Hubbry Logo
Liquid hydrogenLiquid hydrogenMain
Open search
Liquid hydrogen
Community hub
Liquid hydrogen
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Liquid hydrogen
Liquid hydrogen
Liquid hydrogen
View on Wikipedia
from Wikipedia
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 number 1966
  • InChI=1S/H2/h1H checkY
    Key: UFHFLCQGNIYNRP-UHFFFAOYSA-N checkY
  • InChI=1/H2/h1H
  • [H][H]
Properties
H2(l)
Molar mass 2.016 g·mol−1
Appearance Colorless liquid
Density 0.07085 g/cm3 (4.423 lb/cu ft)[1]
Melting point −259.14 °C (−434.45 °F; 14.01 K)[2]
Boiling point −252.87 °C (−423.17 °F; 20.28 K)[2]
Hazards
GHS labelling:[3]
GHS02: FlammableGHS04: Compressed Gas
Danger
H220, H280
P210, P377, P381, P403
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 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. propaneInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazard CRYO: Cryogenic
3
4
0
CRYO
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).
checkY verify (what is checkY☒N ?)

Liquid hydrogen (H2(l)) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.[4]

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).[5] 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.[6]

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.[5]. To avoid that the exothermic heat release occurs in storage, and thereby causes excessive boil-off, catalytic conversion to parahydrogen during liquification 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 liquification facilities use 10–13 kWh/kg (36–47 MJ/kg) compared to a 33 kWh/kg (119 MJ/kg) heating value of hydrogen.[7]. More recent work shows future facilities are expected to cut the specific energy demand by half to 6.5 kWh/kg (23 MJ/kg) [8]

History

[edit]
Further information: Timeline of low-temperature technology
Liquid hydrogen bubbles forming in two glass flasks at the Bevatron laboratory in 1955
A large hydrogen tank in a vacuum chamber at the Glenn Research Center in Brook Park, Ohio, in 1967
A Linde AG tank for liquid hydrogen at the Museum Autovision in Altlußheim, Germany, in 2008
Two U.S. Department of Transportation placards indicating the presence of hazardous materials, which are used with liquid hydrogen

In 1885, Zygmunt Florenty Wróblewski published hydrogen's critical temperature as 33 K (−240.2 °C; −400.3 °F); critical pressure, 13.3 standard atmospheres (195 psi); and boiling point, 23 K (−250.2 °C; −418.3 °F).

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

[edit]
Main article: 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.[9] 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.[9]

Uses

[edit]

Liquid hydrogen is a common liquid rocket fuel for rocketry application and is used by NASA and the U.S. Air Force, which operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons).[10]

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, 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 increase specific impulse, despite the incomplete combustion.

Liquid hydrogen can be used as the fuel for an internal combustion engine or fuel cell. Various submarines, including the Type 212 submarine, Type 214 submarine, and others, and concept hydrogen vehicles have been built using this form of hydrogen, such as the DeepC, BMW H2R, and others. 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. 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, also known as hydrogen-2, for nuclear fusion.

Properties

[edit]

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.[11] 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).[12] Liquid hydrogen also has a much higher specific energy than gasoline, natural gas, or diesel.[13]

The density of liquid hydrogen is only 70.85 kg/m3 (at 20 K), a relative density 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, 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[13]). 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[5] and 7.042 kPa.[14]

Safety

[edit]

Due to its cold temperatures, liquid hydrogen is a hazard for cold burns. Hydrogen itself is biologically inert and its only human health hazard as a vapor is displacement of oxygen, resulting in asphyxiation, and its very high flammability and ability to detonate when mixed with air. Because of its flammability, liquid hydrogen should be kept away from heat or flame unless ignition is intended. Unlike ambient-temperature gaseous hydrogen, which is lighter than air, hydrogen recently vaporized from liquid is so cold that it is heavier than air and can form flammable heavier-than-air air–hydrogen mixtures.

An indirect safety risk exists due to the cryogenic temperature being lower than the boiling point of oxygen. Exposure of insuffiently thermally insulated liquid hydrogen containments can result in air condensing on the outside of the containment, leading to oxygen enrichment that can spontaneously ignite flammable materials.

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Liquid hydrogen

View on Grokipedia
from Grokipedia
Liquid hydrogen (LH₂) is the cryogenic liquid form of the diatomic hydrogen molecule (H₂), produced by cooling hydrogen gas below its boiling point of −252.9 °C (−423.2 °F) at standard atmospheric pressure. This transparent, colorless, odorless, and noncorrosive fluid has a density of 70.8 kg/m³ (4.42 lb/ft³) at its boiling point, providing exceptional gravimetric energy density—approximately 120 MJ/kg, three times that of gasoline—but very low volumetric energy density, about one-fourth that of gasoline. Due to these properties, liquid hydrogen is primarily utilized as a high-performance rocket propellant and holds potential as a clean energy carrier in fuel cells and transportation, though its handling requires specialized cryogenic infrastructure. Liquid hydrogen's physical properties, detailed in subsequent sections, include a freezing point of −259.3 °C (−434.8 °F) and an of 1:845 from liquid to gas at ambient conditions. As a , it presents handling challenges such as risks of burns and flammability of its vapors (4% to 74.2% in air), but it is non-toxic and chemically inert. Production typically involves liquefying high-purity hydrogen gas, primarily obtained via steam-methane reforming of (about 75% of global production) or for low-carbon options, through energy-intensive processes requiring 10–13 kWh/kg. Storage and transportation rely on vacuum-insulated cryogenic systems to minimize boil-off losses, with applications centered on (e.g., engines in NASA's , specific impulse > s) and emerging uses in cells for and .

