and higher at lower temperatures. With the rising of temperature, physisorption decreases and chemisorption increases. For MOF-519 and MOF-520, the isosteric heat of adsorption decreased with pressure increase. For MOF-5, both gravimetric and volumetric hydrogen uptake increased with increase in pressure. The total capacity may not be consistent with the usable capacity under pressure swing conditions. For instance, MOF-5 and IRMOF-20, which have the highest total volumetric capacity, show the least usable volumetric capacity. Absorption capacity can be increased by modification of structure. For example, the hydrogen uptake of PCN-68 is higher than PCN-61. Porous aromatic frameworks (PAF-1), which is known as a high surface area material, can achieve a higher surface area by doping. Modification of MOFs There are many different ways to modify MOFs, such as MOF catalysts, MOF hybrids, MOF 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 effect and inert metals have 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 kinds of modified MOFs show that MOF hybrids are more promising because of the good controllability in selection of 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. In 2009, researchers reached 10 wt% at 77 bar (1,117 psi) and 77 K with MOF NOTT-112. 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 the thermal vibration energy. Varying several factors such as surface area, pore size, catenation, ligand structure, and sample purity can result in different amounts of hydrogen uptake in MOFs. In 2020, researchers reported that NU-1501-Al, an ultraporous metal–organic framework (MOF) based on metal trinuclear clusters, yielded "impressive gravimetric and volumetric storage performances for hydrogen and methane", with a hydrogen delivery capacity