As illustrated in Fig. 1, graphene oxide in the aqueous phase was adsorbed on the surface of MMT via the hydrogen bond due to the surface hydroxyl group. After the graphene oxide was in situ reduced by NaBH4, the surface of MMT was modified with reduced graphene oxide. Reduced graphene oxide modified montmorillonite (RGO-MMT) was used as supporting material to load stearic acid (SA) via vacuum impregnation method. Reduced graphene oxide connects between SA and MMT as thermal transfer bridge at the interface of obtained SA/RGO-MMT form-stable phase change material. For comparison, stearic acid/montmorillonite (SA/MMT) was prepared via similar vacuum impregnation method without graphene.
The morphologies of samples were observed by scanning electron microscope (SEM) and transmission electron microscopy (TEM). As shown in the SEM and TEM images (Fig. 2a and b), MMT exhibits 2D lamellar morphology with 10–20 µm in width and 0.5-2 µm in thickness. The SEM image of RGO-MMT (Fig. 2c) indicates MMT and RGO nanosheets are stacked with each other, which exhibits layered structures. In the TEM image of RGO-MMT (Fig. 2d), RGO nanosheets cover on the surface of MMT, which ensures the uniform distribution of MMT and RGO. In the SA/MMT (Fig. 2e), MMT was wrapped with the continuous SA to form a block. From the SEM image of SA/RGO-MMT (Fig. 2f), the layered structures of RGO-MMT could be observed, and SA is filled in the layered structures, which contributes to the interfacial thermal transfer between SA and the supporting materials.
The phase structures of samples were investigated by X-ray diffraction (XRD), and the XRD patterns are shown in Fig. 3. The reflection at 1.2 nm is found in the XRD pattern of MMT, which is corresponding to (001) basal spacing of Na-montmorillonite[33]. The XRD pattern of SA exhibits two obvious reflections of stearic acid at 21.5° and 23.8°. The XRD pattern of RGO-MMT is similar with that of MMT, and no obvious reflection is attributed to the RGO. The reflections corresponding to MMT and SA could be found in the XRD patterns of SA/MMT and SA/RGO-MMT, and the phase of SA possesses high crystallinity. The reflection intensity of MMT in SA/RGO-MMT is relatively lower than that of SA/MMT, which suggests the higher content of SA and thermal storage capacity in SA/RGO-MMT.
The vibrational bands and interfacial characteristics of samples were studied by Fourier transform infrared (FTIR) spectra[34] (Fig. 4). In the FTIR spectrum of MMT, the bands at 1022 and 471 cm− 1 are corresponding to the stretching and bending vibration of Si-O in silicon-oxygen tetrahedron. The bands at 3620 and 1637 cm− 1 are assigned to the stretching and bending vibration of hydroxyl groups, and the broad bands at 3453 cm− 1 are ascribed to stretching vibrations of H-O-H in water [35, 36]. For SA, the bands corresponding to –CH2 and –CH3 are found at 2918, 2849, 1466 and 723 cm− 1, and the band at 1703 cm is attributed to the vibration of C = O. Compared with FTIR spectrum of MMT, that of RGO-MMT exhibits less hydroxyl groups, which might be because the hydroxyl groups on the surface of MMT are covered or removed due to the modification of RGO. The bands ascribed to MTM and SA are found in the FTIR spectra of SA/MMT and SA/RGO-MMT, and no obvious shift of band was observed, indicating the physical interactions between SA and RGO-MMT. The band intensity of SA in SA/RGO-MMT is relatively higher than that in SA/MMT, which might be due to the higher content of SA in according with the result of XRD.
The energy storage capacity is one of the most critical parameters for phase change material. The phase change properties of SA, SA/MMT and SA/RGO-MMT are analyzed via the differential scanning calorimetry (DSC) method (Fig. 5), and the corresponding phase change temperature and enthalpy are listed in Table 1. The pristine SA has a melting enthalpy of 208 J/g with the melting temperature of 54.9 oC and a freezing enthalpy of 212 J/g with the freezing temperature of 53.0 oC, respectively. The melting and freezing enthalpies of SA/RGO-MMT are 159 and 165 J/g, which are rather higher than that of SA/MMT (59 and 52 J/g). It is because that SA/RGO-MMT has higher content of SA, which has been confirmed by XRD and FTIR analysis.
Table 1
Thermal properties of SA, SA/MMT, SA/RGO-MMT before and after thermal cycling.
