Particle design and oxidation kinetics of iron-manganese oxide redox materials for thermochemical energy storage

High-temperature thermochemical energy storage shows promise in aiding concentrating solar power plants to meet variable, grid-scale electricity demand. In this work, manganese oxide-based mixed metal oxide particles have been designed and tested for thermochemical energy storage. Particles are designed for high energy storage capacity, flowability, and physical and chemical stability. We evaluate the effects of Al₂O₃, Fe₂O₃, and ZrO₂ in Mn₂O₃-based spray-dried particles in a TGA between 650 °C and 1200 °C over six consecutive redox cycles. Results are compared with thermodynamic predictions from 400 to 1400 °C under oxidizing and reducing atmospheres. A mixture of 2:1 Fe₂O₃:Mn₂O₃ formed iron manganese oxide spinel (MnFe₂O₄) on calcination, demonstrated the highest thermochemical activity. Intensive mixing was also investigated as a method for preparing MnFe₂O₄ spinel. Sodium contained in binders/precursors used for spray drying results in the formation of slag phases which negatively impact spray dried particle redox. Spherical particles formed by intensive mixing provided for superior redox reaction. Differential scanning calorimetry (DSC) was performed on MnFe₂O₄ particles prepared by intensive mixing. The oxidation reaction kinetics of MnFe₂O₄ was investigated using solid-state kinetics theory and XRD analysis. The reaction proceeds by two different reaction mechanisms. The reaction first proceeds by a diffusion-controlled reaction mechanism (192 ± 2 kJ mol⁻¹ activation energy) with no phase change, followed by a nucleation-growth reaction mechanism (181.4 ± 0.3 kJ mol⁻¹ activation energy). The work reported here supports using low cost MnFe₂O₄ spinel, formed by intensive mixing of a 2:1 Fe₂O₃:Mn₂O₃ composition, as a desirable thermochemical storage active material. Additionally, these results demonstrate the benefit of operating the redox cycle through a cation-vacancy mechanism where the spinel phase maintains its crystal structure and where the reaction rate is stable/robust regardless of particle size.