Effect of graphene and its derivatives on thermo-mechanical properties of phase change materials and its applications: a comprehensive review

Abstract

Phase change materials (PCMs) play a leading role in overcoming the growing need of advanced thermal management for the storage and release of thermal energy which is to be used for different solar applications. However, the effectiveness of PCMs is greatly affected by their poor thermal conductivity. Therefore, in the present review the progress made in deploying the graphene (Gr) in PCMs in the last decade for providing the solution to the aforementioned inadequacy is presented and discussed in detail. Gr and its derivatives ((Gr oxide (GO), Gr aerogel (GA) and Gr nanoplatelets (GNPs)) based PCMs can improve the thermal conductivity and shape stability, which may be attributed to the extra ordinary thermo-physical properties of Gr. Moreover, it is expected from this review that the advantages and disadvantages of using Gr nanoparticles provide a deep insight and help the researchers in finding out the exact basic properties and finally the applications of Gr can be enhanced.

In this work, Gr and its derivatives based PCMs was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction spectroscopy (XRD), and scanning electron microscopy (SEM) by which crystal structure was known, phase was identified along with the knowledge of surface structure respectively. The increase in the mass fraction (%) of the filler (Gr and its derivatives) led to even better thermo-physical properties and thermal stability. The thermal characterization was also done by differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA) and thermal conductivity tests. The enthalpy of freezing and melting showed that Gr and its derivatives based PCMs had a very high energy storage capability as reflected in its various applications.

Graphical abstract

Keywords

phase change materials (PCMs) / graphene / thermal conductivity / characterization

1 Introduction

The applications of phase change materials (PCMs) are manifold, which can be found mainly in residential and industrial fields [1–4]. Without almost any energy decrement, these smart energy storage systems can store and release energy in the form of freezing/melting latent heat [5,6]. Recently, liquid-vapor [7] and solid-solid PCMs (SSPCMs) [8] have shown improvements in energy storage and release but on the other hand, solid-liquid PCM has a higher phase change enthalpy in comparison with SSPCMs. Much less volume change is also observed in transition of phase when compared with solid-gas [9] and liquid-gas systems.

In general, there exist two types of solid-liquid PCMs, inorganic and organic PCMs [10]. Inorganic PCMs [11] have a high latent heat by volume and an energy storage tendency but organic PCMs have a non-corrosive nature [12], properties of self-nucleation and congruent melting. Other major drawbacks of inorganic PCMs are chemical instability, super cooling, and low availability. These issues are there for industrialization of thermal energy storage applications [13]. The most appealing PCMs are n-alkanes in the family of organic PCMs. Paraffin (Pn) itself is a kind of n-alkane with 12 to 40 carbon atoms. With the increment in the number of carbon atoms in n-alkane, the enthalpy and melting temperature also increase [14]. The problem that both inorganic and organic PCMs face is leakage and low thermal conductivity [15–17].

Graphene (Gr) and its derivatives like Gr oxide (GO), Gr aerogel (GA) and Gr nanoplatelets (GNPs) play a critical role in avoiding the problem of low thermal conductivity in PCMs due to the very high thermal conductivity of such fillers in organic as well as inorganic PCMs. The reason for this high conductivity has been explained further in respective sections of thermal characterization like thermal behavior and thermal stability analysis for different kinds of Gr.

The PCMs [18] leakage can be prevented by microencapsulation with polymer, organic, and inorganic shell during liquid-solid phase transformation [19–22]. There are multiple advantages of microencapsulation like protection against environmental condition, high rate of heat transfer and deduction in volume changes with correct selection of shell materials [23,24]. Nano encapsulation, when used in comparison with microencapsulation, can maximize the thermal conductivity and surface area/volume ratio, increase the melting and solidifying rate [25–28], and reduce super cooling. There are specific challenges facing the shell part in encapsulated PCMs [29]. For instance, the shell must be resistant to volume changes via melting/solidifying processes when large volume changes are expected to occur [30]. The heat transfer, storage, and release of conventional polymer based shells are very low, resulting in a low thermal conductivity. The size and shape of the shell can be controlled through polymerization but volume changes and low thermal conductivity are major problems of nano and microencapsulated PCMs [31–33].

The addition of a conductive class, like carbon based nano-materials, is essentially needed for increasing the thermal conductivity [34] and thermal performance of PCMs. It serves another purpose like enhancing the phase transition performance of the basic PCMs [35]. Such additions of nano-materials can provide shape and structural stability to PCMs [36].

Carbon containing or carbonaceous materials increases the shape stability and thermal conductivity of PCMs [37]. The main carbonaceous materials are expanded graphite (EG), Gr, carbon nano-tubes (CNTs) and GNPs [38], of which, Gr is the best and most unique nano-materials owing to its thermo-physical properties. The Gr based PCMs has its classification under two main types like three-dimensional (3D) structure and nanosheets. The type nanosheets has GO, Gr, reduced GO (rGO), and functionalized Gr. All such types of nanosheets can be combined with PCMs for a large amount of thermal energy storage and release. By the use of carrier transport pathways and very less impurities [39], rGO sheets have very high thermal conductivities as compared to pristine GO nanosheets. Hence, the addition of such carbonaceous or Gr nanosheets (GNSs) improves the overall properties of PCMs. The application of 3D structures results in increasing the overall properties of PCMs nano-composites. These nanostructures have very low densities at very high specific surface areas [40]. Therefore, such nanostructures result in the increase of thermal conductivity and shape stability. The EG or exfoliated graphite has been used multiple times to increase the shape stability and thermal conductivity of PCMs. If the numbers of carbon layers are 10 or more, it is known as GNPs and for few-layered GNS, the number of layers are between 3–10. Figure 1 depicts derivatives of Gr and its thermal and physical properties including the various PCMs used and the ranges of various characterization techniques of PCMs. More specifically, from inwards, the first circle represents Gr and its derivatives. The second circle represents the kinds of Gr used in this review like GA, GO, GNPs and expanded Gr. The third circle indicates the various properties of Gr which makes it a suitable material for studying its thermo-physical properties. The next circle indicates the various kinds of PCMs used in this review. The last circle indicates the thermo-physical range of properties from various characterization techniques of various PCMs doped with Gr and its derivatives. Figure 2 illustrates the highlights of the paper from sample preparation to characterizations (thermal and physical).

Numerous amounts of studies have been done on PCMs, their manufacturing, and their physical, thermal and mechanical characterization. The studies conducted by various researchers over the past five years have been proof to the fact that manufacturing of new PCMs and their characterization leads to extremely wonderful properties and improvement in properties over the pristine PCMs.

Zhou et al. [41] presented a review about machine learning methods and artificial intelligence in PCMs cooling systems and analyzed them systematically with respect to charging and discharging of PCMs. Wong-Pinto et al. [42] used nano-materials to enhance the physical and thermal performance of PCMs. Salt hydrates were used as PCMs in this review. The different types of metallic carbonates and metallic oxides were used as nano-particles. Anupam et al. [43] regulated the temperature extremes in case of asphalt and concrete pavements and deduced that the use of PCMs in pavement related application decreased the cooling and heating rates and delayed the occurrence of extreme peak temperature. Zhou et al. [44] presented an overview about cooling performance increments of PCMs integrated systems.

