## 2 Results and Discussion  
Figure 1 presents the FT-IR spectra of different carbon/OD and carbon/CNTs/OD composites, in comparison with the corresponding spectra of carbon matrices and raw OD. Figure 1: FT-IR spectra of: (**a**) activated carbon samples; (**b**) expanded graphite samples; and (**c**) ceramic carbon foam samples. The FT-IR spectra of activated carbon samples are shown in Figure 1a. The spectrum of the AC sample exhibits characteristic adsorption bands of activated carbon materials and confirms the presence of functional groups on its surface. More specifically, the broad adsorption band at 3440 cm\({}^{-1}\) corresponds to O-H stretching vibrations probably derived from hydroxyl groups and/or water molecules [25]. The adsorption bands, observed at 2920 and 2850 cm\({}^{-1}\), correspond to C-H stretching vibrations of aliphatic methylene groups and suggest the possible presence of residual organic content in the activated carbon [26]. Furthermore, the adsorption bands at 1700 and 1627 cm\({}^{-1}\) are attributed to C=O vibrations of carboxylic groups and C=C stretching vibrations on aromatic groups, respectively [26; 27], while they may also have contributions from vibrations of -COOH and/or -COO\({}^{-}\) functional groups [27]. The small adsorption bands at 1415, 1362 and 1224 cm\({}^{-1}\) are indicative of C-O stretching vibrations in carboxylic, ether, ester or hydroxyl groups, respectively [25; 28]. The functional groups that are observed from the FT-IR spectra of AC contribute to its high surface reactivity and adsorption capacity. On the other hand, the spectra of AC/OD and AC/CNTs/OD composites exhibit additional features, compared with the spectrum of pure AC. The adsorption bands at 2920 and 2850 cm\({}^{-1}\) appear obviously stronger and are similar to those bands observed on the octadecane spectrum indicating that they came from the C-H symmetric and asymmetric stretching vibrations of the methylene groups on the long alkane chain of octadecane [29]. Also, the new bands at 1469 and 722 cm\({}^{-1}\), which are also shown on the OD spectrum, are attributed to C-H bending vibrations of aliphatic hydrocarbons and -CH\({}_{2}\) group's in-plane rocking vibrations of PCM molecules [30; 31]. These features indicate the successful incorporation of PCM on the activated-carbon surface while similar results are also shown in the spectra of the expanded graphite and the ceramic carbon foam samples (Figure 1b and c, respectively). Leaching tests for all composite materials were conducted, in order to evaluate the stability of the materials and Figure 2 shows the filter papers after conduction with the composite materials for 72 h at 80 \({}^{\circ}\)C. In all cases, the paper is clear that no oil stamps are observed indicating that no octadecane leakage occurred under these conditions. Thus, the leaching tests confirm that activated carbon, expanded graphite and ceramic carbon foam, with and without CNTs, effectively stabilize octadecane, offering promising solutions for applications requiring reliable thermal energy storage and minimal environmental impact. The microtomography CT images of the three samples, EG, EG/OD and EG/CNTs/OD are shown in Figure 3a, b and c, respectively. According to Figure 3a, EG exhibits a highly porous structure, as indicated by the predominance of black areas in the image, which correspond to less dense regions filled with air voids. In the EG/OD composite, octadecane (indicated by purple areas) is uniformly distributed throughout the porous structure of EG. In the case of the EG/CNTs/OD composite, octadecane is also uniformly distributed, but some aggregates are observed, likely due to the presence of CNTs in these regions. The corresponding murotomography CT images of the AC and the CCF samples are presented in Figures S1 and S2, respectively.