Observation of a reversible order-order transition in a metal-organic framework – ionic liquid nanocomposite phase change material

Composites composed of ionic liquids (ILs) and metal-organic framework (MOF) materials have been proposed for a range of potential applications, including gas separation, ion conduction, and hybrid glass formation. Here, we discover an order transition in an IL@MOF composite using the commodity compounds CuBTC (copper benzene-1,3,5-tricarboxylate) and EMIMTFSI (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide). This transition absent for the bare MOF or IL provides an extended super-cooling range for latent heat storage at a capacity similar to that of soft paraffins, in the temperature range of ~ 220 C. Structural analysis and in situ monitoring indicate electrostatic interaction of the IL molecules with the Cu paddle-wheels, leading to a decrease in pore symmetry at low temperature. These interactions are released above the transition temperature, what reflects in a reversible volume expansion of the MOF composite on the order of 1 %.