Properties

Physical properties

Liquid hydrogen appears as a colorless, transparent liquid due to its non-polar molecular structure and low density. Its refractive index is approximately 1.11 at the boiling point, reflecting minimal light scattering in the cryogenic medium. The density of liquid hydrogen is 70.8 kg/m³ at its normal boiling point of 20.28 K and 1 atm pressure, making it one of the lightest liquids known and posing unique challenges for storage due to buoyancy effects. Density increases with applied pressure, reaching up to about 80 kg/m³ at higher pressures near the freezing point, while it decreases slightly with rising temperature along the saturation line, emphasizing the need for precise pressure control in containment systems. The normal boiling point is 20.28 K (-252.87 °C) at 1 atm, with the critical point occurring at 33 K and 12.8 atm, beyond which the distinction between liquid and vapor phases disappears. The freezing point at 1 atm is 14.01 K (-259.14 °C), marking the transition to solid hydrogen under standard conditions. Viscosity of liquid hydrogen is low, approximately 13 μPa·s at the , facilitating easy flow but requiring careful of transfer lines to minimize turbulence-induced boil-off in cryogenic storage. is about 1.55 mN/m at 20 , influencing droplet formation and behavior in engineering applications such as systems, where low values promote rapid spreading but complicate . The of liquid hydrogen highlights its narrow liquid-vapor equilibrium region, bounded by the at approximately 13.8 and 0.07 on the low-temperature side, extending along the curve to the critical point at 33 and 12.8 . This equilibrium curve describes the conditions under which liquid hydrogen remains against , with increases allowing liquid to higher temperatures up to the critical limit.

Thermodynamic properties

Liquid hydrogen's thermodynamic properties are governed by its cryogenic state, where intermolecular forces and quantum effects significantly influence , , and phase behavior. These properties determine the energy requirements for maintaining the liquid phase and the of heat exchangers in liquefaction and storage systems. At the normal boiling point of 20.28 K and 1 atm, liquid hydrogen demonstrates a high heat of vaporization, reflecting the energy needed to overcome weak van der Waals forces in the molecular structure. The heat of vaporization is 445.7 kJ/kg at the , indicating the absorbed during the from to saturated vapor. This value is essential for calculating boil-off rates in insulated tanks. The of the phase is approximately 9.9 kJ/kg·K near the , representing the energy required to raise the temperature of the without phase change; this low value contributes to the challenges in thermal management during storage. Thermal conductivity, measured at 0.117 W/m·K at 20 K, facilitates efficient in cryogenic applications but underscores the need for specialized insulation to minimize losses. The enthalpy of the liquid state relative to the gaseous state at the boiling point follows the relation Hliquid=Hgas−ΔHvapH_{\text{liquid}} = H_{\text{gas}} - \Delta H_{\text{vap}}, where ΔHvap\Delta H_{\text{vap}} is the heat of vaporization. For hydrogen, this yields an enthalpy difference of 445.7 kJ/kg between the saturated liquid and vapor phases at 20.28 K, with the liquid enthalpy typically referenced as lower by this amount in thermodynamic tables. Upon vaporization at atmospheric pressure, liquid hydrogen exhibits a volume expansion ratio of approximately 1:845, meaning 1 volume of liquid produces 845 volumes of gas at standard temperature and pressure; this dramatic expansion drives safety considerations in venting systems. In cryogenic conditions, liquid hydrogen's compressibility is low, with the compressibility factor Z=PVRTZ = \frac{PV}{RT} approaching 0.98–1.0 near the boiling point, indicating near-ideal behavior modified by weak intermolecular attractions. The thermal expansion coefficient, α=1V(∂V∂T)P\alpha = \frac{1}{V} \left( \frac{\partial V}{\partial T} \right)_P , is approximately 0.25 K^{-1} at 20 K and low pressures, increasing with temperature and decreasing with pressure up to 70 bar; this high expansivity affects tank design under varying thermal loads. Pressure-volume-temperature (PVT) relations for these conditions are accurately modeled using the modified Benedict-Webb-Rubin equation of state, which accounts for real-gas deviations in high-pressure cryogenic storage.

