- Email: [email protected]
SWCNTs phase change films via colloid aggregation for thermal energy storage
Applied Energy 260 (2020) 114320
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Facile preparation of flexible eicosane/SWCNTs phase change films via colloid aggregation for thermal energy storage
T
Renjie Chena,c,d,1, Xinyu Huangb,1, Weibin Denga,c,d, Ruizhi Zhenga,c,d, Waseem Aftabb, ⁎ ⁎ ⁎ Jinmin Shib, Delong Xiea,c,d, , Ruqiang Zoub, , Yi Meia,c,d, a
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China College of Engineering, Peking University, Beijing 100871, China c Yunnan Provincial Key Laboratory of Energy Saving in Phosphorus Chemical Engineering and New Phosphorus Materials, China d Higher Educational Key Laboratory for Phosphorus Chemical Engineering of Yunnan Province, China b
H I GH L IG H T S
film yielded from the SWCNT hydrogel owned an excellent conductivity. • The films were obtained by volatilization of the eicosane-contained hydrogels. • The solidification temperature of eicosane was reduced when filled in the SWCNT film. • The sample displayed a latent heat and conductivity of 204.8 J/g and 620.3 S/m. • The • The phase change film exhibited high energy storage efficiency as 91.3%.
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change materials Single wall carbon nanotube Colloid aggregation Energy storage Electrothermal
Carbon nanotubes (CNTs) based phase change composites have been widely reported to be used in areas related to energy conversion and storage. However, these bulk-like materials are not convenient for the further applications. Herein, we introduce the preparation of electrically conductive CNT/polymer films into the fabrication of conductive PCM films. The stable single wall carbon nanotube (SWCNT) dispersion was prepared by ultrasonic treatment with the help of surfactant. The SWCNT hydrogel was prepared by the addition of CaCl2 (aq.) via colloid aggregation (diffusion-limited cluster aggregation). Eicosane was infiltrated into the SWCNT skeleton by diffusion after solvent replacement, and the flexible phase change film (PCMF) was easily obtained by volatilization synchronously. According to the SEM images, eicosane was encapsulated in the SWCNT scaffolding. And PCMF exhibited a promising conductivity, flexibility, thermal stability and reversibility during the heat storage and release processes. For example, when the SWCNT ratio was 27.1%, the latent heat and conductivity were 204.8 J/g and 620.3 S/m, respectively. The results of 200 repeated melting and freezing cycles revealed that the sample have stable latent heat performance. The eicosane/SWCNT films could store thermal energy at modulated input voltage, which could be applied by conventional batteries. And the highest energy storage efficiency of samples was up to 91.3%. These flexible electro-driven films have wide prospect in smart electronic device, small scale heater, infrared stealth, wearable devices, etc.
1. Introduction The energy that maintains the function of human society comes mostly from the consumption of fossil fuel [1]. While obtaining fossil fuel is facile and massive, its reserve is limited and regional. Therefore, the improvement of energy efficiency is necessary to secure energy
supply [2]. Energy conversion and storage can mitigate the fluctuation of energy supply [3,4]. It is regarded as an effective way to improve energy efficiency [5]. Among the energy storage materials, phase change material (PCM) plays a dominant part in the use of latent heat storage [6]. By storing/releasing heat during phase transition, PCMs can passively smooth the temperature difference, thus lowers the cost of
⁎
Corresponding authors at: Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China (Y. Mei). E-mail addresses: [email protected] (D. Xie), [email protected] (R. Zou), [email protected] (Y. Mei). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apenergy.2019.114320 Received 10 July 2019; Received in revised form 14 November 2019; Accepted 1 December 2019 0306-2619/ © 2019 Published by Elsevier Ltd.
Applied Energy 260 (2020) 114320
R. Chen, et al.
flexible PCM films were fabricated by volatilization synchronously. The interconnected carbon nanotubes held PCMs in position by nanoconfinement. The microstructure of the CNT film and composite were investigated by XRD, FTIR, SEM and EDS. The thermal properties were measured by DSC, TGA and DTA. The heat storage of the PCM composite could be triggered electrically by inputting a small voltage. The efficiency was valued and compared under different mass percentage and voltage.
