydrogen storage properties have been systematically studied. X-ray diffraction analysis showed that La1-xNdxMgNi4-yCoy alloys belong to the MgCu4Sn-type structure. Hydrogen storage capacity of the alloys is in the 1.1–1.7 wt % H range. The alloy hydrides of are stable in air at room temperature. XRD studies have shown that hydrides retain the crystalline structure of the starting alloy while increasing the cellular volume by 15–23%. Pressure–composition–temperature (PСT) measurements demonstrated that thermodynamic stability of hydrides decreases with an increasing Nd content whereas increasing Co content in the alloys leads to the increase their stability which is demonstrated by the lowering desorption plateau pressure with Co content. Electrochemical studies showed that La-rich electrode materials exhibit the maximum discharge capacity (300–330 mAh/g), but Nd-doped ones show better cyclic stability.

The isothermal sections of the Nd–Mg–Co phase diagram at 300 and 500 °C have been constructed on the basis of X-ray diffraction (XRD), Wavelength Dispersive Spectrometry (WDS) and Electron Probe Micro Analyzer (EPMA) analysis. The existence of two ternary compounds Nd 4 Mg 3 Co 2 (own structure type) and Nd 4 MgCo (Gd 4 RhIn type) was confirmed. The latter has homogeneity range defined as Nd 4- x Mg 1+ x Co with 0 ≤ x ≤ 0.2 at 300 °С and 0 ≤ x ≤ 0.3 at 500 °С. Four new ternary compounds were identified in this work: Nd 23- x Mg 4+ x Co 7 (Pr 23 Mg 4 Ir 7 type; −1.5 ≤ x ≤ 0.1 at 300 °С and −0.6 ≤ x ≤ 0.3 at 500 °С); NdMg 2 Co 9 (YIn 2 Ni 9 type), ∼Nd 77 Mg 14 Co 9 (cubic) and ∼Nd 56 Mg 6 Co 38 (unknown crystal structure). Two binary compounds, NdCo 2 and NdCo 3 , were found to dissolve significant amount of Mg with the formation of substitutional solid solutions Nd 1- x Mg x Co 2 (MgCu 2 type; x ≤ 0.1 at 300 °С and x ≤ 0.15 at 500 °C) and Nd 1- x Mg x Co 3 (PuNi 3 type; x ≤ 0.33 at 300 °С and x ≤ 0.5 at 500 °C), respectively. The Nd 4 Mg 3 Co 2 , Nd 4- x Mg 1+ x Co, Nd 23- x Mg 4+ x Co 7 and Nd 1- x Mg x Co 2 compounds irreversibly absorb hydrogen at room temperature and pressure 1–10 bar to form crystalline or amorphous hydrides with hydrogen content 1.6–2.1 wt. %. Hydrogen thermal desorption from these hydrides in vacuum results in their disproportionation with formation of binary NdH 2 hydride. The Nd 1- x Mg x Co 3 (0 ≤ x ≤ 0.5) alloys reversibly absorb up to 1.3 wt. % of hydrogen. NdMg 2 Co 9 compound has small hydrogenation capacity, absorbing only 0.5 wt. % at room temperature and hydrogen pressure 185 bar.

New Nd 3− x Mg x Co 9 alloys have been synthesized by powder sintering method and their crystal structure, hydrogen storage and magnetic properties have been systematically studied. X-ray diffraction analysis showed that Nd 3− x Mg x Co 9 ( x ≤ 1.5) alloys belong to the PuNi 3 -type structure and NdMg 2 Co 9 is an individual ternary compound with YIn 2 Ni 9 -type structure. Hydrogen storage capacity of the PuNi 3 -type Nd 3− x Mg x Co 9 alloys is in the range 1.1–1.6 wt % H at room temperature, while for NdMg 2 Co 9 it is below 0.5 wt % H. PСT measurements demonstrated that thermodynamic stability of hydrides decrease with increasing Mg content. Hydrides of the alloys with lower Mg content ( x ≤ 1.0) are stable in air at room temperature. XRD study of these hydrides showed preserved PuNi 3 -type structure with hydrogen-induced volume expansion 16.7–24%. The positions of hydrogen atoms were determined by neutron powder diffraction study of Nd 2 MgCo 9 D 10 . In the Nd 2 MgCo 9 D 10 structure D atoms partially occupy four types of interstices, including octahedral Nd 2 Co 4 sites, tetrahedral Nd 2 Co 2 sites and two types of trigonal bipyramids (Nd,Mg) 3 Co 2 . Electrochemical investigations of the Nd 2 MgCo 9 electrode demonstrated rather low discharge capacity, 100 mAh/g, which is about 30% of its theoretical capacity. Magnetic measurements have shown that Nd 2 MgCo 9 is ferromagnetic with high Curie temperature (T C = 633 K) and spin reorientation at 225 K, its hydrogenation causes a significant decrease of the magnetization.

