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Topic summary

Liquid hydrogen

Liquid hydrogen
Names IUPAC name
Hydrogen
Systematic IUPAC name
Liquid hydrogen
Other names
Hydrogen (cryogenic liquid), Refrigerated hydrogen; LH2, para-hydrogen
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
KEGG
RTECS number
  • MW8900000
UNII
UN number1966
  • InChI=1S/H2/h1H
    Key: UFHFLCQGNIYNRP-UHFFFAOYSA-N
  • InChI=1/H2/h1H
  • [H][H]
Properties H2(l)Molar mass2.016 g·mol Appearance Colorless liquid Density0.07085 g/cm (4.423 lb/cu ft) Melting point−259.14 °C (−434.45 °F; 14.01 K) Boiling point−252.87 °C (−423.17 °F; 20.28 K) Hazards GHS labelling: DangerH220, H280P210, P377, P381, P403NFPA 704 (fire diamond)
3
4
0
CRYO
571 °C (1,060 °F; 844 K) Explosive limitsLEL 4.0%; UEL 74.2% (in air)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Liquid hydrogen (H2(l)) 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 its critical point of 33 K. 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). A 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. 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 for some time in thermally insulated containers.

There are two spin isomers of hydrogen. Room temperature hydrogen is 75% orthohydrogen. At cryogenic temperature it converts exothermically to parahydrogen. The thermodynamic lowest energy state for liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen. To avoid the exothermic heat release and resulting excessive boil-off in storage, catalytic conversion to parahydrogen during liquefaction is employed.

Hydrogen requires a theoretical minimum of 3.3 kWh/kg (12 MJ/kg) to liquefy, and 3.9 kWh/kg (14 MJ/kg) including converting the hydrogen to the para isomer. Existing liquefaction facilities use 10–13 kWh/kg (36–47 MJ/kg) compared to a 33 kWh/kg (119 MJ/kg) heating value of hydrogen. More recent work shows future facilities are expected to cut the specific energy demand by half to 6.5 kWh/kg (23 MJ/kg).

Wikipedia source