Development of a continuous solid solution with extended Red-Ox temperature range and unexpected high reaction enthalpies for thermochemical energy storage

Abstract

Thermochemical reactions are considered as a promising approach to be applied for heat storage in next generation of concentrated solar power (CSP) plants where the expected working temperatures will be higher than the current ones. Redox reactions involving multivalent cations have been investigated for high temperature applications in temperature range of 900 - 1200°C. However, only a few number of metal oxides with limited number of reaction temperatures have been identified in that temperature range which has been considered as one of the main barriers for further development of new concepts and systems for thermochemical energy storage. Mixed metal oxides, on the other hand, could provide a flexibility allowing to extend the reaction temperature in a wide temperature range to be adapted for each specific application. With this regard, the study of mixed cobalt and manganese oxides with the general formula Co3-xMnxO4 (0≤x≤3) was carried out, where five pure mixed metal oxides, i.e. Co2.5Mn0.5O4, Co2MnO4, Co1.5Mn1.5O4, CoMn2O4 and Co0.5Mn2.5O4, have been successively synthesized. Their structural characterization demonstrated that a continuous solid solution, with pure spinel structures, was obtained between the two pure cobalt and manganese oxides with an adjustable reduction/oxidation temperature in a large temperature range between 850°C and 1700°C. The obtained redox temperatures for Co2.5Mn0.5O4, Co2MnO4, Co1.5Mn1.5O4, CoMn2O4 and Co0.5Mn2.5O4 are 980, 1129, 1230, 1320 and 1428°C, respectively. Finally, the thermodynamic study has revealed that the enthalpies of mixed metal oxides are higher than the pure oxides ones reaching a maximum value of 1233 J/g for Co1.5Mn1.5O4, which is almost the double of the pure cobalt oxide enthalpy 675 J/g.