In this paper, first, Gr and its derivatives fused PCMs and its applications are introduced. Gr and its derivatives, like GA [45], GNPs, EG and GO morphology (physical structure) and thermal characterization, are also studied. Then, the effect of Gr and GO on the different properties of phase change nanocomposite (PCNC) are presented. Next, the effect of GNPs and GA on the different properties of PCNC is detailed. After that, the applications of Gr and its derivatives fused PCMs are elaborated on. Finally, the conclusions and future outlook has been summarized. This is a very novel review, as the entire physical, structural, and thermal characterizations are discussed in detail.

2 Effect of Gr and GO on different properties of PCNC

Gr has an arrangement of carbon atoms with their lattices in the form of honeycomb. The sp² hybridization makes the structure of Gr very stable which results in a high thermal conductivity (3000–5000 W/(m·K)) [46]. The thermal conductivity tends to the increase of the logarithmic nature, which may be attributed to the stable bonding pattern in the two-dimension (2D) structure of Gr. Henceforth, this nature of Gr molecules allows them to form a strong network in the pool of PCMs, which consequently results in the increase of thermal conductivity of the storage system [47]. Xin et al. [47] stated that GNS can also produce a stable PCMs.

2.1 Characterization of Gr and GO-based PCNC

The characterization techniques which are comprehensively discussed in this section are Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction spectroscopy (XRD), and scanning electron microscopy (SEM)/transmission electron microscopy (TEM). This section deals with the characterization of Gr and GO-based PCNC. By using different characterization techniques, the structure and varied properties were depicted and measured respectively. Yang et al. [48] prepared PCNC by paraffin wax (PW) [49] as solid-liquid PCMs, melamine sponge (MS) as supporting materials, rGO and zirconium carbide (ZrC) as thermal conduction and solar absorption additives. The attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FT-IR) analysis of MS, PW, and MS was conducted [50]. rGO is shown in Fig. 3(a). The stretching vibration and torsional vibration absorption peaks of group for MS is presented at 3320 cm⁻¹ and 973 cm⁻¹. The other stretching vibration peaks are presented at 1500 cm⁻¹ for carbon double bonded with nitrogen. The peak at 3293 cm⁻¹ is seen for MS and rGO. The other peaks at precisely 2915 cm⁻¹ and 2846 cm⁻¹ are presented for PW due to (–CH₂) group stretching vibration. PW/MS was defined as S1 for comparison. The S2 has PW, MS, and RGO as constituents and S6 has the same constituents but with the percentage of ZrC varying. Figure 4(a) shows the XRD outline of PW, ZrC, rGO, MS, and form-stable PCMs (FSPCMs). The diffraction peak for PW is shown at 21.8° and 24.8° whereas for MS it is shown at 36.3° and 42.6°. The peaks of diffraction of ZrC are there at 60.7°, 72.6°, and 76.5° for planes at ((220), (311), (222)). S1 and S2 show peaks which is the same as that of PW. FSPCMs S6 shows similar diffraction peaks as those of PW, MS, and ZrC. Mu and Li [51] prepared polyurethane (PU)-grafted rGO, comprised by 4,4'-diphenylmethane diisocyanate (MDI), polyethylene glycol (PEG) which was a SSPCMs and the rGO was fabricated by in-situ polymerization reduction reaction, and esterification. The esterification reaction was characterized by FT-IR, as shown in Fig. 3(b). In accordance to the FT-IR spectrum of PEG, the absorption peak at 2875 cm⁻¹ and 3467 cm⁻¹ corresponds to the proportioned stretching vibration peak of –CH because of –CH₂ and to the stretching vibration peak of –OH respectively. The bending vibration peak of –CH was denoted by peaks at 1456 cm⁻¹, while the absorption peak at 1277 cm⁻¹ and 1342 cm⁻¹ are denoted by planar zigzag conformation and the helix of the OCH₂–CH₂O structure. The crystalline properties of PU, PEG, and rGO-PU based SSPCMs are displayed in Fig. 4(b). Two very strong diffraction peaks at 19.12° and 23.37° are seen due to the lattice plane at (132) and (120) of pure PEG. The rGO-PU and rGO-PU-1 are SSPCMs having different rGO concentrations.

Zhao et al. [52] studied Gr pickering stabilizer, a series of Pn@Gr/melamine formaldehyde (MF) micro-PCMs made by in-situ polymerization. Finally, micro-PCMs were made and named as micro-PCMs-3Gr, micro-PCMs-2Gr, micro-PCMs-1Gr, and micro-PCMs-0Gr where the concentrations of Gr aqueous suspension were 3 g/L, 2 g/L, 1 g/L, and 0 g/L in pickering emulsion preparation, individually. Meanwhile, micro-PCMs were represented as micro-PCMs-10/1, micro-PCMs-6/1, micro-PCMs-4/1, and micro-PCMs-3/1 when the core/shell ratio of micro-PCMs were 10 : 1, 6 : 1, 4 : 1, and 3 : 1. The FT-IR spectra of micro-PCMs-2Gr, Pn, MF, and modified Gr are shown in Fig. 3(c). Out of plane ring and in plane ring vibrations of triazine show peaks at 815 cm⁻¹ and 1559 cm⁻¹. These are characteristic peaks of MF. Pn shows peaks at 2851 cm⁻¹ and 2958 cm⁻¹ due to the stretching vibration of –CH₂. The peaks at 1089 cm⁻¹ and 1631 cm⁻¹ are caused by the stretching vibration of C–C and C=C groups which are relevant to Gr. The XRD pattern of the composite [53] prepared by Zhao et al. [52] is shown in Fig. 4(c). It discloses the XRD patterns of modified Gr, micro-PCMs-2Gr, Pn, and MF. The formless and shapeless diffraction peak at 23.1° (2θ) belongs to MF. The peak is not noticeable in micro-PCMs-2Gr. The proof may be that the content of MF in the micro-PCMs is slightly low. For Pn, the main sharp diffraction peaks at 2θ = 19.38°, 19.65°, 23.44°, and 24.86° are caused by the four diffractions of crystal planes (010), (011), (100), and (111) of Pn, respectively. The modified Gr shows a strong diffraction peak at 2θ = 26.30°, corresponding to the (002) plane. The (002) plane in the XRD pattern of micro-PCMs, to a reasonable extent, proves that the modified Gr is contained by micro-PCMs. Beyond that, the XRD patterns of micro-PCMs are inclusive of the sharp diffraction peaks of Pn. Nano encapsulation phase change materials (NePCMs) were prepared by Zhu et al. [54]. By polycondensation and interfacial hydrolysis of methyl tri methoxy silane and tetraethyl ortho silicate plus self-assembly, the Gr nano-encapsulated n-octadecane (n-OD) with SiO₂/Gr composite shell was made. The FT-IR spectra of Gr, n-OD, n-OD@SiO₂, and G-SiO₂-0.75 (G-Gr) are shown in Fig. 3(d).

