## 1 Introduction and Background
Encapsulation techniques are assumed to be an efficient way to stabilize PCMs' shape [19]. The incorporation of PCMs into a protective shell leads not only to limiting undesired interaction and preventing their leakage but also to an increase in the specific surface contact area, and therefore, improve the thermal transfer [20]. The encapsulation corresponds to all technologies that aim to envelop a solid, liquid, or gaseous active principle within a coating material. The objectives of encapsulation are multiple and depend on the desired specific application [21]. Primarily, encapsulation aims to protect the active substance from the external environment (reactivity with other materials, pH, temperature, etc.) during the application. Secondly, it prevents the oxidation of the core material and increases its durability. Thus, the core material's thermal, physical, and chemical stability can be improved, whereas their volatility and flammability can be reduced [22]. Capsules resulting from different encapsulation techniques are termed according to their size, i.e., macroencapsulation refers to the encapsulation of PCMs in any type of containers such as tubes, spheres, or panels where the size of these containers is usually larger than 1 cm; the microcapsules range from less than 1 \(\upmu\)m to 1000 \(\upmu\)m in diameter size, and the nanocapsules are synthesized at the nanoscale [23]. Microencapsulation techniques are based on two steps: preparing an emulsion or a suspension of the active ingredient and forming a shell. The emulsion preparation determines the size distribution of the capsules, which depends on the operating conditions (the rate and time of agitation, viscosity, the mass ratio of different phases, etc.) [24]. The formation of the shell depends upon many factors (pH, temperature, solubility, and concentration of shell materials) that should be adapted to the encapsulation method and the employed core material [24]. The main application fields of the encapsulation process are the pharmaceutical [25], cosmetic [26], and food industries [27]. PCMs encapsulation is achieved through different methods. According to the literature survey, PCMs have been gaining research interest since the 1980s [28; 29], including the direct and indirect applications (Figure 1). The number of publications has been steadily increasing until 2016. From 2016 to 2020, extensive efforts were devoted to exploring sustainable PCMs technologies to face the growing demand for energy consumption. It can be depicted by the number of PCMs publications, which reached 17,111 publications in 2019. Indeed, scientific publications concerning PCMs microencapsulation reached 17,615 publications in 2020. In the last few years, there has been a growing interest in the research on storage materials since green energy has become required to respond to the various IPCC (Intergovernmental Panel on Climate Change) reports. They have highlighted the impending problems leading to the acceleration of climate change and have urged immediate action toward renewable energy, efficiency, and limiting greenhouse gas emissions. Therefore, the increase in publications carrying out the enhancement of PCMs performances has allowed their application areas, such as building [30], smart textile [31], micro electro mechanical system [32], air conditioning [33], etc. Microencapsulation of PCMs has been widely studied by several researchers [34; 35; 36; 37]. Several reviews focused on PCMs encapsulation and its properties for energy storage applications. However, recent researches concerning PCMs encapsulation are focused on developing innovative microcapsules with additional functionalities (e.g., pollutants degradation by photocatalytic activity, antibacterial for medical application, photoluminescence for thermosensitive and photosensitive sensors, etc.). Therefore, this article aims mainly to highlight the innovation and particularity of the new types of microcapsules.