The inorganic salt hydrate PCM, CaCl2 center dot 6H(2)O, has a promising potential for thermal energy storage. However, this salt hydrate has tremendous issues of supercooling (the supercooling degree was 25.5 degrees C as found in this research) which restricts its utilization in practical applications. The present study aimed to reduce the supercooling degree of CaCl2 center dot 6H(2)O through the innovative use of hydrophilic graphene oxide (GO) nano-sheets and SrCl2 center dot 6H(2)O as nucleating agents. The percentages of these nucleating agents in PCM were 0.005, 0.01, 0.02, 0.05 and 0.08 wt% for GO and 0.2, 0.5 and 0.8 wt% for SrCl2 center dot 6H(2)O, respectively. Based on the synergistic/additive effects, a novel composite PCM based on CaCl2 center dot 6H(2)O, GO and SrCl2 center dot 6H(2)O was proposed and developed. The test results showed that the composite PCM containing 0.02 wt% GO nano sheets and 0.8 wt% SrCl2 center dot 6H(2)O effectively lowered the supercooling degree of CaCl2 center dot 6H(2)O from 25.5 degrees C to as low as 0.2 degrees C (approximately 99.2% supercooling degree of CaCl2 center dot 6H(2)O was reduced). To the best of authors' knowledge, it was the first time that the combination of GO and SrCl2 center dot 6H(2)O was found effective in reducing supercooling degree of CaCl2 center dot 6H(2)O. Moreover, 99.2% supercooling reduction of CaCl2 center dot 6H(2)O is considered the best achievement so far when compared to other studies related to supercooling reduction of CaCl2 center dot 6H(2)O. Finally, the solidification enthalpy of the composite PCMs was found to be as high as 207.90 J/g. Therefore, the developed composite PCM is an excellent candidate for thermal energy storage applications in buildings. (C) 2017 Elsevier B.V. All rights reserved.