The influence of Co substitution on the structural and magnetic properties of NdMgNi 4− x Co x ( x ⩽ 3) alloys and their hydrides has been investigated. The formation of NdMgCo 4 compound has not been observed. The gas hydrogenation of these alloys resulted in the formation of NdMgNi 4 H 4 , NdMgNi 3 CoH 4.8 , NdMgNi 2 Co 2 H 5.6 and NdMgNiCo 3 H 6.2 hydrides. Transformation from cubic into orthorhombic structure upon hydrogenation has been confirmed for NdMgNi 4 H 4 hydride, whereas it has been found that Co-containing hydrides preserved the parent cubic structure. PC diagrams at 50 °C for NdMgNi 4− x Co x –H 2 ( x = 0, 2, 3) showed only one absorption/desorption plateau, which equilibrium pressure decrease as the cell volume increase. The NdMgCu 4 ternary compound has been synthesized, but it does not interact with hydrogen under normal conditions. Electrochemical investigations demonstrated that substitution of the nickel atoms by the cobalt ones in NdMgNi 4 does not significantly influence on the maximum discharge capacity (244–261 mA h/g) and all alloys are characterized by high cyclic stability. NdMgNi 4 and its hydride display a Pauli paramagnet behavior. NdMgNi 2 Co 2 displays a magnetic order below 50 K whereas its hydride shows a Pauli paramagnet behavior similar to that of NdMgNi 4 H 4 .

New two ternary YMgCo4 and YMgCu4 and one quaternary YMgCo2Ni2 compounds have been synthesized by mechanical alloying with further annealing. The hydrogenation capacity of YMgCo4 reaches 6.8 at. H/f.u. The Pressure-Composition-Temperature studies of YMgCo4–H2 and YMgNi4–H2 systems revealed that introduction of magnesium, accompanied by shrinking of the unit cell, decreases the stability of hydrides comparing to binary YCo2 and YNi2 compounds. The values of heat and entropy of the YMgCo4H6.8 hydride formation were calculated: ΔH = −27.9 ± 0.8 kJ mol–1 H2 and ΔS = −93.4 ± 2.6 J mol−1 H2 K−1. The YMgCo2Ni2–H2 system shows intermediate thermodynamic properties compared to the ternary hydrides (ΔH = −28.8 ± 0.2 kJ mol–1 H2 and ΔS = −117.6 ± 2.4 J mol–1 H2 K−1). The YMgCo4H6.8 and YMgCo2Ni2H4.9 hydrides keep the cubic structure of the parent compounds with a cell volume expansion of 23 and 14.4% respectively. It is shown that the YMgCu4 compound does not interact with hydrogen under normal conditions.

A number of ternary Mg–Ti–Ni alloys have been synthesized by high energy ball milling, followed by annealing heat treatment. On the basis of X-ray phase analysis of these alloys an isothermal section of the Mg–Ti–Ni phase diagram at 500 °C has been constructed. One ternary intermetallic compound with Mg 3 TiNi 2 stoichiometry that adopts an ordered Ti 2 Ni-type structure has been identified. Binary cubic TiNi compound dissolves considerable amount of Mg to form a ternary phase Mg x Ti 1−2/3 x Ni 1−1/3 x crystallizing in the CsCl-type structure and existing over the range of compositions 0 x ≤ 0.5. Other binary phases do not show appreciable solubility of the third component. The hydrogenation behavior of Mg 3 TiNi 2 compound and Mg 0.5 Ti 0.67 Ni 0.83 solid solution phase has been studied.

New ternary Mg–Mn–Ni alloys synthesised by high energy ball milling have been studied as hydrogen storage materials. A new Mg 3 MnNi 2 ternary intermetallic compound of the Ti 2 Ni type has been identified, its structure was studied by neutron diffraction. It has been shown that in the Mg–Mn–Ni system this intermetallic coexists with Mg, forming two-phase Mg–Mg 3 MnNi 2 alloys. The Mg 3 MnNi 2 compound reversibly absorbs about 1 wt.% of hydrogen already at room temperature and forms interstitial type Mg 3 MnNi 2 H ∼3 hydride. Substantially higher hydrogenation capacity has been observed in Mg–Mg 3 MnNi 2 alloys with high Mg content, e.g. 5.4 wt.% for the Mg 88 Mn 4 Ni 8 alloy at 250–350 °C. Hydrogen absorption–desorption kinetics of these alloys are drastically improved compared to undoped magnesium due to catalytic effect of the Mg 3 MnNi 2 phase.