Samples | Melting temperature (Tm,oC) | Freezing temperature (Tf,oC) | Melting enthalpy (ΔHm, J/g) | Freezing enthalpy (ΔHf, J/g) | Extent of supercooling (Tm-Tf,oC) |
SA | 54.9 | 53.0 | 208 | 212 | 1.9 |
SA/MMT | 53.5 | 50.4 | 59 | 52 | 3.1 |
SA/MMT-RGO | 52.8 | 51.4 | 159 | 165 | 1.4 |
SA/MMT-RGO after thermal cycling | 52.9 | 51.6 | 155 | 163 | 1.3 |
Compared with the reported composite PCMs in the literatures (Table 2) [25, 37–40], SA/RGO-MMT possesses highly competitive energy storage capacity. The extent of supercooling of SA/RGO-MMT is 1.4 oC, lower than that of pristine SA (1.9 oC) and SA/ MMT (3.1 oC). The lower extent of supercooling of SA/RGO-MMT is beneficial in energy storage process of phase change, which could be attributed to the higher heat transfer efficiency in SA/RGO-MMT.
Table 2
Comparison of thermal properties of SA/MMT-RGO with the reported composite PCMs.
Composite PCMs | Melting temperature (Tm,oC) | Freezing temperature (Tf,oC) | Melting enthalpy (ΔHm, J/g) | Freezing enthalpy (ΔHf, J/g) | References |
Stearic acid/mesoporous SiO2 microspheres | 71.5 | 62.4 | 135.3 | 133.5 | [37] |
Stearic acid/halloysite | 53.7 | 49.1 | 106 | 107 | [38] |
Stearic acid/activated montmorillonite | 59.9 | 55.1 | 85 | 89 | [25] |
Stearic acid/graphene microcapsule | 72.0 | 64.0 | 166 | 163 | [39] |
Stearic acid/graphite and activated bentonite | 53.4 | 52.9 | 85 | 84 | [40] |
SA/MMT-RGO | 52.8 | 51.4 | 159 | 165 | This work |
Energy storage and release rates of phase change material have great influence in practical application of energy storage. Thermal storage and release performances of SA, SA/MMT and SA/RGO-MMT were evaluated via temperature variation curve over time (Fig. 6). The time period of SA/RGO-MMT from the ambient temperature to melting temperature is 163 s, much less than that of SA (410 s) and SA/MMT (360 s). Compared with SA, MMT and RGO-MMT in SA/MMT and SA/RGO-MMT could enhance heat transfer of composite phase change materials. Furthermore, SA/RGO-MMT possesses faster heating rate than SA/MMT due to the higher interfacial thermal transfer efficiency. As shown in Fig. 7a, the thermal conductivity of MMT-RGO is 0.921 Wm− 1K− 1, which is more than twice that of MMT (0.372 Wm− 1K− 1). The graphene with high thermal conductivity connects between SA and MMT, which could greatly enhance the interfacial thermal transfer in SA/RGO-MMT (Fig. 7b). SA/RGO-MMT has fastest cooling rate from 80 oC to the ambient temperature, the cooling process of which only takes about 1070s. SA/RGO-MMT possesses faster heating and cooling rates, indicating the higher energy storage and release efficiency and less time cost during energy storage utilization.
Figure 7. (a) Thermal conductivity of MMT-RGO and MMT, (b) interfacial structure and thermal transfer of SA/RGO-MMT
Thermal reliability plays a significant role in the energy storage application of phase change material. The properties of SA/RGO-MMT before and after 100 thermal cycling are compared to evaluate the thermal storage and release stability. The XRD patterns and FTIR spectra of SA/RGO-MMT before and after thermal cycling (Fig. 8a and b) exhibit no remarkable difference, indicating the thermal cycling makes little impact on phase structures and interfacial characteristics of SA/RGO-MMT. From the SEM images of SA/RGO-MMT before and after thermal cycling (Fig. 8c and d), the morphology of SA/RGO-MMT appears little change with only a few agglomerations, which is favor for heat transfer. The melting and freezing enthalpies of SA/RGO-MMT have a negligible reduction after thermal cycling (Fig. 8e), and the extent of supercooling decreases slightly. The thermal storage and release curves of SA/RGO-MMT before and after thermal cycling (Fig. 8f) are similar, and the energy storage and release rates remain unchanged. Therefore, SA/RGO-MMT has excellent thermal reliability after 100 thermal cycling.
Figure 8. (a) XRD patterns, (b) FTIR spectra, SEM images of (c) before and (d) after thermal cycling, (e) DSC curves and (f) thermal storage and release curves of SA/RGO-MMT before and after 100 thermal cycling.