## 1 Introduction
Flow-through pellets, perforated monolithic bodies of Co\({}_{3}\)O\({}_{4}\), or composites of Co\({}_{3}\)O\({}_{4}\) with ceria, zirconia, alumina, iron oxide, silicon carbide and manganese oxide, were investigated by [PERSON] et al. (2014) [33] for energy storage applications in CSP plants. Based on their good heat storage capacity and thermo-mechanical stability, both cobalt oxide-alumina and cobalt oxide-iron oxide were considered very promising in this study. On the other hand, the addition of ceria (25 wt%) enhanced the reaction kinetics of cobalt oxide, while the corresponding pellets exhibited poor thermo-mechanical stability. A change in the temperature for the beginning of the reduction/oxidation reaction was also observed with a modification in the added second oxide. The performance of commercial Co\({}_{3}\)O\({}_{4}\) powder in a fixed bed with air flow was appraised in a vertical quartz tube heated between 765 \({}^{\circ}\)C and 930 \({}^{\circ}\)C: full conversion and very good cycling stability over eight successive cycles were observed. A comparable study was conducted with commercial Co\({}_{3}\)O\({}_{4}\) shaped into porous foams placed within a ceramic tube heated by a horizontal furnace. Full conversion and good cyclability were again obtained. The material shaping neither affected the reactivity, nor the Co\({}_{3}\)O\({}_{4}\) stability [34]. Nonetheless, cobalt oxide is a potentially carcinogenic material and is expensive [6], which gives incentives for the development of other alternative materials. The manganese oxide system (Mn\({}_{2}\)O\({}_{3}\)/Mn\({}_{3}\)O\({}_{4}\)) features slow kinetics of the oxidation step, which impacts the cycling reversibility of this material and requires performance improvement, but it is a less expensive raw material than cobalt oxide [35; 36; 37]. This system can achieve a gravimetric energy storage density of 110 to 160 kJ/kg [37; 38] and the theoretical reaction enthalpy is reported to be 202 kJ/kg [37]. While Co\({}_{3}\)O\({}_{4}\)/CoO performs redox cycles with rapid reaction kinetics, Mn\({}_{3}\)O\({}_{4}\) is very slow to re-oxidize and also presents a larger hysteresis than cobalt oxide [35; 36; 37; 38]. Thus, the redox properties of Mn\({}_{2}\)O\({}_{3}\)/Mn\({}_{3}\)O\({}_{4}\) need to be improved further to use this material efficiently for TCES application. A few works investigated the improvement of Mn\({}_{3}\)O\({}_{4}\) re-oxidation kinetics via a change in morphology and shape of the initial material. The impact of the initial particle size on the cyclability of manganese oxide was studied [39], as well as the influence of the morphology and shape on the redox kinetics of the material [38; 39]. The initial particle size influences the redox behavior of the metal oxide as smaller particles are more prone to material sintering and densification, which then hinders O\({}_{2}\) diffusion to the reacting core, then affecting the re-oxidation step [39; 40]. In addition to the influence of initial particle size on the reaction kinetics, a sintering issue can also be encountered especially for smaller particles that favor material sintering [39]. Due to this fact, the material re-oxidation can be observed in two steps, first during the temperature cooling down of the discharge step, then continuing during the next heating up of the following charging step, although the re-oxidation is sometimes not observed at all [7; 38; 39]. The performance of Mn\({}_{2}\)O\({}_{3}\) powders resulting from different synthesis methods (modified Pechini method and precipitation in ammonia [7; 34]) was compared, along with data from commercial powder. The material synthesized by Pechini and commercial powder showed very low re-oxidation, while the reduced powder from the synthesis by precipitation in ammonia was re-oxidized up to 93%. Both synthetic powders were subjected to sintering, however the latter formed a porous structure described as \"coral-like\" [39] which allowed for oxygen circulating within the material.