## 1 Introduction and Background  
Solar energy is one of the green alternatives to fossil energy that could meet the growing energy demand [1]. However, as with many renewable energy sources, solar energy is limited by its fluctuating nature, known as intermittence in daily and seasonal cycles [2]. There are various forms of energy storage, such as mechanical, electrical, thermal, and chemical [3]. Thermal energy storage (TES) was early recognized as a solution to managing the solar irradiation excess and the energy demand with no known environmental damage [4]. Thermal energy storage can be achieved according to three physical principles, i.e., (i) sensible heat thermal energy storage (SHTES) based on raising the temperature of the material (solid or liquid) without involving a phase change [5]: (ii) thermochemical storage (TCS), which refers to the heat absorbed or released during a reversible chemical reaction [6]; and (iii) latent heat thermal energy storage (LHTES) based on the phase change of the material. Each form is characterized by a specific capacity, storage period, efficiency, rate of charge and discharge, and its cost [7]. Phase change materials (PCMs) are recognized as promising LHTES materials that allow absorbing and releasing latent heat during phase change transition [8]. Energy absorption occurs in three consecutive stages ranging from storage by sensible heat and latent heat. As the temperature increases below the phase transition temperature, the material's specific heat (Cp) is absorbed as sensible heat. During the phase change, energy storage occurs isothermally by absorbing heat in the form of latent heat. Finally, above this transition, the temperature of the material increases, and storage by sensible heat takes place [9]. PCMs remain mainly selected for their thermal properties. For a specific application, the phase change temperature should be suitable for the operating one. At the same time, they should present high latent heat to ensure maximum latent heat storage capacity by using a small material amount [10]. PCMs exhibiting high thermal conductivity have a higher heat absorption and release rate than other materials [11]. Moreover, chemical stability is an essential criterion in the selection of PCMs. Therefore, chemically stable materials allow PCMs to operate at the desired temperature without degradation or damage after several heating/cooling cycles [12]. The chemical composition of PCMs and the phase change temperature must be selected in function of the operating conditions to avoid possible reactions with other materials through the application. In addition, the used material can be neither hazardous nor flammable to prevent toxic emissions that can be harmful to the environment and/or humans during its preparation and use. Moreover, the vapor pressure should be as low as possible during the phase change at the operating temperature, and preferably, the volume change during solidification is negligible [13]. PCMs having a small degree of supercooling does not impact the phase transition temperature. Finally, PCMs with suitable crystallization and nucleation rates are expected to be potential material candidates for energy storage [14; 15]. Despite many desirable properties, most PCMs undergo leakage issues during the phase change process and have significant volume change and low thermal conductivity, limiting their direct application [16]. Many efforts have been made to improve the heat transfer of PCMs to overcome these drawbacks. Since 2005, several studies have been focused on the thermal conductivity improvement of PCMs. Different materials are used as heat transfer enhancers, such as copper, nickel, aluminum, and carbon fibers [17]. [PERSON] et al. reviewed different thermal conductivity enhancement techniques, such as developing PCMs composite materials using thermal conductors (e.g., expanded graphite, metallic foams, etc.) or by encapsulation with high thermal enhancer shell materials such as inorganic materials and carbon-based materials [18].