Hydrogen storage is a key issue and a great challenge on the road to the hydrogen economy, especially in the automotive sector. This is because, although hydrogen has a high gravimetric density, it has a low volumetric density compared to other fuels. Developing efficient storage systems is therefore crucial for the extensive use of hydrogen as an energy carrier.
There are several hydrogen storage technologies at different stages of technical and commercial development. Hydrogen can be stored as a compressed gas, in liquid form or in solid-state compounds. The first two methods include well-established but limited technologies. Solid-state hydrogen-storage is a promising option in order to overcome some of these limitations, but it is still far away from widespread industrial application.

Pressurized gas storage systems
Compressed gas cylinders are the simplest as well as the most mature and common hydrogen-storage technique. The pressure at which hydrogen is stored depends on the final use of the gas. As an example, hydrogen for laboratory or industrial use is compressed at about 200 bar, whereas higher-pressure levels (from 350 to 700 bar) are required for use in automotive applications.
Safety and integrity of the tank are major issues when hydrogen is stored as a high-pressure gas. The ability of atomic hydrogen to diffuse into materials may degrade the mechanical performance of the vessels increasing the probability of brittle fractures. This phenomenon takes place according to different mechanisms and is known as hydrogen embrittlement.
At present, four types of tanks are used for hydrogen storage. They are named Type I to Type IV and differ according to the materials employed for the designing and manufacturing of the tank.
Type I is simply a pressure tank made of metal (typically steel or aluminum); Type II is a pressure tank made of metal hoop-wrapped with a fiber-resin composite; Type III is a pressure tank made of a metallic liner fully-wrapped with a fiber-resin composite; Type IV is a pressure tank made of a polymeric liner fully-wrapped with a fiber-resin composite.
The choice of the type of tank depends on the final application as well as on economic considerations. More specifically, Type I and Type II are suitable for stationary applications. Type III and Type IV, which are lighter compared to Type I and Type II, are preferred for mobile applications.
Type I tanks represent the most common and cheapest storage solution. The main advantages are the low production cost (approximately 5 $/l) and large-scale knowledge of the production processes of the vessels. The main limitations are the high weight of the tanks (roughly 1,5 kg/l) and low maximum working pressure of the system, which generally does not exceed 200 bar.
Type II tanks are less heavy, but considerably more expensive than Type I tanks. However, the heavy weight of the vessels as well as the bulky size of the whole storage system make both these two types of tanks not suitable for automotive applications. Particularly, Type II tanks are used for high-pressure stationary storage applications with a working pressure up to 800 bar.
Type III tanks have a nominal working pressure of 350 bar and are less affected by hydrogen embrittlement compared to Type I and Type II vessels. The weight of the tanks is about 0.4 kg/l but their cost is two times higher than Type II vessels and more than three times higher than Type I tanks.
Type IV tanks have a nominal working pressure of 700 bar. The polymeric liner mainly acts as a barrier for hydrogen permeation, whereas the fiber-resin composite ensures the mechanical integrity of the tank. The weight and the cost of this type of vessels are comparable to those of a Type III tank. Research efforts are focused on developing new methodologies to improve the performances of these type of vessels.
An innovative method to store hydrogen gas at high pressure involves the use of tiny hollow glass spheres. The microspheres are warmed to about 300°C and then filled by immersion in hydrogen gas at high pressure (350-700 bar). This process exploits the fact that hydrogen permeability of glass increases with temperature. Next, the glass microspheres are cooled down to room temperature and the hydrogen is trapped inside them. For the hydrogen to be released, the spheres must be re-heated to 200-300°C. Glass microspheres have the potential of being a portable, inexpensive and safe hydrogen storage medium, especially in the automotive sector.

