Evaluation of nanoparticle zirconia supports in the thermochemical water splitting cycle over iron oxides

Abstract

Two-step solar thermochemical water splitting offers a viable solution for employing renewable energy pathways for production of high-purity hydrogen, which can be used in fuel cells or for the production of liquid fuels. One promising thermochemical cycle uses iron oxide as the reactive material to split water in a relatively simple two-step process. However, this cycle requires high temperatures, on the order of 1500 °C. Therefore, to increase stability and thermal shock resistance, iron oxide was deposited onto high-temperature-stable ceramic oxides, such as zirconia (ZrO2) and yttria-stabilized zirconia (YSZ). Nanoparticle zirconia samples (n-ZrO2 and n-YSZ) were used in this study to increase the interactions between the iron oxide and the zirconia, and the resulting materials were compared with an iron oxide supported on a commercially available porous zirconia support (p-ZrO2). After heat treatment, these FeOx/n-ZrO2 and FeOx/n-YSZ materials were surprisingly stable and yielded consistent hydrogen production over several cycles. The hydrogen yield per cycle for the FeOx/n-YSZ material was consistently higher for all iron loadings compared with the FeOx/n-ZrO2 materials, and they both yielded significantly more hydrogen compared to the FeOx supported on the commercial ZrO2 support. Compared with the FeOx/n-YSZ, the iron loading had a larger effect on the hydrogen production over n-ZrO2-supported materials. X-ray diffraction (XRD) data indicated that iron was incorporated into the unit cell lattice of the n-YSZ supported material, which alleviated some of the detrimental iron oxide sintering inherent at the high-temperatures of the thermochemical water splitting reaction, and resulted in a more consistent and higher hydrogen production compared with the FeOx/n-ZrO2 material.