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Hydrogen
Chemical element with atomic number 1 (H)

Hydrogen is a chemical element with the symbol H and atomic number 1, making it the lightest and most abundant element in the universe. It mostly exists as diatomic gas (H2), known as dihydrogen, which is colorless, odorless, and highly combustible. First identified by Henry Cavendish in the 18th century, hydrogen plays a vital role in quantum mechanics and forms covalent bonds with nonmetals, contributing to compounds like water. In nature, hydrogen appears in stars as plasma and on Earth in molecular forms within water and organic compounds. Industrially, it is mainly produced by steam reforming of natural gas and used in fossil fuel processing, ammonia production, and emerging fuel cell technologies.

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Properties

Atomic hydrogen

Electron energy levels

Main article: Hydrogen atom

The ground state energy level of the electron in a hydrogen atom is −13.6 eV,1 equivalent to an ultraviolet photon of roughly 91 nm wavelength.2 The energy levels of hydrogen are referred to by consecutive quantum numbers, with n = 1 {\displaystyle n=1} being the ground state. The hydrogen spectral series corresponds to emission of light due to transitions from higher to lower energy levels.3: 105  Each energy level is further split by spin interactions between the electron and proton into 4 hyperfine levels.4

High precision values for the hydrogen atom energy levels are required for definitions of physical constants. Quantum calculations have identified 9 contributions to the energy levels. The eigenvalue from the Dirac equation is the largest contribution. Other terms include relativistic recoil, the self-energy, and the vacuum polarization terms.5

Isotopes

Main article: Isotopes of hydrogen

Hydrogen has three naturally occurring isotopes, denoted 1H, 2H and 3H. Other, highly unstable nuclei (4H to 7H) have been synthesized in the laboratory but not observed in nature.67

1H is the most common hydrogen isotope, with an abundance of >99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium.8 It is the only stable isotope with no neutrons; see diproton for a discussion of why others do not exist.9

2H, the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in the nucleus. Nearly all deuterium nuclei in the universe is thought to have been produced at the time of the Big Bang, and has endured since then.10: 24.2  Deuterium is not radioactive, and is not a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1H-NMR spectroscopy.11 Heavy water is used as a neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusion.12

3H is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through beta decay with a half-life of 12.32 years.13 It is radioactive enough to be used in luminous paint to enhance the visibility of data displays, such as for painting the hands and dial-markers of watches. The watch glass prevents the small amount of radiation from escaping the case.14 Small amounts of tritium are produced naturally by cosmic rays striking atmospheric gases; tritium has also been released in nuclear weapons tests.15 It is used in nuclear fusion,16 as a tracer in isotope geochemistry,17 and in specialized self-powered lighting devices.18 Tritium has also been used in chemical and biological labeling experiments as a radiolabel.19

Unique among the elements, distinct names are assigned to its isotopes in common use. During the early study of radioactivity, heavy radioisotopes were given their own names, but these are mostly no longer used. The symbols D and T (instead of 2H and 3H) are sometimes used for deuterium and tritium, but the symbol P was already used for phosphorus and thus was not available for protium.20 In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry (IUPAC) allows any of D, T, 2H, and 3H to be used, though 2H and 3H are preferred.21

Antihydrogen (H) is the antimatter counterpart to hydrogen. It consists of an antiproton with a positron. Antihydrogen is the only type of antimatter atom to have been produced as of 2015.2223 The exotic atom muonium (symbol Mu), composed of an antimuon and an electron, is analogous hydrogen and IUPAC nomenclature incorporates such hypothetical compounds as muonium chloride (MuCl) and sodium muonide (NaMu), analogous to hydrogen chloride and sodium hydride respectively.24

Dihydrogen

Under standard conditions, hydrogen is a gas of diatomic molecules with the formula H2, officially called "dihydrogen",25: 308  but also called "molecular hydrogen",26 or simply hydrogen. Dihydrogen is a colorless, odorless, flammable gas.27

Combustion

Hydrogen gas is highly flammable, reacting with oxygen in air, to produce liquid water:

2 H2(g) + O2(g) → 2 H2O(l)

The amount of heat released per mole of hydrogen is −286 kJ or 141.865 MJ for a kilogram mass.28

Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%29 and with chlorine at 5–95%. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).30 In a high-pressure hydrogen leak, the shock wave from the leak itself can heat air to the autoignition temperature, leading to flaming and possibly explosion.31

Hydrogen flames emit faint blue and ultraviolet light.32 Flame detectors are used to detect hydrogen fires as they are nearly invisible to the naked eye in daylight.3334

Spin isomers

Main article: Spin isomers of hydrogen

Molecular H2 exists as two nuclear isomers that differ in the spin states of their nuclei.35 In the orthohydrogen form, the spins of the two nuclei are parallel, forming a spin triplet state having a total molecular spin S = 1 {\displaystyle S=1} ; in the parahydrogen form the spins are antiparallel and form a spin singlet state having spin S = 0 {\displaystyle S=0} . The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form.36 The ortho form is an excited state, having higher energy than the para form by 1.455 kJ/mol,37 and it converts to the para form over the course of several minutes when cooled to low temperature.38 The thermal properties of these isomers differ because each has distinct rotational quantum states.39

The ortho-to-para ratio in H2 is an important consideration in the liquefaction and storage of liquid hydrogen: the conversion from ortho to para is exothermic and produces sufficient heat to evaporate most of the liquid if not converted first to parahydrogen during the cooling process.40 Catalysts for the ortho-para interconversion, such as ferric oxide and activated carbon compounds, are used during hydrogen cooling to avoid this loss of liquid.41

Phases

Liquid hydrogen can exist at temperatures below hydrogen's critical point of 33 K.42 However, for it to be in a fully liquid state at atmospheric pressure, H2 needs to be cooled to 20.28 K (−252.87 °C; −423.17 °F). Hydrogen was liquefied by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask.43

Liquid hydrogen becomes solid hydrogen at standard pressure below hydrogen's melting point of 14.01 K (−259.14 °C; −434.45 °F). Distinct solid phases exist, known as Phase I through Phase V, each exhibiting a characteristic molecular arrangement.44 Liquid and solid phases can exist in combination at the triple point, a substance known as slush hydrogen.45

Metallic hydrogen, a phase obtained at extremely high pressures (in excess of 400 gigapascals (3,900,000 atm; 58,000,000 psi)), is an electrical conductor. It is believed to exist deep within giant planets like Jupiter.4647

When ionized, hydrogen becomes a plasma. This is the form in which hydrogen exists within stars.48

Thermal and physical properties

Thermal and physical properties of hydrogen (H2) at atmospheric pressure4950
Temperature (K)Density (kg/m^3)Specific heat (kJ/kg K)Dynamic viscosity (kg/m s)Kinematic viscosity (m^2/s)Thermal conductivity (W/m K)Thermal diffusivity (m^2/s)Prandtl Number
1000.2425511.234.21E-061.74E-056.70E-022.46E-050.707
1500.1637112.6025.60E-063.42E-050.09814.75E-050.718
2000.122713.546.81E-065.55E-050.12827.72E-050.719
2500.0981914.0597.92E-068.06E-050.15611.13E-040.713
3000.0818514.3148.96E-061.10E-040.1821.55E-040.706
3500.0701614.4369.95E-061.42E-040.2062.03E-040.697
4000.0613514.4911.09E-051.77E-040.2282.57E-040.69
4500.0546214.4991.18E-052.16E-040.2513.16E-040.682
5000.0491814.5071.26E-052.57E-040.2723.82E-040.675
5500.0446914.5321.35E-053.02E-040.2924.52E-040.668
6000.0408514.5371.43E-053.50E-040.3155.31E-040.664
7000.0349214.5741.59E-054.55E-040.3516.90E-040.659
8000.030614.6751.74E-055.69E-040.3848.56E-040.664
9000.0272314.8211.88E-056.90E-040.4121.02E-030.676
10000.0242414.992.01E-058.30E-040.4481.23E-030.673
11000.0220415.172.13E-059.66E-040.4881.46E-030.662
12000.020215.372.26E-051.12E-030.5281.70E-030.659
13000.0186515.592.39E-051.28E-030.5681.96E-030.655
14000.0173215.812.51E-051.45E-030.612.23E-030.65
15000.0161616.022.63E-051.63E-030.6552.53E-030.643
16000.015216.282.74E-051.80E-030.6972.82E-030.639
17000.014316.582.85E-051.99E-030.7423.13E-030.637
18000.013516.962.96E-052.19E-030.7863.44E-030.639
19000.012817.493.07E-052.40E-030.8353.73E-030.643
20000.012118.253.18E-052.63E-030.8783.98E-030.661

