Hydrogen gas was first produced artificially in the 17thcentury by thereaction ofacids with metals.Henry Cavendish, in1766–1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned: this is the origin of hydrogen's name, which means'water-former' (fromAncient Greek:ὕδωρ,romanized: húdōr,lit. 'water', andγεννάω,gennáō, 'I bring forth'). Understanding thecolors of light absorbed and emitted by hydrogen was a crucial part of the development ofquantum mechanics.
Hydrogen, typicallynonmetallic except underextreme pressure, readily formscovalent bonds with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial inacid-base reactions, which mainly involve proton exchange amongsoluble molecules. Inionic compounds, hydrogen can take the form of either a negatively-chargedanion, where it is known ashydride, or as a positively-chargedcation,H+, called a proton. Although tightly bonded to water molecules, protons strongly affect the behavior ofaqueous solutions, as reflected in the importance ofpH. Hydride, on the other hand, is rarely observed because it tends to deprotonate solvents, yieldingH2.
High-precision values for the hydrogen atom energy levels are required for definitions of physical constants. Quantum calculations have identified nine contributions to the energy levels. Theeigenvalue from theDirac equation is the largest contribution. Other terms includerelativistic recoil, theself-energy, and thevacuum polarization terms.[16]
The three naturally-occurring isotopes of hydrogen: hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium)
Hydrogen has three naturally-occurring isotopes, denoted1 H,2 H and3 H. Other, highly-unstablenuclides(4 H to7 H) have beensynthesized in laboratories but not observed in nature.[17][18]
1 H is the most common hydrogen isotope, with an abundance of >99.98%. Because thenucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal nameprotium.[19] It is the only stable isotope with no neutrons (seediproton for a discussion of why others do not exist).[20]
2 H, the other stable hydrogen isotope, is known asdeuterium and contains one proton and oneneutron in the nucleus. Nearly all deuterium nuclei in the universe are thought to have been produced inBig Bang nucleosynthesis, and have endured since then.[21]: 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 calledheavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for1 H-NMR spectroscopy.[22] Heavy water is used as aneutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercialnuclear fusion.[23]
3 H is known astritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying intohelium-3 throughbeta decay with ahalf-life of 12.32years.[24] It is radioactive enough to be used inluminous 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.[25] Small amounts of tritium are produced naturally bycosmic rays striking atmospheric gases; tritium has also been released innuclear weapons tests.[26] It is used in nuclear fusion,[27] as a tracer inisotope geochemistry,[28] and in specializedself-powered lighting devices.[29] Tritium has also been used in chemical and biological labeling experiments as aradiolabel.[30]
Unique among the elements, distinct names are assigned to hydrogen's 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 andT (instead of2 H and3 H) are sometimes used for deuterium and tritium, but the symbolP was already used forphosphorus and thus was not available for protium.[31] In itsnomenclatural guidelines, theInternational Union of Pure and Applied Chemistry(IUPAC) allows any of D, T,2 H, and3 H to be used, though2 H and3 H are preferred.[32]
Understandard conditions, hydrogen is agas ofdiatomic molecules with theformulaH2, officially called "dihydrogen",[36]: 308 but also called "molecular hydrogen",[37] or simply hydrogen. Dihydrogen is a colorless, odorless, flammable gas.[37]
Combustion
Combustion of hydrogen with the oxygen in the air. When the bottom cap is removed, allowing air to enter, hydrogen in the container rises and burns as it mixes with the air.
