# Systematic Analysis of the Hydrogen Value Chain from Production to Utilization
## 3 Hydrogen Storage Technologies
### Material-Based Storage Technologies  
In material-based storage technologies, hydrogen is bonded to materials that are capable of reversibly storing hydrogen in either hydrides or in molecular hydrogen form. The storage process can either result from adsorption, a surface phenomenon where hydrogen molecules accumulate at the surface of a material, or absorption, a bulk phenomenon where hydrogen molecules are broken into hydrogen atoms that penetrate the storage material and are incorporated into the volume of the material [71].  
Adsorption consists of a surface interaction process between a gas and a solid material, mostly reversible, where, depending on the gas component and the temperature and pressure conditions, a physical binding between the two is formed. This bond results from Van der Waals forces and, unlike absorption, there are no chemical bonds formed [66; 71]. Carbon-based materials, such as porous carbons and carbon nanotubes, metal-organic frameworks (MOF), and zeolites, are the most frequently mentioned adsorption materials in the literature.  
The metal hydride storage process is based on the interaction between hydrogen and metals or metal alloys, usually in solid powder, to form metal hydrides through absorption. The hydrogen molecules, at the surface of the material, are dissociated into atomic hydrogen, which is then diffused into the interior of the metallic material and stored in the interstitial space of the metal lattice, forming ionic, metallic, or covalent bonds with the metallic material [71].  
In chemical hydrogen storage, hydrogen forms strong covalent bonds with other elements, generating chemical compounds containing hydrogen that can be used to store hydrogen or as hydrogen carriers [76]. Different substances are frequently mentioned as chemical hydrogen storage materials, among which ammonia, methanol, and liquid organic hydrogen carriers (LOHC) are the most commonly highlighted [75; 77].  
Regarding material-based storage technologies, recent research has focused on improving both hydride and porous material systems to enhance hydrogen storage performance [78]. Regarding hydrides, efforts have been made to develop novel metal hydrides through alloying strategies and nanostructuring techniques intended to improve hydrogen absorption and desorption kinetics. A follow-up approach involves nanoconfinement, where hydrides are embedded into porous scaffolds. This approach has produced ammonia borane, borohydrides, and magnesium hydrides within mesoporous silicas, nanoconfined alanates, nanostructured carbons, and MOFs [78]. Complex hydrides have attracted growing interest due to their high hydrogen content and favorable thermodynamics. Regarding porous materials, enhancements to MOFs, including the incorporation of polymers, nanoparticles, and electron-rich functional groups, or the introduction of oxygen, nitrogen, or any other electron-rich group, have been explored [78]. Carbon-based materials have also gained particular attention [78].  
The main characteristics of each of the storage techniques mentioned are summarized in Table 2.