History

Main article: Timeline of hydrogen technologies

18th century

In 1671, Irish scientist Robert Boyle discovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.5152 Boyle did not note that the gas was inflammable, but hydrogen would play a key role in overturning the phlogiston theory of combustion.53

In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance "phlogiston"5455 and further finding in 1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.5657

In 1783, Antoine Lavoisier identified the element that came to be known as hydrogen58 when he and Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned.59 Lavoisier produced hydrogen for his experiments on mass conservation by treating metallic iron with a stream of H2O through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:

  • Fe + H2O → FeO + H2
  • 2Fe + 3 H2O → Fe2O3 + 3 H2
  • 3Fe + 4 H2O → Fe3O4 + 4 H2

Many metals react similarly with water leading to the production of hydrogen.60 In some situations, this H2-producing process is problematic as is the case of zirconium cladding on nuclear fuel rods.61

19th century

By 1806 hydrogen was used to fill balloons.62 François Isaac de Rivaz built the first de Rivaz engine, an internal combustion engine powered by a mixture of hydrogen and oxygen in 1806. Edward Daniel Clarke invented the hydrogen gas blowpipe in 1819. The Döbereiner's lamp and limelight were invented in 1823. Hydrogen was liquefied for the first time by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask. He produced solid hydrogen the next year.63

One of the first quantum effects to be explicitly noticed (but not understood at the time) was James Clerk Maxwell's observation that the specific heat capacity of H2 unaccountably departs from that of a diatomic gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H2 because of its low mass. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.64

20th century

The existence of the hydride anion was suggested by Gilbert N. Lewis in 1916 for group 1 and 2 salt-like compounds. In 1920, Moers electrolyzed molten lithium hydride (LiH), producing a stoichiometric quantity of hydrogen at the anode.65

Because of its simple atomic structure, consisting only of a proton and an electron, the hydrogen atom, together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of atomic structure.66 The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, in which the electron "orbits" the proton, like how Earth orbits the Sun. However, the electron and proton are held together by electrostatic attraction, while planets and celestial objects are held by gravity. Due to the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.67

Hydrogen's unique position as the only neutral atom for which the Schrödinger equation can be directly solved, has significantly contributed to the understanding of quantum mechanics through the exploration of its energetics.68 Furthermore, study of the corresponding simplicity of the hydrogen molecule and the corresponding cation H+2 brought understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.69

Hydrogen-lifted airship

Because H2 is only 7% the density of air, it was once widely used as a lifting gas in balloons and airships.70 The first hydrogen-filled balloon was invented by Jacques Charles in 1783. Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted airship by Henri Giffard. German count Ferdinand von Zeppelin promoted the idea of rigid airships lifted by hydrogen that later were called Zeppelins; the first of which had its maiden flight in 1900.71 Regularly scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships in the form of blimps were used as observation platforms and bombers during the War II, especially on the US Eastern seaboard.72

The first non-stop transatlantic crossing was made by the British airship R34 in 1919 and regular passenger service resumed in the 1920s. Hydrogen was used in the Hindenburg airship, which caught fire over New Jersey on 6 May 1937.73 The hydrogen that filled the airship was ignited, possibly by static electricity, and burst into flames.74 Following this Hindenburg disaster, commercial hydrogen airship travel ceased. Hydrogen is still used, in preference to non-flammable but more expensive helium, as a lifting gas for weather balloons.75

Deuterium and tritium

Deuterium was discovered in December 1931 by Harold Urey, and tritium was prepared in 1934 by Ernest Rutherford, Mark Oliphant, and Paul Harteck.76 Heavy water, which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in 1932.77

