Aerogel-Directed Energy-Storage Films with Thermally Stimulant

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Aerogel-directed energy-storage films with thermally stimulant multi-responsiveness Jing Lyu, Guangyong Li, Meinan Liu, and Xuetong Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04028 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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Aerogel-directed energy-storage films with thermally stimulant multi-responsiveness Jing Lyu,a† Guangyong Li,a† Meinan Liu,a Xuetong Zhanga,b* aSuzhou

Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou

215123, China. bDivision

of Surgery & Interventional Science, University College London, London, NW3 2PF,

UK. KEYWORDS: phase change, energy storage, paraffin, silica aerogel, temperature-dependent properties.

ABSTRACT: Phase change materials offer enormous potential for thermal energy storage due to their high latent heat and chemical stability. Researchers have developed numerous innovative strategies to overcome the leakage of organic phase change materials and enhance thermal performance. However, manufacturing form-stable, free-standing energy storage films based on phase change materials with high latent heat remains difficult. Therefore, the present study focused on producing of the free-standing, form-stable energy storage films with high phasechange enthalpy and thermally stimulant multi-responsiveness from the simple composite of paraffin with polytetrafluoroethylene/silica (PTFE/SiO2) aerogel framework, where paraffin was effectively confined in PTFE/SiO2. The resulting paraffin/PTFE/SiO2 films exhibit a large phase

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change enthalpy (128 J/g) at a paraffin content of 62.8 wt%. The temperature-dependent wettability, transmittance, and mechanical properties of this composite film have also been investigated.

1. Introduction With the rapid development of renewable energy conversion (including the use of solar and hydrothermal), thermal energy storage technologies have gained tremendous attention for their crucial role in renewable energy storage infrastructures.1–4 Phase change material is regarded as a promising candidate for thermal energy storage application because they are capable of storing and releasing sustained and substantial thermal energy in the form of latent heat during the process of phase transition in a narrow temperature interval.5–7 In addition, phase change material can also be utilized in the fields of industrial waste heat recovery and thermal managements of electronics.8–10 Among various groups of phase change materials, organic phase change materials (e. g. paraffin, polyethylene glycol, stearic acid, and hexadecanol) have attracted much attention due to their high latent heat, practical melting temperatures, large thermal storage capacity, and excellent chemical and thermal stability.5,11–14 However, the inherent liquid leakage during the typical solid-liquid phase transition hinders the practical applications of these phase change materials in thermal energy storage.6,15,16 Innovative strategies like core-shell microencapsulation,17–20 longitudinal confinement,21,2223 and porous confinement24–28 have been adopted to control the leakage. However, it should be noted that these supporting materials are usually inactive, which will inevitably decrease the energy storage density of phase change composites.29,30


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Recently, aerogel has been regarded as an alternative supporting matrix for phase change materials due to their ultralow density, extraordinary adsorption capacity, and strong capillary force.11,25,31 Typically, phase change aerogel composites can be obtained by impregnation of molten phase change materials into the aerogel framework.32–34 However, to the best of our knowledge, all these phase change aerogel composites were 3D monolithic bulks, 1D fibers or 0D microspheres. Shaping these phase change aerogel composites into 2D thin films may give more fascinating properties (such as low area density and favorable flexibility) and open the door for other innovative applications, like thermal management systems in miniaturized electronic devices, and shape memory composites. Moreover, as the temperature increasing, organic phase change materials in the aerogel films transited from solid to liquid, then properties of the composite films including surface wettability, light transmittance and mechanical properties could change simultaneously. In this work, free-standing, form-stable paraffin/PTFE/SiO2 phase change aerogel films were reported. The PTFE/SiO2 aerogel films were formed by pressing the mixture of PTFE dispersion and SiO2 aerogel powder into free-standing films by roller machine. The organic phase change material, paraffin, was infiltrated into PTFE/SiO2 aerogel films to fabricate the form-stable, freestanding phase change aerogel films. Due to the ultralow density, porous structure, and capillary force, PTFE/SiO2 aerogel films can hold high percentage of paraffin and thus effectively prevent the leakage. The resulting paraffin/PTFE/SiO2 nanocomposite exhibited remarkable energy storage properties, with high phase change enthalpy, and superior phase change reversibility. In addition, forming the phase change composites into films has endowed multiple responsiveness to thermal stimuli, including wettability, transmittance, and mechanical properties. This study


