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The influence of additives on the reaction kinetics and for microstructure refinement in LiBH 4-MgH 2 composites is investigated in detail. Indications on the rate limiting processes during the reactions are obtained by comparison of the measured reaction kinetics to simulations with one specific rate limiting process. The kinetics of the sorption reactions are derived from volumetric measurements as well as from in-situ Xray diffraction (XRD) measurements. During desorption, the hydrogen is released at a constant rate, which possibly is correlated to the one-dimensional growth of MgB 2 platelets. In contrast, the kinetic curves of the absorption reactions exhibit the typical shape of contracting-volume controlled kinetics. The microscopical interpretation of kinetic measurements are supported by transmission electron microscopy (TEM) images confirming the formation of additive-nanostructures in the grain boundaries upon cycling. The present investigations underline the importance of the additives as nucleation substrates and the influence of the microstructure on the reaction kinetics.

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A breakthrough in hydrogen storage technology was achieved by preparing nanocrystalline hydrides using high‐energy ball milling and the use of suitable catalysts/additives. These new materials show fast or in case of Mg‐based hydrides very fast absorption and desorption kinetics within minutes, thus qualifying lightweight Mg‐ or Al‐based hydrides for storage applications. This article summarizes our current understanding of the kinetics of Mg‐based light metal hydrides, describes an approach for a cost‐effective processing technology and highlights some promising new developments in lightweight metal hydride research.

The hydrogenation of the CaH 2 +MgB 2 composite and the dehydrogenation of the resulting products are investigated in detail by in situ time-resolved synchrotron radiation powder X-ray diffraction, high-pressure differential scanning calorimetry, infrared, and thermovolumetric measurements. It is demonstrated that a Ca(BH 4 ) 2 +MgH 2 composite is formed by hydrogenating a CaH 2 +MgB 2 composite, at 350 °C and 140 bar of hydrogen. Two phases of Ca(BH 4 ) 2 were characterized: Rand β-Ca(BH 4 ) 2 . R-Ca(BH 4 ) 2 transforms to β-Ca-(BH 4 ) 2 at about 130 °C. Under the conditions used in the present study, β-Ca(BH 4 ) 2 decomposes first to CaH 2 , Ca 3 Mg 4 H 14 , Mg, B (or MgB 2 depending on experimental conditions), and hydrogen at 360 °C, before complete decomposition to CaH 2 , Mg, B (or MgB 2 ), and hydrogen at 400 °C. During hydrogenation under 140 bar of hydrogen, β-Ca(BH 4 ) 2 is formed at 250 °C, and R-Ca(BH 4 ) 2 is formed when the sample is cooled to less than 130 °C. Ti isopropoxide improves the kinetics of the reactions, during both hydrogenation and dehydrogenation. The dehydrogenation temperature decreases to 250 °C, with 1 wt % of this additive, and hydrogenation starts already at 200 °C. We propose that the improved kinetics of the above reactions with MgB 2 (compared to pure boron) can be explained by the different boron bonding within the crystal structure of MgB 2 and pure boron.