temperature manipulation [7,8]. Owing to this outstanding feature, PCMs are extensively applied in solar energy conversion and storage, electronics heat management, building energy conservation, waste heat utilization and off-peak electricity thermal storage systems [9,10]. Solid-liquid organic PCMs have been recognized as an ideal candidate because of their superior properties, including wide phase change temperatures for convenient use, excellent chemical stability, none toxicity, low corrosion, easy availability in natural resources, no phase segregation and negligible subcooling [11,12]. However, their utilizations have been hampered by several challenges. The liquefied PCM will leak if not properly contained [13,14]. And the insufficient thermal conductivities will lower the charging/discharging speed [15,16]. In order to solve these shortages of organic PCMs, two main methods are mainly used. Firstly, different additives have been blended in to prepare phase change composites, which can improve the performances of pure PCMs, such as thermal conductivity [17]. Secondly, form or shape stable technique is used to hold PCMs in position [18]. The skeletons can not only catch the PCMs from leakage, but also add their properties to composites [19–21]. Carbon nanotube (CNT) is one of the most common additives to prepare functional PCMs [22,23]. The cylindrical structure of carbon atoms provides the pathway for phonons and electrons through the composites. Therefore, it can be used as a thermal/electrical conductivity enhancer, which could improve the thermal storage efficiency [24,25]. By adding 5% CNTs, the thermal conductivity of the shapestabilized fatty acids was increased by 56.1% [26]. The thermal conductivity of PCM composite with metal organic framework was doubled by introducing CNTs [27]. The compatibility of CNTs enables the introduction into different forms of composites. CNT incorporated PCMs were used as the phase change slurry in the solar heat transfer utilization systems [28,29]. CNTs could also be the part shell of PCM microcapsules. When it was blended into the polymer based shell, the thermal stability, thermal transfer rate, hardness and Young's modulus of the microcapsules were improved [30]. And the conductive and stable PCM microcapsules could serve in versatile electro-thermal system [31]. CNTs have outstanding electrical conductivity comparable with metal. By fabricating interconnected CNTs framework, PCM composites can be applied in electro-thermal conversion [32,33]. The CNT sponge was used to encapsulate paraffin to fabricated composite that is electrically conductive. The heat storage could be driven by a low input voltage of 1.50–1.75 V. And the electro-to-heat storage efficiency is 40.6–52.5% [9]. When the CNT sponge was replaced by the CNT array, the electro-to-heat conversion efficiency of the sample was improved to 74.7% [32]. The results demonstrated that CNT-based bulk incorporation is an effective way to prepare functional PCMs composites. However, the electro-to-heat storage is limited from textile and foldable electronics. The PCM composite bulk is fragile, where folding could craze the bulk and disrupt energy transportation. To expand the application scenes, flexible PCM films have been designed by using polymer as support. Thermochromic membrane was fabricated by burying PCM microcapsules inside polyvinyl alcohol/polyurethane [34]. The membrane has temperature indication and the energy storage efficiency reached 67.5%. PCL/PCM fibers were electrospun onto polymer film to prepare smart food packaging [35]. The heat capacity was in the range of 88–119 J/g. Flexible PCM loaded polyvinyl alcohol (PVA) matrix with embedded Au nanoparticle was prepared for photothermal conversion [36]. The hydrogen-bonding keeps the PCM from leakage. PCM confined nanofiber aerogel films were prepared for infrared stealth [37]. The foldable films have a melting enthalpy as high as 179.1 J/g and can mimic the thermal radiance of the environment. Herein, we prepared a CNT/PCM film that combine flexibility and efficient heat conversion and storage. The method is based on colloid aggregation [38,39]. Single wall carbon nanotube (SWCNT) porous skeleton was obtained by adding CaCl2 (aq.) as dispersion agent. Eicosane was infiltrated into the SWCNT scaffold by diffusion. And the
2. Materials and methods 2.1. Materials Single wall carbon nanotubes (SWCNTs) were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Science. Sodium dodecyl sulfate, CaCl2, dichloromethane, acetonitrile, and absolute ethyl alcohol were provided by Tianjin Fengchuan Chemical Reagents Technologies Co., Ltd. Eicosane was purchased from Aladdin Industrial China. All dispersion experiments were carried out with deionized water. 2.2. Fabrication of SWCNT dispersion and eicosane/SWCNT film SWCNTs and SDS with the stoichiometric ratio were dispersed in the deionized water by a horn sonicator (SCIENTIZ-1500F, Ningbo Xinzhi Biotechnology Co., Ltd, China) with a cylindrical tip (20 mm end cap diameter). The output power was fixed at 525 W, the frequency was 20 kHz ± 200 Hz, and the duration of ultrasonic treatment was 45 min. The dispersion was stored hermetically at room temperature. The SWCNT hydrogel was obtained quickly by adding CaCl2 aqueous solution to the dispersion with same volume. After aging for 24 h at room temperature, absolute ethyl alcohol was used for replacing the water, and then alcohol was removed by dichloromethane again. Different contents of eicosane dissolved in the dichloromethane and diffused into the SWCNT-gels spontaneously at 30 °C for 6 h. The eicosane/SWCNT films were simply fabricated through volatilization at room temperature. 2.3. Characterizations UV–vis absorption spectra were recorded with a Shimadzu spectrophotometer UV-1800 operating between 200 and 500 nm. Samples were taken regularly during the dispersion (about 80 μl each, thus the solution volume under investigation is almost unchanged during the total experiment) and diluted by 50 times with distilled water, to attain the suitable SWCNTs contents for UV–vis measurements. The FT-IR Spectra were recorded on a Nicolet Nexus 870 from 400 to 4000 cm−1 with are solution of 2 cm−1 using KBr pellets. The SEM images of films were observed on QUANTA430 (Thermo Fisher Scientific Inc.) and energy dispersive spectroscopy (EDS) was obtained by EDAX-GENESIS (AMETEK, Inc.). Powder X-ray diffraction (PXRD) patterns of all the synthesized samples were recorded by a Rigaku SmartLab X-ray diffractometer operating at 45 kV and 200 mA with Cu Kα radiation (λ = 1.5406 Å). Temperature swing X-ray diffraction was performed on a Rigaku Smartlab-9kw. The temperature swing was programmed as 1 °C per minute and stayed constant for 5 min at 25, 30, 40, 50, 60 °C, respectively. The thermo-gravimetric analysis (TGA) was carried out at a STA (NETZSCH STA 449 F3 Jupiter) with a heating rate of 10 °C/min in N2 flow. The weight of samples was kept within 3–5 mg. The temperature was from 40 °C to 800 °C. The differential scanning calorimetry (DSC) data were collected with a differential scanning calorimeter (NETZSCH DSC 214 Polyma) with a heating/cooling rate of 10 °C/min under Ar. All the sample weights were about 5 mg sealed in an aluminum crucible. The conductivity of films was tested by automatic four-point test 2
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 1. Schematic of the formation steps of eicosane/SWCNT film.
