## 4 Conclusions  
In this work, the influence of the amount of water added during the pelletizing process on the redox cycle stability and on the reaction kinetics of iron oxide pellets was studied. To this aim, the water content was varied between wc = 18 to 33 wt.-%. After sintering at 950 \({}^{\circ}\)C, the reaction kinetics of the repeated reduction and oxidation at 800 \({}^{\circ}\)C was investigated. Furthermore, the micro-structure and the open porosity of the produced pellet samples were determined.  
The obtained results indicate a direct correlation between the added water amount, the structure of the pellets, and their reaction kinetics. Pellets with a higher water content tend to have faster kinetics and a higher cycle stability. Considering the sixth redox cycle, sample P1 with the lowest water content of 18 wt.-% needed about 26 min and 19 min to reach a conversion rate of \(\alpha\) = 0.8 during the reduction and oxidation, respectively. In contrast, sample P10 with the highest water content of 33 wt.-% could achieve a similar conversion rate after only 18 min (reduction) and 13 min (oxidation) in the ninth cycle.  
Micro-computed tomography scans before and after the redox cycling showed that samples with lower water contents are subjected to a more agglomerated internal structure with a dense outermost shell. In addition, sample P1 (wc = 18 wt.-%) showed the lowest open porosity of \(\Phi_{P1}\approx 23\%\), compared to samples P8 and P10 (wc = 33 wt.-%) with the highest open porosities of \(\Phi_{P8}\approx 46\%\) and \(\Phi_{P10}\approx 45\%\), respectively. Furthermore, increasing the added water content resulted in a more homogeneously distributed pore structure. A homogeneous distributed pore structure and a higher porosity are associated with higher reaction kinetics and better cycle stability due to an increased specific reaction surface and gas permeability. Indeed, more development work is needed to approve the cycle stability over more redox cycles and to investigate the mechanical stability of the iron oxide pellets.  
Conceptualization, L.H.; methodology, L.H.; software, L.H. and M.H.; validation, L.H., M.H. and B.D.; formal analysis, L.H. and M.H.; investigation, L.H. and M.H.; resources, L.H., B.D. and A.P.; data curation, L.H. and M.H.; writing--original draft preparation, L.H.; writing--review and editing, M.H, B.D., A.P. and M.S.; visualization, L.H. and M.H.; supervision, B.D. and A.P. All authors have read and agreed to the published version of the manuscript.  
The first author would like to acknowledge the funding received by the Nagelschneider Foundation.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
Both first and second authors would like to acknowledge the Regensburg Center of Energy and Resources (RCER) of OTH Regensburg. In addition, the authors would like to gratefully thank both the Regensburg Center of Biomedical Engineering (RCBE) for the support through the use of their uCT facility and the Laboratory for Geotechnics of OTH Regensburg for the received support through the use of their He pycnometer facility.  
The authors declare no conflict of interest.