## 5 Graphene Applications in Thermal Phase-Change Materials  
In this section we briefly outline the approach of [PERSON] and co-workers [29] for graphene's practical applications in thermal PCMs and outline results of other groups that reported the use of graphitic nanoparticles for such applications. As an example material system for the composite matrix we selected a specific paraffin wax. Paraffins or paraffinic hydrocarbons have the general composition of C\({}_{n}\)H\({}_{2n+2}\) and are straight-chain or branching saturated organic compounds [110]. Commonly used in PCMs, paraffin waxes have the advantages of low cost and availability, are chemically stable, and are durable to cycling. Paraffins are suitable for the thermal control of batteries with its large range of melting points and a high latent heat of fusion (\(\sim\)250 kJ/kg). The specific paraffin (IGI-1260) used has a relatively high melting point of _T\({}_{M}\)_-70 \({}^{\circ}\)C. The _n_-alkanes distribution of this paraffin wax predominately consists of C\({}_{34}\)-C\({}_{35}\) hydrocarbons [110]. The paraffin wax is a solid at room temperature and will start to exhibit softening characteristics at elevated temperatures as the long hydrocarbon chains are broken down into smaller ones with the absorption of heat. As previously reported by some of us [29] the graphene-PCM composites were prepared by melting paraffin wax at 70 \({}^{\circ}\)C on a hot plate and performing high-shear mixing with solutions of graphene and FLG synthesized from industry standard liquid-phase exfoliated (LPE) techniques. To avoid the oxidation process of liquid paraffin waxes, resulting in the formation of peroxide radicals and hydroperoxide above 80 \({}^{\circ}\)C, processing temperatures were carefully controlled. The resulting graphene-PCM was poured into molds and allowed to solidify under controlled humidity conditions. Different LPE recipes were used to synthesize the three types of LPE graphene solutions used as filler material. These graphene solutions had different average thicknesses and lateral dimensions. For low loading fractions up to 1%, a graphene solution with an average thickness of one monolayer (0.35 nm) and a lateral size distribution with a range from 150 to 3000 nm with 550 nm average size was used (graphene filler type A). For high loading fractions up to 20%, other types of FLG have been used. Filler type B consisted of a graphene solution with an average thickness of approximately three atomic layers (\(\sim\)1 nm) and an average lateral dimension of \(\sim\)10 \(\upmu\)m. Filler type C had an average flake thickness of 8 nm with a lateral size in the range of 150 and 3000 nm. Figure 3a presents optical images of hybrid graphene-PCM composites with graphene-FLG loading fractions of 0.5 to 20 wt. %. These composites were molded into disks and clearly show changes in color from white to black with the addition of graphene. This addition of graphene was also observed to cause some reduction of _T\({}_{M}\)_. The uniform dispersion and physical coupling of graphene-FLG within the paraffin matrix were verified using scanning electron microscopy (SEM) (see Figure 3b). As confirmed from the SEM images, graphene-FLG fillers are evenly distributed within the matrix. These composites with increasing amounts of graphene were further analyzed and monitored using Raman spectroscopy. Paraffin is known to have a large number of informative bands in its vibrational spectra from which variations are indicative of changes in a paraffin's state and composition [111; 112; 113; 114; 115]. Analysis of the observed changes in the Raman spectrum indicated good attachment of graphene to hydrocarbon chains [29]. Additional confirmation of the increasing concentration of sp\({}^{2}\)-bonded carbon (_i.e._, graphene filler) and changes to the paraffin's hydrocarbon chain were obtained from X-ray photoelectron spectroscopy (XPS) analysis. Such data provides information relating to the elemental composition, empirical formula, and chemical state of the measured sample.