## 2 Hydrogen Value Chain
### Storage on Hydrogen Value Chain
#### 2.2.3 Storage by Chemical Processes  
Regarding chemical storage methods, metal hydride systems in particular store hydrogen in a solid-state--that happens when hydrogen molecularly bonds to the metal--resulting in a much safer and high-volume efficient tankage when compared to liquid or gas [34; 38]; although this being still an emerging technology, it has already reached high levels of safety--proven to be non-reactive even to bullets [41]. This presents as a major advantage over, say, battery storage, which nowadays still has associated risks with explosiveness. Current research on metal hydrides is focused on improving the adsorption properties and decreasing the cost of this system [34; 38], with magnesium hydride appearing as a promising solution due to its favorable properties; however, this system requires relatively high ambient temperatures to (dis-)charge, which may pose a drawback to the transportation sector. Such drawbacks are currently being addressed through research on several different production techniques, as well as the study of catalyst inclusion to the systems [34].  
On the other hand, hydrogen can also be stored in ammonia. This compound is well known and used within the fuel industry, making it easily one highly valued hydrogen storage option. It has a relatively high hydrogen density and a high utilization flexibility--both in stationary and mobile applications. As ammonia's capabilities ensure long-term stable storage and transportation, it can cope with the needs to store energy in time and in space (stationary and export/import energy, respectively) [42]. When comparing these storage methods, liquid ammonia was found able to store volumes of hydrogen larger than, say, liquefied hydrogen itself. Besides, it can also be stored at normal pressure and temperature conditions (1 bar and 25 \({}^{\circ}\)C, respectively)--much lower than compressed hydrogen gas, for example. However, this very high density also means heavier storage and a special transportation demand. Moreover, the process of releasing hydrogen from ammonia downstream consumes great amounts of energy, requiring even more in the case where high-purity hydrogen is needed; this could arguably be the most challenging aspect of high-efficiency hydrogen storage in ammonia, particularly considering that both compressed and liquid hydrogen easily deliver pure hydrogen from design [27]. Methanol could be seen as competitor of ammonia-based hydrogen storage, as it yields higher energy density; however, it has a lower hydrogen content by weight and volume [43].  
Table 1 offers a summary of the described so far, compiling the major advantages and disadvantages of the leading hydrogen storage technologies.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Storage Technology** & **Compression** & **Liquefaction** & **Chemical** & **Ref.** \\ \hline
**Basic Properties** & & & & \\ Storage temperature (\({}^{\circ}\)C) & \(\sim\)25 & \(<\)\(-\)252.90 & \(\sim\)25 & [27; 35] \\ Storage pressure (bar) & 690 & 1 & 9.90 & [27; 35] \\ Density (kg/m\({}^{3}\)) & 39 & 70.80 & 600 & [27; 35] \\ \hline
**Hydrogen Content** & & & & \\ Gravimetric (wt\%) & 100 & 100 & 17.80 & [27; 38] \\ Volumetric (kg-H\({}_{2}\)/m\({}^{3}\)) & 39 & 70.80 & 106.80 & [27; 38] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of current hydrogen storage methods.  
Nowadays, the most advanced technologies for storing hydrogen are cryogenically liquefied and compressed gas [38]; but these methods may not be completely suitable for future widespread hydrogen applications, mainly due to leakage and safety concerns in their pressurized form and energy requirements in the case of liquefaction. Even so, as the push for environmentally friendly solutions is gaining traction, new technologies are constantly being researched and developed to overcome these issues [38].