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.

Automated summary

Calcium nitrate tetrahydrate shows potential prospects as a room temperature phase change material due to its appropriate melting point and high enthalpy. However, the supercooling problem hinders its widespread use in energy storage. In this study, the microscopic structure of liquid calcium nitrate tetrahydrate at different temperatures is examined using X-ray diffraction and Raman spectroscopy. The results are compared with the crystalline structure of calcium nitrate tetrahydrate. It is found that the structure of the liquid state differs from that of the solid state, with the nitrate ions acting as dioxygen bridges in the liquid state and forming longer chains or shorter ring structures. The findings provide a theoretical basis for addressing the supercooling problem.