Chemistry

Reactions of H2

H2 is relatively unreactive. The thermodynamic basis of this low reactivity is the very strong H–H bond, with a bond dissociation energy of 435.7 kJ/mol.78 It does form coordination complexes called dihydrogen complexes. These species provide insights into the early steps in the interactions of hydrogen with metal catalysts. According to neutron diffraction, the metal and two H atoms form a triangle in these complexes. The H-H bond remains intact but is elongated. They are acidic.79

Although exotic on Earth, the H+3 ion is common in the universe. It is a triangular species, like the aforementioned dihydrogen complexes. It is known as protonated molecular hydrogen or the trihydrogen cation.80

Hydrogen reacts with chlorine to produce HCl and with bromine to produce HBr by a chain reaction. The reaction requires initiation. For example in the case of Br2, the diatomic molecule is broken into atoms, Br2 + (UV light) → 2Br•. Propagating reactions consume hydrogen molecules and produce HBr, as well as Br and H atoms:

Br• + H2 → HBr + H H + Br2 → HBr +Br

Finally the terminating reaction:

H + HBr → H2 + Br• 2Br• → Br2.

consumes the remaining atoms.81: 289 

The addition of H2 to unsaturated organic compounds, such as alkenes and alkynes, is called hydrogenation. Even if the reaction is energetically favorable, it does not take place even at higher temperatures. In the presence of a catalyst like finely divided platinum or nickel, the reaction proceeds at room temperature.82: 477 

Hydrogen-containing compounds

Main article: Hydrogen compounds

Hydrogen can exist in both +1 and −1 oxidation states, forming compounds through ionic and covalent bonding. It is a part of a wide range of substances, including water, hydrocarbons, and numerous other organic compounds.83 The H+ ion—commonly referred to as a proton due to its single proton and absence of electrons—is central to acid–base chemistry, although the proton does not move freely. In the Brønsted–Lowry framework, acids are defined by their ability to donate H+ ions to bases.84

Hydrogen forms a vast variety of compounds with carbon known as hydrocarbons, and an even greater diversity with other elements (heteroatoms), giving rise to the broad class of organic compounds often associated with living organisms.85

Hydrogen compounds with hydrogen in the oxidation state −1 are known as hydrides, which are usually formed between hydrogen and metals. The hydrides can be ionic (aka saline), covalent, nor metallic. With heating, H2 reacts efficiently with the alkali and alkaline earth metals to give the ionic hydrides of the formula MH and MH2, respectively. These salt-like crystalline compounds have high melting points and all react with water to liberate hydrogen. Covalent hydrides are include boranes and polymeric aluminium hydride. Transition metals form metal hydrides via continuous dissolution of hydrogen into the metal.86 A well known hydride is lithium aluminium hydride, the [AlH4]− anion carries hydridic centers firmly attached to the Al(III).87 Perhaps the most extensive series of hydrides are the boranes, compounds consisting only of boron and hydrogen.88

Hydrides can bond to these electropositive elements not only as a terminal ligand but also as bridging ligands. In diborane (B2H6), four H's are terminal and two bridge between the two B atoms.89

Hydrogen bonding

Main article: hydrogen bond

When bonded to a more electronegative element, particularly fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a lone pair like oxygen or nitrogen, a phenomenon called hydrogen bonding that is critical to the stability of many biological molecules.90: 375 91 Hydrogen bonding alters molecule structures, viscosity, solubility, as well as melting and boiling points even protein folding dynamics.92

Protons and acids

Further information: Acid–base reaction

In water, hydrogen bonding plays an important role in reaction thermodynamics. A hydrogen bond can shift over to proton transfer. Under the Brønsted–Lowry acid–base theory, acids are proton donors, while bases are proton acceptors.93: 28  A bare proton, H+ essentially cannot exist in anything other than a vacuum. Otherwise it attaches to other atoms, ions, or molecules. Even species as inert as methane can be protonated. The term 'proton' is used loosely and metaphorically to refer to refer to solvated H+" without any implication that any single protons exist freely as a species. To avoid the implication of the naked proton in solution, acidic aqueous solutions are sometimes considered to contain the "hydronium ion" ([H3O]+) or still more accurately, [H9O4]+.94 Other oxonium ions are found when water is in acidic solution with other solvents.95

