## 1 Introduction
Finally, the storage principle offers promising integration possibilities with industrial processes working at elevated temperatures like hydrogen pyrolysis [36] or H\({}_{2}\) production/combustion via solid oxide fuel cell technologies [37]. The overall goal of our research group is to develop a redox storage technology based on promising iron oxide pellets [38] as a stand-alone renewable energy storage and supply system for urban quarters and industry premises. Figure 1 depicts a schematic of the redox-storage integration in one possible complete system. During the charging phase, water is split into H\({}_{2}\) and O\({}_{2}\) in an electrolyzer with the help of the surplus renewable electricity. The produced green hydrogen flows into the redox reactor, where it reduces iron oxide to iron and water vapor according to Equation (2). The heat demand shall be supplied by the waste heat of the electrolyzer. During the storage time, only iron exists inside the reactor. During the discharge phase, water vapor is introduced into the redox reactor, which oxidizes iron to iron oxide again, and hydrogen is released according to Equation (3). H\({}_{2}\) flows into the fuel cell together with O\({}_{2}\) out of its storage to cover both electricity and heat demands. Thermal energy storage shall be introduced inside the redox reactor or outside it to deliver the heat for the endothermic reduction of iron oxide and to store it during the exothermic oxidation of iron. \begin{table}
\begin{tabular}{c c c} \hline \hline
**System** & **Volumetric ESD** & **References** \\ \hline CGH2-35 MPa & 0.8 kWh/L & [5; 6] \\ CGH2-70 MPa & 1.3 kWh/L & [5; 6] \\ LH2-20 K & 2.3 kWh/L & [6; 9] \\ CcH2 & 2.7 kWh/L & [12] \\ LOHC & 1.9 kWh/(L H\({}_{18}\)DBT) & [14] \\ Redox & 2.8 kWh/(L bulk) & This work \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the most widely used hydrogen storage systems with their volumetric energy storage densities (ESDs). CGH2, compressed gas storage of hydrogen; CcH2, cryo-compressed hydrogen; LOHC, liquid organic hydrogen carrier. In order to design the redox reactor and its thermal management system, the first development task, to be carried out, is to establish and validate a dynamic simulation model for the combined heat and mass transfer in such a reactive system. To this aim, the reaction kinetics of both the reduction and oxidation phases shall be measured under different boundary conditions. This work presents, therefore, an experimental study on the redox reaction kinetics of single iron oxide pellets [38] under different process parameters applying a thermo-gravimetric analyzer (TGA). The redox reactions are experimentally investigated at the reaction temperatures of 700 \({}^{\circ}\)C, 800 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C. In addition, the effect of the reactant gas composition at a reactant gas flow rate of 250 mL/min is investigated. Moreover, the effect of the pellet diameter on the redox reactions' kinetics is investigated. Finally, the kinetic performance of the investigated pellets is checked after 29 redox cycles in comparison with the start of life performance. To evaluate the structure of the investigated iron oxide pellets before and after repeated redox cycles, a micro-computer tomography (\(\upmu\)CT) unit is applied.