Hydrogen storage: the smallest molecule, the biggest gas

Hydrogen can be stored in many forms. The main ones are compression (up to 700 bars) and liquefaction below -253.2°C. Storage in the form of hydrides and in underground caves are promising methods.

This article is a part of our dossier on hydrogen, innovation and ecology.

One of the biggest challenges of hydrogen is its storage due to its volume. Under “normal” pressure and temperature conditions, 1 kg of hydrogen occupies about 11,000 liters. Even if it contains, per kg, 4 times more energy than gasoline, the difference in volume is gigantic.

“Under normal temperature and pressure conditions (CSTP) 1 bar, 20°C, hydrogen occupies a volume of 333 litres/kWh and, under the same conditions, gasoline occupies 0.1 litre/kWh. »

Translated from french, CEA, Les technologies de l’hydrogène au CEA, 2012

To use it, especially for its mobile applications, you have to find a way to reduce the volume. The main methods for this are liquefaction (liquid hydrogen stores 70g/litre, i.e. 14 liters to contain one kg) and compression (42g/litre at 700 bars, i.e. 23 liters per kg).

General issues of dihydrogen storage

One of these locks for the use of hydrogen as an energy carrier is its storage. It must provide, on the one hand, a high degree of security and, on the other hand, ease of use (storage capacities – storage/retrieval dynamics) to enable operation under acceptable technical conditions. For hydrogen to become a viable solution, storage processes will therefore have to be safe, economical and suitable for a multitude of uses in the future: mobile applications for transport and portable or stationary devices.

Translated from french, Maxime Botzung, p.17

The hydrogen storage methods studied

There are many ways to store hydrogen. Among those that are based on a physical change, there is gas compression, liquefaction, but also cryocompression. Those that are based on materials are more numerous. They can be based on chemistry. This is the case for ammonia, formic acid and organic liquid hydrogen carriers. They can also be based on physical mechanics. This is the case of metal hydrides, zeolites, Metal Organic Framework (MOF), “glass capillary array” and glass microspheres.

We will only see the main ones:

  • Compressed gazeous hydrogen
  • Liquid Hydrogen
  • Adsorbtion storage
  • Metallic hydrides

Recurring issues

Each of these methods has its own issues and their interest varies greatly depending on their uses. Overall, several questions will systematically arise:

  • How much energy does it take to transform H2 into its stored variant?
  • How much energy does it take to maintain the gas in its stored form? How much energy does it take to bring the gas back from its stored form to a usable form?
  • What are the risks posed by the stored form of the gas ? (e.g. compression implies a danger of explosion, ammonia a risk of pollution, etc.)

Physical hydrogen storage

The most common way to store hydrogen is to “simply” compress it. This can be done at high pressure, low pressure or, for stationary storage, in a salt cavern (common practice for methane).
Hydrogen storage in the form of gas

Mechanical compression of hydrogen at high pressure

The most common hydrogen compression format in vehicles is 350 bars in general (buses, planes, etc.) and 700 bars for hydrogen cars. These are enormous pressures, which require a lot of energy to achieve: the compression of the gas causes around 10% of the energy to be lost.

In addition, extremely strong containers are needed. There were fears in the 1970s that the constraints were such that hydrogen tanks would have to be rounded, which would have been problematic to place in a vehicle.

However, the current containers are cylindrical and do not pose too much of a problem to place (eg under the seats for cars). They are composed of a coating (“liner”), for sealing, and a polymer for the mechanical resistance and protection of the tank.

Hydrogen compression at low pressure

For stationary storage applications, it seems to me that pressures of 50 bar are sufficient. To check.

Underground hydrogen storage

Underground gas storage is already common. Aquifers and salt caverns are already widely used to store natural gas. Nevertheless, hydrogen carries specific storages which require a rethinking of everything.

According to a study of capital costs, the cheapest option would be the depleted deposit ($1.23/kg), before the aquifers ($1.29/kg) and the salt caverns ($1.61/kg). (Tarkowski 2019)

One may wonder whether seismic activity does not represent a risk for underground hydrogen storage.

Storage in aquifers

Gas can be stored in aquifers. Pressurized gas is injected to “replace” the water and take advantage of a naturally impermeable surface that will serve as a reservoir. There are 13 sites in France. It is preferred to storage in salt caverns, because the operating costs are lower and there is no problem of the corrosiveness of the salt.

I don’t believe this is seriously considered as a storage solution for H2. (to check)

Storage in depleted / depleted deposits

Another, similar solution is to store the gas in old hydrocarbon deposits. This would represent >40% of storage sites in Europe. The big problem is the contamination of the gas with sulfur residues, which must then be removed.