The SWCNTs were dispersed in the water with various concentration of sodium dodecyl sulfate (SDS) by a horn sonicator. Fig. 2a showed the UV–vis spectra evolution of SWCNTs (0.05 wt%) /SDS (0.15 wt%) solution as a function of sonication time. There was an absorption peak at 269 nm where the dispersion behavior of SWNTs was monitored. The height was increased with the time of sonication. Therefore, we compared the absorbance of SWCNT dispersion (0.05 wt%) at different SDS concentration and sonication time, as displayed in Fig. 2b. The absorbance of the peak at 269 nm varied according to the sonication time and SDS content. It was obvious that the content of SDS influenced the duration of ultrasonic treatment. The dispersion effect was remarkably improved, when the ratio of SDS/SWNTs was changed from 2 up to 3. And it was not observably promoted when the ratio was increased. The extra SDS content would have negative influence on the performances of SWCNT dispersion and its further application [41]. Therefore, the ratio of SDS/SWNTs was 3 and the duration of ultrasonic treatment was 45 min.
system (Four-PointProbes-RTS8, Guangzhou 4 PROBES Tech. Co.) at room temperature. The electroheat storage performance was measured by electrochemical workstation (Zahnerennium/IM6). Films were loaded on the electrochemical work station with two copper wires stuck to each side as electrodes. A certain bias electric field of 3–7 V was Reversible applied to samples for about 100 s while recording the electric current flow. A thermal sensor (Pt-100) with an accuracy of ± 0.15 °C was planted into the target material to detect the temperatures plots during heating and cooling processes. The tensile stress-strain curves of all samples were recorded by using IBTC-300 in-situ dynamic material system at room temperature. The specimens were loaded at a constant rate of 1 mm min−1 until fracture or yield. 3. Results and discussions 3.1. Preparation of SWCNT dispersion
3.2. Preparation and characterization of SWCNT films
The preparation process of eicosane/SWCNT film was illustrated in Fig. 1. The stable SWCNT dispersion was the basis of the preparation of conductive phase change film. CNTs were easily prone to aggregation and precipitation in aqueous environment because of their high aspect ratio and hydrophobicity. When the surface of SWCNT was coated by surfactants, the re-aggregation of SWCNTs could be prohibited [40]. To obtain the stable SWCNT dispersion, SWCNTs could be exfoliated into the small agglomerates and single tubes after providing enough energy.
The SWCNT dispersion was stabilized by SDS and ultrasonic treatment. SDS could cover the surface of SWCNTs and prevent the agglomeration of them, thus the stable dispersion has obtained [41]. When the concentration of electrolyte was less than the critical coagulation concentration (CCC), the dispersion could keep stable because of the electrostatic repulsion among the SWCNTs. However, the
Fig. 2. (a) UV–vis spectra evolution of SWCNTs (0.05 wt%) /SDS (0.15 wt%) solution, (b) the absorbance of SWCNT aq. (0.05 wt%) with different SDS concentration and sonication time. 3
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 3. Photos of SWNTs dispersion with different CaCl2 solution (a), SWCNT film1 before solvent exchange (b), and SWCNT film2 after solvent exchange (c). Fig. 4. (a) The conductivity of SWCNT film1 and film2, (b) FTIR of SDS, SWCNT film1 and film2.
dispersions would agglomerate or even settle in the presence of electrolyte [42]. And the aggregation would happen due to the disappearance of electrostatic repulsion when the concentration of electrolyte exceeded the CCC according to Derjaguin-Landau-VerweyOverbeek (DLVO) theory [43–45]. Different valence ions could aggregate the stabilized SWCNTs to a certain coagulation concentration [39,40]. Here, the SWCNT hydrogel was prepared by the addition of CaCl2 (aq.) with different concentrations via diffusion limited cluster aggregation (DLCA) [46]. The influence of CaCl2 concentration on the SWCNTs dispersions are list in Table S1 and Fig. 3a. When the concentrations of CaCl2 were less than 6 mmol/L, the dispersions were stable (sample 1 and 2). An interconnected structure could be formed immediately at the proper concentration of CaCl2 solution (6–40 mmol/L, sample 3–6), owing to the compression of the electric double layer of SWCNTs surface [47]. However, excessive CaCl2 would destroy the structure of the hydrogel (sample 7), because the shield of surface charges of SWCNTs would lead to re-aggregation [43]. Therefore, the concentration of CaCl2 (aq.) was chose as 15 mmol/L. After homogenously dispersed in aqueous solution, SWCNT films could be formed by removing water from the hydrogel. Considering that the mass content of SDS was three times as much as SWCNTs, the influence of surfactants on the performance of SWCNT films could not be negligible. Two kinds of SWCNT films are prepared with different methods. The first one was simple evaporation to remove the water directly (film1, Fig. 3b). And the second was volatilization after replacement of water by organic solvents to eliminate the surfactant in solution (film2, Fig. 3c). Based on the results of four probe method (Fig. 4a), the conductivity of film2 was a few times higher than that of film1. The conductivity was improved from 3.152 × 104 S/m to 105 S/m. SDS, film1 and film2 were analyzed by Fourier Transform Infrared Spectroscopy (FTIR). According to the three curves in Fig. 4b, the FTIR spectrum of film2 differed
