Hydrides: A outstanding family of energy related compounds

Hydrogen is the main “vector” chosen to play the main crucial role on the energy transition due to reasons such as sustainability, free-emissions and high energy density and emerges as a leading contender in providing flexibility, while carrying energy to all sectors across the energy landscape. Subsequently, the so-called hydrogen economy (or cycle) is rising. That is based on steps which be briefly enumerated as follows: green hydrogen production by water splitting, purification and separation, compression, transportation, storage, and combustion in different ways (using a fuel cell, an internal combustion engine). 

The synthesis, characterization and development of materials is essential to guarantee the implementation of hydrogen economy. However, they must also be “green”, i.e. non-polluting, abundant and easily recyclable. The hydrides based compounds are the ideal family to accomplish this challenge. Generally speaking, they are compounds with hydrogen bonded to some element. The flexibility of the H-bonding (H-atom may be bond with many elements to form ionic, covalent or interstitial (that is, metallic) systems) together with the small mass and size of hydrogen are the source of its high multi-functionality and variety which has meant that metal hydrides and, more generally, hydrogen-containing compounds are attracting a great deal of attention becoming a fundamental pillar of the hydrogen economy. 

Hydrides has been intensively explored for hydrogen storage which is one the key issued for employing hydrogen as energy vector. Hydrogen is a gas and even when is stored under high pressures (~800 b) the amount of stored hydrogen is far away to many applications. However, if hydrogen is stored into a hydride by simply a chemical reaction (H₂-molecule is absorbed by an element through cleavage of the H-H bond and chemically binding the hydrogen atoms to form a new chemical compound) the amount of hydrogen is drastically increased. For instance, palladium reacts with hydrogen gas forming palladium hydride (Animation 1), and an amount of hydrogen three orders of magnitude greater is stored in the same volume and under normal conditions (1 bar and room temperature). Even if hydrogen is compressed up to 800 b, the volume needed is still three times higher (0,02 kg/l) than those stored on palladium (0,06 kg/l).

It is obvious that palladium is not feasible to be used as hydrogen accumulator but other cheaper and sustainable materials could be used such could be TiFe alloys. In this case, amount of hydrogen stored is even more (0,125 kg/l). However, the gravimetric hydrogen density i.e. hydrogen stored by weight unit is worse than compressed gas. As hydrogen is stored into a solid, the hydride weight is drastically heavier than similar amount of hydrogen into a bottle. Those weight limitations have led that H₂-hydrogen storage are currently restricted to mid-capacity and stationary applications. To overcome this drawback, fundamental and applied research is performed to discover lighter hydrides. Therefore, the last decade has been prolific into synthesis and characterization of different families: magnesium hydride (MgH₂) able to store 0,7 kg in 10 kg of hydride to more complex one such as amino-boranes (NH₃BH₃) able to store (2 kg into 10 kg of hydride). Despite that amount of stored hydrogen is impressive, the needed to use moderate temperatures to drive the reaction are precluding its implementation.

Animation 1. The animation shows hydrogen (grey dots) entering a metal lattice (blue dots).

However, the future of hydrides and therefore of hydrogen is inexorably linked to applications beyond hydrogen storage.  Hydrides could be also employed in electric energy storage and transportation. In fact, the great success of the hydrides has been the negative electrode (an intermetallic hydride) of the massively used Ni-MH alloys during last decade. With the development of novel hydrides, other families (complex hydrides) of them have shown excellent capabilities to be used as solid electrolytes in sodium or lithium batteries.

A metal-insulating transition is usually observed when the metal is hydrogenated.  That transition provides a abrupt change on the optical properties of the metal, becoming transparent when is hydrogenated. This leads to the fact that these materials can be used as hydrogen sensors, or, as transition is usually observed when the metal is hydrogenated. This leads to the fact that these materials can be used as optical hydrogen sensors.

Finally, hydrides as potential superconductors (materials that do not oppose resistance to electric current) are harvesting attraction since a few years. Since the researchers discovered the superconductor character of hydrogen sulphide has a Tc of 203 K when was compressed to a huge pressure (150 GPa), a wave of interest in the compressed hydrides is emerging leading to different labs synthesise novel hydrides i.e. LaH₁₀ or CaH₆ with high number of hydrogen atoms contained.

Briefly, hydrides exhibit very exciting properties and functionalities in many energies related research areas. What else? For sure that the story of the hydrides will continue intimately related to hydrogen but besides it will also be closer to functionalities and properties which will play a crucial role in different ways of energy conversion.

Further reading

Introductory level:

• Jeremy Rifkin. La economía del hidrógeno (2002)
• Andreas Züttel, Andreas Borgschulte, Louis Schlapbach, Hydrogen as a Future Energy Carrier, Wiley-VCH Verlag GmbH & Co. KGaA, Germany, 2008.
• https://www.iea.org/reports/global-hydrogen-review-2021
• https://www.ieahydrogen.org
• https://hycare-project.eu/

Advanced level:

• Q. Wang et al. “The power of hydrides”, Joule 4, 705 (2020)
• R. Mohtadi et al. “The renaissance of hydrides as energy materials” Nat Rev Mater 2, 16091 (2017)