Thermochemical energy storage (TCS) based on reduction–oxidation cycles of multivalent metal oxides is of great interest for concentrating solar facilities, as it can allow enhancing the global plant efficiency and improving the energy generation dispatchability. However, to guarantee the feasibility of the process, selected materials should present long term durability, which requires the evaluation of the redox couple cyclability. In this work we have demonstrated, for the first time, the suitability of the Mn₂O₃/Mn₃O₄ pair for this application during 30 cycles, performed in thermobalance. Nevertheless, an appropriate design of such materials is crucial since it has been found that initial particle size influences the redox behaviour of these oxides. Results showed a 2-fold influence of particle size of the as-prepared materials on the redox reversibility. Firstly, this parameter affects to the thermodynamics and kinetics of the redox reactions. Namely a decrease of the particle size shifted the oxidation temperature to lower values and produced slower reduction and oxidation reactions. Secondly, depending on the particles size, samples followed different sintering mechanisms. This fact influenced dramatically the behaviour of the materials with lower particle size, which suffered from a higher degree of densification that eventually caused a total loss of cyclability.

The barium peroxide-based redox cycle was proposed in the late 1970s as a thermochemical energy storage system. Since then, very little attention has been paid to such redox couples. In this paper, we have revisited the use of reduction–oxidation reactions of the BaO₂/BaO system for thermochemical heat storage at high temperatures. Using thermogravimetric analysis, reduction and oxidation reactions were studied in order to find the main limitations associated with each process. Furthermore, the system was evaluated through several charge–discharge stages in order to analyse its possible degradation after repeated cycling. Through differential scanning calorimetry the heat stored and released were also determined. Oxidation reaction, which was found to be slower than reduction, was studied in more detail using isothermal tests. It was observed that the rate-controlling step of BaO oxidation follows zero-order kinetics, although at high temperatures a deviation from Arrhenius behaviour was observed probably due to hindrances to anionic oxygen diffusion caused by the formation of an external layer of BaO₂. This redox couple was able to withstand several redox cycles without deactivation, showing reaction conversions close to 100% provided that impurities are previously eliminated through thermal pre-treatment, demonstrating the feasibility of this system for solar thermochemical heat storage.