Table 1.

cycles, and cost that are still matter of intensive research. Improving mainly the properties of the hydride forming material and optimizing the weight and operative conditions of the solid hydrogen storage system is challenging. However, there are

unexplored system configurations, which can lead to an efficient and safe hydrogen

The solid storage system is based on hydride forming materials, i.e., the system consists basically in a vessel filled with the hydride forming material. Of course, the complexity of the storage system including pipes, heat transfer system, and control instrumentations is in this simplified description neglected. In this section, it is presented basic concepts regarding the chemical reaction for the hydride formation

Hydrogen interacts with a large number of elements and materials for the formation of hydride compounds. Hydride forming materials can be classified as: metals (alkaline, alkaline earth, transition, and rare earth), intermetallic (Mg2Ni, LaNi5, etc.), non-intermetallic (Mg-Fe, Mg-Co, etc.), and the combination of boron (B), aluminum (Al), or nitrogen (N) with alkaline or alkaline earth metals. Furthermore, a general classification considering the main feature of different hydride compounds is shown in Table 1. There are two main groups of hydride: the room temperature hydrides where hydrogen is located in the interstices of the metal's lattice, without forming a strong metal-hydrogen bond. This kind of hydride works at low temperatures, i.e., 20–50°C, and under low and high pressures, depending on the type of alloy. For instance, LaNi5 alloy works at low pressure, between 10 and 50 bar, while TiCrMo alloy uptake hydrogen under high pressures, over 100 bar. The main constraint of the room temperature hydride compounds is the low gravimetric hydrogen storage capacities ranging from 1 to 3 wt.% H2, though they do have

ries are binary and complex hydrides, where hydrogen is chemically bound. They work in a broad range of temperatures and pressures, but the most common ranges are temperatures over 100°C and pressures from 10 to 200 bar. There are some exceptions such as non-stable hydride compounds at room temperature, as for example Ti(BH4)3, Fe(BH4)2, Ni(AlH4)2, among others. Moreover, several binary and complex hydrides are not even reversible [15–20]. These hydrides show large gravimetric hydrogen storage capacities ranging from 4 to about 20 wt.% H2 and

The chemical reaction for the hydride formation can be described as a reversible gas-solid reaction. For the sake of clarity, the most simple reaction between a solid metal (M(s)) and gas hydrogen (H2(g)) is herein explained. At certain temperature (T) and under certain pressure (P), M(s) reacts with H2(g) to form a hydride com-

Reaction (1) shows the overall process for the reversible formation of a hydride compound without any detail about reaction intermediates. As seen, the hydride

Mð Þ<sup>s</sup> þ ð Þ X=2 H2 gð Þ ⇆ MHx sð Þ þ Heat Tð Þ ; P (1)

. The other catego-

.

considerable hydrogen volumetric capacity of about 100 kg/m<sup>3</sup>

also considerable volumetric hydrogen capacity from 100 to 150 kg/m<sup>3</sup>

several promising materials under research and development, and besides

2. Hydride compounds: basics on the chemical reaction and

storage system for the future hydrogen economy.

Gold Nanoparticles - Reaching New Heights

2.1 Reversible gas: solid chemical reaction

pound (MHx(s)) according to reaction (1):

128

thermodynamics

and its thermodynamics.

General classification of hydride compounds and their main characteristics [15–20].

compound formation is exothermic, while its decomposition is endothermic. Moreover, the formation and decomposition of a hydride compound occur under certain T and P conditions, which depend on the kind of M(s) and the resulting hydride compound. Thus, these T and P conditions are determined by the thermodynamics and kinetics of the metal-hydrogen (M-H2) system.
