**5. Storage mechanism of gas in carbon nanotubes**

CNTs store gas through the process of adsorption. Adsorption of a gas is a process that gains one or more constituents of the gas in the region of the gas-solid interface where the molecules of the gas are bounded to the surface of the adsorbent as illustrated in **Figure 4**. Gas absorption in CNT is superficial, and this phenomenon involves increasing the density of the gas near the contact surface, and because the process is spontaneous, Gibbs free energy changes are negative. And because the entropy change is also negative (decreasing the degree of release of gas molecules during the process), the enthalpy changes are less than zero, so the process is hot. There are two types of absorption: physical adsorption and chemical adsorption. In physisorption, the bounding of the gas molecules is superficial because the gaseous molecules are not chemically bounded to the walls of the adsorbent. Weak van der Waals forces are responsible for holding the molecules of the gas to the adsorbent. The adsorption of methane on CNT is most likely through the process of physisorption where the adsorbents require an elevated exposed surface per gram of material called specific surface area, and it is expressed in cubical centimeters of adsorbate per gram of adsorbent. When the elemental constituents of the solid get smaller, the specific surface area gets larger. CNTs that have diameters in the order of nanometers and with hollow cylindrical surfaces present good specific surface areas and are an excellent adsorbent for adsorption of gases. As the pores of a CNT surface decrease, the gas storage capacity increases [2]. The doping of CNTs with metal atoms (such as Li or K) or functional groups (such as COO or NH3+) is a possible way to enhance their adsorption capability [7].

**Figure 4.** *Representation of the adsorption process of a gas on a solid surface [2].*
