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The Zn/MnO2 battery is safe, low cost and comes with a high energy density comparable to Li-ion batteries. However, irreversible spinel phases formed at the MnO2 electrode limits its cyclability. A viable solution to overcome this inactive phase is to use an aqueous ZnSO4-based electrolyte, where pH is mildly acidic leading to a different reaction mechanism. Most importantly, the addition of MnSO4 achieves excellent cyclability. How accessible Mn2+ ions in the electrolyte enhances the reversibility is presented. With added Mn2+, the capacity retention is significantly improved over 100 cycles. Zn2+ insertion plays an important role on the reversibility and a hydrated layered Zn-buserite structure formed during charge is reported. Furthermore, Zn4SO4(OH)6 · 5H2O precipitates during discharge but is not involved in the electrochemical reaction. This precipitate both buffers the pH and partly insulates the surface. Described in operando study show how the phase transformations and the failure mechanisms depend on the presence of Mn2+-ions in the electrolyte. These results give insight necessary to improve this battery further to make it a worthy contender to the Li-ion battery in large scale energy storage solutions.

Nickel-Metal Hydride batteries (NiMH) have long cycle life. But in the end corrosion of the metal hydride is detrimental for life expectancy. Corrosion reduces the metal hydride capacity, but more severely it consumes water in the electrolyte resulting in increased internal resistance, which is the main cause for cell failure. The corrosion, further, evolves hydrogen, causing an unbalance between anode and cathode, leading to premature internal pressure increase when the cells are approaching end of charge. This accelerates the drying out, if the cells vent through the safety valve. In this study, a controlled addition of oxygen was used to rebalance the electrodes and replenish the electrolyte – as the added oxygen reacts with hydrogen that was formed during the corrosion process. Thus, the two most detrimental factors in cell ageing can be mitigated. To fully restore the electrolyte content as well as electrode balance, both oxygen and hydrogen are needed to compensate for the loss to hydroxide ions OH− formed in the corrosion process. A proper optimization of the gas additions combined with a cell design including an excess amount of MH-alloy to compensate for the corrosion can substantially increase the cycle life of NiMH batteries.

Alkaline iron electrodes present some challenges for use in secondary batteries that are associated with low coulombic efficiency and discharge utilization. Low coulombic efficiency is correlated t ...

Activation behavior of an AB 2 , namely (Ti 0.36 Zr 0.64 ) (V 0.15 Ni 0.58 Mn 0.20 Cr 0.07 ) 2 Laves phase alloy, was investigated with regards to; particle size, ball milling and hot alkaline treatments. Galvanostatic cycling in open cells showed that an untreated alloy initially had almost no capacity, but reached a value of 220 mAh/g after 14 cycles. Experiments with different particle sizes showed that coarse particles activate faster yielding an improved capacity. In terms of activation more pronounced effect was obtained with boiling the alloy powder in a hot KOH solution. A capacity in excess of 300 mAh/g is reached in the first cycle after a 20 min treatment. The capacity was highest after 80 min, yielding a value of 390 mAh/g well above that expected from the gas-phase storage in the alloy. This was attributed to the formation of rough surface in the powder, which may stabilize hydrogen bubbles allowing pressures above 1 atm to be reached locally in the surface.

A surface treatment method has been developed to activate the surface of an AB(5) type (La-20 Ce-7 Pr-1 Nd-4 Al-2 Mn-5 Co-6 Ni-55) alloy. In the process the surface is covered with a porous surface ...

A large active specific surface area is a very efficient way to improve charge/discharge rate capabilities of metal hydride electrodes. This can be promoted by washing oxidized, pre-hydrided AB 5 alloy particles in a hot KOH solution. This leads to magnetization of the alloy powder, making it possible to lift the powder with a simple permanent magnet, indicating that nickel liberated in the surface layer is essential for the improved kinetics. The overpotential of each step in the discharge process is analysed in detail using both half cells and sealed cells. The electrode polarisation depends inversely on the current density per active area. Untreated and treated alloy powders have similar current densities per active area, but treated powder exhibits superior kinetics due to its larger surface area. A NiMH cell with a surface-treated alloy has very good rate capability, only limited by the kinetics of the Ni electrode. At low overpotential, the electrochemical reaction gives the main contribution to the electrode polarisation. If the discharge current is increased to give an overpotential of more than 0.1 V, concentration polarisation will become important and a limiting factor at high current densities.