obviously from the others. The absorption peaks of sulfate group could be found apparently at 1223 cm−1, 1078 cm−1 and 829 cm−1 from spectra of film1 and SDS. The stretching and bending vibration of long chain alkyl group of surfactant could be observed around 2918 cm−1, 2815 cm−1, 1467 cm−1 and 723 cm−1. These characteristic peaks disappeared or was weakened in the infrared spectrum, which indicated that most SDS was washed through solvent exchange. The same phenomenon could also be discovered in the XRD patterns as shown the Fig. S1. The X-ray relative diffraction peak intensity of SDS is weakened after solvent exchange. Further evidence to validate the two-step preparation of CNT films could be confirmed by SEM and EDS test. Three samples were tested. As showed in Fig. 5a, the as-received SWCNTs powders were formed by twisted carbon nanotubes. In Fig. 5b and c, SWCNT film1 was prepared by removing the water directly from the hydrogel, and SWCNT film2 was obtained after solvent exchange. Via ultrasonic treatment and colloid aggregation, the films were fabricated by interconnected individual carbon nanotubes. The interconnect network functioned as the pathway for phonons and electrons transport. Besides carbon, the characteristic elements of SDS and CaCl2 were detected in SWCNT film1 (sulfur, sodium, calcium and chlorine). In comparison, the weight content of carbon was raised and the weight contents of calcium and sodium were decreased in film2. The large amount of SDS and other inorganic salt on the surface would reduce the conductivity of SWCNT [38,41], and solvent exchange process could remove most of the salts, which restored the conductivity of SWCNT film. The tensile stress-strain curves of SWCNT films were tested with insitu dynamic material system at room temperature, as illustrated in Fig. 6. The tensile strength of SWCNT film1 and film2 is 24.10 and 8.13 MPa, respectively. The curves showed a linear elastic region before 4
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 5. SEM and EDS of raw SWCNT powder (a), SWCNT film1 (b) and film2 (c).
flexibility, which could be bent arbitrarily as shown in Fig. 7e and videos. These flexible films can be widely applied in smart electronic device, small scale heater, signal transmitter, etc. PCMFs were also measured by X-ray diffraction (XRD). The XRD pattern was shown in Fig. 8a. The peaks can be attributed separately to eicosane and SWCNT, and their XRD patterns were illustrated in Fig. S2.
break. The lower tensile strength of film2 was attributed to the larger free space between nanotubes. The loose skeleton provides enough room for PCM infiltration. 3.3. Configuration and morphology of eicosane/SWCNT films The schematic preparation of eicosane/SWCNT film was illustrated in Fig. 1. The twisted SWCNTs (Figs. 5a and 7b) were untangled and dispersed in water under pulsed tip-sonication with SDS. The hydrogel (Fig. 7c) was formed by carbon nanotube scaffolding when CaCl2 (aq.) was added to the dispersion. After solvent exchange, different contents of PCM dissolved in the dichloromethane and diffused into the SWCNTgels spontaneously at 30 °C. The phase change films (PCMFs) were obtained through volatilization (PCM content 95%, 90%, 85%, and 80%, named PCMF1, PCMF2, PCMF3 and PCMF4). In Fig. 7d, SWCNT film was a three-dimensional carbon nanotube scaffold, with open channel for PCM infiltration. The SEM image of eicosane/SWCNT film (Fig. 7f) shows that the surfaces of SWCNT were wrapped by eicosane, and the inter-tube pores is filled. Moreover, compared to the reported PCM/nano carbons composites [9,32], the PCMFs exhibited a reversible
3.4. Thermal stability and phase change behavior of eicosane/SWCNT films As depicted in Fig. 9a, the eicosane transformed to liquid quickly at 80 °C. Owing to the capillary adsorption effects of porous skeleton, liquid PCM was held by the various scale pores and surfaces of SWCNT scaffold (Fig. 7f), which avoided leakage during melting process. Even when the ambient temperature was above the melting point of eicosane, the phase change film maintained its shape showing promising stability. The shape-stabilized feature guarantees the long-term use of the composite. Thermogravimetric (TG) and derivative TG (DTG) analyses were used to determine the thermal stability of PCMFs. The TG curve of SWCNT was showed in Fig. S3. Fig. 9b presented that the eicosane 5
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 6. The tensile stress-strain curves of SWCNT films.
Fig. 8. XRD patterns of PCMFs (a) and PCMfilm2 before/after cycling (b).
temperatures of the maximum weight loss rate of PCMFs were also above the pure one. The results illustrated a better thermal stability. In addition, phase change behavior was measured by differential scanning calorimetry (DSC). Fig. 10a and 10b were the DSC curves of pure eicosane and PCMFs with PCM loadings from 66.6% to 78.1%, calculated by TGA. The onset melting points of PCMFs were below that of the eicosane, as listed in Table 1. Because the PCM was embedded into the SWCNT networks uniformly, the skeleton might influence the wrapped organic molecules, leading to the increased phase change point of PCMFs. The enthalpy of eicosane was 232.1 J/g. The enthalpies of PCMFs rose with the increasing contents of PCM, which all were lower than the pure. The reduction of enthalpy has been observed and reported previously in PCM/CNT composites [48,49]. However, the normalized enthalpies were also approaching to the eicosane with increasing content of PCM, which was increased to 204.8 J/g and 211.1 J/g when the contents of PCM were 72.9% and 78.1%, respectively. This phenomenon was influenced by the eicosane loading and the alkane-nanotube interaction [9]. The difference in the DSC patterns was attributed to the change of phase change behavior of eicosane. The phase change process of pure eicosane was presented as a single endothermic peak and a bimodal exothermal peak [50]. The liquid eicosane transferred to a metastable rotator phase before solidification. When eicosane was filled in the CNT films, the solidification temperature was reduced because of physical interactions and confinement. We also provided the XRD pattern of pure eicosane at different temperature and the comparison of eicosane
Fig. 7. The photos of SWCNT (a), SWCNT hydrogel (c) and eicosane/SWCNT film (e), and the SEM images of SWCNT (b), SWCNT film (d) and eicosane/ SWCNT film (f).
