Hydrogen Storage

Hydrogen Storage

Our work is focused on the study of novel light metal hydrides and more recently on complex hydrides as hydrogen storage materials. In particular, we are interested in the kinetic, thermodynamic and structural properties of these compounds and in processes leading to the optimisation of their hydrogen storage properties by different synthesis methods, heat treatments, alloy additions or use of catalysts.

Hydrogen is not a primary energy source, but an energy carrier such as petroleum, natural gas or coal, with an energy content per kilogram of hydrogen three times larger than that for gasoline. Hydrogen also exhibits the highest heating value per mass of all the chemical fuels. The main difference between hydrogen and other energy carriers is that the hydrogen combustion only releases energy and pure water, i.e. hydrogen is a clean fuel.

Figure 1: The circuit of hydrogen
Figure 1: The circuit of hydrogen

The major difficulty for the use of hydrogen as energy carrier is the fact that hydrogen is a gas at room temperature. Under such conditions, the hydrogen energy content per volume is low due to the low density of its gaseous state and does not fulfil the requirements for mobility (e.g. cars and mobile devices) or even, stationary applications (e.g. household applications).

It is thus necessary to find new methods for hydrogen storage leading to a safe and efficient use of hydrogen.

The key factors for hydrogen storage are: lightweight storage media with large storage capacity and small volume, and the possibility to introduce and retrieve the hydrogen from the media at moderate temperatures and pressures with a good cyclic stability.

Hydrogen can be stored as pressurised gas in high pressure gas cylinders (compression up to 800 bar, in the case of lightweight composite cylinders), as liquid hydrogen in cryogenic tanks (liquefaction at 21.2 K and ambient pressure), or as solid, adsorbed on interstitial sites in a host metal (at ambient pressure and temperature) or chemically bonded (absorbed) in covalent and ionic compounds (at ambient pressure). Common solid state storage materials are Metal Hydrides and Complex Hydrides. Hydrogen can also be indirectly stored through the oxidation of reactive metals (e.g. Li, Na, Mg, Al, and Zn) with water to form hydroxides.

Hydrogen storage in solid form offers the safest alternative for transportation and storage of hydrogen. Several promising systems including adsorbed hydrogen on nano-structures (e.g. nano-tubes and metal organic frameworks), and hydrogen absorbed in transition metal based hydrides and complex hydrides are currently being investigated. Metal hydrides are known to possess high volumetric hydrogen densities (of the order of 3 to 8 wt.%) along with the ability to store hydrogen at atmospheric pressure and room temperature. Moreover, many metals and alloys are capable of reversibly absorbing large amounts of hydrogen. Particularly interesting are light metals such as Li, Mg, B, and Al, from groups one and two in the periodic table, that can combine with hydrogen to form a large variety of metal-hydrogen complexes (e.g. MgH2).

Figure 2: Schematic presentation of the gas-solid reaction: dissociation of molecular hydrogen at an interface, penetration and diffusion (interstitial solution) of hydrogen atoms in the bulk.
Figure 2: Schematic presentation of the gas-solid reaction: dissociation of molecular hydrogen at an interface, penetration and diffusion (interstitial solution) of hydrogen atoms in the bulk.

The main difference between complex hydrides and metallic hydrides is that, when absorbed, the hydrogen atom is part of a real ionic or covalent compound instead of being placed on interstitial sites of the metal lattice. In complex boron- and aluminium-based hydrides, for example, the hydrogen atom is often located at the corners of a tetrahedron with boron or aluminium atoms in the centre (see figure below). The negative charge of these formed anions ([BH4]- and [AlH4]-, respectively) is then compensated by a cation (e.g. M=Li or Na). Hydride complexes of borane, the tetrahydroboranes M(BH4), and of alane, the tetrahydroaluminates M(AlH4) are considered to be specially interesting hydrogen storage materials because of their light-weight and high gravimetric hydrogen densities. However, they are very stable and decompose only at elevated temperatures, often above the melting point of the complex. By changing the synthesis conditions of these compounds (e.g. pressure, temperature and use of catalysts), we can optimise their hydrogen storage capabilities and therefore, making them suitable for technological applications.

Figure 3: Typical structure of complex hydrides
Figure 3: Typical structure of complex hydrides