Hydrogen gas is highly flammable, reacting withoxygen in air to produce liquid water:
Hydrogen gas forms explosive mixtures with air in concentrations from4%–74%[39] and with chlorine at5%–95%. The hydrogenautoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).[40] In a high-pressurehydrogen leak, the shock wave from the leak itself can heat air to the autoignition temperature, leading to flaming and possibly explosion.[41]
Hydrogen flames emit faint blue andultraviolet light.[42]Flame detectors are used to detect hydrogen fires as they are nearly invisible to the naked eye in daylight.[43][44]
MolecularH2 exists as twonuclear isomers that differ in thespin states of their nuclei.[45] In theorthohydrogen form, the spins of the two nuclei are parallel, forming a spintriplet state having atotal molecular spin; in theparahydrogen form the spins are antiparallel and form a spinsinglet state having spin. 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.[46] The ortho form is anexcited state, having higher energy than the para form by1.455 kJ/mol,[47] and it converts to the para form over the course of several minutes when cooled to low temperature.[48] The thermal properties of these isomers differ because each has distinctrotational quantum states.[49]
The ortho-to-para ratio inH2 is an important consideration in theliquefaction and storage ofliquid hydrogen: the conversion from ortho to para isexothermic, and produces sufficient heat to evaporate most of the liquid if the conversion to parahydrogen does not occur during the cooling process.[50]Catalysts for the ortho-para interconversion, such asferric oxide andactivated carbon compounds, are therefore used during hydrogen cooling to avoid this loss of liquid.[51]
Liquid hydrogen becomessolid hydrogen atstandard pressure below hydrogen'smelting point of 14.01 K (−259.14 °C; −434.45 °F). Distinct solid phases exist, known as PhaseI through PhaseV, each exhibiting a characteristic molecular arrangement.[55] Liquid and solid phases can exist in combination at thetriple point; this mixture is known asslush hydrogen.[56]
Metallic hydrogen, a phase obtained at extremely high pressures (in excess of 400 million Pa (58,000 psi)), is an electrical conductor. It is believed to exist deep withingiant planets likeJupiter.[55][57]
Whenionized, hydrogen becomes aplasma. This is the form in which hydrogen exists withinstars.[58]
Thermal and physical properties
Thermal and physical properties of hydrogen (H2) at atmospheric pressure[59][60]
In 1671, Irish scientistRobert Boyle discovered and described the reaction betweeniron filings and diluteacids, which results in the production of hydrogen gas.[61][62]Boyle did not note that the gas was flammable, but hydrogen would play a key role in overturning thephlogiston theory of combustion.[63]
In 1766,Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from ametal-acid reaction "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance "phlogiston"[64][65] and further finding in1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.[10][9]
Antoine Lavoisier, who identified the element that came to be known as hydrogen
In 1783,Antoine Lavoisier identified the element that came to be known as hydrogen[66] when he andLaplace reproduced Cavendish's finding that water is produced when hydrogen is burned.[9]Lavoisier produced hydrogen for his experiments onmass conservation by treating metalliciron with a stream of water through an incandescent iron tube heated in a fire. Anaerobicoxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:
Fe + H2O → FeO + H2
2 Fe + 3 H2O → Fe2O3 + 3 H2
3 Fe + 4 H2O → Fe3O4 + 4 H2
Many metals react similarly with water, leading to the production of hydrogen.[67] In some situations, this H2-producing process is problematic, for instance in the case ofzirconium cladding onnuclear fuel rods.[68]
One of the firstquantum effects to be explicitly noticed, although not understood at the time, wasJames Clerk Maxwell's observation that thespecific heat capacity ofH2 unaccountably departs from that of adiatomic 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 inH2 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.[70]
Hydrogen emission spectrum lines in the four visible lines of theBalmer series
Because of its simple atomic structure, consisting only of a proton and an electron, thehydrogen atom, together with thespectrum of light produced from it or absorbed by it, has been central to thedevelopment of the theory of atomic structure.[72] The energy levels of hydrogen can be calculated fairly accurately using theBohr 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 byelectrostatic attraction, while planets and celestial objects are held bygravity. Due to thediscretization ofangular momentum postulated in earlyquantum mechanics byBohr, the electron in theBohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.[73]
Hydrogen's unique position as the only neutral atom for which theSchrödinger equation can be directly solved, has significantly contributed to the understanding of quantum mechanics through the exploration of its energetics.[74] Furthermore, study of the corresponding simplicity of the hydrogen molecule and the correspondingcation,H+2, brought understanding of the nature of thechemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.[75]
BecauseH2 has only 7% the density of air, it was once widely used as alifting gas in balloons andairships.[76] The first hydrogen-filled balloon was invented byJacques Charles in1783. Hydrogen provided the lift for the first reliable form of air-travel following the1852 invention of the first hydrogen-lifted airship byHenri Giffard. German countFerdinand vonZeppelin promoted the idea of rigid airships lifted by hydrogen that later were calledZeppelins, the first of which had its maiden flight in1900.[9] Regularly-scheduled flights started in1910 and by the outbreak of World WarI in August1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships in the form ofblimps were used as observation platforms and bombers during World WarII, especially on theUSEastern seaboard.[77]
The first non-stop transatlantic crossing was made by the British airshipR34 in1919 and regular passenger service resumed in the1920s. Hydrogen was used in theHindenburg, which caught fire overNew Jersey on 6May 1937.[9] The hydrogen that filled the airship was ignited, possibly by static electricity, and burst into flames.[78] Following thisdisaster, commercial hydrogen airship travelceased. Hydrogen is still used, in preference to non-flammable but more expensivehelium, as a lifting gas forweather balloons.[79]
