Thermal stability of Na2CO3-Li2CO3 as a high temperature phase change material for thermal energy storage

The eutectic Na2CO3-Li2CO3 salt is widely used as an electrolyte in the Molten Carbonate Fuel Cell because of its high both ionic conductivity and operating temperature. The major objective of this study was to investigate the thermal stability of the binary salt as a phase change material for thermal energy storage through thermal decomposition analysis in different environments and thermal cycling. The composition of the binary salt was determined using FactSage software 6.4 and its thermophysical properties were experimentally investigated using a Simultaneous Thermal Analyzer and X-Ray Diffraction. The results show that the eutectic Na2CO3-Li2CO3 salt has a melting/solidification point of 498.3 ± 0.1 °C/483.9 ± 0.1 °C, with a heat of fusion of 330.8 ± 0.6 J/g and heat of solidification of 329.2 ± 0.3 J/g. The thermal decomposition analysis indicates that the eutectic molten salt has good thermal stability in a CO2 environment without weight loss at temperatures up to 600 °C. However, 0.8% weight loss is observed at 500 °C in a N2 atmosphere, which is associated with the thermal decomposition of the eutectic salt to produce corresponding metal oxides and CO2. The CO2 environment could inhibit the thermal decomposition, thus resulting in higher thermal stability of the binary salt in CO2 than when in N2. Hence, an operating environment has a large effect on the thermal stability of the eutectic Na2CO3-Li2CO3 salt. The eutectic salt exhibits high thermal stability in CO2 after 500 thermal cycles with little change in its thermal properties compared with the initial sample.