Abstract
<jats:p>Iron is a promising energy carrier for sustainable energy storage via hydrogen-based reduction of iron oxides and energy release through oxidation with air. While it is well known that the cycling of iron powders affect their morphology and reactivity, the associated structural changes and the reaction mechanism remain poorly understood. Here, we investigate the solid-state cycling of iron oxide powder using complementary in situ techniques: synchrotron based Mössbauer spectroscopy, quick scanning X-ray absorption spectroscopy, and time-resolved synchrotron X-ray diffraction. With a time resolution better than 1 min, compositional changes during three consecutive oxidation–reduction cycles were monitored both with respect to changes in the oxidation state of iron and the transformation of the corresponding crystalline phases. During isothermal cycling at 560 °C, the rate of iron oxidation increased with each cycle number, whereas the corresponding reduction rates to metallic iron decreased. The oxidation proceeded initially via Fe → Fe3O4 → Fe2O3 and then involving wustite as reaction intermediate Fe → FeO → Fe3O4 → Fe2O3. Reduction occurred exclusively through Fe2O3 → Fe3O4 → FeO → Fe, though the observed reaction behavior changed in each cycle between initial and advanced reduction phase.</jats:p>