Electronic properties of Ti 4 Fe 2 O oxide, a very promising hydrogen-storage material, were studied both from theoretical and experimental points of view employing the first-principles band-structure augmented plane wave + local orbitals (APW + LO) method with the WIEN2k code as well as X-ray photoelectron spectroscopy (XPS) and soft X-ray emission spectroscopy (SXES). Total and partial densities of states of the constituent atoms of Ti 4 Fe 2 O have been derived from the APW + LO calculations. The XPS valence-band spectra as well as the SXES Ti Lα, Fe Lα and O Kα bands have been measured for a series of Ti 4 Fe 2 O x oxides ( x = 1.0, 0.5, and 0.25). The present APW + LO calculations reveal that, the O 2p-like states are the dominant contributors into the bottom of the valence band, whilst the top of the valence band and the bottom of the conduction band of Ti 4 Fe 2 O are dominated by contributions of the Fe 3d- and Ti 3d-like states. The APW + LO results are confirmed experimentally by a comparison on a common energy scale of the XPS valence-band spectra and the SXES Ti Lα, Fe Lα and O Kα bands of the Ti 4 Fe 2 O x oxides. The XPS Ti 2p, Fe 2p and O 1s core-level binding energies have been measured for the compounds under consideration.

The Zr 9 V 4 SH ∼23 hydride was synthesized by saturation from gaseous hydrogen at room temperature and 0.12 MPa hydrogen pressure. The measured hydrogen absorption capacity of the κ -Zr 9 V 4 S was 1.77 H/M. The refinement of the hydride structure with the use of X-ray powder diffraction data confirmed the preserved symmetry of the metallic matrix (sp.gr. P 6 3 / mmc , a = 9.2715 ( 2 ) , c = 9.0756 ( 4 ) A , Δ a / a = 7.3 % , Δ c / c = 6.1 % , Δ V / V = 22.1 % , Δ V / at.H = 2.62 A 3 ). The analysis of interstices in this compound revealed that without positioning hydrogen atoms in triangular Zr 3 interstices it is impossible to model the structure of k -hydride with the experimentally observed capacity. Neutron powder diffraction study of the Zr 9 V 4 SD ∼23 deuteride (sp.gr. P 6 3 / m m c , a = 9.2779 ( 5 ) , c = 9.0779 ( 5 ) A) revealed deuterium atoms filling two tetrahedral and three triangular interstices. A full occupancy of only one site (D4) was observed in our sample giving the refined deuterium content of 20.8(2) D/f.u. Filling of the D4 site causes a displacement of the sulphur atoms from the three-fold axis (splitting of 2c to 6h position).

This work was focused on the studies of vanadium-based “low-temperature” bcc alloys as H storage materials. Both as cast and annealed at 1100 ° C quaternary alloys V92.5−x−yZr7.5Ti7.5+x−yMy (M = Cr, Mn, Fe, Co, Ni; x = 0 , 10; y = 0 , 7.5) were studied. They were characterised by XRD, PCT hydrogen absorption–desorption measurements ( T = 30 –120 ° C) and thermal desorption spectroscopy studies of H desorption from the saturated hydrides ( T = − 100 to 800 ° C). The thermodynamics of the VH∼2↔ VH∼1 transition and hydrogen sorption capacities ( 1.55 , … , 1.80 H/M) were found to be significantly influenced by the substituting M-element. The reversible hydrogen storage capacity ( T 100 ° C, P > 0.1 bar) reaches 50–60% of its overall value. Complete hydrogen desorption proceeds in vacuum at T 300 ° C. The major future challenge of the work is in increasing of the easy-reversible part of the H storage capacity corresponding to transformation VH∼2↔ VH∼1 by reducing the content of H in the monohydride.

Hydrogenation of RECu2 (RE=Dy, Ho, Y) at room temperature and pressures of 100–1500-bar H2 has not resulted in the formation of ternary hydrides. The interaction of hydrogen with Pr(Cu1−xNix)2 (x=0, 0.1, 0.17, 0.25, 0.32) at room temperature and pressure of 25 bar resulted in the formation of Pr(Cu1−xNix)2H∼3 hydrides. It was found that the PrCu2H3 and Pr(Cu0.9Ni0.1)2H2.9 hydrides are poorly crystallized, but that an increase of the Ni-content leads to improved crystallinity of the hydrides. The hydrides Pr(Cu0.75Ni0.25)2H∼3 and Pr(Cu0.68Ni0.32)2H∼3 preserve, shortly after the hydrogen absorption, the CeCu2 type structure of their metallic matrix with a hydrogen induced volume expansion up to 28% compared to the parent compound. During long-term exposure in the air they undergo a structural transformation from the orthorhombic CeCu2 to the hexagonal Fe2P type with a hydrogen induced volume expansion up to 16.6% compared to the parent compound.