H2 is relatively unreactive. The thermodynamic basis of this low reactivity is the very strongH–H bond, with abond dissociation energy of435.7 kJ/mol.[80] It does form coordination complexes calleddihydrogen complexes. These species provide insights into the early steps in the interactions of hydrogen with metal catalysts. According toneutron diffraction, the metal and two Hatoms form a triangle in these complexes. TheH-H bond remains intact but is elongated. They are acidic.[81]
Although exotic on Earth, theH+3ion is common in the universe. It is a triangular species, like the aforementioned dihydrogen complexes. It is known asprotonated molecular hydrogen or the trihydrogen cation.[82]
Hydrogen reacts withchlorine to produceHCl, and withbromine to produceHBr, via achain reaction. The reaction requires initiation. For example, in the case of Br2, the dibromine molecule is split apart:Br2 + (UV light) → 2Br•. Propagating reactions consume hydrogen molecules and produceHBr, as well as Brand Hatoms:
Hydrogen can exist in both +1 and −1oxidation states, forming compounds throughionic andcovalent bonding. The element is part of a wide range of substances, including water,hydrocarbons, and numerous otherorganic compounds.[85] The H+ion—commonly referred to as a proton due to its single proton and absence of electrons—is central toacid–base chemistry, although the proton does not move freely. In theBrønsted–Lowry framework, acids are defined by their ability to donate H+ions to bases.[86]
Hydrogen forms a vast variety of compounds withcarbon, 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 ashydrides, which are usually formed between hydrogen and metals. The hydrides can be ionic (aka saline), covalent, or metallic. With heating, H2 reacts efficiently with thealkali andalkaline earth metals to give theionic hydrides of the formulasMH and MH2, respectively. These salt-like crystalline compounds have high melting points and all react with water to liberate hydrogen. Covalent hydrides includeboranes and polymericaluminium hydride.Transition metals formmetal hydrides via continuous dissolution of hydrogen into the metal.[87] A well-known hydride islithium aluminium hydride: the[AlH4]−anion carries hydridic centers firmly attached to the Al(III).[88] Perhaps the most extensive series of hydrides are theboranes, compounds consisting only of boron and hydrogen.[89]
Hydrides can bond to theseelectropositive elements not only as a terminalligand but also asbridging ligands. In diborane(B2H6), four hydrogen atoms are terminal, while two bridge between the two boron atoms.[24]
When bonded to a moreelectronegative element, particularlyfluorine,oxygen, ornitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with alone pair like oxygen or nitrogen. This phenomenon, calledhydrogen bonding, is critical to the stability of many biological molecules.[90]: 375 [91] Hydrogen bonding alters molecule structures,viscosity,solubility, melting and boiling points, and evenprotein folding dynamics.[92]
An "A-Tbase pair" in DNA illustrating how hydrogen bonds are critical to thegenetic code. The drawing illustrates that in many chemical depictions,C-H bonds are not always shown explicitly, an indication of their pervasiveness.
In water, hydrogen bonding plays an important role in reaction thermodynamics. A hydrogen bond can shift over to proton transfer.Under theBrønsted–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 chemical species as inert asmethane can be protonated. The term "proton" is used loosely and metaphorically to refer to solvated hydrogencations attached to other solvated chemical species; it is denoted"H+" without any implication that any single protons exist freely in solution 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] Otheroxonium ions are found when water is in acidic solution with other solvents.[95]
The concentration of these solvated protons determines thepH of a solution, alogarithmic scale that reflects its acidity or basicity. Lower pHvalues indicate higher concentrations of hydronium ions, corresponding to more acidic conditions.[96]
In astrophysics, neutral hydrogen in theinterstellar medium is calledHI and ionized hydrogen is calledHII.[100] Radiation from stars ionizes HI to HII, creatingspheres of ionized HII around stars. In thechronology of the universe neutral hydrogen dominated until the birth of stars during the era ofreionization, which then produced bubbles of ionized hydrogen that grew and merged over hundreds of millions of years.[101]These are the source of the 21-centimeterhydrogen line, at1420 MHz, that is detected in order to probe primordial hydrogen. The large amount of neutral hydrogen found in thedamped Lyman-alpha systems is thought to dominate thecosmologicalbaryonic density of the universe up to aredshift ofz = 4.[102]
Protonated molecular hydrogen(H+3) is found in theinterstellar medium, where it is generated by ionization of molecular hydrogen bycosmic rays. This ion has also been observed in theupper 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] Neutraltriatomic hydrogenH3 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 existing withinchemical compounds such ashydrocarbons and water.[24] Elemental hydrogen is normally in the form of a gas,H2, atstandard conditions. It is present in a very low concentration in Earth's atmosphere (around0.53 parts per million on amolar basis[107]) because of its light weight, which enables it toescape the atmosphere more rapidly than heavier gases. Despite its low concentration in the atmosphere, terrestrial hydrogen is sufficiently abundant to support the metabolism of several varieties of bacteria.[108]
Large underground deposits of hydrogen gas have been discovered in several countries includingMali,France andAustralia.[109] As of 2024, it is uncertain how much underground hydrogen can be extracted economically.[109]
Nearly all of the world's current supply of hydrogen gas(H2) is produced from fossil fuels.[110][111]: 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.[112]
Steam reforming
Inputs and outputs of steam reforming (SMR) and water gas shift (WGS) reaction of natural gas, a process used in hydrogen production
Hydrogen is mainly produced bysteam methane reforming(SMR), the reaction of water and methane.[113][114] Thus, at high temperature (1,000–1,400 K [730–1,130 °C; 1,340–2,060 °F]),steam (water vapor) reacts withmethane to yieldcarbon monoxide andH2.