The concentration of these solvated protons determines the pH of a solution, a logarithmic scale that reflects its acidity or basicity. Lower pH values indicate higher concentrations of hydronium ions, corresponding to more acidic conditions.96

Occurrence

Cosmic

Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass97 and >90% by number of atoms.98 In the early universe, the protons formed in the first second after the Big Bang; neutral hydrogen atoms formed about 370,000 years later during the recombination epoch as the universe expanded and plasma had cooled enough for electrons to remain bound to protons.99

In astrophysics, neutral hydrogen in the interstellar medium is called H I and ionized hydrogen is called H II.100 Radiation from stars ionizes H I to H II, creating spheres of ionized H II around stars. In the chronology of the universe neutral hydrogen dominated until the birth of stars during the era of reionization led to bubbles of ionized hydrogen that grew and merged over 500 million of years.101 They are the source of the 21-cm hydrogen line at 1420 MHz that is detected in order to probe primordial hydrogen. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the universe up to a redshift of z = 4.102

Hydrogen is found in great abundance in stars and gas giant planets. Molecular clouds of H2 are associated with star formation. Hydrogen plays a vital role in powering stars through the proton-proton reaction in lower-mass stars, and through the CNO cycle of nuclear fusion in case of stars more massive than the Sun.103

A molecular form called protonated molecular hydrogen (H+3) is found in the interstellar medium, where it is generated by ionization of molecular hydrogen from cosmic rays. This ion has also been observed in the upper atmosphere of Jupiter. The ion is long-lived in outer space due to the low temperature and density. H+3 is one of the most abundant ions in the universe, and it plays a notable role in the chemistry of the interstellar medium.104 Neutral triatomic hydrogen H3 can exist only in an excited form and is unstable.105

Terrestrial

Hydrogen is the third most abundant element on the Earth's surface,106 mostly in the form of chemical compounds such as hydrocarbons and water.107 Elemental hydrogen is normally in the form of a gas, H2. It is present in a very low concentration in Earth's atmosphere (around 0.53 ppm on a molar basis108) because of its light weight, which enables it to escape the atmosphere more rapidly than heavier gases. Despite its low concentration in our atmosphere, terrestrial hydrogen is sufficiently abundant to support the metabolism of several bacteria.109

Large underground deposits of hydrogen gas have been discovered in several countries including Mali, France and Australia.110 As of 2024, it is uncertain how much underground hydrogen can be extracted economically.111

Production and storage

Main article: Hydrogen production

Industrial routes

Nearly all of the world's current supply of hydrogen gas (H2) is created from fossil fuels.112113: 1  Many methods exist for producing H2, but three dominate commercially: steam reforming often coupled to water-gas shift, partial oxidation of hydrocarbons, and water electrolysis.114

Steam reforming

Hydrogen is mainly produced by steam methane reforming (SMR), the reaction of water and methane.115116 Thus, at high temperature (1000–1400 K, 700–1100 °C or 1300–2000 °F), steam (water vapor) reacts with methane to yield carbon monoxide and H2.

CH4 + H2O → CO + 3 H2

Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide.117 The production of natural gas feedstock also produces emissions such as vented and fugitive methane, which further contributes to the overall carbon footprint of hydrogen.118

This reaction is favored at low pressures, Nonetheless, conducted at high pressures (2.0 MPa, 20 atm or 600 inHg) because high-pressure H2 is the most marketable product, and pressure swing adsorption (PSA) purification systems work better at higher pressures. The product mixture is known as "synthesis gas" because it is often used directly for the production of methanol and many other compounds. Hydrocarbons other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly optimized technology is the formation of coke or carbon:

CH4 → C + 2 H2

Therefore, steam reforming typically employs an excess of H2O. Additional hydrogen can be recovered from the steam by using carbon monoxide through the water gas shift reaction (WGS). This process requires an iron oxide catalyst:119

CO + H2O → CO2 + H2

Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the Haber process for ammonia production, hydrogen is generated from natural gas.120

Partial oxidation of hydrocarbons

Other methods for CO and H2 production include partial oxidation of hydrocarbons:121

2 CH4 + O2 → 2 CO + 4 H2

Although less important commercially, coal can serve as a prelude to the shift reaction above:122

C + H2O → CO + H2

Olefin production units may produce substantial quantities of byproduct hydrogen particularly from cracking light feedstocks like ethane or propane.123

Water electrolysis

Electrolysis of water is a conceptually simple method of producing hydrogen.

