One-Step Preparation of Form-Stable Phase Change Material through

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One-Step Preparation of Form-Stable Phase Change Material through Self-Assembly of Fatty Acid and Graphene Amir Reza Akhiani, Mohammad Mehrali, Sara Tahan Latibari, Mehdi Mehrali, Teuku Meurah Indra Mahlia, Emad Sadeghinezhad, and Hendrik Simon Cornelis Metselaar J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 Sep 2015 Downloaded from http://pubs.acs.org on September 24, 2015

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The Journal of Physical Chemistry

One-Step Preparation of Form-Stable Phase Change Material through Self-Assembly of Fatty Acid and Graphene

Amir Reza Akhiani a, Mohammad Mehrali a*, Sara Tahan Latibari a, Mehdi Mehrali a, Teuku Meurah Indra Mahlia b, Emad Sadeghinezhad a, Hendrik Simon Cornelis Metselaar a*

a

Advanced Material Research Center, Department of Mechanical Engineering,

University of Malaya, 50603 Kuala Lumpur, Malaysia. b

Department of Mechanical Engineering, University Tenaga Nasional, 43009 Kajang,

Selangor, Malaysia

*Corresponding authors. Tel.: +60 3 79674451; fax: +60 3 79675317. Email: [email protected] (M.Mehrali), [email protected] (H.S.C. Metselaar).

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Abstract: In this work, for the first time, a one-step (self-assembly) method is introduced to prepare a Phase change material (PCM) composite consisting of palmitic acid (PA) and minimal amount (0.6%) of oleylamine functionalized-reduced graphene oxide (OArGO) as a supporting material. Graphene oxide were reduced and functionalized with long chain alkylamine, oleylamine (OA), to adsorb palmitic acid (PA) and were simultaneously self-assembled into a three-dimensional structure. This technique eliminates common freeze-drying and impregnation steps since the phase change material is adsorbed in situ and there was clearly no shrinkage during drying process. In addition, the connected graphene network provides nuclei for the heterogeneous nucleation and crystallization of PA with an enhanced heat transfer to and from the PCM and retains excellent shape-stable property which prevents the leakage of molten PA. The obtained composites exhibit a large phase change enthalpy (99.6%), enhanced thermal conductivity (150%), excellent cycling performance and substantial sunlight adsorption.

1. Introduction Phase change materials (PCM) for thermal energy storage (TES) have been extensively employed in different applications including thermal management, energy storage and solar energy since they offer numerous benefits for various areas of the industry.1-4 Conversion of sunlight to heat by means of PCMs is particularly attractive, since solar power is the most abundant and lasting renewable source available.5 PCMs are capable of storing and releasing a great amount of thermal energy in the form of latent heat (∆H) during the melting and solidification phase transition in small temperature intervals.6-9 Among the different groups of phase change materials, organic 2 ACS Paragon Plus Environment

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PCMs have been extensively studied due to their practical melting temperatures, high latent heat storage capacity, small volume change and high chemical and thermal stability.10-11 However, low thermal conductivity, low solar harvesting efficiency and seepage of the molten PCM are the main factors that hinder the usage of organic PCMs in practical applications.12-15 Methods like micro-encapsulation,16 electrospinning,17-18 and in situ polymerization19 have been developed to eliminate the storage difficulties of PCMs. In addition, carbonaceous materials, such as expanded graphite,20-22 carbon nanotube (CNT),23 carbon nanofiber,13 carbon nanosphere,24 activated carbon,25 and graphene sheets26-27 have been introduced to compensate the low solar and thermal properties of organic PCMs. The common implemented methods to disperse these fillers in the PCM composites include Vacuum impregnation,28-29 Solution intercalation,30 and Fusion adsorption.31 The dispersed fillers provide a dark surface and transport paths inside the organic matrix, thus contributing to an enhancement of the heat transfer and light harvesting within the PCMs. One critical problem lies in the fact that these additives are usually present as a high mass fraction in the composite PCMs, which leads to a significant decrease of the phase change enthalpy. Recently, carbonaceous aerogels and foams have been researched as an alternative to powder or fiber like fillers. They provide a continuous porous network in the PCMs which leads to the more stable composites.32 Furthermore, the high porosity and low density of carbon aerogels provide a large PCM loading ratio and ultrahigh surface area which retains the PCM inside the structure by surface tension. Preparation of PCM/aerogel composites includes 1) synthesis of hydrogel 2) freeze-drying the hydrogel to form a spongy structure 3) impregnation of molten PCM within the aerogel pores.33-35 PCM-aerogel composites have a considerable thermal conductivity with little loss of latent heat.