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will pave the way for designing and fabricating of phase change aerogel films with expected thermal properties and intelligent responsiveness. 2. Experimental 2.1 Materials. Polytetrafluoroethylene dispersion (60 wt% PTFE), tetraethoxysilane (TEOS), ammonium hydroxide (NH3∙H2O), hexane, and trimethylchlorosilane (TMCS) were purchased from Aladdin and used as-received. Polyethoxydisiloxane (PEDS) was prepared according to literature,35 and the molar ratio of TEOS: water: ethanol was 1:1.3:2.5. Paraffin was obtained from Hangzhou Ruhr New Material Technology Co., Ltd. 2.2 Preparation of paraffin/PTFE/SiO2 phase change aerogel films. SiO2 aerogel powder was synthesized as reported by our group previously.36 In a typical synthesis, 30 g PEDS was dissolved in 200 ml ethanol and stirred for 10 min, then 500 µl NH3∙H2O was added and keep stirring until gelation. After aging, the gel was crushed and vigorously stirred in 200 ml hexane for about 5 h. Collecting the gel by filtration and again stirred in 200 ml hexane for another 5 h. Finally, the gels were modified by TMCS (5 ml TMCS in 100 ml hexane) for 2 h and dried after filtration at 150 °C for 30 min. The obtained SiO2 aerogel powder was mixed with PTFE dispersion in a grinding miller, then the mixture was pressed into free-standing aerogel film by roller machine. Subsequently, the PTFE/SiO2 aerogel film was immersed into paraffin at a temperature of 85 °C (above the melting point of paraffin) in a vacuum oven and placed for 3-4 h to infuse aerogel film with melted paraffin. After that, the composite film was transferred onto filter papers to remove the excess paraffin adhered on surface, until no paraffin leakage can be observed. Finally, the form-stable paraffin/PTFE/SiO2 composite film was obtained.


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2.3 Characterization. The morphologies of PTFE/SiO2 and paraffin/PTFE/SiO2 films were inspected with a Hitachi S-4800 scanning electron microscope (SEM) at an acceleration voltage of 10.0 kV and a Quanta FEG-250 environmental SEM at low voltage (2.0 kV), respectively. The specific surface area of the aerogels was determined at 77 K by the Brunauer-Emmett-Teller (BET) nitrogen (N2) adsorption-desorption (ASAP 2020, Micromeritics, USA). The pore structure and pore size distribution of the aerogels were calculated from the desorption branch of the N2 adsorption isotherm using the Barrett-Joyner-Halenda (BJH) formula. Thermogravimetric (TG) analysis was carried out using a TG 209F1 Libra (NETZSCH) analyzer with a temperature ramp to 600 °C at 10 °C∙min-1 in a nitrogen atmosphere. X-ray diffraction (XRD) patterns of paraffin, PTFE/SiO2 aerogel film and the paraffin/PTFE/SiO2 composite film were collected on a D8 Advance Bruker AXS diffractometer with Cu Kα generated at 40 kV and 40 mA at a scanning rate of 0.05°∙s-1 over a 2θ range of 5°-60°. Differential scanning calorimetry (DSC) analysis was performed on a DSC 200F3 NETZSCH with 10 °C∙min-1 heating and cooling rate in a nitrogen atmosphere. The temperature-time curves of PTFE/SiO2 aerogel film and paraffin/PTFE/SiO2 composite film were recorded by a data collection system. The contact angle was measured by an optical angle meter system (OCA15EC, Dataphysics Inc. Germany). The advancing and receding contact angle were measured by inflating and deflating the droplet. Ultraviolet-visible (UV-Vis) absorption spectra of the phase change film were recorded on a Jasco V-560 spectrometer over a wavelength range of 200-900 nm. The stress-strain measurements were performed by using an Instron 3365 tensile testing machine. 3. Results and discussion For preparation of PTFE/SiO2 aerogel films, SiO2 aerogel powder was mixed with PTFE dispersion with different content. With an increased SiO2 content, a rise in the specific surface


