Two-Step Water Splitting Using Mixed-Metal Ferrites: Thermodynamic Analysis and Characterization of Synthesized Materials

Abstract

We report a comprehensive thermodynamic analysis of thermal oxidation−reduction cycles for producing hydrogen that use metal ferrites with the spinel structure (MFe2O4; M = Fe, Co, Ni, and Zn) as the redox material. Solution phases (both solid and liquid) were included in the calculations as well as the expected line compounds. Omitting solution phases, whose existence is experimentally well-documented, has a very significant impact upon the results of the calculations. Thermodynamic modeling of the three important material-related aspects of the process was performed, including synthesis of the ferrite materials from bulk oxides, thermal reduction at high temperatures, and reoxidation by reaction with steam. An experimental investigation of the NixFe3−xO4 system was performed to provide compositional data for comparison to model predictions. The results indicate that the Fe/Ni ratio, thermal reduction reaction kinetics, and the specifics of the cooling process affect the composition of the synthesized ma...