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However, the complex equipment involved in the synthesis of aerogels confine their large-scale production for industrial applications. Two dimensional (2D) graphene sheets have a large specific surface area (2630 m2 g − 1 36-37

),

high thermal conductivity (∼ 5000 Wm− 1K− 1),37-38 and the capability to self-

assemble into a three dimensional (3D) structure.39-41 Due to these unique characteristics, graphene sheets are promising as thermal conductivity promoter and as packaging material. Moreover, the various oxygen functional groups on a graphene oxide (GO) surface provide essentially infinite possibilities for the modification or functionalization.38, 42 The covalent functionalization of the GO sheets is an effective tool to control its physical and chemical properties for different applications.43 In the present work, for the first time a one-step preparation of a PCM composite consisting of palmitic acid (PA) and minimal amount of oleylamine functionalizedreduced graphene oxide (OA-rGO) needed for shape stabilization is introduced. GO sheets are functionalized with a long chain alkylamine, oleylamine (OA), to adsorb palmitic acid (PA) and simultaneously, graphene sheets self-assembled into the 3D structure. This method eliminates the freeze-drying and impregnation steps as were necessary for all the carbonaceous PCM composites before. In addition, the connected graphene network provides nuclei for the heterogeneous nucleation and crystallization of PA with an enhanced heat transfer to and from the PCM and retains excellent shapestable property which prevents the leakage of molten PA. The influence of the 3D interconnected OA-rGO network on the morphology, structure of the composite were systematically studied and enhancement of thermal conductivity and phase change storage properties were reported.


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2. Experimental 2.1. Materials The graphite flakes used in this project were purchased from Ashbury Inc. The sulfuric acid (H2SO4, 98%), phosphoric acid (H3PO4, 98%), potassium permanganate (KMnO4, 99.9%), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, 37%), PA (C16H32O2, 98%) and OA (C18H37N, 80-90%) were purchased from fisher Scientific. All materials were used as obtained without further purification. 2.2. Preparation of PCM composite GO was prepared from graphite flakes through oxidation with concentrated acids (H2SO4 and H3PO4) and oxidant (KMnO4), following a simplified Hummers method.44 For preparation of PA/OA-rGO composites, GO aqueous dispersion (1.5 mg ml-1, 14 ml) was gradually added to different amounts of melted PA at 80 ºC with constant stirring to ensure GO/PA mass ratios of 0.6, 1, 1.5 and 2 % which were labelled as PG0.6, PG-1, PG-1.5 and PG-2, respectively. Subsequently, 5 mg of OA was added to the unstable emulsions and vigorously stirred for 20 min at 80 ºC in a water bath to obtain stable and homogeneous GO-PA-water emulsions (Figure 1a). The mixtures were sealed in a glass vial and kept in the water bath at 80 ºC for 24 h. With prolonging the reaction time, color of mixtures changed from brown to black and slight gelation took place (Figure 1b). In order to obtain the rigid black monolith of PG composites, temperature was increased to 95 ºC for 36 h (Figure 1c). Finally, the samples were dried in an oven at 100 °C for 12 h. Furthermore, to investigate the effect of OA on the phase change properties of PA and functionalization of GO, PA/OA and OA/GO mixtures were prepared with the same ratio as PG-1 but in the absence of GO (POA-1) and PA (OA-rGO), respectively. 5 ACS Paragon Plus Environment


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(a)

(b)

(c)

(d)