A combined study using neutron diffraction, inelastic neutron scattering, and first-principles calculations describe cobalt with a very low formal oxidation state of (−I) in a slightly distorted tetrahedral Co(−I)H4-complex in BaH2Mg5[Co(−I)H4]2 and in the structurally related RbH2Mg5[Co(−I)H4 Ni(0)H4]. This indicates that the electron “back donating” effect via the polarizable hydride ions to the counterions in the solid state hydrides, can be compared to more conventional “back bonding” able to reduce the oxidation state down to −I. The hydrides were synthesized by hot sintering of transition metal powders with corresponding binary alkali- and alkaline earth hydrides. In the similarly synthesized SrH2Mg2[Co(I)H5], cobalt is formally + I-valent, showing a high sensitivity to differences in the counterion framework, which can also influence electrical properties.

The structure of two isomorphous high pressure synthesized metal hydrides MgZr 2 H 6 and MgNb 2 H 6 is described in the R -3m space group with the hexagonal unit cells a = 3.3592(2) A, c = 25.131(3) A and a = 3.2901(9) A, c = 23.09(1) A, respectively (Z = 3). The metal atom structures were refined from powder XRD data, with Mg in the 3 a site (0, 0, 0,) and Zr and Nb in the 6 c site (0, 0, z) with the only refineable atomic parameter z = 0.225 and z = 0.220, respectively. As only a minute sample could be produced in the anvil cell, a conventional structural determination with neutron diffraction on a deuterated sample could not be made. Instead formation energies of the all proposed structural models were calculated using density functional theory (DFT) for accessing the most stable structure of the lowest energy level. The measured lattice parameters and the refined atomic positional parameters from the powder X-ray diffraction pattern were reproduced well by the DFT calculation. The hydrogen atoms were found to fully occupy an octahedral site coordinated by 3Mg and 3Zr atoms, a tetrahedral site coordinated by 4Zr atoms and another tetrahedral site coordinated by 3Mg and 1Zr atoms. Chemical, mechanical and dynamical stabilities of the MgZr 2 H 6 structure were further discussed based on a chemical potential equilibrium diagram constructed from the calculated formation energies, calculated elastic constants and phonon spectra respectively. It is also shown that a high pressure increases the stability of the new hydrides.

Electrochemical reactions at both positive and negative electrodes in a nickel metal hydride (Ni-MH) battery during charge have been investigated by in situ neutron powder diffraction. Commercially available beta-Ni(OH)(2) and LaNi5-based powders were used in this experiment as positive and negative electrodes, respectively. Exchange of hydrogen by deuterium for the beta-Ni(OH)(2) electrode was achieved by ex situ cycling of the cell prior to in situ measurements. Neutron diffraction data collected in situ show that the largest amount of deuterium contained at the positive electrode is de-intercalated from the electrode with no phase transformation involved up to similar to 100 mA h/g and, in addition, the 110 peak width for the positive electrode increases on charge. The negative electrode of composition MmNi(3.6)Al(0.4)Mn(0.3)Co(0.7), where Mm = Mischmetal, exhibits a phase transformation to an intermediate hydride gamma phase first and then to the beta phase on charge. Unit cell dimensions and phase fractions have been investigated by Rietveld refinement of the crystal structure.

Rechargeable batteries are essentially unstable systems with respect to charging/discharging. The main electrode reactions of all battery chemistries are well known but are valid and reversible only at small currents. When batteries are used with nonzero current, gradients in voltage, current, and temperature will arise and initiate a number of less understood parasitic reactions. If all these rather complicated and interconnected reactions are not reversible upon charging/discharging, the battery will derail after a number of charging/discharging cycles. This article describes two ways to improve performance. One is to choose applications where the battery is not deeply discharged, such as in hybrid electric vehicles. In battery electric vehicle applications, this would correspond to working with a significantly oversized battery. The second way is to improve uniformity and quality of design and materials of metal hydride electrodes; however, this will also drastically increase cost, and for a battery application, the total throughput of available energy over the lifetime cost of the battery must be maximized. Uniform metal hydride particles with a large and uniform reaction surface are examples of how to increase battery performance by making the electrodes work under more ideal conditions, which slows down the deteriorating influence from the parasitic reactions.

