Hydrogen can be stored using several methods. These include using high pressure and very low temperatures, or using special chemical compounds that release hydrogen gas when needed. Although many industries produce large amounts of hydrogen, most of it is used right where it is made, especially for creating ammonia. For many years, hydrogen has been stored as compressed gas or as a liquid kept at extremely cold temperatures. It is then moved in containers like cylinders, tubes, and tanks for use in industry or as fuel for space missions. A major challenge is hydrogen's very low boiling point, which is about 20.268 K (−252.882 °C or −423.188 °F). Reaching such cold temperatures requires using a lot of energy.

Hydrogen gas has a high energy density by weight because it is very light. However, at normal temperatures and pressures, it has very low energy density by volume. To use hydrogen as fuel in vehicles, it must be stored in a form that holds a lot of energy in a small space. Because hydrogen is the smallest molecule, it can easily escape from storage containers. Its estimated global warming potential over 100 years is about 11.6 ± 2.8.

Established technologies

Compressed hydrogen is a way to store hydrogen gas by keeping it under high pressure, which allows more hydrogen to be stored in a smaller space. Hydrogen tanks in vehicles use pressures of 350 bar (5,000 psi) and 700 bar (10,000 psi). These tanks are made using type IV carbon-composite technology. Car companies like Honda and Nissan are working on this method.

Liquid hydrogen is stored in special tanks, such as those used in the BMW Hydrogen 7. Japan has a liquid hydrogen (LH2) storage facility in Kobe Port. To make hydrogen liquid, its temperature is lowered to −253 °C, similar to how liquefied natural gas (LNG) is stored at −162 °C. This process results in an energy loss of 12.79%, or 4.26 kW⋅h/kg out of a total of 33.3 kW⋅h/kg.

Chemical storage

Chemical storage can provide high storage performance because it allows for high storage densities. For example, supercritical hydrogen at 30 °C and 500 bar has a density of 15.0 mol/L, while methanol contains 49.5 mol H₂ per liter of methanol. Saturated dimethyl ether at 30 °C and 7 bar has a density of 42.1 mol H₂ per liter of dimethyl ether.

Regenerating storage materials is a challenge. Many chemical storage systems have been studied. Hydrogen release can happen through hydrolysis reactions or catalyzed dehydrogenation reactions. Examples of storage compounds include hydrocarbons, boron hydrides, ammonia, and alane. A promising method is electrochemical hydrogen storage, where hydrogen release is controlled by applying electricity. Most materials listed can be used directly for electrochemical hydrogen storage.

Nanomaterials, especially those made using ball milling or severe plastic deformation, offer an alternative that addresses two main issues in bulk materials: the speed of hydrogen absorption and activation. High-entropy alloys, such as TiZrCrMnFeNi, show advantages in fast and reversible hydrogen storage at room temperature with good storage capacity for stationary uses.

Improving hydrogen absorption speed and storage capacity can be achieved by using nanomaterial-based catalysts. A study by the Clean Energy Research Center at the University of South Florida examined LiBH₄ doped with nickel nanoparticles. They found that adding more nanocatalyst lowered the release temperature by about 20 °C and increased weight loss by 2-3%. The best result occurred with 3 mol% nickel particles, where the release temperature was around 100 °C and weight loss was significantly higher than in undoped samples.

Hydrogen absorption improves at the nanoscale because of shorter diffusion distances compared to bulk materials. Nanomaterials also have a favorable surface-area-to-volume ratio.

The release temperature of a material is the temperature at which hydrogen begins to leave the material. The energy or temperature needed to release hydrogen affects the cost of chemical storage. If hydrogen is too weakly bound, high pressure is required for regeneration, which cancels energy savings. For onboard hydrogen fuel systems, the target is a release temperature below 100 °C and a recharge pressure below 700 bar (20–60 kJ/mol H₂). A modified van 't Hoff equation relates temperature and hydrogen partial pressure during desorption. The changes to the standard equation account for size effects at the nanoscale.

ln(pH₂) = ΔH(r)/(R*T) + (3Vmγ)/(rRT) + ΔS(r)/R

Where pH₂ is the hydrogen partial pressure, ΔH is the enthalpy of the sorption process (exothermic), ΔS is the entropy change, R is the ideal gas constant, T is the temperature in Kelvin, Vm is the molar volume of the metal, r is the nanoparticle radius, and γ is the surface free energy of the particle.