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Figure 1. Digital image of (a) as prepared mixtures, (b) after 24 h in 80 ºC, (c) After 24h in 95 ºC, (d) After drying in oven (PG-0.6, PG-1, PG-1.5, PG-2 from left to right, respectively). 2.3. Characterization and measurements High-resolution FEI Quanta 200F field emission scanning electron microscopy (FESEM) was used to visually characterize the morphology of the PG composites. Fourier Transform-Infrared (FT-IR) absorption spectra of the composites were recorded using a Bruker FT-IR (Bruker Tensor 27) spectrometer at room temperature in the range 4000–400 cm−1 using ATR mode. The X-Ray diffraction (XRD) pattern of the powders and composites were obtained using an automated X-ray powder diffractometer (XRD, PANalytical's Empyrean) with a monochromated CuKα radiation (λ = 1.54056 Å). Raman spectra were obtained using a Renishaw Invia Raman Microscope using laser excitation at 514 nm. An X-ray photoemission spectrometer (XPS, PHI-Quantera II) with an Al-Ka (hm = 1486.8 eV) X-ray source was used to identify the functionalization and reduction of OA-rGO. The melting and freezing temperatures and latent heats of PG composites were obtained by differential scanning calorimeter (DSC, METTLER TOLEDO 820C-Error ±0.25–1 °C) at a heating rate of 5 °C/min. The weight loss and thermal stability of PCMs were obtained by thermogravimetric analysis (TGA, SETARAM 92 apparatus -Error ±1µg) at a heating rate of 10 °C /min and a temperature of 50–500°C in purified nitrogen atmosphere. The thermal reliability of PG composites was investigated after 500 thermal cycles by using 6 ACS Paragon Plus Environment

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an accelerated thermal cycling system.45 The laser flash technique (Netzsch LFA 447 NanoFlash) was used to measure the thermal diffusivity of prepared samples at room temperature. 2.4. Photo-to-thermal energy conversion 1g of PA and PG composites (1.7 cm×6 mm) were placed into a foam insulation. The samples were directly illuminated by simulated light source, at an ambient temperature of around 28 °C. A pyranometer (CMP10, Kipp & Zonen Co., Netherlands) was used to measure and adjust a solar radiation flux density of the light irradiation from simulated light source. The temperature evolution of the samples during heating and cooling processes was measured with thermocouple (type-K, Omega Co., Singapore) which were connected to the data collecting device (Graphtec, midi logger GL220). The schematic of the light-to-heat conversion process under simulated light source is shown in Figure 2.

Figure 2. A schematic of the photo-to-thermal energy conversion system



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3. Result and discussion The PG composites were successfully prepared through one-step self-assembly method as shown in Figure 1d. The OA-rGO adsorbed all the PA while assembled to the 3D structure and composites have different size and density due to the different amounts of PA. They are not completely cylindrical, since the adsorbed PA has a lower density than water and black monolith floated toward the top of the water surface. Thus, the contact of composites with the glass vial changes their cylindrical form. Interestingly, except for the integrated cylinder, there were no separated PA or graphene sheets elsewhere, and the transparent solution was left in the vessel. Finally, Samples were simply dried at 100 ºC in an oven and no freeze-drying was necessary as the molten PA moisturized the 3D structure of graphene and the PCMs did not shrink while drying. 3.1. Mechanism of PG composite formation GO sheets have both hydrophilic edges and hydrophobic basal plan which they are reasoned to show an attractive amphiphilic nature.46 Therefore, GO are capable of lowering the surface tension and stabilizing the oil-water emulsions, behaving like colloidal surfactants.47 Primary interaction of GO sheets with OA (hydrophobic surfactant) improves its emulsification properties, thus by vigorously stirring, a homogeneous and stable GO-PA-water emulsion can be formed (Figure 1a). It should be noticed that the higher PA concentration in the solution leads to the higher viscosity in the resulting emulsion, which is beneficial for the stability of the oil–water emulsion. It is well known that the hydrophilic carboxyl functional groups are responsible for the stable colloidal dispersion of GO sheets in water. Ionization of carboxylic groups creates electrostatic repulsion between GO sheets which protects them from 8 ACS Paragon Plus Environment

