Thermochemical Heat Storage at High Temperatures using Mn2O3/Mn3O4 System: Narrowing the Redox Hysteresis by Metal Co-doping☆

Abstract Thermal energy storage systems are a key component of concentrated solar power plants, since its implementation increases the energy generation dispatchability. In particular, thermochemical storage through redox cycles of metal oxides is going to play a major role in future plants working with volumetric air receivers, as they are able to store energy at high temperatures, using air as both heat transfer fluid and reactant. One of the most remarkable characteristics of redox cycles of some metal oxides (e.g. Mn 2 O 3 /Mn 3 O 4 and Co 3 O 4 /CoO) is that the forward and reverse reactions start at different temperatures, i.e., a thermal hysteresis exists. Namely, the metal oxide reduction takes place at higher temperatures than the re-oxidation of the reduced phase. In the case of Mn-based redox couple, the temperature difference between reduction and oxidation is of ca. 200 °C, whereas for Co 3 O 4 /CoO is around 50 °C. Narrowing the hysteresis loop for the manganese oxide system means that heat is stored and released in a closer range of temperatures, which will suppose an increase of the charge-discharge energy efficiency. In this work, the effect that co-doping the Mn oxides with Fe and Cu has on the redox temperatures of both reactions has been studied. Materials were prepared by a variation of Pechini method and characterized by XRD and SEM. The capacity to withstand several redox cycles was analyzed by thermogravimetric analyses. It was found that addition of certain amount of both dopants narrowed the thermal hysteresis of such redox couple, presenting stable reversibility.