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area was observed. However, the over high SiO2 loading would make the PTFE/SiO2 aerogel film fragile. The optimized SiO2 content was 40 wt%, where a flexible, free-standing PTFE/SiO2 aerogel film (Figure 1 (a) inset) with an average thickness of 185 µm and a high BET surface area was obtained. Scanning electron microscopy (SEM) images of the microstructure of PTFE/SiO2 aerogel film were presented in Figure 1 (a) and (b). It can be found that the PTFE nanofibers (~200 nm in diameter) and SiO2 aerogel powder are mixed homogeneously, and the network structure of these PTFE nanofibers hold the SiO2 aerogel nanoparticles to form the freestanding films, which effectively enhances the mechanical properties of the as-fabricated aerogel film. The target paraffin/PTFE/SiO2 composite film was obtained when the mass fraction of paraffin was constant (62.8 wt%). It is worth mentioning that the entire manufacturing process was simple, economical, and scalable. The as-prepared paraffin/PTFE/SiO2 film was showed in the Figure 1 (c) inset, and the average thickness of the composite increased to 370 µm. The microstructure of the paraffin/PTFE/SiO2 composite film was revealed by SEM as shown in Figure 1 (c) and (d), it can be observed that the paraffin can be absorbed by the PTFE/SiO2 aerogel film and bounded by the network structure of PTFE nanofibers. Compared with core-shell microcapsule or other confinement strategies, the mass fraction of phase change materials in the paraffin/PTFE/SiO2 composite film was high, which attributed to the aerogel structure of SiO2 powder and the network structure of PTFE nanofibers. Typically, aerogel material, which exhibits extraordinary pore volume and large capillary force, which would adsorb massive phase change materials without leaking during phase transition. To further confirm it, nitrogen sorption measurements were conducted. Figure 2 shows the N2 adsorptiondesorption isotherms and the pore-size distribution curves of the SiO2 aerogel powder and PTFE/SiO2 aerogel film. According to IUPAC classification, the isotherms are Type-IV with


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hysteresis loops, indicating the presence of mesoporous structures.37 The specific surface area of SiO2 aerogel powder and PTFE/SiO2 aerogel film calculated from the Brunauer-Emmet-Teller (BET) equation are 740 m2∙g-1 and 400 m2∙g-1, respectively. The pore-size distribution, calculated from the desorption data from the isotherm using the Barrett-Joyner-Halenda (BJH) model, showed that the SiO2 aerogel powder contained pores in the range from 2 to 60 nm, and most of them were mesopores (~7 nm), while the average pore size and pore volume of PTFE/SiO2 aerogel film decreased, which was consistent with the BET surface area results. To investigate the practical application temperature range of the phase change aerogel composites, thermogravimetric (TG) analysis was conducted at a heating rate of 10 °C∙min-1 under nitrogen atmosphere. In Figure 3 (a), the TG curves of paraffin, PTFE/SiO2 and paraffin/PTFE/SiO2 composites are presented. The paraffin decomposed between 125 °C and 230 °C, while PTFE/SiO2 showed higher thermal stability, weight loss at 520-590 °C, which was attributed to the degradation of PTFE.38 The TG curve of paraffin/PTFE/SiO2 composite film exhibited a distinct two-stage thermal degradation behavior. The first degradation stage occurred at temperature between 150 °C and 275 °C, due to the volatilization of paraffin. The higher volatilization temperature can be attributed to PTFE/SiO2, which served as a protective layer, and thus delayed the escape of the vaporized paraffin. The second stage was caused by the degradation of PTFE, which is similar to degradation behavior of PTFE/SiO2. The results revealed that paraffin/PTFE/SiO2 composites present good thermal stability, which is necessary for practical application. The crystalline structures of paraffin, PTFE/SiO2 aerogel film and synthesized paraffin/PTFE/SiO2 composite were investigated by X-ray diffraction (XRD) as presented in Figure 3 (b). The sharp and intense peaks occurring at around 20°, 24°, and 25° in XRD pattern