Spin isomers

Liquid hydrogen exists in two nuclear spin isomers: ortho-hydrogen, where the protons' spins are parallel (total nuclear spin quantum number I = 1), permitting only odd rotational quantum levels (J = 1, 3, ...), and para-hydrogen, with antiparallel spins (I = 0), restricted to even rotational levels (J = 0, 2, ...). At room temperature, the equilibrium mixture, known as normal hydrogen, consists of approximately 75% ortho-hydrogen and 25% para-hydrogen due to the threefold degeneracy of the ortho spin state. In the liquid state near 20 K, the favors nearly complete conversion to para-hydrogen (about 99.8% para, 0.2% ortho), as higher rotational states become inaccessible at low temperatures. However, without , the ortho-to-para conversion is extremely slow, with a timescale of months to years, necessitating catalysts to achieve full para conversion during liquefaction to minimize generation from the (releasing ~519 J/g for complete conversion). The kinetics of catalyzed spin conversion often follow a of the form d[para]/dt = k ([ortho][para] - [para_eq][ortho_eq]), reflecting a second-order relaxation toward equilibrium concentrations, where k depends on the catalyst and . The spin isomers influence key thermophysical properties of liquid hydrogen. Pure para-hydrogen has a lower normal of 20.26 compared to 20.37 for pure ortho-hydrogen, due to differences in intermolecular potential and rotational contributions to the . Non-equilibrium mixtures with higher ortho content exhibit elevated (up to ~50% higher near 20 ) and , as the ortho form's accessible excited rotational states store additional and affect molecular interactions, leading to increased boil-off rates if not equilibrated. Techniques for separating or enriching spin isomers exploit their differences in adsorption affinity and . Activated charcoal preferentially adsorbs ortho-hydrogen at low temperatures (e.g., 77 K), enabling fractional separation through repeated adsorption-desorption cycles, as demonstrated in early experiments. can also induce population shifts or enrich isomers by interacting with the ortho form's nonzero , with studies showing ortho-para ratio inversion under strong homogeneous fields at cryogenic temperatures. The existence of ortho- and para-hydrogen was first demonstrated in 1929 by Karl-Friedrich Bonhoeffer and Paul Harteck, who separated the isomers using activated charcoal and observed distinct thermal properties, laying the foundation for understanding quantum effects in diatomic molecules. Adalbert Farkas contributed significantly through subsequent experimental and theoretical work, including detailed studies on conversion mechanisms in the early 1930s.

Production

Liquefaction methods

of hydrogen requires cooling the gas from ambient conditions to its boiling point of approximately 20.28 at standard pressure, a that demands sophisticated techniques due to hydrogen's low critical temperature and the need for efficient heat removal. The primary methods exploit thermodynamic cycles that leverage compression, expansion, and heat exchange to achieve the necessary cryogenic temperatures. These techniques must also account for hydrogen's unique molecular properties, such as its spin isomers, to minimize energy losses during the . The Linde process, also known as the Joule-Thomson cycle, is a foundational method for that relies on . In this approach, gas is compressed and precooled using successive stages with refrigerants like (down to about 77 K) and then further cooled by expanding through a throttle valve, where the Joule-Thomson effect causes a drop due to intermolecular forces in the real gas. The precooled gases, including nitrogen and itself in multiple stages, enable a stepwise reduction in until occurs. This method is simple and suitable for moderate-scale operations but has lower efficiency compared to more advanced cycles. The Claude enhances over the Linde method by incorporating work-extracting expansion through rather than relying solely on throttling. Precooled is partially liquefied in a , with a portion of the high-pressure gas directed through an expander to cooling via isentropic expansion; the work extracted from this step is given by the W=∫P d[V](/page/V.)W = \int P \, d[V](/page/V.), where PP is and VV is , contributing to the overall cycle . The expanded gas is then recombined with the main stream, allowing for greater refrigeration capacity and reduced energy input, making it preferable for larger-scale . This cycle typically achieves higher liquefaction yields by recovering mechanical work during expansion. A critical aspect of hydrogen liquefaction is the ortho-para conversion, as normal hydrogen gas at room temperature consists of about 75% ortho-hydrogen (higher spin state) and 25% para-hydrogen (lower state), but the liquid form is stable only with nearly 100% para-hydrogen. Without conversion, the exothermic ortho-to-para relaxation during cooling releases heat, leading to significant energy losses—up to 15% of the total destruction in the process. Catalysts such as activated charcoal, , or rare-earth compounds are employed in heat exchangers to accelerate this conversion, ensuring equilibrium ratios and preventing boil-off or inefficient cooling. This step is integrated into both Linde and Claude cycles to optimize overall performance. The theoretical minimum work required for hydrogen liquefaction sets a fundamental efficiency limit, based on the Carnot principle for reversible processes. This minimum work is calculated as Wmin⁡=T0(ΔSgas−ΔSliquid)W_{\min} = T_0 (\Delta S_{\text{gas}} - \Delta S_{\text{liquid}}), where T0T_0 is the ambient temperature (typically 298 K), and ΔS\Delta S represents the difference between gaseous and liquid states at the ; for from ambient conditions, this equates to approximately 3.3 kWh/kg, though it rises to about 3.9 kWh/kg when including ortho-para conversion . Actual processes operate at 30-50% of this Carnot due to irreversibilities like losses and . For laboratory-scale liquefaction of small quantities (e.g., grams to kilograms), methods like pulse-tube cryocoolers and dilution refrigerators are employed to achieve and maintain temperatures without large infrastructure. Pulse-tube cryocoolers use oscillating pressure waves in to provide cooling down to 20 K, offering vibration-free operation and reliabilities suitable for research environments, with capacities up to a few watts at hydrogen's . Dilution refrigerators, leveraging the of and mixtures, can reach even lower temperatures but are adapted for hydrogen by precooling stages, enabling precise control for experiments in or . These systems prioritize compactness and low power consumption over high throughput.