The absorption peak of Gr at 1566 cm⁻¹ were noticed due to the coupled C–C bond and the absorption peaks at 1091 cm⁻¹ and 3435 cm⁻¹ are due to residual C–OH groups at the edges of Gr. N–OD shows characteristic absorption peaks at 719 cm⁻1, 1469 cm⁻¹, 2855 cm⁻¹, and 2925 cm⁻¹. For n-OD@SiO₂, besides the absorption peaks from n-OD, the absorption peaks at 448 cm⁻¹, 778 cm⁻¹, 1054 cm⁻¹, 1272 cm⁻¹, and 3428 cm⁻¹ were noticed. The crystal arrangement and organization of n-OD, G-SiO₂-5, Gr, and n-OD@SiO₂ are found to be typical by the XRD method, as shown in Fig. 4(d). It can be seen that the n-OD demonstrates a characteristic triclinic crystal structure in accordance with the diffraction peaks at 2θ = 39.7°, 34.7°, 19.8°, 23.5°, 24.9°, and 19.3°. Xia et al. [55] prepared 4,4'-diphenylmethan diisocyanate as the cross-linking agent, and successfully designed and synthesized GO as the skeleton material and SSPCMs with lamellar structures by using PEG as the phase change ingredient. The amount added of GO were 3%, 6%, 9%, and 12% (mass fraction) in comparison with the amount of PEG and the byproduct of the composites were named as SS-PCMs-1, SS-PCMs-2, SS-PCMs-3, and SS-PCMs-4, respectively.

Wang et al. [56] developed a PCNC which had PEG/SiO₂ as form-stable PCMs and well mixed Fe₃O₄-functionalised GNS (Fe₃O₄-GNS) as energy changer which had a high heat storage capability and very high thermal stability. By SEM and TEM, the crystal structure and complete microstructure of the Fe₃O₄-GNS PCNC were examined and compared with the captured images of GO. In Fig. 5(a), the wrinkled and rippled structure of GO is clearly observed. The synthesized Fe₃O₄-GNS PCNC contains numerous spherical Fe₃O₄ nanoparticles attached to the GNS as shown in Fig. 5(a-1). Precisely, the Fe₃O₄ anchored on the GNS is shown in Fig. 5(a-2) and thereby forming a 2D structure. The TEM images of the PCNC further confirmed the SEM results as shown in Fig. 5(a-3) in which the Fe₃O₄ surround the GNS. The sketch of Fe₃O₄ and GNS were clearly depicted. The high resolution TEM shows that Fe₃O₄ nanoparticles in the wrinkled Gr are made up of many magnetite nano-crystals.

Li et al. [57] by using the chemical wrapping method, enfolded GNPs on the wick material, thereby suggesting a low-cost high-efficiency solar steam generator. Figure 5(b) shows the SEM image of No. 3 wick material (fine fluffy cloth) wrapped around with GNPs having 1 mg/mL GO by HI reduction. The fibers of the wick material are uniformly wrapped with GNPs. Hence, due to the chemical wrapping method, the nanoparticles are uniformly scattered and highly stable than the direct smearing method. Mu and Li [58] prepared a very novel form of stable composite phase change materials (FS-CPCMs). Lauric acid (LA) was the PCMs and on the exterior surface of GA, LA was grafted. The SEM images of the GA, LA-GA formed by reacting LA with GO, LA/GA prepared by keeping mass ratio of LA and GA as 95 : 5 and LA/LA-GA which was made by keeping the mass ratio of LA and GO hybrids 1 : 1 FS-CPCMs are shown in Fig. 5(c). From the SEM images as shown in Fig. 5(c-1), it is seen that the GA has a nano-layer structure with an undulated rough morphology due to its very thin features. The converged and stretchy Gr sheets (GS) resulted in a porous morphology and the pores in the GA joined with each other. Figure 5(c-2) shows that the LA-GA has the same structure as that of GA. Figures 5(c-3) and 4 show the fracture surface images of the FS-CPCMs LA/LA-GA and LA/GA FS-CPCMs. Zhao et al. [59] prepared a rGO film by thermally converting GO films for solar cells and related applications. Figure 5(d) shows the TEM image of rGO from the GO films. As like and consistent with the SEM images, the TEM images also show a crumpled and gauze like morphology. The lattice fringes obtained from high resolution TEM shows the inter planar spacing as 0.366 nm analogous with (002) plane of Gr. This directly points out that the conversion of rGO from GO is successfully achieved by the annealing process. Cao et al. [60] overcame the problems of poor shape stability and low thermal conductivity in organic PCMs by styrene-acrylic emulsion/GA supported microencapsulated phase change composite (SGM composite) which was manufactured by freeze drying and vacuum assisted impregnation. As per Fig. 5(e-1), GO completely exfoliates into the single layer GO having a thickness of 1 nm with the help of ultasonication. The coating of GO on the surface of microencapsulated Pn produced a very smooth surface which resulted in the high thermal conductivity and other related thermal properties. The particle size analyzer depicted the particle size between 8 to 15 micro meters as shown in Fig. 5(e-3). Zhang et al. [61] reported that for the preparation of microencapsulated PCMs (MePCMs) by the in-situ polymerization method, the use of emulsifier is very much necessary. The generally used emulsifiers are Span 80, sodium dodecyl sulfate (SDS), syrene-maleic anhydride and Tween 80. Five samples were created and compared. The first one was with PW and GO. The second one was made by combining Pn, Span 80, and Tween 80. The third one contained Pn, SDS and Tween 80, and Span 80. The fourth one had PW, Span 80, Tween 80, and GO as ingredients. The final one was prepared by adding PW, SDS, Tween 80, Span 80, and GO where the last four were emulsifiers. The mixture for all the samples prepared were in a certain ratio as stated by Zhang et al. [61]. The 1st sample which had GO as the emulsifier had a very smooth surface and uniformity in particle size as shown by SEM. The 2nd one, due to the weak interfacial force between the core and the shell displayed an irregular morphology by SEM when Tween 80 and Span 80 were used as emulsifiers. The 3rd one had a very uniform morphology as seen under SEM due to the strong polarity of SDS and the shell side of MePCMs can be interacted through electrostatic repulsion forces. The 4th one showed a very spherical morphology with minimum agglomeration due to GO particles and the integration of Pn droplets and prepolymer particles, where the size of MePCMs was very uniform with a certain amount of agglomeration.

Feng et al. [62] did experimentation and prepared modified-GO (m-GO) and treated it with Pn (P) to prepare P@m-GO-5, P@m-GO-10, P@m-GO-15, and P@m-GO-20 composites where 5, 10, 15, and 20 is the filler weight percent. The SEM micrographs suggests that pristine PW forms less material, i.e., amorphous having thick and large layers whereas m-GO having a curly and small sized laminar morphology. The joining of PW and m-GO can be seen by SEM. The adhesion of PW and m-GO increases with the increase in the volume of m-GO suspension. In the absence or very small amount of m-GO, the coating will be incomplete with a large leakage. The amount of m-GO added should be optimum as too much will result in improper thermal transmissions. By using SEM, the size of filler can also be measured which was found out to be 50 micrometers.

