Hydrogen storage

Several methods exist for storing hydrogen. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H₂ upon demand. While large amounts of hydrogen are produced by various industries, it is mostly consumed at the site of production, notably for the synthesis of [|ammonia]. For many years hydrogen has been stored as compressed gas or cryogenic liquid, and transported as such in cylinders, tubes, and cryogenic tanks for use in industry or as propellant in space programs. The overarching challenge is the very low boiling point of H₂: it boils around 20.268 K. Achieving such low temperatures requires expending significant energy.
Although molecular hydrogen has very high energy density on a mass basis, partly because of its low molecular weight, as a gas at ambient conditions it has very low energy density by volume. If it is to be used as fuel stored on board a vehicle, pure hydrogen gas must be stored in an energy-dense form to provide sufficient driving range. Because hydrogen is the smallest molecule, it easily escapes from containers. Its effective 100-year global warming potential is estimated to be.

Established technologies

Compressed hydrogen

is a storage form whereby hydrogen gas is kept under pressures to increase the storage density. Compressed hydrogen in hydrogen tanks at 350 bar and 700 bar are used for hydrogen tank systems in vehicles, based on type IV carbon-composite technology. Car manufacturers including Honda and Nissan have been developing this solution.

Liquefied hydrogen

tanks for cars, producing for example the BMW Hydrogen 7. Japan has a liquid hydrogen storage site in Kobe port. Hydrogen is liquefied by reducing its temperature to −253 °C, similar to liquefied natural gas which is stored at −162 °C. A potential efficiency loss of only 12.79% can be achieved, or 4.26 kW⋅h/kg out of 33.3 kW⋅h/kg.

Chemical storage

Chemical storage could offer high storage performance due to the high storage densities. For example, supercritical hydrogen at 30 °C and 500 bar only has a density of 15.0 mol/L while methanol has a hydrogen density of 49.5 mol H₂/L methanol and saturated dimethyl ether at 30 °C and 7 bar has a density of 42.1 mol H₂/L dimethyl ether.
Regeneration of storage material is problematic. A large number of chemical storage systems have been investigated. H₂ release can be induced by hydrolysis reactions or catalyzed dehydrogenation reactions. Illustrative storage compounds are hydrocarbons, boron hydrides, ammonia, and alane etc. A most promising chemical approach is electrochemical hydrogen storage, as the release of hydrogen can be controlled by the applied electricity. Most of the materials listed below can be directly used for electrochemical hydrogen storage.
Nanomaterials, particularly those produced by ball mill and severe plastic deformation, offer an alternative that overcomes the two major barriers of bulk materials, rate of sorption and activation. High-entropy alloy materials such as TiZrCrMnFeNi also show advantages of fast and reversible hydrogen storage at room temperature with good storage capacity for stationary applications.
Enhancement of sorption kinetics and storage capacity can be improved through nanomaterial-based catalyst doping, as shown in the work of the Clean Energy Research Center in the University of South Florida. This research group studied LiBH₄ doped with nickel nanoparticles and analyzed the weight loss and release temperature of the different species. They observed that an increasing amount of nanocatalyst lowers the release temperature by approximately 20 °C and increases the weight loss of the material by 2-3%. The optimum amount of Ni particles was found to be 3 mol%, for which the temperature was within the limits established and the weight loss was notably greater than the undoped species.
The rate of hydrogen sorption improves at the nanoscale due to the short diffusion distance in comparison to bulk materials. They also have favorable surface-area-to-volume ratio.
The release temperature of a material is defined as the temperature at which the desorption process begins. The energy or temperature to induce release affects the cost of any chemical storage strategy. If the hydrogen is bound too weakly, the pressure needed for regeneration is high, thereby cancelling any energy savings. The target for onboard hydrogen fuel systems is roughly <100 °C for release and <700 bar for recharge. A modified van 't Hoff equation, relates temperature and partial pressure of hydrogen during the desorption process. The modifications to the standard equation are related to size effects at the nanoscale.
Where is the partial pressure of hydrogen, is the enthalpy of the sorption process, is the change in entropy, is the ideal gas constant, T is the temperature in Kelvin, is the molar volume of the metal, is the radius of the nanoparticle and is the surface free energy of the particle.
From the above relation we see that the enthalpy and entropy change of desorption processes depend on the radius of the nanoparticle. Moreover, a new term is included that takes into account the specific surface area of the particle and it can be mathematically proven that a decrease in particle radius leads to a decrease in the release temperature for a given partial pressure.

Hydrogenation of CO₂

Hydrogenation of CO₂ to methanol has been evaluated for hydrogen storage. Barriers of CO₂ hydrogenation includes purification of captured CO₂, H₂ source from splitting water and energy inputs for hydrogenation. For industrial applications, CO₂ is often converted to methanol. Until now, much progress has been made for CO₂ to C1 molecules. However, CO₂ to high value molecules still face many roadblocks and the future of CO₂ hydrogenation depends on the advancement of catalytic technologies.

