Complex hydrides are mixed ionic–covalent compounds that can serve as reversible H2 storage media only when they are catalyzed by a transition metal such as Ti. As the prime example, the phenomenology of Ti-catalyzed sodium alanate (NaAlH 4 ) is reviewed from a historical perspective. Dehydriding yields a theoretical 5.6 wt% H 2 during two-step decomposition, NaAlH 4 → Na 3 AlH 6 → NaH + Al, although 100% recovery of that H 2 is not currently possible. H 2 can be discharged and recharged at practical rates at 125°C. More work is needed on the alanates, in particular, as well as the identification and optimization of the catalytic mechanism and a broad extension of the concept to other than Na-based alanates. The possibility of an even further extension of the concept to other complex hydrides (e.g., the borohydrides and transition-metal complexes) is discussed.

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Complex metal hydrides such as sodium aluminohydride (NaAlH4) and sodium borohydride (NaBH4) are solid-state hydrogen-storage materials with high hydrogen capacities. They can be used in combination with fuel cells as a hydrogen source thus enabling longer operation times compared with classical metal hydrides. The most important point for a wide application of these materials is the reversibility under moderate technical conditions. At present, only NaAlH4 has favourable thermodynamic properties and can be employed as a thermally reversible means of hydrogen storage. By contrast, NaBH4 is a typical non-reversible complex metal hydride; it reacts with water to produce hydrogen.

For the continuous production of electricity with solar heat power plants the storage of heat at a temperature level around 400 °C is essential. High temperature metal hydrides offer high heat storage capacities around this temperature. Based on Mg-compounds, these hydrides are in principle low-cost materials with excellent cycling stability. Relevant properties of these hydrides and their possible applications as heat storage materials are described.

A novel type of complex rare-earth aluminum hydride was prepared by mechanochemical preparation. The crystal structure of the REAlH(6) (with RE = La, Ce, Pr, Nd) compounds was calculated by DFT methods and confirmed by preliminary structure refinements. The trigonal crystal structure consists of isolated [AlH(6)](3-) octahedra bridged via [12] coordinated RE cations. The investigation of the rare-earth aluminum hydrides during thermolysis shows a decrease of thermal stability with increasing atomic number of the RE element. Rare-earth hydrides (REH(x)) are formed as primary dehydrogenation products; the final products are RE-aluminum alloys. The calculated decomposition enthalpies of the rare-earth aluminum hydrides are at the lower end for reversible hydrogenation under moderate conditions. Even though these materials may require somewhat higher pressures and/or lower temperatures for rehydrogenation, they are interesting examples of low-temperature metal hydrides for which reversibility might be reached.

The so-called “one-step direct synthesis method” – simultaneous ball-milling (b.m.) of NaH–Al powder doping agent mixtures under H2 pressure – turned out to be a simple way for the preparation of metal-doped sodium alanate hydrogen storage materials, leading to solids with high storage capacity and excellent kinetics. The method has been published until now only for TiCl3 as a doping agent. In combination with ScCl3 or CeCl3 as doping agents, the one-step direct synthesis method delivers materials with hydrogen storage properties which come close to those required by the car industry for hydrogen supply of PEM fuel cells. With respect to the kinetics of the chemical reaction, hydrogenation rates corresponding to 3–5min time for refuelling of a hydrogen tank can be realized, although removal of the resulting hydrogenation heat in such a short time posses a severe engineering problem. Release of all stored hydrogen in a time compatible with handling of a car is possible without additional heating devices, if instead of the current fuel cells, advanced designs on the basis of polybenzimidazole membranes are used. These fuel cells work at temperatures of 150–200°C, so that their waste heat temperature level is sufficiently high for desorption of hydrogen from both dehydrogenation steps of the NaAlH4 system. Additionally, it has been reported that using mischmetal with 42at.% of Ce as a dopant for NaAlH4, at 150°C, ∼5wt.% of hydrogen can be desorbed in ∼3h. Moreover, in a 95 cycles de- and re-hydrogenation test the present Ce-doped NaAlH4 storage material showed stable storage properties.

NaAlH4 is amongst the most advanced and intensely investigated hydrogen storage materials today. It reaches a storage capacity of more than 5wt.% at around 100°C. By suitable doping with catalysts, the re- and de-hydrogenation kinetics can be dramatically improved so that re-hydrogenation times on the order of minutes can be achieved. The salient features of doped NaAlH4 are discussed first. Then the findings are extrapolated to other possible alanate-based hydrogen storage materials.

AbstractDurch Hochenergievermahlen unter Wasserstoffdruck werden hochreaktive Metallhydride und Wasserstoffspeichermaterialien innerhalb von kürzester Zeit synthetisiert. Zur Reaktionskontrolle dieser Synthesen wird ein Telemetriesystem vorgestellt, das die Beobachtung von Druck und Temperatur während des Mahlprozesses erlaubt.

The most favorable dopants for 1 solid-state hydrogen-storage materials are still considered to be compounds of titanium in spite of intensive research on other catalysts. Here, we show that NaAlH4 doped with SCCl3 is a highly efficient dopant, both with respect to storage capacity and kinetics: 2 mol % ScCl3 exhibits a nearly theoretical hydrogen-storage capacity (4.9 wt %, see graph). Other rare-earth metal trichlorides are also investigated.

Titan erleichtert die Dissoziation von Wasserstoff an der Oberfläche des Wasserstoffspeichermaterials Natriumalanat sowie seine Diffusion in den Festkörper, wie H/D-Austauschexperimente auf Ti-dotiertem NaAlH4 belegen (siehe Schema). Nach kinetischen Studien ist der geschwindigkeitsbestimmende Schritt bei der Beladung/Entladung offenbar der Massentransfer in den Festkörper.

Pressure-concentration isotherms have been recorded for NaAlH4 with different doping levels of titanium. It is well known that titanium accelerates the hydrogenation and dehydrogenation reactions in this system. Our studies have shown that the titanium doping also significantly alters the thermodynamics of the system, which is demonstrated by the change of the dissociation pressure with doping level. This can be explained by changes in the systems energy by dilution of the TiAl-alloy present after doping: Such an alloy forms as a result of the doping reaction. Aluminum generated during the dehydrogenation reaction dilutes this alloy, which gives an additional contribution to the free energy of the system.

Es wird ein Verfahren zur Synthese von Verbindungen, in welchem Feststoffe mit einem oder mehreren Gasen zur Reaktion gebracht werden, beansprucht, in welchem die Feststoffe in Gegenwart des Gases oberhalb von Atmospharendruck vermahlen werden. Zur Durchfuhrung des Verfahrens wird vorzugsweise eine Muhle mit einem das Mahlgut (10) aufnehmenden Mahlraum (1) verwendet, in welcher der Mahlraum (1) auch gegenuber grosen Innendrucken (P 1 ) druckdicht verschliesbar ist.

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