Cryogen storage tanks are typically cylindrical, double-walled containers with capacities of up to 10 kg of liquid hydrogen (LH₂) (i.e., 170 L of volume) for passenger cars ^[1][50]. The insulation system, for example, can be made of 70 layers of aluminum foils or aluminized polymers, separated by glass fibers or polymer spacers with a total thickness of approximately 30 mm ^[1][50]. It is designed to allow a boil-off loss lower than 1.5% per day ^[1][50]. The double-walled filling tube, the pressure relief valve, and the safety vent pass through the insulation system. Generally, a super-insulated tank operates at a pressure of around 4 bar, and the maximum allowable pressure before venting is 7 bar ^[1][50]. The internal pressure is controlled through an electric heating device ^[2][3][51,52]. Michel et al. suggested an improved pressure management system, in which part of the gas is heated and routed back inside the tank to transfer its heat to the liquid fuel depending on the pressure measured in the exit line. This solution avoids any electricity consumption and recovers part of the waste heat from the engine ^[4][53]. An example of an LH₂ tank for passenger cars was fabricated in 2006 by the Austrian company Magna Steyr for the BMW Hydrogen 7. It is constituted of a double-walled steel vessel with an internal wall 2 mm thick, a 30 mm high-vacuum super-insulation system, and another 2 mm external wall. The pressure in the vacuum jacket is 0.1 bar at 20 K ^[5][54]. A normal dormancy period for these cryogenic tanks is slightly lower than 12 days.

On the other hand, large amounts of LH₂ are stored in double-walled spherical tanks to maximize the volume-to-surface ratio. One example is the largest LH₂ tank in the world installed at the NASA Kennedy Space Center. This tank has a volume of 3800 m³ and it has been in operation since the 1960s ^[6][46]. Instead of multi-layer insulation (MLI), the vacuum jacket is filled with perlite powder. Recently, NASA commissioned the construction of a larger stationary tank with a volume of 1.25 million gallons (approx. 5683 m³) and with insulation composed of glass bubbles which have been demonstrated to perform better than perlite, to support future missions to the Moon and Mars ^[7][55]. Moreover, an innovative heat exchanger system called integrated refrigeration and storage (IRAS) will be installed in the tank to minimize the boil-off and the scheduled releases through the pressure relief valve (PRV) ^[8][9][56,57].

Copious amounts of liquid hydrogen can be transferred using a cryogenic pump. This component needs to reach a pressure slightly higher than 5 bar (maximum operating pressure of the receiving tank), but the desired flow rate ranges between 300 and 1200 kg/min. An alternative method of transferring the LH₂ is to use a pressure-build loop, in which a certain amount of liquid hydrogen is evaporated and then returns as gaseous hydrogen to the top of the tank, pressurizing it enough to drive the flow. This method does not require power, does not involve moving parts, but increases the pressure and heat content of the storage tank, thus increasing overall boil-off losses ^[10][58].

The refueling stations should also have a flow rate meter to keep track of fueling, pressure sensors, relief devices for safety reasons, a dispenser hose, and a connector to make the connection to the tender car. Vessels for cryogenic fuels are not designed to withstand high pressures and adding liquid to the tank would rapidly over-pressurize the remaining gas phase. Therefore, the gaseous hydrogen must be removed from the empty tank as it is filled with liquid. This can be done by simply venting the gas into the atmosphere. The total component cost for each refueling station is driven by the cost of the super-insulated LH₂ tank, depending on the size of the storage system.

Due to the extremely low temperature of liquid hydrogen (20 K), special materials are required for storage and transportation containers. The main requirements are the adaptability of materials in a liquid hydrogen environment, resistance to hydrogen embrittlement, mechanical properties, and thermophysical properties at cryogenic temperatures ^[11][8]. Considering the wide range of applications of LH₂ technologies, the requirements for container materials are not the same for maritime, aerospace, automotive sectors, or stationary applications.