Metal hydrides

, such as MgH₂, NaAlH₄, LiAlH₄, LiH, LaNi₅H₆, TiFeH₂, ammonia borane, and palladium hydride represent sources of stored hydrogen. There are three main classes of metal hydrides:

Inter-metallic Hydrides: exhibit fast kinetics and moderate hydrogen capacities. Such as LaNi₅H₆, TiFeH₂.
Complex Hydrides: capable of higher hydrogen storage capacities but require catalysts. Such as NaAlH₄, LiBH₄.
Lightweight Hydrides: offer high gravimetric hydrogen storage but require high temperatures for desorption. Such as MgH₂, CaH₂.

Here are the properties of some metal hydrides:

Metal Hydride	H₂ Capacity	Absorption Temp	Desorption Temp	Applications
LaNi₅H₆	1.5-2.0	30-60	50-100	Stationary Storage, Fuel Cells
NaAlH₄	5.6	100-150	200-250	Solid-State Hydrogen Batteries
MgH₂	7.6	300-400	>300	High-Density Hydrogen Storage

Again the persistent problems are the % weight of H₂ that they carry and the reversibility of the storage process. Some are easy-to-fuel liquids at ambient temperature and pressure, whereas others are solids which could be turned into pellets. These materials have good energy density, although their specific energy is often worse than the leading hydrocarbon fuels.
An alternative method for lowering dissociation temperatures is doping with activators. This strategy has been used for aluminium hydride, but the complex synthesis makes the approach unattractive.
Proposed hydrides for use in a hydrogen economy include simple hydrides of magnesium or transition metals and complex metal hydrides, typically containing sodium, lithium, or calcium and aluminium or boron. Hydrides chosen for storage applications provide low reactivity and high hydrogen storage densities. Leading candidates are lithium hydride, sodium borohydride, lithium aluminium hydride and ammonia borane. A French company McPhy Energy is developing the first industrial product, based on magnesium hydride, already sold to some major clients such as Iwatani and ENEL.
Reversible hydrogen storage is exhibited by frustrated Lewis pairs.
Image:Phosphinoboranehydrogenstorage 2.png|center|600px|Phosphino borane hydrogenstorage
The phosphino-borane on the left accepts one equivalent of hydrogen at one atmosphere and 25 °C and expels it again by heating to 100 °C. The storage capacity is 0.25 wt%.

Selected advances using metal hydrides

Aluminium

Hydrogen is produced by hydrolysis of aluminium. It was previously believed that, to react with water, aluminium must be stripped of its natural oxide passivation layer, or mixing with gallium. It has since been demonstrated that efficient reaction is possible by increasing the temperature and pressure of the reaction. The byproduct of the reaction to create hydrogen is aluminium oxide, which can be recycled back into aluminium with the Hall–Héroult process, making the reaction theoretically renewable. Although this requires electrolysis, which consumes a large amount of energy, the energy is then stored in the aluminium.

Magnesium

Traditional MgH₂ stores 7.6 wt% hydrogen, but its high desorption temperature limits applications. Mg-Ti-V nanocomposites can lower the desorption temperature to below 200 °C. Carbon-coordinated MgH₂ exhibits 80% of improvement on cycling stability over 1000 cycles.
LiBH₄ + MgH₂ composites stored about 11 wt% of hydrogen, one of the highest capacities reported. And ammonia borane releases 12 wt% hydrogen at moderate temperatures.
Mg-based hydrogen storage materials include pure Mg, Mg-based alloys, and Mg-based composites. Nonetheless, the inferior hydrogen absorption/desorption kinetics rooting in the overly undue thermodynamic stability of metal hydride make the Mg-based hydrogen storage alloys currently not appropriate for the real applications, and therefore, massive attempts have been dedicated to overcoming these shortages. Some sample preparation methods, such as smelting, powder sintering, diffusion, mechanical alloying, the hydriding combustion synthesis method, surface treatment, and heat treatment, etc., have been broadly employed for altering the dynamic performance and cycle life of Mg-based hydrogen storage alloys. Besides, some intrinsic modification strategies, including alloying, nanostructuring, doping by catalytic additives, and acquiring nanocomposites with other hydrides, etc., have been mainly explored for intrinsically boosting the performance of Mg-based hydrogen storage alloys. Like aluminium, magnesium also reacts with water to produce hydrogen.
Of the primary hydrogen storage alloys progressed formerly, Mg and Mg-based hydrogen storage materials are believed to provide the remarkable possibility of the practical application, on account of the advantages as following: 1) the resource of Mg is plentiful and economical. Mg element exists abundantly and accounts for ≈2.35% of the earth's crust with the rank of the eighth; 2) low density of merely 1.74 g cm-3; 3) superior hydrogen storage capacity. The theoretical hydrogen storage amounts of the pure Mg is 7.6 wt %, and the Mg2Ni is 3.6 wt%, respectively.