Core-Sheath Paraffin-Wax-Loaded Nanofibers by Electrospinning for

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Energy, Environmental, and Catalysis Applications

Core-sheath Paraffin Wax-loaded Nanofibers by Electrospinning for Heat Storage Yuan Lu, Xiudi Xiao, Yongjun Zhan, Changmeng Huan, Shuai Qi, Haoliang Cheng, and Gang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02057 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Applied Materials & Interfaces

Core-sheath Paraffin Wax-loaded Nanofibers by Electrospinning for Heat Storage Yuan Lu†,‡,§,ǁ, Xiudi Xiao†,‡,§,*, Yongjun Zhan†,‡,§, Changmeng Huan†,‡,§,ǁ, Shuai Qi†,‡,§,⊥, Haoliang Cheng†,‡,§ and Gang Xu†,‡,§,#

† Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China ‡ Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, PR China § Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China ǁ University of Chinese Academy of Sciences, Beijing 100049, PR China ⊥ Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, PR China # Tibet New Energy Research and Demonstration Centre, Lhasa, Tibet, 850000, PR China

*Corresponding author. Tel: 86-20-37279344. Fax: 86-20-37278821. E-mail address: [email protected]

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ABSTRACT Paraffin wax (PW) is widely used for smart thermo-regulation materials due to its good thermal performance. However, the leakage and low thermal conductivity of the PW suppress its application in the heat storage field. Accordingly, developing effective methods to address these issues is of great importance. In this study, we explored a facile approach to obtain PW-loaded core-sheath structured flexible nanofibers film via coaxial electrospinning technique. The PW as the core layer was successfully encapsulated by the sheath layer PMMA. The diameter of fiber core increased from 395 nm to 848 nm as the core solution speed rate increased from 0.1 ml/h to 0.5 ml/h. In addition, it can be seen that the higher core solution speed rate could lead to higher PW encapsulation efficiency according to the TEM results. The core-sheath nanofibers films, moreover, possessed the highest latent heat of 58.25 J/g and solidifying enthalpy of 56.49 J/g. Also, we found that after 200 thermal cycles, there was little change in latent heat, which demonstrated it is beneficial for the PW-loaded core-sheath structure to overcome the leakage issue and enhance thermal stability properties for the thermo-regulation film.

KEYWORDS: Phase change material; Paraffin wax; Electrospinning; Core-sheath; Heat storage

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1. INTRODUCTION The discovery and improvement of materials for energy storage and conversion are important for providing clean and renewable power. The phase change material (PCM), owing to great potential for heat storage, plays important roles in various application fields such as energy storage, thermal protection, thermo-regulation, energy saving etc. 1-3. Notably, PCMs can absorb and store thermal energy with latent heat and then release the stored heat at the constant phase transition temperature 4-5. It has been reported that PCMs including eutectics, inorganic and organic compounds could keep the constant thermal property after multiple solid-liquid state transitions

6-7

. Although the PCMs, such as pentaerythritol (PE), polyethylene glycol

(PEG), were widely investigated for energy storage 8-10, however, a significant barrier that these materials possess high water-solubility restricts their practicability. Besides, the organic PCMs alkanes with high latent heat enthalpy were also applied, but the high price limited their applicability. As well known, paraffin wax (PW), composed of a mixture of alkanes (C18-C30), is a common heat storage organic PCM due to obvious advantages in terms of high storage density, high latent heat, low cost, negligible supercooling and noncorrosiveness properties

11-12

. Nevertheless, it is still significant

challenge that PW suffers from low thermal conductivity and leakage issues during solid-to-liquid phase change process. In past, considerable efforts have been also devoted to overcome aforementioned limits. On the one hand, the higher thermal conductivity materials such as carbon and metal materials were introduced to enhance thermal conductivity, especially considerable effort has reported that the thermal 3



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conductivity of CNTs was about 3000 W/mK, which is higher than that of other materials 13-16. On the other hand, the encapsulation was a kind of good technique for solving the leakage of the PW which strongly suppressed the usable lifetime. More recently, the stable wax-loaded materials have been prepared by different approaches such as sol-gel, microencapsulation and electrospinning etc.

