Metal hydrides are metals able to absorb hydrogen or give it back depending on the pressure and temperature conditions. It is a method of hydrogen storage that allows high volume densities, is stable and efficient. However, it is still at an experimental stage, notably because of the difficulty of maintaining the storage conditions.
Hydrogen has a lot of applications to fight against global warming and protect the environment. Indeed, this gas has the capacity, if we manage to overcome certain challenges, to decarbonize difficult sectors, such as the steel industry or heavy mobility. It could also, combined with an electrolyser and a fuel cell, absorb the intermittency of renewable energies, such as wind and solar power. However, one of its greatest weaknesses is its transport and storage. Indeed, in gaseous form, hydrogen in “normal” conditions (~1 bar, 20°C), it occupies 11,000 liters per kg! To be stored, it needs to be compressed or transformed into liquid form. These processes are however energy consuming.
One possibility is to transform it into hydrides. These are molecules that can absorb hydrogen and release it when the pressure and temperature conditions are varied. In fact, they serve as a hydrogen reservoir. The process could have the highest efficiency: >95%. Its main problems are its mass, which is not a problem for ships or for stationary storage (see what we have just said about renewable energies) and the maintenance of suitable conditions. Nevertheless, if there has been a lot of research in the last 30 years, there are no large-scale applications today.
Be careful not to confuse them with metallic hydrogen, which is a physical state of hydrogen.
Definition of metal hydrides
Metal hydrides are metals that can absorb hydrogen or give it back depending on pressure and temperature conditions.
A metal hydride is a solid intermetallic compound formed by the direct action of hydrogen gas on a metal or metal compound M following the reversible reaction : M + x/2 H2 <=> MHx + ΔH (heat)
M.Botzung 2008
“Complex metal hydrides are compounds for which hydrogen atoms establish covalent or ionic bonds with nearby metal atoms.”
L’hydrogène dans tous ses états: du solide au gaz en passant par le liquide
It is a process of absorption (chemical exchange)
Absorption, or chemisorption, is the reversible chemical combination of hydrogen with atoms of a wide variety of metals or alloys to form metal hydrides or hydrogen-metal complexes.
Crédits : AFHYPAC, Stockage solide de l’hydrogène, fiche 4.4
It differs from adsorption, where hydrogen simply “settles” on the molecule.
Some examples
There is an infinity of studied tracks (sodium alanates NaALH4; LiBH4; borazanes NH3BH3; Li2NH …). They are classically distinguished in 5 families: AB, A2B, AB2, AB5 and the BCC. Here are some examples presented by M.Botzung :
| Molecule | Class | Reversible mass capacity | Temperature (°C) at 1 bar | Kinetics | ΔH (kJ/mol) | Cost for 200 kg ($) |
| FeTi | AB | 1.5 | -8 | Fast | -28.1 | 936 |
| LaNi5 | AB5 | 1.28 | 12 | Very fast | -30.8 | 1974 |
| Mg2Ni | A2B | 3.3 | 255 | Average | -64.5 | 1252 |
| Mg | 7.6 | 279 | Very slow | -74.5 | 850 |
A stable way to store hydrogen
The molecule does not need to be stored at very high pressures or extreme temperatures. They can often operate at near-normal conditions.
Storing hydrogen efficiently
This stability also gives them a higher efficiency: there is no need to compress or liquefy the hydrogen.
The CEA Grenoble has announced a 97% efficiency of its storage system!
The realization of magnesium hydride tanks is possible for the storage of large quantities of hydrogen. The next step is to increase the storage efficiency: currently, the energy dissipated during hydrogen absorption is lost.
The recovery of this energy in a phase change material should provide the energy necessary for desorption without external energy input. The storage energy balance would be very close to 100%.
Albin Chaise 2008
The constraints of the reservoirs
One of the properties of these molecules is that they expand and shrink depending on whether they absorb or release hydrogen.
Therefore, free space should be provided in the hydride tank to anticipate this expansion.
Developments and applications of metal hydrides
I will briefly present the history of this hydrogen storage solution, before going on to discuss its application.
Note that I will not talk about a technology using hydrides: nickel-metal hydride batteries (NiMH) (notably developed by Panasonic and Duracell). Indeed, it is not a hydrogen storage technology.
