Due to their high hydrogen content, tetraborohydrides are discussed as potential synthetic energy carriers. On the example of lithium borohydride LiBH4, we discuss current approaches of direct, solvent free synthesis based on gas solid reactions of the elements or binary hydrides and/or borides with gaseous H2 or B2H6. The direct synthesis from the elements requires high temperature and high pressure (700°C, 150bar D2). Using LiB or AlB2 as boron source reduces the required temperature by more than 300 K. Reactive milling of LiD with B2H6 leads to the formation of LiBD4 already at room temperature. The reactive milling technique can also be applied to synthesize other borohydrides from their respective metal hydrides.

The direct and solvent-free synthesis of yttrium borohydride was achieved by reactive ball milling of yttrium hydride in diborane/hydrogen atmosphere. The product contains only the solid elemental hydride as remaining contaminant. Yields above 75% were obtained. The product crystallizes in the cubic alpha phase and releases hydrogen above 460 K. The decomposition was measured by in situ X-ray diffraction and the hydrogen release was monitored gravimetrically in conjunction with infrared gas analysis. No diborane was detected during the decomposition. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We report on maximum entropy method study of the experimental atomic and ionic charges of LiBD4 in its low-temperature orthorhombic phase. Synchrotron radiation x-ray powder diffraction data, neutron powder diffraction data, and density functional calculations were used. The atomic and ionic charges were determined for both experimental and theoretical results using the Bader analysis for atoms in molecules. The charge transfer from the Li cation to the BD4 anion is 0.86(+/- 9) e, which is in good agreement with the ab initio calculated value of 0.895 e. The experimental accuracy was determined considering the differences between results obtained for data collected at 10 and 90 K, different experimental setups (high-resolution diffractometer or image plate diffractometer), and different structural models used for the prior density distributions needed for accurate maximum entropy calculations (refined using only synchrotron radiation x-ray powder diffraction data or combined with neutron powder diffraction data).

We present the design and construction of a high-pressure (200 bars) and high-temperature (600 °C) x-ray diffraction (XRD) cell for the in situ investigation of the hydrogen sorption of hydrides. In combination with a pressure, composition, and temperature system, simultaneous XRD and volumetric measurements become accessible. The cell consists of an x-ray semi-transparent hemispherical beryllium (Be) dome covering a heatable sample stage, which simultaneously allows sample temperatures of up to 600 °C in an applied hydrogen atmosphere of up to 200 bars. The system volume is as low as possible to maximize the precision of the volumetric measurements. Due to the high thermal conductivity of hydrogen, and in order to preserve the mechanical stability of the beryllium, the cell is water cooled. Its operability was studied on the example of the hydrogen absorption of Mg(2)Ni. The advantages and limitations of the proposed design are discussed.

We have solved and refined the crystal structure of the orthorhombic gamma-phase of Ca(BD4)(2) by combined synchrotron X-ray powder diffraction, neutron powder diffraction, and ab initio calculations. Among five structural candidates giving the same quality of the fit of the diffraction data, the structural model with the highest symmetry and space group Pbca is the most appropriate. This is supported by the implicit presence of the Pbca symmetry operations in the low-symmetry space groups in both experimental and DFT calculated structures. The Ca atoms are surrounded by six BD4 groups that have similar distortions as reported for the beta-phase of Ca(BD4)(2). On the basis of the experimental structures, free energies of the alpha-, beta-, and gamma-phases are calculated in the range 300 K < T < 620 K. The phase transitions are observed in the same temperature range by means of X-ray diffraction on Ca(BD4)(2) + MgD2 and pure Ca(BD4)(2) samples. According to the ab initio calculations, the alpha-phase is the ground state at 0 K. At room temperature, the calculated free energies of the alpha-phase, beta-phase, and gamma-phase were found to be within similar to 0.13 eV/f.u., in agreement with the observed coexistence of these phases. Moreover, calculations provide insight into trends for relative stabilities of alpha-, beta-, and gamma-phases. That is, with increasing temperature, the beta-phase becomes more stable, while the metastable gamma-phase becomes more destabilized with respect to the alpha-phase.

The preparation of Li11BD4 from Al11B2, LiD and D2 and several subsequent hydrogen sorption cycles according to 2LiD+AlB2+3D2↔2LiBD4+Al starting from the desorbed state are investigated by in situ neutron diffraction. Absorption is carried out at T=450° C and at p=50bar D2, while desorption is performed at T=450° C using a back-pressure of 1bar. Compared to the Al free system, the absorption is observed at much lower temperatures and pressures. We attribute the increased deuterium uptake to the enhanced chemical reactivity of the elemental boron released in the dissociation of AlB2. The desorption process on the other hand is similar to the desorption of pure LiBD4 and seems not to be influenced by the present aluminum. The hydrogen capacity decreases from cycle to cycle, due to an incomplete re-formation of AlB2. We attribute this to a phase separation of Al and B during the decomposition of LiBD4.