17-19

. Among

these methods, coaxial electrospinning, using concentrically aligned spinnerettes linking to separate channels for different solutions, can offer a straightforward manner to generate core-sheath nanofibers 20-26. Xia et al. encapsulated the alkanes via melting coaxial electrospinning with an additional complicated heating and temperature controller system

27

, while there was no need extra heating equipment and

temperature controller in the electrospinning setup for the solution coaxial electrospinning

28-32

. By now the solution electrospinning is still the most popular

electrospinning technique where the solvent evaporated, left-behind polymer fibers were solidified and collected 33. It can provide a facile method to encapsulate PW to obtain core-sheath nanofibers film. For instance, Yu et al. chose the soy wax as encapsulated material and the obtained latent heat of the material was 36.47 J/g

12

.

Jassal et al. fabricated the nanoencapsulated octadecane nanofibers and the latent heat was 4.3 J/g 34. Unfortunately, the latent heat enthalpy of these obtained materials was low, which indicates that it is of great necessity to further improve the encapsulation efficiency of them. Furthermore, there was scarce study that investigates the paraffin wax encapsulated material via coaxial electrospinning in the past. Therefore, the mixture of paraffin wax, almost the same latent enthalpy with n-alkane, was chosen 4


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and employed in this work. Here, we present a study of the paraffin wax-loaded core-sheath nanofibers using the simple solution coaxial electrospinning method. After exploring the appropriate solvents for PW, toluene with competitive solubility was chosen as solvent for the core layer. Besides, in order to improve the thermal conductivity, CNTs with higher thermal conductivity was added in the sheath layer. It is worthwhile to mention that the well-defined nanofiber film shows some attractive advantages such as flexibility, high latent heat, mechanical strength, specific surface area etc. Additionally, the shape of nanofiber can be maintained via the polymeric sheath and the leakage cannot occur. In this work, consecutive 200 recycle phase change experiments have been conducted to estimate the reutilization of the PW-loaded electrospun nanofibers membrane. Thus, we demonstrated the PW-loaded electrospun core-sheath nanofibers film displayed good thermal storage capacity by encapsulating PW into PMMA using solution coaxial electrospinning. 2. EXPERIMENTAL SECTION 2.1 Materials The chemicals reagents in this research were obtained commercially. The Polymethylmethacrylate (PMMA, Mw=120000) was purchased from Shanghai Macklin Biochemical Technology Co., Ltd., China. Paraffin wax (PW, melting point of 52-54 ºC) and Multi-walled carbon nanotubes (CNTs, length of 10-20 µm, internal diameter of 5-15 nm, and outer diameter of 50 nm) were purchased from Aladdin Chemistry Co., Ltd. Toluene and N, N-dimethylformamide (DMF) were obtained 5



from Guangzhou Chemical Reagent Factory. Other reagents used were analytical grade. 2.2 Fabrication of the electrospun core-sheath nanofibers film The core-sheath nanofibers film with PW was fabricated via coaxial electrospinning technique as reported previously

35

. Briefly, the PMMA solution as

the sheath layer precursor was obtained by dissolving the PMMA in N, N-dimethylformamide (DMF) with gentle magnetic stirring at ambient temperature with concentration of 20% (w/w) for 12 h. And the core layer precursor solution was prepared using PW dissolved in toluene with different concentrations changing from 10% (w/v) to 50% (w/v). Furthermore, to improve the thermal conductivity, the CNTs and PMMA with different ratio (1% (w/w), 5% (w/w) and 10% (w/w)) was respectively added into the DMF with ultrasonic treatment of 2 h and then the mixture solution was stirred at room temperature for 24 h. Then, the PMMA and the PW solution were mounted in the double spinneret to produce the sheath and core layer, respectively. The double spinneret consisted of a stainless-steel needle with inner needle (outer diameter of 0.9mm and inner diameter of 0.5 mm) and outer needle (outer diameter of 2.0mm and inner diameter of 1.2 mm). The solutions were fed into two 10 ml plastic syringe equipped with a blunt metal needle (Beijing Technova Technology Co., Ltd) with internal diameter of 0.8 mm, which was driven by two syringe pumps (TYD02-04, Baoding Lead Fluid Technology Co., Ltd; WZ-50c6, Zhejiang Medical Instrument Co. Ltd., China). The positive electrode of the high voltage DC power supply (High Voltage Technology Institute, 6


