Abstract
Improvement in the reduction kinetics and stabilities resulting in longer lifetimes are required for practical application of energy conversion and storage systems using redox reaction of metal oxides such as chemical looping systems. It has been previously reported that using oxide ion conductors as supports can improve the redox reactions of metal oxides. To evaluate which physical properties (such as oxide ion transport and electron transport) can affect the redox reaction kinetics of Fe2O3, reduction kinetic analyses of Fe2O3 reduction by hydrogen were performed. The experiments were conducted using various supports such as CaTi1–xFexO3 (CTFO, as a mixed ionic and electronic conductor), yttlia-stabilized zirconia (YSZ, as a pure oxide ionic conductor) and Al2O3 as a reference (insulator). The results showed that reduction of Fe2O3 with CTFO or YSZ as a support was greatly improved compared with Al2O3. Oxidation measurements of the Fe/supports by water vapor, i.e. hydrogen production by a steam-iron reaction, showed that CTFO was highly effective at increasing oxidation. Modeling analyses based on the oxygen chemical potential distributions of the Fe2O3/supports were also performed to analyze the effect of the supports quantitatively. Calculations assuming high electron and oxygen transport at the interface supported the experimental results. The results suggest that the transport properties at the interface were derived from a highly conducting phase and that oxide ionic conductivity is the predominant factor in the improvement. Sensitivity analysis using various parameters was also conducted and the mechanism for the improvements was discussed.
