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Research Papers

# Progress in Understanding Factors Governing the Sodium Manganese Ferrite Thermochemical Cycle

[+] Author and Article Information
C. Alvani, M. Bellusci, A. La Barbera, F. Padella, L. Seralessandri

ENEA Italian National Agency for New Technologies, Energy and the Environment, C. R. Casaccia, Via Anguillarese 301, 00123 Roma, Italy

F. Varsano

ENEA Italian National Agency for New Technologies, Energy and the Environment, C. R. Casaccia, Via Anguillarese 301, 00123 Roma, Italyfrancesca.varsano@enea.it

J. Sol. Energy Eng 132(3), 031001 (Jun 04, 2010) (5 pages) doi:10.1115/1.4001401 History: Received September 19, 2008; Revised March 30, 2009; Published June 04, 2010; Online June 04, 2010

## Abstract

The mixed sodium manganese ferrite thermochemical cycle for sustainable hydrogen production is reviewed. Both the hydrogen production step and the reaction that leads to the regeneration of initial reactants are described as multistep reactions. The chemical cyclability of the reactive system has been demonstrated at $750°C$.

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## Figures

Figure 1

Scheme of the experimental setup utilized for thermal analysis measurements and to analyze the regeneration reaction. (a) Netzsch STA 409 thermobalance (TB) coupled to a Balzer quadrupole MS. (b) Modified TPD/TPR apparatus to quantify hydrogen production. Purified Ar gas (bottle P) is water-saturated in reactor W, before entering the sample holder (C) inside the furnace (F). Unreacted water and carbon dioxide are trapped in (T) before the gas flow enters the TCD detector.

Figure 2

Thermogravimetric curve of MnFe2O4/Na2CO3 mixture heated in pure Ar. Carbon dioxide evolution during the experiment was followed by mass spectroscopy.

Figure 3

XRD pattern of MnFe2O4/Na2CO3 powder mixture heated in pure Ar. Rietveld refinement (Rwp=0.1085, Rp=0.088, χ2=1.806), difference curve, and identified phases are reported.

Figure 4

(a) Lamellar NaMn1/3Fe2/3O2 and (b) spinel MnFe2O4 structures (15). In the layered compound (a), sodium and transition metals Mn and Fe occupy octahedral positions. Manganese and iron are positioned in the same plane. The high in-plane sodium mobility favors its diffusion out of the lamellar structure when exposed to CO2 gas. In the spinel structure (b), manganese atoms occupy tetrahedral sites.

Figure 6

Weight gain of Na(Mn1/3Fe2/3)O2 following carbon dioxide (0.2 atm) exposure. To support the existence of the sodium carbonate formation/decomposition equilibrium, the temperature has been varied (±10°C). A weight gain is observed subsequently to a temperature decrease (sodium carbonate formation is an exothermic reaction).

Figure 7

A schematic view representing possible equilibria in the regeneration of manganese ferrite and sodium carbonate. The first equilibrium is strictly related to carbon dioxide pressure and temperature. The second step is the result of lamellar to spinel oxide structural transformation.

Figure 8

Hydrogen production yield (mol H2/molMnFe2O4 according to reaction 1 stoichiometry) during successive cycles. Both production (1 h) and regeneration (2 h) reactions have been performed at T=750°C. Water content in Ar stream is ∼15.000 μl/l. Carbon dioxide pressure for the duration of the regeneration step is 1 atm.

Figure 5

XRD patterns representative of the different powder compositions observed. Symbols correspond to (1) MnFe2O4, (2) Na2CO3, (°) O3-Na1−x(Mn1/3Fe2/3)O2−δ, ( ∗)P3−Na1−x(Mn1/3Fe2/3)O2−δ phases. The reported spectra correspond to powders reacted in the following experimental conditions: (a) T=800°C and pCO2=1 atm, (b) T=800°C and pCO2=0.5 atm, and (c) T=600°C and pCO2=0.5 atm(8).

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