## 1 Introduction
Furthermore, with an increase in density from 40 g/L (1.3 kWh/L) for CGH2 and 70 g/L (2.3 kWh/L) for LH2 to 80 g/L (2.7 kWh/L) for CcH2, larger amounts of hydrogen can be stored [6,12]. In addition to the described hydrogen storage options, various research institutions around the world are working on further physical and or chemical sorption storage systems in liquids [9,11,13,14] or solids [15,16,17,18,19,20,21,22]. Under specific temperature and pressure conditions, hydrogen can be stored via a sorption process in organic, as well as inorganic materials. Such reversible processes offer some advantages in volumetric ESD and safety compared to physical storage. Organic carrier systems, e.g., dibenzyltoluene (H\({}_{0}\)DBT)/perhydro dibenzyltoluene (H\({}_{18}\)DBT), are also known as liquid organic hydrogen carriers (LOHCs) [9]. Charging and discharging take place by means of a catalytic hydrogenation and dehydrogenation. During hydrogenation, the reaction takes place typically at temperatures around 200 \({}^{\circ}\)C and 7 MPa and in discharge at up to 310 \({}^{\circ}\)C and a slight overpressure [11]. The resulting hydrogenated liquid has physico-chemical properties comparable to diesel and can, therefore, be stored and transported easily [23]. This provides the main advantage of LOHC technology: hydrogen can be safely stored and transported in a chemically bound form under ambient conditions [9]. Furthermore, an ESD of up to 6.2 wt% of hydrogen related to the total weight of the carrier is possible, whereby a volumetric ESD of up to 1.9 kWh/(L H\({}_{18}\)DBT) is reachable [14,23]. One of the challenges of LOHC technology is still the thermal management of the process. For efficient process design, heat, arising during the exothermic hydrogenation, must be reused during the endothermic dehydrogenation process [9]. Additionally, a purification of the hydrogen released from the carrier is needed after the discharge process [11]. Inorganic hydrogen carriers are also available in a solid state such as nanostructured materials and metal hydrides [7,24,25]. Nanostructured materials such as carbon nanotube and metal organic framework (MOF) systems can store hydrogen on materials with a high specific surface area by adsorption processes. MOFs are porous crystalline materials with a specific internal surface area of up to 4800 m\({}^{2}\)/g. They consist of organic linkers and inorganic metal oxide units with zirconium oxide \([\text{ZnO}_{4}]^{6+}\)[26]. Such an MOF may exhibit a theoretical ESD of up to 3.0 kWh/ (L MOF) [9]. Up to now, no system can be tested at a demonstration scale, as the technology is still in the laboratory stage [27]. Compared to nanostructured materials, metal hydrides store hydrogen chemically by an exothermic absorption processes. Since the desorption is endothermic, an intelligent thermal control system is necessary. Research and development activities are still going on to improve the kinetics and stability of materials, as well as reducing the metal hydrides' costs [28; 29]. A promising approach for hydrogen storage is the thermochemical reduction and oxidation of metals (\(M\))/metal oxides (\(M_{n}O_{m}\)) [11; 15; 30], which can be generally described by:  
\[M_{n}O_{m}+m\text{H}_{2}\leftrightarrow nM+m\text{H}_{2}\text{O}\,. \tag{1}\]  
In contrast to previous systems, the solid material does not work as a carrier material for hydrogen. The solid mass acts as a kind of catalyst for the oxidation of hydrogen to steam and backwards for the reduction of steam to hydrogen. Various metals, like Ce, Cu, Fe, Mn, Ni, and W, have been tested as the reaction mass [31]. From the holistic consideration of the material parameters--availability, costs, kinetics, and ESD--iron has been found to be the most adequate oxide carrier material [15; 31; 32]. The basis of the iron redox process was developed at the beginning of the 20 th Century for the production of hydrogen, whereby the process also received its name \"steam iron process\" [33; 34].