[ materials ]
Fig . 2 . Physisorption and chemisorption mechanisms for hydrogen storage 6
Liquid-phase storage Liquid hydrogen storage requires extremely low temperatures (< - 20 ° K / -253 ° C ), as the boiling point of hydrogen gas is around 20 ° K / -253 ° C at atmospheric pressure . This storage technique poses two main obstacles : the liquefying operation and the liquid boiloff phenomenon . The boil-off phenomenon occurs when heat from a variety of sources transfers into the tank , causing the liquid to boil and the pressure in the tank to rise . Although liquid hydrogen storage has high storage efficiency and higher density compared to gas-phase storage , the energy consumption for maintaining the liquid state , insulation requirements , and suitable materials of construction for cryogenic temperature operation create genuine difficulties for its application . 4 Estimates indicate that the cooling and compressing processes for liquid hydrogen storage consume 30 % more energy than that in the stored liquid hydrogen itself . Although there have been recent improvements in the efficiency of liquefying systems , liquid hydrogen storage is still not commonplace due to aforementioned difficulties in achieving and maintaining the critically low temperature requirement .
storage can be categorized into two classes , as shown in Figure 2 and described below .
• Physisorption : hydrogen gas molecules are physically attracted ( i . e ., adsorbed ) to the surface of the storing material , owing to the Van der Waals forces effect .
• Chemisorption : hydrogen gas molecules chemically ingress ( i . e ., are absorbed ) into the storing material . The chemisorption interaction is more profound than physisorption .
Materials for solid-state hydrogen storage
Thermodynamic Aspects Solid-state hydrogen storage using metal hydrides and complex hydrides is hindered by high thermodynamic stability of the storage solids , high decomposition temperature , and poor reversibility . Over the years , numerous approaches have been developed to improve the hydrogen storage properties of metal hydrides and complex hydrides . For example , nanostructuring by ball milling , doping with catalyst , and destabilizing with other hydrides ( i . e ., reactive hydride composites ). 8 , 9 Properties such as large surface area , satisfactory thermal stability , high porosity , and good mechanical behavior are important when selecting a material for solid-state hydrogen storage . 5
The reaction between a metal ( M ) and molecular hydrogen ( H 2
) gas is described by :
Solid-stage storage The solid-state storage approach has attracted global attention as an ideal hydrogen storage method . Solid-state materials , especially metal and complex hydrides , offer high gravimetric capacity which enables the storage of a large amount of hydrogen . The storage of hydrogen in solid phase is usually carried out by either absorbing or reacting with metals or a chemical compound , or by storing in another chemical form . 5 Solid-state hydrogen
Fig . 3 . The Lennard-Jones one-dimensional potential of atomic and molecular hydrogen 9
40 Hydrogen Tech World | Issue 8 | February 2023