**2. Metal hydrides for hydrogen storage**

Metal hydrides, such as palladium, zirconium, or titanium, form an important group of reversible‐sorbing materials. The main advantage of metal hydrides over other hydrogen storage media, such as activated carbon, carbon nanotubes, or zeolites, is that they can be reversible hydride, and they can also release hydrogen of the metal hydrides in an extremely pure way, which is an important factor when considering hydrogen for mobile applications [1].

The process of hydrogen absorption (or desorption) by metals is reversible and involves the surface and the bulk of the material, through several steps. In the gas phase, the reaction involves the following mechanism:

**1.** Surface physisorption (desorption) of molecular hydrogen (H2). In this step, the hydrogen molecules adsorb (desorb) at (from) the surface of the metal (M).

$$\text{M} + \text{H}\_{2} \leftrightarrow \text{M} - \text{H}\_{2}$$

**2.** Surface dissociative chemisorption (recombination). This leads to the formation (desorption) of atomic hydrogen at the surface of the metal.


burned, because its by‐product is water. For this reason, hydrogen can be considered as a clean

Hydrogen can be stored in fuel cells to produce electricity and can be used in applications as diverse as transportation or electronics industry in batteries for laptops and cell phones applications. Undoubtedly, one of the most important challenges that the hydrogen‐based economy has is its storing. In order to use hydrogen as a fuel, it needs to be safely stored in a medium that allows absorption, storage, and desorption, besides an easy transport. There are several ways of storing hydrogen, among them are metal hydrides, hydrides ceramics, carbon‐

As a consequence of the so called "Oil Crisis," in the 1970s, metal hydrides started to be considered as good candidates for using in hydrogen energy storage, due to their large capacity to accommodate an extremely high density of hydrogen in their structures. It is possible to pack more hydrogen into a metal hydride than into the same volume of liquid. The reason is that when a metal that forms a hydride is brought in contact with gaseous hydrogen, the hydrogen molecules are adsorbed onto the surface of the material. If enough energy is given to the system, the hydrogen molecules can dissociate into hydrogen atoms, which tend to enter to the crystal lattice of the metal and occupy interstitial sites. As the energy given to the system increases, hydrogen atoms are forced into the crystal until the metal becomes saturated with hydrogen. At this stage, the material goes into a new phase: the metal hydride, which allows

Metal hydrides, such as palladium, zirconium, or titanium, form an important group of reversible‐sorbing materials. The main advantage of metal hydrides over other hydrogen storage media, such as activated carbon, carbon nanotubes, or zeolites, is that they can be reversible hydride, and they can also release hydrogen of the metal hydrides in an extremely pure way, which is an important factor when considering hydrogen for mobile applications [1].

The process of hydrogen absorption (or desorption) by metals is reversible and involves the surface and the bulk of the material, through several steps. In the gas phase, the reaction

**1.** Surface physisorption (desorption) of molecular hydrogen (H2). In this step, the hydrogen

M H M–H + «2 2

**2.** Surface dissociative chemisorption (recombination). This leads to the formation (desorption)

molecules adsorb (desorb) at (from) the surface of the metal (M).

fuel.

based materials, etc.

the material to absorb hydrogen in larger amounts.

210 New Advances in Hydrogenation Processes - Fundamentals and Applications

**2. Metal hydrides for hydrogen storage**

involves the following mechanism:

of atomic hydrogen at the surface of the metal.

**3.** Surface absorption (desorption). During this step, the adsorbed atomic hydrogen is absor‐ bed (desorbed) by (from) the metal in its sub‐surface.

$$\mathrm{H}\_{\mathrm{ad}} \leftrightarrow \mathrm{H}\_{\mathrm{ab}}$$

**4.** Diffusion. The atomic hydrogen is transported from subsurface to the bulk region.

$$\mathcal{H}\_{\text{sub}} \leftrightarrow \mathcal{H}\_{\text{bulk}}$$

**5.** Phase transformation. Leads to the precipitation (dissolution) of the metal hydride phase.

$$\text{H}\_{\text{bulk}} \leftrightarrow \text{MH}$$

Pure titanium is considered to be an element with a high affinity to hydrogen; however, high temperature has to be used in order to absorb hydrogen in this material [2]. To avoid this inconvenience, some titanium alloys, such as TiFe, Ti2Ni, TiMn2, or Ti‐6Al‐4V, have attracted interest for storage hydrogen because they can absorb and release hydrogen in large amounts and at lower temperature than pure titanium.

