## 2 Material and Methods
### Experimental Setup  
Figure 2 illustrates the experimental setup for cycling the produced storage pellets and investigating their kinetic performance during the repeated redox reactions. The core component of the setup was the simultaneous thermal analyzer (STA 449 Jupiter F3, Netzsch, Selb, Germany). The supply of hydrogen and nitrogen, as a carrier gas, could be regulated in the range of 0 to 250 mL min\({}^{-1}\)\(\pm\) 0.5% through a mass flow control (MFC) device for each gas. Moreover, a steam generator (aSTEAM DV2, aDROP) was applied to produce the required steam during the oxidation reaction. The amount of steam could be set within a range from 0 to 25 g h\({}^{-1}\) with an accuracy of 1%. The accuracy of the weight measurement by the STA device amounted to \(\pm\)0.1 ug, while the temperature was adjustable within a tolerance limit of \(\pm\)0.3 \({}^{\circ}\)C. All devices were connected to a computer for process control as well as data monitoring and recording. The redox reactions were conducted with the use of a thermogravimetric (TG) analyzer device in an atmosphere of 50% nitrogen and 50% hydrogen (reduction) or 50% steam (oxidation). In order to achieve fast aging in the investigated samples (in less than nine redox cycles), the TG measurements were carried out at 800 \({}^{\circ}\)C [30]. To analyze the measured reaction kinetics, the conversion rate for the reduction (\(\alpha_{red}(t)\)) and oxidation (\(\alpha_{ox}(t)\)) was determined by Equations (4) and (5). Thereby, the time-dependent mass change of the sample (\(\Delta m(t)\)) was divided by the maximum mass change after a complete conversion (\(m_{max}-m_{min}\)). The maximum mass (\(m_{max}\)) represents the mass of the sample in the fully oxidized state (Fe\({}_{3}\)O\({}_{4}\)), considering the amount of the supporting material, while the minimum mass (\(m_{min}\)) is the wight of the pellet in its completely reduced state (Fe). Under the tested process conditions (800 \({}^{\circ}\)C, \(P_{O_{2}}\) <10\({}^{-14}\) bar), hematite is not stable. This implies a difference between initial sample mass and measured maximum mass of about 3% [29; 30]. A fixed data point at \(\alpha_{red}\) = 0.8 and \(\alpha_{ox}\) = 0.8 was chosen to compare the kinetic behavior of the tested storage samples. \[\alpha_{red}(t)=\frac{m_{max}-m(t)}{m_{max}-m_{min}} \tag{4}\] \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**No.** & **P1** & **P2** & **P3** & **P4** & **P5** & **P6** & **P7** & **P8** & **P9** & **P10** \\ \hline wc/wt.-\% & 18 & 20 & 21 & 25 & 26 & 27 & 27 & 33 & 33 & 33 \\ \(m_{start}/g\) & 1.47 & 1.34 & 1.45 & 1.44 & 1.37 & 1.35 & 1.38 & 1.43 & 1.37 & 1.37 \\ \hline \end{tabular}
\end{table} 
Table 1: Production parameters of the investigated samples with the initial sample mass after sintering (\(m_{start}\)) and the water content (wc). Furthermore, the structure of the cycled storage samples was investigated in a micro-computed tomography facility (\(\mu\)CT; Phoenix V \(|\) tome \(|\) x s240/180 from Waygate Technologies Digital Solutions Baker Hughes, Houston, TX, USA). The voxel size for the analysis was about 8 \(\mu\)m, and the number of images was 3000. The scanning parameters for the \(\mu\)CT scans were as follows: 140-160 kV voltage, 50-60 \(\mu\)A current, and 0.1 mm Cu filter. Reconstructed volumes were processed using the manufacturer's software phoenix datos l x 2 reconstruction 2.4.0. The software VG Studio Max 2022.2 from Volume Graphics was used to determine the parameters of the 3D images, including their surface area and volume. To measure the open porosity of the samples (\(\Phi\)), the volume of void space (\(V_{v}\)) was determined and divided by the total volume (\(V_{total}\); see Equation (6)). To this aim, the density (\(\rho\)) of each sample was measured by a He-pycnometer (Pynomatic ATC, Thermo Fisher Scientific) with an accuracy of 0.01%. For the density measurements, the maximum standard deviation was about 0.3%. The total volume of the sample was estimated by the data from the \(\mu\)CT scans.