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China) was clamped to the metal needle tip of the syringe with 16 kV and the distance of 15 cm from tip to collector. The sheath feed speed rate was 1ml/h, while the core feed speed rate varied from 0.1 to 0.5ml/h with relative humidity (45%) and room temperature (25 oC). Finally, the prepared electrospun core-sheath nanofibers films were dried at room temperature under vacuum to remove solvent. 2.3 Thermal cycles test Furthermore, to explore the thermal stability of the as-obtained film, the melting-cooling cycles were conducted in a thermostatic oven between 25 and 80 °C. The films were heated above melting temperature and then cooled to room temperature as a melting and solidifying thermal periodical cycle. The above mentioned process was carried out consecutively until the times of thermal cycle was 50, 100, 150 and 200. Then the DSC was applied to characterize the thermal stability of the samples. 2.4 Characterizations The morphology of as-prepared nanofibers was observed by Field Emission Scanning Electron Microscope (FE-SEM, S-4800, FEI Ltd., Japan) at an acceleration voltage of 2 kV. All samples were coated with a thin layer of gold prior to FE-SEM observations. The core-sheath structure of the nanofibers was investigated by Transmission Electron Microscope (TEM, JEM-2100, JEOL, Japan) under an acceleration voltage of 200 kV. To prepare the samples for TEM analysis, the nanostructures were deposited onto 400 mesh carbon-coated Cu grids. The thermal conductivity of the nanofibers film was characterized by transient plane source 7



method (TPS2500S, Hot Disk, Sweden) 36. The samples were cut into 5 cm*5 cm*1 cm, and then a disk sensor (Kapton) were imbedded into the sample. Thermal conductivity was measured by placing the sensor between two samples with parallel plane surfaces to ensure completely contact. Heat transfer was assumed purely conductive and thermal transport properties were determined by the instrument considering the constant electric current supplied by the sensor to monitor the resistance. Differential scanning calorimetry (DSC214, Netzsch, Germany) was carried out in nitrogen flow (20 mL/min). The samples were heated from 30 °C to 120 °C at a heating rate of 5 K/min and then cooled from 120 °C to 30 °C. 3. RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the coaxial electrospinning technique used to 8


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fabricate the PW-loaded core-sheath nanofibers film. The typical coaxial electrospinning technique was applied to fabricate the PW-loaded core-sheath nanofibers as shown in Figure 1, coaxial electrospinning using concentrically aligned spinnerettes linking with two separate channels for different solutions. Two kinds of liquids were fed into the inner (PW solution) and outer (PMMA solution or PMMA-CNTs blend solution) metallic capillaries, where the PMMA served as a support material to obtain the PW encapsulated core-sheath nanofibers. The suitable high voltage was applied between the spinneret and the collector, and the nanofibers could be obtained in the surface of the collector after solvent evaporation resulting from the stretched solution jet by electrostatic forces.

9



Figure 2. FE-SEM images of the electrospun core-sheath PW-loaded nanofibers film with different core feed rates: (a), (d), (j), (g) and (m) 0.1 ml/h. (b), (e), (h), (k) and (n) 0.3 ml/h. (c), (f), (i), (l) and (o) 0.5 ml/h. And samples with different PW concentration (w/v): (a-c) 10%, (d-f) 20%, (g-i) 30%, (j-l) 40% and (m-o) 50%. The sheath feed speed rate of all samples was 1 ml/h. Scale bars= 1 µm. Figure 2 shows the morphology of the PW-loaded nanofibers films with different PW core concentrations and the PMMA sheath feed speed rates. And the 10


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corresponding detailed average fiber diameters can be seen in Figure 3. When the PW concentrations were 10% (w/v) and 20% (w/v), the films were straight and cylindrical fibers with bead-free structure with the average diameters of fiber were 544±70 nm and 556±55 nm, respectively. While the PW concentration increased to 30% (w/v) and 40% (w/v), the fibers became obviously heterogeneous especially the concentration of 40% (w/v) (Figure 2j-l), which was attributed to the change of viscosity and conductivity with the augmentation of the PW loaded content. Unfortunately, when the PW concentration was 50% (w/v), the big spindle-like beads appeared and some PW was exposed on the surface of the nanofibers because the redundant PW cannot be encapsulated by the outer polymer layer. In addition, the porous structure can be obtained which is due to the solvent volatilization of the PW solution. Hence, the PW concentration of 50% (w/v) was not appropriate for the fabrication of the PW-loaded nanofibers films due to leakage of the PW.

