Self-Assembled Structure Evolution of MnFe Oxides for High Temperature Thermochemical Energy Storage

Thermochemical energy storage (TCES) materials have emerged as a promising alternative to meet the high-temperature energy storage requirements of concentrated solar power plants. However, most of the energy storage materials are facing challenges in redox kinetics and cyclic stability. Iron-doped manganese oxide attracts raising attention due to its non-toxicity, low cost, and high energy capacity over 800 °C. However, there are few investigations on the reversibility enhancement of the redox reaction from the microstructural-evolution-mechanism point of view. Herein, bixbyite-type (Mn_0.8 Fe_0.2 )₂ O₃ is synthesized and extruded into honeycomb units, which can maintain an 85% initial capacity after 100 redox cycles. It is also found that a self-assembled core-shell MnFe₂ O₄ @Mn_2.7 Fe_0.3 O₄ structure forms during the reduction step, and then transforms into a homogeneous solid solution of (Mn_0.8 Fe_0.2 )₂ O₃ in the following oxidation step. During the reduction step, shells are formed spontaneously from the Mn_2.7 Fe_0.3 O₄ with the MnFe₂ O₄ as cores due to the lower surface energy, which facilitates the oxygen adsorption and dissociation during subsequent oxidation step. Through the density functional theory calculation, it is revealed that the lower formation energy of oxygen vacancies in the shell contributes to the improvement of oxygen diffusion rate. This study can provide a guideline to design prospective materials for high-temperature TCES.