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Top1. Introduction
The metal ions clusters and organic multidentate ligands form Metal-organic frameworks (MOFs), and these materials are highly crystalline and porous (Yaghi et al., 1995; Capková et al., 2020). MOFs emerged as potential materials for application in science like catalysis (Miao et al., 2019), storage (Jia et al., 2019), biomedical, and sensors (Shet et al., 2021; Kreno et al., 2012) from the past decade. MOFs can be changed into many varieties by altering the ligands or metal ions, as their structures can be tuned by changing the pore sizes. Additionally, MOFs consist of substantial surface areas and volumes, making them attractive hydrogen storage candidates (Broom et al., 2019; Hu & Zhang, 2010; Suh et al., 2012). Hydrogen storage might be crucial to achieving a hydrogen economy that can be used as a fuel carrier for the fuel cells. Even though many researchers study several materials for the storage application of hydrogen, to date, no material was achieved "DOE" ("US Department of Energy") targets, i.e., volumetric capacities (40 g/L) and gravimetric capacities (5.5 wt%) at the ambient temperatures (Dai et al., 2020). MOFs and COFs (Covalent organic frameworks) are considered suitable sorption materials for hydrogen as they have significant surface areas and porosities. MOFs and COFs utilize weak Vander Waals interactions to enable reversible/fast discharge, which might store hydrogen.
Nevertheless, due to these interactions at ambient temperatures, significant amounts of hydrogen cannot be stored. According to the literature available, several researchers have been that at cryogenic temperatures, the hydrogen storage capacities have reached above 5.5 wt %; however, none of the studies said that hydrogen uptake capacities reached around two wt % at room temperature conditions (Wang et al., 2020; Guo et al., 2020; Kaye et al., 2007; Furukawa et al., 2007; Lin et al., 2009; Koh et al., 2009; Furukawa et al., 2010). Insertion of cations with alkaline nature to the MOFs Nano space has gained many researchers' attention to overcome the common storage problems associated with hydrogen storage. Specifically, cations like lithium are promising materials as these compounds have a lower molecular weight and provide an affinity for the molecules of hydrogen as they induce dipole interactions (Lochan and Head-Gordon, 2006). According to the literature available, the researchers have proposed several theoretical theories by doping lithium to COFs and MOFs to achieve a hydrogen storage wt % of 6 at ambient temperatures (Han & Goddard, 2007; Cao et al., 2009). Many research groups have been demonstrated the experiments by doping lithium with MOFs and revealed that these ions' doping enhanced the storage capacities of hydrogen at non-cryogenic temperatures (Mulfort & Hupp, 2007; Yang et al., 2008; Mulfort et al., 2009a; Mulfort et al., 2009b; Nouar et al., 2009; Himsl et al., 2009; Yang et al., 2009; Li et al., 2010; Xiang et al., 2011). These studies used MOFs consisting of specific functionalized groups such as hydroxyls to form lithium alkoxides by removing the protons with lithium cations (Mulfort et al., 2009b; Nouar et al., 2009). MOFs with specific functionalized groups are limited; thus, new methods like doping of lithium might be adopted to develop wide varieties of MOFs.