Understanding Redox Kinetics of Iron-Doped Manganese Oxides for High Temperature Thermochemical Energy Storage

Thermochemical heat storage based on redox oxides has been proposed as suitable alternative for future concentrating solar plants working at high temperatures. In particular, the (Mn0.8Fe0.2)(2)O-3/(Mn0.8Fe0.2)(3)O-4 redox couple is a promising system owing to its reduced cost, adequate thermodynamic characteristics, and high stability over prolonged cycling. As demonstrated in this work, such redox materials can withstand over 75 reduction/oxidation (charge/ discharge) chemical loops. The outstanding durability that this system exhibits has prompted a more comprehensive assessment of the kinetics of both charging and discharging reactions. The goal of this work has been twofold. First, on the basis of data extracted from thermogravimetric analysis, we propose a rate law model for both reduction and oxidation reactions that could help to the future reactor design. Second, aided by in situ XRD and Raman spectroscopy, we have gained further insight into the crystallographic transformations that take place during such redox processes and about the role of Fe incorporation on the oxidation improvement. Kinetic modeling results indicated that both reactions might be well described by a nucleation and growth mechanism. In situ XRD confirmed the presence of two spinel phases (cubic and tetragonal) in the reduced form. Finally, Raman spectroscopy analyses suggested that Fe incorporation alter the metal oxide bond lengths; namely, Fe doping induced an enlargement of Mn-O bonds. This structural modification can facilitate the rearrangement of the coordination polyhedra, and it correlates well with a rise in oxidation rate with increasing the amount of Fe incorporated into the Mn oxide.