**1. Introduction**

Phase change materials (PCMs) are pivotal in advancing sustainability across various industries, offering enhanced energy efficiency, reduced environmental impacts, and bolstered sustainability. Central to thermal energy storage (TES) and thermal management application (TMA), PCMs facilitate innovative solutions in cooling systems, electronics, and building designs. Their application ranges from conserving water in power plant condensers to minimizing the environmental footprint of data centers through efficient cooling strategies. Additionally, PCMs revolutionize building architecture by improving energy efficiency and lessening dependence on traditional heating, ventilation, and air conditioning (HVAC) systems, aligning with sustainability objectives. This chapter delves into the application of PCMs in

TES systems, exploring their use with additives like nucleators and gelling agents to improve performance in both organic and inorganic forms. It addresses material compatibility with various metals and alloys to ensure the long-term reliability of PCMbased solutions. Moreover, the chapter underscores the potential of machine learning (ML) in enhancing the sustainability, performance, and reliability of PCMs. Through the Cold Finger Technique (CFT) and ML, it tackles challenges such as supercooling, showcasing the significant role of PCMs in sustainability efforts and the innovative impact of ML on optimizing their application and environmental benefits.
