Surface evolution of eutectic MgCl2·6H2O-Mg(NO3)2·6H2O phase change materials for thermal energy storage monitored by scanning probe microscopy

Abstract

Surface evolution of eutectic salt hydrate phase change materials (PCMs) plays an important role in progressive degradation, and its tracking and control are crucial to the cycle stability in latent heat storage. In this study, we combined Peak Force quantitative nanomechanics with differential scanning calorimetry to monitor the topography evolution, nanomechanical performances, and corresponding thermal properties of the eutectic MgCl2·6H2O-Mg(NO3)2·6H2O PCM (EPCM). Topography of the EPCM was highly similar to those of the corresponding single crystallohydrates, accounting for its congruent melting and solidifying behavior. Surface evolution-induced initial phase separation at the nanoscale level was directly imaged. This deterioration is rooted in adsorption/evaporation of the surface water molecules, and can be restored accurately. Based on this, a universal strategy to eradicate phase segregation was proposed, with no significant degradation after 1500 thermal cycles. Moreover, evolution of the EPCM containing thickener hydroxyethyl cellulose and its effects on local supercooling due to the formed crosslinking networks were explored, and stable supercooling inhibition (≤0.6 °C) was achieved. This study provides a better understanding of the evolution mechanism and its effects on the thermal behavior, and can be used as an effective guide for improving the long-term stability of low-cost salt hydrate-based latent heat storage materials.