History of hydrogen storage projects in hydrides
1974: The first hydrogen storage in the form of metal hydrides took place in 1974 at the CEA Grenoble: 900kg of TiFe could contain 14.5kg of H2.
2000: HDW imagined (produced?) in Germany a hydrogen storage system in the form of hydrides for a submarine (compressed gas being to be avoided in this context). These molecules are particularly adapted for this use, the weight not being a problem. The system included 18 tanks each weighing 4.4 tons for 1200 liters containing 55kg of dihydrogen. They were of the TiFe or TiMn type.
2003: LabTech developed a 1.33kg H2 storage in LaNI5 hydrides in Bulgaria as part of the HELPS (Hydrogen based Electrical energy system for Local Power Storage) project. Charging lasted 42 hours at a pressure of 15 bar and a temperature of at most 25°C. Unloading lasted 5 hours at a pressure of 3 bars and a temperature of 80°C at the most.
2003: Ovonic develops in the United States a storage system that can store 10kg of hydrogen. It could be charged in 13 hours.
2005: Hera (Canada) and Caterpillar (US) designed a hydrogen powered construction machine storing 13.2kg of H2 in AB2 hydrides discharging at a temperature of 60 to 75°C and a pressure of 1.8bar.
2005: Treibacher, a producer of metal powders, designed a building project (“Glashusett”) in Austria equipped with a H2 production / consumption system. They have developed a metal hydride storage (TiVCr) that can hold 1.33kg of H2 in 90 liters.
2005: Toyota tested in Japan a 4×4 with a hydride tank of type
Ti-Cr-V type hydride tank. The heat management (to hold, then desorb the hydrogen) was very difficult.
2005: Ovonic modified a Prius in the US to run on a hydride tank containing 3.3 kg of H2 operating between 20 and 35 bar. It had a range of 320 km.
Magnesium (Mg) hydrides
Until now, the use of magnesium hydrides in the form of nanomaterials has proven to be the most successful and industrially applicable process for static storage in a wide range from tens to hundreds of kg of hydrogen. It is even possible to apply this process to mobile units when the weight is not essential (large vehicle, train, ship, submarine…) and in conditions where the cost constraints remain marginal. AFHYPAC, Solid hydrogen storage, sheet 4.4
Magnesium “represents a good potential candidate for hydrogen storage” and is the subject of much attention: more than 2000 publications over the last 40 years! (Liv Pall’s thesis) It has a good mass absorption capacity (7.6%) and is abundant (and therefore inexpensive).
Mg+H2 <=> MgH2
The absorption of H2 releases heat: 75 kJ/mol. This reaction is said to be exothermic.
A process developed in France
The work of a team from the Neel Institute of the CNRS in Grenoble in collaboration with the SME “MCP Technologies” within the framework of the European projects Hystory and Neshy would have resulted in the development of a storage solution. AFHYPAC describes the production process:
It has developed a manufacturing process by microgrinding of a mixture of nanostructured powders of magnesium hydride and transition metals with sufficiently fast adsorption and desorption kinetics for application to hydrogen storage with a performance of 7.6% by mass. These powders are then mixed with expanded graphite and the whole is compacted in the form of wafers (diameter 50cm, thickness 2cm, each containing 0.6 Nm3, i.e. 50g, of hydrogen). These are stacked in thermally insulated storage cylinders.
AFHYPAC, Stockage solide de l’hydrogène, fiche 4.4
The absorption/desorption reaction would ideally occur between 350 and 370°C for pressures ranging from 1 to 10 bars. The thesis of Albin Chaise from the Néel Institute of the CNRS, validated by the Joseph-Fourier University of Grenoble and published in 2008, showed the feasibility of this process (Albin Chaise 2008). The McPhy Energy company was created from these patents.
The McPhy Energy solution
McPhy Energy has developed a hydrogen storage system in the form of magnesium hydrides that would have an extraordinary energy yield of 97%. The system would absorb or reject hydrogen depending on whether the system is above or below an equilibrium pressure (quite low, between 10 and 2 bars).
The company has concluded several important partnerships:
- With Enel, Europe’s 2nd largest electricity producer, for a 2kg hydrogen storage, to conduct experiments in Italy.
- With Iwatani Corporation, Japan’s leading hydrogen producer, for a 4kg hydrogen storage.