2 H2O(l) → 2 H2(g) + O2(g)

Commercial electrolyzers use nickel-based catalysts in strongly alkaline solution. Platinum is a better catalyst but is expensive.124 The hydrogen created through electrolysis using renewable energy is commonly referred to as "green hydrogen".125

Electrolysis of brine to yield chlorine126 also produces high purity hydrogen as a co-product, which is used for a variety of transformations such as hydrogenations.127

The electrolysis process is more expensive than producing hydrogen from methane without carbon capture and storage.128

Innovation in hydrogen electrolyzers could make large-scale production of hydrogen from electricity more cost-competitive.129

Methane pyrolysis

Hydrogen can be produced by pyrolysis of natural gas (methane), producing hydrogen gas and solid carbon with the aid a catalyst and 74 kJ/mol input heat:

CH4(g) → C(s) + 2 H2(g) (ΔH° = 74 kJ/mol)

The carbon may be sold as a manufacturing feedstock or fuel, or landfilled. This route could have a lower carbon footprint than existing hydrogen production processes, but mechanisms for removing the carbon and preventing it from reacting with the catalyst remain obstacles for industrial scale use.130: 17 131

Thermochemical

Water splitting is the process by which water is decomposed into its components. Relevant to the biological scenario is this simple equation:

2 H2O → 4 H+ + O2 + 4e−

The reaction occurs in the light reactions in all photosynthetic organisms. A few organisms, including the alga Chlamydomonas reinhardtii and cyanobacteria, have evolved a second step in the dark reactions in which protons and electrons are reduced to form H2 gas by specialized hydrogenases in the chloroplast.132

Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to more efficiently generate H2 gas even in the presence of oxygen.133 Efforts have also been undertaken with genetically modified alga in a bioreactor.134

Relevant to the thermal water-splitting scenario is this simple equation:

2 H2O → 2 H2 + O2

More than 200 thermochemical cycles can be used for water splitting. Many of these cycles such as the iron oxide cycle, cerium(IV) oxide–cerium(III) oxide cycle, zinc zinc-oxide cycle, sulfur-iodine cycle, copper-chlorine cycle and hybrid sulfur cycle have been evaluated for their commercial potential to produce hydrogen and oxygen from water and heat without using electricity.135 A number of labs (including in France, Germany, Greece, Japan, and the United States) are developing thermochemical methods to produce hydrogen from solar energy and water.136

Natural routes

Biohydrogen

Further information: Biohydrogen and Biological hydrogen production (Algae)

H2 is produced by enzymes called hydrogenases. This process allows the host organism to use fermentation as a source of energy.137 These same enzymes also can oxidize H2, such that the host organisms can subsist by reducing oxidized substrates using electrons extracted from H2.138

The hydrogenase enzyme feature iron or nickel-iron centers at their active sites.139 The natural cycle of hydrogen production and consumption by organisms is called the hydrogen cycle.140

Some bacteria such as Mycobacterium smegmatis can use the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking. Their hydrogenase are designed with small channels that exclude oxygen and so permits the reaction to occur even though the hydrogen concentration is very low and the oxygen concentration is as in normal air.141142

Confirming the existence of hydrogenases in the human gut, H2 occurs in human breath. The concentration in the breath of fasting people at rest is typically less than 5 parts per million (ppm) but can be 50 ppm when people with intestinal disorders consume molecules they cannot absorb during diagnostic hydrogen breath tests.143

Serpentinization

Serpentinization is a geological mechanism that produce highly reducing conditions.144 Under these conditions, water is capable of oxidizing ferrous (Fe2+) ions in fayalite, generating hydrogen gas:145146