Vivekananthan and Amirtham [63] prepared nanoparticle dispersed PCMs by mixing nanoparticles in different ratios (0.1%, 0.5%, and 1%) into erythritol based PCMs which were named as sample A, sample B, and sample C. The addition of 1% of Gr led to the increment of 53.1% in the thermal conductivity. Field emission scanning electron microscopy (FESEM) was used to study the morphology and microstructure of erythritol based PCMs. In the three samples prepared as stated above, the GNPs which are distributed randomly, hold all the particles together. Under 5000 times of magnification, the size of sample A was found to be between 28 nm and 35.8 nm. The range of size for sample B was found to be between 24.9 nm and 31.9 nm. Similarly, the size for sample C was found in the range of 20.1 to 29.5 nm. The Gr particles were distributed uniformly in erythritol as per FESEM images.

From the FT-IR, XRD and SEM/TEM analysis done in this section, it can be summarized that FT-IR is a time saving, nondestructive and rapid method which is sensitive to fluctuations in molecular structure and has the ability to detect various functional groups. Besides, the information about the physical state of the sample and chemical composition is provided by FT-IR. In addition, unit cell dimension and phase identification is provided by XRD. Moreover, the morphology of the sample is well provided by SEM/TEM. Furthermore, the range of FT-IR peaks is from 900 cm⁻¹ to 3500 cm⁻¹ and that of XRD is from 18° to 72° while SEM/TEM shows wrinkled, rough, and crumpled morphology PCNC, the reason for which is stated above in SEM/TEM description.

The various structural and morphological characterization techniques like FT-IR, XRD, SEM/TEM and thermal characterization techniques like thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC) and thermal conductivity test, the sample preparation method (PCNC fabrication method), sonication details like the sonication time as well as speed which varies from 5 minutes to 24 hours and 800 to 1800 r/min as well as the type of Gr with associated PCMs are presented in Table 1.

2.2 Thermal behavior analysis

The DSC and thermal conductivity increment are discussed in this section lengthily. The behavior of materials as the function of temperature and time is the thermal analysis of the materials. Similarly, the increment in thermal conductivity by addition of Gr and its derivative to the PCMs is also studied in this section. Yang et al. [48] used PW as PCMs, rGO and ZrC as thermal conduction additives and supporting material as MS. They developed four samples, i.e., S3 to S6 with varying contents of mass fractions of ZrC. The ratio of PW, MS, and rGO was kept the same in samples S2, S3, S4, S5, and S6. S1 was composed of PW and MS whereas S2 was composed of PW, MS, and rGO. The rest of the samples from S3 to S6 had different weight fractions of ZrC along with PW, MS, and rGO. The thermal conductivity of MS/PW and PW were 0.27 W/(m·K) and 0.28 W/(m·K) respectively, whereas that of MS@rGO/PW was found to be 0.54 W/(m·K) which was 92.9% more than pristine PW as shown in Fig. 6(a). The thermal conductivities of S3, S4, S5, and S6 were shown in Fig. 6(a-2) with different contents of ZrC. The reason behind the drop in the thermal conductivity of MS/PW composite was the very poor thermal conductivity of MS itself whereas the increase of 92.9% in the thermal conductivity of MS@rGO/PW was due to the addition of GO as well as the ZrC content.

Ramakrishnan et al. [73] developed Pn/hydrophobic expanded perlite (EPOP) with Gr as heat transfer promoter. The thermal conductivity increased by 49.2% in the case of EPOP/GNPs as compared to EPOP. The reason is the very high thermal conductivity of GNPs as shown in Fig. 6(b).

Hussain et al. [74] made a novel material graphene coated nickel (GcN) foam saturated with Pn for thermal management of lithium ion battery which is a significant part of hybrid energy vehicles. The intrusion of PCMs PW into the metal formed the composite and the freezing and melting curves were shown in Fig. 7(a). The heating process and cooling process are shown by the positive heat flow and negative heat transfer respectively. The main peak in the curve shows the liquid-solid phase change whereas the neighboring peaks are solid-solid transition peaks.

Zhou et al. [75] manufactured PU-based SSPCMs by cross linking it with GO. The PCMs GO/PU-SSPCMs were named as GPCMs-x, where x is the sample number having different molar ratios of PEG and different contents of GO (% (mass fraction)). There were six types of different composites prepared from GPCMs-0 to GPCMs-5. The crystallization temperature was increased due to heterogeneous nucleation whereas phase change enthalpy does not get increased. Therefore, within a certain composition (≤14.5% (mass fraction)) of GO, the confinement almost equals the nucleation effect, thereby increasing the crystallization temperature from 23.8°C to 34°C and decreasing the phase change enthalpy from 78 J/g to 71.5 J/g as depicted in Fig. 7(b). Liu and Rao [76], for enhancing the thermal performance of Pn in thermal energy storage, made composites by mixing PW and GS where the mass fraction of exfoliated GS and Gr varied from 0%–2% (mass fraction). The DSC tests were carried out to get the phase change enthalpy of the composite materials as shown in Fig. 7(c). Figure 7(c) suggested lower melting point of composites material in comparison with pure PW. The melting enthalpy of pristine Pn was 135.65 J/g. When GS were mixed into the Pn with mass fraction of 0.2%, 0.5%, 1.0%, 1.5% and 2.0%, the corresponding melting enthalpies were 141.15 J/g, 146.85 J/g, 162.89 J/g, 155.89 J/g and 137.85 J/g for Gr/PW composites, respectively. For the same mass fraction, the enthalpies of exfoliated graphite sheet/PW were 141.61 J/g, 163.28 J/g, 169.02 J/g, 161.17 J/g and 143.83 J/g, respectively. As the graphite and GS were without latent heat the addition of GS in PW was able to lessen the enthalpy.

To limit the heat dissipation in case of electronics equipments, thermal contact resistance is one of the biggest problems. Liu et al. [77] prepared Gr-olefin block copolymer/Pn filled with Gr (<4.0% (mass fraction)) which reduced the thermal contact resistance from 8–20 (K·cm²)/W to 0.1–0.2 (K·cm²)/W. This was due to the good wettability of contact surfaces. Cao et al. [78] studied the thermal transport of water-Gr interface by using molecular dynamics simulations and found that with Gr layer numbers decreasing, the thermal contact resistance decreases too. Zou and Cao [79] used Gr as the substrate material on hexagonal boron nitride and investigated its effect by using molecular dynamics simulations. The thermal conductivity of supported and free Gr were 2200 W/(m·K) and 3517 W/(m·K) respectively. Big reduction of flexural phonon life times is the reason for this reduction by 37.4%.

By DSC, the thermal management of lithium ion batteries were made possible for use in hybrid energy vehicles. The DSC curves provided information about heating curves and cooling curves which helps in finding the phase change enthalpy of the PCNC. The negative heat transfer and positive heat transfer gave information about phase transition temperatures of the system.

As the content of Gr and GO in PCMs increases, the thermal conductivity of the PCNC shows huge increments due to the covalent sp² bonded carbon atoms of Gr.

