*2.3. E*ff*ect of Conductivity on CO2 Decomposition*

As we suggested a mechanism in our previous report, CO2 decomposition is considered to be affected significantly by the conductivity of the sample [17]. To decompose CO2 effectively, the metals in the ferrite should easily provide electrons for CO2; therefore, the electronic conductivity plays an important role at first. As the oxygen in neutral CO2 has sufficient electrons, electron transfer from metals on the surface of the ferrite to CO2 is not likely to occur naturally. Therefore, the activation process should be performed before exposure to CO2, and the produced oxygen vacancies become the driving force of the redox reaction. Based on such reasoning, the amount of oxygen vacancies could be the most important factor in this reaction. In addition, the activation process would be easier if the metal oxide contained metals with variable oxidation number.

Once the oxygen ions (i.e., O2<sup>−</sup>) fill the oxygen vacant sites on the sample surface, the ability to decompose CO2 to CO or C is lost owing to the saturation (or deactivation) of the sample surface. However, if oxygen ions migrate well inside the lattice and oxygen vacancies are reformed on the sample surface, CO2 decomposition could be continued until all vacancies are filled. This is why oxygen ionic conductivity should also be considered. It would be interesting to determine how much of a role oxygen ion conductivity plays in the decomposition of carbon dioxide; however, this will be reported in a separate paper. After all, the oxygen ions accepted through the electronic conductivity effect could move to inside defects via oxygen ionic conducting properties. Therefore, samples with good electrical and ionic conductivity would decompose CO2 more effectively. The total conductivity of SrFeO3−<sup>δ</sup> is 31.6 S cm−<sup>1</sup> at 800 ◦C [32], which is higher than that of SrFeCo0.5Ox (17 S cm−1) [33]. The total conductivity of SrFeO3−<sup>δ</sup> shows good agreement with the reference value, and it was determined to be 33.9 S cm−<sup>1</sup> at 800 ◦C from our own measurement. This feature was the reason that SrFeO3−<sup>δ</sup> was selected for the CO2 decomposition experiment in this paper.
