Reactivity of CeO2-based ceramics for solar hydrogen production via a two-step water-splitting cycle with concentrated solar energy

Ce0.9M0.1O2−δ ceramics (M = Mg, Ca, Sr, Sc, Y, Dy, Zr and Hf) were synthesized by a polymerized complex method. X-ray powder diffraction (XRD) patterns indicate that solid solutions with a fluorite structure were formed after the synthesis, and this structure was retained after redox cycles. An analysis of the redox cycles using a direct gas mass spectrometer (DGMS) suggests that the reactivity of CeO2-based ceramics in the O2-releasing step could be enhanced by doping the ceramics with cations with a higher valence and a smaller effective ionic radius. The investigation of two-step water-splitting cycles indicates that the amount of H2 evolved in the H2-generation step is dominated by the amount of O2 (Ce3+) evolved in the O2-releasing step. Electrochemical impedance spectroscopy (EIS) investigations show that the higher bulk conductivity of CeO2-based ceramics at intermediate temperatures could promote reactivity by enhancing the molar ratio of H2–O2 that is evolved during the two-step water-splitting cycles. The highest reactivity, both in the redox and in the two-step water-splitting cycles, is exhibited by Ce0.9Hf0.1O2.