2.3 Thermal stability analysis

TGA measured the thermal stability as the mass of the sample prepared did not vary over time and temperature as shown in Fig. 8. TGA is a thermal analysis technique for finding out the thermal stability of the sample. The TGA was conducted on the thermo-gravimetric analyzer. The TGA also gives the idea about oxidation and combustion of the sample. The thermal stability was established by TGA of GcN foam [74]. TGA was done at a heating rate of 2°C/min and nitrogen atmosphere of 25 mL/min. There was not any reasonable decay in GcN foam and nickel foam with rising temperature. The materials were stable even on a temperature rise of 900°C, as shown in Fig. 8(a). The composite prepared and PW had their TGA done as shown in Fig. 8(a-2). The TGA curves [75] as shown in Fig. 8(b) was the minimal amount of degradation at 280°C due to the urethane group; PEG degraded at 340°C. The maximum weight loss temperature was 410°C. When the contents of GO reached 44.6%, the weight loss before 280°C was more than others as shown in Fig. 8(b) due to urethane groups and oxygen containing groups GO. The inference of the thermal stability [76] of PW/Gr composites were shown in Fig. 8(c-1). On reaching 150°C, the total mass of composite material began to fall. The weights of these composites were lost heavily from 250°C to 340°C. The mass loss reduced very slowly at temperatures above 350°C due to thermal decomposition. The trends of curve for PW graphite sheets were the same as those of the Pn/Gr composites as shown in Fig. 8(c-2).

From this section, the trend of the curves as shown by TGA are due to the different groups presented in the material like oxygen containing group as discussed above. No mass change indicates that the PCNC is thermally stable and the minimal amount of degradation with respect to temperature indicates that the PCNC is really thermally stable. For example, the PCNC was stable even at a high temperature of 900°C.

The major advantages of using Gr based PCMs (Gr and GO) are that they are the thinnest material and the strongest, pliable and transparent, very good conductors of heat and electricity, which can be used in the production of high speed electronic devices and as chemical sensors for detecting explosives (bombs). The main disadvantage as a catalyst is that they are susceptible to oxidative environments and are toxic in nature [80].

3 Effect of GNPs and Gr/aerogel on the different properties of PCNC

GA are created by pristine Gr layers. The mixture of CNTs and Gr were left in a mold to solidify and dehydrate which left behind GA [81]. The material so left had a supreme absorption and elasticity. GNPs had an average width of 5–10 nanometers. These nanoparticles consist of short bundles of platelet shaped Gr. Due to its unique morphology and structure, it has super mechanical properties. Due to the presence of graphite, it has very good thermal conductivity and properties.

3.1 Characterization of GNPs and GA PCNC

This section deals with characterization techniques like SEM/TEM, FT-IR and XRD for GNPs and GA based PCNC lengthily. Wei et al. [82] prepared PCNC by combining microcrystalline cellulose [83] (MCC)/GNPs hydro gels by solution compounding, gelling and solvent exchanging successively. The microstructure has a 3D-oriented structure which is porous as shown in Figs. 9(a-1) and 9(a-2). The porous structure is present due to the increase in the amount of GNPs which facilitates the growth of a such structure. Tang et al. [84] prepared a form-stable phase change material by mixing palmitic acid (PA) with high density polyethylene (HDPE) and GNPs.

Nine different types of PCMs were formed by Tang et al, including HPCM-1, HPCM-2, and HPCM-3 with HDPE to a PA ratio of 1 : 9, 2 : 8, and 3 : 7 with Gr percentage as 0% (mass fraction), respectively. On the other hand, CPCM-1, CPCM-2, CPCM-3, CPCM-4, CPCM-5, and CPCM-6 were fabricated by varying the ratio of HDPE to PA and addition of GNPs from 1% to 4%. The SEM image of the microstructure of HPCM-2 and CPCM-6 with 4% of GNPs is depicted in Figs. 9(b-1) and 9(b-2). The dark part and the grey part represent the PA and HDPE, respectively. PA is properly wrapped with a network structure of HDPE which prevents leakage. The CPCM-6 microstructure is irregular and layered.

Silakhori et al. [85] prepared a form-stable phase change material by the addition of PA and polypyrrole (PPy) and the thermal conductivity was increased by the addition GNPs. Figures 9(c-1) and 9(c-2) depict the SEM images of GNPs, form-stable PA/PPy, and PA/PPy/GNPs. The high area on the surface side of GNPs leads to interconnected composites with mechanical reinforcement. PA and PPy are the latent heat storage material and supporting materials respectively. The structures of PA/PPy are granular. PA/PPy/GNPs has a rough surface area compared with the smooth surface of GNPs. The various composites prepared are S1, S2, S3, and S4 with varied amount of GNPs, PPy, and PA.

Mehrali et al. [86] used nitrogen doped Gr (NDG) for shape stabilization and escalating of the thermal conductivity of PA. The liquid PA was mixed with NDG from mass fraction (1%–5% (mass fraction)) using powerful ultrasonication. The prepared composite PCMs with 1%–5% (mass fraction) NDG were named S1–S5, respectively. The varying weight percentage NDG and form-stable PCMs are shown in Figs. 9(d-1) to 9(d-6) by FESEM image. At a lower nano-filler loading, the NDG particles in the nanocomposites are easier to identify because of their particular structure as shown in Figs. 9(d-1) and 9(d-2). Figures 9(d-3) to 9(d-5) show almost only white areas, signifying that the PA is adsorbed well into the NDG network. It can be clearly observed in Fig. 9(d-6) that NDG layers possess the extensive capability to soak up the melted PA once the temperature is higher than the melting point of PA.

The FT-IR results [84] show that there are six absorbtion peaks. The first two peaks at 2921 cm⁻¹ and 2846 cm⁻¹ are due to the antisymmetric stretching vibration of the group and the symmeteric stretching of the group. The other peaks at 1701 cm⁻¹ and 947 cm⁻¹ are caused by the carbon oxygen stretching vibration and the out-of-plane wagging vibration of the hydroxyl group. The scissoring bending vibration and rocking vibration are the reason behind the absorption peak of 1471 cm⁻¹ and 717 cm⁻¹. Figure 10(a) shows the entire FT-IR spectrum. The FT-IR spectra [85] of PPy, GNPs, PA, and form-stable PCMs are shown in Fig. 10(b). The peaks at 1698 cm⁻¹ is caused by the carbon oxygen double bond stretching vibration while those at 2914.2 cm⁻¹ and 2848.6 cm⁻¹ result from the symmetric stretching vibration of PA. The FT-IR transmittance spectra of composite FS-PCMs and pristine GNPs [87], UPR, PEG, are shown in Fig. 10(c). As can be seen from the figure of pure PEG, the absorption peak at 3435 cm⁻¹ is due to the stretching vibration of the group while the stretching vibration results in the sharp peak at 2881 cm⁻¹. Moreover, the obtained bands at 1642 cm⁻¹ and 1110 cm⁻¹ represent the stretching vibration and the asymmetric stretching vibration, respectively because the bending vibration results in the pointed peak at 841 cm⁻¹.