Fe2SiO4 + H2O → 2 Fe3O4 + SiO2 + H2

Closely related to this geological process is the Schikorr reaction:

3 Fe(OH)2 → Fe3O4 + 2 H2O + H2

This process also is relevant to the corrosion of iron and steel in oxygen-free groundwater and in reducing soils below the water table.147

Laboratory syntheses

H2 is produced in laboratory settings, such as in the small-scale electrolysis of water using metal electrodes and water containing an electrolyte, which liberates hydrogen gas at the cathode:148

2H+(aq) + 2e− → H2(g)

Hydrogen is also often a by-product of other reactions. Many metals react with water to produce H2, but the rate of hydrogen evolution depends on the metal, the pH, and the presence of alloying agents. Most often, hydrogen evolution is induced by acids. The alkali and alkaline earth metals, aluminium, zinc, manganese, and iron react readily with aqueous acids.149

Zn + 2 H+ → Zn2+ + H2

Many metals, such as aluminium, are slow to react with water because they form passivated oxide coatings of oxides. An alloy of aluminium and gallium, however, does react with water. At high pH, aluminium can produce H2:150

2 Al + 6 H2O + 2 OH− → 2 [Al(OH)4]− + 3 H2

Storage

If H2 is to be used as an energy source, its storage is important. It dissolves only poorly in solvents. For example, at room temperature and 0.1 Mpascal, ca. 0.05 moles dissolves in one kilogram of diethyl ether.151 The H2 can be stored in compressed form, although compressing costs energy. Liquifaction is impractical given its low critical temperature. In contrast, ammonia and many hydrocarbons can be liquified at room temperature under pressure. For these reasons, hydrogen carriers - materials that reversibly bind H2 - have attracted much attention. The key question is then the weight percent of H2-equivalents within the carrier material. For example, hydrogen can be reversibly absorbed into many rare earth and transition metals152 and is soluble in both nanocrystalline and amorphous metals.153 Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice.154 These properties may be useful when hydrogen is purified by passage through hot palladium disks, but the gas's high solubility is also a metallurgical problem, contributing to the embrittlement of many metals,155 complicating the design of pipelines and storage tanks.156

The most problematic aspect of metal hydrides for storage is their modest H2 content, often on the order of 1%. For this reason, there is interest in storage of H2 in compounds of low molecular weight. For example, ammonia borane (H3N−BH3) contains 19.8 weight percent of H2. The problem with this material is that after release of H2, the resulting boron nitride does not re-add H2, i.e. ammonia borane is an irreversible hydrogen carrier.157 More attractive, somewhat ironically, are hydrocarbons such as tetrahydroquinoline, which reversibly release some H2 when heated in the presence of a catalyst:158

C9H10NH ⇌ C9H7N + 2H2

Applications

See also: Hydrogen economy

Petrochemical industry

Large quantities of H2 are used in the "upgrading" of fossil fuels. Key consumers of H2 include hydrodesulfurization, and hydrocracking. Many of these reactions can be classified as hydrogenolysis, i.e., the cleavage of bonds by hydrogen. Illustrative is the separation of sulfur from liquid fossil fuels:159160

R2S + 2 H2 → H2S + 2 RH

Hydrogenation

Hydrogenation, the addition of H2 to various substrates, is done on a large scale. Hydrogenation of N2 produces ammonia by the Haber process:161

N2 + 3 H2 → 2 NH3

This process consumes a few percent of the energy budget in the entire industry and is the biggest consumer of hydrogen. The resulting ammonia is used in fertilizers critical to the supply of protein consumed by humans.162 Hydrogenation is also used to convert unsaturated fats and oils to saturated fats and oils. The major application is the production of margarine. Methanol is produced by hydrogenation of carbon dioxide; the mixture of hydrogen and carbon dioxide used for this process is known as syngas. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. H2 is also used as a reducing agent for the conversion of some ores to the metals.163164

Fuel

The potential for using hydrogen (H2) as a fuel has been widely discussed. Hydrogen can be used in fuel cells to produce electricity,165 or burned to generate heat.166 When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor.167 When burned, hydrogen produces relatively little pollution at the point of combustion, but can lead to thermal formation of harmful nitrogen oxides.168