The maximum hydrogen capacity of the material depends on the number of hydrogen sites that are available for hydrogen occupation. For instance, the Ti‐6Al‐4V alloy is composed by a biphasic structure: the beta phase (Body‐centered cubic –BCC‐structure) surrounding the alpha phase [hexagonal closed packed (HCP) structure]. The fact that this alloy has a BCC structure eases the hydrogen absorption, due to the way hydrogen atoms enter the crystalline structure and fit in the interstitial sites of the crystal. In the case of the beta‐phase of the Ti‐ 6Al‐4V alloy, the BCC structure has 6 octahedral sites and 12 tetrahedral sites per unit cell where hydrogen can fit. The octahedral and tetrahedral sites in a BCC structure are three times more than the ones found in the HCP and face‐centered cubic (FCC) lattices [3]. It has also been observed that the rate of hydrogen diffusion in a BCC structure is several orders of magnitude higher than in a HCP or a FCC structure [4]. **Figure 1** shows the scanning electronic microscopy (SEM) micrographs of (a) pure titanium and (b) the Ti‐6Al‐4V alloy. Pure titanium has a hexagonal closed packed (HCP) crystalline structure; meanwhile, the Ti‐6Al‐4V alloy shows an alpha phase (HCP) composed by Ti and Al, which looks as a dark zone in **Figure 1b**, surrounded by the beta phase (BCC) composed by Ti and V.

#### **2.1. The importance of an accurate quantification of oxygen and hydrogen in the materials used for hydrogen storage**

The initial mechanisms of hydrogen absorption depend on the crystalline structure of the metal, but also on its surface quality. The uptake rate of hydrogen can be reduced by a combination of metal oxides and hydroxides on the metal surface, acting as a diffusion barrier [5]. In this way, accurate oxygen and hydrogen measurements must be conducted in order to correlate the storage capacity of the metal with sample oxidation. In the literature, several techniques are used to detect oxygen and hydrogen in materials; nevertheless, these methods are destructive and have limitations for the determination of concentrations and depth profiles. The ion beam analysis (IBA) techniques, such as Rutherford backscattering spectrometry (RBS) [6] and elastic recoil detection analysis (ERDA) [7], are non‐destructive nuclear techni‐ ques that allow performing a very accurate measurement of the concentration and depth profile of oxygen and hydrogen, respectively.

**Figure 1.** SEM micrographs of (a) pure titanium and (b) Ti‐6Al‐4V alloy [8].

The physical principle of RBS consists of impinging a beam of collimated and monoenergetic particles (usually light ions such as hydrogen or helium) on the material to be analyzed. As the result of the interaction, part of the energy of the incident particle is transferred to the nucleus of the atom in the sample; so that the backscattered particle contains information of the target, because the reduction in energy of the incident particle depends on the mass of the projectile and the nucleus.

The ERDA technique is used to quantify the concentration of light elements, such as hydrogen and carbon. During an ERDA experiment, a beam of collimated and monoenergetic ions heavier than those who are to be detected, impinges on a material. The projectiles collide elastically with the nuclei of the sample, and as they are lighter than the projectiles, they acquire enough energy to leave the material. Simultaneously with the light ions, heavy projectiles could leave the sample and can reach the detector. For this reason, it is essential to place an absorber of Mylar or aluminum before the detector in order to stop the heavier projectiles and only let to lighter recoils to reach it.

In both techniques, the element that will be analyzed is identified by the energy and the type of the emitted particles that comes from the RBS or the ERDA reactions.

In the case of oxygen measurements, an oxygen resonance energy must be used in order to quantify the element concentration. This is because the Rutherford cross section of the oxygen is quite low, causing its RBS signal to be veiled by the signal of other elements present in the material. Oxygen resonances are presented at different energies, being the most common the ones located at 3.045 and 6.585 MeV. These resonances increase the cross section of the element by several times, making possible to improve the oxygen signal and quantify its concentration. In the next section, the 6.585 MeV oxygen resonance was used during the RBS experiment. This energy allowed obtaining the carbon signal as well, which is usually completely veiled by other elements.
