**4. Conclusions**

We demonstrated that SrFeO3−<sup>δ</sup> could serve as a promising material for effective CO2 decomposition in a continuous gas-flow system. In this study, we performed three categorized CO2 treatment experiments using SrFeO3−δ: nonisothermal, isothermal, and stability tests. In nonisothermal experiments, the maximum CO2 decomposition rate reached ~90% and the decomposition rate (≥80%) lasted for around 170 min. Both CO2 decomposition and CO generation by SrFeO3−<sup>δ</sup> exhibited better performance than those of NiFe2O3−<sup>δ</sup> or SrFeCo0.5Ox. The results of isothermal tests indicated that the optimized temperature for CO2 decomposition should range between 650 and 700 ◦C. As the operating temperature increased from 625 to 650 ◦C, the amount of CO2 decomposition increased unusually, even though no special structural phase or weight changes occurred. In addition, SrFeO3−<sup>δ</sup> maintained the CO2 decomposition efficiency during isothermal cyclic and reproducibility experiments. Therefore, SrFeO3−<sup>δ</sup> is expected to have the capacity to contribute to the mitigation of CO2, a greenhouse gas. Nonetheless, studies of CO2 utilization should be continued because many unsolved problems remain in this field.

**Author Contributions:** Conceptualization, C.Y.P. and S.-C.N.; methodology, J.S.; investigation, S.-H.K. and J.-Y.K.; data curation, C.Y.P. and J.S.; writing—original draft preparation, J.S.; writing—review and editing, K.B.L. and C.Y.P.; supervision, C.Y.P.; project administration, S.-C.N.; funding acquisition, S.-C.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to acknowledge the financial support of the National Research Foundation under the "Next Generation Carbon Upcycling Project" (Project No. 2017M1A2A2043109) of the Ministry of Science and ICT, Republic of Korea.

**Acknowledgments:** The analytical support from the Platform Technology Laboratory at Korea Institute of Energy Research is much appreciated, especially the invaluable assistance provided for in-situ XRD and surface analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
