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

• We focus on (Ce0.9Hf0.1)1−xPrxO2−δ for H2 production via two-step water splitting cycle.
• The enhancement on H2 production of Ce–Hf oxide by doping Pr was confirmed.
• It is because of the multivalent properties of Pr.
• It can reduce the influence of contamination air and enhance the reaction rate.
• (Ce0.9Hf0.1)0.9Pr0.1O2−δ yields the highest H2 yield.

We synthesized (Ce0.9Hf0.1)1−xPrxO2−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 Ce0.9Hf0.1O2 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 Ce0.9Hf0.1O2 (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+).

(a) Online DGMS profiles of O2 and H2 for (Ce0.9Hf0.1P)0.9Pr0.1O2−δ during two-step water-splitting cycles (O2-releasing step was done in Ar and oxidation (H2-generation) step in steam) and (b) temperature program of two-step water-splitting cycles.Figure optionsDownload as PowerPoint slide