## 1 Introduction  
The intense research in the field of energy, especially towards the energy efficiency of the buildings and industrial sector, and the massive exploitation of renewable energy sources (RES) cannot leave aside the development of storage materials and components: RES or industrial waste heat sources are intrinsically fluctuating and their utilization for heating and cooling purposes requires the coupling with a thermal energy storage (TES) [1]. In addition, it has been proved that the application of a TES to renewable-based energy systems improves their efficiency [2]. Different technologies for storing thermal energy have been proposed, such as sensible heat, latent heat, and heat of reaction [3]. The latter class usually takes the name of thermochemical heat and exploits reversible physical and chemical reactions. Particularly, in the low-to-medium temperature operating range (i.e., 70-150 \({}^{\circ}\)C), this technology is mainly based on hydration/dehydration reactions of salt hydrates [4]. It is considered the most attractive solution for its high-energy storage density and the possibility to keep energy almost indefinitely stored as long as the sorbent component (e.g., salt) is kept separated from the sorbate component (e.g., water vapor). The class of salt hydrates for thermochemical storage (TCS) has been intensively studied [5], in order to realize a database of materials and their properties [6; 7], to gain data on the storage density [8], and to evaluate the application in lab-scale reactors [9]. However, several limitations exist when using pure salt hydrates, such as swelling and agglomeration of the salt during hydration, that reduce mass transfer; low thermal conductivity, that reduces heat transfer; significant volume changes and degradation under cycling [10]. A common approach proposed to solve such issues is the use of composites, where a matrix is used in order to work as a support structure and a heat and mass transfer enhancer for the salt. Several combinations of salt hydrates and matrices for thermal energy storage exist. One of the salts that have been extensively evaluated in the scientific literature is MgSO\({}_{4}\cdot\)7H\({}_{2}\)O, whose theoretical energy storage density is 2.8 GJ/m\({}^{3}\)[11] and can be used under both ambient pressure and sub-atmospheric pressure [12], thus being suitable for the application in open cycles or closed cycles. The salt can be efficiently regenerated also at temperatures \(<\) 150 \({}^{\circ}\)C, compatible with the integration in low-grade heat recovery systems or solar systems in buildings [13]. Several matrices have been proposed in combination with MgSO\({}_{4}\cdot\)7H\({}_{2}\)O, such as clay, pumice [14], zeolites and zeotypes [14; 15; 16], silica gel, activated carbon, and vermiculite [17]. Results demonstrated that the use of composites allows for an increase of the TES potentiality, in terms of energy storage and dynamic performance, thanks to the increase in the thermal conductivity of the material [14; 17; 18]. Nonetheless, the investigated matrices present two main issues: they are rigid structures, that can therefore limit the mechanical stability of the material over cycling, especially in view of the expansion/contraction of the grains of the salt during hydration/dehydration; and the amount of salt that is possible to insert inside the structure is limited. In order to overcome such an issue, a different approach was proposed in the literature, i.e., the use of flexible foams as porous matrices for salts. In such a context, an innovative concept was proposed by [PERSON] et al. [19], employing a polymeric macro-porous foam to host MgSO\({}_{4}\cdot\)7H\({}_{2}\)O salt. Among the advantages offered by foams, they allow further increase of the heat transfer surface with respect to porous materials and, in case of polymeric foams, they are flexible enough to host sufficient quantities of salt inside the pores, thus obtaining a high volumetric storage density and high mechanical stability.