Figure 11(a) [84] presents the XRD patterns of CPCM-1–6, HPCM-1–3, GNPs, PA, and HDPE. Due to the crystallization of PA, two sharp diffraction peaks are noticed at 21.8° and 24.1°. The peak at 26.6° depicts the regular crystallization of GNPs. The rest of the figure shows the XRD peaks of PA/HDPE composites. The XRD results in Fig. 11(b) [85] showing the peaks at 13°, 24°, 25°, 27° confirm the formation of highly crystalline PA. The peaks grow slightly wider at 24° and 25° which further suggest that the crystalline phase is affected by the ratio of PA to PP. Figure 11(c) displays the XRD pattern of pure PA/NDG, NDG, and PA composite. The broad peak centered at around 24.2° confirming the recovery of a graphitic crystal structure is observed for the NDG sample. The XRD patterns from the composite PCMs usually illustrate that the pure PA [86] and crystal formation of the PA within the composite PCMs are alike. Figure 11(d) presents the XRD patterns of the composite FS-PCMs [87] and pristine GNPs, UPR, and PEG. The wide amorphous diffraction peak shown in the range of 16.3°–27.6° for UPR recommends that UPR possesses a formless structure. In the XRD pattern of PEG [88], there are two sharp diffraction peaks appearing at 23.21° and 19.18°, indicating that the polymer is highly crystalline. The sharp peaks at 23.19° and 19.17° associated with PEG also emerge in the GNPs doped FS-PCMs and the patterns of the PEG/UPR composite.

The SEM/TEM analysis conducted in this section indicates the structure/morphology of various PCNC and its composition which helps the researcher to understand its surface topography. The FT-IR analysis done in this section has peaks in the range of 717 cm⁻¹ to 3500 cm⁻¹ due to the bending and scissoring vibration of the functional groups as stated in this very section. In this section the XRD peaks yield the information about the position of the atoms in the lattice structure and its crystalline behavior having its peaks in the range of 13° to 30°.

The FT-IR, XRD, EDX/XPS, and SEM/TEM details are presented in Table 2. The various characterization technique peaks are mentioned and a detailed table is made where Gr and its derivatives combined with various PCMs are discussed comprehensively.

3.2 Thermal behavior analysis

The description of the thermal conductivity and DSC curves is presented for GA and GNPs. This section describes their thermal behavior in detail.

GNPs [82] is a thermal conductive filler and as its percentage increases, the thermal conductivity increases, as depicted in Fig. 12(a). The thermal conductivity of composite (HPCM-2) with various percentages [84] of Gr was depicted in Fig. 12(b) and it is observed that as the GNPs mass faction (%) increases, the overall thermal conductivity of the various composites increases.

Figure 12(c) [85] indicates that with increased GNPs particles, the thermal conductivity of the PA/PPy/GNPs form-stable PCMs rises linearly. From Fig. 12(d) [86], it can be seen that compared to the pure PA, the thermal conductivities of the PA/NDG composites (S1–S5) clearly improve due to the high thermal conductivity of NDG. The thermal conductivity [87] dominates the rate of releasing and storing energy.

The Gr types, the PCMs used, the volume fraction or weight percentage of Gr types, the sample preparation methods, and the thermal conductivity are as described in Table 3.

It is very important to determine the efficiencies of thermal energy storage/release measurement of enthalpy of melting and crystallization. The common procedures are compounding of PCMs and thermal conductive fillers result in decrease of enthalpy during the phase change process. The melting and crystallization behavior is shown in Figs. 13(a-1) and 13(a-2) [82]. The melting and solidification process of PA, HDPE, and pure HDPE composites is shown in Figs. 13(b-1) and 13(b-2) [84].

Figures 13(c-1) and 13(c-2) [84] show the melting and solidifying curves of GNPs doped form-stable PCMs. The onset melting temperature of HDPE and PA are 124.79°C and 62.71°C, respectively, as shown in the figures. Figure 13(d) [85] depicts the DSC curve and notably signifies one major exothermic and endothermic peak for the every sample with a peak temperature between 60°C and 63°C. Figure 13(d) also signifies that the enthalpy of form-stable PPy/PA/GNPs decreases from 163 J/g to 151 J/g, which is more than that of the prior form-stable PCMs. The thermal energy storage [104] properties of the prepared PA/NDG composite PCMs and the pure PA were studied with DSC [86]. All DSC curves in Fig. 13(e) show a single big endothermic peak with T_peak located in the temperature range of 64°C–67°C, which is attributed to the melting of PA. The role of storing thermal energy was performed by PA.

The various thermal characterization techniques like DSC and thermal conductivity values are presented in Table 4 along with applications. The DSC test as mentioned in the Table 4 enlists the melting temperature, freezing temperature, and the respective phase change enthalpies of PCNC. For nearly almost all the works presented in Table 4, the PCMs are found to be thermally stable with 5% weight loss temperature in the range of 170°C to 220°C. The rise in the thermal conductivity by the addition of Gr and its derivatives in PCMs is due to the covalent sp² bonded carbon atoms of Gr and also due to the large grain size, the high flatness, and the weak interlayer binding energy between the layers. The high thermal conductivity of the single layer Gr which is 3000–5000 W/(m·K) also plays a part in it. The melting or heating and solidification or cooling curves of DSC determine the heat flow as well as the heating and cooling temperatures.

3.3 Thermal stability analysis

In this section, TGA curves are presented to determine the thermal stability of the used samples and they are different from Gr and GO based PCNC because of the presence of Gr and its derivatives like GA and GNPs.

The thermal reliability is depicted by TGA curves [84] of PA/HDPE, PA, HDPE, composite and GNPs doped PA/HDPE composites. The analysis also yields the temperatures of the percentage of the weight loss, the maximum thermal degradation rate, and the amount of residue, as shown in Figs. 14(a) and 14(b). Figure 14(c) [85] shows the thermal stability results of PPy, PA, and form-stable PCMs.

Figure 14(d) exhibits the one-step weight loss starting at 201.48°C and maximum weight loss temperatures at 275.16 °C, caused by the evaporation of PA. The one-stage weight loss behavior is indicated by PA/NDG samples while the temperatures at the highest weight loss rate increases up to 349.21°C [86]. For instance [87], the T_end (ending temperature) and T_max (maximum temperature) of S4 are 468.3°C and 409.1°C, respectively, which are slightly higher than those of pure PEG which represents the complex structure of the cured UPR and confines the decay of PEG. The mass loss processes of both S4 and pure PEG occurs in only one step which indicates that the prepared FS-PCMs composed of GNPs, UPR and PEG are mixed unvaryingly as shown in Fig. 14(e).

The thermal stability was checked for MCC/PEG/GNPs-1.51PCMs for 25 to 50 DSC heating and cooling cycles. Figure 15(a) shows the curve. It can be observed that after 25 to 50 heating and cooling cycles, there is very less change in the crystallization and melting parameters, which indicates that the PCMs have a higher thermal stability [82].

Sensible and adequate changes [87] in latent heat and temperature indicate that the so-formed UPR/PEG/GNPs composite FS-PCMs after a large number of thermal cycles can be utilized as suitable latent heat thermal eonnergy storage (LHTES) materials and possess an admirable thermal reliability, as shown in Fig. 15(b). Figure 15(c) illustrates that after 1, 50, and 100 thermal cycles, there are no obvious changes in either the exothermal or endothermal curves. Therefore, LA/LA-GA FS-CPCMs exhibit a good thermal reliability.