Liquid hydrogen storage
The storage of hydrogen in liquid form allows storing large quantities of hydrogen. This is because cryogenic hydrogen has a considerably higher energy density than gaseous hydrogen.
In terms of application, liquid hydrogen is currently used in NASA’s space program, petroleum refining and ammonia production. Cryogenic vessels are also used for the storing and transporting of medical gases.
Storage of hydrogen as a liquid needs very low temperatures below -253°C (hydrogen boiling point). Consequently, it is necessary to use high efficiency insulated tanks. Cryogenic vessels are normally vacuum insulated and strongly reinforced. Aluminum layers or perlite powder are used to further increase the thermal insulation of the structure.
The liquefaction of hydrogen is a rather energy-intensive process. Many different approaches can be followed to cool hydrogen gas down to -253°C. The simplest process is the Linde cycle. The gas is initially compressed at ambient temperature and is then cooled to about 78 K in a heat exchanger using liquid nitrogen. Subsequently, it is undergone to an isenthalpic expansion, producing some liquid. In order to reduce evaporation losses, catalysts are added to promote the conversion of ortho-hydrogen to para-hydrogen.
The storage of hydrogen in liquid form requires the adoption of a number of specific safety precautions. Generally, however, the storage of liquid hydrogen poses fewer risks than storage of hydrogen gas. More precisely, in case of tank failure, cryogenic hydrogen disperses into the atmosphere more slowly than pressurized hydrogen gas.

Solid-state hydrogen storage
Some safety and technical issues relating to the storage of hydrogen in gaseous and liquid form can be addressed by solid-state hydrogen storage.
Hydrogen can be stored in solid state either physically or chemically. In the first case (physisorption), hydrogen molecules are physically adsorbed on the surface of a solid whereas, in the second case (chemisorption), hydrogen chemically reacts with a solid forming a hydride.
Hydrogen physisorption is driven by weak van der waals forces. As physisorption is a surface mechanism, researchers and industries have investigated high surface area materials over the years. Researched materials include graphitic nanofibres, carbon nanotubes, metal organic frameworks (MOFs) and zeolites.
Graphitic nanofibres are produced from the dissociation of hydrocarbons or carbon monoxide over catalytic surfaces. Depending on the catalytic used, graphitic sheets are arranged as platelets, ribbons or herringbone.
Carbon nanotubes are made of graphite sheets rolled-up in cylindrical shape. Depending on the number of graphite layers making up the structure, carbon nanotubes are classified into multi-walled carbon nanotubes (MWNTs) and single- walled carbon nanotubes (SWNTs).
MOFs are synthetic nanoporous materials consisting of metal ions or clusters connected by organic ligands to form mono-, bi- or three-dimensional structures with very high porosity. A remarkable feature of these materials is the high specific surface area, which can exceed 6000 m2/g.
Zeolites are three-dimensional microporous minerals commonly used as catalysts and adsorbents. Zeolites contain aluminum, silicon, and oxygen combined to form open tetrahedral structures with different framework types. At ambient temperature, zeolites have a low hydrogen storage capacity, which can be increased by reducing the temperature down to 77 K.
In chemisorption, hydrogen forms a chemical bond with a solid material. Most metals and alloys can react reversibly with hydrogen to form a metal hydride. Some of them include iron, titanium, nickel, manganese, etc. When hydrogen molecules come into contact with the surface of a metal, they dissociates and hydrogen atoms are inserted within the metal lattice. This is an exothermic process, in which heat is released. This heat must be removed in order to maintain the continuity of the reaction. Conversely, the reverse reaction is endothermic and small amounts of heat must be supplied in order to release hydrogen.
Hydrogen storage in metal hydrides depends on several factors. The most important include the surface structure and purity of the metal, which affect the amount of hydrogen that can be stored per unit of mass and per unit of volume, the energy that needs to be supplied for the hydrogen release process, overall efficiency, kinetics, process stability and costs.
At present, metal hydrides represent a promising but not mature technology. A major advantage concerns the safety aspect. Damage to a hydride tank would not lead to a fire or explosion because hydrogen would be trapped within the metal. On the other hand, the metal hydride gravimetric storage density is rather low. This disadvantage is particularly noticeable where weight represents a problem, for example in the automotive sector.