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were diffraction peaks of paraffin.39 The peak at approximately 18° in XRD pattern was characteristic diffraction peak of PTFE, indicating a long-range ordering in (100) lattice plane.40 The same diffraction peaks (18°, 20°, 24°, and 25°) were observed in XRD pattern of the paraffin/PTFE/SiO2 composite, indicating that paraffin and PTFE/SiO2 aerogel film were physically incorporated during the fabrication process. Thermal properties, especially phase change enthalpy and phase change reversibility are quite important for the application of paraffin/PTFE/SiO2 composite films. Herein, phase change behavior of paraffin and this composite film was performed by using differential scanning calorimetry (DSC) method. The DSC curves of paraffin and paraffin/PTFE/SiO2 composite films present one peak in endothermic and exothermic process, as shown in Figure 4 (a). Pure paraffin displayed an onset melting temperature of approximate 42.0 °C with a peak position at 50.5 °C, and the calculated melting enthalpy was 243 J/g. The DSC curve of the paraffin/PTFE/SiO2 composite film exhibited similar peak shapes and close melting temperature (onset at ~42.2 °C with a peak position at ~50.0 °C) to those of pure paraffin, and the phase-change enthalpy of paraffin/PTFE/SiO2 films was 128 J/g. The theoretical calculated phase-change enthalpy of paraffin/PTFE/SiO2 films was 152.6 J/g, higher than the experimental value. It can be deduced that the crystalline degree of paraffin in paraffin/PTFE/SiO2 films was slightly decreased or partial paraffin was in the presence of amorphous state, which was attributed to the nanoconfinement effect. Table 1 illustrated the comparison of thermal properties of paraffin/PTFE/SiO2 films with other common phase change composites in terms of melting temperature, freezing temperature and latent heats.21,41–44 The results indicated that the paraffin/PTFE/SiO2 films possess satisfactory thermal storage capacity and applicable phase change temperatures. Noteworthy that the latent heat of commercial phase-change fabric Outlast


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is only ~4 J/g, much lower than that of our paraffin/PTFE/SiO2 films. To assess the phase change reversibility, the paraffin/PTFE/SiO2 composite film was tested by the cyclic DSC at heating/cooling rate 10 °C∙min-1. The typical results were shown in Figure 4 (b), where 20 cyclic DSC curves of the paraffin/PTFE/SiO2 composite were virtually identical, indicating the extraordinary cyclic reversibility of the composite film. The large thermal storage capacity and cyclic stability endow the composite film with extensive application prospects. The thermal energy storage/release property of paraffin/PTFE/SiO2 composite films was further investigated under the constant heating power. As shown in Figure 5, a latent heat storage process occurred between two inflection points appearing in the temperature-time curve of paraffin/PTFE/SiO2 and the stored latent heat was employed to change the paraffin phase from solid to liquid, which corresponding to the DSC curves in Figure 4 (a). It took about 10.5 s to complete the phase transition, as labeled in red frame (Figure 5). The sensible heat took place before the first inflection point and after the second inflection point was used to change the temperature, without phase transition. The temperature-time curve of paraffin/PTFE/SiO2 reached the equilibrium state (approximately 87 °C at 60 s) when input energy balanced with the heat dissipation from the composite film to the environment. Once the heating power was terminated, the temperature of paraffin/PTFE/SiO2 composite films decreased immediately to the solidifying point of paraffin (~44 °C), then the heat release process appeared, which is consistent with DSC analysis results in Figure 4. The heat release process took about 100 s, as labeled in blue frame (Figure 5). However, in the endothermic and exothermic phase, no storage/release inflection point appeared in the temperature-time curve of PTFE/SiO2 aerogel film. A rapidly increase in temperature of PTFE/SiO2 aerogel film was observed during the initial heating stage and achieved an equilibrium. When the heating process was withdrawn, the temperature dropped


