Inge Lindemann

Al3Li4(BH4)13: a complex double-cation borohydride with a new structure.

The new double-cation Al-Li-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (approximately 70 degrees C) combined with a high hydrogen density (17.2 wt%). It was synthesised by high-energy ball milling of AlCl(3) and LiBH(4). The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) A). The unexpected composition Al(3)Li(4)(BH(4))(13) can be rationalized on the basis of a complex cation [(BH(4))Li(4)](3+) and a complex anion [Al(BH(4))(4)](-). The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at approximately 70 degrees C, forming LiBH(4). The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane.

Enhanced reversibility of H2 sorption in nanoconfined complex metal hydrides by alkali metal addition

Complex metal hydrides, containing up to 18 wt% H2, are attractive candidates for on-board hydrogen storage. However, only limited reversibility of H2 desorption is achieved under mild conditions, especially in the absence of catalysts. Nanoconfining the materials in porous matrixes facilitates rehydrogenation, but still full reversibility has been rarely achieved. We reveal the factors that limit the reversibility using NaAlH4 in a porous carbon matrix as a model system. Relatively large Al crystallites (>100 nm) are formed after desorption, migrating out of the mesopores of the matrix. However, their formation does not fundamentally limit the reversibility, as these crystallites react with Na(H) and H2 reforming nanoconfined NaAlH4 under relatively mild conditions. We show for the first time that the main limiting factor for the decayed cycling capacity is the loss of active alkali metal species. Evaporation losses are minor, even when dehydrogenating at 325 °C in vacuum. Significant losses (30–40%) occur upon the first hydrogen desorption run, and are attributed to the reaction of Na species with impurities in the carbon matrix. A one-time addition of extra Na compensates for this loss, leading to close to full reversibility (>90%) at 150 °C under 55 bar H2 pressure. A similar effect is found when adding extra Li species to nanoconfined LiBH4. For nanoconfined complex metal hydrides irreversible loss of the reactive alkali metal species due to reaction with impurities can act as a major loss mechanism. However, the one-time addition of extra alkali metal species is very effective in resolving this issue, leading to close to full cycling reversibility under relatively mild conditions even in the absence of catalysts.

Novel sodium aluminium borohydride containing the complex anion [Al(BH4,Cl)4]-.

The synthesis of a novel alkali-metal aluminium borohydride NaAl(BH4)xCl4-x from NaBH4 and AlCl3 using a solid state metathesis reaction is described. Structure determination was carried out using synchrotron powder diffraction data and vibrational spectroscopy. An orthorhombic structure (space group Pmn2(1)) is formed which contains Na+ cations and complex [Al(BH4,Cl)4]- anions. Due to the high chlorine content (1 < or = x < or = 1.43) the hydrogen density of the borohydride is only between 2.3 and 3.5 wt.% H2 in contrast to the expected 14.6 wt.% for chlorine free NaAl(BH4)4. The decomposition of NaAl(BH4)xCl4-x is observed in the target range for desorption at about 90 degrees C by differential scanning calorimetry (DSC), in situ Raman spectroscopy and synchrotron powder X-ray diffraction. Thermogravimetric analysis (TG) shows extensive mass loss indicating the loss of H2 and B2H6 at about 90 degrees C followed by extensive weight loss in the form of chloride evaporation.

Synthesis of LiNH2 + LiH by reactive milling of Li3N

The hydrogen sorption properties of Li3N under reactive milling conditions have been investigated in- and ex-situ as a function of polytype structure (alpha vs. beta), focusing on the influence of the micro-structure and/or the crystal structure upon hydrogen uptake. LiNH2 and LiH were synthesized by reactive milling of Li3N at 20 bar hydrogen pressure for 4 h. Reactive milling represents a quick and effective technique to produce LiNH2 by hydrogenation of Li3N at low hydrogen pressure and without any need for heating. As to our knowledge, we present a full hydrogenation of Li3N under the aforementioned conditions for the first time. The (de)hydrogenation and rehydrogenation behaviour of milled amides was evaluated using a combination of powder X-ray diffraction, differential scanning calorimetry, thermogravimetry and in situ Raman spectroscopy. In situ Raman spectroscopy showed a shift in the lithium amide stretching modes upon hydrogenation supporting a non-stoichiometric storage mechanism consistent with the literature. The microstructure and polytype composition of the Li3N dehydrogenated materials had no effect on the hydrogenation products and only minor effects on the hydrogen uptake profile during milling.

In situ Raman cell for high pressure and temperature studies of metal and complex hydrides.

A novel cell for in situ Raman studies at hydrogen pressures up to 200 bar and at temperatures as high as 400 °C is presented. This device permits in situ monitoring of the formation and decomposition of chemical structures under high pressure via Raman scattering. The performance of the cell under extreme conditions is stable as the design of this device compensates much of the thermal expansion during heating which avoids defocusing of the laser beam. Several complex and metal hydrides were analyzed to demonstrate the advantageous use of this in situ cell. Temperature calibration was performed by monitoring the structural phase transformation and melting point of LiBH(4). The feasibility of the cell in hydrogen atmosphere was confirmed by in situ studies of the decomposition of NaAlH(4) with added TiCl(3) at different hydrogen pressures and the decomposition and rehydrogenation of MgH(2) and LiNH(2).