**3. Tailoring thermodynamics of light weight metal hydrides**

While there are plenty of known hydrides with suitable thermodynamics for hydrogen uptake and release at ambient conditions (several bar equilibrium pressure at or nearby room temperature) currently no hydride is known which combines suitable thermodynamics and kinetics with such a high gravimetric storage capacity that a hydrogen storage tank based on such a material could compete with a 700 bar compressed composite vessel in regard to weight. Depending on the working temperature and pressure as well as the reversible gravimetric storage capacity of the selected hydride the achievable capacity of a metal hydride based storage tank is usually better than half of the capacity of the metal hydride bed itself (Buchner & Povel, 1982). Since modern composite pressurized gas tanks meanwhile show gravimetric hydrogen storage capacities of around 4 wt.% according to conservative extrapolations the possible choice of hydrides should be limited to those having the ability to reversibly store at least 6 wt.%H2. All currently known high capacity hydrides, however, show either too small values of the respective reaction enthalpy and are therefore not reversible or would require several thousand bar hydrogen pressure or alternatively electrochemical loading or on the other hand are too stable and have an equilibrium pressure which around room temperature is much below the required pressures. The value of reaction enthalpy aimed at is between 20 and 30 kJ/mol H2. Fig. 9 shows the potentially available hydrogen content of some well known hydrides plotted against their hydrogen reaction enthalpies.

Fig. 9. Theoretically achievable reversible storage capacities and reaction enthalpies of selected hydrides. LaNi5H6 and FeTiH2 are taken as examples for conventional room temperature hydrides. The reaction enthalpies and achievable hydrogen storage capacities are H = -31 kJ/mol H2, CH,max = 1.4 wt.% for LaNi5H6 and for the Fe-Ti system H = - 31.5 kJ/mol H2, CH,max = 1.8 wt.%(average over two reaction steps with H(FeTiH2) = - 28 kJ/mol H2 and H(FeTiH) = -35 kJ/mol H2 respectively) (Buchner, 1982). The respective values for NaAlH4 are H = -40.5 kJ/mol H2, CH,max = 5.6 wt.%(average over two reaction steps with H(NaAlH4) = -37 kJ/mol H2 and H(NaAl3H6) = -47 kJ/mol H2 (Bogdanovic et al., 2009)), for MgH2: H = -78 kJ/mol H2 (Oelerich, 2000) and CH,max = 7.6 wt.%, for LiBH4: H = -74 kJ/mol H2 (Mauron, 2008) and CH,max = 7.6 wt.%, for Mg(BH4)2: H = - 57 kJ/mol H2 (Li, 2008) and CH,max = 14.9 wt.%.

As shown in Fig. 9 none of the plotted hydrides, neither the conventional room temperature hydrides with their rather low gravimetric capacity nor the sophisticated novel chemical hydrides with their unsuitable reaction enthalpy, show the desired combination of properties. Therefore the tailoring of the thermodynamic properties of high capacity light weight and complex hydrides is a key issue, an imperative for future research in the area of hydrides as hydrogen storage materials.