Figure 3. Diameter distribution of the electrospun core-sheath nanofiber films 11



prepared by different conditions To further explore the influence of the core feed speed rate on the film morphology, the different feed speed rate at different PW concentration were discussed. The diameter of the core-sheath nanofibers were measured by the software named “Nano Measure”. And the averaged diameter and deviation has been calculated via the 100 randomly measured fiber diameter. As can be seen from Figure 3, with the PW concentration of 10% (w/v), there were no obvious changes for the films gained from different rates, indicating the rate have no effect on morphology at this condition. However, according to the fiber diameter deviation value changed from 71 nm to 135 nm, the fibers showed relatively a little heterogeneous with the speed rate increased from 0.1 ml/h to 0.5 ml/h at the PW concentration of 20% (w/v). Interestingly, the same change trend of the film morphology could be observed between the PW concentration of 30% (w/v) and 40% (w/v) with the rate of 0.1 ml/h to 0.5 ml/h for core feed (Figure 3g-l). The obvious non-uniform can be seen especially for the sample with PW concentration of 40% (w/v) exhibiting the increased deviation of fiber diameter ranging from 130 nm (0.1 ml/h) to 267 nm (0.5 ml/h). It can be explained that the higher speed rate could induce higher encapsulation efficiency and the more easily evaporated process of PW solution can also produce increased diameter deviation. It follows that the PW concentration and the core feed speed rate play a great role in the morphology of the films. Take the latent heat into consideration, the obtained film with PW concentration of 40% (w/v) and core feed speed rate of 0.5 ml/h was chosen to the further study. 12


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Figure 4. FE-SEM images of (a) PMMA nanofibers with polymer concentration of 20% (w/w), (b) CNTs, and the coaxial electrospun nanofibers film with PW concentration of 40% (w/v) and CNT concentrations (w/w) of (c) 1%, (d) 5% and (e) 10%. (f) The corresponding thermal conductivity of the nanofibers films containing CNTs. In order to improve the thermal conductivity of materials, the CNTs with high thermal conductivity was incorporated into the PMMA. The morphology of the PMMA nanofibers and CNTs could be observed in Figure 4a and 4b, respectively. The effect of CNTs concentrations on the morphology of the nanofibers was shown in 13



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Figure 4c-e. Obviously, the as-prepared film with CNTs-loaded core-sheath nanofibers displayed continuous and randomly oriented fibers. When the added content was 1% (w/w), the fiber surface was still smooth without protuberances, maybe because a little amount CNTs could not change the viscosity and conductivity of the solutions to keep a delicate balance

37

. However, the CNT content was

increased to 5% (w/w) and 10% (w/w), the nanofibers became much rougher with some irregular protuberances, which was ascribed to the increase of the uniform charge density of the fiber surface 38. Besides, it might be explained that high content CNTs cannot stretch well and some can be exposed on the surface of nanofibers, which could induce irregular protuberances. Theoretically, the exposure of CNTs on the surface is beneficial for the improvement of thermal conductivity

39

. As displayed in Figure 4f, the thermal

conductivity increased after adding various contents of CNTs. Unexpectedly, the thermal conductivity was no obviously enhanced. It is known that the phonon scattering, having a great effect on the thermal conductivity of polymer, could come from the correlation in the spatial fluctuation of the sound velocity at the interfaces between the crystallites and the amorphous matrix in the acoustic mismatch polymer 16, 40

. Besides, some researches have reported that disorder structure especially the

micro or nano meters material could also decrease the thermal conductivity

40-41

.

Though thermal conductivity of the film was increased with CNTs addition, the change of film conductivity by CNTs was not sufficient to relieve the influence of the phonon scattering. Hence, the amorphous polymer film with disordered nanoscale 14


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fiber just possessed low thermal conductivity. Further investigations on this question would be currently underway in our future work.