Usually metallic films for metal hydrides research applications are deposited on hard and flat substrates such as silicon, magnesium oxide, fused silica or quartz glass. With the film thickness increases all films during hydrogenation deals with typical problems such as film brittleness and cracking. In this paper we demonstrate that metal Mg–Ni films for hydrogen storage can be successfully deposited on to the flexible low surface energy expanded PTFE substrates. The received results for soft substrates (expanded PTFE) are compared to films being deposited on crystalline silicon substrate with and without plasma pretreatment. It is observed that different interface zone between substrate and film has great affect on both film crystallinity and its reaction with hydrogen. It is also demonstrated that modifying surface of the substrate might have affect on film microstructure before and after hydrogenation.

The kinetics of oxygen and hydrogen gas recombination on the metal hydride electrode in a NiMH cell is investigate as function of gas pressure, temperature, cycling and a hydrophobic additive. Both oxygen and hydrogen gas recombination rates in the NiMH cells are found to be linearly proportional to the gas pressure, indicating that surface processes with a constant number of reaction sites could be the rate limiting step. The rate of oxygen gas recombination is, however, more than 10 times faster than that of hydrogen at the same pressure. As the temperature increases, both reaction rates increase very rapidly, even though the solubilities of the gases in the KOH electrolyte decrease with temperature. The activation energies for oxygen and hydrogen gas recombination are 32.6 kJ mol −1 and 27.0 kJ mol −1 , respectively. The reactions rates also increase as the cells are cycled. This indicates, that the number of active sites increases, as the surface area increases, due to the continuously decrepitating of the alloy particles. The hydrogen gas absorption rate increases dramatically after a large number of cycles due the drying of the MH electrode. A hydrophobic PTFE additive in the MH electrode can significantly improve hydrogen absorption efficiency in NiMH cells.

A number of new quaternary iridium based hydrides and deuterides AAeIrH/D(6) (where A = Na and K; Ae = Ca, Ba, Sr, and Eu) have been synthesized by direct combination of the alkali, alkaline earth or europium binary hydrides/deuterides and iridium powder. The crystal structures were determined by Guinier-Hagg X-ray and neutron powder diffraction and a new cubic structure type was found. The structure is described in space group F (4) over bar 3m, but related to the K(2)PtCl(6) type structure. The new structure can be described as consisting of cubes of A(+) and Ae(2+) ions, where the A(+) and Ae(2+) ions alternates so that they occupy opposite corners in the cube. Every second cube contains a regular octahedral [Ir(III)H(6)](-3)-complex and the adjacent is empty. Solid-state IR spectroscopy was used to determine the Ir-allowed stretching and bending frequencies for the [Ir(III)H(6)](-3) complex with different counterions. These were also compared with the corresponding stretching frequencies for Fe(II)H(6), Ru(II)H(6), Os(II)H(6), and Ir(II)H(5) complexes in similar solid state hydrides, which increased when going from Fe through Ru, Os to Ir. The frequencies scaled almost linearly with the inverse of size of the cube surrounding the complexes but showed no significant dependence of the formal oxidation state or the experimentally obtained Ir-D distances. However, this was mainly because of difficulties in obtaining enough accurate atomic positions. The ab initio DFT calculations could reproduce the stretching frequencies within a few 10 cm(-1), indicating that experimental vibrational frequencies offer a sensitive test of DFT results.

The first examples of a new class of gallium hydride clusters with direct Ga-Ga bonds and common hydrocarbon structures are reported. Neutron powder diffraction was used to find a Ga[GaH(3)](4)(5-) ...

