The shape-stabilized phase change materials composed of polyethylene glycol and graphitic carbon nitride matrices

Thermochimica Acta 612 (2015) 19–24

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The shape-stabilized phase change materials composed of polyethylene glycol and graphitic carbon nitride matrices Lili Feng a, * , Ping Song b , Shicheng Yan c , Haibo Wang a , Jie Wang a a Key Laboratory of Urban Stormwater System and Water Environment (Ministry of Education), Beijing Climate Change Response Research and Education Center, Beijing University of Civil Engineering and Architecture, Beijing 100044, PR China b National Research Center for Geoanalysis, Beijing 100037, PR China c National Laboratory of Solid State Microstructures, College of Engineering and Applied Science, Ecomaterials and Renewable Energy Research Center (ERERC), Nanjing University, Nanjing 210093, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 March 2015 Received in revised form 6 May 2015 Accepted 10 May 2015 Available online 14 May 2015

New shape-stabilized phase change materials (PCMs) composed of polyethylene glycol (PEG) and graphitic carbon nitride materials (bulk-C3N4 and CNIC) were prepared by a blending and impregnating method. The structural and thermal properties of the composites were investigated using various characterization techniques. The highest PEG content stabilized in the composite PCMs is 40 wt% for bulk-C3N4 and 60 wt% for CNIC. The crystallinity and phase change property of PEG/bulk-C3N4 PCMs are entirely damaged by the bulk-C3N4 stabilizer. For the PEG/CNIC PCMs with various PEG content, the phase change temperatures and the extent of supercooling show little difference and are much lower than those of pure PEG. The phase change temperatures, Tm and Tc, respectively decrease by
24  C and
19  C compared with pure PEG. The phase change enthalpies increase with higher PEG content. 60 wt% PEG/CNIC PCM has a relatively larger phase change enthalpy of 45.8 J g1. This study suggests that the graphitic carbon nitride supporting materials are in favor of remarkably lowering the phase change temperature and the extent of supercooling of PCMs and will provide insights into the design of composite PCMs through employing new shape stabilization matrices. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Shape-stabilized phase change materials Graphitic carbon nitride Polyethylene glycol Bulk-C3N4 C3N4-based intercalation compounds

1. Introduction Phase change materials (PCMs) are very attractive for their thermal energy storage and temperature control properties [1–3]. Thermal energy can be stored as latent heat in PCMs and the temperature is almost constant during phase change. Polyethylene glycol (PEG) is an outstanding PCM for thermal energy storage [4–5]. It has high latent heat and suitable melting temperature range from 3.2  C to 68.7  C. PEG is also non-toxic, non-corrosive and relatively stable. However, it is mostly employed in solid– liquid PCMs, which is less convenient than solid–solid phase change in practical use due to its leakage during the solid–liquid phase change. In addition, the low thermal conductivity limits the application of PEG. Shape-stabilized PCMs composed of PEG and supporting materials can solve the problems. Many researches on PEG based shape-stabilized PCMs have been carried out by introducing an inorganic material, such as silica [6–14], expanded graphite [15,16], active carbon [4], graphene [17–19], ordered

* Corresponding author. Tel.: +86 10 68322124; fax: +86 10 68322124. E-mail addresses: [email protected], sharpfl@buaa.edu.cn (L. Feng). http://dx.doi.org/10.1016/j.tca.2015.05.001 0040-6031/ ã 2015 Elsevier B.V. All rights reserved.

mesoporous materials [5,16,20], diatomite [21] and so on, as the supporting material. Graphitic carbon nitride (g-C3N4) is predicted to be a promising candidate to complement carbon materials in various potential applications due to its unique properties [22–29]. It was predicted that C3N4 should have a bulk modulus comparable to diamond. The structure of C3N4 leads to a very high thermal conductivity as well as mesoporous property, which can overcome the drawbacks of PEG as PCM. g-C3N4 can be prepared in large scale by condensation of some precursors, such as cyanamide, dicyandiamide, melamine, and ammonium thiocyanate, etc. [30–33]. g-C3N4 is also the most stable one of CNs under ambient conditions, which can be stable in air up 600  C. It also demonstrates good stability in aqueous and organic solvents. Generally, there are structural defects and surface terminations in the structure of g-C3N4 for incomplete condensation of precursors. The study of applying g-C3N4 to support PCMs is still lacking. In this work, we prepare stabilized PEG based heat storage systems employing graphitic carbon nitride materials (bulk-C3N4 and CNIC) as shape stabilization matrices, by a simple blending and impregnation process. The structural and thermal properties of the composites are characterized by various techniques. This study

