Chemical equilibria involved in the oxygen-releasing step of manganese ferrite water-splitting thermochemical cycle

Sodium ferrimanganite carbonatation reaction was investigated at different temperatures/carbon dioxide partial pressures to evaluate the feasibility of the thermochemical water-splitting cycle based on the MnFe2O4/Na2CO3/Na(Mn1/3Fe2/3)O2 system. After thermal treatments in selected experimental conditions, the obtained powder samples were investigated by using the X-ray diffraction (XRD) technique and Rietveld analysis. Two different lamellar Na1−xMn1/3Fe2/3O2−δ phases were observed together with the expected MnFe2O4/Na2CO3 mixture. Different equilibrium regions among sodium-depleted lamellar phases, manganese ferrite and sodium carbonate were found as a function of the different reaction conditions. A hypothesis concerning the regeneration mechanism of the initial compounds is proposed. Chemical equilibrium between stoichiometric and sub-stoichiometric forms of sodium ferrimanganite and sodium carbonate formation/dissociation appears to be essential factors governing the oxygen-releasing step of the manganese ferrite thermochemical cycle.