## 1 Introduction
As for the copper oxide (CuO/Cu\({}_{2}\)O), it has been studied for air separation applications [23], but has also attracted attention for TCES application as it presents high energy storage capacity, 811 kJ/kg [21; 25] and the reaction temperature is suitable to be used with CSP [21; 22; 25]. However, the melting point of copper oxide is very close to the transition temperature and sintering issues are increased, which is not convenient to achieve a good cycling stability [25]. The onset temperature for the reduction of copper oxide was measured around 1028 \({}^{\circ}\)C under 20% O\({}_{2}\)/Ar (Table 1) during the sample heating up [7]. The onset temperature measured for the re-oxidation during the next cooling down step was around 1002\({}^{\circ}\)C. The material showed a good reversibility, however not total, with a maximum conversion of 88% observed with temperature kept constant at 950 \({}^{\circ}\)C. In another TGA study in air, a re-oxidation of 92% for one cycle has been observed [23], with reduction and oxidation temperatures at 1075 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C, respectively. The suitability of CuO/Cu\({}_{2}\)O for TCES was also tested in a rotary kiln reactor [21], in which the material suffered from sintering and thus to loss in capacity, with incomplete reduction (80% in Ar, 40% in air) and low re-oxidation (\(\sim\)9%). Lastly, the Fe\({}_{2}\)O\({}_{3}\)/Fe\({}_{3}\)O\({}_{4}\) redox pair may also be a very attractive material for its low price and high availability. The Fe\({}_{2}\)O\({}_{3}\)/Fe\({}_{3}\)O\({}_{4}\) redox pair also presents a high reaction enthalpy but the transition in air occurs at very high temperature (1392 \({}^{\circ}\)C [41]). The development of an efficient and suitable material for TCES application by incorporating iron oxide would help obtaining a more cost-effective energy storage medium with enhanced properties [6]. The reduction of Fe\({}_{2}\)O\({}_{3}\) to Fe\({}_{3}\)O\({}_{4}\) was observed under 20% O\({}_{2}\) atmosphere around 1391 \({}^{\circ}\)C (Table 1), and reduction at lower temperature can be achieved under inert atmosphere (958 \({}^{\circ}\)C in Ar). Fe\({}_{3}\)O\({}_{4}\) was then capable to regain 92 wt% of the previously released oxygen. When studied in air, Fe\({}_{3}\)O\({}_{4}\) could be re-oxidized up to 80% (750-900 \({}^{\circ}\)C), and a decrease in conversion rate was noticed and attributed to the formation of an oxidized impervious layer which became thicker with increasing temperature [42]. Iron-based materials offer high gravimetric energy storage density as iron is a widely available and cheap material. However, used as such, raw Fe\({}_{2}\)O\({}_{3}\) is reduced at very high temperature and thus presents a high risk to encounter sintering. Nowadays, the development of alternative TCES materials with suitable properties for long-term exploitation in CSP plants remains a challenge. Single-metal oxide systems can hardly compete with Co\({}_{3}\)O\({}_{4}\)/CoO in terms of energy storage density, reaction kinetics and cycling stability. It is possible to achieve better redox performance with a metal oxide by modifying its morphology (for example through tailored synthesis), or through the addition of a secondary transition metal. In previous works, this method has been applied to modify single metal oxides, chiefly Co\({}_{3}\)O\({}_{4}\) and Mn\({}_{2}\)O\({}_{3}\), and the impacts of this modification on the tuning of reaction kinetics, cycling stability and reaction temperature, were reported [34; 35; 36]. The present work aims to compare experimental assessment of mixed metal oxide systems based on Co\({}_{3}\)O\({}_{4}\)/CoO and Mn\({}_{2}\)O\({}_{3}\)/Mn\({}_{3}\)O\({}_{4}\) through binary mixing between Co, Mn, Fe and Cu oxides to provide an overview of the most suitable systems and to propose perspectives for future development of oxide-based TCES technology.