## 1 Introduction  
The intermittent nature of solar energy used to drive concentrated solar power (CSP) plants calls for a suitable energy storage system, which will grant us the freedom to produce and dispatch electricity according to the population needs. Thermal energy storage (TES) can serve this purpose to match the variable electricity demand with the intermittent energy source supply, and much progress has been achieved with latent heat and sensible heat storage systems already in use in CSP plants [1; 2; 3; 4]. Alongside these conventional modes of heat storage, thermochemical energy storage (TCES) is an attractive option to store solar energy at higher temperatures (400-1200 \({}^{\circ}\)C) and to be compatible with next generation CSP plants, such as pressurized air-based solar tower receivers for power generation via gas turbines. These high-temperature processes associated with thermodynamic power cycles will enable higher solar to electricity conversion efficiency. TCES offers higher energy storage densities and possible heat storage at room temperature in the form of stable solid materials, due to energy storage in the form of chemical bonds, which then relates to long-term/seasonal storage. Its main application goal is still related to day/night operation of the power block in a CSP plant for continuous electricity generation. This technology is based on reversible solid-gas reactions, in which the heat transfer fluid is also the reactive gas of the reaction, and the energy required for the endothermic reaction is provided by CSP (Equation (1)). The reverse reaction (exothermic) can then release the stored energy to be delivered to the heat transfer fluid. \[\text{AB(s)}+\text{Heat (CSP)}\Rightarrow\text{A(s)}+\text{B(g)} \tag{1}\]  
Active research is ongoing for the development of suitable TCES systems and for their commercial implementation. In this endeavor however, additional research and development is required to achieve long-term cost-effective systems as a suitable way to implement TCES in solar power plants [5]. Recently, [PERSON] et al. (2018) [6] reported an interesting economical evaluation of 17 potential systems for TCES implementation in solar power plants. As a few systems are known to hold suitable properties (reversibility, high energy storage density, availability at low cost and non-toxicity) for TCES application (such as Ca(OH)\({}_{2}\)/CaO, Sr(OH)\({}_{2}\)/SrO and Ba(OH)\({}_{2}\)/BaO; carbonate looping with CaCO\({}_{3}\)/CaO and SrCO\({}_{3}\)/SrO; redox with BaO\({}_{2}\)/BaO), their implementation would also be cost competitive with molten salts in the near term [6]. Various systems can be considered, other than metal oxides [3; 7]. For example, hydroxides, and especially calcium hydroxide [8; 9; 10; 11; 12; 13; 14] shows promising results for TCES. The systems based on carbonates are also relevant for TCES application. Calcium [15; 16; 17; 18] and strontium carbonate [18; 19] systems are promising but still require structure stabilization to hinder sintering at high reaction temperatures. In the specific case of single metal oxides, air can be used as heat transfer fluid (Equations (2) and (3)), which allows operating with an open-loop system. \[\text{MO}_{\text{(ox)}}+\text{Heat}\rightarrow\text{MO}_{\text{(red)}}+\text{ \text{\text  
of 800-1000 \({}^{\circ}\)C and compared to Co\({}_{3}\)O\({}_{4}\) powder [28]. The full reduction/oxidation of the total mass of Co\({}_{3}\)O\({}_{4}\) coated on the supports (Al\({}_{2}\)O\({}_{3}\), ZrO\({}_{2}\) and cordierite) was observed, as opposed to Co\({}_{3}\)O\({}_{4}\) on SiC supports. Moreover, no detrimental interaction between the cobalt oxide and the support, such as Al\({}_{2}\)O\({}_{3}\), ZrO\({}_{2}\) and cordierite, was reported. Furthermore, the protection of SiC with a SiO\({}_{2}\) layer also proved to offer a full conversion of the material.