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will provide insights into the design of shape-stabilized PCMs through employing novel supporting matrices.

3. Results and discussion 3.1. Structural properties

2. Experimental 2.1. Synthesis of graphite carbon nitride materials Two graphitic carbon nitride materials were selected as the shape stabilizers of polyethylene glycol: bulk carbon nitride (bulkC3N4) and carbon nitride intercalation compounds (CNIC). The bulk C3N4 samples were obtained by directly heating melamine at 520  C for 4 h. The CNIC samples were synthesized through molten salt method with calcination temperature of 500  C [34]. Melamine was used as a precursor. The eutectic mixture of LiClH2O–KCl– NaCl (1:1:1 weight ratio) was selected as a solvent. The mixture of eutectic salts and melamine was prepared with a weight ratio of 15:1 for eutectic salts to melamine and finely ground in a mortar. The resulting mixture was transferred into a quartz glass beaker, and then the powder mixtures were heated to 500  C for 1 h under semiclosed environment. After it was cooled to room temperature, the obtained yellow powders were washed thoroughly with deionized water several times, and then the product was collected by centrifugation and dried at 60  C for 10 h. All the chemicals used were analytical grade reagents without further purification. 2.2. Preparation of the composite PCMs Chemically pure polyethylene glycol (PEG; Mw 6000) was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd., PEG based phase change materials with different PEG contents of 40–90 wt% were prepared by a physical blending and impregnating method. First, x g of PEG (x = 0.2–0.45) was melted and dissolved in 20 ml of absolute ethanol (AR, >99.7% purity) to form a homogeneous solution. Then, (0.5-x) g of the carbon nitride material was added to the above PEG solution while stirring. The resulting solution was stirred vigorously for 4 h. Finally, the mixture was dried at 80  C for 72 h, so that the ethanol solvent would completely evaporate and the shape stabilization of the composites above PEG’s melting point could be investigated. It was found that there were no PEG leakages at 80  C from 60 wt% PEG/CNIC PCMs and 40 wt% PEG/bulk-C3N4 PCMs.

The phase structures of the samples were investigated by XRD. Fig. 1(a) shows that bulk-C3N4 and PEG/bulk-C3N4 PCMs have similar diffraction patterns, which is in good agreement with the reports on g-C3N4 prepared by polymerization of cyanamide, dicyandiamide and melamine [25,28,35–37]. The peak at 13.2 which corresponds to an interlayer distance of d = 0.67 nm and is indexed as (1 0 0), is associated with an in-plane ordering of tri-striazine units [35,36]. The high-intensity peak at 27.5 , corresponding to d = 0.32 nm, is a characteristic interlayer stacking peak of aromatic segments and can be indexed as the (0 0 2) peak observed for graphitic materials [35–36]. It is observed that no crystalline peaks of PEG appear in PEG/bulk-C3N4 PCMs, indicating that the crystal growth of PEG is completely hindered by the bulkC3N4 supporting material. Consequently, no crystalline peaks can be observed. Fig. 1(b) shows that the diffraction peaks of the PEG/CNIC PCMs are composed of those of pure PEG and CNIC, meaning that the CNIC stabilizer do not affect the crystal structure of PEG in the composites, which indicates that no chemical reactions take place between PEG and CNIC. Additionally, the relative intensity of the peaks at 19.11 and 23.31 for PEG in PEG/CNIC PCMs are weakened as the mass fractions of CNIC increased, so CNIC interferes with the crystallization of PEG [4,10]. When the mass fraction of PEG is 40%, there are characteristic peaks of PEG in PEG/CNIC PCMs, whereas no peaks of PEG are observed in PEG/bulk-C3N4 PCMs (in Fig. 1(a)), suggesting that the influence of the bulk-C3N4 stabilizer on the