Reaction of single-walled carbon nanotubes (SWNTs) with hydrogen gas was studied in a temperature interval of 400-550 °C and at hydrogen pressure of 50 bar. Hydrogenation of nanotubes was observed for samples treated at 400-450 °C with about 1/3 of carbon atoms forming covalent C-H bonds, whereas hydrogen treatment at higher temperatures (550 °C) occurs as an etching. Unzipping of some SWNTs into graphene nanoribbons is observed as a result of hydrogenation at 400-550 °C. Annealing in hydrogen gas at elevated conditions for prolonged periods of time (72 h) is demonstrated to result also in nanotube opening, purification of nanotubes from amorphous carbon, and removal of carbon coatings from Fe catalyst particles, which allows their complete elimination by acid treatment.

The hydrogen storage capacity of Mg–Ti–H films is approximately five times that of conventional metal hydride electrodes in NiMH-batteries. Mg and Ti are considered to be immiscible in the bulk and the ambient pressure phase diagram of Mg and Ti indicates that no binary stable bulk compounds are formed. However, in the presence of hydrogen, an Mg–Ti–H phase has been obtained by Kyoi et al. using a high pressure synthesis – where magnesium hydride is compacted with different TM-hydrides in an anvil cell at pressures of the order several GPa (4–8 GPa) and at a temperature of 873 K. In this work, we have proved the feasibility of in situ powder diffraction using the Paris–Edinburgh high pressure cell for the observation of structural changes on this system and we propose modifications to improve the output of the experiment.

Lab-size rechargeable metal hydride–air (MH–air) cells with a gas management device were designed in order to minimize the loss of electrolyte. An AB5-type hydrogen storage alloy was used as anode materials of the MH–air. The thickness of the metal hydride electrodes was in the range of 3.0–3.4 mm. Porous carbon-based air electrodes with Ag2O catalysts were used as bi-functional electrodes for oxygen reduction and generation. The electrodes were first examined in half-cells to evaluate their performance and then assembled into one MH–air cell. The results showed the good cycling stability of the rechargeable MH–air cell with a capacity of 1990 mAh. The discharge voltage was 0.69 V at 0.05–0.1 C. The charge efficiency was about 90%. The specific and volumetric energy densities were about 95Wh kg−1 and 140 Wh L−1, respectively.

By substituting Si − with (Al–H) − in trigonal BaSi 2 , a series of hydrides has been synthesized with compositions BaAl 2− x Si x H 2− x (0.4 x 2 and BaAl 2 H 2 are conductors whereas the intermediate BaAlSiH is a semiconductor. The cell parameters for the trigonal cell vary linearly as a function of the Si − /(Al–H) − substitution in this MgB 2 related structure type, opening up the possibility of continuously tunable electric properties.

Hydrogen storage alloy MmNi(3.6)Mn(0.4)Al(0.3)Co(0.7) (MH) was tested as anode material in a metal hydride-air cell. The cathode was a non-noble metal air electrode based on a mixture of perovskite ...

The band gap in semiconducting Mg2NiH4 was found to be dependent on subtle structural differences. This was discovered when investigating if thin film samples of Mg2NiH4 could be used in a switchable mirror or window device by utilizing a high to low temperature transition at about 510K. In powder samples; this transition between an FCC high temperature phase, with dynamically disordered NiH4-complexes, and a monoclinic distorted low temperature phase, with ordered Mg2NiH4-complexes, has been demonstrated in a mechanical reversible conductor-insulator transition (Blomqvist and Noreus (2002) [7]). Black monoclinic Mg2NiH4 powders were found to have a band gap of 1.1 eV. Pressed tablets of black monoclinic Mg2NiH4 powders are conductive, probably from doping by impurities or non-stoichiometry. Thin film Mg2NiH4 samples were produced by reacting hydrogen with magnetron sputtered Mg2Ni films on quartz glass or CaF2 substrates. The Mg2NiH4 films on the other hand were orange, transparent with a band gap of 2.2 eV and a cubic unit cell parameter almost identical to the disorder HT phase but with lower symmetry. If black Mg2NiH4 powder is heated above the phase transition at 510K and subsequently cooled down, the conductivity is lost and the powder turns brown. After this heat treatment TEM pictures revealed a multiple stacking fault having a local pseudo-cubic arrangement separating regions of monoclinic symmetry. The loss of conductivity and colour change is attributed to a higher band gap in the strained areas. The structure on each side of the stacking fault is related by a mirror plane as a consequence of the possibility for the NiH4-complexes to order with different orientations. This leads to a mismatch in the long range ordering and strain is probably creating the stacking faults. Strain is important for forming the cubic modification. A severely strained film was revealed with optical microscopy in reflected light, indicating that strain prevents it from relaxing back into the monoclinic structure. This was supported by multiple twinned red translucent Mg2NiH4 crystals grown with cubic symmetry at elevated temperatures in a LiH flux. When cooled to ambient conditions, the ""crystals"" had the same cubic symmetry as the films, probably held together by their neighbours. When they were ground to a fine powder to prepare TEM samples, they relaxed and reverted back to the conventional monoclinic unit cell. This interesting nanoscale modulated resistance could possibly be developed into novel memory devices if properly controllable.

