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• Synthesized PCMs with unzipped multiwalled carbon nanotube oxides (UMCNOs) and PEG. • Thermal conductivity enhanced by 111.7 % with 6 wt% UMCNOs addition. • Heat storage and release times reduced up to 40 % and 30 %, respectively. • Thermally and chemically reliable for over 100 thermal cycles. Polyethylene glycol (PEG) as a phase change material (PCM) is being explored for latent heat energy storage systems due to its high thermal storage capacity. In the current study, PCM composites were synthesized with different mass fractions of unzipped multiwalled carbon nanotube oxides (UMCNOs) in PEG by melt mixing method. The influence of UMCNOs loading on the thermal conductivity (TC), phase change properties such as temperatures and latent heats of melting and solidifying, thermal energy storage and release rates were investigated. At a loading of 6 mass% of UMCNOs, the TC of the prepared PCM composite increased by 2.1 times as compared to that of pure PEG (0.21 W/m-K). Differential scanning calorimetry analysis showed that as the concentration of UMCNOs increased, there was a slight decrease in the melting temperature. Additionally, the latent heats of melting were found to be reduced by 1.11–9.46 % with the addition of 0.5–6 mass% UMCNOs. Further, the thermal energy storage and release rates were improved with the addition of UMCNOs in PCM composites. In addition, there was negligible change in phase change properties after 100 melting and solidifying cycles of PCM composite, indicating the great prospects for the latent heat energy storage application. [ABSTRACT FROM AUTHOR]