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aggregation in aqueous solutions.48 During chemical reduction of GO by OA, its amine (–NH2) functional group reacts with epoxy and carboxyl functional groups of GO via ring opening reaction and condensation reaction, respectively (Figure 3a).49-50 Reduction of GO by OA diminishes the repulsion force between graphene sheets, therefore the OA-rGO become hydrophobic.51 Conversely, the combination of π-π stacking, hydrogen bonding and hydrophobic interactions enhance the bonding force between graphene sheets which consequently promote gelation.52 Simultaneously, PA is adsorbed and retained in the graphene gel network via hydrophobic Van der Waals interaction between the OA and PA aliphatic chains (Figure 3b).53 As the reduction continues, the functionalized graphene nanosheets and PA assembled more tightly to form 3D composites with an interconnected network structure. In addition, the applied temperature keeps the PA in the melting state and increase the reaction of GO with OA.

Figure 3. Schematic illustration of (a) Functionalizing GO. (b) Reduction, functionalization and self-assembly process



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3.2. Morphology of PG composites FESEM images of GO and PG composites are displayed in Figure 4. GO possesses a nanosheet structure with a rippled rough morphology due to its ultrathin feature (Figure 4a). The overlapping and interlocking flexible graphene sheets resulted in the formation of an interconnected 3D structure of OA-rGO and PA. In Figure 4b-f, the OA-rGO sheets are wrapped by the phase change material which made the rough surface and sharp edges of graphene nanosheets smooth. The interfaces of the PA and functionalized graphene combined compactly due to the hydrophobic interaction and large contact area arising from the 2D geometry of OA-rGO. (a)

(b)

100µm

500nm

(d)

(c)

(e)

100µm

100µm

(f)

100µm

5µm

Figure 4. FESEM images of (a) GO, (b) PG-0.6, (c) PG-1, (d) PG-1.5, (e) PG-2, and (f) PG-2 with higher magnification. 3.3. Structural properties of PG composites In the FTIR spectra of pure PA (Figure 5), the sharp peaks at 2914 and 2849 cm−1 represent the asymmetrical vibration and symmetrical vibration of –CH2 group. The


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peak at 1465 cm−1 signifies the deformation vibration of its –CH3 and –CH2 group. The absorption peak at 1695 cm−1 indicates the C=O stretching vibration. The broad absorption band at 2600–3200 cm−1 shows the stretching vibration of the –OH functional groups of PA. The peak at 1299 cm−1 corresponds to the in-plane bending vibration of the –OH group of PA, the band at 941 cm−1 belongs to the out of-plane bending vibration of the –OH functional group and the peak at 723 cm−1 represents the in-plane swinging vibration of the –OH functional group.54 In the spectra of PG nanocomposites, it is clear that all the main absorption peaks of PA functional groups appeared and no shift can be observed. However, a new peak appeared around 1539 cm−1 in the FTIR spectra of POA-1 (Figure 5), which belongs to the –NH bending vibrations. This indicates that the amidation reaction accrued between OA and PA in the absence of GO for POA-1 sample. This peak is shifted to 1581 cm−1 in the PG composites and can be ascribed to the asymmetric stretching vibration of –COO−, Suggesting that the PA molecules were deprotonated and transformed into carboxylate anions and no amidation reaction was observed.55 This peak is very weak for PG-0.6 and PG-1 composites due to low ratio of OA/PA. Based on these results, the amine functional group of OA reacts, mainly, with the functional groups of GO and there was only a slight interaction with the carboxylic functional group of PA. At first, the emulsion kept at 80 ºC as PA molecules require large activation energy to react with OA. By the time the temperature increased to 95 ºC most of OA was already reacted with the functional groups of GO which minimized the possible modifications in the PA chemical structure.