The design and construction of a high-pressure (200 bar) and high-temperature (600 degrees C) heat-flow differential scanning calorimeter (DSC) for the in situ investigation of the hydrogenation and dehydrogenation reactions of hydrides is presented. In combination with a pressure-concentration-temperature (pcT) system, simultaneous thermodynamic and volumetric measurements become accessible. Due to the high thermal conductivity of hydrogen, only the sample cell and the reference cell are exposed to hydrogen and the remaining system is under ambient conditions. This separation has the advantage that the calibration factor is independent of the hydrogen pressure. The internal empty volume of the combined system is as low as possible to maximize the precision of the pcT measurements. The calorimetric block of the DSC is designed with a silver/copper alloy and the temperature measurements are made resistively with platinum temperature sensors (Pt 100). The instrument was calibrated and its operability was successfully studied on the example of the hydrogen sorption behavior of LaNi(5).

The direct synthesis of Li[BH4] and Li[B11D4] from the corresponding elements at T=700°C and p(H2) (or p(D2))=150bar is demonstrated. The chemical inertness of boron is thought to prevent its reaction with lithium and hydrogen to LiBH4 at lower temperatures. We show, however, that the reaction kinetics can be enforced by a LiB3-like alloy and a Li7B6 intermetallic compound, which are formed in inert argon atmosphere at 330 and 450°C, respectively. The synthesis is performed by exposing the Li–B mixtures to a defined hydrogen pressure in a high pressure stainless steel vessel and the hydrogen adsorption is monitored by the pressure decrease. The product is identified by X-ray powder diffraction.

We report on neutron powder-diffraction experiments, inelastic incoherent neutron-scattering experiments, and density-functional calculations on dynamics, order and disorder properties of LiBH4 and LiBD4. From refinement of LiBD4 structure at 10 and 302 K, we found an almost ideal tetrahedral geometry of BD4 ions (difference between shortest and longest interatomic distances is less than 4% for B-D bond, and less than 3% for D-D bond), close to the calculated geometry. A quantitative agreement was found between experimental and calculated anisotropic temperature factors of individual atoms. For phonon energies <15 meV, the phonon density of states of LiBH4 in the low-temperature phase depends quadratically on the phonon energy while for the high-temperature phase a linear dependence is observed, revealing a high lattice anharmonicity in the high-temperature phase. Moreover, an increased phonon density of states at low energies in the high-temperature phase compared to the low-temperature phase give a direct evidence for disorder in the high-temperature phase of LiBH4 of the hydrogen sublattice which can originate from orientational disorder of BH4 units. Potential energy landscape for rotation of BH4 indicates that fairly localized minima and barriers higher than 0.6 eV exist in the low-temperature phase, i.e., ordered BH4 ions. The high-temperature structure shows shallow barriers of similar to 0.2 eV without distinct energy minima, i.e., orientation of a single BH4 unit cannot be precisely defined. This corroborates the large thermal displacements observed in diffraction studies and high disorder of BH4 ions deduced from experimental partial phonon density of states in the high-temperature phase.

We have investigated the crystal structure of Ca(BD4)2 by combined synchrotron radiation X-ray powder diffraction, neutron powder diffraction, and ab initio calculations. Ca(BD4)2 shows a variety of structures depending on the synthesis and temperature of the samples. An unknown tetragonal crystal of Ca(BD4)2, the beta phase has been solved from diffraction data measured at 480 K on a sample synthesized by solid-gas mechanochemical reaction by using MgB2 as starting material. Above 400 K, this sample has the particularity to be almost completely into the beta phase of Ca(BD4)2. Seven tetragonal structure candidates gave similar fit of the experimental data. However, combined experimental and ab initio calculations have shown that the best description of the structure is with the space group P4(2)/m based on appropriate size/geometry of the (BD4)tetrahedra, the lowest calculated formation energy, and real positive vibrational energy, indicating a stable structure. At room temperature, this sample consists mainly of the previously reported alpha phase with space group Fddd. In the diffraction data, we have identified weak peaks of a hitherto unsolved structure of an orthorombic gamma phase of Ca(BD4)2. To properly fit the diffraction data used to solve and refine the structure of the beta phase, a preliminary structural model of the gamma phase was used. A second set of diffraction data on a sample synthesized by wet chemical method, where the gamma phase is present in significant amount, allowed us to index this phase and determine the preliminary model with space group Pbca. Ab initio calculations provide formation energies of the alpha phase and beta phase of the same order of magnitude (delta H < or = 0.15 eV). This indicates the possibility of coexistence of these phases at the same thermodynamical conditions.

The synthesis of Li[(11)BD(4)] from LiB and D(2) (p = 180 bar) is investigated by in situ neutron diffraction. The onset of the Li[(11)BD(4)] formation is observed far below the temperatures reported so far for the reaction from the pure elements, indicative of a lower activation barrier. We attribute the improved formation behavior to the breaking of the rigid boron lattice and intermixing of the elements on an atomic level when forming the binary compound LiB. The reaction starts with the decomposition of the initial LiB compound and the formation of LiD. At 623 K LiBD(4) starts to form. However, under the given experimental conditions (maximal temperature = 773 K) a complete reaction was not achieved; there is still residual LiD present.