From this equation, we see that the enthalpy and entropy changes during desorption depend on the nanoparticle radius. A new term accounts for the particle's surface area, and it can be shown mathematically that reducing the particle radius lowers the release temperature for a given hydrogen pressure.

Hydrogen storage through CO₂ hydrogenation to methanol has been studied. Challenges include purifying captured CO₂, obtaining hydrogen from water splitting, and energy requirements for hydrogenation. For industrial use, CO₂ is often converted to methanol. Progress has been made in converting CO₂ to C1 molecules, but creating high-value molecules remains difficult. Future success depends on advances in catalytic technologies.

Metal hydrides, such as MgH₂, NaAlH₄, LiAlH₄, LiH, LaNi₅H₆, TiFeH₂, ammonia borane, and palladium hydride, are sources of stored hydrogen. Metal hydrides fall into three main categories:

• Intermetallic Hydrides: These have fast absorption and moderate storage capacity. Examples include LaNi₅H₆ and TiFeH₂.
• Complex Hydrides: These can store more hydrogen but require catalysts. Examples include NaAlH₄ and LiBH₄.
• Lightweight Hydrides: These offer high gravimetric storage but need high temperatures for desorption. Examples include MgH₂ and CaH₂.

Persistent issues include the percentage of hydrogen stored and the reversibility of the storage process. Some materials are liquids at room temperature and pressure, while others are solids that can be made into pellets. These materials have good energy density, though their specific energy is often lower than that of leading hydrocarbon fuels.

Lowering desorption temperatures can be achieved by adding activators. This method has been tested with aluminum hydride, but the complex synthesis process makes it less attractive.

Proposed hydrides for hydrogen storage include simple hydrides of magnesium or transition metals and complex hydrides containing sodium, lithium, or calcium with aluminum or boron. Hydrides chosen for storage must be safe and have high hydrogen storage densities. Leading candidates include lithium hydride, sodium borohydride, lithium aluminum hydride, and ammonia borane. A French company, McPhy Energy, is developing an industrial product based on magnesium hydride, which is already sold to clients like Iwatani and ENEL.

Reversible hydrogen storage is demonstrated by frustrated Lewis pairs. A phosphino-borane compound can absorb and release hydrogen at 1 atm and 25 °C, with a storage capacity of 0.25 wt%.

Hydrogen can be produced by reacting aluminum with water. Previously, it was thought that aluminum needed to be stripped of its oxide layer or mixed with gallium to react efficiently. However, recent studies show that increasing reaction temperature and pressure allows efficient hydrogen production. The byproduct, aluminum oxide, can be recycled into aluminum using the Hall–Héroult process, making the reaction theoretically renewable. Although electrolysis is energy-intensive, the energy is stored in the aluminum and released when it

Physical storage

Hydrogen remains in physical forms, such as gas, supercritical fluid, adsorbate, or molecular inclusions. Theoretical limits and experimental results are examined regarding the volumetric and gravimetric capacity of glass microvessels, microporous, and nanoporous materials, as well as safety and refilling-time requirements. Because hydrogen is the smallest molecule, it easily escapes from containers and during transfer between containers. While it does not directly contribute to radiative forcing, hydrogen is estimated to have an effective 100-year global warming potential of 11.6 ± 2.8 due to its impact on processes such as atmospheric methane oxidation and tropospheric ozone production.

