Mn Disproportionation, Not Redox Cycling, Explains Low-Temperature, Thermochemical “Water Splitting” with MnFe2O4/Na2CO3

Abstract The development of redox-active materials for thermochemical water splitting to produce hydrogen is an attractive alternative to water electrolysis due to the low cost of heat and generally favorable scaling relationships of heat-driven processes. Recently, a material system composed of MnFe2O4 and Na2CO3 reacting to form NaMn1/3Fe2/3O2 has been identified, which reportedly operates at reasonable temperatures (∼750 °C) for both oxidation and reduction steps of the water-splitting cycle, a marked improvement over alternative cycles involving metal oxides. The cycle has been proposed to involve the Mn2+/Mn3+ redox couple, which is mediated by Na+ intercalation and deintercalation during oxidation and reduction, respectively. However, an issue in these reports is the apparent degradation of H2 production even after only one oxidation–reduction cycle, which has been attributed to physical effects such as sintering. Here, we employ a combination of on-line mass spectrometry, cerimetric titration, X-ray diffraction, X-ray photoelectron spectroscopy, and density functional theory calculations to provide quantitative insight into the chemical redox processes, phase evolution, and gas evolution behavior of the Na–Mn–Fe–O system during attempted thermochemical water splitting. Our work elucidates that while CO2 uptake and release are observed and coincide with substantial crystallographic evolution between spinel and layered oxide phases, as previously reported, mass spectrometry reveals that these structural changes are decoupled from H2 and O2 production, highlighting that phase and mass change are insufficient to describe the chemical state of the system. We find no evidence of O2 release from NaMn1/3Fe2/3O2 materials during CO2 exposure, contradicting prior reports which assumed O2 release on the basis of the product crystal phases. We further propose that in the presence of CO2, Mn3+ disproportionation results in more stable Mn4+ and Mn2+ species, relieving lattice strain caused by Jahn–Teller distortions. It is the disproportionation-formed Mn2+ species that results in the formation of MnFe2O4 upon Na+ deintercalation, rationalizing the confusion of expected reduction in prior reports. Thus, since this Mn2+ formation is balanced by Mn4+ formation, no net reduction occurs in NaMn1/3Fe2/3O2 upon CO2 treatment at 750 °C. The absence of net redox changes to Mn eliminates the possibility of forming a thermochemical cycle for water splitting using MnFe2O4/Na2CO3 and provides a chemical description for failed cyclability in previous works, rather than a physical phenomenon.