## 2 Material and Methods
### Manufacturing Process  
The manufacturing of the iron oxide pellets follows a three-stage process. In the first stage, iron oxide and the support material are dried at 105 \({}^{\circ}\)C for 24 h and mixed in a double cone laboratory mixer (J. Engelsmann AG, Ludwigshafen, Germany) for 60 min at a rotation speed of 50 min\({}^{-1}\)(100% power of the mixer). After this stage, the pelletizing disk (illustrated in Figure 1) is applied to produce the so-called green pellets. Due to the rotation of the pelletizing disk and the alternating dosing of the powder mixture and water, agglomerated green pellets are formed. The internal structure of the pellets is affected by the way the pellets form, e.g., their coalescence, breakage, and abrasion transfer, which directly influences both the mechanical strength and the porosity of the pellets [31; 32; 33]. The inclination angle \(\beta\) is the angle between the surface of the pelletizing disk and the horizontal. The literature recommends keeping this angle between about 40 and 60\({}^{\circ}\)[31; 32; 34; 35]. In a dedicated investigation campaign on the influence of \(\beta\) on the shape of the manufactured pellets, different angles were tested within the range between 30 and 65\({}^{\circ}\). It turned out that, with \(\beta\) lower than 35\({}^{\circ}\), the pellets tended to have an oval shape. With \(\beta\) between 50 and 60\({}^{\circ}\), almost spherical pellets could be formed. Further increasing \(\beta\) above 60\({}^{\circ}\) led to increased material loss due to more ejection of the material from the pelletizing disk during rotation. Accordingly, \(\beta\) was set to 52\({}^{\circ}\) during the manufacturing of all produced and tested samples in this work. The optimum rotation speed \(n_{opt}\) of the pelletizing disk was defined by [34; 35] as in the range between 0.6 to 0.75 of the critical speed \(n_{c}\). The latter can be obtained by setting equal the centrifugal and weight forces acting on the material during the rotation of the pelletizing disk. Solving the resulting equation for the rotational speed yields Equation (3), in which \(g\) represents the gravitational acceleration in m s\({}^{-2}\), \(\beta\) represents the inclination angle in \({}^{\circ}\), and \(D_{Disk}\) represents the diameter of the pelletizing disk in m.  
\[n_{c}=\frac{1}{\pi}\sqrt{\frac{g\cdot sin(\beta)}{2\cdot D_{Disk}}} \tag{3}\]  
The laboratory pelletizing disk has a diameter of 0.4 m. Considering the previously set inclination angle of 52\({}^{\circ}\), the optimum rotation speed \(n_{opt}\) is estimated to be in the range between 35.6 and 44.5 min\({}^{-1}\). During our preliminary pelletizing runs, the rotation speed was set between about 35 and 40 min\({}^{-1}\). Within this range, no influence on the stability or on the shape of pellets could be identified. Figure 1: Schematic view of the pelletizing disk for the production of the storage composites (inspired by [34]), including the rotation speed (\(n\)), the disk angle (\(\beta\)), and the filling zone where water and powder mixture are added. As mentioned before, water dosing takes place alternately with the dosing powder mixture during the pelletizing process. In order to determine the amount of water that exists in the green pellets, the difference between the mass of the pellets immediately after being pelletized (wet green pellets) and after drying was measured. Thereby, the mass of the wet green pellets represents the 100% reference mass. The lowest amount of water by which pellets could still be formed was 18 wt.-%, while an amount of more than 33 wt.-% led to less stable and deformed wet pellets. Accordingly, pellets with a water amount between 18 and 33 wt.-% were produced and investigated. The last manufacturing step is the sintering of the dried samples to increase their mechanical strength. The sintering temperature was selected to be higher than the maximum process temperature during the redox reactions in order to minimize the possibility of having the samples being sintered during the redox reactions.