**2.2 Hydrogen storage in light weight hydrides**

900 Thermodynamics – Interaction Studies – Solids, Liquids and Gases

Fig. 8. Hydride and non hydride forming elements in the periodic system of elements.

rare earths:

classes.

Even better agreement with experimental results than by use of Miedema's rule of reversed stability is obtained by applying the semi-empirical band structure model of Griessen and Driessen (Griessen & Driessen, 1984) which was shown to be applicable to binary and ternary hydrides. They found a linear relationship of the heat of formation H = H0f of a metal hydride and a characteristic energy E of the electronic band structure of the host metal which can be applied to simple metals, noble metals, transition metals, actinides and

with *E* = *E*F-*E*S (*E*F being the Fermi energy and *E*S the center of the lowest band of the host

As described above, most materials experience an expansion during hydrogen absorption, wherefore structural effects in interstitial metal hydrides play an important role as well. This can be and is taken as another guideline to tailor the thermodynamic properties of interstitial metal hydrides. Among others Pourarian et al. (Pourarian, 1982), Fujitani et al. (Fujitani, 1991) and Yoshida & Akiba (Yoshida, 1995) report about this relationship of lattice parameter or unit cell volume and the respective plateau pressures in different material

Intensive studies let to the discovery of a huge number of different multinary hydrides with a large variety of different reaction enthalpies and accordingly working temperatures. They are not only attractive for hydrogen storage but also for rechargeable metal hydride electrodes and are produced and sold in more than a billion metal hydride batteries per year. Because of the high volumetric density, intermetallic hydrides are utilized as hydrogen storage materials in advanced fuel cell driven submarines, prototype passenger ships,

metal, = 59.24 kJ (eV mol H2)-1 and = -270 kJ (mol H2)-1 and *E* in eV.

forklifts and hydrogen automobiles as well as auxiliary power units.

*H E* α β (18)

Novel light weight hydrides show much higher gravimetric storage capacities than the conventional room temperature metal hydrides. However, currently only a very limited number of materials show satisfying sorption kinetics and cycling behaviour. The most prominent ones are magnesium hydride (MgH2) and sodium alanate (NaAlH4). In both cases a breakthrough in kinetics could be attained in the late 90s of the last century / the early 21st century.

Magnesium hydride is among the most important and most comprehensively investigated light weight hydrides. MgH2 itself has a high reversible storage capacity, which amounts to 7.6 wt.%. Furthermore, magnesium is the eighth most frequent element on the earth and thus comparably inexpensive. Its potential usage initially was hindered because of rather sluggish sorption properties and unfavourable reaction enthalpies. The overall hydrogen sorption kinetics of magnesium-based hydrides is as in case of all hydrides mainly determined by the slowest step in the reaction chain, which can often be deduced e.g. by modelling the sorption kinetics (Barkhordarian et al, 2006; Dornheim et al., 2006). Different measures can be taken to accelerate kinetics. One important factor for the sorption kinetics is the micro- or nanostructure of the material, e.g. the grain or crystallite size. Because of the lower packing density of the atoms, diffusion along grain boundaries is usually faster than through the lattice. Furthermore, grain boundaries are favourable nucleation sites for the formation and decomposition of the hydride phase. A second important parameter is the outer dimension of the material, e.g. in case of powdered material, its particle size. The particle size (a) determines the surface area, which is proportional to the rate of the surface reaction with the hydrogen, and (b) is related to the length of the diffusion path of the hydrogen into and out of the volume of the material. A third major factor by which hydrogen sorption is improved in many hydrogen absorbing systems is the use of suitable additives or catalysts. In case of MgH2 it was shown by Oelerich et al. (Oelerich et al., 2001; Dornheim et al., 2007) that already tiny amounts of transition metal oxides have a huge impact on the kinetics of hydrogen sorption. Using such additives Hanada et al. (Hanada et al., 2007) could show that by using such additives hydrogen uptake in Mg is possible already at room temperature within less than 1 min. The additives often do not just have one single function but multiple functions. Suitable additives can catalyze the surface reaction between solid and gas. Dispersions in the magnesium-based matrix can act as nucleation centres for the hydride or the dehydrogenated phase. Furthermore, different additives, such as liquid milling agents and hard particles like oxides, borides, etc. , can positively influence the particle size evolution during the milling process (Pranzas et al., 2006; Pranzas et al., 2007; Dornheim et al, 2007) and prevent grain i.e. crystallite growth. More detailed information about the function of such additives in MgH2 is given in (Dornheim et al., 2007). Beyond that, a preparation technique like high-energy ball milling affects both the evolution of certain particle sizes as well as very fine crystallite sizes in the nm range and is also used to intermix the hydride and the additives/catalysts. Thus, good interfacial contact with the light metal hydride as well as a fine dispersion of the additives can be achieved.

