# Hydrogen Materials and Technologies in the Aspect of Utilization in the Polish Energy Sector
## 2 Physicochemical Properties of Hydrogen  
Of all the elements, hydrogen is the lightest in any state of matter. The density of gaseous hydrogen (273 K, 1.0 atm) is 0.09 g/dm\({}^{3}\), liquid hydrogen 71 g/dm\({}^{3}\), and crystalline hydrogen 88 g/dm\({}^{3}\). Under standard conditions, it occurs in the form of diatomic H\({}_{2}\) molecules; it is a colourless and odourless gas with a boiling point of 20 K and a freezing point of 14 K. Due to its properties, it is difficult to detect with the human senses, and its small size contributes to high diffusivity in addition to its tendency to penetrate many solid materials. Hydrogen is a flammable gas and should be stored away from heat sources or open flames. The reaction of hydrogen with oxygen is highly exothermic (Equation (1)). However, it should be remembered that it takes place very slowly at room temperature. This reaction occurs at a noticeable rate only at the temperature of 180 \({}^{\circ}\)C, and at 450 \({}^{\circ}\)C, it is explosive. The process is particularly vigorous when the volume ratio of hydrogen to oxygen in the mixture is 2:1 (the so-called fulminant mixture). \[2\mathrm{H}_{2}\;(\mathrm{g})+\mathrm{O}_{2}\;(\mathrm{g})\to 2\mathrm{H}_{2} \mathrm{O}\;(\mathrm{g})\quad\Delta\mathrm{H}=-483.6\;\mathrm{kJ} \tag{1}\]  
As can be seen from Figure 1, hydrogen is an extremely attractive fuel, owing to its high gravimetric energy density (143 MJ/kg) being approximately three times greater than the energy density of conventional fuels such as liquefied natural gas (LNG) (55.5 MJ/kg), compressed natural gas (CNG) (55.5 MJ/kg, petrol (46.4 MJ/kg), or diesel oil (45.6 MJ/kg) [4; 5; 6; 7]. On the other hand, the value of hydrogen combustion heat related to a unit of volume is small. Hydrogen gas compressed to a pressure of 700 atm at room temperature has an energy density of 5.6 MJ/L, which is approximately six times less than gasoline or diesel oil. During the process of liquefaction, its volume decreases, raising the energy density of liquid hydrogen to 10 MJ/L. Nonetheless, compressing hydrogen is a costly process, and converting it into a liquid form requires the temperature of hydrogen to be reduced below 33 K (the critical temperature of hydrogen). It has been estimated that the hydrogen liquefaction process requires the equivalent of approximately 36% of the energy stored in it [8]. The very low volumetric energy density of gaseous hydrogen makes its transport and storage inefficient and therefore significantly more expensive than currently used liquid fuels. Hence, the vigorous development of efficient storage and transport technologies is crucial in order to promote sustainable development. It is expected that multiphase composites can extend the temperature/pressure range of hydrogen storage materials. Attention should also be paid to other parameters describing the physicochemical properties of hydrogen for use as fuel. As can be seen from Table 1, the only negative feature of hydrogen fuel is its wide range of flammability in comparison to methane, diesel, or petrol fuel. If the volume concentration of hydrogen is from 4 to 75% at a temperature of 293 K, the probability of the spontaneous combustion of such a mixture is very high. The upper flammability limit of hydrogen-enriched methane mixtures increases with the hydrogen content in the fuel. The share of hydrogen does not significantly affect the value of the lower flammability limit of methane. Replacing a certain amount of natural gas with hydrogen is also a method of reducing CO\({}_{2}\) emissions. In this way, it is possible to effectively use the excess electricity generated by renewable energy sources and reduce the consumption of natural gas. Although mixing hydrogen with natural gas offers potential benefits, levels above 15% (\(\
u/\
u\)) of hydrogen pose many challenges in terms of the pipeline materials, safety, and modification of the final receiving equipment [9].