weight ratios of films were from 66.6% to 78.1% as list in Table 1, which were lower than the designed values (from 80% to 95%). The loss results from the diffusion and volatilization process. Both TG curve and DTG curves exhibited that the thermal stability of PCM was improved by infiltrating into the SWCNT gel. The thermal gravity temperatures (the temperature of 5% weight loss, T-5%) of PCMFs were higher than eicosane’s. Comparing the values of maximum weight loss rate (Fig. 9c), the PCMFs were lower than that of eicosane. And the 6
Applied Energy 260 (2020) 114320
R. Chen, et al.
(Fig. S4a) and composites at 25 and 60 °C (Fig. S4b and S4c). The results demonstrated that the change of XRD patterns of composites corresponds to the change of the phase change of eicosane. Furthermore, the reversibility without decomposition was of importance for application of PCMs during the heat storage and release processes. The repeated test of PCMF2 was measured for 200 cycles at the same condition, which displayed a stable performance for each melting and freezing processes, as illustrated in Fig. 10c and 10d, and the results of the rest samples could be found in Fig. S5. The XRD patterns of PCMF2 before and after cycling were illustrated in Fig. 8b. The angles of peaks were identical, indicating the chemical structure stability of PCMF2 after repeatedly phase change cycles. Therefore, chemical stability and the good thermal storage/release cyclability of the films could reinforce their application for longer-term heat storage through frequent use. 3.5. Conductivity and electro-to-heat storage of eicosane/SWCNT films The result of four probe method showed that the SWCNT film has an outstanding conductivity of 100,000 S/m. According to the SEM images, eicosane not only covered the surface of SWCNTs, but also infiltrated into the pores of inter-tubes. These composite constructions would have an effect on the conductance of the PCMFs due to the coating of insulator. As illustrated in Fig. 11a, the conductivities of samples were 531.2 S/m, 620.3 S/m, 965.4 S/m and 3927.3 S/m respectively, which were increased with the increasing ratio of SWCNT. The resistances of different films could be calculated by their sizes and conductivities. And the resistance values of PCMFs were 73.3 Ω, 49.5 Ω, 43.4 Ω and 25.2 Ω respectively. Owing to the highly conductive SWCNT skeleton, electro-to-heat storage of our flexible phase change films could be triggered through applying an electric potential directly. According to the temperaturetime plots of PCMFs, when voltage was applied, the current past through the film could heat the nanotubes, where after joule heat was generated. When there was a voltage (3 V, 4 V or 5 V) applied on the sample at an environmental temperature of 26 °C, the temperature of the film would go up. The energy transferred from SWCNT skeleton to wrapped eicosane by heat conduction. The heat could be stored by phase transition process, which also could be released again. As shown in Fig. 11b, the temperature curves of PCMF1 rose gently during the initial time and reach a balance where the heat dissipation and the energy input achieved equilibrium state from the heated SWCNT skeleton to the circumstance. The highest temperatures of these three curves were below the melting point of eicosane, due to its lower electrical conductivity. If the input voltage was further enhanced to 6 V and 7 V, the temperature of sample could exceed the phase change point, which led to the heat storage process, as illustrated in Fig. S6. With the increasing of conductivity, the temperature-time plots of PCMF2 and PCMF3 differed from PCMF1, when the input voltages were 4 V and 5 V. The slopes gradually decreased until an inflection point was met, which indicated that the phase transition process occurred at the plateau region of temperature-time curve (Fig. 11c and d). The temperatures were between 33 °C and 36 °C, in accordance with the melting point of the same infiltrating composite. At the termination of phase change, film temperature continued to rise quickly, and met a balance again. However, when the voltage was 3 V, the temperatures curves were below the phase change point, revealing that input energy was insufficient to melt eicosane in the SWCNT film and the voltage threshold was higher. After switching off the applied voltages, the temperatures of all the curves decreased to the solidifying point rapidly (about 34 °C), and the temperatures maintained to release heat until the end of freezing. Besides, when the sample conductivity was further improved, the critical voltage of PCMF4 was reduced to 3 V, and the phase change process could be found in Fig. 11f. Here, the efficiency of electro-heat storage was calculated. In the calculation, all the infiltrate PCM is assumed to take part in the phase
Fig. 9. (a) Photos of eicosane and PCMFs, TGA (b) and DTG (c) curves of eicosane and PCMFs. Table 1 . TGA and DSC details of eicosane and PCMFs. Sample
T-5%oC
Residue (330°C) %
PCM %
T m oC
△Hm J/g
△Hn J/g
PCMF1 PCMF2 PCMF3 PCMF4 Eicosane
201.8 199.9 198.2 197.4 192.8
21.9 27.1 29.2 33.4 0.0
78.1 72.9 70.8 66.6 100.0
36.5 36.7 36.7 36.8 37.2
164.9 149.3 127.0 86.4 232.1
211.1 204.8 179.4 129.7 232.1
7
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 10. DSC curve of eicosane (a) and PCMFs (e), recycling test (c) and phase change stability (d) of PCMF2.
