Elements. The elements class includes carbon-based materials such as activated carbon, carbon nanotubes, graphitic nanofibers, graphene, graphite and fullerene. Carbonaceous sorbents are promising materials for hydrogen storage due to their low densities, good chemical stability, high surface area, and porosity. Carbonaceous sorbents research focuses on increasing effective adsorption temperature through increase of the hydrogen binding energies and improving volumetric and gravimetric storage capacities through optimizing the material's pore size, increasing pore volume and surface area, and investigating effects of material densification. Hydrogen can be physically adsorbed on activated carbon and be "packed" on the surface and inside the carbon structure more densely than if it has just been compressed. The best results were achieved with carbon nanotubes. Carbon nanotubes, when decorated with metal or metal oxide nanoparticles, show a significantly improved hydrogen storage capacity. For instance, multi-walled carbon nanotubes (MWCNTs) decorated by Dy3Fe5O12 nanoparticles can store H2 at temperatures as low as 196.15°C and pressures as low as 6 MPa, providing a gravimetric density of 10.8 wt% and volumetric density of 41 kg/m 3 . Fullerenes are potential hydrogen storage material based on the ability to react with hydrogen via hydrogenation of carbon-carbon double bonds. The theory predicts that a maximum of 60 hydrogen atoms can be attached to both the inside and outside of the fullerene spherical surface. A stable C60H60 isomer can be formed with the theoretical hydrogen content of ∼7.7 wt %. It seems that the fullerene hydride reaction is reversible at high temperatures. The 100% conversion of C60H60 indicates that 30 moles of H2 gas will be released from each mole of fullerene hydride compound. However, this reaction is not possible because it requires a high temperature, about 823-873 K. A new trend in carbonaceous sorbents is preparing porous carbon materials from biomass pyrolysis. It has been shown that the pyrolysis temperature, pyrolysis heating rate and carbon-containing precursor strongly affect the yield and structure of the resulting porous carbons. Another attempt is to fabricate highly porous carbon sorbents by carbonizing highly crystalline metal-organic frameworks (MOFs) without any carbon precursors. Other widely used elements are transition metals which can be applied alone or together with noble metals (Pt, Pd) as dehydrogenation catalysts for metal hydrides and liquid organic hydrogen carriers. It was shown that using nickel nanoparticles as ammonia borane dehydrogenation catalysts is a promising step toward a feasible hydrogen storage medium for fuel cells. Coordination compounds. In coordination compounds such as metal-organic frameworks, H2 is physiosorbed on the surface of the pores. In general, the interactions of the type "H2-MOF" are very weak. Therefore, MOFs' high storage capacities can be achieved at liquid nitrogen temperature and high pressures. Still, MOFs have been extensively studied as promising hydrogen storage materials. Various metal ions such as Zn(II), Cu(II), Mn(II), Cr(III), and La(III), and ligands such as carboxylates, imidazolates, triazolates, tetrazolates have been employed for the construction of the MOFs. Promising H2 storage data were reported for MOF-5 (4.5 wt % at 77 K and 1 atm), prepared from benzene-1,4-dicarboxylate (BDC) and Zn(II) salt. Significant research is focused on densified hybrid MOF@CNT and MOFs impregnated with fullerene composites for hydrogen storage. Various MOFs such as MOF-5, MIL-101, Zr-MOFs and HKUST-171 have been compacted and their hydrogen adsorption capacities evaluated. After compaction at ∼700 MPa, the densified UiO-66 achieved a total H2 uptake of 5.1 wt% at 100 bar and 77 K compared to 5.0 wt% for the UiO-66 powder. Metal Hydrides. Light and complex metal hydrides are prominent hydrogen storage materials as they require comparatively feasible working temperatures and have good hydrogen storage capacity. To form light metal hydrides, hydrogen interacts with metals through different bonds. Ionic or covalent hydrides, such as LiH and MgH2 are quite suitable for hydrogen storage due to their high hydrogen storage capacity but require a very high operating temperature. Research on hydrides focuses on improving the volumetric and gravimetric capacities, hydrogen adsorption/desorption kinetics, cycle life, and reaction thermodynamics of potential material candidates. Pristine MgH2 can store ∼7.6 wt.% hydrogen. However, both MgH2 hydrogenation and dehydrogenation reactions are very slow. Magnesium hydride forms ternary and quaternary hydride structures by reacting with various transition metals (Fe, Co, Ni, etc.) and thus improved kinetics. Moreover, the nanoscale version of these transition metal particles offers an additional hydrogen sorption mechanism via its active surface sites. Noble metals (Pt, Ag, Au, Pd, and Ru) are beneficial to improve dynamical stability and dehydrogenation properties of LiH. Doping nanoparticles of Fe and Ti with a few mol% of carbon nanotubes improves the sorption behavior of MgH2. Ball milling is widely used for modification of the hydrogenation/dehydrogenation process in magnesium-based hydrogen storage materials . The phenomenon of mechanical milling helps to pulverize the particles of MgH2 into micro-or nano-crystalline phases and thus leads to lowering the activation energy of desorption. The activation energies of the H2 sorption and dehydrogenation temperatures for the bulk MgH2, mechanically milled MgH2 and nanocatalyst-doped MgH2, are shown. Both alkali metals and alkaline-earth metals borohydrides have a significant hydrogen storage capacity. For example, LiBH4 demonstrates the highest gravimetric (18.5 wt. % H2) and volumetric (121 kg H2 m -3 ) hydrogen storage capacity. However, pristine LiBH4 undergoes different changes with the effect of temperature. It was shown that hexagonal boron nitride (h-BN) additive greatly improves the dehydrogenation of LiBH4. Sodium borohydride (NaBH4) is an excellent hydrogen storage material, but its application is restricted by irreversibility of hydrolysis and high cost of regeneration. To regenerate NaBH4 with high yield and low costs, the hydrolytic product NaBO2 reacts with CO2, forming Na2B4O7•10H2O and Na2CO3, both of which are ball-milled with Mg under ambient conditions to form NaBH4 in high yield (close to 80%). This method is expected to effectively close the loop of NaBH4 regeneration and hydrolysis, enabling a wide deployment of NaBH4 for hydrogen storage. 96