In situ high-pressure angle-dispersive synchrotron X-ray diffraction and high-pressure mid-infrared (IR) spectrum measurements of C60H18 were carried out up to 32 and 10.2 GPa, respectively. Our diffraction data indicated that the fcc structure of C60H18 was stable up to 32 GPa. The bulk modulus B0 was determined to be 21 ± 1.16 GPa, about 40% higher than that of C60. The C−H vibrations still existed up to 10.2 GPa, and the vibrational frequencies decreased with increasing pressure. IR-active vibrational frequencies and their corresponding eigenvectors of C60H18 were simulated by DMOL3. The effects of the hydrogen atoms attached to the fullerene molecular cage on the stability of the structure under high pressure are discussed.

Two kinds of metal hydride alloys as the bi-functional catalyst concept for hydrogen generation and oxidation in hydrogen-diffusion electrodes were investigated. The AB5-type hydride electrode shows much higher catalytic activities than the Zr-based AB2-type hydride electrode. However, the activity of Zr-based hydride electrodes can be improved only after removal of zirconium oxides on surface by a 1.0 M HF solution. The experiments demonstrated that the both metal-hydride hydrogen-diffusion electrodes for cycles of hydrogen generation (12 h) and oxidation (12 h) had good stability under the current densities of 100 and 50 mA/cm2, respectively. The results also showed that small amounts of oxygen below 500 ppm and moisture up to 145,000 ppm in the hydrogen gas have little effect on the activity. It indicated that the hydride alloys as the non-noble-metal bi-functional catalysts in a reversible MH-air fuel cell have potential applications. Keywords: Metal hydride, Catalyst, Hydrogen, Gas-diffusion electrode, Air-MH fuel cell

Nanocrystalline Mg–Ni alloy powder was produced via the magnetron co-sputtering technique. The samples obtained showed crystallisation of Mg and Mg 2 Ni and an overall morphological heterogeneity with different enriched zones, as evidenced by microstructural analysis. A number of pressure-composition-isotherms (PCI) were carried out on these samples at a range of temperatures. Following hydrogenation, different hydrogen sorption behaviours were observed depending on the samples’ relative composition in terms of Mg and Mg 2 Ni. Hydrogenation/dehydrogenation cycling is believed to have caused modification of the nanocrystalline materials investigated. The derived dissociation enthalpies of the corresponding hydrides, MgH 2 and Mg 2 NiH 4 , were found to be coherent with values reported in literature, with the exception of two samples. In addition, one of these samples showed a third plateau pressure pointing to the presence of a hydrogenated metastable phase.

Structural, thermodynamical and electrochemical properties of metallic hydrides belonging to the pseudo-binary family A–Mg–Ni (A: rare earths) are reviewed and compared. Technology aspects of bipolar cells are also discussed.

