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MgH2-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO2, and Ti4Fe2Ox suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH2 and hydrogenation of nano-Ti and Ti4Fe2Ox. The possible reduction of TiO2 during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO2 compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH2 by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH2 down to 74 and 67 kJ/mol for MgH2 modified with nano-TiO2 and Ti4Fe2O0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH2. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti4Fe2O0.3 nanocomposite, ΔHdes = 76 kJ/mol H2 and ΔSdes = 138 J/K·mol H2, correspond to the reported literature values for pure polycrystalline MgH2. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO2 and Ti4Fe2Ox additives. Cycling stability of Mg/Ti4Fe2Ox was substantially improved by introduction of 3 wt% graphite into the composite.

The influence of substitution Pr for La and Ni for Co on hydrogen storage properties of Pr1-xLaxMgNi4-yCoy (х = 0; 0.5, у = 0–3) alloys were studied. The existences of solid solutions have been found. It is shown that the synthesized alloys absorb hydrogen at room temperature and hydrogen pressure 0.1–10 bar. For some of the studied compounds, the formation of hydrides with cubic and orthorhombic structures was found. Hydrogen capacity for Pr1-xLaxMgNi4-yCoy alloys increases with Co content increasing and reaches 6.6 H/f.u. for PrMgNi2Co2. For electrochemical hydrogenation different trend was observed. With increasing of Co content discharge capacity slightly increases only to y = 0.5, and after y > 0.5, decreases. Highest discharge capacity is equal to 305 mА∙h/g for Pr0.5La0.5MgNi3.5Co0.5, and 268 mА∙h/g for PrMgNi3.5Co0.5 at current densities 50 mА/g and 200 mA/g, respectively. Influence of Co and number of activation cycles on HRD value of PrMgNi4-yCoy alloys was investigated. Additionally, obtained results of the electrochemical properties were compared with related compounds.

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 structure of hydrogen sublattice of the Mg2NiH0.3 phase has been studied by computational methods to overcome some ambiguities of the experimental method. Seven types of possible hydrogen sites (marked as X1 to X7) were found. Models with occupation of X1 or X2+X3 sites were found in the literature. Further analysis of geometry and electronic density of the Mg2Ni compound revealed three additional types of voids X4-X6. Models having from 1 to 3 hydrogen atoms per unit cell were tested. The optimization of some of the models with occupied X2 and X5 sites led to shift of hydrogen atoms into the new type of voids X7. It was revealed that two the most energetically favorable ordered models have two types of tetrahedral sites occupied by hydrogen: X3 (H@Mg3Ni) and X7(H@Mg2Ni2). It was suggested that the real structure was a superposition of these models with either two X3 or both X3+X7 voids occupied. The developed structure is close to and has the advantage over one of the models mentioned in the literature, since the trigonal X2 site presented in the literature model is split here into two adjacent tetrahedral X7 sites.

The isothermal section of the Tb–Mg–Co phase diagram at 500 °C has been built on the basis of XRD analysis of forty samples prepared by powder metallurgy. The existence of two ternary compounds Tb4Mg3Co2 and Tb4MgCo was confirmed. The formation of two solid solutions, Tb1−xMgxCo3 (0≤x≤0.4) and Tb1-−xMgxCo2 (0≤x≤0.6), was found for the first time. It is shown that Tb5Mg24 also dissolves a small amount of Co. Other binary compounds do not dissolve the third component. The Tb4MgCo and TbMgCo4 compounds form hydrides (12.7 and 5.3 at.H/f.u. capacity, respectively) that retain the original structure of metallic matrices. Upon thermal desorption the Tb4MgCoH12.7 hydride was stable up to 300 °C and disproportionated at higher temperature. Two other hydrides, Tb4Mg3Co2H∼4 and Tb2MgCo9H12, are unstable in air and decompose into the initial compounds.

The hydrogen absorption properties of the R 4 MgCo compounds ( R =Y, La, Nd, Tb; str. type Gd 4 RhIn; sp.gr. F 4 ¯ 3 m ) have been studied for the first time. It was shown that their hydrogen storage capacity reaches about 2 wt%. At low pressure hydrogenation and moderately elevated temperatures the formed hydrides preserve the original structure of the metallic matrix. The crystal structure of the R 4 MgCoH x hydrides have been determined by XRD. Experimental hydrogen storage capacity (12 at.H/f.u. for Y 4 MgCo) is in good agreement with the theoretically calculated models, which allow also to estimate the distribution of H-atoms in metal lattice. TDS and DSC experiments demonstrated the multistep desorption process. XRD studies of the Tb 4 MgCoH x sample after TDS demonstrated the formation of TbH 2 as the main phase and disproportionation of the parent compound.

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.

