Abstract

At present, there is a clear interest in developing redox materials with improved properties for high temperature thermochemical energy storage. Chemical modification of manganese oxides with cations such Li and Cu can produce, among other phases, LiMn2O4 and CuMn2O4 spinels, which are feasible candidates for heat storage due to their redox capacity. In this work, these materials were synthesized by Pechini method, and the characterization results confirmed the formation of the targeted phases with some minor contribution of Mn3O4. Thermogravimetrical redox tests in air established that both materials experience fully reversible redox transformations when the temperature is varied between 900 and 1000 °C. These assays showed the stability of both Cu and Li mixed oxides after five consecutive redox cycles and, in accordance, the XRD confirmed that the two samples retaining their spinel crystal structure after the treatment. However, in these conditions reduction temperatures are higher than 940 °C for both oxides and the enthalpies of these transformations are modest, with a maximum value of 36 kJ/kg for LiMn2O4. Alternatively, if the reduction is performed in argon and the oxidation in air, it is possible to increase the amount of oxygen exchanged in the gas-solids reactions and, accordingly, the heat storage capacity. Therefore, the heat recovered in the re-oxidation of CuMn2O4 at 700 °C was 144 kJ/kg (34 kJ/mol), while LiMn2O4 showed an enthalpy of 209 kJ/kg (37 kJ/mol). These changes in the composition of the atmosphere do not affect to the stability of the system and the conversion is maintained after five consecutive cycles. In all cases, the initial spinel phase is recovered after reoxidation, which takes place at remarkably fast rates. Analysis of the intermediate reduced materials reveals a significant complexity of the redox transformations, which imply the formation of LiMnO2 and CuMnO2, among other phases. Accordingly, considering the stability of these systems, as well as the relatively high enthalpies CuMn2O4 and LiMn2O4 appear to be promising materials for thermochemical energy storage.

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