Industrial processes

Industrial-scale production of liquid hydrogen (LH2) primarily involves the liquefaction of gaseous derived from large-scale sources such as methane reforming (SMR) or . In SMR, reacts with at high temperatures (over 700°C) to produce and carbon monoxide, followed by a water-gas shift reaction to increase yield, accounting for about 76% of global in 2023. , particularly (PEM) or alkaline powered by renewable , generates by splitting into and oxygen, with growing for low-carbon LH2 to meet decarbonization goals. The gaseous is then purified and compressed before entering the process, ensuring compatibility with cryogenic cooling cycles like the Claude or Brayton process for efficient scaling. Major commercial facilities operated by companies like Air Liquide and Linde exemplify the infrastructure for LH2 production, with capacities typically ranging from 6 to 35 tons per day per plant. Air Liquide's North Las Vegas plant in the USA, opened in 2022, produces 30 tons of LH2 per day, serving clean transportation and industrial sectors, while utilizing hydroelectric power for partial green integration. Linde's McIntosh facility in Alabama, expanded in 2023 with a $90 million investment, also achieves 30 tons per day, focusing on high-purity output for electronics and aerospace. In Europe, Air Liquide operates multiple hydrogen plants integrated with SMR, contributing to a network producing over 1.2 million tons of hydrogen annually (including gaseous forms), though dedicated LH2 capacities are similarly in the 20-30 tons per day range per site. These facilities highlight the shift toward modular, scalable plants to meet rising demand, with total U.S. LH₂ capacity of approximately 794 tons per day across operators (as of 2024). Energy requirements for LH2 production, encompassing compression, purification, and , range from 10 to 15 kWh per kg, with alone consuming about 10-12 kWh/kg due to the need for multi-stage to reach 20 K. Purification steps, such as , add 1-2 kWh/kg to remove impurities like oxygen or , which is critical before cryogenic cooling. As of the , production costs for LH2 vary from $2 to $5 per kg for gray hydrogen from SMR, rising to $3-8 per kg for green LH2 integrated with renewables, influenced by prices and scale efficiencies; U.S. Department of targets aim for $1-2 per kg by 2030 through technological improvements. Purity standards for rocket-grade LH2 typically require at least 99.99% by volume, with ultra-high grades approaching 99.999% to prevent operational issues in applications. Impurities such as or can solidify at LH2 temperatures (20 K), leading to blockages in transfer lines or engines, as even trace levels (below 10 ppm) cause freezing and reduced flow efficiency in cryogenic systems. These standards are achieved through advanced purification in industrial plants, ensuring reliability for high-stakes uses. Post-2020 advancements have focused on enhancing efficiency and in LH2 manufacturing. Integration of sources, such as solar or wind-powered , has enabled green LH2 production at scale, with facilities like Air Liquide's Bécancour plant in (operational since 2021), using for a 20 MW PEM electrolyzer to produce up to 8.2 tons per day of low-carbon . These developments, supported by DOE workshops and international collaborations, emphasize modular liquefaction units and ortho-para conversion optimization to lower boil-off losses and support the global transition. As of 2024, the U.S. Department of Energy targets liquefaction efficiencies of 6-7 kWh/kg for plants exceeding 100 tons per day through advanced cycles.

History

Early discovery

The liquefaction of gases advanced significantly in the late , building on earlier successes with more easily condensable substances. In 1877, Louis-Paul Cailletet and Raoul Pictet independently achieved the first liquefaction of oxygen, producing misty droplets through rapid expansion and compression techniques, which demonstrated that even diatomic gases could transition to the liquid state under extreme conditions. This breakthrough shifted scientific focus toward "permanent gases" like , previously thought incapable of liquefaction due to their low boiling points. Theoretical foundations for hydrogen's liquefaction were laid by in 1873, whose accounted for molecular volume and intermolecular forces, predicting a critical for around 33 K—far below but above , confirming its condensability unlike assumptions. Adaptations of the highlighted hydrogen's exceptionally low critical , requiring cooling below approximately 80 K for effective via expansion methods. These predictions motivated experimental efforts despite hydrogen's unique challenges, including its low Joule-Thomson inversion of 202 K, above which isenthalpic expansion causes heating rather than cooling, necessitating pre-cooling with or to enable the throttling process. Early experimental attempts faced substantial hurdles. In January 1884, Zygmunt Wróblewski and Karol Olszewski in conducted the first targeted experiments on , achieving transient in a dynamic state by expanding compressed gas cooled by boiling oxygen, but they could not collect stable due to insufficient overall cooling and the fleeting nature of the mist formed near the critical point. Their work confirmed , such as a of about 0.03 g/cm³ at the critical point, but the lack of insulation and limited persistence of the phase. The definitive breakthrough came in 1898 with at the Royal Institution in , who successfully produced the first stable samples of liquid hydrogen using a continuous-flow apparatus incorporating his newly invented vacuum-insulated flask to minimize ingress. Pre-cooling gaseous hydrogen with to below 202 allowed the Joule-Thomson expansion to cool it further to the of approximately 20.4 , yielding about 20 liters of liquid hydrogen in initial runs. This achievement overcame prior limitations by combining high-pressure compression, precise throttling, and insulation, enabling storage and study of the liquid for the first time.