If hydrogen is produced with low or zero greenhouse gas emissions (green hydrogen), it can play a significant role in decarbonizing energy systems where there are challenges and limitations to replacing fossil fuels with direct use of electricity.169170

Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals, thus contributing to the decarbonization of industry alongside other technologies, such as electric arc furnaces for steelmaking.171 However, it is likely to play a larger role in providing industrial feedstock for cleaner production of ammonia and organic chemicals.172 For example, in steelmaking, hydrogen could function as a clean fuel and also as a low-carbon catalyst, replacing coal-derived coke (carbon):173

2FeO + C → 2Fe + CO2 vs FeO + H2 → Fe + H2O

Hydrogen used to decarbonize transportation is likely to find its largest applications in shipping, aviation and, to a lesser extent, heavy goods vehicles, through the use of hydrogen-derived synthetic fuels such as ammonia and methanol and fuel cell technology.174 For light-duty vehicles including cars, hydrogen is far behind other alternative fuel vehicles, especially compared with the rate of adoption of battery electric vehicles, and may not play a significant role in future.175

Liquid hydrogen and liquid oxygen together serve as cryogenic propellants in liquid-propellant rockets, as in the Space Shuttle main engines. NASA has investigated the use of rocket propellant made from atomic hydrogen, boron or carbon that is frozen into solid molecular hydrogen particles suspended in liquid helium. Upon warming, the mixture vaporizes to allow the atomic species to recombine, heating the mixture to high temperature.176

Hydrogen produced when there is a surplus of variable renewable electricity could in principle be stored and later used to generate heat or to re-generate electricity.177 It can be further transformed into synthetic fuels such as ammonia and methanol.178 Disadvantages of hydrogen fuel include high costs of storage and distribution due to hydrogen's explosivity, its large volume compared to other fuels, and its tendency to make pipes brittle.179

Nickel–hydrogen battery

The very long-lived, rechargeable nickel–hydrogen battery developed for satellite power systems uses pressurized gaseous H2.180 The International Space Station,181 Mars Odyssey182 and the Mars Global Surveyor183 are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the Hubble Space Telescope is also powered by nickel-hydrogen batteries, which were finally replaced in May 2009,184 more than 19 years after launch and 13 years beyond their design life.185

Semiconductor industry

Hydrogen is employed to saturate broken ("dangling") bonds of amorphous silicon and amorphous carbon that helps stabilizing material properties.186 Hydrogen, introduced as a unintended side-effect of production, acts as a shallow electron donor leading to n-type conductivity in ZnO, with important uses in transducers and phosphors.187188 Detailed analysis of ZnO and of MgO show evidence of four and six-fold hydrogen multicentre bonds.189 The doping behavior of hydrogen varies with the material.190191

Niche and evolving uses

Other than the uses mentioned above, hydrogen is also used in smaller scales in the following applications:

  • Cryogenic research: Liquid H2 is used in cryogenic research, including superconductivity studies.196
  • Leak detection: Pure or mixed with nitrogen (sometimes called forming gas), hydrogen is a tracer gas for detection of minute leaks. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries.197 Hydrogen is an authorized food additive (E 949) that allows food package leak testing, as well as having anti-oxidizing properties.198

Safety and precautions

Main article: Hydrogen safety

In hydrogen pipelines and steel storage vessels, hydrogen molecules are prone to react with metals, causing hydrogen embrittlement and leaks in the pipeline or storage vessel.204 Since it is lighter than air, hydrogen does not easily accumulate to form a combustible gas mixture.205 However, even without ignition sources, high-pressure hydrogen leakage may cause spontaneous combustion and detonation.206

Hydrogen is flammable when mixed even in small amounts with air. Ignition can occur at a volumetric ratio of hydrogen to air as low as 4%.207 In approximately 70% of hydrogen ignition accidents, the ignition source cannot be found, and it is widely believed by scholars that spontaneous ignition of hydrogen occurs.208

Hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burns.209 Hydrogen is non-toxic,210 but like most gases it can cause asphyxiation in the absence of adequate ventilation.211

See also

Further reading

References

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