## 1 Introduction
The current research and development activities are focusing mainly on increasing the hydrogen production rates, e.g., by reducing the sintering tendency of the iron-based active material [16]. During the repeated redox process, the pure metals undergo an aging process, which decreases the reaction behavior and long-term mechanical stability. To avoid aging, different inert components, e.g., Al\({}_{2}\)O\({}_{3}\), CaO, SiO\({}_{2}\), and/or ZrO\({}_{2}\), can be used as stabilizers to improve both the chemical and mechanical properties [16; 31]. The endothermic charging phase, in which the simplified reduction of magnetite (Fe\({}_{3}\)O\({}_{4}\)) to Fe is shown, can be described as:  
\[\text{Fe}_{3}\text{O}_{4}+4\text{H}_{2}\to 3\text{Fe}+4\text{H}_{2}\text{O}\,. \tag{2}\]  
Hydrogen reduces Fe\({}_{3}\)O\({}_{4}\) first to wustite (simplified with FeO) and then to Fe. Hereby, H\({}_{2}\) reacts to steam. The corresponding oxidation reaction, which follows also the two-stage reactions, is given by:  
\[3\text{Fe}+4\text{H}_{2}\text{O}\rightarrow\text{Fe}_{3}\text{O}_{4}+4\text{ H}_{2}\,. \tag{3}\]  
Based on the mass ratio of H\({}_{2}\) to Fe in Equations (2) and (3), a theoretical maximum storage of hydrogen in iron of 4.8 wt% can be achieved, which results in a gravimetric ESD of 1.6 kWh/(kg Fe). Depending on the composite ingredients (iron/stabilizer material) and the material geometry, the volumetric ESD varies considerably. A mixture of 90%-wt. iron oxide and 10%-wt. stabilizer material in a spherical geometry with \(d=12\) mm leads to a theoretical volumetric ESD of up to 4.4 kWh/ (L Fe). Due to the fact that the spherical storage masses are to be operated later in a tubular reactor, it is reasonable to assume a fixed bed porosity of 36%. Accordingly, the expected volumetric ESD amounts to 2.8 kWh/ (L bulk). Table 1 presents a summary of the volumetric storage densities for the discussed hydrogen storage technologies so far. The resulting high ESD of the redox storage process, as well as the low-cost advantage of the storage material \"iron oxide\" provide an excellent basis for the development of an efficient thermochemical energy storage concept. For these reasons, various research groups have been dealing with this type of hydrogen storage. One focus of the research work so far is the intermediate storage of hydrogen in a rechargeable oxide battery (ROB) [17; 35]. To this aim, different small-scale samples (approximately 1 mm thick rectangular tape-cast form) have been developed and investigated in gas mixtures consisting of a H\({}_{2}\)O/H\({}_{2}\) ratio of 1:4 during fuel cell mode and 4:1 during electrolyzer mode at an operating temperature of 800 \({}^{\circ}\)C. Different additive materials like Al\({}_{2}\)O\({}_{3}\), CaCO\({}_{3}\), Cuo, MgO, ZrO\({}_{2}\), and 8 YSZ have been applied to enhance the cyclic stability under the described process parameters. The best stability results of 200 cycles, each having a 70 min duration, could be obtained by applying 30% of 8 YSZ to the iron oxide form, however at the cost of sharply reducing the ESD from 1340 Wh/kg to 900 Wh/kg due to the excessive amount of additive needed. Here, the ESD is related to the higher heat value, as well as to the educt side of Equation (3). Furthermore, 8 YSZ is extremely expensive, so that the authors, themselves, recommended not to apply it in future work [16]. In further publications, they focused on the application of clearly less expensive additives like CaCO\({}_{3}\)[30]. One of the main challenges in the storage process via the steam iron process is to develop an iron oxide mixture that is capable of sustaining long-term thermal and reactive cycling processes between 700 \({}^{\circ}\)C and 900 \({}^{\circ}\)C. On the other hand, the redox storage of H\({}_{2}\) could offer high storage performance due to the strong binding of hydrogen and the high ESD [26; 27]. Furthermore, hydrogen is not stored directly, which provides a considerable safety advantage against, e.g., hydrogen compression storage.