The hydrogen, discovered in 1776 by Henry Cavendish, is the most abundant element in the universe, but is not present in the atmosphere, being 14.4 times lighter than air. It is also one of the most abundant on the Earth's crust along with silicon and oxygen. Combined with oxygen, nitrogen and carbon is one of the key elements making up the plant and animal life. It is the lightest element, it consists only of a proton and an electron. In nature there is also the isotope 2, called deuterium, with a neutron, but the hydrogen 1 is the predominant, constituting the 99.98 % of the natural mixture. The hydrogen 3, called tritium, with two neutrons, is produced only during nuclear interactions. As many gaseous elements hydrogen is diatomic, but dissociates at high temperature. The gaseous hydrogen is a mixture of two different forms, the orthohydrogen (nuclei with parallel spins), which is about the 75% of the mixture, and the parahydrogen (with antiparallel spins). The hydrogen has the lowest boiling and melting points than any other substance except helium: solidifies at -259.2 ° C and liquefies at 252.77 ° C. At 0 °C and at the pressure of 1 atmosphere, it’s in the gaseous form with a density of 0.089 g / liter. The atomic weight is 1.007 u.a. It combines easily with oxygen to form water. The reaction occurs slowly at low temperature but with explosive trend above 550 ° C. The limit of flammability is very broad, being between 4 and 75% by volume, like the limit of detonation is between 18.5 and 59% by volume. The energy content of hydrogen per unit mass is higher than any other current fuel. Table 1.1 shows the main physical properties of molecular hydrogen:

 

Table 1.1 Main physical properties of molecular hydrogen.

Molecular mass	2.01588 g/mole
Melting point	-259 °C (1,013 bar)
Boiling point	-253 °C (1,013 bar)
Critical temperature	-239,9 °C (12,98 bar)
Density (as gas in standard conditions)	0.0838 Kg/m3
Density (as liquid at 253 °C)	70 Kg/m3
Higher heating value	141.8 KJ/g
Lower heating value	119.9 KJ/g

In table 1.2, the heating values of hydrogen and thos of some common fuels are compared::

 

Table 1.2 Heating values of some common fuels.

Fuel	Higher heating value	Lower heating value
Hydrogen	141.8 KJ/g	119.9 KJ/g
Gasoline	47.5 KJ/g	44.5KJ/g
Diesel	47.5KJ/g	42.5 KJ/g
Methane	55.5 KJ/g	50.0 KJ/g

As can be seen, the heat value of hydrogen is by far the highest, which means that the same mass as hydrogen produce an amount of heat, and therefore energy, proportionally higher. For example, 1 kg of hydrogen contains the same amount of energy of 2.1 kilograms of natural gas or 2.8 kg of gasoline