## 5 Discussion and Conclusions  
Thermochemical energy storage (TCES) is a promising route for the long-term storage of solar energy in the form of chemical bonds involving metal oxide systems. Indeed, the use of metal oxides for this application brings interesting alternative to other solid-gas systems because air can advantageously be used in open loop as both the reacting gas (O\({}_{2}\)) in the reversible reactions and the heat transfer fluid (energy carrier from or to the storage medium). To cope with the noticeable challenges for developing reliable energy storage materials applied to solar power plants, binary and ternary metal oxide systems are considered. Their utilization could indeed alleviate the main critical issues related to singles oxides (cost and toxicity issues for Co oxide, poor reversibility due to slow oxidation rate for Mn oxide), and improved materials characteristics could be achieved such as resistance to sintering, compatible transition temperature, complete reaction reversibility, and suitable energy storage capacity. Cobalt-based and manganese-based oxides have been investigated and compared with recent literature data in order to summarize the suitability of mixed oxides systems for TCES application at high temperature. The properties of mixed oxides regarding their energy storage capacity, redox reaction reversibility/cyclability, reaction temperature and hysteresis gap, were evaluated for the following systems: Co-Fe-O, Co-Cu-O, Mn-Fe-O, Mn-Cu-O and Mn-Co-O. Concerning cobalt-based systems, the addition of iron greatly decreases the OSC of the material, and the respective pure oxides show a higher reaction enthalpy than the mixed oxides of the Co-Fe-O system. However, the reaction temperature is positively impacted, as the hysteresis gap is reduced for compositions with 0.1 \(\leq\) x(Fe) \(\leq\) 0.4. In addition, small amounts of added Fe, x(Fe) \(\leq\) 0.1, may contribute to the thermal stability of Co\({}_{3}\)O\({}_{4}\). From these observations, the incorporation of Fe around x(Fe) = 0.1 might be relevant to obtain a TCES material with enhanced stability, lower temperature gap between reaction steps, still high enough energy storage density (even though lower than that of the pure Co oxide), slightly lower cost, and possible use for higher temperature application, when compared to pure Co\({}_{3}\)O\({}_{4}\). The mixed oxides investigated in the Co-Cu-O system retain high reaction enthalpy (over 200 kJ/kg) and show good stability over several redox cycles. Similar to the Co-Fe-O system, the reaction enthalpy reported for the Co-Cu mixed oxides decreases with increasing amounts of added Cu. The OSC of the mixed oxides follows the same trend, except for a very small Cu amount, x(Cu) = 0.03, for which the OSC is unchanged when compared to pure Co\({}_{3}\)O\({}_{4}\). Furthermore, previous literature data attracted attention to low Cu-content compositions, especially around x(Cu) = 0.1, which was reported to hinder the material sintering, thus enhancing the stability of Co\({}_{3}\)O\({}_{4}\) over repeated redox cycles, and improving its reaction kinetics. In addition, the addition of Cu to cobalt oxide reduces the transition temperature, thus providing a tunable option for storage systems requiring a different operating temperature. The incorporation of Co into Mn\({}_{2}\)O\({}_{3}\) contributed to the enhancement of the material properties, but both the OSC and energy storage capacity decrease with increasing amounts of Mn. The Mn-Co-O mixed oxides with a majority of cobalt in the composition might be an alternative, as these materials show good reaction reversibility and suitable reaction enthalpies. They could further reduce the cost of the storage system, as compared to pure Co\({}_{3}\)O\({}_{4}\), taking into account that their performances are however by far lower than those of pure Co\({}_{3}\)O\({}_{4}\).