Zeolites are microporous and highly crystalline aluminosilicate materials. Their cage and tunnel structures allow the encapsulation of non-polar gases like H₂. In this system, hydrogen is physisorbed on the surface of zeolite pores through a mechanism involving hydrogen being forced into the pores under pressure and low temperature. Similar to other porous materials, hydrogen storage capacity depends on the BET surface area, pore volume, interaction of molecular hydrogen with the internal surfaces of micropores, and working conditions such as pressure and temperature.

Channel diameter is one of the parameters determining this capacity, especially at high pressure. An effective material should have a large pore volume and a channel diameter close to the kinetic diameter of the hydrogen molecule (d_H = 2.89 Å).

The table below shows the hydrogen uptake of several zeolites at liquid nitrogen temperature (77K):

Activated carbons are highly porous amorphous carbon materials with high apparent surface area. Hydrogen physisorption can be increased by raising the apparent surface area and optimizing pore diameter to around 7 Å. These materials are of interest because they can be made from waste materials, such as cigarette butts, which have shown potential as precursor materials for high-capacity hydrogen storage.

Graphene can store hydrogen efficiently. The H₂ adds to the double bonds, forming graphane. The hydrogen is released upon heating to 450 °C.

Hydrogen carriers based on nanostructured carbon (such as carbon buckyballs and nanotubes) have been proposed. However, hydrogen content amounts up to ≈3.0-7.0 wt% at 77K, which is far from the value set by the U.S. Department of Energy (6 wt% at nearly ambient conditions).

To realize carbon materials as effective hydrogen storage technologies, carbon nanotubes (CNTs) have been doped with MgH₂. The metal hydride has a theoretical storage capacity (7.6 wt%) that meets the U.S. Department of Energy requirement of 6 wt%, but has limited practical applications due to its high release temperature. The proposed mechanism involves the creation of fast diffusion channels by CNTs within the MgH₂ lattice. Fullerene substances are other carbonaceous nanomaterials tested for hydrogen storage. Fullerene molecules have a C₆₀ close-caged structure that allows hydrogenation of the double-bonded carbons, leading to a theoretical C₆₀H₆₀ isomer with a hydrogen content of 7.7 wt%. However, the release temperature in these systems is high (600 °C).

Metal–organic frameworks (MOFs) are another class of synthetic porous materials that store hydrogen and energy at the molecular level. MOFs are highly crystalline inorganic-organic hybrid structures containing metal clusters or ions (secondary building units) as nodes and organic ligands as linkers. When guest molecules (solvent) occupying the pores are removed during solvent exchange and heating under vacuum, the porous structure of MOFs can be achieved without destabilizing the frame, and hydrogen molecules will be adsorbed onto the surface of the pores by physisorption. Compared to traditional zeolites and porous carbon materials, MOFs have a very high number of pores and surface area, allowing higher hydrogen uptake in a given volume.

Temperature, pressure, and composition of MOFs can influence their hydrogen storage ability. The adsorption capacity of MOFs is lower at higher temperatures and higher at lower temperatures. As temperature rises, physisorption decreases and chemisorption increases. For MOF-519 and MOF-520, the isosteric heat of adsorption decreases with pressure increase. For MOF-5, both gravimetric and volumetric hydrogen uptake increase with pressure. The total capacity may not match the usable capacity under pressure swing conditions. For example, MOF-5 and IRMOF-20, which have the highest total volumetric capacity, show the least usable volumetric capacity. Absorption capacity can be increased by modifying the structure. For example, the hydrogen uptake of PCN-68 is higher than PCN-61. Porous aromatic frameworks (PAF-1), a high surface area material, can achieve a higher surface area by doping.

There are many ways to modify MOFs, such as MOF catalysts, MOF hybrids, MOFs with metal centers, and doping. MOF catalysts have high surface area, porosity, and hydrogen storage capacity. However, the active metal centers are low. MOF hybrids have enhanced surface area, porosity, loading capacity, and hydrogen storage capacity. Nevertheless, they are not stable and lack active centers. Doping in MOFs can increase hydrogen storage capacity, but there might be steric effects and inert metals with inadequate stability. There might be formation of interconnected pores and low corrosion resistance in MOFs with metal centers, while they might have good binding energy and enhanced stability. These advantages and disadvantages for different modified MOFs show that MOF hybrids are more promising because of the good controllability in selecting materials for high surface area, porosity, and stability.