2.3. Property analysis Powder X-ray diffraction (XRD) patterns were collected at a scanning rate of 5 /min in the 2u range 5–80 on a D8-advance Bruker diffractometer using Ni-filtered Cu Ka radiation (l = 0.1541 nm) and operating at 40 kV and 100 mA. The Fourier transform infrared (FTIR) spectra of the samples (KBr pellets) were recorded in the transmission mode on a Bruker VERTEX 70 FTIR spectrometer. The surface morphology of the samples was examined using an S-5500 Hitachi scanning electron microscope (SEM). The BET surface area, total pore volume and average pore size of the carbon nitride materials were measured by the nitrogen adsorption method at liquid nitrogen temperature on a Quantachrome Autosorb-iQ-MP gas adsorption analyzer. The phase change temperature and enthalpy of the samples were determined using a Q2000 differential scanning calorimetry (DSC, Thermal Analysis Corporation, USA). The samples were heated and cooled between 40  C and 90  C at a rate of 10  C min1 in a nitrogen atmosphere. Thermal stability of the samples was measured on a simultaneous Q600 SDT thermogravimetric analysis (TGA) Instruments (Thermal Analysis Corporation, USA) from 0  C to 1200  C at a rate of 10  C min1 under dry nitrogen.

Fig. 1. XRD patterns of pure PEG, carbon nitride stabilizers and the composite PCMs.

L. Feng et al. / Thermochimica Acta 612 (2015) 19–24

crystallization of PEG is much stronger than that of the CNIC stabilizer. The XRD pattern of CNIC in Fig. 1(b) shows a series of diffraction peaks. By analogy to graphite intercalation compounds [38], series of diffraction peaks, at 8.2 , 12.0 , 21.3 , 27.8 , 32.1, 36.0 and 44.2 , can be respectively assigned to the (0 0 2), (0 0 3), (0 0 5), (0 0 7), (0 0 8), (0 0 9) and (0 0 1 0) reflections, suggesting that a carbon nitride intercalation compound (CNIC) was successfully synthesized by the simple thermal polycondensation of melamine in molten salts [34]. In the PEG/CNIC composites, the aforementioned (0 0 1) diffraction peak slightly shifts to higher angle compared with the CNIC matrix, indicating that the interlayer spacing becomes smaller with the addition of PEG. The corresponding interlayer distance d is listed in Table 1. From Table 1, the d values for CNIC in PEG/CNIC PCMs are slightly lower than those for the CNIC matrix. This may result from the interaction between PEG and CNIC. The chemical structures of the samples were demonstrated by FTIR spectra. As shown in Fig. 2, in the spectra of pure PEG, we can observe the triplet peak of the C O C stretching vibration at 1143 cm1, 1108 cm1 and 1066 cm1 with a maximum at 1108 cm1. The peaks at 3437 cm1 and 1629 cm1 are assigned to the stretching vibration of hydroxyl and water, respectively. The peaks at 2881 cm1, 954 cm1 and 841 cm1 are caused by the stretching vibration of the CH2 functional group, the crystal peak of PEG [13] and C C O bonds. In the spectra of the bulk-C3N4 stabilizer, two types of vibration are exhibited. The characteristic peaks of the tri-s-triazine units lie at about 810 cm1 [38]. The other band in the 1244–1629 cm1 regions is due to the skeletal stretching vibrations of the CN heterocycles [39]. These peaks of bulk-C3N4 can also be found in the spectra of the PEG/bulk-C3N4 composite. Nevertheless, some absorption peaks of the main functional groups of PEG disappear in the spectra of the composite, which is consistent with the results of XRD that the bulk-C3N4 stabilizer damages the PEG crystal. In the spectra of the CNIC stabilizer, in addition to above two types of vibration of bulk-C3N4, we can observe the peaks at 3437 cm1 and 2178 cm1 assigned to the stretching vibration of cyano group ( NH). The peak at 1161 cm1 represents the stretching vibration of C–N functional group. The peak at 645 cm1 is caused by the bending vibration of  NH [34]. The adsorption peaks of PEG and CNIC can basically be found in the spectra of the PEG/CNIC composites. Moreover, no new peaks lie in the spectra of the composites other than characteristic peaks of PEG and CNIC. This indicates that the interaction between PEG and CNIC was physical [5–8], corresponding to the XRD results. The C–N absorption peak at 1161 cm1 in the spectra of CNIC shifts to lower wave number of 1108 cm1 in the spectra of the composites owing to the strong interactions probably formed by hydrogen bonds between bridging nitrogen atoms of CNIC and the terminal hydroxyl group of PEG. The micro structures of the samples were depicted by SEM images. From Fig. 3, the bulk-C3N4 stabilizer reveals irregular bulk plates (Fig. 3(a)) and is uniformly clothed by PEG in PEG/bulk-C3N4 PCMs (Fig. 3(b)). A homogeneous and smooth morphology without

