## 3 Results and Discussion
### Structural Investigations
However, the storage material of the samples is nearly homogeneously distributed without agglomerations, such as in sample P1. The average porosity is about 35% (cf. Table 3). This suggests that increasing the added water amount during the pelletizing process from about 20 to 27 wt.-% does have a positive effect on the sample's structure. Figure 6: (**a**) Sample P1 (wc = 18 wt.-%) after the 6 th cycle, (**b**) sample P2 (wc = 20 wt.-%) after the 6 th cycle, (**c**) sample P3 (wc = 21 wt.-%) after the 9 th cycle. Micro-computed tomography scans of samples P1, P3, and P2 after several redox cycles. The \(\mu\)CT scans of samples P1 and P2 exhibit separated spherical ares with material accumulations and large cavities in the interior part. Sample P3 shows comparably smaller accumulations. After six (P1, P2) or nine (P3) redox cycles, the formation of a dense external shell can be observed. In addition to the \(\upmu\)CT scans after six or nine cycles, the structures of the samples P2, P5, and P8 were investigated right after the sintering phase and before the start of the redox cycling tests. The obtained results are depicted in (Figure 9). In Figure 9a, the formation of a dense outer shell and the agglomerated internal structure of sample P2 is already visible. This indicates that these pellets were formed primarily by the coalescence of compressed agglomerates. A reason for this could be the small amount of added water, since agglomerates with a low moisture content are comparatively more brittle, leading to more frequent breakage and abrasion [32; 34]. With further cycling, the cavities in the internal structure expand (cf. Figures 6b and 9a), which is assumed to take place because of the increasing sintering effects of iron/iron oxide during the repeated redox cycles. Figure 9b demonstrates that an increased water content has a positive effect on the distribution of storage material and small pores during the pelletizing process. Figure 9c illustrates the initial structure of sample P8 with the highest added water amount of 33 wt-%. Compared to sample P2 and P5, sample P8 shows a much higher porosity, most specifically in the outermost layers. The results indicate that a higher open porosity can be generated by adding a larger amount of water during the pelletizing process. Indeed, all investigated samples have shown visible aging effects. Compared to the \(\upmu\)CT images before the redox cycling, a more compact structure is formed after few redox cycles, especially in the outermost layers. The sintering effect due to the execution of the redox cycles at 800 \({}^{\circ}\)C can also be determined when comparing the sample volume of the initial (\(V_{start}\)) and the re-oxidized (\(V_{rodx}\)) samples. In fact, the volume of sample P2 is reduced from \(V_{start_{2}}=630\) mm\({}^{3}\) down to \(V_{rodx_{P2}}=387\) mm\({}^{3}\), while the sample P5 contracts from \(V_{start_{P5}}=1064\) mm\({}^{3}\) to \(V_{rodx_{P5}}=452\) mm\({}^{3}\). Considering the sample P8, it is reduced from \(V_{start_{2}}=1180\) mm\({}^{3}\) to \(V_{rodx_{P8}}=571\) mm\({}^{3}\) after nine redox cycles. Moreover, it is noticeable that the cracks within the sample structure are mostly in the shape of a shell, which can be attributed to the step-wise agglomeration of the storage material in layers during the applied pelletizing process. The obtained results indicate that increasing the amount of added water during the pelletizing process results in a more homogeneous distribution and a higher open porosity of the produced pellet samples. This promotes faster reaction kinetics and an increased gas permeability [30; 39; 40]. Figure 9: (**a**) Sample P2 (wc = 20 wt-%) after sintering. (**b**) sample P5 (wc = 26 wt-%) after sintering. (**c**) sample P8 (wc = 33 wt-%) after sintering. Micro-computed tomography scans of the samples P2, P5, and P8 after sintering. The structure of sample P2 displays a dense external shell and the formation of accumulations in its interior already after the sintering process.