CH4 + H2O → CO + 3 H2
Producing onetonne of hydrogen through this process emits6.6–9.3tonnes of carbon dioxide.[115] The production of natural gas feedstock also produces emissions such asvented andfugitive methane, which further contributes to the overall carbon footprint of hydrogen.[116]
This reaction is favored at low pressures but is nonetheless conducted at high pressures(2.0 MPa [20 atm; 590 inHg]) because high-pressureH2 is the most marketable product, andpressure 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 ofmethanol 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 ofcoke or carbon:
CH4 → C + 2 H2
Therefore, steam reforming typically employs an excess ofH2O. Additional hydrogen can be recovered from the steam by using carbon monoxide through thewater gas shift reaction(WGS). This process requires aniron oxide catalyst:[114]
CO + H2O → CO2 + H2
Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In theHaber process forammonia production, hydrogen is generated from natural gas.[117]
Partial oxidation of hydrocarbons
Other methods for CO andH2 production include partial oxidation of hydrocarbons:[45]
2 CH4 + O2 → 2 CO + 4 H2
Although less important commercially, coal can serve as a prelude to the above shift reaction:[114]
C + H2O → CO + H2
Olefin production units may produce substantial quantities of byproduct hydrogen, particularly fromcracking light feedstocks likeethane orpropane.[118]
Water electrolysis
Inputs and outputs of the electrolysis of water production of hydrogen
Commercialelectrolyzers usenickel-based catalysts in strongly alkaline solution.Platinum is a better catalyst but is expensive.[119] The hydrogen created through electrolysis using renewable energy is commonly referred to as "green hydrogen".[120]
Innovation inhydrogen electrolyzers could make large-scale production of hydrogen from electricity more cost-competitive.[124]
Methane pyrolysis
Hydrogen can be produced bypyrolysis ofnatural gas (methane), producing hydrogen gas and solid carbon with the aid of a catalyst and74 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.[125]: 17 [126]
Thermochemical
Water splitting is the process by which water is decomposed into its components. Relevant to the biological scenario is this equation:
Efforts have been undertaken togenetically modify cyanobacterial hydrogenases to more efficiently generateH2gas even in the presence of oxygen.[128] Efforts have also been undertaken with genetically‐modified alga in abioreactor.[129]
Relevant to the thermal water-splitting scenario is this simple equation:
H2 is produced in organisms by enzymes calledhydrogenases. This process allows the host organism to usefermentation as a source of energy.[132] These same enzymes also canoxidizeH2, such that the host organisms can subsist by reducing oxidized substrates using electrons extracted fromH2.[133]
Some bacteria such asMycobacterium smegmatis can use the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking. Their hydrogenases feature small channels that exclude oxygen from the active site, permitting the reaction to occur even though the hydrogen concentration is very low and the oxygen concentration is as in normal air.[107][136]
Confirming the existence of hydrogenase‐employing microbesin the human gut,H2 occurs in human breath. The concentration in the breath of fasting people at rest is typically under5 parts per million(ppm), but can reach50 ppm when people with intestinal disorders consume molecules they cannot absorb during diagnostichydrogen breath tests.[137]
Hydrogen is also often a by-product of other reactions. Many metals react with water to produceH2, 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. Thealkali andalkaline earth metals as well asaluminium,zinc,manganese, andiron, react readily with aqueous acids.[96]
Zn + 2 H+ → Zn2+ + H2
Many metals, such as aluminium, are slow to react with water because they formpassivated oxide coatings. An alloy of aluminium andgallium, however, does react with water. In high-pH solutions, aluminium can react withH2:[96]
