Structure of Phase Change Energy Storage Material Ca(NO3)2·4H2O Solution

Calcium nitrate tetrahydrate, Ca(NO3)(2)center dot 4H(2)O, has the potential prospects as a room temperature phase change material due to appropriate melting point and high enthalpy. However, the supercooling problem prevents its widespread use in an energy storage field. In this work, the microscopic structure of liquid Ca (NO3)(2)center dot 4H(2)O at different temperatures are studied by X-ray diffraction and Raman spectroscopy and compared with that of Ca(NO3)(2)center dot 4H(2)O crystal. The first coordination layer of a calcium ion contains 3 water molecules and 4-5 nitrate ions. A nitrate ion acts as a dioxygen bridge to link two calcium ions in a monodentate fashion and form a longer chain or a shorter ring structure in the liquid state. In the crystalline state, two oxygen atoms of a nitrate ion bind with a calcium ion in a bidentate manner. The structure and stability of [Ca-m(2)+(NO3)(n)H2O2)q] clusters are examined by DFT calculations to confirm the results obtained by X-ray diffraction and Raman spectroscopy. The key factor of severe supercooling problem would be the conversion from edge-sharing mono-oxygen bridge bonds of Ca-NO3(Ca-O(NO)O-Ca) in liquid to corner-sharing dioxygen bridge bonds of Ca-NO3(Ca-O(NO2)-Ca) in solid. This work provides a theoretical basis for solving the supercooling problem. (C) 2022 Elsevier B.V. All rights reserved.