# Hydrogen Materials and Technologies in the Aspect of Utilization in the Polish Energy Sector
## 4 Hydrogen Storage
There is evidence that tanks for storing liquid hydrogen, despite the complexity of their structure, are less expensive per mass of stored hydrogen in comparison to tanks for pressurised gaseous hydrogen. However, it is essential to bear in mind the high costs related to the energy demand for hydrogen liquefaction and the capital required to build a liquefaction plant, which constitute a significant portion of the total costs of liquefaction, even for larger plants [44]. Unlike storing hydrogen in a gaseous or liquid form, most tanks based on hydrogen adsorption have to date only been used on a laboratory scale. As a consequence of the weakness of the van der Waals bond between the adsorbent and molecular hydrogen, low temperatures (liquid nitrogen) and elevated pressures (10-100 atm) must be used to obtain a significant hydrogen storage density by adsorption. Many porous materials based on carbon [45] have been proposed for storing hydrogen, porous materials composed of metal ions connected with organic ligands, so-called organometallic structures [46], porous polymer materials [47], and zeolites [48]. Some activated carbons and organometallic structures adsorb hydrogen in an amount of 8-10% m/m at a temperature of \(-\)196 \({}^{\circ}\)C [49; 50]. Nevertheless, owing to the fact that the employed adsorbents have a low density, the volumetric hydrogen storage density is low. Moreover, to ensure that the appropriate degree of adsorption is achieved, the generated heat must be effectively removed. In order to remove the heat of adsorption of 1 kg of hydrogen using an adsorbent that has a hydrogen adsorption enthalpy of 4 kJ/mol, at least 10 kg of liquid nitrogen must be evaporated [28]. In metal hydrides, hydrogen can be directly bonded to a metal atom (elemental metal hydrides and intermetallic hydrides) or may be part of a complex ion which is bonded to a metal atom (complex metal hydrides). Chemical hydrides, on the other hand, consist exclusively of non-metallic elements. Most elemental metal hydrides are unsuitable for hydrogen storage, resulting from their ability to store small amounts of hydrogen, their high temperature, and slow kinetics of the hydrogen absorption/desorption reaction. The most promising and widely researched metal hydrides for large-scale hydrogen storage are, among others, MgH\({}_{2}\) and AlH\({}_{3}\)[51; 52]. MgH\({}_{2}\) is an appealing hydrogen storage material due to its high theoretical hydrogen storage capacity of 7.6% m/m in addition to the fact that magnesium is non-toxic, inexpensive, and widely available. On the other hand, magnesium and hydrogen form a strong bond and the kinetics of the hydrogenation/dehydrogeneration reaction are slow. Because of these kinetic and thermodynamic limitations, in order to achieve a reasonable rate of MgH\({}_{2}\) dehydrogenation, temperatures over 300 \({}^{\circ}\)C must be applied [53]. The hydrogen storage properties of AlH\({}_{3}\) are different. The dehydrogenation enthalpy for aluminium hydride is about 11 times lower than for magnesium hydride and is only 7 kJ/mol. Hydrogen binds relatively weakly with aluminium, and the dehydrogenation reaction, in which theoretically 10.1% m/m hydrogen is released, proceeds quickly already at a temperature of 100 \({}^{\circ}\)C. However, this is an irreversible reaction, and extremely high pressures are required to rehydrogenerate Al from the gas phase [54]. The methods of obtaining aluminium hydride based on electrochemical regeneration [55] have proven promising but have not to date been implemented on an industrial scale. In practise, the number of intermetallic structures used to store hydrogen is relatively limited. Most often they are AB\({}_{5}\), AB\({}_{2}\), and AB alloys capable of absorbing a maximum of 2% by weight hydrogen.