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Figure 5. FTIR spectra of POA-1, PG composites and pure PA The XRD patterns of pure PA, PG-2 and POA-1 are presented in Figure 6a. Sharp and intense peaks at 7.36º, 12.31º, 21.5º, 23.98º are the characteristics of PA and they are all observed in PG-2.56 This indicate that the PA has been successfully adsorbed by the 3D structure of OA-rGO. The peak positions are identical for PA and PCM composite (PG-1) which shows that the crystal structure of PA did not change in the preparation process. However, the crystallinity of PA in the composite is different, as PG-2 and POA-1 have different peak intensities from pure PA. The percentage of PA crystallinity for these samples was determined by calculating the ratio of amorphous area of X-ray diagram to the total area.57 PA Crystallinity was 25.57±0.24% and increased to 28.09±0.84% for PG-2 while it was decreased to 24.74±0.36 for POA-1. This demonstrates that the crystallinity of PA improved in the nanocomposite 3D network, whereas the direct interaction with OA reduced its crystallinity. These findings are in agreement with the previously discussed FTIR results.


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The low amount of OA-rGO in the composites was not detected with XRD and FTIR. Therefore, Raman spectroscopy were employed to investigate the composite structure. The well-known D band and G band appeared around 1346 cm−1 and 1600 cm−1 in the Raman spectra of GO and PG-2, respectively (Figure 6b). The Raman spectra of composite can be used to investigate the graphene structure, since in the PA Raman graph there are no peaks around the D band and G band of graphene. The D band arises from the breathing mode of κ-point phonons of A1g symmetry which can be associated with structural defects and partially disordered structures of the sp2 domains, while the G band is related to the E2g vibration mode of sp2 carbon atoms in the graphitic structure.58-59 Commonly, the intensity ratio of D band to G band (ID/IG) is used to measure the disorder degree of sp2 clusters in a network of sp3 and sp2 bonded carbons.60 The ID/IG for GO decreases from 0.75 to 0.5 in PG-2 which indicates the restoration of sp2 domains and improvement of the graphitic degree of graphene nanosheets in PG composites due to the reduction effect.61-62 The peaks at 2845 and 2885 are the symmetric and asymmetric vibration mode of –CH2 and together with peaks at 1063, 1102, 1131, 1179, 1299 and 1441 cm−1 can be assigned to PA characteristics.63-64 In the Raman spectra of PG composite all the peaks for PA are observed which is consistent with XRD and FTIR results and confirms the composition of PA and OA-rGO.



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(a)

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(b)

Figure 6. (a) XRD patterns of the pure PA, PG-2 and POA-1. (b) Raman spectra of GO, PG-2 and pure PA The XPS analysis was performed for better understanding the reduction and functionalization of GO with OA. The survey XPS spectra of GO and OA-rGO in Figure 7a shows that most of oxygen functional groups were removed by reduction and C/O atomic ratio increased from 1.6 (GO) to 12.4 (OA-rGO).65 The new peak (N1s) in the survey spectra of OA-rGO (Figure 7b) can be attributed to presence of nitrogen element, indicating the functionalization of GO with OA. The C1s high-resolution XPS spectra of GO (Figure 7b) can be deconvoluted into four peak components which are associated with carbon atoms in aromatic/conjugated C (C=C, 284.7 eV), hydroxyl/epoxide (C–O/C–O–C, 286.9 eV), carbonyl (C=O, 288.1 eV) and carboxyl (O–C=O, 288.7 eV) functional groups.66 The peak intensities of oxygen-containing functional groups dramatically decreased upon reaction of GO with OA (Figure 7c), implying the considerable deoxygenation of GO and partial restoration of the sp2 carbon sites through the reducing process.67 As shown in Figure 7c, the newly appearing peak for C–N (258.9 eV) accompanying the dramatic decrease of C–O/C–O– C signal indicates the reaction of OA with epoxide and carboxyl groups on the basal plane of GO.49 The high resolution N 1s spectra of OA-rGO (Figure 7d) consists of three components at the binding energies of 399.03 (=N–), 399.7 (–NH–) and 401.01 14 ACS Paragon Plus Environment

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eV (–N+–).68 It is clear that the most intense peak assigned to the –NH– in Figure 7d, confirms that the primary reaction between OA and GO is via ring-opening amination of basal plane epoxides. Meanwhile, the existence of –N+– bond shows that the OA could also interact with carboxylic acid on the edge of GO. The results corroborate the proposed mechanism for reduction and functionalization of GO to form OA-rGO which was discussed earlier. (a)

(c)

(b)

(d)