However, I do not believe that this is envisaged for hydrogen. (to check)

Storage in salt caverns

Storage in salt cavities consists of artificially creating a cavity by injecting water into a salt deposit. The latter dissolves, leaving a hollow in which gas can be placed. The practice is already common for the storage of natural gas: there are 170 salt caverns used for this purpose in Germany. It is also planned to store hydrogen in these salt cavities. The big advantage of this approach is the tightness of the device, the walls being covered with saline crystals. The salt would not react with the hydrogen and there would be few losses related to microorganisms. There are several sites that already exist (3 in the US, 1 in the UK and 1 in France), but this is still experimental.

Overall, Tarkoswski (2019) is quite pessimistic about these storage modes:

“Underground hydrogen storage is not yet and will not be in the next few years a viable and technically feasible way to store energy.”

For further, you can consult our article on salt caverns.

Liquid hydrogen

Hydrogen liquefies at a very low temperature (20.28°K, or -252.87°C). This liquefaction requires a lot of energy and releases a lot of heat. However, liquid hydrogen is much denser than in its gaseous form: its density, at atmospheric pressure, goes from 0.08988 g/l to 70.973 g/l.

The big problem with H2 liquefaction is that it consumes a lot of energy and time: up to 40% of the energy can be lost in the process! It is also necessary to maintain this extremely low temperature over time, which is an extremely difficult challenge.

“Some car manufacturers have become interested in cryogenic storage of hydrogen in liquid form. This technique currently offers the best performance in terms of mass and volume (hydrogen occupies a volume of 0.38 litres/kWh) but has two major drawbacks: liquefaction is very energy-intensive and tank safety is more difficult to ensure (boil-off phenomenon and the fragility of the reservoirs). »

CEA, Les technologies de l’hydrogène au CEA, 2012

Material-based physical storage

Even higher storage densities could be obtained by storing hydrogen “in” (by adsorption) or “on” (by adsorption of materials.

If the processes are interesting, especially metal hydrides, they are not yet mature. Thus, Moradi and Groth (2019) conclude their review:

“Material-based storage methods are still in an early stage of development and need more time to demonstrate that they are viable long-term solutions.”

Hydrogen storage by adsorption

Definition of adsorption: “The adsorption of a gas such as hydrogen by a solid, or physisorption, is the increase in the density of this gas at the surface of the solid by the effect of intermolecular forces.” (AFHYPAC, file 4.4)

It increases with the pressure and with the surface of the solid and decreases with the temperature. The reaction is purely physical, there is no change in the composition of the molecules: the hydrogen is charged or discharged by changing the pressure and/or the temperature.

It requires a solid with a large interaction surface, both very porous and very divided. They may in particular be activated carbons or carbon nanotubes.

“Thus activated carbons made up of entangled micro-crystals of graphite form a network of pores with diameters of the order of a nanometer. When combined, the surfaces of these pores represent an active surface that can reach several thousand m2 per gram of carbon. »

AFHYPAC, Stockage solide de l’hydrogène, fiche 4.4

However, activated carbons could only contain 2% of their mass in hydrogen at room temperature.

Hydrogen storage by absorption: metal hydrides

The storage of hydrogen in the form of metal hydrides (metals capable of absorbing hydrogen) is freed from the constraints linked to high pressures and liquefaction. That allows him:

  • Very good efficiency (McPhy technology, based on magnesium hydrides, claims that the absorption-desorption process restores 97% of the hydrogen)
  • Stability in pressure / temperature situations (relatively) close to normal

However, it is still at an experimental stage and is handicapped by its weight.

The main challenges are:

  • The exothermic (heat-releasing) nature of hydrogen absorption and endothermic (heat-requiring) nature of its desorption
  • The need to maintain precise conditions to absorb/desorb hydrogen.

To further, you can read our article on metal hydrides.

Le stockage chimique d’hydrogène

We have seen “physical” hydrogen storage, i.e. without a chemical reaction (assuming that the absorption of hydrides is not one, I have a doubt). Now we are going to see the chemical storage of hydrogen, ie through a transformation into another molecule. The main ones are ammonia (NH3), methane (CH4) and methanol.

Ammonia (NH3)

Ammonia (be careful not to confuse it with aqueous ammonia NH4OH or NH3) is probably the most promising molecule. First of all, it has already been produced since 1909 from hydrogen by the Bosch-Haber process to make fertilizer. It is therefore a mature technology framed by the necessary infrastructure, regulations, etc. Then, its characteristics would be interesting:

Apart from solid-state materials, hydrogen can be stored indirectly in otherchemicals such as methanol, methane or ammonia. Out of these, ammonia can be considered as a much cleaner option owing to its carbon-free structure
and high hydrogen content of 17.7 wt%.

Sankir et Sankir 2018, p.62

Its boiling point would indeed be -33.5°C, so it could easily be stored in liquid format at a pressure of 8 bars. However, it is not currently used due to its toxicity, a point which could be addressed by storage in solid form, inside molecules such as halides or borohydrides. (Sankir and Sankir 2018, p.62)