The metal ions clusters and organic multidentate ligands form Metal-organic frameworks (MOFs), and these materials are highly crystalline and porous [1, 2]. MOFs emerged as potential materials for application in science like catalysis [3], storage [4], biomedical, and sensors [5, 6] 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 [7–9]. 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 [10]. 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 [11–17]. 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[18]. 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 [19, 20]. 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 [21–29]. These studies used MOFs consisting of specific functionalized groups such as hydroxyls to form lithium alkoxides by removing the protons with lithium cations [24, 26]. MOFs with specific functionalized groups are limited; thus, new methods like doping of lithium might be adopted to develop wide varieties of MOFs.
Composite MOFs are prepared to enhance hydrogen uptake capacities by doping carbonaceous materials[30]. The developed composites materials have significant mechanical properties and moisture stabilization; It was also observed that doping of carbonaceous materials with transition metals enhanced hydrogen uptake capacities at room temperature. The enhancements in uptake capacities are attributed to the spillover mechanism - the MOFs act as secondary receptors consisting of a larger surface area for hydrogen atoms [30, 31]. Theoretical investigations have also been reported in the literature on the cation alkali metals doped to the MOFs consisting of organic linkers to the fullerenes and carbon nanotubes to enhance uptake capacities [32]. Lithium cations are the most common materials doped to the MOFs as they can effectively donate the electrons to the linkers of MOF and these ions are easy to dope as they have low molar masses[33]. Some computational studies on the MOFs doped with lithium revealed that Li donates the electrons to MOFs' linkers and leads to the high binding energy of hydrogen, which exhibited a solid affinity for MOFs nearer to Li[34]. Meng et al. reported that hydrogen sorption capacities in computational studies achieved at 298 K with a 100 bar pressure attained a weight of 4 wt% with a simulation doping lithium with IRMOF9. In the theoretical studies at ambient temperatures, MOFs and COFs doped with lithium also achieved a weight of 6 % hydrogen[35]. Klontzas et al. conducted theoretical calculations and reported that utilization of functionalized organic linkers consisting of lithium atoms showed enhanced MOFs' performance to store hydrogen. It was also found that enhanced hydrogen uptake capacities were observed in the experimental approaches. In his empirical studies[36], Li et al. revealed that hydrogen uptake capacities in conjugated polymers (microporous) with lithium over 1 bar pressure at 77 K were a weight percent of 6.1; adopting these mechanisms to real-time applications are not possible due to inconsistent repeatability[37]. Mulfort et al. reported enhanced uptake capacities of hydrogen over 1 bar pressure at 77 K, i.e., up to 75 % in the MOFs doped with lithium than the pristine MOFs; further reported that the organic inkers might increase these capacities with functionalized groups[38]. Himsl et al. revealed that lowering the pressures enhanced the uptake capacities of hydrogen from a weight of 0.5 to 0.7 % by the post-synthesis formation of lithium alkoxides in MIL-53 (Al) hydroxyl functionalized groups[39]. By immersing MOF with lithium chloride solutions, Yang et al. prepared NOTT 200 and further doped it with the lithium, showing enhanced efficiency in hydrogen uptake[40].
Further, experimental and theoretical studies confirmed that doping lithium is the most effective method for enhancing MOFs' H2 sorption capacities. Even though many studies are available in the literature on uptake capacities of hydrogen at lower pressures (up to 1 bar) in the MOFs doped with the lithium; however it is challenging to find the Li doped MOFs' studies at high pressures. Thus, paucity in the literature about the sorption capacities of hydrogen at high pressures motivated us to perform the current research, mainly focused on the uptake capacities of hydrogen at cryogenic temperature and room temperatures up to a pressure of 70 bar inside MOFs doped with the lithium. For the present study, MIL-53 (Al) was selected; as these materials consist of high thermal stabilities, exhibits stability to moisture, and high surface area/volume, which helps in the dispersal of ions doped with the MOFs. MIL-53 (Al) is a MOF doped with aluminum consists of trans chains where the corners are shared with the octahedra of AlO4(OH)2, which are interconnected by the BDC (benzene dicarboxylate) linkers. These frameworks consist of 1-D channels (one dimensional) removed by the solvents and benzene dicarboxylate. In the present study, MIL-53 (Al) was synthesized by doping the ions of lithium. Adsorption isotherms of hydrogen at high pressure were performed at 253 and 298 K up to 70 bar pressure to determine doped lithium's effect on hydrogen storage at high-pressure capacity in these materials [41–45].