Probing crystal structures of dicarbamate phase change materials to inform structural design

Abstract

The isothermal nature of latent energy storage makes phase change materials (PCMs) uniquely suited to facilitate highly efficient “Thermal Batteries” for the storage of renewable energy. Nevertheless, suitable PCMs for this application are scarce, which is largely due to the challenges of targeting a high enthalpy of fusion among the various other property requirements of this application, including a T_m between 100 and 220 °C and the excellent thermal stability required for long-term operation at these temperatures. Key to targeting the diverse suite of properties needed is a comprehensive understanding of the structure–property relationships that govern them. Here, we probe the molecular origins of the thermal properties of five aliphatic dicarbamate PCMs that melt in this intermediate temperature range, for two of which we have previously demonstrated excellent thermal stability indicating long lifetimes in this application. With the use of single-crystal X-ray crystallography, in situ powder X-ray diffraction and static solid-state nuclear magnetic resonance spectroscopy, for the first time we study the solid-state structures of the dicarbamate PCMs and correlate structural features with packing arrangements and intermolecular interactions with thermal properties. The results elucidate structural features that can be incorporated into new PCMs to achieve favourable thermal properties for renewable energy applications, as well as some features which may encourage solid-state transitions in these compounds.