Reactivity of Iron-Containing YSZ for a Two-Step Thermochemical Water Splitting Using Thermal Reduction Temperatures of 1400–1500°C

Abstract

A thermochemical two-step water splitting cycle using a redox system of iron-based oxides or ferrites is one of the promising processes for converting solar energy into clean hydrogen in sunbelt regions. Fe3O4 supported on YSZ (Yttrium-Stabilized Zirconia) or Fe3O4/YSZ is a promising working material for the two-step water splitting cycle. In the water splitting cycle, an iron-containing YSZ or Fe2+-YSZ is formed by a high-temperature reaction between Fe3O4 and YSZ support at 1400°C in an inert atmosphere. The Fe2+-YSZ reacts with steam and generate hydrogen at 1000°C, to form Fe3+-YSZ that is re-activated by a thermal reduction in a separate step at 1400°C under an inert atmosphere. In the present work, the thermal reduction was performed in a higher temperature range of 1400–1500°C while the hydrolysis reaction was carried out at 1000°C. It was confirmed by XRD analysis that the cyclic redox reactions occurred based on the same reaction mechanism when using a thermal reduction temperature between 1400 and 1500°C. The conversions of Fe3O4 to Fe2+-YSZ were 20, 26 and 47% when the thermal reduction temperature were 1400, 1450, and 1500°C respectively, indicating that the x values in the formed Fe2+-YSZ or Fex2+YyZr1−yO2−y/2+x were 0.08, 0.11, and 0.19 respectively, where y = 0.15. The conversions of Fe2+-YSZ to Fe3+-YSZ in the hydrolysis reaction (at 1000°C), however, decreased from 90% to 60% when the thermal reduction temperature increased from 1400 to 1500°C. As the results, the hydrogen production reactivity of Fe3O4 supported on YSZ increased from 5.6 × 10−4 to 7.5 × 10−4 g per gram of Fe3O4/YSZ for one cycle on the cycle average by elevated thermal reduction temperature from 1400 to 1500°C.