Hydrogen storage methods
Hydrogen storage remains a significant challenge that hinders the widespread application of hydrogen in mobile as well as stationary applications.
The transition from fossil fuel to hydrogen in the transportation sector greatly depends upon cost-effective storage.
To achieve an acceptable driving range for vehicular applications, the hydrogen storage system should pose both high volumetric and gravimetric hydrogen storage density.
These densities determine the physical space and weight of the storage system, which are crucial parameters in the case of vehicular application. In contrast, the weight and size of the storage are not that critical for stationary and static applications.
The hydrogen storage methods used and explored by industry and the academic community can be broadly divided into two types
- Physical-based storage (hydrogen is physically contained)
- Material-based storage (hydrogen interacts with storage material)
Two major storage methods have been commonly used in physical-based storage; they are gaseous and liquid hydrogen storage.
Gaseous Hydrogen Storage
Hydrogen tanks before being installed to a Toyota Motor Corp. Mirai fuel-cell vehicle (FCV) in Japan on Oct. 13, 2016. Photographer: Tomohiro Ohsumi/Bloomberg
Hydrogen is very light and a low density (0.084 kg/m3) gas, which can be compressed and stored in a gaseous form.
At ambient temperature and pressure, a large volume of hydrogen is required for storage. The volumetric density of hydrogen is less in compressed gas storage, and it is a function of pressure.
The hydrogen storage tank (350 bar and 700 bar) made up of Type IV carbon fiber is now being widely used for vehicles.
The volumetric density corresponding to 350 bar is about 18 kg/m3, and 700 bar is 35 kg/m3 [1,2]. U.S. Department of Energy (DOE) Technical targets for onboard hydrogen storage for 2020 indicate volumetric densities of 30 kg/m3, but ultimately 50 kg/m3 is planned .
The hydrogen pressure within the tank reaches more than 1500 bar to achieve a volumetric density of 50 kg/m3.
To withstand such a high pressure, the storage tank material should have very high strength and to compress hydrogen to such high pressure requires large compression work.
The material cost, compression cost, and other costs add up, making this storage very expensive.
Hydrogen storage tank design Image credit: Process Modeling Group, Nuclear Engineering Division, ANL
Hydrogen reacts with the material used for the construction of a vessel at high pressure and makes them brittle.
Therefore, to avoid failure of the material, they are generally built of particular alloys and with strengthened composite carbon fiber.
This type of storage has considerable potential energy and is risky and has the possibility of an explosion because of high pressure.
But the use of suitable blast walls and different sensors and monitoring equipment to ensure safe-limit of pressure inside the vessels can certain safety to a greater extent.
Some of the important compressed hydrogen storage tank specifications 
|Nominal Pressure||350 and 700 bar|
|Tank Liner||Aluminum (Type III) High-Density Polyethylene (Type IV)|
|Maximum Filling Pressure||350-bar: 438 bar; 700-bar: 875 bar|
|“Empty” Pressure||20 bar|
|Tank Size (water capacity)||350-bar: 258 L; 700-bar: 149 L|
|Carbon Fiber (CF) Type||Toray T700S|
|Tank Liner Thickness||5 mm HDPE (Type IV); 7.4 mm Al (Type III, 350-bar); 12.1 mm Al (Type III, 700-bar)|
Some of the manufacturers, such as Toyota, Hyundai, Honda, etc., have used this storage technique with the key goal of 500 km driving range and demonstrated their prototype vehicle.
This compressed gas method of storage is used not only in vehicles for road transportation but also in the stationary application at refueling stations for dispensing hydrogen and for stationary power generation.
The comparatively low hydrogen density, as well as the high gas pressures requirement, are significant limitations of this technically straightforward and well-established storage system.
Liquid Hydrogen Storage
Hydrogen is liquefied by reducing its temperature to -253◦C and stored at ambient pressure in a cryogenic tank.
The vessels are effectively shielded by an internal pressure vessel and an outer shielding jacket to maintain a temperature at -253◦C .
The volumetric density of liquid storage is far better than gaseous storage. Yet heat leakage causes liquid storage to boil-off. Boil-off losses are proportional to the surface-to-volume ratio, as the storage tank size increases, the evaporation rate decreases.
This storage method is more suitable for use, as in delivery tankers with a higher volume than onboard applications.
For liquefaction, approximately 40% of the LHV of hydrogen is needed, compared with 10% for compressed hydrogen .
It is costly to use in vehicles because of the need for reliable low temperatures, insulation, and large tank sizes.
