THERMODYNAMICS

It is known that some metals and alloys are able to absorb chemically hydrogen and then to reversibly release it [1-12]. The thermodynamic aspects of hydride formation from gaseous hydrogen are described by the PCI (pressure-composition isotherm) curves as shown in Fig. 1a. These curves are obtained in the following way. At a given temperature and with a low hydrogen pressure, the host metal begins to dissolve a small quantity of hydrogen in solid solution (α phase), after the dissociation of the H₂ molecule into atomic hydrogen at the surface of the material. When the pressure increases, the hydrogen concentration in the metal undergoes small increments and then the local interactions between hydrogen atoms become more and more important up to the nucleation and growth of the hydride phase β. As long as the solid solution and the hydride coexist, the isotherm curves (hydrogen pressure at a given temperature as a function of hydrogen concentration in the material) present a plateau; the length of this plateau represents the hydrogen amount which can be reversibly stored at that temperature by small pressure changes. When the α→β transition is completed, the hydrogen pressure begins again to sharply increase with the concentration. The region of the diagram where the two phases coexist ends at a critical point T_c, over which the α→β transition is continuous. The equilibrium pressure (position of the plateau) strongly depends on temperature and is related to the enthalpy and entropy changes DH and DS, respectively, by the Van’t Hoff relation, reported analytically and graphically in Fig. 1b.

Figura 1a	Figura 1b

 

The enthalpy changes related to the hydride formation or dissociation can obtain experimentally from the slope of Van’t Hoff's plots. While the enthalpy term depends on the metal-hydrogen bond stability, the entropy term corresponds essentially to the transition from molecular hydrogen to atomic hydrogen, necessary for the passage from gaseous to solid phase and is similar for all the known hydrides. The working temperature of a metal/hydride system is fixed by the thermodynamic equilibrium pressure and by the overall reaction kinetics. In order to make metal hydrides interesting for the use in hydrogen reservoirs, the working pressure and temperature should be in the ranges 1-10 bar and 20-100 °C, respectively, corresponding to an enthalpy change between 15 and 24 kJ/molH. A further problem, already mentioned, concerns the weight of the absorbing material, thus light metal hydrides containing a high amount of hydrogen are preferable. Table 1 presents some features of the main hydrides studied so far [1]. Besides the mentioned thermodynamic aspects, also the hydrogen absorption and desorption kinetics, i.e. the rate at which these processes occur, have a primary importance for practical applications. However, none of the currently studied hydrides presents at the same time all the required characteristics for the practical functionality of a hydride-based hydrogen reservoir.
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Table 1. Some of the most studied hydrides: mass percent of hydrogen contained in the hydride and temperature at which the gas/solid equilibrium pressure is 1 bar.
Hydrides	H2 (w %)	T (°C) (peq=1 bar)
LaNi5H6	1.37	12
FeTiH2	1.89	-8
Mg2NiH4	3.59	255
MgH2	7.6	279

 

COMMERCIAL HYDRIDES

Hydrides of AB₅ (e.g. LaNi₅), AB (e.g. FeTi), AB₂ (e.g. ZrV₂) and A₂B (e.g. Mg₂Ni) types alloys are already commercially available and are supplied in special vessels. AB₅ alloys absorb quickly and reversibly hydrogen at a pressure of few bars at room temperature or close to it. Moreover, they stand repeated cycles of absorption/desorption without loss of storage capacity [2]. Their weak point is the low weigh percent of stored hydrogen (less than 1.5), which makes the reservoir too heavy. Among the goals that, according to the U.S. DOE (Department of Energy), should be achieved within 2015 with the materials used for hydrogen storage, there are a hydrogen wt% of 5.5 (referred to the entire storage system), which is equivalent to 1.8 kWh/kg. The other 2015 targets are 40 g/L (1.3 kWh/L) and 3.3 min (1.5 kgH₂/min) for the system volumetric density and the system fill time for 5-kg fill, respectively. Therefore, the AB₅ alloys are not ideal for the use in a hydrogen reservoir, while they are more suitable as electrodes in a fuel cell [3]. The Fe-Ti alloy, studied since the 70’s and cheaper than the LaNi₅ alloy, forms the hydrides FeTiH and FeTiH₂ [4]. It allows absorption and desorption operations in thermodynamic favourable conditions, but requires a too high activation temperature and the weight percent of stored hydrogen (less than 2) also in this case is too low. AB₂ alloys present, with respect AB₅ alloys, a better reaction kinetics and a lower cost, but are more sensitive to contaminants. Mg₂Ni alloy has a higher hydrogen capacity (up to 3.6 wt%), but the required working temperature for absorption and desorption reactions is higher than 200 °C.

 

LIGHT METALS BASED HYDRIDES

A high quantity of stored hydrogen can be achieved only by using light elements like magnesium, which forms the hydride MgH₂ with a theoretical hydrogen weight percent of 7.6 [5]. The use of magnesium presents two main difficulties. First, due to the high stability of Mg-H bond, the plateau pressure of the system is too low in the temperature range of applicative interest (it is only 0.36 mbar at 100 °C). In order to get desorption pressures near atmospheric, it is necessary to raise the temperature to about 300 °C. Moreover, even at 300 °C the hydrogenation and dehydrogenation reactions are sluggish. It has been shown that the use of nanostructured magnesium hydride produced by high energy milling is more convenient than the massive material [6]: the presence of sub-micrometric grains reduces the hydrogen diffusive path in solid phase and the high concentration of defects and grain boundaries offer preferential paths for gas escape and also nucleation sites for metallic Mg. Good absorption and desorption kinetics were achieved around 230 °C by milling metallic magnesium or its hydride with or without the addition of catalysts [7-13].
Alanates like NaAlH₄ and mixtures based on MgH₂+LiNH₂ have hydrogen mass capacity around 5%, but present reversibility problems and not low enough decomposition temperatures (above 150°C). These materials are promising because the addition of some suitable catalyst makes reversible the hydrogen a/d reaction at lower temperatures [14-15]. Complex hydrides based on boron are being intensively studied because exhibit high weight capacity for hydrogen (e.g. 18 wt% for LiBH₄) but are typically characterized by a high desorption temperature (more than 400 °C for LiBH₄) and by a complex mechanism of recycling [10, 16]. In order to improve thermodynamics and kinetics of dehydrogenation of light metal hydrides, Vajo and Olson [17] have proposed confining them in nanoporous scaffolds, exploiting the favourable properties of nanostructured materials and avoiding sintering and agglomeration during cycling. The beneficial effect of nanoconfinement of hydrides has been proved also in our laboratory [18].

 

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18. F. Agresti, A. Khandelwal, G. Capurso, S. Lo Russo, A. Maddalena, G. Principi, Nanotechnology 21 (2010) 065707.