Reactive Ceramics of Ce_xSc_1−xO_2−Δ for Solar Hydrogen Production by Two-Step Water Splitting

This paper describes the enhancement of the H₂-generation reactivity by Sc doping in the CeO₂. The evolved O₂ gas (3.48ml/g) and H₂ gas (7.00ml/g) for Ce_0.97Sc_0.03O_1.985 are higher than those of Ce_0.9Zr_0.1O₂, indicating that Sc-doping enhances the two step water splitting reactivity compared to the Zr-doping. The molar ratio of the H₂ and O₂ gases is nearly two, which suggests nearly completion of each step reaction for the H₂O decomposition. The O₂-releasing reactivity of Sc-doped ceria (Ce_0.97Sc_0.03O_1.985) is nearly the same as that of Zr-doped one (Ce_0.9Zr_0.1O₂), whereas the H₂-generation reactivity is much different due to improvement of the H₂-generation reactivity by Sc-doping. To examine the reason for the improvement, the ionic conductivity of Sc-doped and Zr-doped ceria has been measured with AC impedance spectroscopy. The activation energies of bulk conduction estimated from the impedance measurement are 1.09eV (Sc-doped) and 1.48eV (Zr-doped), respectively, in the temperature range of 400–800°C. This indicates that the oxygen deficient site movement of the Sc-doped ceria is more active than Zr-doped ceria. It could be said that the anion site can move more readily toward the surface of the ceramics by Sc-doping. This would facilitate the oxygen storage process (the H₂-generation reaction).