Stainless steel is the cryogenic material most widely used for liquid hydrogen vessels in applications where the weight of the containment system is not a constraining factor. Austenitic stainless steels are usually the first choice for liquid hydrogen transportation vessels due to their reliable performance at cryogenic temperatures ^[12][59]. The crystal structure of austenitic steels is face-centered cubic, thus implying a superior plastic deformation ability. With the decrease in temperature, the strength of the material tends to be improved while maintaining acceptable plasticity and impact resistance ^[13][60]. The macro properties of each grade of stainless steel are determined by the composition elements of the alloy, thus determining their suitability for different applications. In general, greater stability of the alloy at low temperatures can be obtained by adding a higher content of Ni and Cr ^[14][61].

The Cr-Ni austenitic stainless steels (300 series) are widely used for the storage of cryo-liquefied gases due to their superior overall performance. The difference in the alloy elements of stainless steel directly affects the final application of materials. For example, 316L stainless steel is suitable for the marine environment, since the greater content of Mo improves the resistance of steel to chloride ion corrosion; 321 stainless steel is used in environments where high corrosion resistance and heat resistance are required since the added Ti element improves the resistance to intergranular corrosion and the strength at elevated temperature ^[13][60]. The metallic materials suitable for cryogenic applications and hydrogen service are summarized in Table 1. The material costs have been categorized based on the market values found in ^[15][62].

The materials exposed to hydrogen manifest a detrimental effect on the tensile properties, fracture mechanical properties, and fatigue performance. This phenomenon is widely known as hydrogen embrittlement and results in material crack initiation and subsequent fracture due to the absorption and permeation of hydrogen atoms through the metal lattice ^[16][17][18][63,64,65]. For austenitic stainless steels, the elevated stability at low temperatures ensures good resistance to hydrogen embrittlement. However, this material damage is more likely to occur in metastable 304 stainless steel than in 310 and 316 thanks to their superior phase stability. The different microstructures and surface treatments have a significant influence on the material’s susceptibility to hydrogen embrittlement ^[19][66]. Fan et al. studied the effect of grain refinement on hydrogen embrittlement of 304 stainless steel, demonstrating that a smaller grain size can significantly reduce the stress concentration and the susceptibility to hydrogen embrittlement ^[20][67]. Hence, tight control of all the aspects of material forming, the maximization of the stability of the alloy, and the reduction of concentrated stresses are particularly important to increase the hydrogen embrittlement resistance of austenitic stainless steels.

Metals exposed to cryogenic temperatures generally manifest an increase in elastic modulus, tensile strength, and yield strength, along with an increase in fatigue performance and endurance limit ^[21][68]. The ductile–brittle transition characteristic of materials influences their plasticity at low temperatures ^[11][8]. Most metallic materials with body-centered cubic structures show a sharp decrease in plasticity below the ductile–brittle transition temperature; they cannot be used under cryogenic conditions since the significant decrease in the plasticity can induce brittle crack initiation and fractures ^[22][69]. On the other hand, for materials without brittle transition, the elongation tends to increase with decreasing temperature. To assess the suitability of materials to operate in a low-temperature environment, low-temperature impact toughness tests, drop weight tests, full thickness tests, and fracture mechanics tests are commonly performed. It is often necessary to carry out low-temperature impact toughness testing on the base metal, welds, and heat-affected zones (HAZs) for liquid hydrogen storage tank materials. The performance of weldments proves to be often worse than that of the base metal.

The plasticity and toughness of austenitic stainless steels do not show a significant reduction with the decrease in temperature, thanks to the face-centered cubic crystal structure.