In 2006, chemists achieved hydrogen storage concentrations of up to 7.5 wt% in MOF-74 at a low temperature of 77 K. MOF NOTT-112 exhibits 10 wt% at 77 bar (1,117 psi) and 77 K. Most articles about hydrogen storage in MOFs report hydrogen uptake capacity at a temperature of 77K and a pressure of 1 bar because these conditions are commonly available and the binding energy between hydrogen and the MOF at this temperature is large compared to thermal vibration energy. Varying factors such as surface area, pore size, catenation, ligand structure, and sample purity can result in different amounts of hydrogen uptake in MOFs.

NU-1501-Al, an ultraporous metal–organic framework (MOF), has a hydrogen delivery capacity of 1

Stationary hydrogen storage

Storing hydrogen in places that don't move, like buildings, isn't a big issue because hydrogen's density isn't a major problem there. For moving applications, like vehicles, stationary storage can use proven methods:
– Compressed hydrogen (CGH₂) in a tank
– Liquid hydrogen (LH₂) in a cryogenic tank
– Slush hydrogen in a cryogenic tank

Underground hydrogen storage involves keeping hydrogen in caverns, salt domes, or old oil and gas fields. For many years, companies like ICI have safely stored large amounts of gaseous hydrogen in caverns without problems. Storing liquid hydrogen underground can help store energy for the power grid. This method has a round-trip efficiency of about 40% (compared to 75–80% for pumped-hydro storage), and the cost is slightly higher than pumped hydro if only a small amount of storage is needed. A European study found that for large-scale storage, hydrogen stored in salt caverns using an electrolyser and combined-cycle power plant is the cheapest option at €140/MWh for 2,000 hours of storage. A project called Hyunder in 2013 said that storing wind and solar energy would need 85 more caverns because systems like pumped hydro and compressed air energy storage cannot handle all the storage needs. A German study found that if Germany stored its extra power (7% by 2025 and 20% by 2050) as hydrogen underground, it would need about 15 caverns of 500,000 cubic meters each by 2025 and 60 caverns by 2050—about one-third of the gas caverns currently used in Germany. In the U.S., Sandia Labs is researching storing hydrogen in old oil and gas wells, which could hold large amounts of renewable hydrogen since there are over 2.7 million such wells.

Underground hydrogen storage includes caverns, salt domes, and old oil/gas fields. Large amounts of hydrogen have been stored in caverns for many years. Storing hydrogen underground in salt domes, aquifers, or mines can help power grids store energy, which is important for a hydrogen-based energy system. Using a turboexpander, the energy needed to compress hydrogen to 200 bar is about 2.1% of the hydrogen's total energy.

The Chevron Phillips Clemens Terminal in Texas has stored hydrogen in a salt cavern since the 1980s. The cavern is 2,800 feet underground, shaped like a cylinder with a 160-foot diameter, 1,000-foot height, and a usable hydrogen capacity of 1,066 million cubic feet (30.2 × 10⁶ m³), or 2,520 metric tons.

Salt caverns are made by injecting water into rock salt from the surface. Rock salt is a material made of NaCl (halite). Salt domes or bedded salt layers are often chosen for caverns. Salt caverns can be up to 2,000 meters deep and hold up to 1,000,000 cubic meters. They can be used 10 to 12 times a year, and hydrogen leaks are about 1%.

Salt caverns have advantages because rock salt has low water content, low porosity, and does not react with hydrogen. Permeability, or how easily hydrogen can move through the rock, is important for sealing hydrogen underground. Some studies show that cracks can increase permeability, but salt crystals can heal over time, helping the cavern stay tight. Salt's flexibility prevents cracks from forming, which is important for storage. However, salt caverns have lower storage capacity, need a lot of water, and may corrode. A cushion of gas is needed to prevent pressure loss when hydrogen is removed, though this is only about 20% of the total storage. Construction and operation costs are still high.