Table 1 The interlayer distance d for the CNIC matrix and CNIC in PEG/CNIC PCMs. Sample

CNIC 40% PEG/CNIC 50% PEG/CNIC 60% PEG/CNIC

d (nm) (0 0 2)

(0 0 3)

(0 0 5)

(0 0 7)

(0 0 8)

(0 0 9)

(0 0 1 0)

1.080 1.060 1.080 1.080

0.736 0.721 0.728 0.736

0.424 0.417 0.417 0.420

0.320 0.318 0.319 0.320

0.279 0.276 0.275 0.271

0.249 0.248 0.248 0.249

0.207 0.200 0.201 0.202

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Fig. 2. FTIR spectra of pure PEG, carbon nitride stabilizers and the composite PCMs.

voids between PEG and bulk-C3N4 can be observed. This indicates good cohesion and strong interfacial adhesion between PEG and bulk-C3N4, which confines the free motion of PEG and hinders its crystal formation, so no crystalline peaks of PEG in PEG/bulk-C3N4 PCMs can be seen in Fig. 1(a). The CNIC stabilizer exhibits a porous structure which is aggregated by nanorods (Fig. 3(c)). The results of nitrogen adsorption–desorption for two carbon nitride stabilizers in Table 2 also show that the specific surface area of CNIC reaches 77 m2 g1, much higher than 5 m2 g1 of bulk-C3N4, owing to the porous structure. The SEM images of PEG/CNIC PCMs in Fig. 3 (d)–(f) show the morphology of the encapsulated CNIC by PEG, PEG and CNIC. Since a part of PEG segments are in unconfined state, it can crystallize, which is accordant with the XRD results in Fig. 1(b) that the crystalline peaks of PEG in PEG/CNIC PCMs can be seen. 3.2. Thermal properties Fig. 4 shows the DSC curves of pure PEG and the composite PCMs with various PEG weight percentages during the heating and cooling scan. The detailed results of onset melting/crystallization temperature (Tmo/Tco), end melting/crystallization temperature (Tme/Tce), peak melting/crystallization temperature (Tm/Tc) and melting/crystallization enthalpy (DHm/DHc) obtained are presented in Table 3. As shown in both Fig. 4 and Table 3, the solid to liquid phase change of PEG occurs between 53.3  C and 75.2  C with a peak temperature at 67.8  C, and the heat of fusion is 194.3 J g1. In the cooling process, the phase change temperature ranges from 42.8  C to 21.3  C with a peak temperature of 31.7  C and the released heat is 173.2 J g1. The PEG/ bulk-C3N4 PCM exhibits neither endothermic nor exothermic peak, which is consistent with the XRD result, due to the interference of bulkC3N4 by acting as an impurity in perfect PEG crystallization. Tmo,Tco, Tme, Tce, Tm and Tc of the PEG/CNIC PCMs with various PEG content show little difference. The phase change temperatures, Tm and Tc, respectively reduce by
24  C and
19  C and much more than those of other reported shape-stabilized PCMs compared to pure PEG [4–21]. The phase change enthalpy, DHm: 35.0–45.8 J g1 and DHc: 29.3–42.7 J g1, of the PEG/CNIC PCMs decreased with the addition of CNIC and evidently less than their theoretical value (the product of the enthalpy of pure PEG and PEG weight percentage in the composites PCMs, e.g., for 40% PEG/CNIC PCM, melting enthalpy: 194.3 J g1  40% = 77.7 J g1, crystallization enthalpy: 173.2 J g1  40% = 69.3 J g1). This mainly resulted from the interference of CNIC matrix with the crystallization of PEG. The CNIC matrix impedes the regular arrangement of the PEG chains into crystal lattices owing to the confinement effect of mesoporous and

