No 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. An iron-containing yttrium-stabilized zirconia (YSZ) or Fe-YSZ is a promising working redox material for the two-step water-splitting cycle. Fe2+-YSZ is formed by a high-temperature reaction between YSZ and Fe3O4 supported on the YSZ at 1400°C in an inert atmosphere. Fe2+-YSZ reacts with steam and generates hydrogen at 1000–1100°C to form Fe3+-YSZ that is reactivated by thermal reduction in a separate step at temperatures above 1400°C under an inert atmosphere. In the present study, ceramic foam coated with Fe-YSZ particles is examined as the thermochemical water-splitting device to be used in a solar directly irradiated receiver/reactor system. The Fe-YSZ particles were coated on an Mg-partially stabilized zirconia foam disk, and the foam device was tested during the two-step water-splitting cycle; this was performed alternately at temperatures between 1100°C and 1400°C. The foam device was irradiated by concentrated visible light from a sun simulator at a peak flux density of 925 kW/m2 and an average flux density of 415 kW/m2 (total power input on the surface of the foam was 0.296 kW) in a N2 gas stream; subsequently, it was reacted with steam at 1100°C while heating by an infrared furnace. Hydrogen successfully continued to be produced in the repeated cycles.

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. An iron-containing YSZ (Yttrium-Stabilized Zirconia) or Fe-YSZ is a promising working redox material for the two-step water splitting cycle. The Fe2+ YSZ is formed by a high-temperature reaction between YSZ, and Fe3O4 supported on the YSZ at 1400°C in an inert atmosphere. The Fe2+-YSZ reacts with steam and generate hydrogen at 1000–1100°C, to form Fe3+-YSZ that is re-activated by a thermal reduction in a separate step at temperatures above 1400°C under an inert atmosphere. In the present work, a ceramic foam coated with the Fe-YSZ particles is examined as the thermochemical water splitting device for use in a solardirectly-irradiated receiver/reactor system. The Fe-YSZ particles were coated on an Mg-partially-stabilized zirconia foam disk and the foam device was tested on the two-step water splitting cycle being performed alternately at temperatures between 1100 and 1400°C. The foam device was irradiated by concentrated visible light from a sun-simulator at the peak flux density of 1000 kW/m2 and the average flux density of 470 kW/m2 in a N2 gas stream, and then, was reacted with steam at 1100°C while heating by an infrared furnace. Hydrogen successfully continued to be produced in the repeated cycles.

A two-step water-splitting thermochemical cycle using redox working material of iron-based oxide (ferrite) particles has been developed for converting solar energy into hydrogen. The two-step thermochemical cycle for producing a solar hydrogen from water requires a development of a high temperature solar-specific receiver-reactor operating at 1000–1500°C. In the present work, ferrite-loaded ceramic foams with a high porosity (7 cells per linear inch) were prepared as a water splitting device by applying ferrite/zirconia particles on a MgO-partially stabilized Zirconia (MPSZ) ceramic foam. The water splitting foam device was prepared using a new method of spin coating. A spin coating method we newly employed that has advantages of shortening preparation period and reducing of the coating process in comparison to previous preparation method reported. The water-splitting foam devices, thus prepared, were examined on hydrogen productivity and reactivity through a two-step thermochemical cycle. NiFe2O4/m-ZrO2/MPSZ and Fe3O4/c-YSZ/MPSZ foam devices were firstly tested for thermal reduction of ferrite using the laboratory scale receiver-reactor by a sun-simulator to simulate concentrated solar radiation. Subsequently, with another quartz reactor the light-irradiated device was reacted with steam by infrared furnace. As a result, it was possible to perform cyclic reactions over several times and to produce hydrogen through thermal-reduction at 1500°C and water-decomposition at 1100–1200°C. In further experiments, the NiFe2O4/m-ZrO2/MPSZ foam device was successfully demonstrated in a windowed single reactor for cyclic hydrogen production by solar-simulated Xebeam irradiation with input power of 1 kW. The NiFe2O4/m-ZrO2/MPSZ foam device produced hydrogen of 70–190μmol per gram of device through 6 cycles and reached ferrite conversion of 60% at a maximum.