(i) Hydrogen economy : The energy is transported and stored in the form of liquid or gaseous hydrogen is the principle due to which hydrogen is considered to be a possible source of clean energy. This proposal to use hydrogen as fuel is called hydrogen economy.
(ii) Hydrogenation : The process of addition of hydrogen to unsaturatedhydrocarbons is known as hydrogenation.

H2C=CH2

Ethene

+H2

Ni‌or‌Pt‌or‌Pd

→

473K

H3C−CH3

Ethane

H−C≡C−H

Ethyne

+2H2

Ni‌or‌Pt‌or‌Pd

→

473K

H3C−CH3

Ethane

(iii) Syngas : The mixture of CO and H2 is used for the synthesis of methanol and a number of hydrocarbons, it is also called synthesis gas or syngas. Nowadays, syngas is produced from sewage, saw dust, scrap wood, newspapers, etc.
(iv) Water-gas shift reaction : The production of dihydrogen can be increased by reacting CO of syngas mixtures with steam in the presence of iron chromate as catalyst.
CO(g)+H2O(g)

673K

→

Catalyst

CO2(g)+H2(g)
This is called water-gas shift reaction.
(v) Fuel cell : It is a commercial cell in which the chemical energy produced during the combustion of fuel is converted directly into electricity, e.g., H2–O2 fuel cell. A fuel cell is used as a source of electrical energy in the space vehicles.