1.4 Hydrogen storage
1.4.3 Hydrides
%. (1.9)
where H/M is the hydrogen-to-metal or material host atom ratio, MH is the molar mass of hydrogen, and MHost is the molar mass of the host material or metal.
• volumetric storage capacity: it defines the amount of hydrogen stored per unit volume of material. It is defined as the number of hydrogen moles absorbed in the unit cell volume, assuming that during hydrogen uptake process the crystal lattice does not expand considerably. In reality, this additional effect should be taken into account in the calculation of the volumetric density.
Both quantities can be measured by gravimetric and volumetric techniques. A more detailed explanation of these two methods is given in [11].
1.4.2 Storage target requirements
In order to make hydrogen a safe a reliable fuel for on-board applications, some requirements have to be satisfied in order to become a commercial alternative to fossil fuels.
Some requirements have been set by the IEA (International Energy Agency) in the “IEA-HIA Task 32/17 - Hydrogen-Based Energy Storage” in 2006 [12, 13]. Also, the Department of Energy of United States (DOE) set some system targets, which have been revised in 2015 [14]. Some values are reported in table 1.3.
1.4.3 Hydrides
Hydrides are promising candidates for many stationary and mobile hydrogen storage appli-cations. Current applications vary from nickel-metal hydrides rechargeable batteries, aircraft
fire-detectors, isotope separation, synthesis of magnetic materials, switchable mirrors [7].
Hydrogen can be either adsorbed at the surface of the materials or chemically bonded with the host material. In metal hydrides the hydrogen is bounded with the structure of the material with a chemical bond.
The first metal hydride was discovered by Graham, who observed a large hydrogen uptake from palladium. Formation of metal hydrides is a chemical process, therefore the thermody-namics of this process will be shortly presented.
The reaction can be described by the following expression:
M + x
2H2 ←→MHx+Q, (1.10)
whereMis the hydride-forming metal andQis the heat of reaction in the formation process.
The uptake process of hydrogen at a constant temperature can be better visualized in the Pressure-Composition Isotherms (PCI) plot.
For each (constant) temperature, the pressure is plotted as a function of the hydrogen uptake (or concentration).
Three different regions can be identified:
• α-phase: at low H2 concentrationsx , hydrogen molecules dissociate at the surface of the metal and start to form a solid solution.
At this stage, the thermodynamic equilibrium conditions is given by:
1
2µH2(p, T) =µH(p, T, cH), (1.11) where µH2 and µH are the chemical potentials of molecular and atomic hydrogen, re-spectively. cH is the hydrogen concentration.
• α+β phase: as the hydrogen concentration increases, hydrogen atoms start to diffuse inside the lattice and the interaction H–H starts to be significant. In this region, a new phase nucleation takes place, characterized by high concentration, called β phase.
During this nucleation, the pressure does not increase with increasing H2concentration.
In this region, the equilibrium pressure Peq at the α → β transformation is given by the van’t Hoff equation:
lnpeq = ∆H
RT − ∆S
R , (1.12)
where ∆H and ∆S are, respectively, the enthalpy and entropy changes. Plotting the plateau pressure as a function of the inverse of the temperature, the van’t Hoff plot is obtained. A linear fit of the values leads to a slope and an intercept that are, respectively, proportional to the enthalpy and entropy changes. Therefore, these two values can be obtained and they characterize the reaction thermodynamics.
• β phase: once the phase transformation is completed, the αphase disappears and the pressure rises as the hydrogen concentration increases.
Figure 1.4: Left: PCI measurements. Right: van’t Hoff plot of the plateau values as a function of inverse of the temperature. The straight line is a linear fit in order to extract the enthalpy (slope) and entropy (intercept) changes.
The three phases above described are strictly valid for interstitial metal hydrides, e.g.
palladium hydride or LaNi compounds. However, PCI measurements can be performed also in other hydrides.
Different hydrides can be formed, depending on the metal-hydrogen bond. They can be grouped in three different categories: Ionic Hydrides,Covalent Hydridesand Metallic Hydrides.
The division made in three categories is not strict. In fact, most of metal hydrides don’t have a precise bonding type, but they might exhibit a mixture of different bondings. For example, lithium hydride (LiH) is not a pure ionic hydride, since it shows significant covalent bonding.
For the same reason, in magnesium hydride (MgH2), the interaction with magnesium and hydrogen is partly ionic partly covalent.
1.4.3.1 Ionic Hydrides
Ionic hydrides are characterized by an ionic bond between hydrogen and the host metal.
Usually alkali and alkaline earth metals are forming ionic hydrides. Examples are sodium hydride (NaH) or calcium hydride (CaH2). These compounds show a quite high decomposi-tion temperature, and therefore they are not suitable for hydrogen storage applicadecomposi-tions.
1.4.3.2 Covalent Hydrides
Covalent hydrides are formed by hydrogen and a non-metal. The bonding between hydrogen and the non-metal is covalent. Most covalent hydrides are liquid or gaseous at room tem-perature. In fact, they are characterized by low melting and boiling points. Examples are hydrogen sulfide (H2S), methane (CH4), water (H2O).
1.4.3.3 Metallic Hydrides
These compounds are formed by transition metals, including rare earth and actinide series.
The nature of the bonding between hydrogen and host lattice is metallic. Metallic hydrides have a wide variety of stoichiometric and non-stoichiometric compounds. Examples are pal-ladium or neodymium hydrides.