In the past, LH₂ has mainly been used in the aerospace industry, especially in the last 60 years due to space missions ^[1][50]. Despite LH₂ utilization being proposed already in 1903 by Tsiolkovsky ^[23][70], it has only been investigated for aerospace applications since 1945 ^[1][50]. LH₂ has been mostly used together with liquid oxygen (LOX) in rocket engines. Even though LH₂/LOX systems have an exceptionally high specific impulse performance, their utilization was always implemented in rocket upper stages due to their low density which results in bulky tanks and additional weight and drag (aerodynamic resistance) for the rocket. Therefore, LH₂/LOX propellants were used for the first time in the upper stages of the Centaur and Saturn space programs in 1958–1959. Centaur was an unmanned space missions program while Saturn was a manned moon voyage one ^[23][70]. Additional details on the development of LH₂/LOX rocket engines can be found in ^[1][50]. After several technical difficulties and a few failures, the first successful launch of a NASA Atlas-Centaur rocket occurred in 1963. Between 1967 and 1972, during the Apollo moon flights, LH₂/LOX engines were installed in the second and third stages of the Saturn-V rockets ^[1][50]. In parallel, the European space agency began to test LH₂/LOX engines for the Ariane rocket program in 1964. The LH₂/LOX Vulain-2 engine was then installed on the Ariane-V rocket. Also in Japan, LH₂ has been used for rockets since 1986 ^[24][71]. LH₂ and LOX were used again in the US as propellant fuels on board the space shuttle. Finally, it has been demonstrated that LH₂ can be employed as a fuel for spaceplanes since it is a light fuel and its extremely low temperature can be exploited to cool the aerodynamic surface heated by air friction ^[1][50]. It must be added that NASA has employed hydrogen to power fuel cells (polymer electrolyte and alkaline types) to generate all the electricity and drinkable water on board spacecraft since 1965 ^[25][72].

Despite LH₂ having been considered as airplane fuel already in 1938 by Sikorski, the first successful flight test was carried out in 1957 by a B-57 aircraft with one of the two engines powered by hydrogen stored on board in an LH₂ tank with a volume of 1.7 m³ and pressurized by helium ^{[1][23][26][27][28][29][30]}[50,70,73,74,75,76,77]. LH₂ was used as a fuel to power one of the three engines of the Tu-155 aircraft developed by the Russian Tupolev company for the first time in 1988 ^[1][26][29][50,73,76]. The same year, a small four-seat Grumman-American “Cheetah” was the first and only airplane that flew solely powered by LH₂ stored in a 40-gal tank (approx. 182 l) ^[1][29][30][50,76,77]. Even though the flight lasted barely 36 s at an altitude of 100 ft (30.5 m), it was recognized by the US National Aeronautic Association as a world record ^[30][77]. Other concepts of LH₂ fueled airplanes were proposed as part of different projects, such as the Suntan project started in 1956 where a supersonic high-altitude reconnaissance aircraft was designed ^[1][50], or the Cryoplane project where passenger planes were being considered to be converted to LH₂ propulsion ^[1][31][50,78]. Thorough descriptions of the abovementioned LH₂ airplanes can be found in ^{[1][23][29][30][32]}[50,70,76,77,79]. In 2012, Boeing developed and tested an LH₂-powered high altitude long endurance (HALE) unmanned aerial vehicle (UAV) called Phantom Eye ^[33][34][80,81]. This UAV is designed to carry 450 lb (240 kg) of payload, fly at an altitude of 65,000 ft (approx. 19.8 km) at a cruise speed of 150 kt (approx. 292 m/s), with an endurance of 4 days ^[34][35][81,82]. The Ion Tiger is another UAV, developed by the Naval Research Laboratory (NRL), which has been the only UAV powered by LH₂ ^[36][83]. More specifically, the Ion Tiger was a fixed-wing UAV with a 20.46-l LH₂ tank installed on board and it established the endurance world record for small electric UAVs by continuously flying for 48 h and 1 min ^[37][84]. Recently, the study carried out jointly by the Clean Sky 2 Joint Undertaking and Fuel Cell and Hydrogen 2 Joint Undertaking ^[38][85] concluded that interest in LH₂ powered airplanes has increased. Other proof of this are the three concepts for the world’s first zero-emission commercial aircraft powered by hydrogen revealed by Airbus in 2020 ^[39][4].