Hydrogen storage capacity and crystal structure of hydrogen-free Zr3V3O0.6 and its saturated deuteride, Zr3V3OxD9.6, have been studied using both X-ray and neutron powder diffraction. Experimental data together with the geometrical analysis of the crystal structure of the deuteride indicate that oxygen non-stoichiometry plays a significant role in the 2-fold increase of hydrogen storage capacity of Zr3V3O0.6 compared with the stoichiometric oxygenated Zr3V3O.

Crystal chemistry aspects of hydrogen behaviour in the zirconium–iron intermetallic deuterides (hydrides), Zr2FeD1.80–5.00, Zr3FeD1.27–6.70 and Zr4Fe2O0.6H7.80, were studied with a focus on the application of high resolution powder neutron diffraction. The effects of crystal structure, chemical composition of the metal matrices, temperature and hydrogen contents on preferences in the interstices occupation and H ordering were investigated and discussed in relation to the H absorption–desorption properties. The Hydrogenation–Disproportionation–Desorption–Recombination process was successfully applied to all materials studied, including the first reported example of an oxygen-containing compound, Zr4Fe2O0.6.

The hydrogen storage capacity of η-phases Zr 3 V 3 O x and Zr 4 Fe 2 O x has been found to depend substantially on the oxygen content. Increase of the hydrogen storage capacities to H/f.u. ∼10.5 and 9.4 with the decrease of O-content have been observed for Zr 3 V 3 O x and Zr 4 Fe 2 O x , respectively. The hydrogen concentration in the title phases is higher than that in the corresponding ZrV 2 H 4.9 and Zr 2 FeH 4.5 saturated hydrides. The maximum hydrogen storage capacity is observed at x =0.6 for η-Zr 3 V 3 O x and x =0.3 for η-Zr 4 Fe 2 O x . Based on these results, it is believed that other Zr-based oxygen-stabilised η-phases could also display large hydrogen storage capacity due to partial occupation of 16(c) or 8(a) sites by oxygen atoms.

Experimental studies on the series of ternary intermetallic compounds, Zr6MeX2, where Me=Fe, Co, and Ni, and X=Al, Ga, and Sn, show that they have large hydrogen storage capacities ranging from 9.3 to 10.8 H-atoms per formula unit. The crystal structure of the parent intermetallic compounds changes upon hydrogenation with a two-fold increase of the crystallographic c-axis length and a corresponding change in space group from P 6 m2 to P 6 2c . Full profile (Rietveld) refinement using X-ray powder diffraction data indicates that the crystal structures of the new hydrides, Zr6CoAl2H10, Zr6NiAl2H9.7 and Zr6NiSn2H10.8 are analogous to Zr6FeAl2D10, which was examined by neutron powder diffraction. Thermal desorption of hydrogen in all hydrides occurs in the temperature range between 400 and 900 K and is characterized by two similar events. The compounds Zr6FeAl2Hx (x=0, 10 and 0.1–0.2) order magnetically at 45, 155 and 10 K, respectively. Electronic structure calculations are carried out on Zr6FeAl2Hx (0≤x≤10) to interpret their structural and thermal behavior. Site energies for hydrogen incorporation, energetics of hydrogen desorption, as well as quantitative details of the densities of states are evaluated. Site energies and metal–metal bonding largely affect the maximum hydrogen content, while H–H repulsions constitute the greatest driving force for the change in space group upon hydrogenation.

The intermetallic compound Zr6FeAl2 was hydrogenated (deuterated) at room temperature and investigated by X-ray and neutron powder diffraction. The hydride (deuteride) crystallizes with hexagonal symmetry (space group P62c; deuteride: a = 8.1354(4) A, c = 7.0940(6) A, V = 406.62(5) A3, Z = 2, refined composition Zr6FeAl2D9.82(3)). Its metal atom substructure derives from an ordered Fe2P type host structure (space group P62m) by small atomic displacements that lead to a doubling of the c axis. Deuterium occupies two types of tetrahedral [Zr4], one type of tetrahedral [Zr3Fe] and one type of trigonal bipyramidal [Zr3Fe2] interstices, thus avoiding the neighbourhood of aluminium. The metal-deuterium bond distances are in the range Zr-D = 2.03–2.22 A and Fe-D = 1.73–1.77 A, while the shortest Al-D distance is 2.90 A. The hydrogen sorption properties of Zr6FeAl2 are intermediate to those of Zr2Fe and Zr2Al.