Storing hydrogen is more complex than storing natural gas. Hydrogen can move through solids, which limits salt cavern storage. Microbial activity, like bacteria that produce methane, is being studied because it can cause hydrogen loss. Methanogenic bacteria use hydrogen and carbon dioxide to make methane, reducing stored hydrogen.

• In 2011, Sandia National Laboratories created a framework to study the cost of storing hydrogen underground.
• In 2013, the Hyunder project said 85 more caverns would be needed for wind and solar energy storage.
• In 2015, ETI reported that the UK has enough salt resources to store tens of gigawatts of energy.
• In 2017, RAG Austria AG completed a hydrogen storage project in an old oil/gas field and is working on a second project.

A cavern 800 meters tall and 50 meters wide can hold hydrogen equivalent to 150 gigawatt-hours.

Power to gas is a method that converts electricity into gas fuel. Two methods are used:
1. Split water into hydrogen using electricity and inject the hydrogen into the natural gas grid.
2. Combine hydrogen and carbon dioxide to make methane using electrolysis and the Sabatier reaction.
A third method uses hydrogen and carbon (from sources like biogas or captured carbon dioxide) to make methane in an anaerobic environment with microbes. This method is efficient because the microbes can reproduce and only need low heat (60°C) to work.

SoCalGas has developed a simpler method to convert carbon dioxide in biogas to methane in one step, turning extra renewable energy into storable natural gas.

The UK has surveyed its gas grid and plans to inject hydrogen, as the grid once carried "town gas" (a 50% hydrogen-methane mix). KPMG found that switching to hydrogen gas could save £150–200 billion compared to rewiring homes for electric heating.

Extra energy from wind or solar power can balance the grid. In Canada, Hydrogenics and Enbridge are using the natural gas system to develop a power-to-gas system.

Hydrogen can be stored in natural gas pipelines. Before switching to natural gas, Germany used "town gas," which was mostly hydrogen (60–65%). The German gas network can store over 200,000 gigawatt-hours of energy, enough for several months.

Automotive onboard hydrogen storage

Portability is a major challenge in the automotive industry because storing hydrogen in high-density systems is difficult due to safety concerns. High-pressure hydrogen tanks are much heavier than the hydrogen they contain. For example, in the 2014 Toyota Mirai, a full tank holds only 5.7% hydrogen, with the rest of the weight coming from the tank itself.

System storage densities are often about half of the material’s storage capacity. This means that even if a material can store 6% hydrogen by weight, the actual system using that material may only store 3% when the weight of tanks, temperature control equipment, and other components are included.

Hydrogen is a clean fuel option because it burns without polluting. Using hydrogen could reduce greenhouse gases like carbon dioxide (CO₂), sulfur dioxide (SO₂), and nitrogen oxides (NOₓ). However, hydrogen fuel cells face challenges such as efficiency, size, and safe storage of the gas. Other issues include cost, operability, and durability, which need improvement. To solve these problems, scientists are exploring the use of nanomaterials. These materials could increase storage density and help vehicles reach a target driving range of 300 miles. Research focuses on carbon-based materials like carbon nanotubes and metal hydrides because they are versatile, strong, and cost-effective.

Using nanomaterials in hydrogen storage systems could be a major advancement in the automotive industry. Nanomaterials can also improve other parts of fuel cells. For example, adding TiO₂/SnO₂ nanoparticles to Nafion membranes in fuel cells enhances their performance. This happens because the nanoparticles improve how quickly hydrogen splits and how fast protons move through the cell. These improvements make fuel cells with nanoparticle membranes a promising alternative.

Another use of nanomaterials is in water splitting. Researchers at Manchester Metropolitan University in the UK developed screen-printed electrodes using a material similar to graphene. Similar systems have been created using photoelectrochemical techniques.