The vibrational properties of the recently discovered aluminum hydrides SrAl2H2 and SrAlSiH have been investigated by means of inelastic neutron scattering (INS) and first-principles calculations. Both compounds contain Al-H units being part of a two-dimensional polyanionic layer, [(AlH)(AlH)]2- and [Si(AlH)]2-, respectively. The INS spectrum of SrAlSiH is characterized by very weakly dispersed Al-H modes with well-resolved overtones, while SrAl2H2 yields a solid-state dispersed phonon spectrum. The frequency of the stretching mode of the Al-H unit in SrAlSiH is the hitherto lowest observed for a terminal Al-H bond. At the same time, SrAlSiH displays the highest decomposition temperature known for an aluminum hydride compound. It is proposed that the stability of solid-state aluminum hydrides correlates inversely with the strength of Al-H bonding.

Mg–Ni films were grown on a silicon substrate using two magnetron sputter deposition sources and simultaneous Ar ion irradiation. X-ray diffraction microstructure and phase composition, EDX elemental composition and atomic force microscopy surface topography analysis showed that under low-energy Ar ion irradiation (bias voltages from 0 to −120 V), the Mg2Ni phase was dominant and on the contrary with the increase of ion energy (bias voltages from −120 to −200 V), the MgNi2 phase appeared. The Mg content changed from 63 at% down to 42 at% in films grown under bias voltages of 0 and −200 V, respectively. During hydrogenation at 8 bar, 270 °C for 3 h, films with a dominant phase of Mg2Ni were transformed into Mg2NiH4. Hydrogen in MgNi2 films was mainly in interstitials and tended to form bubbles.

Mg2Na2NiH6 was synthesized by reacting NaH and Mg2NiH4 at 310 degrees C under hydrogen pressure. The novel structure type was refined from neutron-diffraction data in the orthorhombic space group Pnma (No. 62), with unit cell dimensions of a = 11.428(2), b = 8.442(2), and c = 5.4165(9) Angstrom and a unit cell volume = 523 Angstrom(3) (Z = 4). The structure can be described by (Mg2H2)(2+) layers intersected by (Na2NiH4)(2-) layers. The [NiH4](4-) complex is approximately tetrahedral, indicating formal zerovalent nickel. This is the first example of a solid-state hydride where a [NiH4](4-) complex is directly stabilized by alkali metal ions instead of the more polarizing Mg(2+) ions. A rather long nickel-hydrogen bond distance of 1.65 Angstrom indicates a weaker Ni-H bond as a result of the weaker support from the less polarizing alkali metal counterions.

Effect of long-term overcharge and operated temperature on perfomance of rechargeable NiMH cells

Evaluation of nano-crystal sized alpha-nickel hydroxide as an electrode material for alkaline rechargeable cells

Two new magnesium–niobium hydrides, Mg6.5NbH∼14 with a face centred cubic unit cell a = 9.548(1) A (space group F m 3 ¯ m (No. 225), Z = 4, V = 870.49 A3) and MgNb2H∼4 with a C-centred monoclinic unit cell a = 5.685 (4) A, b = 3.2914(6) A, c = 7.924(2) A and β = 103.82(3)° (space group C2/m (No. 12), Z = 2, V = 143.98 A3), were synthesized by reacting MgH2 and Nb in a high-pressure anvil cell at 8 GPa and 600 °C. The structures of the new hydrides were refined from synchrotron radiation data. Mg6.5NbH∼14 has a structure related to Mg7TiH∼16 (a = 9.532(2) A space group F m 3 ¯ m (No. 225)) (D. Kyoi, T. Sato, E. Ronnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noreus, T. Sakai, J. Alloys Compd. 372 (2004) 213). Where the metal atom structure is of the Ca7Ge type. Mg6.5NbH∼14, however, has some vacancies on one of the magnesium sites similar to the recently discovered Mg6VH∼14 (D. Kyoi, T. Sato, E. Ronnebro, Y. Tsuji, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noreus, T. Sakai, J. Alloys Compd. 375 (2004) 253). The metal atoms in MgNb2H∼4 have the AuTe2 type structure.

Nickel-metal hydride (NiMH) batteries offer some advantages in the aspects of power, cycle life and environment. However, they are encountering challenge from other rechargeable batteries such as Li-ion batteries. In this paper, the possibility of performance improvements and cost reduction of NiMH batteries were analyzed. Some approaches to improve energy densities and reduce the battery cost were discussed. An attempt to increase the energy density of NiMH batteries was made through improvements in the specific capacity of rare earth-based AB 5 -type hydride alloys in the negative electrodes and capacity enhancement of nickel hydroxide electrode. A relatively lower cost small cylindrical NiMH cell was constructed and its properties were evaluated.