The first example of an LH₂ fueled road vehicle dates back to 1971 when the Perris Smogless Association in the USA converted a Ford F250 pickup into the world’s first LH₂/LOX fueled car. Two LH₂ tanks (for a total capacity of 300 L) and a LOX tank were installed on the truck. The car was capable of running 160 km and 25 h ^[6][46]. Research on LH₂-fueled vehicles and refueling technologies was conducted between 1979 and 1981 in cooperation between the German Aerospace Center (DLR) and the Los Alamos National Laboratories (LANL). DLR provided the onboard LH₂ tank and the refueling technology. A Buick Century four-door sedan was adapted by LANL and proved its ability to run for 133 h, 3540 km, and to be refueled at least 60 times ^[40][86]. In 1967, General Motors and Opel started their research on hydrogen vehicles with the construction of an electro van equipped with cryo-tanks for both LH₂ and LOX ^[41][87].

General Motors presented in 2000 a hydrogen-fueled car (HydroGen1) based on the Opel Zafira. The third generation HydroGen3 was the first car to get permission to operate on public roads. It was equipped with either a GH₂ or an LH₂ tank system to power a 60-kW electric engine. The super-insulated tank can store 4.6 kg or 68 L of LH₂, sufficient to run 400 km. The entire system weight including mounting brackets reaches up to 90 kg. The maximum allowable speed was approximately 160 km/h ^[42][88].

In 1978, BMW started its research on hydrogen vehicles with a prototype internal combustion engine. One year later, the DLR presented a hybrid BMW 518 powered by hydrogen and gasoline. The car development was focused on the improved design of the LH₂ storage system ^[43][89]. In 1988, BMW launched a prototype of the BMW 735i converted to hydrogen and equipped with a 120-L tank for LH₂, capable of running 200 km. In 2000, a fleet of 15 BMW 750hL adapted with a cryo-tank to store 8 kg of LH₂ on board was put into operation ^[44][90]. The latest generation of hydrogen-powered vehicles developed by BMW is the model Hydrogen 7 (adapted from the BMW 760iL). It is equipped with an 8 kg LH₂ tank for a cruising range of about 200 km and average H₂ fuel consumption of 3.6 kg per 100 km. The Hydrogen 7 goes from 0 to 100 km/h in 9.5 s (which is the average acceleration for a car of that power class), reaches a maximum speed of 230 km/h, and has a refueling time lower than 8 min. For a half-filled tank the holding time, i.e., the time for the complete emptying of the tank due to boil-off, is 9 days ^[45][91]. The BMW H₂R, a racecar adapted to run on LH₂, was developed in 2004 and equipped with an LH₂-powered 210 kW engine. The acceleration from 0 to 100 km/h can be achieved within 6 s, and the maximum speed exceeded 300 km/h. A special 11 kg capacity LH₂ tank with a design pressure of 3 bar, provided with a boil-off valve and two safety valves, was constructed for this racing car ^[4][53].

The Mercedes-Benz Group started its activities with hydrogen-driven road vehicles (including passenger cars, light-duty cars, and buses) in the mid-1980s with H₂ and gasoline-fueled internal combustion engine cars ^[1][50]. In 1999, in the NECAR-4 prototype, LH₂ storage was used to supply a 70-kW fuel cell powertrain. The cruising range was 450 km at a maximum speed of 145 km/h.

The Musashi Institute of Technology in Japan developed its first hydrogen-driven vehicle in 1971 and, except for the first car, all the following light-duty vehicles had an LH₂ storage tank installed on board. A significant example is the Musashi-9, a refrigerator truck where the LH₂ was not only used to feed the internal combustion engine but also to keep the transported goods at low temperatures, thus efficiently recovering the cold energy. The latest model is the Musashi-10 and dates back to 1997.

Hydrogen has been used for the last two decades in the railway sector. The rail vehicles which use onboard hydrogen fuel as a source of energy for the traction motors of the auxiliaries are known as hydrail ^[46][92]. These vehicles can be powered either by burning hydrogen directly in an internal combustion engine or oxidizing hydrogen in a fuel cell system. In 2002, the first hydrogen-powered locomotive powered by Nuvera Fuel Cells was demonstrated in Val-d’Or (Quebec, Canada). It was a mining locomotive of 3.6 tons with a 17 kW fuel cell powertrain ^[47][93]. After that, a variety of pilot projects have been initiated all over the world. In April 2006, the world’s first hydrail railcar was developed by the East Japan Railway Company ^[48][94]. In November 2010, Southwest Jiaotong University in China demonstrated its first hydrail prototype. In 2016, Alstom revealed their newly developed iLint trains that were then deployed in Germany in 2017 ^[49][95]. Hydrogen is stored on board this fuel cell-powered train as compressed gas and it has a total autonomy of 1000 km with a maximum speed of 140 km/h ^[49][95].