The thermal stability and reliability was examined in this section with the help of TGA and heating and cooling curves. Two main outcomes were found. To begin with, the thermal stability of Gr doped PCMs is very good even after undergoing more than 50 thermal cycles. In addition, the TGA analysis provides the temperature of thermal degradation and temperature for percentage of weight loss and amount of residue as shown in the first set of figures of this section.

The main advantages of GA and GNPs are that they can be used as membranes for separation of gases by having nano-pores inside them and havinge their benefits in high frequency transistors. They also have advantage in being used as display screens of high speed mobile devices and also in light emitting diodes, used in fast charging lithium-ion batteries, storing hydrogen for fuel cells propelled cars and in low cost water desalination [80]. GA and GNPs main disadvantage is that being a good conductor of electricity it still does not have any band gap. Researchers are trying to find out the reason for this phenomenon. In buildings, the cooling potential is based on intelligent cooling strategies and PCMs integrated form such as coupled and distributed systems and combined strategies such as ventilation, high reflective coatings and radiative cooling walls. By doping with Gr and its derivatives, the PCMs thermal and mechanical properties are improved [115].

4 Applications

Solar energy can be harnessed directly and indirectly by solar photovoltaic cells and by solar thermal systems respectively [116]. The following section deals with applications of Gr based PCMs in the area of solar thermal energy [117].

4.1 Gr coatings and Gr based PCMs in solar power generation

Direct steam generation and the simple reheat Rankine cycle is utilized by Gr based PCMs and Gr coatings in solar thermal power plants to store thermal energy. The condenser of a Rankine cycle based power plant can be quite inefficient in collection of steam, hence they are coated with Gr based PCMs coatings and Gr coatings for incrementing the thermal efficiency of a power plant [118] due to the hydrophobic nature of Gr. The energy stored is released in the time of need, making the system highly cost effective. There is surplus amount of solar energy stored due to the fact that Gr based PCMs and Gr coatings have a light absorption efficiency of 85%–90% because of its black surface [119], and they can also reduce the charging and discharging time due to the presence of Gr in the PCMs as a result of its very high thermal conductivity. This type of energy can be utilized for solar energy generation systems used in house hold applications. The schematic diagram of a concentrated solar thermal power plant is described in Fig. 16 [120].

4.2 Gr based PCMs in solar water heating systems

The area where the solar energy is highly exploited is the water heating system. Solar water can be used in commercial, industrial, and residential sectors. A solar water heater is easy to maintain, very inexpensive, and easy to fabricate. A solar water heating system is composed of a PCMs contained heater storage unit and a solar water heater system. The operation of the solar water heater in the day time is that the water heater collects solar energy to heat the water and Gr based PCMs absorbs thermal energy and stores it. In night time when the sun shine is unavailable, the thermal energy is retrieved from Gr based PCMs to heat the water. The schematic diagram of a solar water heater is shown in Fig. 17.

Kumar et al. [121] examined the functionalized Gr with Pn PCMs for water heating and thermal energy storage applications. It was reported that 1% (mass fraction) of Gr with PCMs absorbs heat well due to its high thermal conductivity. In this system, the water temperature was increased up to 90°C at a water flow rate of 12.5 mL/s.

The experimental study of hybrid PCMs with Gr for application of thermal energy storage and water heating was examined by Bharadwaj et al. [18]. The overall efficiency was improved by 30.26% with the use of hybrid nano-PCMs. The heat transfer fluid temperature was reached up to 80°C at 1% (mass fraction) of Gr and the hot water can be used for household and industrial applications.

4.3 Gr based PCMs in solar green house

Gr based PCMs have been used in solar green house for storing solar energy for drying, curing, and production of plants. The solar energy storage unit during day time collected energy and released it during the night time. During the off-sunshine hours, the solar energy was released leading to proper production of plants.

The one pot method is used to prepare composite nano-PCMs and the thermal properties were analysed by Ge et al. [122]. It was reported that EmPEG/PDA-rGO-3 gave a better structure with a consistent behaviour after more than 100 cycles and there is no leakage. In addition, the prepared nanocomposite gives a consistent rate of thermal charging, discharging, and reliability. This type of low temperature PCMs are promising in the field of green house, waste heat recovery, and textiles.

4.4 Gr based PCMs in buildings

Gr based PCMs have been used in buildings for serving the purposes like wallboards, trombe walls, under floor heating systems, shutters, and ceiling boards [123]. That means the heating and cooling of buildings basically used Gr based PCMs in heat and cold storage units and walls of buildings. Gr based PCMs are used in floors and wall partition ceilings to serve the purpose of regulators of temperature in the Gr based PCMs trombe wall. The wall boards are not that expensive and can be used in a variety of applications like thermal comforts unit in all kinds of buildings. The shutters containing PCMs are placed outside of the windows and are exposed to solar radiation, which causes PCMs to melt and at night when the shutter is closed and the windows are slided, by introducing the heat into the room. The heating and cooling of buildings floor is a significant part of the system. The under-floor heating systems are used for thermal comfort in the buildings. The significant part of the roof is the ceiling boards used for heating as well as cooling of the buildings.

Xu et al. [124] analyzed CaCl₂·6H₂O PCMs with SrCl₂·6H₂O and GO as a nucleating agent for building energy storage. It was found that the addition of nucleating agents reduced the supercooling up to 99.2%. In addition, the freezing enthalpy of the PCMs was found to be 207.9 J/kg. The developed nano-enhanced PCMs is an exceptional material to store thermal energy in buildings.

Sayyar et al. [125] analyzed the performance of Gr based nano-PCMs for energy efficiency in buildings. It was observed that the integration of nano-enhanced PCMs into wallboards minimized the energy consumption to maintain the temperature.

The inorganic PCMs Na₂SO₄·10H₂O and Na₂HPO₄·12H₂O with a weight ratio of 8 : 2 as PCMs and nucleating agent as GO was analyzed by Li et al. [126]. This composite PCMs was applicable for building energy saving projects due to their best packing factor and higher packing factor to solve the leakage problem.

Prabakaran et al. [127] examined fatty acid PCMs with functionalized Gr platelets for air conditioning of buildings. It was observed that the maximum thermal conductivity enhancement was 102% at 0.5% (mass fraction) of Gr nanoparticles. The PCMs nanocomposite was more beneficial to enhance the thermal energy storage system.

4.5 Gr based PCMs in battery thermal management system

The control of temperature differences and operating temperature for thermal management of lithium ion batteries used in electric vehicles is very important and essential. Commonly used PCMs suffer from the low thermal conductivity. Therefore, doping of PCMs with Gr and its derivatives is very much needed for transfer of heat from the battery thermal management system (BTMS). This also results in the increment of lifespan and thermal safety of the batteries [128]. The essential flow diagram of BTMS is shown in Fig. 18.

