Determining the Role of Fe-Doping on Promoting the Thermochemical Energy Storage Performance of (Mn 1- x Fe x ) 3 O 4 Spinels.

Mn oxides are promising materials for thermochemical heat store, but slow reoxidation of Mn 3 O 4 to Mn 2 O 3 limits efficiency. In contrast, (Mn 1- x Fe x ) 3 O 4 oxides show an enhanced transformation rate, but fundamental understanding of the role played by Fe cations is lacking. Here, nanoscale characterization of Fe-doped Mn oxides is performed to elucidate how Fe incorporation influences solid-state transformations. X-ray diffraction reveals the presence of two distinct spinel phases, cubic jacobsite and tetragonal hausmannite for samples with more than 10% of Fe. Chemical mapping exposes wide variation of Fe content between grains, but an even distribution within crystallites. Due to the similarities of spinels structures, high-resolution scanning transmission electron microscopy cannot discriminate unambiguously between them, but Fe-enriched crystallites likely correspond to jacobsite. In situ X-ray absorption spectroscopy confirms that increasing Fe content up to 20% boosts the reoxidation rate, leading to the transformation of Mn 2+  in the spinel phase to Mn 3+ in bixbyite. Extended X-ray absorption fine structure shows that FeO length is larger than MnO, but both electron energy loss spectroscopy and X-ray absorption near edge structure indicate that iron is always present as Fe 3+  in octahedral sites. These structural modifications may facilitate ionic diffusion during bixbyite formation.