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Fig. 3. SEM images of carbon nitride stabilizers and the composite PCMs with various PEG weight percentages. (a) bulk-C3N4; (b) 40 wt% PEG/bulk-C3N4; (c) CNIC; (d) 40 wt% PEG/CNIC; (e) 50 wt% PEG/CNIC; (f) 60 wt% PEG/CNIC.

Table 2 Porous characteristics of the selected carbon nitride stabilizers. Matrix

SBET (m2 g1)

Vpore (cm3 g1)

Dpore (nm)

Bulk-C3N4 CNIC

5 77

0.04 0.32

31.6 16.5

Fig. 4. DSC curves of pure PEG and the composite PCMs with various PEG weight percentages.

strong intermolecular hydrogen bonding and surface adsorption, leading to a decline in phase change enthalpy [5,12]. In practical applications, the supercooling of PCMs must be considered. Using the DSC measurement results in Fig. 4, the extent of supercooling was evaluated as the difference between the melting point and crystallization temperature [5,11]. The comparison of the extent of supercooling for different PEG/CNIC PCMs is shown in Fig. 5. The extent of supercooling in pure PEG was larger compared with that in the PEG/CNIC PCMs. This suggests that the extent of supercooling of PEG can be favorably reduced by blending of porous CNIC with pure PEG. Thermal stability is characterized by TGA. In Fig. 6(a), PEG lost weight in one step from 300  C to 428  C, which indicates that PEG have good thermal stability at 300  C. The bulk-C3N4 stabilizer also lost weight in one step and did not decompose below 510  C. Therefore, for the PEG/bulk-C3N4 PCMs, there are two steps of losing weight corresponding to that of PEG and the bulk- C3N4 stabilizer. In addition, note that there were 3 wt% residual unknown substances that remained for pure PEG, and a 63 wt% residual remained after the decomposition of PEG in 40 wt% PEG/bulk-C3N4 PCMs at 428  C, proving that the as-prepared PEG/bulk-C3N4 PCMs were very homogeneous and contained 40 wt % PEG (no loss of PEG in the preparation process of the composite PCMs). From Fig. 6(b), the CNIC stabilizer lost weight in three steps, and all the PEG/CNIC PCM samples lost weight in four steps corresponding to one weight-lost step of PEG and three weightlost steps of CNIC. Three TGA curves of the PEG/CNIC PCMs with

L. Feng et al. / Thermochimica Acta 612 (2015) 19–24

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Table 3 Thermal properties of pure PEG and PEG/CNIC PCMs with various PEG content under thermal cycling. Samples

Tmo ( C)

Tme ( C)

Tm ( C)

4Hm (J g1)

Tco ( C)

Tce ( C)

Tc ( C)

4Hc (J g1)

Pure PEG 40% PEG 50% PEG 60% PEG

53.3 33.3 33.5 32.9

75.2 48.8 48.1 47.9

67.8 44.4 43.9 43.8

194.3 35.0 40.6 45.8

42.8 21.9 22.3 23.0

21.3 3.6 3.2 2.3

31.7 13.7 12.3 12.0

173.2 29.3 37.9 42.7

60 wt% PEG/CNIC PCMs at 428  C after the decomposition of PEG, also proving that the as-prepared PEG/CNIC PCMs were homogeneous and had no loss of PEG in the preparation. 4. Conclusions

Fig. 5. Comparisons of the phase change temperature and the extent of supercooling for pure PEG and the PEG/CNIC composites.