Figure 7. (a) Survey XPS spectra; C 1s of (b) GO, (c) OA-rGO and (d) N 1s of OArGO 3.4. Shape-stable properties of PG composites The shape-stabilized properties of the pure PA and PCM composites were examined using a hot plate and a digital camera. The PG composites were placed on the hot plate and the temperature was increased to 120 ºC. After 30 min, pure PA added and changes 15 ACS Paragon Plus Environment


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in the samples shape were observed by digital camera. The black monoliths remained solid unlike the pure PA, which melted completely into liquid (Figure 8a, b and video S1). The surface of the composites was moist, but no liquid leakage from the composites was detected during the heating process. The hydrophobic surface of OArGO and capillary effect of pores in the composite structure prevented the molten PA from spreading out. Based on the above evidence, it can be concluded that the functionalized graphene matrix endows the PG composites with good shape-stable properties. (a)

(b)

Figure 8. PG-0.6, PG-1, PG-1.5, PG-2 (from left to right, respectively) and pure PA (a) before and (b) after (holding at 120 ºC) 3.5. Thermal properties of PG composites In shape-stabilized PCMs, it is important to utilize the lowest amount of additives to minimize the loss of the latent heat, since supporting materials do not undergo phase transition. Thermal energy storage properties of pure PA (as a reference), POA-1 and PG composites with different OA-rGO/PA mass fractions are examined with DSC. All the composite shows one peak in endothermic and exothermic DSC curves in Figure 9, indicating the latent thermal energy storage of PCMs during melting and solidifying. The corresponding thermal properties are summarized in Table 1. It is found that the phase change properties of PCM composites are slightly affected by OA and supporting material (OA-rGO) during self-assembly process. In addition, The DSC curve of POA1 was also investigated to distinguish the effect of OA from OA-rGO on the phase


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change behavior of PA in the composites. It is clear that the fusion latent heat of the PG composites decreases as additive content increases. However, the decrease of the latent heat of the PG composites cannot be attributed to the lower fraction of PA alone. Another factor leading to the loss of the latent heat is the interaction of carboxylic group of PA with amine group of OA as it was discussed through the FTIR results. Obviously, only crystalized PA would contribute to latent heat during melting and solidifying. Thus, the mass fraction of crystalized PA in composites and POA-1 is determined by the following equation:54, 56

=

(1)

∆ × 100 ∆

Where, ∆ and ∆ are the endothermic latent heat of PG composites and pure PA, respectively. The mass fraction of crystalized PA in POA-1 sample and composites suggests that the higher OA/PA fraction leads to lower amount of crystalized PA. The broader DSC peaks for PCM composites can be explained by increasing the crystallinity of PA in interaction with the functionalized surface of supporting material and it was confirmed through the XRD analysis earlier.



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Figure 9. DSC curves of pure PA, POA-1 and PG composites. Table 1. Phase change behaviour of Pure PA, POA-1 and PG composites Melting Tpm (ºC)

Tom (ºC)

∆Hm(J g-1)

Tps (ºC)

Solidifying Tos ∆Hs (J g-1) (ºC)

PA

64.28

61.65

-203.0

58.92

56.26

214.1

100

POA-1

63.52

59.65

-174.5

58.35

56.38

178.3

85.9

PG-0.6

64.06

61.2

-196.6

59.35

56.99

203.0

96.9

PG-1

64.33

61.12

-187.5

59.26

57.26

194.3

92.4

PG-1.5

64.74

61.02

-184.9

59.04

57.2

190.0

91.1

PG-2

64.77

60.99

-181.9

59.1

57.13

189.3

89.6

Sample

ω% a

a

Tom: onset melting, Tos: onset solidifying, Tpm: peak melting, Tps: peak solidifying temperature, ∆Hm : latent heat of melting and ∆Hs : latent heat of solidifying, ω : mass fraction of crystalized PA

It is discussed that the PCMs store and release thermal energy while melting and solidifying. The rate of storing and releasing thermal energy are highly related to PCMs thermal conductivities. Pure PA has a low thermal response due to its low thermal conductivity. As presented above, reduction of GO during the self-assembly process leads to removal of the functional groups and partial restoration of the conjugated structure of graphene sheets. The OA-rGO sheets have a high thermal conductivity and 18 ACS Paragon Plus Environment

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supply a continuous path for heat to transfer throughout the PA rapidly. The thermal diffusivity was measured by the laser flash method and the thermal conductivity can be calculated as follows:13 (2)

= . .