Increasing hydrogen gas pressure improves its energy density by volume, allowing for smaller storage tanks. Steel is the standard material for storing pressurized hydrogen in tube trailers because it does not weaken when exposed to hydrogen gas. Tanks made of carbon and glass fiber-reinforced plastic, like those in the Toyota Mirai and Kenworth trucks, meet safety standards. However, few materials are suitable for hydrogen storage because hydrogen molecules are small and can leak through many types of plastic. In 2020, most vehicles stored hydrogen at a pressure of 700 bar (70 MPa), but compressing hydrogen to this pressure requires significant energy.

Pressurized gas pipelines are always made of steel and operate at much lower pressures than tube trailers.

As an alternative, liquid hydrogen or slush hydrogen could be used. Liquid hydrogen is stored at extremely cold temperatures (20.268 K, or −252.882 °C). While cryogenic storage reduces weight, it requires large amounts of energy to cool hydrogen to this temperature. The liquefaction process, which involves pressurizing and cooling hydrogen, is energy-intensive. Liquid hydrogen has lower energy density by volume than gasoline, with about four times less energy per liter. This is because gasoline contains more hydrogen atoms per liter (116 grams) than liquid hydrogen (71 grams). Liquid hydrogen storage tanks must also be well insulated to prevent heat from causing the liquid to boil away.

Japan has a liquid hydrogen storage facility in Kobe and planned to receive the first shipment of liquid hydrogen via a specialized carrier in 2020. Hydrogen is cooled to −253 °C, similar to liquefied natural gas (LNG), which is stored at −162 °C. However, this process results in an efficiency loss of 12.79%, or 4.26 kWh/kg of energy, compared to the 33.3 kWh/kg needed for liquefaction.

Research

The study of hydrogen storage materials is a large and complex field with many research papers published over the years. From 2000 to 2015, researchers collected and analyzed data from the Web of Science using a tool called VantagePoint. This analysis showed that research on hydrogen storage materials grew quickly between 2000 and 2010. After 2010, the growth slowed, and by 2015, the amount of research reached a steady level. Some countries, such as those in the European Union, the United States, and Japan, produced fewer papers after 2010. Other countries, like China and South Korea, continued to increase their research output until 2015. China, the European Union, and the United States published the most papers on hydrogen storage materials, with China leading throughout the entire period.

Among different types of materials, Metal-Organic Frameworks (MOFs) were the most studied. Next were Simple Hydrides. Three patterns of research were observed:
1. New materials, such as MOFs and Borohydrides, which were mainly studied after 2004.
2. Classic materials, like Simple Hydrides, which were researched throughout the entire period and saw increasing interest.
3. Materials, such as AB5 alloys and Carbon Nanotubes, where research activity remained the same or decreased over time.

Currently, physisorption technologies for storing hydrogen are not yet ready for use in real-world applications. Most experiments use small samples, less than 100 grams. These methods typically require high pressure or very low temperatures. Because of these requirements, they are not seen as a new technology on their own but as a helpful addition to existing methods like compression and liquefaction.

Physisorption involves hydrogen attaching to the surface of materials through weak forces. This process is reversible because it does not require energy to start. In materials like MOFs, porous carbons, zeolites, clathrates, and organic polymers, hydrogen is stored on the surfaces of tiny pores. The ability of these materials to store hydrogen depends on their surface area and the size of their pores. However, the weak forces between hydrogen and the material’s surface limit their effectiveness. These materials can store large amounts of hydrogen at very low temperatures and high pressures, but their storage capacity drops significantly at normal temperatures and pressures.

Liquid Organic Hydrogen Carriers (LOHC) are a promising method for storing hydrogen. LOHC are organic compounds that can absorb and release hydrogen through chemical reactions. These compounds can store and release hydrogen in a cycle. In theory, any organic molecule with double or triple bonds (like those in unsaturated compounds) can absorb hydrogen during a chemical process called hydrogenation. LOHC systems prevent hydrogen from being released into the atmosphere, making them safe. This method could be useful for storing energy from wind and solar power in liquid form, similar to how fossil fuels are used today.