# Reversible Hydrogen Storage Using Nanocomposites
## 1 Introduction
However, none of these materials fulfill all the hydrogen storage criteria set by US Department of Energy (DOE) such as (1) high hydrogen content; (2) favorable thermodynamics to achieve system fill-time less than 5 min; (3) operation below 85 \({}^{\circ}\)C and above \(-\)40 \({}^{\circ}\)C for hydrogen delivery; and (5) cyclic reversibility (1500 cycles) at ambient temperatures. \begin{table}
\begin{tabular}{l c c} \hline
**Storage Parameter.** & **2020** & **2025** \\ \hline Gravimetric Hydrogen Storage Capacity & & \\ (kg H\({}_{2}\)-kg\({}^{-1}\) material) & & 0.045 & 0.055 \\ Volumetric Hydrogen Storage Capacity & & \\ (kg H\({}_{2}\)-L\({}^{-1}\) material) & & \\ Hydrogen Delivery Temperature (\({}^{\circ}\)C) & \(-\)40/85 & \(-\)40/85 \\ System Fill Time (min; for 5 kg of H\({}_{2}\)) & 3–5 & 3–5 \\ Cycle Life (1/4 Tank to Full; non-dim) & 1500 & 1500 \\ \hline \end{tabular}
\end{table} 
Table 1: Selected US Department of Energy’s (DOE’s) hydrogen storage systems-targets for 2020 and 2025. Adapted with web policies copyright, restrictions, and permissions notice of DOE [15]. Carbon based materials, on the other hand, possess salient properties that make them interesting for storing hydrogen. Due to the abundance of carbon and the simplicity by which one can produce various forms of it, such as carbon nanotubes, carbon nanofibers, nanobells, or graphite [33; 34], there is virtually an unlimited amount of materials that can be investigated for hydrogen storage. One of the most important advantages of carbon-based materials is that they do not need to be kept in an inert atmosphere. It was initially thought that carbon materials are environmentally friendly and benign to humans, however, recent reports on the health risks of carbon nanotubes needs considerable attention [35; 36]. Graphite nanofibers, in its herringbone structure, have been shown to store hydrogen [37], though the exact structure required to store the hydrogen is still being investigated. Hydrogen storage in carbon nanotubes [38], carbon nanobells [39], carbon nanofibers [40], and especially doped carbon nanotubes [41; 42] have shown reversible hydrogen storage at 77 K. Carbon based materials possess another important feature, namely a hydrogen binding energy between 10 and 50 kJ-mol\({}^{-1}\). This is an ideal value for the binding of hydrogen, since a material with weak bonding of molecular hydrogen with binding energy \(<\)10 kJ-mol\({}^{-1}\) is not a suitable candidate material, as the hydrogen can be readily and accidentally released. Materials that have high energy hydrogen bonds, such as chemical hydrides (400 kJ-mol\({}^{-1}\)) or metal hydrides (50-100 kJ-mol\({}^{-1}\)), can store a large amount of hydrogen, but they will require an excessive energy, either as heat or pressure difference, to store or release the hydrogen. The carbon structures that possess this binding energy namely graphene, carbon nanotubes, and fullerenes. This enhanced physisorption, \"spillover\", and \"Kubas\" binding represent the ideal hydrogen bonds. In this review, among the different storage options, we have compared and contrast the salient features and underlying applications of nanostructured variants for the reversible hydrogen storage. In this review article, the most notable solid-state hydrogen storage materials and their nanocomposites are discussed in detail. The structure of the present review is as follows. In Section 2, we have introduced the definition of nanocomposites and their key features for reversible hydrogen storage. In Section 2.1 MgH\({}_{2}\) nanoparticles and catalytically doped MgH\({}_{2}\) nanocrystalline forms for lowering the decomposition temperatures is discussed. Section 2.2 focuses on Mg\({}_{2}\)FeH\({}_{6}\) and the role of Fe in reversible hydrogen storage, additionally this section explains the effects of Ti doping on kinetics at moderate temperatures. Nanoengineering of LiBH\({}_{4}\)/MgH\({}_{2}\) complex hydrides via different synthesis schemes are discussed in Section 2.3.