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sharply close to the environmental temperature. The results indicated that the paraffin/PTFE/SiO2 composite has actual heat storage and thermal management ability. Besides thermal energy storage capability, thermally stimulant multi-responsiveness has been observed in paraffin/PTFE/SiO2 composite films. The surface wettability of the paraffin/PTFE/SiO2 film was investigated before and after paraffin melting, as illustrated in Figure 6. The water contact angle was 152.4° at room temperature (Figure 6 (a)), indicating the promising super-hydrophobic performance of paraffin/PTFE/SiO2 films. The corresponding optical photograph was exhibited in Figure 6 (b). The advancing contact angle and receding contact angle were subsequently studied, and their average values were 153.1° and 149.7°, respectively. Hence, the calculated low contact angle hysteresis (3.4°) further confirmed the super-hydrophobicity of the paraffin/PTFE/SiO2 composite film. When the solid composite film was heated till melting, the water contact angle decreased sharply from 152.4° to 86.0° as observed in Figure 6 (c) and (d). The corresponding advancing and receding contact angle for melting paraffin/PTFE/SiO2 were 96.3° and 73.4°, respectively. This behavior was attributed to the change of surface tension on the paraffin/PTFE/SiO2 film. Thus, the paraffin/PTFE/SiO2 composite film would be applied to smart materials with controllable surface wettability. The optical properties of paraffin/PTFE/SiO2 composite films were investigated by ultravioletvisible (UV-Vis) transmittance spectra in the wavelength ranging from 300 to 850 nm as depicted in Figure 7. The transmittance of solid paraffin/PTFE/SiO2 composite film was extremely low in the whole UV-Vis wave band. Because the aggregation of paraffin on the surface increased the surface roughness and further decreased the transmittance by increasing Mie scattering that incident light was scattered back and forth (Figure 7 (a)). For melting paraffin/PTFE/SiO2 composite films, the transmittance depended on the wavelength. The higher


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transmittance values obtained at longer wavelengths. Specifically, the transmittance of the paraffin/PTFE/SiO2 composite film at 550 nm was around 10 %, while at 800 nm was 29%. To further understand the temperature-dependent transmittance, the transmittance of paraffin/PTFE/SiO2 composite films at different wavelength depending on the temperature was investigated. As shown in Figure 7 (b), the transmittance increased sharply with the increase of temperature, and decreased synchronously with temperature decreasing. The strict correspondence relationship between transmittance and temperature enabled this composite film to be an excellent material with controllable transmittance. Mechanical properties are very important for practical applications especially for free standing thin films. To evaluate the mechanical performances of these composite films, uniaxial tensile test was conducted. As can be seen in Figure 8(a) inset, a sharp yield point of PTFE/SiO2 aerogel film occurred at low strain and marked the onset of plastic deformation. For solid paraffin/PTFE/SiO2 composite film, the tensile stress improved one order of magnitude, but the tensile strain was one order of magnitude lower than that of PTFE/SiO2 aerogel film, which was attributed to the paraffin crystal. As expected, the stress-strain curve of melting paraffin/PTFE/SiO2 composite film was similar to that of PTFE/SiO2 aerogel film, except the higher tensile stress and strain. Thermo-mechanical analysis (TMA) of paraffin/PTFE/SiO2 composite film was conducted with a load of 5 mN as shown in Figure 8 (b). The length of the sample started to increase after the onset melting temperature, which was consist with the stressstain results (Figure 8(a)). The paraffin/PTFE/SiO2 composite films undoubtedly exhibit signs of candidate for smart materials. 4. Conclusions


