Iron oxide redox reaction with oxide ion conducting supports for hydrogen production and storage systems

Improvement of redox reaction kinetics is important for the application of metal oxide redox reactions in energy conversion systems such as chemical looping systems and hydrogen storage. In this study, we focused on physical properties in supports that can affect the redox reaction kinetics of metal oxides. The redox reaction of iron oxide by hydrogen and water vapor was studied with various support materials, ZrO2, CeO2, yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC), which have different physical properties such as oxide ion and electronic conductivities. Oxide ion conductors such as YSZ and GDC clearly increased the rate of Fe2O3 reduction (Fe2O3→Fe3O4→FeO→Fe). This result suggests that oxygen vacancies and good oxide ion transport properties in the supports may enhance removal of oxygen from iron oxide. In addition, CeO2 and GDC significantly improved the reduction kinetics in the reduction step from FeO to Fe. Furthermore, it was found that these supports have an effect on the oxidation reaction kinetics of iron by water vapor in comparison with ZrO2; that is, they increase the rate of the steam-iron reaction for hydrogen production. The mechanism of the improvement was discussed in terms of oxide ion and electronic conductivity in the supports and water dissociation on the support surfaces.