## 1 Introduction
This shifts the reaction equilibrium towards higher pressures and leads to a decreased storage capacity unless the storage tank is cooled. The stored hydrogen can be released from the metal hydride during the desorption phase (endothermic discharging). The process temperature and pressure depend on the properties of the used metal/metal hydride, especially the absorption/desorption properties [4; 13]. Metal hydrides such as magnesium hydride (MgH\({}_{2}\)) can reach a high storage density of about 6 wt.-% (theoretically 7.6 wt.-%). A temperature of about 300 \({}^{\circ}\)C and a pressure of 30 bar must be applied for charging, whereas the discharging temperature is about 350 \({}^{\circ}\)C [14]. A promising approach for hydrogen storage is represented by the thermochemical reduction and oxidation (redox) of metal oxides (M\({}_{n}\)O\({}_{m}\))/metals (M). In such a hydrogen storage process, hydrogen is not chemically stored within the metal bulk. Instead, hydrogen reduces the metal oxide to metal and is oxidized to steam (H\({}_{2}\)O). Due to its high availability, low cost, and thermodynamic behavior, iron is considered to be the most suitable oxide carrier material [15]. The basis of the iron redox storage process was developed in the early 20 th test century with the so-called \"steam-iron\" process, which was developed to produce hydrogen with a high purity and established on an industrial scale [16; 17]. In the redox storage process, iron oxide/iron is repeatedly reduced and oxidized at atmospheric pressure conditions in a hydrogen and a steam gas stream, respectively. During the charging, iron oxide (magnetic, Fe\({}_{3}\)O\({}_{4}\)) is first reduced to wuestite (simplified as FeO). In the next step, it is further reduced to elemental iron (Fe). During this process, oxygen (O\({}_{2}\)) is removed from the iron oxide and reacts with hydrogen (H\({}_{2}\)) to create steam (H\({}_{2}\)O). The discharging represents the reverse process in a steam atmosphere, where iron is oxidized back to iron oxide (see Equations (1) and (2)). To obtain a higher reaction speed, the redox reaction takesplace in a temperature range between 600 and 800 \({}^{\circ}\)C. Due to the process temperature and an oxygen partial pressure (\(p_{\text{O}_{2}}\)) of less than 10\({}^{-14}\) bar, hematite (Fe\({}_{2}\)O\({}_{3}\)) cannot be formed, i.e., Fe\({}_{3}\)O\({}_{4}\) cannot be oxidized to Fe\({}_{2}\)O\({}_{3}\) by water vapor [18; 19; 20; 21; 22]. The absence of elementary hydrogen in the charged system represents a major safety advantage in comparison with the state-of-the-art technologies. In addition, redox storage systems can achieve a high volumetric energy density of about 2.8 kWh \(\text{I}_{bulk}^{-1}\)[23]. \[\text{Fe}_{3}\text{O}_{4}+\text{H}_{2}\rightleftharpoons 3\,\text{FeO}+\text{H}_{2}\text{O} \tag{1}\]  
\[3\,\text{FeO}+3\,\text{H}_{2}\rightleftharpoons 3\,\text{Fe}+3\,\text{H}_{2}\text{O} \tag{2}\]  
However, the repeated redox reactions usually result in aging effects, such as the compaction of the porous storage material. As a consequence, the reactive surface area decreases, and the gas diffusion of the reacting gas slows down. This implies a reduction in the energy storage capacity and an increase in the reaction time, which is considered as a decrement in cycle stability. To improve the cycle stability, support materials can be added [15; 24]. Several researchers in the field of chemical looping (CL) processes [25; 26; 27], solid oxide fuel cells (SOFC), and oxide ceramic batteries (ROB) [15; 28; 29] have shown that this aging can be reduced by the addition of supporting materials, e.g., silica, alumina, or calcium oxide. An improvement of both reaction kinetics and cycle stability during the redox storage process by adding different support materials was also proven in our previous work [30]. Therein, the pure iron oxide sample already required nine times more time for the second reduction compared to the first reduction.