*3.1. SrFeO3*−<sup>δ</sup> *Preparation and Characterization*

SrFeO3−<sup>δ</sup> perovskite-type metal oxide was prepared using solid-state synthesis for use as a CO2 decomposition material. The metal oxide was purchased from K-ceracell Co. Ltd. (Taejeon, Korea), and SrCO3 (Alfa Aesar, >99%, Ward Hill, MA, USA) and Fe2O3 (Alfa Aesar, >99.9%, Ward Hill, MA, USA) were used as starting materials. After weighing the appropriate amount of materials, the powders

were mixed in ethanol (Samchun Chemicals, >99.9%, Seoul, Korea). The mixed powder was ball-milled with zirconia balls (ϕ3–5 mm) for 48 h and then dried to remove the solvent. SrFeO3−<sup>δ</sup> powder was heated at 1000 ◦C for 3 h in ambient air. The final powders were obtained by ball-milling with ethanol for 24 h and then drying in an oven at 80 ◦C for 24 h.

The synthesized powders were characterized using X-ray powder diffraction (XRD, Rigaku D/MAX-2500, Tokyo, Japan), scanning electron microscopy (SEM, S-4800 Hitachi, Hitachi, Japan), and energy-dispersive X-ray spectroscopy (EDS, Thermo Scientific, Waltham, MA, USA) to analyze the phase structure and purity, surface morphology, and chemical composition, respectively. In-situ XRD (Rigaku D/MAX-2500, Tokyo, Japan) and thermogravimetric analysis (TGA, TA Instruments, Milford, MA, USA) were used to investigate the reduction behaviors of the synthesized powder. In-situ XRD and the change in weight were measured with <sup>≈</sup>100 mL min−<sup>1</sup> of 3.5 vol% H2/Ar.
