## 3 Results and Discussion
### Structural Investigations  
In the flowing \(\upmu\)CT scans, the areas with no storage material (only air) are marked in black. Gray areas represent the material of the storage samples. The brightness of the parts is an indicator of their density [36]. For iron oxide samples, a higher brightness can be attributed to areas with a more dense structure or material with a higher density, e.g., iron compared to iron oxide particles. Due to the fully oxidized state of the investigated samples, it is assumed that areas with higher brightness are areas with a more dense structure. Through \(\upmu\)CT scans, the internal macroscopic structure of the samples can be obtained without the necessity of destroying them, which makes it possible to study the sample structure before and after cycling. Figure 5: (**a**) Reduction at 800 \({}^{\circ}\)C. (**b**) Oxidation at 800 \({}^{\circ}\)C. Measured conversion rates over time of the samples P2, P3, P6, and P9 with different water contents during the reduction and oxidation reactions in the 2 nd, 6 th, or 9 th redox cycle at 800 \({}^{\circ}\)C. The dashed horizontal lines mark the conversion rates of \(\alpha_{red}\) and \(\alpha_{ox}=0.8\) in each diagram. Comparing the conversion rates of the different samples shows that with increased water content, the samples attain a higher conversion rate and a higher cycle stability. Despite the similar water content of samples P2 (wc = 20 wt.-%) and P3 (wc = 21 wt.-%), sample P3 achieved a clearly higher conversion rate and cycling stability. In Figure 6, the \(\mu\)CT images of the samples with the lowest water contents (\(\leq\)21 wt.-%), namely P1 and P2 after the sixth redox cycle, as well as P3 after the ninth cycle, are presented. In particular, the internal structure of sample P1 (wc = 18 wt.-%; Figure 6a) exhibits separated spherical areas with material accumulations (\(\approx\) 0.1-1.8 mm) and cavities in between. On the other hand, the outermost shell appears as a continuous dense layer. Sample P2 shows also an agglomerated internal structure (Figure 6b). In contrast to sample P1, a higher amount of smaller agglomerates with a diameter of about 0.3 mm can be recognized. Accumulations up to about 1.3 mm can also be observed. Compared to sample P1 and P2, sample P3 shows less and smaller material accumulations \(\leq\) 0.3 mm, which appear around a porous core (see Figure 6c). However, a dense exterior shell also forms after the ninth redox cycle. The obtained results indicate that the agglomerated structure and the formation of an outward dense layer lead to the presented slowdown of the reaction kinetics. This is caused by the correlated less effective gas transport of H\({}_{2}\) or H\({}_{2}\)O towards the sample structure during the reduction and oxidation reactions, respectively. These results are in line with our previously published results [30] as well as [15; 37]. To verify the suggestion of a structure with a higher density, the open porosity \(\Phi\) of the investigated samples was determined according to Equation (6). This measuring campaign resulted in an open porosity of about 23% (P1: wc = 18 wt.-%), 25% (P2: wc = 20 wt.-%) and 31% (P3: wc = 21 wt.-%). Compared to the open porosity of the other samples, the named three samples acquire the lowest porosities (cf. Table 3). Furthermore, sample P3 after the ninth redox cycle does record a higher porosity than samples P1 and P2 after six cycles. This is consistent with the observed higher kinetics and cycle stability of sample P3. The results of the \(\mu\)CT scans in combination with the determined porosity indicate that the cavities inside the internal structure of samples P1 and P2 are mostly closed and barely contribute to the enhancement of the reaction kinetics [38]. Figure 7 illustrates the \(\mu\)CT images of the samples P5 and P7 after nine redox cycles. The structure also shows internal cracks and the formation of an external shell with a higher density compared to the interior.