Figure 5. TEM images of core-sheath nanofibers with PW concentration of 40% (w/v) at core feed rate of (a) 0.1 ml/h, (b) 0.3 ml/h, (c) 0.5 ml/h. (d) Core-sheath nanofibers with PW concentration of 40% (w/v) and CNTs concentration of 10% at core feed rate of 0.5 ml/h. And the illustration of PW-loaded electrospun core-sheath nanofibers fabricated by different core speed rate. As displayed in the TEM images of the PW-loaded composite nanofiber films prepared by coaxial electrospinning with different core feed rates (Figure 5), the PW was completely covered by polymer forming ideal core-sheath structure. In addition, 15



the encapsulated PW content could be adjusted by the feed speed rates. As the feed rate increased from 0.1 ml/h to 0.5 ml/h, the diameter of inner core layer were increased from 395 nm to 848 nm, illustrating higher speed rate could induce higher encapsulated content of PW, at the same time, the corresponding outer layer diameters changed from 999 nm to 1095 nm. Hence, it can be further verified that the PW concentration of 40% (w/v) and core feed speed rate of 0.5 ml/h was optimal condition for the encapsulated core-sheath nanofibers film. In addition, as shown in Figure 5d, the TEM image demonstrated the CNT distributed in the nanofibers obviously and the CNT in the nanofibers induced some irregular protuberances, which is consistent with the results of SEM.

Figure 6. DSC curves of the PW, PMMA and electrospun core-sheath nanofibers films. (a) The melting process and (b) the cooling process To study the thermal property and latent heat storage capacity of film with the core-sheath nanofiber, the film was measured in nitrogen flow via DSC. The melting and cooling process of the films were shown in the Figure 6, with the corresponding summarized data in Table 1, where CNT5% or CNT10% represented the CNTs 16


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concentration with 5% (w/w) or 10% (w/w) and V0.1 or V0.5 represented the core speed rate of 0.1 ml/h or 0.5 ml/h. It can be found that a weak peak for pristine PMMA film appear at 104.4 ºC, which is the glass transition temperature. However, the glass transition peak could not be observed for the film with the core-sheath nanofibers, which is attributed to the strong melting peak of PW. In addition, the melting temperature (Tm) and melting enthalpy (∆Hm) of PW was 47.26 ºC and 135.18 J/g. The PW-loaded core-sheath nanofibers possessed the ∆Hm of 17.46 J/g and 57.65 J/g at the core feed speed rates of 0.1ml/h and 0.5ml/h, respectively, indicating that higher core feed speed rate could induce higher latent heat capability as a result of more effectively encapsulation of PW into the polymer. Furthermore, the nanofibers films containing the CNTs content of 5% (w/w) and 10% (w/w) were investigated. And there is no obvious influence on the melting temperature and latent heat of the films. Table 1. Enthalpies of the films fabricated by different conditions Sample

Tm (ºC)

∆Hm (J/g)

Tc (ºC)

∆Hc (J/g)

PW

47.26

135.18

50.90

-134.19

PW-PMMA-CNT5%-V0.5

46.56

56.77

51.60

-48.14

PW-PMMA-CNT10%-V0.5

47.04

58.25

51.51

-56.49

PW-PMMA-V0.5

46.98

57.65

51.44

-47.57

PW-PMMA-V0.1

46.15

17.46

48.67

-7.82

17



Figure 7. (a) Heat stored process of the core-sheath nanofibers fabricated at core feed rate of 0.5 ml/h and with PW concentration of 40% and CNTs concentration of 10%. And (b) Simulation of the heat transfers at temperature gradient. The storage of thermal energy can be achieved by means of latent heat via PW changes from solid to liquid phase as presented (Figure 7a). The PW-loaded thermo-regulated films can regulate the temperature as the ambient temperature altered by absorbing or releasing heat when the PW changed the phases from solid to liquid or from liquid to solid. In Figure 7b, the temperature gradient (T1>T2>T3) was defined as the color change. When the PW-loaded films (melting point of 50 ºC, defined as T2) were located in the environment of the temperature of T1 as shown in Figure 7b, the solid PW can change into liquid after absorbing heat from temperature T1 area with the heat collected in PW. Meanwhile, the temperature of the PW is constant during the phase change. Then the stored heat can be released to T3 area due to solidifying. In order to test and verify thermal stability of the film, the thermal cycles have also been conducted for many times.

