Hydrogen is the lightest and most abundant element in the universe, a colorless, odorless gas at room temperature with a density roughly one fourteenth that of air, and one of the most promising energy carriers for the ongoing energy transition. Its largest industrial use is ammonia synthesis: via the Haber-Bosch process, hydrogen reacts with nitrogen at high temperature and pressure over a catalyst to form ammonia, which is further processed into urea, ammonium nitrate and other nitrogen fertilizers that underpin global food production. The refining industry consumes enormous volumes of hydrogen for hydrocracking, hydrodesulfurization and hydrotreating, converting heavy oil into gasoline and diesel while removing sulfur, nitrogen and metals to produce cleaner fuels.
In the energy sector, hydrogen fuel cells electrochemically combine hydrogen and oxygen to generate electricity with only water as the exhaust, powering fuel-cell vehicles, trams, backup power systems and distributed stations; green hydrogen is also regarded as a key alternative fuel for hard-to-abate industries such as steel, chemicals and heavy shipping. Aerospace uses liquid hydrogen with liquid oxygen as the main rocket propellant, delivering high thrust with clean exhaust. Hydrogen also hydrogenates liquid vegetable oils into margarine and shortening, protects the tin bath in float glass production from oxidation, and serves as a reducing and shielding gas in the growth of silicon and germanium single crystals. Hydrogen is chosen because it has the highest gravimetric heat of combustion, an extremely strong reducing power, and clean reaction products.
Water (H2O) is hydrogen's best-known compound, covering drinking, irrigation, industrial cooling, power generation and chemical solvents across virtually every sector, and forming the medium of life itself. Ammonia (NH3) is not only a direct nitrogen fertilizer but also the feedstock for nitric acid, soda ash, acrylonitrile and many amines; liquid ammonia itself is a common low-temperature refrigerant and a reducing agent in flue-gas denitrification. Hydrogen peroxide (H2O2), commonly called peroxide water, decomposes into only water and oxygen, so it is widely used as a bleaching agent for pulp and textiles, a disinfectant for drinking water, a food-processing bleaching agent and an environmentally friendly oxidizer, and it is also heavily used in semiconductor cleaning. Hydrogen chloride (HCl) dissolves in water to form hydrochloric acid, a fundamental strong acid used in steel pickling, oil-well acidizing, metal surface treatment, and the production of chloromethanes and PVC, as well as in pH control and preparing chloride salts.
