2. Hydride compounds: basics on the chemical reaction and thermodynamics

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 and its thermodynamics.

#### 2.1 Reversible gas: solid chemical reaction

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 considerable hydrogen volumetric capacity of about 100 kg/m<sup>3</sup> . The other categories 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 also considerable volumetric hydrogen capacity from 100 to 150 kg/m<sup>3</sup> .

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

$$\mathbf{M}\_{\mathbf{(s)}} + (\mathbf{X}/2)\mathbf{H}\_{2(\mathbf{g})} \leftrightharpoons \mathbf{MH}\_{\mathbf{x(s)}} + \mathbf{Heat}\left(\mathbf{T}, \mathbf{P}\right) \tag{1}$$

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

Thermodynamic properties for the M-H2 systems are usually characterized by measuring pressure-composition-isotherms (PCIs). Figure 3A shows ideal PCIs (without slope and hysteresis), where the x-axis is the hydrogen concentration (CH) expressed as the ratio between atomic hydrogen and metal (H/M), and y-axis is the hydrogen pressure (pH2). The procedure to measure a PCI consist in introducing the hydride forming metal or material in a sealed vessel connected to hydrogen supply and increase steeply the hydrogen pressure at a constant temperature. There are different steps involved during the hydrogen absorption process in a metal under equilibrium conditions. For example, taking the PCI at T2 (Figure 3A), a detailed description of the process during PCI characterization can be done as

and kinetics of the metal-hydrogen (M-H2) system.

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

Tailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage

DOI: http://dx.doi.org/10.5772/intechopen.82433

2.2 Thermodynamics

Table 1.

follows [4, 21]:

129

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

Tailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage DOI: http://dx.doi.org/10.5772/intechopen.82433

