Effect of the Partial Substitution of Fe by Ni on the Structure and Activity of Nanocrystalline NixFe3-xO4 Ferrites for Hydrogen Production by Two-Step Water-Splitting

The influence of the partial substitution of Fe by Ni in nanocrystalline NixFe3-xO4 ferrites (x = 0.25, 0.50, 0.75 and 1) on their structure and ability as redox compounds to produce hydrogen by two-step water-splitting thermochemical cycle was investigated. The physicochemical characterization of the NixFe3-xO4 ferrites samples revealed modifications in their structure, crystalline size and surface area with the incorporation of nickel into the ferrite lattice. The XRD analysis of NixFe3-xO3 ferrites showed a single crystalline cubic phase with a lattice parameter decreasing with increasing the amount of nickel, which indicates that the Ni(II) was occupying the Fe(II) site in the crystalline structure. The partial substitution of Fe by Ni in ferrites leads to an decrease in their crystalline size (from 25 to 21 nm) together with a concomitant increase in the external surface area of the samples (from 34 to 73 m(2)/g). It is observed that the Fe substitution by Ni in the ferrite changes the reduction behaviour and the resistance to sinter of the ferrite at high temperature. The modifications in the characteristics of the ferrites induced by the insertion of Ni directly affect their behaviour for the two-step thermochemical water splitting reaction. It was found that the increase in the partial substitution of Fe by Ni in the ferrites improves the two-step water-splitting reactivity, increasing the hydrogen produced (mmol per mol of ferrite) from 24.1 in the case of the Ni0.25Fe2.75O4 ferrite to 43.6 in the case of the NiFe2O4 ferrite. Characterization of fresh and used samples disclosed the key role of the surface characteristics and reactivity of the NixFe3-xO3 ferrites as concerns the achieved two-step water-splitting activities.