Cryo-compressed storage utilizes the properties of liquid and compressed gaseous hydrogen methods to store hydrogen.
This technique can lessen the boil-off rate of hydrogen and, altogether, uphold a high energy density. A cryo-compressed tank generally stores hydrogen at -253◦C temperature and high pressure at about 300 bar .
A substitute for physical-based storage is material-based storage.
In material-based storage, hydrogen is stored in three different media: the first one is metal hydrides that are solid media; second is liquid hydrogen carriers, and third is surface storage.
Nonetheless, most of these storage processes are still in the research phase.
Source: Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Department Hydrogen Technology, Dresden, Germany 
The molecular hydrogen is initially adsorbed to the metal surface in metal hydride storage systems, then elemental (H) introduced into a metal lattice by heat output and released with the heat input.
The benefits of metal hydride are numerous in contrast with conventional physical-based storage, for instance, high volumetric density, safe, low operating temperature and pressure, etc.
Most hydrides have volumetric density well above 80 kg/m3, which exceeds the ultimate target of DOE .
The metal hydrides of intermetallic compounds are especially important. Intermetallic hydrides are represented by formula AmBnHx.
The interaction between metal atoms and the interstitial hydrogen atoms defines the characteristics of an intermetallic compound and relies primarily on the compound’s crystal structure.
Some examples of intermetallic compounds are AB5, AB, A2B, and AB2. Amongst this AB2, AB5 and A2B have better hydrogen-absorbing characteristics .
There is another group of hydrides called complex hydride. These are the hydrides containing Nitrogen (amides) and Boron (borohydride).
They form a strong bond with hydrogen. Such hydrides have a high hydrogen weight storage capacity, which can easily fulfill the ultimate objectives laid down by the DOE.
Nevertheless, there are many issues like high operating temperature, reversibility, slower kinetics, and decomposition products (borane, ammonia, etc.).
These hydrides are not currently feasible for storage, but if more work is carried out to solve problems, it can be used for both mobile and stationary hydrogen storage.
Liquid Organic Hydrogen Carriers
Liquid organic hydrogen carriers (LOHCs) are liquids or low melting solids, which can, in the presence of a catalyst, be reversibly hydrogenated and dehydrogenated at higher temperatures.
In the underlying reaction systems, catalysis plays a key role. LOHCs can be inexpensive, safe, and easy to handle.
They also allow for long-term energy storage and uncomplicated transport without boil-off or other hydrogen losses. Nonetheless, this form of storage requires a very high operating temperature.
Surface Storage Systems (Sorbents)
Hydrogen may also be reversibly stored as a sorbate, by adsorption, in materials with very specific surface areas through van der Waals forces. Some of the sorption materials include metal-organic frameworks (MOFs), zeolites, carbon nanotubes, etc.
The required operating temperature is low, while the pressure is high for this form of storage. Surface storage systems also suffer low volumetric density.
In short, researchers and industries from all over the world are designing and exploring these storage technologies.
Some companies are working on reinforced carbon fiber and other materials for compressed gas tanks. Some groups focus on liquid organic hydrogen carriers, while others concentrate on the surface storage method.
However, the majority of researchers are now drawn to metal hydride storage as it has a wide bag of materials available and several advantages over other storage areas. It thus seems apparent that the storage problem will be resolved soon.
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 Barthelemy, H., Weber, M. and Barbier, F., 2017. Hydrogen storage: Recent improvements and industrial perspectives. International Journal of Hydrogen Energy,
42(11), pp. 7254 – 7262, special issue on The 6th International Conference on Hydrogen Safety (ICHS 2015), 19-21 October 2015, Yokohama, Japan.
 Züttel, A., 2003. Materials for hydrogen storage. Materials Today, 6(9), pp. 24–33.
 DOE Technical Targets for Onboard Hydrogen Storage for Light-Duty Vehicles, https://www.energy.gov/eere/fuelcells/doe-technical-targets-onboard-hydrogen-storage-light-duty-vehicles [accessed: August 05, 2020]
 Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Department Hydrogen Technology, Dresden, Germany (2018) Available at https://www.ifam.fraunhofer.de/en/Profile/Locations/Dresden/HydrogenTechnology/hydrides/_jcr_content/contentPar/sectioncomponent/sectionParsys/gallery.vimg.4col.large.jpg/ifam/en/images/dd/H2/Bildleiste_Hydride/010_hydrierung_3d.jpg [accessed: August 05, 2018].