Metal hydrides typically absorb 1-2 hydrogen atoms per metal atom and exhibit very large volumetric storage densities of up to 150 kg H-2 m(-3) (e.g. Mg2FeH6). However, due to the large atomic mass of the transition metals the gravimetric hydrogen density is limited to less than 5 mass%. Light weight group 3 metals, e.g. Al, B, are able to bind four hydrogen atoms and form together with an alkali metal an ionic or at least partially covalent compound. These compounds are rather stable and often desorb the hydrogen only above their melting temperature. LiBH4 has a gravimetric hydrogen density of 18.5 mass% and a volumetric hydrogen density of 121 kg H-2 m(-3). The compound was first synthesized by Schlesinger and Brown [J. Am. Chem. Soc. 62 (1940) 3429] in an organic solvent. According to the work of Stasinevich and Egorenko [Russian J. Inorg. Chem. 13(3) (1968) 341] hydrogen desorbs from LiBH4 at temperatures greater than 470 degreesC. We have successfully identified the low temperature structure of LiBH4: orthorhombic, space group Pnma (not equal62), the unit cell contains four molecules and has the dimensions a= 7.1730 Angstrom, b = 4.4340 Angstrom, c = 6.7976 Angstrom at 25 degreesC. A slight hydrogen desorption was observed during the structure transformation around 100degreesC and the major hydrogen desorption (13.5 mass%) starts at approximately 200 degreesC when SiO2-powder is added to the LiBH4 sample. (C) 2002 Elsevier B.V. All rights reserved.

The carbon nanotubes (CNT) show promising electrochemical characteristics particularly for electrochemical energy storage. The electrochemical double-layer (ECDL) capacitor is a new type of capacitor with features intermediate between those of a battery and a conventional capacitor. ECDL capacitors have been made using various types of CNT and activated carbon (a-C) as electrode material. The specific capacitance per surface area of the electrodes depends on the thickness and the specific surface area of the active material. The CNT electrodes show a specific capacitance from 0.8 and 280mFcm−2 and 8 to 16Fcm−3, respectively. Increasing the mass density also helps to increase the capacitance. Commercially available activated carbon (a-C) electrodes were also tested in order to study their specific capacitance as a function of their physical properties. The various a-C electrodes have specific capacitance per surface area ranging from 0.4 to 3.1Fcm−2 and an average specific capacitance per volume of 40Fcm−3 due to their larger mass density.

Carbon nanotube composite materials were produced by catalytic decomposition of gaseous carbon sources (such as carbon monoxide or hydrocarbons) on nanometer-size metal clusters of iron, cobalt and nickel embedded in matrices of inert metal oxide particles. The resulting multiwalled carbon nanotubes are several micrometers long with tube diameters ranging from 5 to 20 nm. A fluidised bed reactor was developed for a large-scale synthesis of the carbon nanotube/metal oxide composite (CMC) material. Hydrogen storage capacities of these materials were tested by volumetric and electrochemical methods.

We have analyzed the hydrogen storage capability of a set of carbon samples including a variety of carbon nanotubes, in the gas phase and in the electrolyte as well. The nanotube samples synthesized in our laboratory by pyrolysis of acetylene are of the multi-wall type. The hydrogen sorption properties of our synthesized nanotubes were compared with the properties of commercially available nanotubes and high surface area graphite as well. The nanotube samples and the high surface area graphite as well absorb hydrogen up to 5.5 mass% at cryogenic temperatures (77 K). However, at room temperatures this value drops to approximate to0.6 mass%. The electrochemical experiments on the carbon samples showed a maximum discharge capacity of 2.0 mass% at room temperature (298 K). The hydrogen tends to covalently bind to carbon when the absorption takes place at elevated temperatures (>573 K). Therefore, hydrocarbons desorbed from the sample were analyzed by means of temperature programmed desorption measurements. We conclude that the adsorption of hydrogen on nanotubes is a surface phenomenon and is similar to the adsorption of hydrogen on high surface area graphite. (C) 2002 Elsevier Science B.V. All rights reserved.

Well-aligned carbon nanotubes films were synthesized by a pyrolytic method with aluminum and silicon as substrates. The substrate was coated with a thin film of Fe(NO3)3. This film was transformed by subsequent heating into Fe2O clusters with a diameter of a few nanometers. Nanotubes were synthesized from acetylene at a temperature between 630°C and 750°C. The nanotubes observed are “multiwall” type with a length in the range of 1–10 μm and a diameter of 5–100 nm. The growth of the nanotubes is a function of the film thickness of deposited Fe(NO3)3 film as well as the temperature. The nanotubes deposited on aluminum exhibit excellent properties as electrode material in electrochemical double layer capacitors (ECDLs).