LiBH4 is a complex hydride and exhibits a high gravimetric hydrogen density of 18.5 wt %. Therefore it is a promising hydrogen storage material for mobile applications. The stability of LiBH4 was investigated by pcT (pressure, concentration, and temperature) measurements under constant hydrogen flows and extrapolated to equilibrium. According to the van 't Hoff equation the following thermodynamic parameters are determined for the desorption: enthalpy of reaction DeltarH = 74 kJ mol-1 H2 and entropy of reaction DeltarS = 115 J K-1 mol-1 H2. LiBH4 decomposes to LiH + B + 3/2H2 and can theoretically release 13.9 wt % hydrogen for this reaction. It is shown that the reaction can be reversed at a temperature of 600 degrees C and at a pressure of 155 bar. The formation of LiBH4 was confirmed by XRD (X-ray diffraction). In the rehydrided material 8.3 wt % hydrogen was desorbed in a TPD (temperature-programmed desorption) measurement compared to 10.9 wt % desorbed in the first dehydrogenation.
The standard approach for the search of new hydrogen-storage materials is to synthesize bulk samples and to use volumetric [1,2] or gravimetric [3] techniques to follow their hydrogenation reaction and to record pressure-concentration isotherms (p-c isotherms). The equilibrium pressure of the metal-to-hydride transition is determined from the plateau of the p-c isotherm. The enthalpy of hydride formation is extracted from the temperature dependence of the equilibrium pressure, by means of the Van 't Hoff relation [4] lnwhere DH is the enthalpy of formation in kJ (mol H 2 ) -1 , DS 0 is the entropy of formation in JK -1 (mol H 2 ) -1 at standard pressure, R the gas constant, the absolute temperature, p 0 = 1.013 × 10 5 Pa the standard pressure, and p eq the H 2 equilibrium plateau pressure of the p-c isotherm. The great disadvantage of this approach is that a bulk sample is needed for each investigated chemical composition. Thin films provide an interesting alternative to bulk, as their nanostructure is controlled by the deposition conditions. Because of the small amount of material and large surfaces present, diffusion and local heating issues are minimized, the kinetics are fast, and the measurement time is reduced drastically. [5] Moreover, a large number of different chemical compositions can be deposited on a single substrate in a combinatorial way. The fact that hydrogen absorption in a metal leads to large optical changes [6,7] is the basis of a new combinatorial method that we call hydrogenography. With a straightforward optical setup, hydrogenography makes it possible to monitor hydrogen ab-and desorption simultaneously on thousands of samples under exactly the same experimental conditions. [8][9][10] We show here that hydrogenography is much more than a monitoring technique, as it also provides a high-throughput method to measure quantitatively the key thermodynamic properties (enthalpy and entropy) of hydride formation. We describe the essential ingredients of hydrogenography with the Mg-Ti-H system and demonstrate its combinatorial power with the Mg-Ti-Ni-H system. We show in particular that there is a relatively narrow range of compositions in the ternary Mg-Ti-Ni phase diagram with a remarkable combination of favorable properties for light-weight hydrogen storage. Pure MgH 2 would in principle be an attractive system for hydrogen storage as it can contain as much as 7.6 wt % of hydrogen. However, its large negative enthalpy of formation (-74 kJ (mol H 2 ) -1
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