Table 4.

Summary of previously reported studies on the mechanisms of hydrogen storage using physical, chemical and electrochemical methods.

Reference	Synthesis methods	Compound	Storage attribute
(i) Metal-Organic Frameworks (MOFs)
[169]	Solvothermal method	MOF-5	BET surface area of 839 m2/g was reported using Et3N as a solvent
[155]	Sonochemical synthesis	TMU-7	BET surface area of 393 m2/g was reported
[170]	Sonochemical synthesis	MOF-5	Langmuir surface area of 3208 m2/g was reported
[171]	Microwave synthesis	MIL-101	BET surface area of 3891 m2/g was observed
[172]	Electrochemical method	Cu3(BTC)2	Langmuir surface area of 1150 m2/g was reported
[173]	Mechanochemical methods	Cu3(BTC)2 [HKUST-1]	BET surface area of 278 m2/g was reported
(ii) Polymer and Metal nanostructures
[174]	Doping-undoping-redoping route	Polyaniline and polypyrrole	6–8 wt% H2 sorption
[175]	Hypercrosslinking route	Polyaniline nanostructure	BET surface area of 20–632 m2/g and H2 storage capacity of 2.2 wt% were reported
[151]	Sonochemical route	Copper oxide	A hydrogen storage capacity of 1.84% was reported
(iii) Chemical Hydrides
[176]	Solvent mediated milling	Ti-doped NaAlH4	A storage capacity of 4 wt% was observed
[177]	Mechanical milling	LiBH4 + 1/2 MgH2, 2–3 mol% of TiCl3	A reversible storage capacity of 8–10 wt% of H2 was reported
[178]	Sonochemical route	MgH2-Fluorographene	H2 uptake of 6 wt% was reported
B. Electrochemical storage mechanism
[156]	Thermal decomposition	ZnAl2O4	Hydrogen storage capacity in terms of peak discharge capacity was 4000 mAh/g
[167]	Ultrasonic synthesis	CdSnO3-Graphene nanocomposite	Peak discharge capacity was 2550 mAh/g at 1 mA
[159]	Combustion route	Sr3Al2O6	Discharge capacity was 2500 mAh/g after 15 cycles
[168]	Chemical precipitation route	Zn2GeO4/graphene nanocomposite	The discharge capacity of the electrode reached 2695 mAh/g after 29 cycles
[161]	Thermal decomposition	Co3O4-CeO2 nanocomposite	Discharge capacity was 5200 mAh/g after 20 cycles