Technological development

The development of liquid hydrogen (LH₂) technology accelerated during through the , where interest arose in its potential for nuclear applications, including as a coolant and moderator in early reactor designs and for concepts. Theoretical work on hydrogen-based fusion began amid the project's atomic bomb efforts, laying groundwork for post-war advancements. By the late 1940s, this interest spurred the construction of initial large-scale LH₂ production facilities under the U.S. Atomic Energy Commission, with the first major plant at the National Bureau of Standards achieving operational capacity around to support expanding nuclear needs. In the 1960s, the drove significant advancements in LH₂ technology, with contracting companies like to build commercial-scale , such as the first dedicated facility in New Orleans operational by , capable of producing hundreds of tons annually to fuel the rocket's J-2 engines. The J-2, developed by starting in 1960, was a / delivering 1,033 kN and a of 421 seconds, enabling upper-stage propulsion for lunar missions and marking a leap in cryogenic handling and production efficiency. NASA played a pivotal role in advancing LH₂ for propulsion starting in the late 1950s, with the Centaur upper-stage rocket program initiated in 1958 at the Lewis Research Center. The Centaur utilized LH₂ and (LOX) propellants, powered by RL10 engines that delivered a of approximately 444 seconds, enabling higher efficiency than kerosene-based alternatives. Early challenges included managing LH₂'s low density and boil-off, but these were addressed through rigorous testing, culminating in the program's first successful orbital flight on November 27, 1963, via the Atlas-Centaur AC-2 mission, which validated LH₂ performance and paved the way for the Surveyor lunar program. Key innovations in the enhanced LH₂ storage and handling viability. Multilayer insulation (MLI), pioneered around in and refined for cryogenic applications by the late , consisted of alternating reflective foil layers in a jacket, drastically reducing leak in LH₂ tanks to below 0.5% daily boil-off. Concurrently, para-hydrogen conversion catalysts, such as hydrous ferric oxide granules, were developed to accelerate the ortho-to-para shift, minimizing exothermic release during and storage; tests in 1958 demonstrated conversion rates exceeding 95% para content at high flow rates, critical for efficient large-scale operations. Internationally, the pursued LH₂ engine technology in the 1970s amid its program, with the 's beginning in 1976 at the KBKhA bureau. This closed-cycle, fuel-rich , optimized for LH₂/, achieved a chamber of 21.5 MPa and over 450 seconds, representing the USSR's first major hydrolox powerplant and enabling high-payload orbital missions by the 1980s.

Applications

Cryogenic fuel in rocketry

Liquid hydrogen (LH₂) is widely used as a cryogenic fuel in rocketry, primarily in combination with liquid oxygen (LOX) as the oxidizer, forming a high-performance bipropellant known as LH₂/LOX. This pairing leverages LH₂'s low molecular weight to produce exhaust primarily composed of water vapor, enabling superior efficiency in space propulsion. The typical mixture ratio for LH₂/LOX is approximately 6:1 by mass (oxidizer to fuel), which optimizes performance and yields a vacuum specific impulse (Isp) of about 450 seconds. In operational engines, this ratio balances combustion efficiency and thrust, as seen in various upper-stage designs. The combustion process follows the reaction 2H2+O2→2H2O+energy2H_2 + O_2 \rightarrow 2H_2O + \text{energy}, where the exothermic formation of water vapor generates the high-temperature gases expelled through the nozzle. A prominent example is the Main Engine (SSME), or , which delivered approximately 1.8 MN of at using LH₂/ in a . Three SSMEs together provided the core propulsion for the , demonstrating LH₂'s role in high-, reusable engine architectures. The primary advantage of LH₂ in rocketry stems from its contribution to a high , driven by the low molecular weight (18 g/mol) of the H₂O exhaust, which allows for greater velocity and compared to denser fuels. However, LH₂'s extremely low —about 70 kg/m³ at —poses challenges, requiring oversized tanks that increase vehicle dry mass, aerodynamic drag, and overall structural demands. Several major launch vehicles have incorporated LH₂/LOX in upper stages to capitalize on these efficiency gains for payload delivery to orbit and beyond. The Delta IV rocket's upper stage (retired in 2024) used the RL10 engine with LH₂/LOX for precise orbital maneuvers, while Ariane 5's ESC-A upper stage (retired in 2023) similarly employed this combination for geostationary transfers. NASA's Space Launch System (SLS) features LH₂/LOX in its Interim Cryogenic Propulsion Stage (ICPS), powered by an RL10B-2, to support deep-space missions like Artemis. In August 2025, Air Products completed a major liquid hydrogen delivery to NASA's Kennedy Space Center to fuel upcoming SLS launches. Looking ahead, LH₂ remains central to prospects for reusable rocketry, with emerging designs focusing on high-performance, rapidly reusable systems to reduce launch costs. For instance, Stoke Space's Andromeda vehicle is developing LH₂/ propulsion for fully reusable upper stages, emphasizing efficiency and turnaround times for frequent missions.

