## 5 Graphene Applications in Thermal Phase-Change Materials
Here, the XPS resultsindicate the presence of hydrocarbons with a binding energy (BE) of 284.9 eV. The transition centred at 284.9 eV is characteristic of this paraffin's C\({}_{1s}\) spectrum and corresponds to the H-C and C-C bonds. As the relative composition of graphene increases in the hybrid graphene--PCM composite the intensity of the C\({}_{1s}\) peak changes. The XPS spectra indicate that as more graphene is added and mixed in with the paraffin wax the sp\({}^{1}\) hydrocarbon quantity decreases. The reduction in the number of these H-C chains in the hybrid composites suggests a reaction is occurring between the graphene and the alkane chains of the paraffin in the hybrid-PCM composite. There are other examples of the use of graphitic nanoparticles for enhancement of paraffin waxes. Additions of nanoparticles to PCMs have been used for temperature regulation of property contrasts between the liquid and solid states. In a specific hexadecane--graphite suspension, a sharp rise in thermal conductivity was observed as the host hexadecane solidified at its transition temperature of 18 \({}^{\circ}\)C [116]. The change in thermal conductivity by a factor of about three near the transition temperature was achieved with a 0.8% volume fraction of exfoliated graphite flakes processed through sulfuric acid intercalation, microwave expansion, and ultrasonic dispersion [116]. The addition of nanoparticles to PCMs not only aims to improve heat transfer during the melt phase but has also shown to enhance thermal energy storage and solidification of paraffin-based PCMs as with graphite nanofibers (GNFs) in _n_-tricosane [117]. Such GNFs were grown by catalytic deposition of hydrocarbons and carbon monoxide on an iron catalyst in a tube furnace. The resulting composites with 10 wt. % of GNF provided for a solidification time reduction by 61% over paraffin samples, demonstrating its benefits to pulsed power profile systems. Another composite of 1 vol % multilayer graphene suspended in hexadecane demonstrated a solid-liquid thermal conductivity contrast ratio range between \(\sim\)2 and 3.0 by freezing rate control [118]. In all aforementioned composites for practical applications and their regulating properties, the thermal transport mechanism occurred through both the nanoparticles and host medium at both liquid and solid host phases. Several instances demonstrate that the adjustment of particulate concentration of the graphite nanoparticles and combinations with a  
Figure 3: Hybrid graphene—paraffin phase change material. **(a)** Optical image of the PCM samples showing the change in color from white to black with increasing graphene content; **(b)** Scanning electron microscopy image of the hybrid graphene-PCM indicating uniform distribution of the graphene flakes. one-dimensional CNTs and bulk material owing to the large difference in the phonon density of states. The attachment need not be covalent for improved heat conduction between fillers and matrix [133]. Thermal coupling in the case between graphene fillers and paraffin matrix is likely to be stronger than other matrix-filler combinations. The density function theory (DFT) calculations and molecular dynamics (MD) simulations indicated a possibility of strong increase of the thermal conductivity in ordered graphene composites with organic matrix where the heat transport is along the direction of the graphene atomic planes. Enhancements of up to \(K\)/\(K_{m}\approx 360\) have been reported for a graphene loading of 5% [135]. The exceptionally strong anisotropic increase in \(K\) was attributed to graphene's planar geometry and good coupling to the octane molecules [135, 136, 137]. The heat carrying phonon modes excited in graphene coupled well to those in the organic molecules of the matrix material. Although a direct comparison between the experimental data for the graphene-paraffin composite and the composite studied in Ref.