Enhanced hydrogen production by doping Pr into Ce0.9Hf0.1O2 for thermochemical two-step water-splitting cycle

We synthesized (Ce_0.9Hf_0.1)_1−xPr_xO_2−δ (x=0, 0.05 and 0.1) using the polymerized complex method. The synthesized samples, as well as the samples after thermochemical two-step water-splitting cycles have a fluorite structure and Pr exists in the solid solutions with both trivalent and tetravalent states, as suggested by powder X-ray Diffraction (XRD) Patterns. The reduction fraction of Ce⁴⁺ in redox cycles (oxidation step in air) and two-step water-splitting cycles (oxidation step in steam) indicates that the addition of Pr into Ce–Hf oxide solid solution cannot improve the reduction fraction of Ce⁴⁺ during the redox cycles but both the reduction fraction of Ce⁴⁺ and H₂ yield are significantly enhanced during two-step water-splitting cycles. The chemical composition of 10 mol% Pr doped Ce_0.9Hf_0.1O₂ exhibits the highest reactivity for hydrogen production in H₂-generation step by yielding an average amount of 5.72 ml g⁻¹ hydrogen gas, which is much higher than that evolved by Ce_0.9Hf_0.1O₂ (4.50 ml g⁻¹). The enhancement effect of doping Pr on the performance during two-step water-splitting cycles is because of the multivalent properties of Pr, which can: (1) reduce the amount of Ce³⁺ oxidized by contamination air (contamination air eliminated by partial oxidation of Pr³⁺ to Pr⁴⁺) in H₂-generation step; (2) enhance the reaction rate in H₂-generation step by improving the ionic conductivity (extrinsic oxygen vacancies created by the substitution of Ce⁴⁺ by Pr³⁺).