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The safest way to store hydrogen is in solid form, physically entrapped in molecular form in highly porous materials, or chemically bound in atomic form in hydrides. Among the different families of these compounds, alkaline and alkaline earth metals alumino-hydrides (alanates) have been regarded as promising storing media and have been extensively studied since 1997, when Bogdanovic and Schwickardi reported that Ti-doped sodium alanate could be reversibly dehydrogenated under moderate conditions. In this review, the preparative methods; the crystal structure; the physico-chemical and hydrogen absorption-desorption properties of the alanates of Li, Na, K, Ca, Mg, Y, Eu, and Sr; and of some of the most interesting multi-cation alanates will be summarized and discussed. The most promising alanate-based reactive hydride composite (RHC) systems developed in the last few years will also be described and commented on concerning their hydrogen absorption and desorption performance.

In this work, a thermodynamic picture of the dehydrogenation mechanism of NaBH 4 in NaBH 4 -MgH 2 mixtures with 2:1 and 1:2 molar ratio is drawn, for the first time in literature, thanks to coupled manometric-calorimetric measurements up to 580 °C. Such a new approach also allows, after the measurement of the borohydride melting enthalpy, the evaluation of the dehydrogenation enthalpy of the complex hydride in the mixtures. The thermodynamics of the 2:1 sample (where the borohydride decomposition takes place mainly in liquid state) is more favorable than that of the 1:2 mixture, where the process evolves fully in solid state. The kinetics of the systems is studied at 450 °C, the minimum temperature at which the borohydride decomposition takes place. In these conditions, the 1:2 system is kinetically favored. Several additives (fluorides; chlorides; hydroxides) have been tested as possible destabilizing/catalyzing agents. These substances react with the component hydrides upon discharging, forming stable binary and ternary compounds that do not change the macroscopic desorption pathway of the composites (separate decomposition of the component hydrides) but lead to variations in the desorption temperature and kinetics. In particular, MgF 2 is found to improve the desorption kinetics of both the component hydrides and to reduce the decomposition temperature and enthalpy of NaBH 4 in the 2:1 system. On the contrary, none of the tested dopants exerts any positive effect on the 1:2 system.

We report the extensive investigation of Li and H dynamics in Li 6 C 60 and Li 6 C 60 H y , by combining 7 Li and 1 H solid state NMR measurements with DC/AC conductivity, in order to evaluate the potential application of these systems for energy-storage purposes. 7 Li NMR results show a local motion of Li ions above 200 K in both pristine and hydrogenated compounds, with activation energies of 90-150 meV and correlation times of about 30 ps. Evidences of Li interdiffusive dynamics are given by conductivity measurements in Li 6 C 60 already above 120 K, with activation energies of 240 meV, suggesting that ionic conductivity is of the order of 10 −5 S•cm −1 at room temperature, with correlation times of about 150 ps. On the other hand, the Li 6 C 60 H y behaves like a semiconductor with a high energy gap (ca. 2.5 eV), suggesting that diffusion of intercalated Li ions is prevented. 1 H NMR measurements indicate the absence of H motions for the whole temperature range investigated (up to 360 K), neither on macroscopic or local scale. Li 6 C 60 good properties for H 2-storage are confirmed in terms of absorption capacity (5 wt% H 2), moreover we found that around 35% of lithium segregates in LiH form, leaving Li 4 C 60 H 40 as the final hydrogenation product.

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Thermodynamic and heat transfer properties of the 2LiBH 4-MgH 2 composite (Li-RHC) system are experimentally determined and studied as a basis for the design and development of hydrogen storage tanks. Besides the determination and discussion of the properties, different measurement methods are applied and compared to each other. Regarding thermodynamics, reaction enthalpy and entropy are determined by pressure-concentration-isotherms and coupled manometric-calorimetric measurements. For thermal diffusivity calculation, the specific heat capacity is measured by high-pressure differential scanning calorimetry and the effective thermal conductivity is determined by the transient plane source technique and in situ thermocell. Based on the results obtained from the thermodynamics and the assessment of the heat transfer properties, the reaction mechanism of the Li-RHC and the issues related to the scale-up for larger hydrogen storage systems are discussed in detail.