Liquid hydrogen

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Liquid hydrogen
IUPAC name
Liquid hydrogen
Other names
Hydrogen (cryogenic liquid); hydrogen, refrigerated liquid; LH2, para-hydrogen
  • 1333-74-0 YesY
3D model (JSmol)
  • Interactive image
  • CHEBI:33251 YesY
  • 762 YesY
  • C00282 YesY
PubChem CID
  • 783
RTECS number MW8900000
  • 7YNJ3PO35Z YesY
UN number 1966
Molar mass 2.02 g·mol−1
Appearance Colorless liquid
Density 70.85 g/L (4.423 lb/cu ft)[1]
Melting point −259.14 °C (14.01 K; −434.45 °F)[2]
Boiling point −252.87 °C (20.28 K; −423.17 °F)[2]
Highly flammable (F+)
NFPA 704
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g., propane Health code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g., sodium chloride Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazard CRYO: Cryogenic.NFPA 704 four-colored diamond
571 °C (1,060 °F; 844 K)[2]
Explosive limits LEL 4.0%; UEL 74.2% (in air)[2]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Liquid hydrogen (LH2 or LH2) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.

To exist as a liquid, H2 must be cooled below hydrogen's critical point of 33 K. However, for hydrogen to be in a fully liquid state without boiling at atmospheric pressure, it needs to be cooled to 20.28 K[3] (−423.17 °F/−252.87 °C).[4][5] One common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form of hydrogen storage. As in any gas, storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. However, the liquid density is very low compared to other common fuels. Once liquefied, it can be maintained as a liquid in pressurized and thermally insulated containers.

There are two spin isomers of hydrogen; liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen.[6]


In 1885 Zygmunt Florenty Wróblewski published hydrogen's critical temperature as 33 K; critical pressure, 13.3 atmospheres; and boiling point, 23 K.

Hydrogen was liquefied by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask. The first synthesis of the stable isomer form of liquid hydrogen, parahydrogen, was achieved by Paul Harteck and Karl Friedrich Bonhoeffer in 1929.

Spin isomers of hydrogen

The two nuclei in a dihydrogen molecule can have two different spin states. Parahydrogen, in which the two nuclear spins are antiparallel, is more stable than orthohydrogen, in which the two are parallel. At room temperature, gaseous hydrogen is mostly in the ortho isomeric form due to thermal energy, but an ortho-enriched mixture is only metastable when liquified at low temperature. It slowly undergoes an exothermic reaction to become the para isomer, with enough energy released as heat to cause some of the liquid to boil.[7] To prevent loss of the liquid during long-term storage, it is therefore intentionally converted to the para isomer as part of the production process, typically using a catalyst such as iron(III) oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds, chromium(III) oxide, or some nickel compounds.[7]


Liquid hydrogen is a common liquid rocket fuel for rocketry applications — both NASA and the United States Air Force operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons).[8] In most rocket engines fueled by liquid hydrogen, it first cools the nozzle and other parts before being mixed with the oxidizer — usually liquid oxygen (LOX) — and burned to produce water with traces of ozone and hydrogen peroxide. Practical H2–O2 rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust, which can actually increase specific impulse, despite the incomplete combustion.

Liquid Hydrogen
DOT Hazardous Material Placard liquid hydrogen.jpg
RTECS MW8900000
PEL-OSHA Simple asphyxiant
ACGIH TLV-TWA Simple asphyxiant

Liquid hydrogen can be used as the fuel for an internal combustion engine or fuel cell. Various submarines (Type 212 submarine, Type 214 submarine) and concept hydrogen vehicles have been built using this form of hydrogen (see DeepC, BMW H2R). Due to its similarity, builders can sometimes modify and share equipment with systems designed for liquefied natural gas (LNG). However, because of the lower volumetric energy, the hydrogen volumes needed for combustion are large. Unless LH2 is injected instead of gas,[clarify] hydrogen-fueled piston engines usually require larger fuel systems. Unless direct injection is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses.

Liquid hydrogen is also used to cool neutrons to be used in neutron scattering. Since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum (elastic collision). Finally, superheated liquid hydrogen was used in many bubble chamber experiments.

The first thermonuclear bomb, Ivy Mike, used liquid deuterium (Hydrogen-2), for nuclear fusion.


A large hydrogen tank in a vacuum chamber at Lewis Research Center in 1967

The product of its combustion with oxygen alone is water vapor (although if its combustion is with oxygen and nitrogen it can form toxic chemicals), which can be cooled with some of the liquid hydrogen. Since water is harmless to the environment, an engine burning it can be considered "zero emissions." Liquid hydrogen also has a much higher specific energy than gasoline, natural gas, or diesel.[9]

The density of liquid hydrogen is only 70.99 g/L (at 20 K), a relative density of just 0.07. Although the specific energy is around twice that of other fuels, this gives it a remarkably low volumetric energy density, many fold lower.

Liquid hydrogen requires cryogenic storage technology such as special thermally insulated containers and requires special handling common to all cryogenic fuels. This is similar to, but more severe than liquid oxygen. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away (typically at a rate of 1% per day[9]). It also shares many of the same safety issues as other forms of hydrogen, as well as being cold enough to liquefy, or even solidify atmospheric oxygen, which can be an explosion hazard.

The triple point of hydrogen is at 13.81 K[3] 7.042 kPa.[10]


Due to its cold temperatures, liquid hydrogen is a hazard for cold burns. Apart from that, elemental hydrogen as a liquid is biologically inert and its only human health hazard as a vapor is displacement of oxygen, resulting in asphyxiation. Because of its flammability, liquid hydrogen should be kept away from heat or flame unless ignition is intended.

See also

Tank for liquid hydrogen of Linde, Museum Autovision, Altlußheim, Germany


  1. ^ Thermophysical Properties of Hydrogen ,, accessed 2012-09-14
  2. ^ a b c d Information specific to liquid hydrogen Archived 2009-07-17 at the Wayback Machine.,, accessed 2009-06-12
  3. ^ a b IPTS-1968,, accessed 2009-06-12
  4. ^ Chemical elements data references
  5. ^ Properties Of Gases. Retrieved on 2011-08-28.
  6. ^ Liquid Air/LH2. Retrieved on 2011-08-28.
  7. ^ a b "Liquefaction of "Permanent" Gases" (PDF of lecture notes). 2011. Retrieved 2017-10-16. 
  8. ^ Flynn, Thomas (2004). Cryogenic Engineering, Second Edition, Revised and Expanded. CRC Press. p. 401. ISBN 978-0-203-02699-1. 
  9. ^ a b Hydrogen As an Alternative Fuel. Retrieved on 2011-08-28.
  10. ^ Cengel, Yunus A. and Turner, Robert H. (2004). Fundamentals of thermal-fluid sciences, McGraw-Hill, p. 78, ISBN 0-07-297675-6
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