## 1 Introduction  
Thermal energy storage (TES) for space heating and cooling of buildings is becoming more and more important due to the rising cost of fossil fuels and environmental concerns. It saves premium fuels and leads to the system being more cost effective by reducing the waste of energy and capital costs [1; 2]. Sensible heat storage (SHS) using thermal mass of the structural elements is the most common way to storage thermal energy in buildings. It can be used for the storage and release of thermal energy in a passive way. However, in comparison to latent heat storage, a much larger volume of material is required to store the same amount of energy by changing the phase of a storage material. Therefore, an effective way to reduce the building's energy consumption for heating and cooling is by incorporating phase change materials (PCMs) in latent heat thermal energy storage (LHTES) systems of various building elements such as walls, windows, ceilings and floors [3; 4].  
PCMs are generally divided into organic PCM, inorganic PCM, and eutectic PCM. Typical inorganic PCMs include water, aqueous solution, salt hydrate, and molten salt, while organic PCMs include paraffin, fatty acid, ester and sugar alcohol. Besides, eutectic mixtures of two or more miscible pure PCM constituents that solidify simultaneously out of the liquid at a minimum freezing point have been widely developed to enrich the diversity of PCMs [5].  
Commonly, inorganic PCMs suffer from decomposition and supercooling, which can further affect their phase change properties [6]. In contrast, organic PCMs are more stable and incorrodible than inorganic PCMs in the surrounding container [7]. In particular, paraffin PCMs are more widely used for application in building because they possess large latent heat capacity, low vapor pressure, and good thermal stability [8; 9]. Also, many fatty acid ester PCMs have lower latent heat capacity than paraffin PCMs, but most of them are more commercial and economical than paraffin PCMs [10]. However, fatty acid ester and paraffin PCMs also have low thermal conductivity and leakage of liquid problems that disturb their application to energy storage [11; 12; 13]. Manufacturing shape-stabilized PCM (SSPCM) by impregnating the PCMs into high thermal conductivity materials such as carbon materials with porous structure resolves these problems. Exfoliated graphite nanoplatelets (xGnP) which are porous and nano-sized carbon materials are suitable [14; 15; 16; 17; 18]. Additionally, they have a layered structure and are very cost effective. The low coefficient of thermal expansion of xGnP, is a desirable property for composite structural applications [19; 20; 21; 22].  
In this research, we prepared Mixed SSPCMs by impregnating two types of PCMs, coconut oil and \(n\)-hexadecane into the xGnP at different proportions to improve thermal performance and investigate the difference between their performances. We used a vacuum impregnation process that guarantees high heat storage of fatty acid ester PCM and paraffin PCM due to capillary forces and surface tension forces during the incorporation process [23]. Further, we analyzed the microstructure, chemical stability, thermal storage performance, thermal stability, and thermal conductivity of the Mixed SSPCMs by using SEM, FT-IR, DSC, TGA, and TGi.