Determining the Role of Fe-Doping on Promoting the Thermochemical Energy Storage Performance of (Mn1- x Fex )3 O4 Spinels

Mn oxides are promising materials for thermochemical heat store, but slow reoxidation of Mn₃ O₄ to Mn₂ O₃ limits efficiency. In contrast, (Mn_{1- x} Fe_x )₃ O₄ 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.

Unraveling the Dissolution-Mediated Reaction Mechanism of α-MnO2 Cathodes for Aqueous Zn-Ion Batteries

Aqueous Zn/α-MnO₂ batteries have attracted immense interest owing to their high energy density, low cost, and safety, making them desirable for future large-scale energy application. Despite these merits, the comprehensive understanding of their reaction mechanism has been elusive due to the limitations of standard bulk characterization. Here, via transmission electron microscopy, the dissolution-mediated reaction mechanism of a Zn/α-MnO₂ system is discovered and explored in full scope to involve reversible formation of Zn₄ SO₄ (OH)₆ ·xH₂ O and "birnessite-like" Zn-MnO_x phase upon cycling. Overall, α-MnO₂ acts primarily as a source for cell activation through dissolution and thus is not directly involved in the Zn redox chemistry. This microscopic study offers a unique knowledge on the unconventional reaction chemistry of Zn/α-MnO₂ batteries.

A "MOF-Protective-Pyrolysis" Strategy for the Preparation of Fe-N-C Catalysts and the Role of Fe, N, and C in the Oxygen Reduction Reaction in Acidic Medium

M-N-C electrocatalysts for the oxygen reduction reaction (ORR) have been considered as the most promising alternatives to precious metal catalysts. However, the catalytic activity of M-N-C especially in an acidic medium is still unable to meet the practical requirement, and the corresponding ORR mechanism also remains unclear. Herein, an "MOF-protective-pyrolysis" strategy was adopted to synthesize an Fe-N-C catalyst (P-FeMOF@ZIF-8) with Fe-N4 as the predominant Fe species. The P-FeMOF@ZIF-8 displays high catalytic activity in both acidic and alkaline mediums. The experimental results further reveal the different importance of the Fe species in different mediums. Based on the results, a distinct mechanism of the ORR the in acidic medium was proposed, where the synergistic effect of the Fe-N4 site and carbon in improving the kinetics of the ORR were well explained. The findings and the corresponding perspective may provide a new guidance for the design of highly efficient ORR electrocatalysts.