Currently, most hydrogen trains commercialized in Europe, Asia, and America are fueled with compressed gaseous hydrogen. Nevertheless, the idea of developing liquid hydrogen-based locomotives has gradually gained ground in recent years. The main advantage of LH₂ is the possibility of storing it at atmospheric pressure while having a volumetric energy density almost twice that of compressed gaseous hydrogen (CGH₂) at 700 bar. The high energy storage density is advantageous given the nature of trains that may run a long distance, and the high-pressure hazard of CGH₂ can be removed. In addition, LH₂ has a high transfer efficiency and a quick charging speed, which allows for minimizing the number of refueling stations since a single stationary tank is able to fuel a large number of trains. Such advantages make LH₂ an attractive option in the railway sector ^[50][96].

By 2021, the Korea Railroad Research Institute and Hyundai Rotem were implementing the core technology of the world’s first LH₂-based locomotive. The aim of this project is to develop an LH₂-fueled train capable of running a distance of 1000 km at a peak speed of 150 km/h with a single load. This technology would have a drive distance 60% higher than a train fueled with CGH₂ compressed at 700 bar and a fuel charge time 20% lower. The 2.7 MW fuel cell propulsion system is constituted of several modules of 390 kW and is designed to supplement a conventional diesel engine. The first stage of the project will be the development of a hybrid LH₂-diesel propulsion system, a high-insulated storage system for cryogenic fuel, and high-speed charging technology. This prototype will be tested on trams in the second half of 2022. After this phase, a large-capacity train and LH₂ supply technology will be developed for commercialization ^[51][97].

Despite a volumetric energy density higher than that of CGH₂, LH₂ requires greater storage volumes than any liquid fossil fuel. Some rail applications are likely to store cryogenic hydrogen on board the locomotive, but to have more fuel storage capacity a fuel tender (i.e., a vehicle hauled to the locomotive containing the fuel) must be used. The main advantages of using a tender are the possibility for the locomotive to run a much greater distance and the possibility to refuel the tender separately from the engine, meaning that a train could exchange an empty tender for a full one without waiting to refuel. It is also noteworthy that the components for LH₂-dispensing are not common, particularly for large freight rail designs. Large-sized cryogenic tanks, high-capacity cryo-pumps, hose connectors, and super-insulated piping are not commercially available, meaning that custom components would be needed for initial demonstration projects. Hence, the costs and layouts of these refueling facilities should be viewed with great uncertainty ^[10][58].

There are still many technical challenges in developing solutions based on LH₂ in the railway sector. Recently, Madovi et al. ^[52][98] carried out a feasibility study for hydrogen fuel cell technology using the Piedmont intercity service (North Carolina, USA) as a case study. Six train configurations and powertrain options as well as nine energy supply options were considered and compared with the traditional diesel supply. The results demonstrated that a hydrail is feasible and a low-carbon hydrogen supply is possible. Despite these promising findings, the least favorable pathways have been shown to be liquid delivery using electrolysis with electricity provided by the grid and by renewable sources; liquid delivery using steam-methane reforming and biomass for hydrogen production have no substantial advantages over a conventional diesel system ^[52][98].

Previously, only vehicles powered by LH₂ were reported and described. However, hydrogen must be delivered from the production site to the end users, and this can be done through pipelines, trucks, trains, ships, or barges ^[6][46]. Therefore, these types of vehicles can be employed for hydrogen transport and can be either fueled by hydrogen or other fuels. Hydrogen delivery is outside the scope of this paper, and it has not been described further. Additional information on hydrogen delivery can be found in ^[11][8].