Industrial and scientific uses

Liquid hydrogen has potential as an efficient storage and transport medium in emerging applications for industrial hydrogenation processes, such as synthesis and , where is growing. In via the Haber-Bosch , hydrogen comprises about 75% of the feedstock by volume, and LH₂ could enable economical delivery to remote sites as infrastructure develops. Similarly, in , hydrogen is essential for hydrocracking and desulfurization, accounting for roughly 25% of global hydrogen consumption, with LH₂ offering higher density for future large-scale needs compared to compressed gas. In applications, liquid hydrogen supports systems for grid-scale power generation, offering long-duration storage potential despite a round-trip efficiency of approximately 40% when integrating for production and conversion back to . This efficiency arises from losses in (about 30% of the hydrogen's lower heating value) and operation (typically 50-60%), making it lower than systems but for seasonal storage to higher volumetric density. Liquid hydrogen's cryogenic nature also allows integration with for combined heat and power, recovering boil-off gases to improve overall system performance. Scientifically, liquid hydrogen functions as a in reactors and spallation sources, slowing fast s to thermal energies around 20 K for enhanced in materials analysis and scattering experiments. Facilities like the NIST Center for Neutron and the employ liquid hydrogen sources to shift neutron spectra, providing up to tenfold intensity gains for low-energy beams used in and . In superconductivity , liquid hydrogen is investigated as a for high-temperature superconductors like MgB₂, operating at 20 K to enable cost-effective magnets without relying on scarce liquid helium, with potential applications in particle accelerators and magnetic resonance systems. Emerging applications in the green hydrogen economy position liquid hydrogen for grid storage pilots in Europe during the 2020s, leveraging renewable electrolysis to produce and store excess power for decarbonized industrial clusters. The European Hydrogen Backbone initiative outlines infrastructure for transporting hydrogen via pipelines to storage hubs, supporting projects like those in the Netherlands and Germany aiming for gigawatt-scale integration by 2030. In August 2025, a new LH₂ testing facility in the Netherlands began enabling safe experimentation for maritime and other non-aerospace applications.

Safety and handling

Hazards and risks

Liquid hydrogen presents several inherent hazards due to its extreme physical properties and chemical reactivity. One primary is cryogenic burns, as the liquid exists at a temperature of -253 °C, which can cause severe tissue damage upon direct contact with skin or eyes, resulting in or . Even brief exposure to the cold vapor or frosted equipment can lead to similar injuries by rapidly freezing moist tissues. Another significant danger is asphyxiation in confined spaces, where the rapid of hydrogen—expanding to 845 times its liquid volume at standard conditions—displaces breathable oxygen, potentially reducing concentrations below 19.5% and causing unconsciousness or death without warning, as hydrogen is odorless and non-irritating. This risk is compounded by its wide flammability limits of 4% to 75% by volume in air, allowing ignition across a broad concentration range and exacerbating the potential for or in oxygen-depleted environments. The explosive potential of liquid hydrogen is particularly acute, given its low ignition energy (as low as 0.017 mJ) and of 585 °C (858 K), which enables under moderate heat conditions. In hydrogen-air mixtures, deflagrations can transition to detonations propagating at velocities of approximately 2 km/s, generating overpressures up to 20 times atmospheric levels and causing structural damage over large areas. Hydrogen-induced embrittlement poses a integrity risk, where atomic diffuses into metals like high-strength steels used in storage systems, reducing and promoting cracking under stress, particularly at temperatures below ° relevant to cryogenic handling. This can lead to catastrophic failures in pipelines or tanks if incompatible materials are employed. Environmentally, uncontrolled venting of liquid hydrogen can indirectly contribute to stratospheric ozone depletion, as leaked hydrogen transported to the upper atmosphere reacts to form water vapor, enhancing hydroxyl radical concentrations that catalyze ozone destruction cycles; however, this impact remains minimal relative to other cryogenic fluids due to hydrogen's short atmospheric lifetime.

