Which hydrogen storage method is more advantageous?

Which hydrogen storage method is more advantageous?

Hydrogen storage technology runs through the hydrogen energy end of the industrial chain to the fuel cell end, and is an important link in controlling the cost of hydrogen.  The storage method of hydrogen is a matter of great concern to people. Currently, the commonly used hydrogen storage technologies mainly include physical hydrogen storage, chemical hydrogen storage and other hydrogen storage.

1. Physical hydrogen storage
Physical hydrogen storage technology refers to the technology of increasing the density of hydrogen by changing the hydrogen storage conditions to achieve hydrogen  storage. This technology is a purely physical process, does not require a hydrogen storage medium, has low cost, is easy to release hydrogen, and has a high hydrogen concentration. It is mainly divided into high-pressure gaseous hydrogen storage and low-temperature liquefied hydrogen storage.
1) High-pressure gaseous hydrogen storage
High-pressure gaseous hydrogen storage technology refers to compressing hydrogen under high pressure and storing it in a high-density gaseous form. It has the characteristics of low cost, low energy consumption, easy dehydrogenation, and wide working conditions. This is the most mature and most commonly used hydrogen storage technology. However, it has a small reserve, high energy consumption, requires a pressure-resistant container, and has unsafe factors such as hydrogen leakage and container explosion. The hydrogen storage density of this technology is greatly affected by pressure, and the pressure is limited by the material of the storage tank. Therefore, the current research hotspot is the improvement of storage tank materials. At 30-40 MPa, the increase is faster, and when the pressure is greater than 70 MPa, the change is very small. Therefore, the working pressure of the storage tank must be between 35 and 70 MPa. Therefore, finding a lightweight, high-pressure resistant hydrogen storage tank has become the key to high-pressure gaseous hydrogen storage. At present, high-pressure gaseous hydrogen storage containers are mainly divided into four types: pure steel metal bottles (Type I), steel liner fiber hoop-wound bottles (Type II), aluminum liner fiber fully wrapped bottles (Type III) and plastic liner fiber wrapped bottles (Type IV). Among them, Type III and Type IV bottles have the advantages of small weight-to-volume ratio and high hydrogen storage density per unit mass, and have been widely used in hydrogen fuel cell vehicles. The working pressure of high-pressure hydrogen storage bottles is generally 35-70 MPa. Domestic on-board  high-pressure hydrogen storage systems mainly use 35 MPa Type III bottles, while foreign countries mainly use 70 MPa Type IV bottles.
2) Cryogenic liquid hydrogen storage
Cryogenic liquid hydrogen storage technology uses the characteristics of hydrogen liquefaction under high pressure and low temperature conditions, and its volume density is 845 times that of gaseous state, to achieve efficient hydrogen storage, and its transportation efficiency is higher than that of gaseous hydrogen. However, in order to ensure low temperature and high pressure conditions, not only are there requirements for the material of the storage tank, but also a matching strict insulation scheme and cooling equipment are required. Therefore, the storage tank volume of cryogenic liquefied liquid hydrogen storage is generally small, and the mass density of hydrogen is about 10%. Cryogenic liquid hydrogen storage technology is mainly used in the military and aerospace fields, and commercial research and application have just begun. However, due to its advantages in large-scale and long-distance storage and transportation, with the formal implementation of my country's three national standards for liquid hydrogen and the continuous advancement and cost reduction of hydrogen storage technology, cryogenic liquid hydrogen storage may coexist with high-pressure gaseous hydrogen storage in the future.

2. Chemical hydrogen storage
Chemical hydrogen storage technology is a technology that uses hydrogen storage media to react with hydrogen under certain conditions to generate stable compounds,  and then releases hydrogen by changing conditions. It mainly includes organic liquid hydrogen storage, liquid ammonia hydrogen storage, coordinated hydride hydrogen storage, inorganic hydrogen storage and methanol hydrogen storage.
1) Organic liquid hydrogen storage
Organic liquid hydrogen storage technology is based on the hydrogenation reaction of unsaturated liquid organic matter under the action of a catalyst to generate stable  compounds, and then dehydrogenation reaction when hydrogen is needed. Commonly used unsaturated liquid organic matter and its properties are shown in the table. .
2) Liquid ammonia hydrogen storage
Hydrogen and nitrogen are synthesized into liquid ammonia under the action of a catalyst and stored and transported in the form of liquid ammonia. Liquid ammonia decomposes and releases hydrogen at normal pressure and about 400 ℃. Compared with the extremely low hydrogen liquefaction temperature of -253℃ required by  low-temperature liquid hydrogen storage technology, the liquefaction temperature of ammonia at one atmosphere is much higher at -33℃, and the "hydrogen-ammonia-hydrogen" method consumes less energy, is less difficult to implement and transport, and is less difficult. At the same time, the volumetric hydrogen storage density in liquid ammonia hydrogen storage is 1.7 times higher than that of liquid hydrogen, and is much higher than the long-tube trailer gaseous hydrogen storage technology. This technology has certain advantages in long-distance hydrogen energy storage and transportation. However, liquid ammonia hydrogen storage also has many disadvantages. Liquid ammonia is highly corrosive and toxic, and there are potential risks of harm to equipment, human body and environment during storage and transportation; the synthetic ammonia process is relatively mature in my country, but there is a certain proportion of loss in the process conversion; the equipment for synthetic ammonia and ammonia decomposition and the terminal industry equipment still need to be integrated.
3) Methanol hydrogen storage
Green methanol has high energy density and is an ideal liquid energy storage and transportation method. Using renewable energy to generate green hydrogen and then  combining it with carbon dioxide to generate green methanol that is easy to store and transport is an important path to zero carbon emissions.
3) Coordination hydride hydrogen storage
Coordination hydride hydrogen storage uses alkali metals to react with hydrogen to generate ionic hydrides, which decompose into hydrogen under certain conditions. The  following table shows the hydrogen storage properties of common coordination hydrides. 
4) Hydrogen storage by inorganic compounds
Inorganic hydrogen storage materials are based on the mutual conversion between bicarbonate and formate to achieve hydrogen storage and release.

Summary: Currently, high-pressure gaseous hydrogen storage is the mainstream, but its development has been restricted due to safety, and the hydrogen storage density is low, which is not suitable for large-scale long-distance transportation; low-temperature liquid hydrogen storage faces many difficulties in its development in China due to its high cost and difficulty in storage and transportation; liquid organic hydrogen storage technology has great advantages in safety, hydrogen storage density, and storage and transportation efficiency. Under the premise that both 70MPa high-pressure hydrogen storage and low-temperature liquid hydrogen storage in my country are lagging behind, it is expected to become one of the main ways of hydrogen storage and transportation in my country in the future. As a hydrogen storage material, hydride has the advantage of storing hydrogen at a high density under low pressure compared with high-pressure hydrogen.