## 3 Results
### Effect of the Process Temperature  
Another important parameter for controlling the energy storage process is the process temperature. Figure 8 presents the temporal reduction and oxidation fractions of sample S1, which were measured at the process temperatures of 700 \({}^{\circ}\)C, 800 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C applying a reactive gas mixture flow rate of 250 mL/min and the reactant gas concentration ro4 according to Table 2.  
As the oxidation is an exothermic reaction, it is expected that enhancing the reaction temperature would imply slowing down the reaction kinetics, which is indeed observed in Figure 8b. Increasing the reaction temperature of S1 from 700 \({}^{\circ}\)C to 800 \({}^{\circ}\)C leads to an increase in the time needed for the oxidation fraction to reach \(\gamma_{\text{Ox}}=0.8\) from 23 to 51 min (221.7%).  
It can be concluded that the reduction reaction gets faster with increasing the reaction temperature, which is common for such an endothermic reaction [43; 48; 49]. To reach a reduction fraction of \(\gamma_{\text{Re}}=0.2\) for the investigated S1 pellet (Figure 8a), the reaction time  
Figure 8: Effect of the reaction temperature on the temporal reduction (**a**) and oxidation (**b**) fractions of S1 at a reactant flow rate of 250 mL/min and the gas concentration ro4 as defined in Table 2; T7 = reaction temperature 700 \({}^{\circ}\)C, T8 = reaction temperature 800 \({}^{\circ}\)C, T9 = reaction temperature 900 \({}^{\circ}\)C, Cycle 1, T9-2 = reaction temperature 900 \({}^{\circ}\)C, Cycle 2.  
Figure 7: Effect of the pellet size of S1 (\(d_{\text{sl}}\approx 12.7\) mm) and S2 (\(d_{\text{s2}}\approx 9.8\) mm) on the temporal reduction (-) and oxidation (- -) fractions at a reactant flow rate of 250 mL/min and the gas concentration ro4 (cf. Table 2).  
is reduced by 9.4% (from 34 to 30.8 min) by increasing the temperature from 700 \({}^{\circ}\)C to 800 \({}^{\circ}\)C. Increasing the temperature further to 900 \({}^{\circ}\)C results in additional acceleration of the reduction reaction by 25.3% (from 30.8 to 23 min). Figure 8b presents the effect of the temperature on the temporal change of the oxidation fraction \(\gamma_{\text{Ox}}\) for the sample S1. The first oxidation run at 900 \({}^{\circ}\)C (T9) follows the same trend, namely a further increase of the time to reach 80% of the final oxidation fraction by 153.5%. Repeating the oxidation reaction at 900 \({}^{\circ}\)C (T9-2) shows, however, a substantial degradation in the oxidation performance of the sample S1. Recalling that the oxidation is an exothermic reaction, it is obvious that a partial sintering of S1 took place, which defines 900 \({}^{\circ}\)C as a limiting temperature for oxidation reactions. Dedicated oxidation runs follow to exactly define the upper temperature limit for oxidation operation, in order to avoid such performance deterioration.  
Figure 9 presents a comparison between the temporal redox behavior of sample S1 in Cycles 2 and 29 at a reactant gas flow rate of 250 mL/min, a temperature of 700 \({}^{\circ}\)C, and the gas concentration no4 (cf. Table 2). The relative mass change and the time needed to reach 100% reduction or oxidation were not changed between the two presented redox cycles. Moreover, a slight increase in the redox kinetics of about 5% was observed to reach 80% of the final redox performance. This increase results from the structural change of the pellet due to the repeated redox reactions, which was observed through the uCT analysis carried out on the raw and the investigated S1-pellet after 29 redox cycles, which will be discussed in the following subsection.