## 1 Introduction  
Hydrogen possesses a gravimetric energy density of \(\sim\)120 MJ kg\({}^{-1}\), is abundant, and has an environmentally friendly nature, as it is derived from renewable resources, making it more economically convenient in comparison to many other fuels [1, 2]. It can also replace conventional fuels in industrial and home applications, either to offer high temperatures upon burning and/or to catalyze hydro-cracking in petrochemical industry [3, 4]. Using hydrogen as an energy source in automobiles releases no harmful emissions and greatly reduces carbon dioxide emissions [5]. The reported travel distance range for a fuel cell electric bus (FCEB) is between 100-225 miles for fueling events, depending on fulfilling of the set-point pressure of 350 bars of hydrogen storage tanks; these are still lower than the ultimate target of DOE of 300 miles [6]. Hydrogen has the drawback of a low density (0.08 g L\({}^{-1}\)) at STP, and thus a large volume for high-pressure tanks is required in the abovementioned application [7, 8]. In addition, \"some serious safety problems\" are faced when it is stored under high pressure, in case of automobile accidents [9]. Liquefaction techniques can be used at ambient pressure to store hydrogen as a liquid at 21.2 K, but hydrogen boiling creates technical hitches in addition to the complexity and large charge of the cryogenic tanks [9; 10; 11]. An alternative storing method is to store hydrogen as hydrides, whose chemical bonds stabilize hydrogen thermodynamically, and hence a large amount of energy is needed for its release during desorption processes, creating serious technical issues regarding the heat exchange management and its complex coupling to the storage system [12]. As an example, a temperature of 561 K is needed to dissociate MgH\({}_{2}\) to release hydrogen. Furthermore, its formation kinetics are very slow because of the difficult dissociation of hydrogen molecules over the surface of magnesium metal to form a MgH\({}_{2}\) compound [13; 14]. Moreover, stress can be accumulated within the storage reactors due to densification and pulverization of hydrides upon hydrogenation cycles [15]. Furthermore, safety and hazard has to be considered when dealing with some hydrides such as NaAlH\({}_{4}\)[16], while cost has to be considered for the heat transfer management and safety requirements [14; 17]. Physical adsorption, in porous materials, occurs when the pores generate weak interaction energy between the adsorbate gas molecules and their walls, offering several benefits to overcome shortcomings of the aforementioned methods [18]. Various porous adsorbents such as zeolites [19], metal-organic frameworks (MOFs) [20], and activated carbons (ACs) [21] are good candidates for physical adsorption due to their high surface area. ACs may be the most suitable porous materials due to their abundance, intoxication, stability, cheapness, low density and their high surface area [22; 23; 24; 25; 26; 27]. In addition, they can be synthesized from waste materials and from natural products which would be beneficial for the environment [28; 29]. During the last decades, several theoretical and experimental works were performed to evaluate H\({}_{2}\) uptake in different carbon materials such as nanotubes, nanofibers, activated carbons, etc. for selecting the most adequate receptor for hydrogen storage applications. Starting with carbon nanotubes (CNTs), it was reported that hydrogen uptake was favored at the end sites of the CNTs and on their outer surface sites in comparison to the inner surface (endohedral) sites within the tubes [30; 31]. The researchers were attracted toward the use of CNTs for hydrogen storage when [PERSON] et al. [32] reported a high storage percentage of hydrogen (~10 wt.%) on SWCNTs at room temperature. Later, [PERSON] et al. [33] claimed a value of up to 5.0 wt.% with pretreated SWCNTs. For MWCNTs, the situation did not vary too much in comparison to the SWCNTs, where the highest value of 2.0, 3.7, 4.0, and 4.6 weight percentages were reported [34; 35].