The improvement of energy performance by using nano-enhanced PCMs for battery thermal management was analyzed by Zou et al. [69]. In this research, Pn was used as PCMs and Gr and MWCNT were used as a nanoparticle. The researcher reported that composite PCMs with a Gr/MWCNT mass ratio of 7/3 could be the best enhancement in heat transfer. Besides, PCMs had the highest increase/decrement rate of temperature which could be reduced by 63.30% and 50% than pristine PCMs. The nano-PCMs had a higher potential in thermal management in lithium ion battery.

4.6 Gr based PCMs in electronics cooling

Joseph and Sajith [129] analyzed the hybrid cooling of electronics components at pulsed/uniform thermal loads. Figure 19 shows the line diagram and photograph of nano-PCMs based electronic cooling system. In this system, Pn and Gr was used as PCMs and nanoparticles were used for energy savings of heat sink up to 23%. The thermal conductivity was increased up to 60% for 0.5% (mass fraction) of Gr. Additionally, the use of Gr based PCMs reduces the fan function, thereby saves energy.

The thermal management of electronics systems use nano-PCMs based heat sink was done for reduction of base line temperature. In this system, RT44HC (Rubitherm Technologies with melting point 44°C high capacity) and RT64HC (Rubitherm Technologies with melting point 64°C high capacity) PCMs and Gr nanoparticles were used. Figure 20 shows the electronics cooling using nano PCMs. It was found that RT44HC/GNPs with 0.005% (mass fraction) was the best mass fraction as it decreased the temperature by 13.60%, whereas 0.008% (mass fraction) of RT64HC gave a temperature reduction of 8.935%. Hence, it can be concluded that the RT44HC PCMs are suitable for low heating loads and RT64HC is suitable for higher heating loads [130].

4.7 Gr based PCMs in thermal energy storage

Zhang et al. [131] examined the improvement of thermal conductivity of stearic acid PCMs using GO-attapulgite aerogel for thermal storage applications. Yuan et al. [132] analyzed the effect of functionalized Gr and octadecane on PCMs thermal properties. It was found that phase change temperature, diffusion coefficient, thermal conductivity and heat capacity could be altered by both pristine and functionalized one. The composite Gr/n-OD PCMs can be used in aerospace industry and microelectronics as thermal energy storage materials due to their higher thermophysical performance. Yang et al. [48] examined MS/PW (MS is modified form of reduced Gr). PCMs was used for store thermal energy and photothermal energy conversion applications. It was reported that the photothermal efficiency could be improved by 81%. Wang et al. [133] examined and analyzed inorganic PCMs with Gr for thermal energy storage applications. Tu et al. [134] analyzed PEG with grafted GO nanocomposite for higher improvement in thermal conductivity. The PEG-grafted GO acted as a plasticizer and improved its crystallizability. Due to the higher thermal performance and latent heat, it is highly efficient for thermal energy storage applications.

Qi et al. [111] analyzed PW with hierarchical Gr form and found that it gave a higher thermal conductivity of 744% than pure PCMs. Moreover, it was observed that the developed composite PCMs exhibited a higher thermal energy storage density, negligible phase change temperature, and thermal reliability. Because of its exceptional thermal properties, it is applicable for thermal energy storage.

Vivekananthan and Amirthan [63] analyzed Gr nanoparticle with erythritol PCMs for thermal energy storage applications. It was observed that with the addition of 1.0% (mass fraction) of Gr, the thermal conductivity was increased by 53.1% and solidification temperature was increased by 18.67% than pristine erythritol PCMs.

The various PCMs and their applications are summa-rized in Table 5. Gr based PCMs are apt for thermal energy storage applications like water heating, heating ventilation and air conditioning (HVAC), battery thermal management, thermal management in buildings, green house, electronic cooling etc., due to their higher thermal conductivity. From the above review, it is observed that very few studies have been conducted in Gr based nano-PCMs for TES applications. That is the reason for the fact that there is a need for more experimental studies on Gr based nanocomposite PCMs for TES applications. Case studies can be conducted especially in thermal power plants, space heating, water heating, HVAC, electronic cooling etc. on modern advanced technologies and assessing its cost effectiveness.

5 Conclusions and future outlook

5.1 Conclusions

This paper deals with characterization of PCNC, thermal characterization, and behavior of PCNC and its thermal stability analysis for GNPs, GA, standalone Gr, and GO based PCMs. Better thermal properties are obtained for Gr mixed PCMs because of the high thermal conductivity of Gr and its derivatives. The most appealing part is the increase in system performance due to the energy storage capability of PCMs. The Gr and its derivative based PCMs were identified and characterized by FT-IR, XRD, and SEM by which the structures and crystal structures were known and chemical bonds were identified, along with their phase identification and surface structure respectively. With increasing mass fraction (%) of Gr and its derivatives in PCNC, the thermo-mechanical properties are improved. The thermal stability was increased with the increase in the mass fraction of Gr in PCNC as predicted by TGA, the latent heat of melting and latent heat of freezing too were increased as a function of time and temperature by DSC. Such enthalpy changes showed that the PCNC had a high energy storage capability. Gr and its derivatives with PCMs like microcrystalline cellulose, PW, fatty acids, PU, and PEG etc. had peaks for FT-IR in the range of 1000–3700 cm⁻¹, had peaks for XRD in the range of 10°–70°, thermal conductivity in the range of 0.3 W/(m·K) to 10 W/(m·K), latent heat of melting and freezing in the range of 90 to 250 J/g and melting and freezing temperature varied from 15°C to 60°C. These technologies are very efficient for humans and energy conservation. The use of Gr based nanoparticles and different PCMs are scattered throughout various journals and literature. This paper is an effort to bring and gel together Gr and its derivatives with different types of PCMs for structural, physical, and morphological characterization, its different applications, and its effect on thermo physical properties of the PCNC.

5.2 Future outlook

The future research must focus on increasing and optimizing the heat transfer ability of Gr based PCMs. Gr needs to be studied so that they may play an important role in thermal energy storage and thermal energy release. Furthermore, due to the high cost of Gr, other PCMs and additives need to be developed so that the whole process of charging and discharging of PCMs becomes very cost-effective. The future requirements can be met by storing excess energy and releasing it whenever it is required i.e., intermittent supply of energy can be met by PCMs with nanofillers like Gr and its derivatives. The amount of energy stored and released is in proportion with the latent heat of storage of Gr based PCMs. In the period of absence of sunshine, the absorbed solar energy can be utilized due to the presence of Gr based PCMs in solar thermal power plants, solar water heating systems, solar cookers and solar dryers, and solar green house and buildings. These systems are highly efficient and have enhanced thermal properties in case of PCMs filled with Gr nanofillers which can manipulate the future of renewable energy systems. Such PCMs nanocompsites can be used in electronic devices for better thermal management too. The synergistic effect of such PCMs combined with Gr and its derivatives needs to be investigated so as to minimize the interfacial thermal resistance. Machine learning tools, for example artificial neural networks, can also be applied to predict the properties of PCNC.

The nanofillers distribution and dispersion which remain constant in the process of undergoing phase transformation still need serious research attention. The orientation of Gr based nanofillers in PCMs matrix affects the mechanical and physical properties, hence, they also need to be studied.

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