Innovations in the field of Hydrogen storage in metal hydrides are moving at such a fast pace at the moment that is difficult to stay up to date. This post gives an overview of solid-state hydrogen storage in metal hydrides.

What is a metal hydride?

Metal hydrides are compounds of one or more metal cations (M⁺) and one or more hydride anions (H⁻). When pressurised, most metals bind strongly with hydrogen, resulting in stable metal hydrides that can be used to store hydrogen conveniently on board vehicles. Examples of metal hydrides are LaNi₅H₆, MgH₂, and NaAlH₄. Metal hydrides can be liquids or powders that are usually stored in tanks at approximately 1 MPa. As the pressure is reduced or the temperature is increased (between 120 °C and 200 °C), hydrogen is released. The metal hydride can be recharged without the use of a high pressure compressed gas or cryogenic liquid. When designing efficient metal hydride systems, the critical material properties to manipulate are thermal conductivity, heat of reaction and activation energy (Jorgensen, 2011).

Metal hydride storage has a low risk of accidental leaks since the hydrogen is stored within the metal hydride crystal and requires energy to be released. In addition, the energy spent in storing hydrogen using metal hydrides is about half as much as that of compression (70 MPa) and about a sixth as much as that of liquefaction. Thus, the CO₂/kg of hydrogen is the lowest of any storage method due to the low storage and release energies involved

Renewable energies, such as photovoltaic and wind power, are characterized by intermittent production, so their storage is necessary for efficient management. Among several solutions proposed, the use of hydrogen as an energy carrier is under investigation. Compared to batteries, hydrogen allows for storing larger amounts of energy in small volumes, over a long time, i.e., no self-discharge issues, with low environmental impact.

Hydrogen can be absorbed in the form of a metal hydride under mild conditions, i.e., close to room temperatures and atmospheric pressure. This solution ensures safe storage and reduces the volume required for storing even large quantities of hydrogen. Solid-state hydrogen storage based on hydrides has been investigated in recent years, with the goal to improve hydrogen gravimetric and volumetric density and to match thermodynamic requirements necessary for dehydrogenation reactions with an equilibrium close to ambient conditions.

Although MH have been known and studied for more than four decades (Van Vucht et al., 1970), no consensus has been reached on the optimum metal hydride to be used for hydrogen storage in stationary applications. This is because of the wide range of possible stationary applications resulting in diverse requirements in terms of hydrogen release/absorption rates, pressure and temperature operation of the metal hydride tank. For example, in the case of power-to-power systems, short-term ramp-up and down to smooth-out the effects of voltage sags and rapid changes in renewable energy generation requires very fast responding MH, i.e., fast H₂ desorption kinetics. In contrast, for seasonal storage, very cheap MH with a very high H₂ density for the bulk storage of renewable are considered more attractive. Upon the selection of appropriate hydride materials, several other important parameters should also be considered. These include the ease of activation of the hydride material toward hydrogen uptake/release, its uptake/discharge H₂ kinetics, the H₂ hysteresis effects (dictating the pressure for hydrogen uptake and release), the heat of formation of the hydride (as this will involve additional heat management issues controlling the rate of H₂ uptake/release and thus the overall efficiency), the H₂ cyclic stability and cycle life, safety associated with the hydride production, handling and operation, and its sensitivity to gas impurities as this will affect the practical H₂ storage capacity