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A free-standing, form-stable energy-storage composite film based on phase change material has been prepared by integrating paraffin into PTFE/SiO2 aerogel film, where PTFE/SiO2 played as aerogel framework to effectively confine the heat storage component paraffin. The resulting paraffin/PTFE/SiO2 composite film exhibited large phase transition enthalpy (128 J/g) and extraordinary cyclic reversibility. When paraffin in the composite film was heated to melt, the contact angles decreased. In addition, the transmittance of the composite film increased with the increase of temperature, while decreased synchronously with the temperature decreasing. The mechanical properties of paraffin/PTFE/SiO2 composite films also responded to temperature stimuli. This promising paraffin/PTFE/SiO2 composite film may have potential applications in energy storage related devices and smart materials.


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Figure 1. (a) Low-resolution and (b) high-resolution SEM image of as-prepared PTFE/SiO2 aerogel film. The inset in (a) is the photograph of PTFE/SiO2 aerogel film. (c) Low-resolution and (d) high-resolution SEM image of paraffin/PTFE/SiO2 aerogel film. The inset in (c) is the photograph of paraffin/PTFE/SiO2 aerogel film. The scale bars for insets are 2 cm.


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Figure 2. (a) Nitrogen adsorption and desorption isotherms and (b) pore volume of the SiO2 aerogel powder and PTFE/SiO2 aerogel film.


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Figure 3. (a) TG curves and (b) XRD patterns of paraffin, PTFE/SiO2 aerogel film and the paraffin/PTFE/SiO2 composite film.


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Figure 4. (a) DSC curves of the pure paraffin and paraffin/PTFE/SiO2 composite. (b) DSC curves of paraffin/PTFE/SiO2 composite tested for 20 cycles.

Figure 5. The temperature-time relationship of PTFE/SiO2 aerogel film and paraffin/PTFE/SiO2 composite film.


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Figure 6. (a) Contact angle test and (b) photograph of 2 μL water droplet on solid paraffin/PTFE/SiO2 composite film. (c) Contact angle test and (d) photograph of 2 μL water droplet on melting paraffin/PTFE/SiO2 composite film


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Figure 7. (a) UV−vis transmittance spectra of paraffin/PTFE/SiO2 composite film before and after melting. (b) UV−vis transmittance changes of paraffin/PTFE/SiO2 composite film depending on the temperature at different wavelength.


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Figure 8. (a) Stress-strain characteristics of uniaxial tensile tests on PTFE/SiO2 film and paraffin/PTFE/SiO2 composite film before and after melting. The inset was the partial enlarged details of (a). (b) TMA of paraffin/PTFE/SiO2 composite film.


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Table 1. Comparison the thermal performance of paraffin/PTFE/SiO2 with other phase change composites. Phase change composites

Melting

Freezing

Latent

Reference

temperature (°C)

temperature (°C)

heat (J/g)

PEG (50 wt%)/diatomite

27.7

32.2

87.1

41

PHDA/GO

32.5

31.8

104

42

n-nonadecane (50 wt%)/cement

31.9

31.8

69.1

43

Paraffin (25.9wt%)

28.0

25.0

29.8

44

(CA-LA) (CA-PA)/PET

234.3

214.4

54.7

21

Paraffin (62.8 wt%)/PTFE/SiO2

42.0

42.3

128

This work

/hydrotalcite

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] or [email protected] Author Contributions †

These authors contributed equally to this work.

Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51572285), the National Key Research and Development Program of China (2016YFA0203301), the Natural Science Foundation of Jiangsu Province (BK20170428), the China Postdoctoral Science Foundation, and the Jingsu Planned Projects for Postdoctoral Research Funds. REFERENCES (1)

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ToC figure:


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Aerogel-Directed Energy-Storage Films with Thermally Stimulant

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