In summary, new shape-stabilized PCMs were prepared by the direct blending of PEG with graphitic carbon nitride materials (bulk-C3N4 and CNIC) and an impregnating method. The maximum PEG content stabilized in the composite PCMs at 80  C above the melting point of PEG is 40 wt% for PEG/bulk-C3N4 PCMs and 60 wt% for PEG/CNIC PCMs. The structural and thermal properties of the as-prepared composite PCMs were investigated. It is shown that the crystallization and phase change behavior of PEG in PEG/bulkC3N4 PCMs are completely hindered by the bulk-C3N4 stabilizer due to the interference of bulk-C3N4 by acting as an impurity in perfect PEG crystallization. The phase change temperatures, Tm and Tc, of the PEG/CNIC PCMs with various PEG content show little difference and respectively decrease by
24  C and
19  C compared to pure PEG. Meanwhile, the extent of supercooling of PEG in the PEG/CNIC PCMs is much lower than that of pure PEG. The phase change enthalpies of the PEG/CNIC PCMs increase with higher PEG content. The largest phase change enthalpies, DHm and DHc, obtained from 60 wt% PEG/CNIC PCMs are respectively 45.8 J g1 and 42.7 J g1. Capillary forces, surface areas and hydrogen bonding are considered to be significant factors for the crystallization and phase change behavior of PEG/CNIC PCMs. This study suggests that the graphitic carbon nitride supporting materials are in favor of remarkably lowering the phase change temperature and the extent of supercooling of PCMs. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51206009), the Ministry of Science and Technology of China (No. 2009CB939902), China Scholarship Council (No. 201408110015) and Beijing Higher Education Young Elite Teacher Project (No. YETP1663). References

Fig. 6. TGA curves of pure PEG, carbon nitride stabilizers and the composite PCMs.

various PEG weigh percentage are perfectly overlapped before the complete decomposition of PEG. There were respectively 55 wt%, 45 wt% and 35 wt% residual substances for 40 wt%, 50 wt% and

[1] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renew. Sust. Energy Rev. 13 (2009) 318–345. [2] J.C. Su, P.S. Liu, A novel solid–solid phase change heat storage material with polyurethane block copolymer structure, Energy Convers. Manage. 47 (2006) 3185–3191. [3] J.F. Wang, H.Q. Xie, Z. Xin, Thermal properties of paraffin based composites containing multi-walled carbon nanotubes, Thermochim. Acta 488 (2009) 39–42. [4] L.L. Feng, J. Zheng, H.Z. Yang, Y.L. Guo, W. Li, X.G. Li, Preparation and characterization of polyethylene glycol/active carbon composites as shapestabilized phase change materials, Sol. Energy Mater. Sol. Cells 95 (2011) 644–650. [5] L.L. Feng, W. Zhao, J. Zheng, S. Frisco, P. Song, X.G. Li, The shape–stabilized phase change materials composed of polyethylene glycol and various mesoporous matrices (AC, SBA–15 and MCM–41), Sol. Energy Mater. Sol. Cells 95 (2011) 3550–3556. [6] H.Z. Yang, L.L. Feng, C.Y. Wang, W. Zhao, X.G. Li, Confinement effect of SiO2 framework on phase change of PEG in shape-stabilized PEG/SiO2 composites, Eur. Polym. J. 48 (2012) 803–810.