Hydrogen absorption properties of ZrVO alloys where some of the Zr or V is substituted by Ti, Hf, or Nb, Fe, respectively have been investigated. Phase structural characteristics of these alloys and their hydrides have been determined by X-ray powder diffraction. The interdependence of obtained crystallographic and hydrogenation characteristics has been discussed.

A novel low-temperature hydride, Mg3MnNi2D3−x with filled Ti2Ni-type structure has been synthesised and its crystal structure has been studied in detail by powder neutron diffraction. Refinement of the structures of β-Mg3MnNi2D1.83 and α-Mg3MnNi2D0.23 deuteride phases showed that D atoms partially fill 16d Mg6 octahedral and 32e Mg3Ni tetrahedral interstices in the cubic metal matrix (sp.gr. View the MathML sourceFd3¯m). The β-hydride was found to be rather unstable, during storage in air it transforms into α-hydride.

The effect of Cu-for-Ni substitution on the structural and hydrogenation properties of the LaCu2 compound with hexagonal AlB2 type structure has been studied. An extended solid solution was found from LaCu2 to La(Cu0.35Ni0.65)2. The La(Cu1−xNix)2 compounds easily absorb hydrogen at room temperature under 1 bar pressure, after preliminary activation treatment by heating in vacuum. Hydrogenation of LaCu2 resulted in the formation of amorphous LaCu2H3.3 hydride. For all Ni-substituted compounds formation of crystalline hydrides with CeCu2 or AlB2 type structure of the metallic sublattice and hydrogen content up to 4.4 at.H/f.u was observed. As found, long-term exposure of the La(Cu0.4Ni0.6)2H4.2 and La(Cu0.35Ni0.65)2H4.0 hydrides to the air or their heating under vacuum up to 100 °C is accompanied by structural transition of the metallic sublattice to Fe2P type structure, previously observed for Pr(Cu,Ni)2-based hydrides.

It is shown that the oxygen-stabilised compounds Zr3Fe(Co)Ox (x = 0–1.0) interact with hydrogen at ambient temperature and pressure forming saturated hydrides with filled Re 3B type structure. The hydrogen storage capacity decreases with increasing oxygen content from 6.7 H/f.u. for Zr3Fe down to 5.35 H/f.u. for Zr3FeO1.0 and from 6.9 H/f.u. for Zr3Co down to 5.3 H/f.u. for Zr3CoO1.0. A small change of the unit cell volumes for the Zr3Fe(Co)Ox parent compounds and a substantial increase of these parameters for the corresponding saturated hydrides were observed with increasing oxygen content. The partial hydrogen-induced lattice expansion, � V/at. H, increases from 2.25 A 3 for Zr3FeH6.7 up to 3.38 A 3 for Zr3FeO1.0H5.35 and from 2.08 A 3 for Zr3CoH6.9 up to 3.25 A 3 for Zr3CoO1.0H5.3. Rietveld refinement using neutron powder diffraction data for Zr 3FeO0.4D6.25 showed a distribution of deuterium atoms and a redistribution of oxygen atoms from octahedral to tetrahedral sites in a similar way as in Zr3NiOxDy. Both 57 Fe Mossbauer spectroscopy and magnetic susceptibility measurements of Fe-containing hydrides indicated weak hydrogen-induced magnetic ordering at low temperatures. The ordering temperatures of Zr3FeO0.2H6.52 and Zr3FeO0.6H6.25 are 105 and 140 K, respectively. © 2004 Elsevier B.V. All rights reserved.

To predict the possibility of formation of η-Zr 4 Fe 2 O x ternary intermetallics of Ti 2 Ni type structure during interaction between ZrFe melt and added oxide, the thermodynamic calculations for ZrFeR x O y systems were done. The influence of oxide addition on the hydrogen absorption properties of Zr 2 Fe alloys was studied.

Hydrogen absorption and desorption properties of the (Hf,Zr) 2 Fe quasibinary alloys (Ti 2 Ni structure type) and the oxygen-stabilised compound Zr 4 Fe 2 O 0.6 with the filled-Ti 2 Ni type of structure, have been studied and discussed in relation to their phase-structural and chemical composition. The crystallographic characteristics, hyperfine parameters and magnetic properties of the hydrogenated alloys have been determined.