# Hydrogen Materials and Technologies in the Aspect of Utilization in the Polish Energy Sector
## 4 Hydrogen Storage
Owing to the constantly developing techniques and the huge number of ideas for storing H\({}_{2}\), the United States Department of Energy has prepared specific guidelines for hydrogen tanks that will be used in so-called light vehicles--mainly passenger cars--but also bicycles and scooters. These requirements include, among others, the tank charging time, its efficiency, cost, and mass capacity, which should ultimately be 65 g of H\({}_{2}\)/kg of the compound, i.e., 6.5% by mass [56]. According to these requirements, the gravimetric densities of hydrogen storage by intermetallic compounds are too low to be used in passenger cars, but they are used as stationary hydrogen storage facilities. Assuming that the intermetallic compound reversibly absorbs only 2 wt% hydrogen, a material weighing 300 kg can store 6 kg of hydrogen fuel, which should cover a distance of approximately 500 km. For comparison, the total weight of the Tesla Model 3 battery pack with a capacity of 75 kWh is as much as 480 kg [57]. A significant problem related to the use of intermetallic hydrides is their price [58]. Nonetheless, despite their high costs, intermetallic hydrides are successfully utilised for hydrogen storage, as anode materials in Ni-MH cells, and in hybrid energy systems [59]. Complex metal hydrides, unlike intermetallic hydrides, consist chiefly of light elements. Thanks to this, they have the ability to gravimetrically store large amounts of hydrogen. The main complex hydrides utilised for hydrogen storage include alanates, borohydrides, and nitrates, which are extensively covered in the literature [60; 61]. Lithium borohydride (LiBH\({}_{4}\)), with its theoretical gravimetric hydrogen storage capacity of 18.5% by weight, is the highest among complex hydrides [62]. Nevertheless, during its dehydrogenation, very stable LiH is formed, reducing the useful hydrogen capacity [61]. Furthermore, quite high temperatures are required for the dehydrogenation of most complex hydrides by thermolysis, whereas only a few can be dehydrogenated reversibly, generally only in the presence of appropriate catalysts [28; 35; 63; 64; 65]. As a consequence, the direct use of borohydrides is not profitable for hydrogen storage. Chemical hydrides, like complex hydrides, consist of lighter elements. Under standard conditions, they are generally liquids, which makes them easier to store and transport. Currently, ammonia and methanol, with high gravimetric and volumetric hydrogen storage capacities, are considered as hydrogen storage facilities [35]. The usefulness of these compounds commonly synthesised from natural gas is not the best for hydrogen storage. A significant quantity of energy in the form of heat typically needs to be introduced to release hydrogen when it is chemically bonded to a storage material. Additionally, it must be cleaned, which usually requires additional energy input [64; 65; 66; 67; 68]. Finding a hydrogen storage material with high capacity and suitable hydrogen sorption/desorption kinetics while maintaining cyclic stability and economic viability faces significant obstacles. An alternative to compressing hydrogen, apart from its liquefaction, may be metal hydrides. Their greatest advantage is the higher volumetric density of hydrogen compared to free hydrogen in the gas or liquid phase. Storing hydrogen in metal hydrides is an expensive technology that has not been fully commercialised, with the exception of hydrogen storage alloys used in NiMH cells. In order to reduce the cost of producing metal hydrides, new low-temperature synthesis methods are being developed, described in [69]. It is expected that solid-state hydrogen storage will become widespread in the next 10-15 years. Currently, China has launched several demonstration projects of solid-state hydrogen storage power plants using AB\({}_{2}\)-MOF composite materials and an MgH\({}_{2}\)-LiBH\({}_{4}\) system with a capacity to store 100 tons of hydrogen [69]. In Poland, hydrogen storage is carried out using high-pressure metal tanks of large volume.