Sodium Manganese Ferrite Water Splitting Cycle: Unravelling the Effect of Solid-Liquid Interfaces in Molten Alkali Carbonates

Abstract

In this work, the Na₂CO₃ of the sodium manganese ferrite thermochemical cycle was substituted by different eutectic or eutectoid alkali carbonate mixtures. Substituting Na₂CO₃ with the eutectoid (Li_0.07Na_0.93)₂CO₃ mixture resulted in faster hydrogen production after the first cycle, shifting the hydrogen production maximum toward shorter reaction times. Thermodynamic calculations and in situ optical microscopy attributed this fact to the partial melting of the eutectoid carbonate, which helps the diffusion of the ions. Unfortunately, all the mixtures exhibit a significant loss of reversibility in terms of hydrogen production upon cycling. Among them, the nonsubstituted Na mixture exhibits the highest reversibility in terms of hydrogen production followed by the 7%Li-Na mixture, while the 50%Li-Na and Li-K-Na mixtures do not produce any hydrogen after the first cycle. The loss of reversibility is attributed to both the formation of undesired phases and sintering, the latter being more pronounced in the eutectic and eutectoid alkali carbonate mixtures, where the melting of the carbonate is predicted by thermodynamics.

Keywords: atomic substitution; carbonation; decarbonation; hydrogen production; sodium manganese ferrite cycle; thermochemical water splitting.

Figure 1

Scheme of the procedure used to prepare the MnFe₂O₄-carbonate mixtures.

Figure 2

Scheme of the TGA setup used for the H₂ production experiments.

Figure 3

(a–d) Decarbonation (dotted line) and carbonation (full line) thermograms of the Na, 7% Li-Na, 50% Li-Na, and Li-K-Na mixtures during 20 decarbonation-carbonation cycles performed in dynamic conditions between 500 and 750 °C. Red arrows indicate the increase of cycle number, i–e, from cycle 1 to 20. (e) Amount of CO₂ desorbed (wt %) by the different mixtures during 20 decarbonation-carbonation cycles. For the sake of simplicity, only cycle nos. 1 (black), 10 (orange), and 20 (blue) are compared for each mixture. (f) XRD analysis of the cycled mixtures.

Figure 4

(a) T_onset, (b) T_offset, and (c) ΔT (T_offset – T_onset) of the four mixtures during cycle nos. 1, 10, and 20. T_onset refers to the 1 wt % loss, while T_offset to 90% of the total mass loss during the cycle.

Figure 5

(a) Amount of CO₂ desorbed (wt %) and (b) of H₂ produced (mmol/g) by the four mixtures during 5 consecutive cycles at 750 °C. The H₂ produced under the same experimental conditions by the 2Mn_0.95Zn_0.05Fe₂O₄-3Na₂CO₃ mixture (Zn-Na) is also reported as a term of comparison. Reproduced with permission from ref (25). Copyright 2022 Elsevier. (c–f) H₂ concentration (micromoles per liter of carrier gas) detected in the exhaust gases of the STA during the WS step of the four investigated mixtures.

Figure 6

XRD analysis of the four mixtures after 5 H₂ production cycles performed under isothermal conditions at 750 °C.

Figure 7

SEM images of the four mixtures after 5 H₂ production cycles under isothermal conditions at 750 °C. Backscattered electron (BSE) images of (a, b) Na, (c) 7% Li-Na, (d) 50% Li-Na, and (e, f) Li-K-Na samples are reported.

Figure 8

(a) Thermograms and the corresponding evolution of the hydrogen concentration in the exhaust gases during the heating and the first WS step at 750 °C for the four mixtures. The temperature profile is also reported (dashed line). Evolution of the H₂ hydrogen concentration in the exhaust gases for the Na (black) and 7% Li-Na (red) mixtures for the 2nd (b) and 3rd (c) cycles at 750 °C.

Figure 9

Schematic representation of the microstructural evolution in the 7% Li-Na mixture during the 1st water splitting (WS), the 1st reduction (RE) step, and the 2nd WS step performed under isothermal conditions at 750 °C.

Figure 10

In situ high-temperature optical microscopy experiments performed for the Na and 7% Li-Na systems. (a) Scheme of the experimental setup/methodology (full details in section 2.5). In situ microstructural evolution of (b) Na₂CO₃ and (c) (Na_0.93Li_0.07)₂CO₃ particles deposited on MnFe₂O₄ pellets upon heating from RT to 760 °C under CO₂. Backscattered SEM micrographs acquired at increasing magnifications after the in situ experiments of the (d–f) Na and (g–i) 7% Li-Na samples. (j) Schematic representation of the capillary imbibition of porous MnFe₂O₄ during the melting of the (Na_0.93Li_0.07)₂CO₃ particles upon heating.