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Figure 8. DSC curves of electrospun nanofibers films prepared at core feed rate of 0.5 ml/h and with PW concentration of 40% (w/v) and CNTs concentration of 10% (w/w) before and after different thermal cycles. The 200 melting-cooling cycles of the optimal film were conducted and the corresponding DSC curves were measured as shown in Figure 8. The phase transition temperature range was calculated between onset temperature and the peak temperature 7, and then the melting/freezing point and latent heat and solidifying could be obtained in Table 2. The results indicated that a little change can be found for the melting (from 45.56 ºC to 45.14 ºC) and freezing point (from 51.38 ºC to 52.11 ºC) with the negligible enthalpy changes after 50 and 200 thermal cycles. And for the latent heat ∆Hm, it just reduced a little from 60.79 J/g to 56.64 J/g with decreasing of ∆Hc from -59.69 J/g to -55.71 J/g, indicating the enthalpy value of the nanofibers film did not decrease obviously after 200 cycles. Hence, it could be seen that the 19



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core-sheath nanofibers film show good thermal stability and storage capacity and the PW cannot leak from the sheath layer. Table 2. Enthalpies of the film prepared at core feed rate of 0.5 ml/h and with PW concentration of 40% (w/v) and CNTs concentration of 10% (w/w) before and after heating-cooling thermal cycles Times

Tonset (ºC)

∆Hm (J/g)

Tc (ºC)

∆Hc (J/g)

50 cycles

45.56

60.79

51.38

-59.69

100 cycles

45.45

57.11

51.46

-58.14

150 cycles

45.35

56.64

51.94

-55.71

200 cycles

45.14

56.52

52.11

-55.29

Figure 9. SEM images of electrospun core-sheath nanofibers films prepared at core feed rate of 0.5 ml/h and with PW concentration of 40% (w/v) after different thermal 20


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cycles (a) 50 cycles, (b) 100 cycles, (c) 150 cycles and (d) 200 cycles. Scale bar=1 µm The morphology of the core-sheath nanofibers films after different thermal cycles was shown in Figure 9. No obvious change was found for the core-sheath nanofibers film after 150 thermal cycles, for the reason that the heated temperature (80 ºC) was lower than the glass transition temperature of PMMA. However, for 200 times (Figure 9d) thermal cycles, some fibers merged together with adjacent fibers in the cross part, indicating that the morphology of the core-sheath nanofibers could be influenced due to frequently thermal cycles. The fibers surface was smooth and no PW could be seen. It illustrated that no PW leakage occurred in the surface of the nanofibers, which is consistent with the results of DSC. The results further exhibited the practicability of the core-sheath nanofibers film. 4. CONCLUSIONS In summary, the PW was encapsulated by PMMA successfully forming well-defined core-sheath structure via coaxial electrospinning technique, addressing the leakage issue of the PW as latent heat materials. The SEM image showed that the film was continuous and randomly oriented fibers with higher PW core concentration and feed speed rates that could induce the higher encapsulation efficiency of PW and the inhomogeneity of the fibers. Furthermore, the clear core-sheath structures consisting of PW as core and PMMA as sheath can be demonstrated by the results of TEM images. Moreover, the core-sheath nanofibers films possessed the best latent heat of 58.25 J/g and solidifying enthalpy of 56.49 J/g from the DSC results. After 200 times thermal cycles, the films still keep the constant latent heat, illustrating the 21



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PW-loaded core-sheath materials exhibit a good performance in energy thermal storage capacity and stability. This work offered good support for the fabrication method of phase change material for the heat storage field. ACKNOWLEDGEMENTS This study was supported by the Major Science and Technology Project of Guangdong Province (No. 2013A011401011), Association

CAS

(No.2017400),

Special

Youth Innovation Promotion

Plan

of

Guangdong

Province

(2015TQ01N714), Pearl River Star of Science and Technology (No. 2014J2200078), Science and Technology Project of Guangdong Province (No.2014A010106018), the National Natural Science Foundation of China (No. 51506205), Project of Science and Technology Service Network initiative (No. KFJ-STS-QYZD-010), Major Science and Technology Project of Tibet Autonomous Region (No. ZD20170017) RERERENCE: 1.

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Hengstberger, F.; Zauner, C.; Resch, K.; Holper, S.; Grobbauer, M., High

temperature phase change materials for the overheating protection of facade integrated solar thermal collectors. Energy Build. 2016, 124, 1-6. 3.

Chen, C.; Wang, L.; Huang, Y., A novel shape-stabilized PCM: Electrospun

ultrafine fibers based on lauric acid/polyethylene terephthalate composite. Mater. Lett. 2008, 62 (20), 3515-3517. 22


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