24

L. Feng et al. / Thermochimica Acta 612 (2015) 19–24

[7] B.T. Tang, J.S. Cui, Y.M. Wang, C. Jia, S.F. Zhang, Facile synthesis and performances of PEG/SiO2 composite form-stable phase change materials, Sol. Energy 97 (2013) 484–492. [8] L.H. He, J.R. Li, C. Zhou, H.Z. Zhu, X.J. Cao, B.M. Tang, Phase change characteristics of shape-stabilized PEG/SiO2 composites using calcium chloride-assisted and temperature-assisted sol gel methods, Sol. Energy 103 (2014) 448–455. [9] B.T. Tang, C. Wu, M.G. Qiu, X.W. Zhang, S.F. Zhang, PEG/SiO2–Al2O3 hybrid form-stable phase change materials with enhanced thermal conductivity, Mater. Chem. Phys. 144 (2014) 162–167. [10] J.G. Li, L.H. He, T.Z. Liu, X.J. Cao, H.Z. Zhu, Preparation and characterization of PEG/SiO2 composites as shape-stabilized phase change materials for thermal energy storage, Sol. Energy Mater. Sol. Cells 118 (2013) 48–53. [11] T.T. Qian, J.H. Li, H.W. Ma, J. Yang, The preparation of a green shape-stabilized composite phase change material of polyethylene glycol/SiO2 with enhanced thermal performance based on oil shale ash via temperature-assisted sol–gel method, Sol. Energy Mater. Sol. Cells 132 (2015) 29–39. [12] G.Q. Qi, C.L. Liang, R.Y. Bao, Z.Y. Liu, W. Yang, B.H. Xie, M.B. Yang, Polyethylene glycol based shape-stabilized phase change material for thermal energy storage with ultra-low content of graphene oxide, Sol. Energy Mater. Sol. Cells 123 (2014) 171–177. [13] W.L. Wang, X.X. Yang, Y.T. Fang, J. Ding, Preparation and performance of formstable polyethylene glycol/silicon dioxide composites as solid–liquid phase change materials, Appl. Energy 86 (2009) 170–174. [14] W.L. Wang, X.X. Yang, Y.T. Fang, J. Ding, J.Y. Yan, Enhanced thermal conductivity and thermal performance of form-stable composite phase change materials by using b-aluminum nitride, Appl. Energy 86 (2009) 1196–1200. [15] W.L. Wang, X.X. Yang, Y.T. Fang, J. Ding, J. Yan, Preparation and thermal properties of polyethylene glycol/expanded graphite blends for energy storage, Appl. Energy 86 (2009) 1479–1483. [16] C.Y. Wang, L.L. Feng, W. Li, J. Zheng, W.H. Tian, X.G. Li, Shape-stabilized phase change materials based on polyethylene glycol/porous carbon composite: the influence of the pore structure of the carbon materials, Sol. Energy Mater. Sol. Cells 105 (2012) 21–26. [17] C.Y. Wang, L.L. Feng, H.Z. Yang, G.B. Xin, W. Li, J. Zheng, W.H. Tian, X.G. Li, Graphene oxide stabilized polyethylene glycol for heat storage, Phys. Chem. Chem. Phys. 14 (2012) 13233–13238. [18] H.R. Li, M. Jiang, Q. Li, D.N. Li, Z.Y. Chen, W.P. Hu, J. Huang, X.Z. Xu, L.J. Dong, H.A. Xie, C.X. Xiong, Aqueous preparation of polyethylene glycol/sulfonated graphene phase change composite with enhanced thermal performance, Energy Convers. Manage. 75 (2013) 482–487. [19] B.T. Tang, Y.M. Wang, M.G. Qiu, S.F. Zhang, A full-band sunlight-driven carbon nanotube/PEG/SiO2 composites for solar energy storage, Sol. Energy Mater. Sol. Cells 123 (2014) 7–12. [20] B.M. Abu-Zied, M.A. Hussein, A.M. Asiri, Development and characterization of the composites based on mesoporous MCM-41 and polyethylene glycol and their properties, Compos. B 58 (2014) 185–192. [21] S. Karaman, A. Karaipekli, A. Sarı, A. Bicer, Polyethylene glycol(PEG)/diatomite composite as a novel form-stable phase change material for thermal energy storage, Sol. Energy Mater. Sol. Cells 95 (2011) 1647–1653.

