Solar hydrogen production by two-step thermochemical cycles: Evaluation of the activity of commercial ferrites

In this work, we report on the evaluation of the activity of commercially available ferrites with different compositions, NiFe2O4, Ni0.5Zn0.5Fe2O4, ZnFe2O4, Cu0.5Zn0.5Fe2O4 and CuFe2O4, for hydrogen production by two-step thermochemical cycles, as a preliminary study for solar energy driven water splitting processes. The samples were acquired from Sigma–Aldrich, and are mainly composed of a spinel crystalline phase. The net hydrogen production after the first reduction–oxidation cycle decreases in the order NiFe2O4 > Ni0.5Zn0.5Fe2O4 > ZnFe2O4 > Cu0.5Zn0.5Fe2O4 > CuFe2O4, and so does the H2/O2 molar ratio, which is regarded as an indicator of potential cyclability. Considering these results, the nickel ferrite has been selected for longer term studies of thermochemical cycles. The results of four cycles with this ferrite show that the H2/O2 molar ratio of every two steps increases with the number of cycles, being the total amount stoichiometric regarding the water splitting reaction. The possible use of this nickel ferrite as a standard material for the comparison of results is proposed.