Microsphere Structure Composite Phase Change Material with Anti‐Leakage, Self‐Sensing, and Photothermal Conversion Properties for Thermal Energy Harvesting and Multi‐Functional Sensor

Abstract The low thermal conductivity and liquid melt leakage of phase change materials are long‐standing bottlenecks for efficient and safe thermal energy harvesting. Although high thermal conductivity materials combined with phase change materials can address the thermal conductivity problem, ensuring no leakage and no reduction in latent heat in the meantime remains challenging. Here, a strategy to synthesize microsphere‐structured phase change composites by encapsulating phase change materials in graphene via emulsion polymerization (no additional emulsifier) and chemical reduction is proposed. Multiple graphene sheets are connected to construct an efficient thermally conductive (increase 58.5 times in thermal conductivity) and electrically conductive network. The composite microspheres exhibit no leakage (<0.5%) and superior phase transition behavior after 1500 heating‐cooling cycles, and sense external environments such as temperature changes and water drops falling, allowing them to be engineered into devices for temperature monitoring. In addition, it converts electrical energy into thermal energy to achieve rapid temperature increases. The incorporation of polydopamine improves the photothermal efficiency of the phase change microspheres and senses light irradiation, offering a promising route to extend the single source of thermal energy. This method provides new insight into the photothermal integration and intelligent sensing of phase‐change materials.