As in case of MgH2 dopants play also an important role in the sorption of Na-Al-hydride, the so-called Na-alanate. While hydrogen liberation is thermodynamically favorable at moderate temperatures, hydrogen uptake had not been possible until in 1997 Bogdanovic et al. demonstrated that mixing of NaAlH4 with a Ti-based catalyst leads to a material, which can be reversibly charged with hydrogen (Bogdanovic, 1997). By using a tube vibration mill

Thermodynamics of Metal Hydrides: Tailoring Reaction Enthalpies of Hydrogen Storage Materials 903

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 = -

**3.1 Thermodynamic tuning of single phase light weight hydrides** 

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

The traditional way of tailoring the thermodynamic properties of metal hydrides is by formation of alloys with different stabilities as described in chapter 2.1. Thereby the value of reaction enthalpy can be reduced by stabilising the dehydrogenated state and/or destabilising of the hydride state, see Fig. 10 a. Accordingly, the total amount of reaction enthalpy is increased by destabilising the dehydrogenated state and/or stabilising the

One of the first examples using this approach for tuning the thermodynamic properties of light weight metal hydrides was the discovery of the Mg-Ni –system as potential hydrogen storage system by Reilly and Wiswall (Reilly & Wiswall, 1968). Mg2Ni has a negative heat of

This approach has been successfully applied to light weight metal hydrides also.

57 kJ/mol H2 (Li, 2008) and CH,max = 14.9 wt.%.

hydrides as hydrogen storage materials.

hydride, see Fig. 10 b.

**Mg-based hydrides** 

of Siebtechnik GmbH Eigen et al. (Eigen et al., 2007; Eigen et al., 2008) showed that upscaling of material synthesis is possible: After only 30 min milling under optimised process conditions in such a tube vibration mill in kg scale, fast absorption and desorption kinetics with charging/discharging times of less than 10 min can be obtained. The operation temperatures of this complex hydride are much lower than compared to MgH2 and other light weight hydrides. Fast kinetics is achieved at 100 °C to 150 °C which is much less than what is required in case of MgH2, however, still significantly higher than in case of the conventional hydrides which show only a very limited storage capacity. Such hydride working temperatures offer the possibility for combinations of metal hydride tanks based on these complex hydrides with e. g. combustion engines, high temperature PEM fuel cells or other medium to high temperature fuel cells. However, compared to MgH2 the gravimetric hydrogen storage capacity is significantly reduced. Having a maximum theoretical storage capacity of about 5.6 wt. % NaAlH4 exhibits a long term practical storage capacity of 3.5-4.5 wt. % H2 only. Furthermore, in difference to MgH2 NaAlH4 decomposes in two reaction steps upon dehydrogenation which implies two different pressure plateaus instead of just one:

$$\text{NaAlH}\_4 \leftrightarrow \text{1/3 Na}\_3\text{AlH}\_6 + \text{2/3 Al} + \text{H}\_2\text{(g)} \leftrightarrow \text{NaH} + \text{Al} + \text{3/2 H}\_2\text{(g)}\tag{19}$$

The first decomposition step has an equilibrium pressure of 0.1 MPa at 30 °C, the second step at about 100 °C (Schüth et al., 2004). A maximum of 3.7 wt.% H2 can be released during the first desorption step, 5.6 wt.% in total. The remaining hydrogen bonded to Na is technically not exploitable due to the high stability of the respective hydride.

While the reaction kinetics was optimized significantly, the desorption enthalpy of NaAlH4 of 37 kJ/molH2 and Na3AlH6 of 47 kJ/mol H2 respectively remains a challenge. For many applications even this value which is much below that of MgH2 is still too large.