Storage and transportation protocols

Liquid hydrogen storage requires specialized vessels to maintain its cryogenic temperature of approximately 20 K and minimize heat ingress, which causes boil-off. These vessels are typically double-walled, vacuum-insulated tanks filled with perlite powder between the walls to provide thermal insulation, achieving boil-off rates of less than 0.5% per day under standard conditions. Such designs are common in large-scale facilities, like NASA's ground storage tanks with capacities up to 3,220 m³, where the vacuum jacket and perlite combination effectively reduces evaporative losses. Materials for these storage tanks must withstand the extreme low temperatures and potential for without losing ductility. Austenitic stainless steels, such as 304 or 316 grades, and aluminum alloys like 6061 are preferred due to their face-centered cubic , which resists hydrogen-induced cracking and brittle fracture at cryogenic temperatures. These materials ensure structural integrity during prolonged exposure to liquid hydrogen, preventing failures that could lead to leaks or ruptures. Transportation of liquid hydrogen employs cryogenic tankers designed for road, rail, or maritime use, incorporating systems to manage boil-off during transit and maintain . Cryogenic trucks often utilize ISO-standard frames, such as 30-foot containers with capacities around 10,000 gallons, equipped with and refrigeration units to limit losses over distances up to several hundred kilometers. For , specialized ships feature vacuum-insulated cargo tanks compliant with ISO 11326, which outlines procedures for liquid hydrogen on hydrogen carriers, ensuring handling during loading, voyage, and unloading. Operational protocols emphasize continuous monitoring and rapid response to maintain safety. Leak detection systems incorporate hydrogen sensors, such as electrochemical or catalytic types, placed at potential leak points like valves and flanges to identify concentrations as low as 0.1% by volume, triggering alarms and shutdowns. Emergency venting systems are integrated into storage and transport vessels, allowing controlled release of vapors through relief valves or flares to prevent overpressurization, with designs ensuring vaporization before atmospheric discharge to mitigate ignition risks. Regulatory frameworks govern these practices to standardize safe handling. The (OSHA) standard 29 CFR 1910.103 provides guidelines for hydrogen systems, including requirements for ventilation, separation distances, and (PPE) such as insulated gloves rated for cryogenic exposure to protect against during transfers. NASA's Safety Standard for Hydrogen and Hydrogen Systems (NSS 1740.16) complements this by detailing , operation, and procedures for liquid hydrogen facilities, mandating features like redundant and trained personnel for all phases of storage and transport.

