Ceria Doped with Zirconium and Lanthanide Oxides to Enhance Solar Thermochemical Production of Fuels

Abstract

Developing an efficient redox material is crucial for thermochemical cycles that produce solar fuels (e.g., H2 and CO), enabling a sustainable energy supply. In this study, the effects of varying the rare-earth content y in Ce0.85‑yZr0.15REyO2−0.5y with RE = Y, La, Sm and Gd on the fuel productivity and long-term stability were investigated. Compared to the none-RE-doped reference material, Ce0.85Zr0.15O2, none of the compositions exhibits higher performances. However, long-term cycling of more than 80 cycles reveals enhanced performance due to rare-earth doping. Ce0.85Zr0.15O2 suffers from linear degradation of the yields and of the CO:O2 ratio r, which is attributed to declining oxidation kinetics, whereas for instance Ce0.82Zr0.15Sm0.03O1.99 features stable yields and kinetics. The suggested rationale behind is found in a vacancy-depleted region that occurs in the grains of Ce0.85Zr0.15O2. While cycling, the specific surface decreases and the impact of these regions on the reaction rate increases which leads to declining oxidation kinetics. In contrast, Ce0.82Zr0.15Sm0.03O1.99 displays structural vacancies corresponding to the Sm3+, which remain during oxidation. Because of these structural vacancies the oxygen bulk transport is enhanced resulting in a constant reaction rate. Because of the long-term cycling, rare-earth doping is in particular beneficial for the oxidation kinetics and hence, important for the technical realization of the process.