PCM3 was 42.3% under 4 V and 83.8% under 5 V. When the applied voltage is increased, the Joule heat is increased as well. However, the heat dissipation from the sample to the atmosphere is limited. Therefore, the charging time is remarkably shortened, which improved the efficiency in turn [9,13]. When used in a real circumstance, the flexible film will cover the substance, whose thermal conductivity is higher than the air. And the efficiency will be lower than it is shown in the present
transition, so electro-heat storage efficiency (η) was defined as mΔHR η = 2 , where m was the mass of sample, △H was the apparent enU t thalpy, R was the resistance, U was input voltage, and t was time of the phase change process obtained by the tangential method (Fig. S7). The efficiencies of various samples were summarized in Table S2 and Fig. 11f. Comparing the three samples, the energy storage efficiency was much higher at 5 V applied bias. For example, the efficiency of
Fig. 11. (a) Conductivity of four PCMFs, (b) Temperature evolution of PCMF1, (c) Temperature evolution of PCMF2, (d) Temperature evolution of PCMF3, (e) Temperature evolution of PCMF4, (f) energy storage efficiencies of three samples. 8
Applied Energy 260 (2020) 114320
R. Chen, et al.
interests or personal relationships that could have appeared to influence the work reported in this paper.
case. As revealed in Fig. 11f, the efficiency of the composite decreased as the increase of CNT percentage under the same voltage. The reason is that the energy capacity m·ΔH and the resistance R are improved as PCM percentage increased. However, the charging time is also lengthened. As the PCM percentage improves, the efficiency would meet a point when the negative effect of charging time appears. The effect should also be considered when designing the application device. The efficiencies of PCMF2, PCMF3 and PCMF4 were 91.3%, 83.8% and 51.1% under 5 V input voltage, and 80.1%, 42.3% and 30.2% under 4 V input voltage, respectively. It was known that PCM content and sample conductivity assumed inverse ratio connection. Too much eicosane covered on the surface of nanotubes could greatly increase the resistance of film, which needs a higher input voltage for electro-driven thermal storage. A suitable coating rate of eicosane of SWCNT network could minimize the convection heat loss from heated SWCNT to air under smaller voltage. Among all the conditions, the maximum value was 91.3% of PCMF2 at 5 V. The electro-driven heating property enabled the film to be used as wearable devices for heat conservation. In a recent model, PCM was embedded in soft polymer to provide thermal protection of biological tissues [51]. Similarly, the SWCNT-incorporated composite can be fabricated as a wearable device to keep human-body warm. Besides, the flexible electro-driven films also have wide possibility in smart electronic device, small scale heater, infrared stealth, etc.
Acknowledgments The work has been financially supported by the Natural Science Foundation of China (No. 51603096, and No. 21663015). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apenergy.2019.114320. References [1] Waseem A, Huang X, Wu W, Liang Z, Asif M, Zou R. Nanoconfined phase change materials for thermal energy applications. Energ Environ Sci 2018;11:1392–424. [2] Shchukina EM, Graham M, Zheng Z, Shchukin DG. Nanoencapsulation of phase change materials for advanced thermal energy storage systems. Chem Soc Rev 2018;47:4156. [3] Zhang L, Zhou K, Wei Q, Ma L, Ye W, Li H, et al. Thermal conductivity enhancement of phase change materials with 3D porous diamond foam for thermal energy storage. Appl Energy 2019;233–234:208–19. [4] Gasiaa J, Maldonadoa JM, Galatib F, Simoneb MD, Cabezaa LF. Experimental evaluation of the use of fins and metal wool as heat transfer enhancement techniques in a latent heat thermal energy storage system. Energ Convers Manage 2019;184:530–8. [5] Zheng Z, Chang Z, Xu G, McBride F, Ho A, Zhuola Z, et al. Microencapsulated phase change materials in colar-thermal conversion systems: understanding geometrydependent heating efficiency and system reliability. ACS Nano 2017;11:721–9. [6] Sheng N, Zhu R, Dong K, Nomura T, Zhu C, Aoki Y, et al. Vertically aligned carbon fibers as supporting scaffolds for phase change composites with anisotropic thermal conductivity and good shape stability. J Mater Chem A 2019;7:4934–40. [7] Merlin K, Soto J, Delaunay D, Traonvouez L. Industrial waste heat recovery using an enhanced conductivity latent heat thermal energy storage. Appl Energy 2016;183:491–503. [8] Gasiaa J, Graciaa A, Zsembinszki G, Cabeza LF. Influence of the storage period between charge and discharge in a latent heat thermal energy storage system working under partial load operating conditions. Appl Energy 2019;235:1389–99. [9] Chen L, Zou R, Xia W, Liu Z, Shang Y, Zhu J, et al. Electro- and photodriven phase change composites based on wax-infiltrated carbon