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, atroom temperature and 0.1millipascals (9.9×10−10atm),approx.0.05 moles of hydrogen dissolve into one kilogram (2.2 lb) ofdiethyl ether.[87] H2 can be stored in compressed form, although compressing costs energy. Liquefaction is impractical given hydrogen's lowcritical temperature. In contrast, ammonia and many hydrocarbons can be liquified at room temperature under pressure. For these reasons, hydrogencarriers—materials that reversibly bindH2—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 manyrare earths andtransition metals[142] and is soluble in both nanocrystalline andamorphous metals.[143] Hydrogensolubility in metals is influenced by local distortions or impurities in thecrystal lattice.[144] These properties may be useful when hydrogen is purified by passage through hotpalladium disks, but the gas's high solubility is also a metallurgical problem, contributing to theembrittlement of many metals,[145] complicating the design of pipelines and storage tanks.[146]
The most problematic aspect of metal hydrides for storage is their modest H2content, often on the order of1%. For this reason, there is interest in storage of H2 in compounds of lowmolecular weight. For example,ammonia borane (H3N−BH3) contains 19.8weight percent ofH2. 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.[147] More attractive arehydrocarbons such astetrahydroquinoline, which reversibly release someH2 when heated in the presence of a catalyst:[148]
Hydrogenation, the addition ofH2 to various substrates, is done on a large scale. Hydrogenation ofN2 produces ammonia by theHaber process:[150]
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 extensively infertilizer production; these fertilizers have become essential feedstocks in modern agriculture.[151] Hydrogenation is also used to convertunsaturated fats andoils to saturated fats and oils. The major application is the production ofmargarine.Methanol is produced by hydrogenation of carbon dioxide; the mixture of hydrogen and carbon dioxide used for this process is known assyngas. It is similarly the source of hydrogen in the manufacture ofhydrochloric acid.H2 is also used as areducing agent for the conversion of someores to the metals.[152][96]
Fuel
The potential for using hydrogen(H2) as a fuel has been widely discussed. Hydrogen can be used infuel cells to produce electricity,[153] or burned to generate heat.[154] When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor.[154] When burned, hydrogen produces relatively little pollution at the point of combustion, but can lead to thermal formation of harmfulnitrogen oxides.[154]
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.[155][123]
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 aselectric arc furnaces for steelmaking.[156] However, it is likely to play a larger role in providing industrial feedstock for cleaner production of ammonia and organic chemicals.[155] For example, insteelmaking, hydrogen could function as a clean fuel and also as a low-carbon catalyst, replacing coal-derivedcoke (carbon):[157]
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 asammonia andmethanol and fuel cell technology.[155] For light-duty vehicles including cars, hydrogen is far behind otheralternative fuel vehicles, especially compared with the rate of adoption ofbattery electric vehicles, and may not play a significant role in future.[158]
ASpace Shuttle Main Engine burns hydrogen with oxygen, producing a nearly invisible flame at full thrust.
Hydrogen produced when there is a surplus ofvariable renewable electricity could in principle be stored and later used to generate heat or to re-generate electricity.[160] It can be further transformed intosynthetic fuels such asammonia andmethanol.[161] 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 toembrittle materials.[116]
Leak detection: Pure or mixed with nitrogen (sometimes calledforming gas), hydrogen is atracer gas fordetection of minute leaks. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries;[182] it also allows for leak testing in food packaging.
In hydrogen pipelines and steel storage vessels, hydrogen molecules are prone to reacting with metals, causinghydrogen embrittlement and leaks in the pipeline or storage vessel.[186] Since it is lighter than air, hydrogen does not easily accumulate to form a combustible gas mixture.[186] However, even without ignition sources, high-pressure hydrogen leakage may cause spontaneous combustion anddetonation.[186]
Hydrogen is flammable when mixed even in small amounts with air. Ignition can occur at avolumetric ratio of hydrogen to air as low as 4%.[187] 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.[186]
Hydrogen fire, while being extremely hot, is almost invisible to the human eye, and thus can lead to accidental burns.[44] Hydrogen is non-toxic,[188] but like most gasesit can cause asphyxiation in the absence of adequate ventilation.[189]
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