Where K is the thermal conductivity (Wm-1k-1), is thermal diffusivity (m2 s-1), is density (kg m-3) and Cp is specific heat capacity (J kg-1 K-1)). Figure 10 shows the thermal conductivity values for PCM composites loaded with different mass fractions of OA-rGO. The results show that the thermal conductivity of the PG composites increases with the amount of OA-rGO loading. In contrast to the findings of Warzoha and Fleischer69 it is obvious that a “kink” in the thermal conductivity distribution of PG composites reveals a shift from the dilute regime to the percolating regime at 1.5 wt %. Thermal conductivity of the PG-2 with 2% of the supporting material is 1.5 times higher than that of the pure PA which is consistent with the previous literatures.33, 70

Figure 10. Thermal conductivity of pure PA and PG composites The TGA and derivative TG (DTG) analysis were performed to examine the thermal stability of PG composites (Figure 11a and b). A two-step thermal degradation processes can be observed for PCM composite in DTG curve. However, the second step



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cannot be recognized in the TGA diagram of composite as it happened continuously. The first step corresponds to the degradation of PA chains. The TGA curve shows that pure PA decomposes between 177 to 300 ºC, while PG-2 shows a weight reduction at a higher temperature around 180 to 330 ºC. The further weight loss at 332 to 400ºC shows the elimination of the remaining OA-rGO functional groups at high temperature. In the composite, the 3D structure of OA-rGO acts as a protecting layer and the interaction of PA with the surface of OA-rGO delays the escape of the vaporized PCM during thermal degradation. The results show that the PG composites exhibit high thermal stabilities.

(a)

(b)

Figure 11. (a) TG and (b) DTG curves of pure PA and PG-2 composite To prove the reversibility of PG composites, the solid–liquid phase transition cycling tests were performed for 500 cycles. The phase transition properties of the composites before and after 500 thermal cycles are listed in Table S1. The DSC curves of the composites after thermal cycling in Figure 12a and Figure S1 have one transition peak, in agreement with the corresponding samples before cycling. Unlike most of the literature in which the latent heats remains unchanged after cycling, in this case, the phase change enthalpy increased by 2.7, 6.6, 7.8 and 8% for PG-0.6, PG-1, PG-1.5 and PG-2, respectively. Moreover, their phase change temperatures decreased by 0.5, 0.9, 1.3 and 1.4 °C. This phenomenon can be explained by investigating the chemical


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structure of PA in the structure of composites after thermal cycling. Comparing the FTIR diagrams of the composites before and after cycling in Figure S2 shows that no thermal degradation of PA occurred during heating and cooling cycles. The absorbance peak for stretching vibration of –COO− around 1581 cm−1 is disappeared after thermal cycling as shown in Figure 12b (from 1480 cm−1 to 1670 cm−1). Based on these results, we believe that during cycling the fatty acid carboxylate anions were protonated and transformed back into fatty acid.Therefore, the percentage of the crystalized PA increased which leads to the higher phase change enthalpy. It is expected that the thermal conductivity of PCMs improves after thermal cycling because of the further reduction of OA-rGO during cycling.

(a)

(b)

Figure 12. (a) DSC curves of PG-2, and (b) FTIR spectra of PG composites (1480 cm−1 to 1670 cm−1) before and after cycling tests 3.6. Sunlight irradiation The photon-to-thermal energy conversion of pure PA and PG composites was investigated under the simulated sunlight radiation (with intensity of 111.6 mW cm-2) as shown in Figure 13. The temperature of the PG composites rapidly rose by adsorbing the radiation energy and equilibrium reached at 68.6, 69.4, 69.6 and 70.4 °C for PG-0.6, PG-1, PG-1.5 and PG-2, respectively. The inflation at the melting point of composites 21 ACS Paragon Plus Environment


