Effect of atomic substitution on the sodium manganese ferrite thermochemical cycle for hydrogen production

This work presents the effect of atomic substitution on the MnFe2O4-Na2CO3 thermochemical cycle for H-2 production. The non-oxidative decarbonation/carbonation reaction of the MnFe2O4-Na2CO3 mixture is investigated as the starting reference. Repeated cycling results in a 30% loss of reversibility due to an overall reduction of the reactive interfaces. The substitution of Na2CO3 for Li2CO3 decreases the decarbonation onset temperature by about 100 degrees C, but almost no reversibility is observed during the cycles due to the irreversible Li+ intercalation. The effect of partial Mn substitution for Ca, Ni, and Zn is presented. The 5% Zn mixture shows the best decarbonation/carbonation reversibility and is tested for H-2 production together with MnFe2O4-Na2CO3. The reference mixture produces more H-2 during the first cycle (asymptotic to 1.1 vs. 0.7 mmol/g), but its production drastically drops by two orders of magnitude upon cycling and becomes negligible after 5 cycles. By contrast, the Zn-doped mixture exhibits a stable H-2 production of 0.22 mmol/g with no decreasing trend observed from cycle 2 to cycle 5. As result, in the fifth cycle, the Zn-doped mixture produces 23 times more H-2 than MnFe2O4-Na2CO3. Thermogravimetry and X-ray diffraction confirm that doping with Zn significantly improves the regeneration of the reactants. (C) 2022 The Author(s). Published by Elsevier Ltd.