Determining the Role of Fe‐Doping on Promoting the Thermochemical Energy Storage Performance of (Mn_1−xFe_x)₃O₄ Spinels

Abstract

Mn oxides are promising materials for thermochemical heat store, but slow reoxidation of Mn3O4 to Mn2O3 limits efficiency. In contrast, (Mn1−xFex)3O4 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 Mn2+ in the spinel phase to Mn3+ 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 Fe3+ in octahedral sites. These structural modifications may facilitate ionic diffusion during bixbyite formation.

Thermodynamic and Structural Effects of Fe Doping in Magnesium Manganese Oxides for Thermochemical Energy Storage

“…MM has high agreement with the crystal structure Mn 3 O 4 with a displacement toward larger 2θ angles due to the incorporation of Mg into the spinel in the tetrahedral and octahedral sites. We observe further peak shifts to larger 2θ values in Fe-doped samples, where the shift increases with Fe concentration (Figure b), consistent with the previous work . Average calculated lattice parameters, listed in Table , show a decrease in lattice parameter with increasing Fe.…”

“…We observe further peak shifts to larger 2θ values in Fe-doped samples, where the shift increases with Fe concentration (Figure 2b), consistent with the previous work. 39 Average calculated lattice parameters, listed in Table 2, show a decrease in lattice parameter with increasing Fe. Calculated lattice parameters at each peak are listed in Supporting Information, Table S1.…”

“…A variety of TCES materials have been previously studied, including metallic hydrides, , hydroxides, carbonates, − and metal oxide materials. ,− The metal oxide materials are compatible with high-temperature power cycles and are operable in an open loop system, eliminating the reactant gas storage needed for the other systems . Mn 3 O 4 /Mn 2 O 3 was one of the earliest proposed metal oxide TCES systems because of its high reduction temperatures and its low cost and toxicity .…”

Improving the Discharge Rate of Co3O4-Based Thermochemical Energy Storage Material with Eutectic Doping of Zr

“…This influences the fine structure of the TCES medium and facilitates the transition of the system from high-valence elements to low-valence states. Such effects have been observed in (Mn 1– x Fe x ) 3 O 4 spinel systems . Significant changes in the TCES performance of cobalt oxides have been achieved by doping the systems with Mg and Ni .…”

Catalytic enhancement of production of solar thermochemical fuels: opportunities and limitations

“…Further development of in situ and operando studies of these catalytic systems can be of high value not only from a fundamental point of view to understand the mechanisms at the molecular scale of these processes, but likewise to rationally improve them. However, the extreme operation temperatures impose significant limitations for some spectroscopies measurements, although XAS 97 and Raman 71 measurements are possible for redox systems at relevant conditions. Furthermore, in situ XRD, which provides information about phase evolution, can be also of interest for investigating thermochemical processes.…”