(Chemical Equation Presented) Stable and semiconducting: Polyanionic hydrides represent a new class of main-group-metal hydrides with unforeseen hydrogen-coordination environments. The hydrogen atom in SrAlSiH is attached exclusively to Al and forms part of a layered [AlHSi]2- polyanion (see figure). This material is the first narrow bandgap semiconductor hydride and combines the high thermal stability of saline hydrides with the air and moisture stability of interstitial transition-metal hydrides.

[chemical structure: see text]. C60H18 has been produced by hydrogenation of C60 at 100 bar H2 pressure and 673 K for 10 h. We have investigated the crude material without any purification by use of 1H NMR, 13C NMR, and IR spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry. We show that the crude material consists of 95% of the C3v isomer of C60H18.

Hydrides with the perovskite structure: General bonding and stability considerations and the new representative CaNiH3

The structure of the title compound was refined from neutron powder diffraction data in the cubic space group P-43m (215). The unit cell contains one formula unit with 3 structurally equivalent [Co(I)H-5](4)-complexes as well as 4 interstitial hydride (H-) ions. The presence of the larger and less polarizing Sr2+ ions weakens the bond in the complexes and probably also the stability of the structure. Attempts to synthesize the corresponding Ba analogue failed in contrast to using smaller and more polarizing Ca2+ and Yb2+ counterions. AuthorCount:3

The stability of a negatively chargedpolyethylene-structured [GaH2]nn−cluster ion was inves-tigated by varying the K+/Rb+ratio in (KxRb1−x)n[GaH2]n(0≤x≤1). Neutron, X-ray, and IR spectroscopies wer ...

Development of a non-noble-metal catalyst electrode is an important issue in the research and development of fuel cells. The catalytic activity and durability of AB5-type hydrogen storage alloys used in hydrogen-diffusion electrodes for alkaline fuel cells are evaluated. The experiments demonstrate that the activity and stability for hydrogen oxidation is greatly improved if the particle size is decreased from 5−30 to 1−10 μm. This also improves the electrode stability. SEM and XRD analyses show that no disintegration of the smaller catalyst particles is observed in long-term tests. A novel H2/hydride//perovskite/O2 alkaline fuel cell was constructed as a non-noble-metal fuel cell concept. The results showed that both gas-diffusion electrodes had high catalytic activities and good stability. A maximum power density of 54 mW/cm2 was achieved at 120 mA/cm2 when using H2/O2 and 49 mW/cm2 at 100 mA/cm2 when using H2/air.

This thesis is focused on structural investigations of novel interstitial ternary alkaline-earth–transition metal hydrides formed by immiscible metals that are held together by the presence of hydrogen atoms. The contents are divided in two parts: Metastable Mg-based hydrides, in which transition-metal additives improve hydrogen storage properties; and a new Ca–Ni hydride, CaNiH3, with a perovskite structure, in which hydrogen enables a Ca:Ni =1:1 ratio although no such alloy composition exists.New ternary Mg-based hydride with an additive of either Ti, Zr, Hf, V, Nb or Ta were synthesized in a high-pressure anvil cell at 4–8 GPa and 600 oC. All hydrides had an FCC metal atom structure with an axis of about 4.8 A. This corresponds to an assumed high-pressure MgH2 phase that is stable at a pressure above 3.6 GPa (P. Vajeeston, P. Ravindran, A. Kjekshus, H. Fjellvag, Phys. Rev. Lett. 89 (2002) 175506). Ordering of the metal atoms in the lattice leads to a doubling of the FCC unit cell all axis, and the metal atom structure of the hydrides (excluding Zr doped hydride) can be described by the Ca7Ge-type structure. Hydrogen atom arrangements at two tetrahedral sites were found from a powder X-ray diffraction pattern obtained with synchrotron radiation. One of the tetrahedral sites coordinates one TM atom and three Mg atoms, and the other coordinates four Mg atoms.CaNiH3 crystallizes with a cubic unit cell: a = 3.544(1) A, V = 44.51(4) A3 and Z = 1 in space group Pm-3m (No. 221). The hydrogen positions were determined from CaNiD3, using powder neutron diffraction, and the hydride was of an ideal perovskite ABH3 type. It was stabilized by a polyanionic framework of [NiH3]2- counterbalanced by Ca2+ cations. Total-energy calculations of the electronic structure showed that the hydride was a metallic conductor due to a half-filled Ni-H antibonding eg band.