[22] R.C. Dante, M.R. Pablo, C.G. Adriana, M.G. Jesús, Synthesis of graphitic carbon nitride by reaction of melamine and uric acid, Mater. Chem. Phys. 130 (2011) 1094–1102. [23] S.B. Yang, Y.J. Gong, J.S. Zhang, L. Zhan, L.L. Ma, Z.Y. Fang, R. Vajtai, X.C. Wang, P. M. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light, Adv. Mater. 25 (2013) 2452–2456. [24] J.Q. Tian, Q. Liu, A.M. Asiri, A.O. Al-Youbi, X.P. Sun, Ultrathin graphitic carbon nitride nanosheet: a highly efficient fluorosensor for rapid ultrasensitive detection of Cu2+, Anal. Chem. 85 (2013) 5595–5599. [25] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation performance of g-C3N4 fabricated by directly heating melamine, Langmuir 25 (2009) 10397–10401. [26] Y. Zheng, J. Liu, J. Liang, M. Jaroniecc, S.Z. Qiao, Graphitic carbon nitride materials: controllable synthesis and applications in fuel cells and photocatalysis, Energy Environ. Sci. 5 (2012) 6717–6731. [27] X.C. Wang, S. Blechert, M. Antonietti, Polymeric graphitic carbon nitride for heterogeneous photocatalysis, ACS Catal. 2 (2012) 1596–1606. [28] A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.O. Müller, R. SchÅgl, J.M. Carlsson, Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts, J. Mater. Chem. 18 (2008) 4893–4908. [29] Y.J. Zhang, Z. Schnepp, J.Y. Cao, S.X. Ouyang, Y. Li, J.H. Ye, S.Q. Liu, Biopolymeractivated graphitic carbon nitride towards a sustainable photocathode material, Sci. Rep. 3 (2013) 1–5. [30] H.Z. Dai, X.C. Gao, E.Z. Liu, Y.H. Yang, W.Q. Hou, L.M. Kang, J. Fan, X.Y. Hu, Synthesis and characterization of graphitic carbon nitride sub-microspheres using microwave method under mild condition, Diamond Relat. Mater. 38 (2013) 109–117. [31] C. Li, C.B. Cao, H.S. Zhu, Preparation of graphitic carbon nitride by electrodeposition, Chin. Sci. Bull. 48 (2003) 1737–1740. [32] E.G. Wang, A new development in covalently bonded carbon nitride and related materials, Adv. Mater. 11 (1999) 1129–1133. [33] L.P. Guo, Y. Chen, E.G. Wang, L. Li, Z.X. Zhao, Identification of a new C–N phase with monoclinic structure, Chem. Phys. Lett. 268 (1997) 26–30. [34] H.L. Gao, S.C. Yan, J.J. Wang, Y.A. Huang, P. Wang, Z.S. Li, Z.G. Zou, Towards efficient solar hydrogen production by intercalated carbon nitride photocatalyst, Phys. Chem. Chem. Phys. 15 (2013) 18077–18084. [35] F. Dong, L.W. Wu, Y.J. Sun, M. Fu, Z.B. Wu, S.C. Lee, Efficient synthesis of polymeric g-C3N4 layered materials as novel efficient visible light driven photocatalysts, J. Mater. Chem. 21 (2011) 15171–15174. [36] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation of rhodamine B and methyl orange over boron-doped g-C3N4 under visible light irradiation, Langmuir 26 (2010) 3894–3901. [37] Q.J. Xiang, J.G. Yu, M. Jaroniec, Preparation and enhanced visible-light photocatalytic H2-production activity of graphene/C3N4 composites, J. Phys. Chem. C 115 (2011) 7355–7363. [38] M.S. Dresselhaus, G. Dresselhaus, Intercalation compounds of graphite, Adv. Phys. 51 (2002) 1–186. [39] S.X. Min, G.X. Lu, Enhanced electron transfer from the excited eosin Y to mpgC3N4 for highly efficient hydrogen evolution under 550 nm irradiation, J. Phys. Chem. C 116 (2012) 19644–19652.