## 2 Hydrogen Storage Systems
Comparing the different vessel types, Type I and Type II cannot be used in the vehicles owing to low gravimetric H\({}_{2}\) storage density [33] and H\({}_{2}\) embrittlement (HE) effect, which affects the metal (steel or metal alloy) of which they are made [34]. HE occurs when metals become brittle because of the diffusion of H\({}_{2}\) into the material. The embrittlement degree is affected both by the amount of H\({}_{2}\) absorbed and the microstructure of the material. Consequently, there is a loss of ductility and a reduction of load-bearing capability of the metal due to the absorption of H\({}_{2}\) atoms or molecules by it. The effect of HE is that components crack and fracture at stresses less than the yield strength of the metal. The HE is a dangerous phenomenon depending on different factors such as gas pressure, temperature, and loading conditions, so the effects of HE on materials properties are fundamental for their possible application with high-pressure H\({}_{2}\) gas [35; 36]. Differently, Type III and Type IV types are largely used in the automotive sector due to their low weight, where H\({}_{2}\) is stored at a pressure of 35 MPa (buses, trains, goods movement) and 70 MPa (passenger vehicles and heavy transport). High pressures are required to achieve high energy density and the need to reduce the transportable weight pushes towards lightweight composite solutions [37]. Despite its diffusion and the presence of different standards on the design of materials and vessels, the cGH\({}_{2}\) storage still requires significant improvements in terms of technical and practical applications. In metal vessels, there is the necessity of a deeper study of fatigue life and the HE effects. In the case of composite vessels, of a more recent conception, several issues still need to be explored: gas permeation, thermo-mechanical performance, fiber breaks, delamination, matrix cracking, liner blistering, fire-resistant, inspection and re-testing criteria, and the development of techniques using low-cost fibers. Another conventional storage method is based on the H\({}_{2}\) liquefaction at 20 K, which has high storage efficiency. Compared to cGH\({}_{2}\), H\({}_{2}\) allows higher densities (70 kg/m\({}^{3}\), approximately double compared to H\({}_{2}\) stored in gaseous form at a pressure of 70 MPa and triple compared to storage at 35 MPa). H\({}_{2}\) liquefies at ambient pressure and a temperature of 20 K, therefore the containers for the transport and storage of liquefied H\({}_{2}\) must have very strict thermal insulation requirements. The simplest liquefaction process is the Linde cycle or Joule-Thompson expansion cycle. Liquid H\({}_{2}\) containers have reached a high degree of development and are generally designed with a spherical or cylindrical shape because they are the ones that allow a smaller surface space for the same volume. From a technical point of view, vessel design methods are described on ISO 21009 and ISO 13985. The vessels used for IH\({}_{2}\) are composed of an inner vessel and an external protective jacket. The thermal conductivity between the two layers can be minimized by applying perlite or wrapping it with layers of aluminum films. So, the vessel design must consider, in addition to the topics in common with the vessels for cGH\({}_{2}\), the low temperature which significantly affect mechanical properties causing expansion and contractions phenomena. [32; 38]. Ultimately, the storage of liquid H\({}_{2}\), while representing a commercial solution and having the advantage of allowing a greater energy density than other solutions, is time-consuming, energy-intensive, and affected by gas loss and so is mainly used in specific sectors, such as space programmes, in the medical or chemical industry, and for gas transportation by tracks. CCH\({}_{2}\) storage method combines the properties of both cGH\({}_{2}\) and IH\({}_{2}\) systems to reduce the boil-off loss of IH\({}_{2}\) and maintain a high energy density.