Surface-Modified Compressed Expanded Graphite for Increased Salt Hydrate Phase Change Material Thermal Conductivity and Stability

Thermal energy storage (TES) will play an essential role in the push toward efficient, electrified buildings, and phase change materials (PCMs) offer a high potential to fill that need. While organic PCMs have been studied extensively for TES, the literature on inorganic PCMs such as salt hydrates is scarce despite their advantages. Calcium chloride hexahydrate (CaCl2 6H2O, CCH) has a phase transition temperature of interest for buildings' heating/cooling applications but like other salt hydrates, suffers from supercooling, phase separation, and low thermal conductivity. Researchers have proposed strontium chloride hexahydrate (SrCl2 6H2O, SCH) as an effective lattice-matched nucleating agent for CCH; additionally, porous compressed expanded graphite (EG) offers thermal conductivity enhancement along with form stability. This paper presents the fabrication of a PCM composite consisting of CCH, nucleating agent SCH, and surface-modified EG using a nonionic surfactant, which addresses the concerns related to salt hydrates while providing excellent cycling stability on a larger scale. The contributions of this study include (1) high thermal conductivity (4 +/- 0.2 W/m K), (2) a significant reduction in supercooling (less than 1 degrees C), (3) high composite latent heat (up to 183 J/g), and (4) eliminated phase separation demonstrated via improved cycling stability with no reduction in latent heat for up to 200 cycles.