McPhy Energy has already signed contracts with major electric utilities such as Enel in Italy and E.ON in northeast France. The company recently acquired the Italian company Piel, which has nearly 3,000 electrolysers in operation worldwide. 5 million from the Fonds Écotechnologies, a Caisse des Dépôts fund for innovative small and medium-sized businesses dedicated to green technologies.
Boron hydrides
The boron hydride (or borohydride) track was promoted by Bor4Store, a European project launched in 2012 funded with €2.27 million from the Hydrogen and Fuel Cell Joint Technology Initiative (FCH JU).
H2 Circular Fuel has developed a sodium borohydride (NaBH4) powder to store hydrogen at room temperature. It is a salt that releases the gas by dissolving in water. This gas can then be used to power the fuel cell that powers the vehicle’s engine. The process is exothermic. Its density would be three times higher than compressed H2 at 700 bars (126kg/m3 against 43). Neo Orbis, a hydrogen-powered ship from Amsterdam using this storage solution, is scheduled to be launched in June 2023.
Researchers at Deakin University (Australia) have discovered a method for storing solid hydrogen in boron nitride (Srikanth Mateti, Chunmei Zhang, Aijun Du, Selvakannan Periasamy, Ying Ian Chen, Superb storage and energy saving separation of hydrocarbon gases in boron nitride nanosheets via a mechanochemical process, Materials Today, Volume 57, 2022, Pages 26-34, ISSN 1369-7021, https://doi.org/10.1016/j.mattod.2022.06.004). It is basically a process to separate olefin and kerosene gases: steel balls are placed in the presence of the gases to be separated and boron nitride powder. By agitating them, the gases would combine with the material to form hydrides. The process would consume 76.8 KJ/s of energy to separate 1000L of gas [rq: why KJ per second?] It would also allow to store hydrogen with a limited energy consumption and a good density. They would have tested only a few over liters of gas. This is to be confirmed, I did not have access to the article and the university is quite evasive. The same goes for the article in The Conversation. According to one of the researchers, the mass density of the storage would be 6.5%.
LaNi5 hydrides
The PLUSPAC project consisted in testing the interest of metal hydrides for intermittent energy storage and mobility. The choice was made for the LaNi5 hydride.
The powder used would work at 75°C, would absorb hydrogen at 1.5bars and desorb it at 1.5bars. The kinetics would be fast and the material would swell by 12%. The prototype, consisting of 10kg of hydrides, could absorb or desorb 100g of hydrogen in 2h at respectively 80°C and 70°C. (Thesis of M. Botzung)
The aliminium – lithium hydride (LiAlH4, lithium tetrahydruroaluminate)
Lithium tetrahydruroaluminate, composition LiAlH4, but often noted LAH, is a hydride already used in organic chemistry as a reducing agent (it removes oxygen). Hydrogen represents 10.6% of its weight. However, it is not stable enough, requiring a pressure of 10 000 bars to be maintained. I have not seen any practical application.
The challenges of hydrides
The main challenges of hydrides are going to be their weight, rate of absorption/desorption and temperature/pressure management. Here are the important variables:
- Reversible hydrogen storage capacity as a function of mass or volume
- The rate of absorption and desorption. This is also called kinetics (= the reactivity of the system).
- The thermal conductivity of the material, to evacuate the heat produced by the hydridation reaction (= hydrogen absorption).
- Little heat to be supplied to desorb the hydrogen (low enthalpy of formation)
- An equilibrium temperature close to the ambient temperature
- An equilibrium pressure adapted to the use and as close as possible to 1bar.
- Obviously, the price and the abundance of the materials used.
The two most promising ways to solve these problems are based on the development of systems based on new materials with high storage capacity: reversible metal hydrides (metal alloys or complex compounds depending on the conditions of the targeted application) and porous materials (nanostructures, activated carbons, etc.) which both allow hydrogen storage at reduced pressures (<5 MPa).
Maxime Botzung, Conception et intégration d’un stockage d’hydrogène sur hydrures métalliques
To further
Bibliography:
- Albin Chaise. Etude expérimentale et numérique de réservoirs d’hydrure de magnésium. Energie électrique. Université Joseph-Fourier – Grenoble I, 2008
- Maxime Botzung, Conception et intégration d’un stockage d’hydrogène sur hydrures métalliques, thèse présentée le 29 Avril 2008