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+).

Graphical abstract(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 full-size imageDownload as PowerPoint slide