A magnesium based titanium doped hydride was prepared in a high-pressure anvil cell by reacting a mixture of MgH2 and TiH1.9 at 8 GPa and 873 K. The metal structure has a Ca7Ge type structure (a=9.532(2) A, space group Fm 3 m (no. 225), Z=4, V=866.06 A3). The refined metal atom composition Mg7Ti was almost in line with EDS analysis. This means that the new magnesium–titanium hydride has a structure that is more related to TiH1.9 than to MgH2. The thermal properties of the new compound were also studied by TPD analysis. The new hydride, Mg7TiHx exhibits 5.5 mass% (x≈12.7) and decomposes into Mg and TiH1.9 upon releasing 4.7 mass% of hydrogen around 605 K, that is at a 130 and 220 K lower desorption temperature compared to MgH2 and TiH1.9, respectively.

Very small amounts of air have been observed to oxidise the surface of the ternary hydride Mg 2 NiH 4 . This leads to a profound color change from orange to brown and to a sealing of the surface with respect to desorption, which can be mistaken for a stability increase of the bulk hydride. An unoxidised sample starts to desorb significant amounts of hydrogen when it is heated above 450 K, as is expected from the van’t Hoff relation for the hydride. A sample with a slightly oxidised surface may need to be heated above 700 K before hydrogen is released. The subsequent hydrogen desorption is, on the other hand, very rapid at this elevated temperature.

With modern X-ray diffraction refinement methods it was possible to identify the first phase in the Mg–Cr–H system, Mg 3 CrH ∼6 from minute sample volume in spite of poor crystallinity and coexisting impurity phases. The new hydride was synthesized at 8 GPa in a high-pressure multi-anvil cell at 873 K. An orthorhombic unit cell was found with a =9.635(2) A, b =8.615(2) A, c =4.8040(7) A, space group Pnam (no. 62), Z =4, V =398.77 A 3 . Chromium is surrounded by a distorted cube of magnesium atoms with average Cr–Mg distances of 2.9(1) A. The hydrogen positions were not possible to determine, as only a small sample amount could be prepared. If the metal atom structure is compared to already known metal hydrides it can be concluded that Mg 3 CrH ∼6 consists of chromium hydrido complexes counterbalanced by magnesium ions. A high hydrogen content was confirmed with thermal desorption spectroscopy.

Zr7Ni10 has three hydrogen occlusion phases, α, β and γ, and the following unusual features are known for the phase transitions in the Zr7Ni10–H2 system: (1) The intermediate hydride phase (β) appears only during dehydrogenation but not during hydrogenation, and (2) The continuous hydrogen solid solution phase (α) exhibits a much higher hydrogen solubility during hydrogenation than during dehydrogenation. In order to clarify the mechanism about the difference in the hydrogen solubility of the α phase, the relation between the pressure-composition isotherms and corresponding structural change has been examined by a conventional volumetric method and X-ray diffraction. Through the examination, we discovered that the crystal structure of the α phase, which undergoes hydrogenation followed by dehydrogenation, is different from that of its pure metal phase, where the crystal structure of the dehydrogenated α phase changes from an orthorhombic structure to a tetragonal structure. The conditions causing the structural change were then examined, and it has been found that the α phase maintains its original orthorhombic structure as long as it is hydrogenated so as not to absorb enough hydrogen to change it to the hydride with a higher hydrogen content (γ). The phenomenon can be understood as one of the hydrogen-assisted phase transitions such as hydrogen-induced amorphization (HIA) in the sense that the phase transition requires hydrogenation under special conditions.