Composites carbon foams based on sucrose-based char matrix and graphite filler were prepared and characterised with the aim of hosting sugar alcohols as phase-change materials (PCMs) in the context of thermal energy storage (TES). Seasonal solar TES demands an excellent undercooling of the molten PCM infiltrated in the foam, so that the heat can be stored as long as possible. The present paper demonstrates how the surface of such composite foams, i.e., comprising two carbon phases of different reactivities, can be modified for promoting undercooling. For that purpose, 8 different hydrophobisation treatments were applied, and the results were compared with those of the non-treated foam, in which heterogeneous nucleation could not be avoided. We show that one kind of functionalisation was successful, i.e., it fully preserved the melting point and the enthalpy of melting of the hosted phase-change material and completely avoided the heterogeneous nucleation of the PCM, while maintaining the thermal conductivity in the range required for this kind of application.