First-principles band-structure augmented plane wave +local orbitals (APW+LO) calculations as incorporated in the WIEN2k code as well as X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES) measurements are performed to elucidate the electronic structure of Zr4Fe2O oxide, a very prospective hydrogen-storage material. Total and partial densities of states for Zr4Fe2O are derived from the APW+LO calculations. The XPS valence-band spectrum as well as the XES Zr Lβ2,15, Fe Lα and O Kα bands are measured for Zr4Fe2O0.6 oxide. Our APW+LO calculations show that the O 2p-like states make the major contributions at the bottom of the valence band, whilst the Fe 3d- and Zr 4d-like states are the dominant contributors at the top of the valence band and at the bottom of the conduction band of Zr4Fe2O. Peculiarities of the occupation of the valence band of Zr4Fe2O predicted by the APW+LO calculations are confirmed experimentally by comparison, on the common energy scale, of the XPS valence-band spectrum and the XES bands representing the energy distributions of the Zr 4d-, Fe 3d- and O 2p-like states in the compound under consideration. In addition, the XPS Zr 3d, Fe 2p and O 1s core-level binding energies are derived for Zr4Fe2O0.6 oxide.

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.

Electronic properties of Zr 3 V 3 O oxide, a very promising hydrogen-storage material, were studied both from theoretical and experimental points of view employing the full potential linearized augmented plane wave (FP-LAPW) method as well as X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). Total and partial densities of states of the constituting atoms of Zr 3 V 3 O have been derived from the FP-LAPW calculations. These data indicate that, the O 2p-like states are the dominant contributors in the bottom of the valence band, whilst the top of the valence band and the bottom of the conduction band of Zr 3 V 3 O are dominated by contributions of the V2 3d-like states, with slightly smaller contributions of the V1 3d-like states as well. Significant contributions of the Zr 4d-like states throughout the whole valence-band region and near the bottom of the conduction band are also characteristic of the electronic structure of Zr 3 V 3 O. The XPS valence-band spectra and the XES Zr Lβ 2,15 , V L α and O K α bands have been derived and compared on a common energy scale for Zr 3 V 3 O and Zr 3 V 3 O 0.6 oxides. This comparison of the experimental spectra was found to be in excellent agreement with the results of the FP-LAPW calculations. In addition, the XPS Zr 3d, V 2p and O 1s core-level binding energies have been measured for Zr 3 V 3 O and Zr 3 V 3 O 0.6 oxides.

Two new ternary intermetallic compounds, CeMgCo4 (C15b pseudo-Laves phase, MgCu4Sn type) and Ce2MgCo9 (substitution derivative of PuNi3 type) were synthesized by mechanical alloying method. The structural and hydrogenation properties of these compounds were studied by X-ray diffraction and Pressure–Composition–Temperature measurements. Both compounds absorb hydrogen at room temperature and pressures below 10 MPa forming hydrides with maximum compositions CeMgCo4H6 and Ce2MgCo9H12. Single plateau behavior was observed in P–C isotherm during hydrogen absorption/desorption by Ce2MgCo9 alloy. The CeMgCo4–H2 system is characterized by the presence of two absorption/desorption plateaus corresponding to formation of β-CeMgCo4H4 and γ-CeMgCo4H6 hydride phases. The structure of β-hydride CeMgCo4H(D)4 was determined from X-ray and neutron powder diffraction data. In this structure initial cubic symmetry of CeMgCo4 is preserved and hydrogen atoms fill only one type of interstitial sites, triangular MgCo2 faces. These positions are occupied by 70% and form octahedron around Mg atom with Mg–D bond distances 1.84 Å.

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.

By means of infrared and Raman spectroscopy with the use of element and thermogravimetry analyses, the structure of the product of cross oxidation polycondensation of aniline and o -nitroaniline in 1 M aqueous solution of HCl with the use of 0.11 M (NH 4 ) 2 S 2 O 8 as the oxidant has been studied. The most possible structure of the product (four-chain fragment with ‘head-to-tail’ junction of monomer units) has been proposed. This fragment has been modeled with the use of quantum chemistry and, in agreement with the experimental results, can be described as emeraldine with not more than 25% oxidation level. The synthesized products were used as positive electrodes in a couple with Mg negative electrode: Mg |2 M MgCl 2 in H 2 O + CH 3 CN (3:1)| PANI ( co - polymer ) The specific capacity of 420.74 A h kg −1 and specific energy of 424.95 W h kg −1 was characteristic for the 8:2 co-polymer used as a cathode at 1.65 V tension of the open circuit and 0.75 V average voltage.

Polyaniline and its co-polymers with o-nitroaniline have been synthesized by the chemical oxidation of aniline and o-nitroaniline in 0.22 M (NH4)2S2O8+1 M HCl aqueous solution. The yield of product was in the range 51.1–75.2%. Considering a close similarity between oxidation in chemical and electrochemical processes, the nature of the intermediates of polyaniline synthesis have been proposed based on cyclic voltammograms. The first stage of the polyaniline synthesis in both electrochemical and chemical oxidation of aniline was found to be the formation of C6H5–NH2+ cation-radical through the transfer of electron from 2s energy level of nitrogen atom to the electrode during electrochemical oxidation. The electronic structure of the initial components and intermediate particles of different charge and multiplicity have been calculated and the most energy suitable structures and the paths of their transformations have been determined. The mechanism of the step-by-step condensation of aniline includes one-electron aniline oxidation into corresponding cation-radical with subsequent recombination with the prior radical. The recombination proceeds according to the ‘head-to-tail’ type, which fact is indicated by the minimum value of the total and electronic energy equal to −205 402 and −1 204 688 kJ mol−1, respectively. Nitroanilines by themselves condense under neither chemical nor electrochemical oxidation, although there are no reasons for it regarding their electronic structures.

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.

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.