nanotube sponges. ACS Nano 2012;6:10884–92. [10] Wu W, Huang X, Li K, Yao R, Chen R, Zou R. A functional form-stable phase change composite with high efficiency electro-to-thermal energy conversion. Appl Energy 2017;3:474–80. [11] Yu D, He Z. Shape-remodeled macrocapsule of phase change materials for thermal energy storage and thermal management. Appl Energy 2019;247:503–16. [12] Ye S, Zhang Q, Hu D, Feng J. Core–shell-like structured graphene aerogel encapsulating paraffin: shape-stable phase change material for thermal energy storage. J Mater Chem A 2015;3:4018–25. [13] Chen R, Yao R, Xia W, Zou R. Electro/photo to heat conversion system based on polyurethane embedded graphite foam. Appl Energy 2015;152:183–8. [14] Long L, Ye H, Gao Y, Zou R. Performance demonstration and evaluation of the synergetic application of vanadium dioxide glazing and phase change material in passive buildings. Appl Energy 2014;136:89–97. [15] Chen R, Cui Y, Tian H, Liu Z, Shu Y, Li C, et al. Controllable thermal rectification realized in binary phase change composites. Sci Rep 2015;5:8884. [16] Geng X, Li W, Wang Y, Lu J, Wang J, Wang N, et al. Reversible thermochromic microencapsulated phase change materials for thermal energy storage application in thermal protective clothing. Appl Energy 2018;217:281–94. [17] Sari A, Bicer A, Al-Sulaiman F, Karaipekli A, Tyagi V. Diatomite/CNTs/PEG composite PCMs with shape-stabilized and improved thermal conductivity: preparation and thermal energy storage properties. Energ Build 2018;164:166–75. [18] Li C, Yu H, Song Y, Wang M, Liu Z. A n-octadecane/hierarchically porous TiO2 form-stable PCM for thermal energy storage. Renew Energ 2020,;145:1465–73. [19] Yang J, Jia Y, Bing N, Wang L, Xie H, Yu W. Reduced graphene oxide and zirconium carbide co-modified melamine sponge/paraffin wax composites as new form-stable phase change materials for photothermal energy conversion and storage. Appl Therm Eng 2019;163:114412. [20] Wu H, Li S, Shao Y, Jin X, Qi X, Yang J, et al. Melamine foam/reduced graphene oxide supported form-stable phase change materials with simultaneous shape memory property and light-tothermal energy storage capability. Chem Eng J 2020,;379:122373. [21] Chen T, Liu C, Mu P, Sun H, Zhu Z, Liang W, et al. Fatty amines/graphene sponge form-stable phase change material composites with exceptionally high loading rates and energy density for thermal energy storage. Chem Eng J 2020,;382:122831. [22] Li M, Chen M, Wu Z, Liu J. Carbon nanotube grafted with polyalcohol and its influence on the thermal conductivity of phase change material. Energ Convers Manage 2014;83:325–9. [23] Li B, Nie S, Hao Y, Liu T, Zhu J, Yan S. Stearic-acid/carbon-nanotube composites with tailored shape-stabilized phase transitions and light–heat conversion for
4. Conclusion In conclusion, a novel, facile, and efficient strategy has been introduced to fabricate electrical conductive and flexible phase change films via colloidal aggregation. The SWCNT dispersion was prepared by ultrasonic treatment with SDS. The SWCNT hydrogel was fabricated by the addition of CaCl2 (aq.). Eicosane was diffused into the porous SWCNT scaffolds spontaneously at 30 °C, after the water of hydrogel was replaced by organic solvent. The phase change films were obtained through volatilization. Due to the capillarity, the composites displayed excellent form stability against leakage, when the ambient temperature was higher than the phase transition point of eicosane. Moreover, the phase change films showed a promising heat storage density (204.8 J/g for PCMF2 with a SWCNT mass ratio of 27.1% and 211.1 J/g for PCMF1 with a SWCNT mass ratio of 21.9%). In addition, these films presented the nice thermal stability and reversibility during the long-term heat storage and release processes. According to results by the four probe method, the electrical conductivities of phase change films were 531.2 S/m, 620.3 S/m, 965.4 S/m and 3927.3 S/m respectively, so they could store thermal energy at modulated input voltage. And the highest energy storage efficiency of the samples was up to 91.3% (PCMF2 under 5 V input voltage). Owing to the flexibility of samples and unique performances of SWCNTs, our flexible electro-driven films have wide possibility in smart electronic device, small scale heater, infrared stealth, wearable devices etc. CRediT authorship contribution statement Renjie Chen: Conceptualization, Methodology, Validation, Data curation, Writing - original draft, Visualization. Xinyu Huang: Conceptualization, Methodology, Validation, Data curation, Writing review & editing, Visualization. Weibin Deng: Investigation, Writing original draft. Ruizhi Zheng: Investigation. Waseem Aftab: Investigation. Jinmin Shi: Investigation. Delong Xie: Writing - review & editing, Supervision, Funding acquisition. Ruqiang Zou: Supervision, Funding acquisition. Yi Mei: Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial 9
