Hydrogen (H) is the simplest and most abundant element in the universe, making up over 90% of all visible matter. It is the first element on the periodic table, with just one electron and one proton and with an average atomic mass of 1.00794.

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Hydrogen was discovered by the British scientist Henry Cavendish in 1766, although in 15th century the physician Paracelsus produced hydrogen by dissolving iron in sulfuric acid. The terms hydrogen comes from the combination of the two ancient Greek words hydro and genes, meaning water and former respectively.
At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless and non-toxic gaseous element.
Protium, deuterium and tritium are three naturally occurring hydrogen isotopes. Protium has no neutrons and is known as normal hydrogen; deuterium and tritium have one and two neutrons, respectively, and since they have a radioactive nature some nuclear devices was built containing these materials.
Since hydrogen has a very low volumetric density of 0.089 kg/m3, it needs to be stored in pressure or, alternatively, in liquid form at temperatures below -253°C. On the other hand, the energy per mass of hydrogen is more than two times the energy per mass of methane and about three times the energy per mass of liquid hydrocarbon fuels. This makes hydrogen one of the best options for replacing fossil fuels in the medium/long-term.
Unlike coal, oil and natural gas, hydrogen does not exist in a natural state but it must be produced using a hydrogen-rich source. For this reason, hydrogen is not a primary energy source but an energy carrier like electricity.
Hydrogen can be produced from hydrocarbons, biomass or water. Anyway, all hydrogen production methods require energy in form of heat, electricity or light. Hydrocarbons-based technologies are well developed and are used to produce industrial hydrogen. Nowadays, more than 95% of hydrogen is produced by steam reforming, gasification or partial oxidation of hydrocarbons and only 4% is produced by water electrolysis.
Apart from thermal cracking of hydrocarbons, the production of hydrogen from fossil fuels generates CO2 emissions. On the other hand, water electrolysis produces no emissions and is particularly suitable to be coupled with stochastic renewable energy sources, such as photovoltaic and wind power systems.
Hydrogen can be stored as gas, liquid or solid. Conventionally, hydrogen is stored as a compressed gas in special high-pressure cylinders or in liquid form within cryogenic tanks. Metal hydrides, graphitic nanofibres and carbon nanotubes are the most promising solid-state hydrogen storage options.
Hydrogen can be transported over both short and long distances by using pipelines, tankers, rail, or barges. Presently, there are some small hydrogen-pipeline systems, up to 200 km, which mainly operates in the United States and Europe and which are used to connect hydrogen production sites to hydrogen utilization sites. Moreover, existing natural gas pipeline can be used to carry a blend of hydrogen and natural gas, commonly named Hythane.
Hydrogen energy can be converted into other useful energy forms (electrical energy, heat energy, etc.) more efficiently than any fossil fuel and has the potential to be used in all economic sectors (industry, agriculture, building, automotive, etc.).
Internal combustion engines, turbines and boilers can be fueled with hydrogen. The use of hydrogen in internal combustion engines leads to an increase in efficiency of about 20-25% over gasoline. Moreover, hydrogen-powered engines emit less pollutants (mainly nitrogen oxides) compared with gasoline engines.
An attractive use of hydrogen is as a fuel for fuel cells. Basically, a fuel cell is a device that converts the chemical energy stored in a fuel (hydrogen, natural gas, methanol, ethanol, etc.) into electricity through an electrochemical reaction with oxygen or another oxidizing agent. Water vapor and heat are the mainly byproducts. Very small amounts of nitrogen oxides and other emissions are produced depending on the fuel source.
Unlike a conventional engine, a fuel cell is not limited by the Carnot cycle. It generates electricity through a single-step process without burning the fuel. This allows the fuel cell to operate more efficiently and cleanly than internal combustion engines.
The efficiency of a fuel cell is typically 50-60% and raises up to 85% when heat is recovered for useful purposes (fuel cell based combined heat and power systems).
Fuel cells are flexible and easy to be installed and maintained. Moreover, they are quieter and more reliable than internal combustion generators. Fuel cells are therefore suitable to be used in a lot of applications including providing power to public and private buildings such as schools, hospitals, municipal buildings, private dwellings, etc.
The world energy demand is constantly increasing and there is a need for clean, versatile and efficient energy solutions to replace depleting fossil fuels. Hydrogen energy appears to be an appropriate answer to this issue. It can help to reduce local and global greenhouse emissions, reduce the dependence on foreign fuels and facilitate a large scale penetration of intermittent renewables into the electrical power system.
Hydrogen has the potential to play a major role as a clean energy carrier in the next years. Even more, it is believed to be a key tool to support the global transition towards a low carbon and sustainable future.