References

  1. https://www.energy.gov/eere/fuelcells/articles/safetygram-9-liquid-hydrogen
  2. https://www.eia.gov/energyexplained/hydrogen/
  3. https://afdc.energy.gov/fuels/hydrogen-production
  4. https://www.hydrogen.energy.gov/docs/hydrogenprogramlibraries/pdfs/9013_energy_requirements_for_hydrogen_gas_compression.pdf
  5. https://www.iea.org/reports/global-hydrogen-review-2024
  6. https://www.nasa.gov/news-release/nasa-awards-contract-for-liquid-hydrogen/
  7. https://www.eia.gov/energyexplained/hydrogen/use-of-hydrogen.php
  8. https://www.webelements.com/hydrogen/physics.html
  9. https://pubs.aip.org/aip/jcp/article/49/7/3096/84203/Refractive-Index-of-Gaseous-and-Liquid-Hydrogen
  10. https://nvlpubs.nist.gov/nistpubs/Legacy/MONO/nbsmonograph168.pdf
  11. https://www.engineeringtoolbox.com/hydrogen-d_1419.html
  12. https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir7352.pdf
  13. https://wtt-pro.nist.gov/wtt-pro/index.html?cmp=hydrogen
  14. https://uspas.fnal.gov/materials/10MIT/Lecture_1.3.pdf
  15. https://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-204-88-LiquidH2.pdf
  16. https://www.osti.gov/etdeweb/servlets/purl/20599211
  17. https://webbook.nist.gov/chemistry/fluid/
  18. https://pmc.ncbi.nlm.nih.gov/articles/PMC11396052/
  19. https://pmc.ncbi.nlm.nih.gov/articles/PMC6768196/
  20. https://pmc.ncbi.nlm.nih.gov/articles/PMC7453723/
  21. https://ntrs.nasa.gov/api/citations/20240008350/downloads/SNP-DOC-0046_v05_pH2_database_final.pdf
  22. https://pubs.acs.org/doi/10.1021/acs.iecr.4c02059
  23. https://academic.oup.com/book/59528/chapter/502299983
  24. https://www.researchgate.net/publication/257590366_On_the_Variation_of_Ortho-hydrogen_and_Para-hydrogen_Ratio_with_Magnetic_Field_Strength_at_Low_Temperature
  25. https://pure.mpg.de/rest/items/item_3285370/component/file_3332341/content
  26. https://www.hydrogen.energy.gov/docs/hydrogenprogramlibraries/pdfs/19001_hydrogen_liquefaction_costs.pdf
  27. https://nvlpubs.nist.gov/nistpubs/jres/60/jresv60n3p221_A1b.pdf
  28. https://trc.nist.gov/cryogenics/Papers/Review/2020-Review_of_Refrigeration_Methods.pdf
  29. https://www.power-h2.com/insights/hydrogen-production
  30. https://www.eia.gov/energyexplained/hydrogen/production-of-hydrogen.php
  31. https://world-nuclear.org/information-library/energy-and-the-environment/hydrogen-production-and-uses
  32. https://advancedbiofuelsusa.info/us-air-liquide-opens-worlds-largest-liquid-hydrogen-production-plant-it-can-serve-40000-fuel-cell-vehicles
  33. https://statzon.com/editors-highlights/liquefied-hydrogen-storage-market-report-2024-2032
  34. https://engineering.airliquide.com/technologies/hydrogen-liquefaction
  35. https://www.blackridgeresearch.com/blog/list-of-top-green-hydrogen-producing-companies-in-the-world
  36. https://www.hydrogen.energy.gov/docs/hydrogenprogramlibraries/pdfs/24003-hydrogen-liquefaction-capacity-united-states.pdf?sfvrsn=b894666d_1
  37. https://www.iea.org/reports/global-hydrogen-review-2021/executive-summary
  38. https://css.umich.edu/publications/factsheets/energy/hydrogen-factsheet
  39. https://www.burckhardtcompression.com/about/news-stories-and-whitepapers/article/the-hydrogen-economy-what-it-costs-to-produce-hydrogen/
  40. https://h2tools.org/faq/hydrogen-purity-requirements
  41. https://www.preprints.org/frontend/manuscript/27105a45321a32ff6e52736d145412e1/download_pub/supplementary
  42. https://www.process-insights.com/wp-content/uploads/2024/01/Process-Insights_App-Note-22A-ASU-Process-4-2023.pdf
  43. https://usa.airliquide.com/sustainability/hydrogen/hydrogen-production
  44. https://www.energy.gov/sites/default/files/2022-07/liquid-hydrogen-tech-workshop-report-2022.pdf
  45. https://www.nature.com/articles/017169b0
  46. https://pmc.ncbi.nlm.nih.gov/articles/PMC3826198/
  47. https://physicstoday.aip.org/features/little-cup-of-helium-big-science
  48. https://farside.ph.utexas.edu/teaching/sm1/Thermalhtml/node67.html
  49. https://www.sciencedirect.com/topics/engineering/joule-thomson-coefficient
  50. https://www.europhysicsnews.org/articles/epn/pdf/2010/04/epn2010414p21.pdf
  51. https://royalsocietypublishing.org/doi/10.1098/rspl.1898.0031
  52. https://www.aps.org/publications/apsnews/201201/physicshistory.cfm
  53. https://ahf.nuclearmuseum.org/ahf/history/hydrogen-bomb-1950/
  54. https://www.tandfonline.com/doi/full/10.1080/15361055.2025.2503035
  55. https://ntrs.nasa.gov/api/citations/19790008823/downloads/19790008823.pdf
  56. https://www.nasa.gov/history/saturn-v/
  57. https://www.nasa.gov/history/centaur-americas-workhorse-in-space/
  58. https://www.nasa.gov/wp-content/uploads/2023/04/sp-4230.pdf
  59. https://taylorandfrancis.com/knowledge/Engineering_and_technology/Mechanical_engineering/MLI/
  60. https://www.sciencedirect.com/topics/engineering/liquid-rocket-engine
  61. http://lpre.de/resources/articles/AIAA-1995-2540.pdf
  62. http://www.braeunig.us/space/comb-OH.htm
  63. https://ntrs.nasa.gov/api/citations/19970024861/downloads/19970024861.pdf
  64. https://www.nasa.gov/wp-content/uploads/2024/12/3i.pdf
  65. https://airandspace.si.edu/collection-objects/engine-rocket-space-shuttle-main-ssme/nasm_A20040205000
  66. https://www.marssociety.ca/2021/03/25/rocket-fuel/
  67. https://www.ulalaunch.com/about/heritage-rockets/delta-iv
  68. http://www.astronautix.com/l/loxlh2.html
  69. https://spaceflightnow.com/2021/07/14/upper-stage-added-to-sls-stack-in-vehicle-assembly-building/
  70. https://www.airproducts.com/company/news-center/2025/08/0821-air-products-liquid-hydrogen-nasa-kennedy-space-center
  71. https://www.stokespace.com/introducing-andromeda/
  72. https://internationalfinance.com/aviation/start-up-week-stoke-space-eyes-strengthening-us-rocket-launch-capabilities/
  73. https://wha-international.com/hydrogen-in-industry/
  74. https://www.sandia.gov/app/uploads/sites/163/2022/03/ESHB_Ch11_Hydrogen_Headley.pdf
  75. https://www.ncnr.nist.gov/summerschool/ss09/pdf/SS2009_Williams.pdf
  76. https://pmc.ncbi.nlm.nih.gov/articles/PMC9017651/
  77. https://ehb.eu/files/downloads/2020_European-Hydrogen-Backbone_Report.pdf
  78. https://www.tno.nl/en/newsroom/insights/2025/08/liquid-hydrogen-testing-facility/
  79. https://h2tools.org/bestpractices/laboratory-safety/laboratory-design/cryogenic-liquid
  80. https://cameochemicals.noaa.gov/chemical/3606
  81. https://seshydrogen.com/en/safety-of-hydrogen-systems/
  82. https://www.sciencedirect.com/science/article/pii/S0360319922060244
  83. https://h2tools.org/bestpractices/material-compatibility/hydrogen-embrittlement
  84. https://ntrs.nasa.gov/citations/20160005654
  85. https://acp.copernicus.org/articles/22/9349/2022/
  86. https://ntrs.nasa.gov/api/citations/20170006481/downloads/20170006481.pdf
  87. https://h2tools.org/bestpractices/material-compatibility
  88. https://www.osti.gov/servlets/purl/1985749
  89. https://www.iso.org/standard/83810.html
  90. https://h2tools.org/hydrogen-leak-detection
  91. https://ntrs.nasa.gov/api/citations/20070018005/downloads/20070018005.pdf
  92. https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.103
  93. https://ntrs.nasa.gov/api/citations/19970033338/downloads/19970033338.pdf
Add your contribution
Related Hubs
User Avatar
No comments yet.