Summary In this work, diatomite (DM) was calcined at 400°C to obtain the pure surface and pores (DM‐1); then, the three kinds of shape‐stable composite phase change materials (ss‐CPCMs) of CaCl2·6H2O (CCH)/DM‐1, CH3COONa·3H2O (SAT)/DM‐1, and Na2SO4·10H2O (SSD)/DM‐1 were prepared by impregnation method. The hydrated salts were uniformly adsorbed on the surfaces and into the pores of DM‐1 by capillary action and surface tension. The addition of nucleating agent effectively reduced the supercooling degrees of hydrated salts, and the heterogeneous nucleation mechanism was employed to explain the supercooling suppression. The results showed that the supercooling degrees of the three hydrated salt ss‐CPCMs with optimal nucleating agent were less than or equal to 0.6°C. Moreover, the thermal conductivities of ss‐CPCMs were significantly improved by adding graphite, and the addition of 10 wt% graphite could increase the thermal conductivity of hydrated salt ss‐CPCMs by at least 70%. The package capacity of CCH, SAT, and SSD in three ss‐CPCMs with appropriate contents of nucleating agent and graphite was 58.1, 56.1, and 56.3 wt%, respectively. The DSC results showed that the phase change temperatures of the three ss‐CPCMs were approximately 28.8 to 57.8°C, and the latent heat was approximately 108.7 to 149.4 J/g. The XRD and FT‐IR results exhibited that the three ss‐CPCMs indicated understanding chemical compatibility. In addition, the results of 200 melt‐solidification cycles demonstrated that the ss‐CPCMs had excellent thermal cycle reliability. Thus, the hydrated salt/DM‐1 ss‐CPCMs showed great potential as heat storage materials for various heat storage applications.

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