## 2 Hydrogen Storage Systems  
At the state of the art, the most common ways to store H\({}_{2}\) are cGH\({}_{2}\), IH\({}_{2}\), CeH\({}_{2}\), and SSMs, as reported in Table 1, where gravimetric and volumetric capacity values, at specific pressure and temperature, pros and cons of each storage method are listed [20; 31]. The important criteria for an H\({}_{2}\) storage system are the gravimetric capacity (the amount of H\({}_{2}\) adsorbed per unit mass, expressed as kg H\({}_{2}\) kg-1 or as a weight percentage (wt.%), which decrease with the pressure increases), the volumetric capacity (the amount of H\({}_{2}\) adsorbed per unit volume, as g H\({}_{2}\) m\({}^{-3}\), which increases with the pressure increases), the pressure and the temperature. Gravimetric and volumetric capacities influence the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Storage** & **Gravimetric** & **Volumetric** & & & & \\
**Method** & **Capacity** & **Capacity** & **T [K]** & **P [MPa]** & **Pros** & **Cons** \\ \hline CGH\({}_{2}\) & 6 & \(<\) 40 & RT & 70 & \begin{tabular}{c} High efficiency, \\ convenient, \\ mature technology \\ \end{tabular} & \begin{tabular}{c} Expensive cylinder and \\ the immature technology \\ of fast filling. \\ Large consume of energy \\ and time, low \\ temperature, \\ boil-off \\ High costs \\ \end{tabular} \\ SSMs -Phys & 2 & 20 & 77 & 10 & \
\begin{tabular}{c} High efficiency, \\ convenient, \\ mature technology \\ \end{tabular} &
\begin{tabular}{c} Expensive cylinder and \\ the immature technology \\ of fast filling. \\ Large consume of energy \\ and time, low \\ temperature, \\ boil-off \\ High costs \\ \end{tabular} \\ SSMs -Phys & 15 & 150 & RT & 2 & H\({}_{2}\) good reversible cycle & H\({}_{2}\) capacity and \\ performance, large & the lifetime \\ volume of H\({}_{2}\) density & of tank \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hydrogen storage systems comparison. weight and the volume of a storage tank, respectively, while pressure and temperature determine the operation condition influencing, besides the geometry of the vessel, the costs and safety of the full system. cGH\({}_{2}\) is the most deep-rooted storage solution in different sectors since the end of the 19 th century so that regulations, codes and several standards have been set up for different applications. Considering the storage applications, pressure vessels are typically used in the industrial stationary field as storage and buffering and in the mobility field for on-board power supply and storage in refueling stations. This method requires deepened studies on materials and geometries to design the more appropriate and safe tanks useful to use in transportation, stationary and portable systems. The storage pressure affects the thickness of the container walls, the size and weight of the containers, the choice of materials, and the costs. Currently, the cGH\({}_{2}\) can be stored in four different types of pressure vessels (Type I to Type IV), that can be pressurized up to 25, 35, or 70 MPa [32]. Type I metallic (generally steel or metal alloys) pressure vessel is mostly used for industrial applications with a pressure of 20-30 MPa, but it is only able to store about 1 wt% of H\({}_{2}\). These are also used in refueling station where the storage of large volumes at medium pressure (about 20 MPa) and high and very high pressure (about 50 and 100 MPa) are foreseen to quickly refuel the vehicles. These are designed for large supplies of H\({}_{2}\) (up to 8 tons/day and more) with the need for large storage volumes. In Type II, the cylindrical metal (mostly steel or aluminum) part of the vessel is wrapped with fiber resin composite, while the Type III and Type IV are fully composite wrapped vessels, in which the composite is plastic or carbon fibers embedded in a polymer matrix and the inner liner is metallic (generally aluminium) or plastic, respectively.