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illustrates that the phase transition of samples has accrued during solar radiation, due to the fact that OA-rGO adsorbed the light radiation and converted it into the heat. The highest temperature for pure PA is 47.6 °C and no visible melting was observed because of its low solar to thermal conversion efficiency. It is obvious that the pure PA had a white surface and it would reflect the irradiated light, which means that the temperature of the pure PA can’t reach its melting point to store the solar energy. However in the PG composites, the black surface of OA-rGO captured the photon energy and heated the PA molecules, then the PA stored the thermal energy through its phase transition.71 The temperature of composites drops rapidly, after removing the light source. The freezing plateau shows the solidification process of PA in the composites. The cooling rate for PG composites is higher than for pure PA due to the enhanced thermal conductivity of the composites. Based on the results, PG composites show high performance compare with pure PA in the light-to-heat transduction.

Figure 13. Obtained temperature of Pure PA and PG composites (light irradiation, ambient temperature 28 °C)


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Moreover, the comparison of thermal properties and preparation methods of the PG composite with recently reported PCM composites is shown in Table 2. It can be seen that functionalizing GO with OA helped to introduce the new method for preparing composite PCMs. This new method significantly enhanced the heat storage capacity of PCMs by using the minimal amount of additive. The low graphene content in PG-2 composite should be mainly responsible for the modest enhancement of thermal conductivity since other composite PCMs utilized high load of graphene. Table 2. Comparison of shape-stabilized composite PCMs

Composite

Method

∆Hm

(J g )

%

-1

K a Ref. (W m-1 K-1)

Polyethylene glycol/graphite foam

Vacuum impregnation

76.1

44

3.5

28

1-octadecanol/annealed graphene

Solution intercalation

221

88

1.32

27

Polyethylene glycol/ GO

Solution intercalation 142.8

90

-

30

Solution intercalation 161.93

76

0.6

22

91.34

44

2.262

31

Vacuum impregnation 188.98

92

2.11

21

Vacuum impregnation 100.21

50

1.24

29

97.6

0.419

Palmitic acid/polyaniline/ xGnP Palmitic acid/graphene-nickel Palmitic acid/ xGnP

b

Palmitic acid/GO Palmitic acid/GO/OA (PG-2) a

b

Fusion adsorption

Self-assembly

198.0

∆Hm : latent heat of melting, ω : mass fraction of crystalized PCM, K: thermal conductivity ,

This work b

xGnP:

exfoliated graphite nanoplatelets

4. Conclusion A novel PCM composite has been prepared through a simple one-step self-assembly process. The composite has a good shape stability, even with only 0.6% of supporting material. The connected graphene network enhances heat transfer across the composite



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1.5 times compared to the pure PA with a 2% additive loading. The low amount of supporting material preserved the energy storage capacity of PA and composites showed high phase transition enthalpy (196.6 J g-1), especially after thermal cycling (202 J g-1). Additionally, prepared composites exhibited high light to energy conversion capacity and good thermal stability. The peculiarity of this method provides a new platform for developing shape stabilized PCMs, which have promising potential applications in many energy-related devices. Acknowledgements This work has been financially supported by Ministry of High Education (MOHE) of Malaysia, Grants number UM.C/HIR/MOHENG/21-(D000021-16001) “Phase Change Materials (PCM) for Energy Storage System” and University of Malaya research Grant No. UMRG RP021-2012A. Supporting Information Detailed phase change properties of composite before and after thermal cycling (table and figures), FTIR spectra of Composites before and after thermal cycling. Web enhanced Video to show the shape-stable property of composites. References (1) Domański, R.; Fellah, G., Thermoeconomic Analysis of Sensible Heat, Thermal Energy Storage Systems. Appl. Therm. Eng. 1998, 18, 693-704. (2) Hasnain, S. M., Review on Sustainable Thermal Energy Storage Technologies, Part I: Heat Storage Materials and Techniques. Energy Convers. Manage. 1998, 39, 11271138.


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Table of Contents Graphic

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One-Step Preparation of Form-Stable Phase Change Material through

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