A Reactive Fe-YSZ Coated Foam Device for Solar Two-Step Water Splitting

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.

Solar Thermochemical CO2 Splitting Utilizing a Reticulated Porous Ceria Redox System

“…Both the efficiency and the cycling rates in the solar reactor were limited largely by thermal losses, resulting from poor conductive and radiative heat transfer across the ceria structure. Several metal oxide structures and supports such as monolithic vertical pins, textured plates, foams, − 3D ordered porous structures, honeycombs, felts, and monolithic and lattice type structures , have been examined for solar thermochemical applications. Microporous structures with pore size in the μm range, such as monoliths or felts, display rapid oxidation rates thanks to their high specific surface areas but are limited by their heat transfer rates because of opacity to incident radiation, leading to undesired temperature gradients across the structure. , In contrast, macroporous structures with pore size in the millimeter range, such as foams and honeycombs, can achieve uniform heating thanks to deeper penetration and volumetric absorption of concentrated solar radiation.…”

Thermochemical CO2 splitting via redox cycling of ceria reticulated foam structures with dual-scale porosities

“…On the other hand, the second oxidation step with H 2 O or CO 2 has been shown to be largely surface dependent, and as such large specific surface area (SSA) is desired to enhance reaction kinetics. 19,20 However, the porous structure supporting the redox material generally features only one of the two desired properties: either high SSA [18][19][20][21] or low optical thicknesses, 13,[22][23][24][25][26] as the former comes at the expense of high radiative opacity while the latter comes at the expense of lower SSA.…”

“…In contrast, porous structures with void sizes in the mm-range, e.g. reticulated foams [22][23][24][25][26] or honeycombs, 13 can absorb incident thermal radiation more homogeneously. Because of their relatively low optical thicknesses, radiation can penetrate and be absorbed volumetrically, which in turn leads to more uniform temperature profiles across the structure thickness.…”

Nickel ferrite supported on calcium-stabilized zirconia for solar hydrogen production by two-step thermochemical water splitting

“…On the contrary, for the Ca-doped sample, 7% Fe is occupying Zr ion positions in the support lattice (table 2). As previously commented, this fact would enhance the activity and stability, by maximizing the active phase dispersion and by avoiding sintering of this lattice stabilized Fe cations [16], [17] . Since the size of Fe 3+ (rVIII = 0.78 Å) is slightly smaller than Zr 4+ (rVIII = 0.84 Å), we could expect a reduction of the lattice in comparison to F-Z sample, however this is not appreciable because the increment of the cell due to the introduction of Ca 2+ (rVIII = 1.12 Å)…”

“…It has been reported an improvement in hydrogen yield and ciclability using stabilized zirconia, by the addition of yttria (YSZ), that stabilizes the high temperature cubic phase during the cooling to room temperature [16] . The improvement in hydrogen production on ferrite/YSZ materials is associated with the redox transition of some Fe 2+ -Fe 3+ ions in the YSZ lattice, which alleviates the deactivation of the redox oxide due to sintering at a high-temperature [2], [17], [18] . When doped-ZrO2 stabilized in the cubic polymorph was used as support for NiFe2O4, the Fe 2+ ions from ferrite could be dispersed into the cubic zirconia lattice during the thermal reduction step.…”