2025 AIChE Annual Meeting

(405a) Non-Redox Phase Change of MnFe2O4/Na2CO3 Inhibits Oxygen Release in Thermochemical Water Splitting

Authors

Justin Marlowe - Presenter, Rutgers, The State University of New Jersey
Yikyeom Kim, University of California Santa Barbara
Phillip Christopher, University of California Santa Barbara
Eric McFarland, University of California
Thermochemical water splitting has been proposed as a scalable, cost-effective alternative to water electrolysis for “green” H2 production. While multiple chemical paradigms have been proposed (CeOx, sulfur-iodide, transition metal chloride), the most desirable systems avoid corrosive species, operate below 1000˚C, and require minimal temperature changes in the thermodynamic cycle. The only system currently reported [1-2] to satisfy these conditions involves MnFe2O4 and Na2CO3, comprised of the general reactions:

  1. 3MnFe2O4 + 3Na2CO3 + 2H2O -> 6NaMn1/3Fe2/3O2 + 3CO2 + 2H2
  2. 6NaMn1/3Fe2/3O2 + 3CO2 -> 3Na2CO3 + 2MnFe2O4 + O2

However, satisfactory H2 production via Reaction (1) has only ever been observed for one cycle, inhibiting its implementation as a viable process. Though some have speculated that sintering explains this loss of productivity, questions persist as to the exact cause of performance degradation, caused in part by reliance on thermogravimetric techniques which have limited the chemical information obtained during reaction.

In this work, we couple on-line mass spectrometry (MS) with previously utilized ex situ techniques like X-ray diffraction (XRD) to clarify that the rapid degradation of H2 productivity is caused by incomplete reduction of NaMn1/3Fe2/3O2 via Reaction (2). MS results indicate that previously observed mass changes likely correspond only to reversible Na2CO3 formation that is not intrinsically correlated with O2 release as previously thought. The ineffective redox of NaMn1/3Fe2/3O2 is also demonstrated by ex situ cerimetric titration, which indicates that the average oxidation states of Mn and Fe after Reaction (2) remain at +3. In summary, NaMn1/3Fe2/3O2 undergoes non-redox phase changes under CO2, preventing subsequent re-oxidation by H2O to form H2, suggestive that further work is needed to realize cyclable performance utilizing this sytem.

References:

[1] Deng, Y., et al. RSC Adv. 2022, 12, 31392.

[2] Udaeta, J., et al. ACS Appl. Mater. Interfaces 2024, 16, 26, 33270-33284.