Liquid hydrogen (LH2 or LH₂) is the

liquid state A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, an ...

of the element

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

. Hydrogen is found naturally in the

molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

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 .IPTS-1968
iupac.org, accessed 2020-01-01 A common method of obtaining liquid hydrogen involves a

compressor A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...

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 in pressurized and thermally insulated containers. There are two spin isomers of hydrogen; liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen.

History

In 1885,

Zygmunt Florenty Wróblewski Zygmunt Florenty Wróblewski (28 October 1845 – 16 April 1888) was a Polish physicist and chemist. Biography Wróblewski was born in Grodno (Russian Empire, now in Belarus). He studied at Kiev University. After a six-year exile for participati ...

published hydrogen's critical temperature as 33K; critical pressure, 13.3 atmospheres; and boiling point, 23K. 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 Spin or spinning most often refers to: * Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning * Spin, the rotation of an object around a central axis * Spin (propaganda), an intentionally b ...

states. Parahydrogen, in which the two

nuclear spin In atomic physics, the spin quantum number is a quantum number (designated ) which describes the intrinsic angular momentum (or spin angular momentum, or simply spin) of an electron or other particle. The phrase was originally used to describe th ...

s 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. 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 Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally ...

activated carbon Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that increase the surface area avail ...

, platinized asbestos, rare earth metals, uranium compounds, chromium(III) oxide, or some nickel compounds.

Uses

Liquid hydrogen is a common

liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...

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). In most rocket engines fueled by liquid hydrogen, it first

cools Cools is a Dutch language, Dutch patronymic surname meaning "son of Cool", Cool being an archaic nickname for Nicholas. The name is particularly prominent in the Belgian province of Antwerp (province), Antwerp. People with the surname include: ...

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 can be used as the fuel for an internal combustion engine or

fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...

. Various submarines ( Type 212 submarine, Type 214 submarine) and concept

hydrogen vehicle A hydrogen vehicle is a vehicle that uses hydrogen fuel for motive power. Hydrogen vehicles include hydrogen-fueled space rockets, as well as ships and aircraft. Power is generated by converting the chemical energy of hydrogen to mechanical energ ...

s have been built using this form of hydrogen (see

DeepC The DeepC is a hydrogen-fueled Autonomous Underwater Vehicle (AUV), power-assisted by an electric motor that gets its electricity from a fuel cell. It debuted in 2004. The project was funded by the German Federal Ministry for Education and Researc ...

, BMW H2R). Due to its similarity, builders can sometimes modify and share equipment with systems designed for liquefied natural gas (LNG). Liquid hydrogen is being investigated as a zero carbon fuel for aircraft. However, because of the lower volumetric energy, the hydrogen volumes needed for combustion are large. 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.

Properties

The product of hydrogen combustion in a pure oxygen environment is solely water vapor. However, the high combustion temperatures and present atmospheric nitrogen can result in the breaking of N≡N bonds, forming toxic NOx if no exhaust scrubbing is done. Since water is often considered harmless to the environment, an engine burning it can be considered "zero emissions". In aviation, however, water vapor emitted in the atmosphere contributes to global warming (to a lesser extent than CO₂). Liquid hydrogen also has a much higher specific energy than gasoline, natural gas, or diesel.Hydrogen As an Alternative Fuel
. Almc.army.mil. Retrieved on 2011-08-28. The density of liquid hydrogen is only 70.85 g/L (at 20 K), a

relative density Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

of just 0.07. Although the specific energy is more than twice that of other fuels, this gives it a remarkably low volumetric

energy density In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or . Often only the ''useful'' or extract ...

, many fold lower. Liquid hydrogen requires

cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...

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). 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 7.042 kPa.Cengel, Yunus A. and Turner, Robert H. (2004). ''Fundamentals of thermal-fluid sciences'', McGraw-Hill, p. 78, File:Liquid_hydrogen_bubblechamber.jpg, Liquid hydrogen bubbles forming in two glass flasks at the Bevatron laboratory, c. 1950s File:Hydrogen Tank - GPN-2000-001458.jpg, A large hydrogen tank in a vacuum chamber at Lewis Research Center in 1967 Image:Linde-Wasserstofftank.JPG, Tank for liquid hydrogen of

Linde Linde may refer to: Places *Lindes and Ramsberg Mountain District, a former district in Sweden, see Lindesberg Municipality *Lipka, Złotów County, a village in Poland, called Linde before World War II Rivers *Linde (Tollense), a river of Meckle ...

, Museum Autovision, Altlußheim, Germany

Safety

Due to its cold temperatures, liquid hydrogen is a hazard for cold burns. 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.

References