Applied Energy 260 (2020) 114320
R. Chen, et al.
2017;164:188–92. [37] Lyu J, Liu Z, Wu X, Li G, Fang D, Zhang X. Nanofibrous kevlar aerogel films and their phase change composites for highly efficient infrared stealth. ACS Nano 2019;13:2236–45. [38] Zhang C, Wen B, Gao L, Chen Y, Yang M. Fabrication of surfactant-removed polymer composites with single-walled carbon nanotube networks. J Appl Polym Sci 2011;119:155–61. [39] Wu Q, Hu J. Waterborne polyurethane based thermoelectric composites and their application potential in wearable thermoelectric textiles. Compos B 2016;107:59–66. [40] Islam M, Rojas E, Bergey D, Johnson A, Yodh A. High weight fraction surfactant solubilization of single-wall carbon nanotubes in water. Nano Lett 2003;3:269–73. [41] Yu J, Lu K, Souty E, Grossiord N, Koning C, Loos J. Characterization of conductive multiwall carbon nanotube/polystyrene composites prepared by latex technology. Carbon 2007;45:2897–903. [42] Li M, Huang C. Stability of oxidized single-walled carbon nanotubes in the presence of simple electrolytes and humic acid. Carbon 2010;48:4527–34. [43] Smith B, Wepasnick K, Schrote K, Cho H, Ball H, Fairbrother D. Influence of surface oxides on the colloidal stability of multi-walled carbon nanotubes: a structureproperty relationship. Langmuir ACS J Surf Colloids 2009;25:9767–76. [44] Amal R, Raper J, Waite T. Effect of fulvic acid adsorption on the aggregation kinetics and structure of hematite particles. J Colloid Interf Sci 1992;151:244–57. [45] Ding P, Pacek A. Effect of pH on deagglomeration and rheology/morphology of aqueous suspensions of goethite nanopowder. J Colloid Interf Sci 2008;325:165–72. [46] Xie D, Wu H, Zaccone A, Braun L, Chen H, Morbidelli M. Criticality for shear-induced gelation of charge-stabilized colloids. Soft Matt 2010;6:2692–8. [47] Lin D, Liu N, Yang K, Zhu L, Xu Y, Xing B. The effect of ionic strength and pH on the stability of tannic acid-facilitated carbon nanotube suspensions. Carbon 2009;47:2875–82. [48] Zeng J, Cao Z, Yang D, Xu F, Sun L, Zhang X, et al. Effects of MWNTS on phase change enthalpy and thermal conductivity of a solid-liquid organic PCM.J. Therm Anal Calori. 2009;95:507–12. [49] Bayramoglu E. Thermal Properties and Stability of N-octadecane based composites containing multiwalled carbon nanotubes. Polym Compos 2011;904–909. [50] Liu H, Niu J, Wang X, Wu D. Design and construction of mesoporous silica/n-eicosane phasechange nanocomposites for supercooling depression and heat transfer enhancement. Energy 2019;188:116075. [51] Shi Y, Wang C, Yin Y, Li Y, Xing Y, Song J. Functional soft composites as thermal protecting substrates for wearable electronics. Adv Funct Mater 2019;29:1905470.
thermal energy storage. Energ Convers Manage 2015;98:314–21. [24] Wang Y, Tang B, Zhang S. Single-walled carbon nanotube/phase change material composites: sunlight-driven, reversible, form-stable phase transitions for solar thermal energy storage. Adv Funct Mater 2013;23:4354–60. [25] Tang Y, Alva G, Huang X, Su D, Liu L, Fang G. Thermal properties and morphologies of MA–SA eutectics/CNTs as composite PCMs in thermal energy storage. Energ Build 2016;127:603–10. [26] San A, Bicer A, Al-Ahmed A, Al-Sulaiman F, Zahir M, Mohamed S. Silica fume/ capric acid-palmitic acid composite phase change material doped with CNTs for thermal energy storage. Sol Energ Mat Sol C 2018;179:353–61. [27] Wang J, Huang X, Gao H, Li A, Wang C. Construction of CNT@Cr-MIL-101-NH2 hybrid composite for shape stabilized phase change materials with enhanced thermal conductivity. Chem Eng J 2018;350:164–72. [28] Chena Y, Zhang Q, Wend X, Yin H, Liu J. A novel CNT encapsulated phase change material with enhanced thermal conductivity and photo-thermal conversion performance. Sol Energ Mat Sol C 2018;184:82–90. [29] Nie C, Tong X, Wu S, Gong S, Peng D. Paraffin confined in carbon nanotubes as nano-encapsulated phase change materials: experimental and molecular dynamics studies. RSC Adv 2015;5:92812. [30] Huang Y, Zhang H, Wan X, Chen D, Chen X, Ye X, et al. Carbon nanotube-enhanced double-walled phase-change microcapsules for thermal energy storage. J Mater Chem A 2017;5:7482–93. [31] Zheng Z, Jin J, Xu G, Zou J, Wais U, Beckett A, et al. Highly stable and conductive microcapsules for enhancement of joule heating performance. ACS Nano 2016;10:4695–703. [32] Liu Z, Zou R, Lin Z, Gui X, Chen R, Lin J, et al. Tailoring carbon nanotube density for modulating electro-to-heat conversion in Phase change composites. Nano Lett 2013;13:4028–35. [33] Guo X, Liu C, Li N, Zhang S, Wang Z. Electrothermal conversion phase change composites: the case of polyethylene glycol infiltrated graphene oxide/carbon nanotube networks. Ind Eng Chem Res 2018;57:15697–702. [34] He Y, Li W, Han N, Wang J, Zhang X. Facile flexible reversible thermochromic membranes based on micro/nanoencapsulated phase change materials for wearable temperature sensor. Appl Energy 2019;247:615–29. [35] Chalco-Sandoval W, Fabra MJ, López-Rubio A, Lagaron JM. Development of polystyrene-based films with temperature buffering capacity for smart food packaging. J Food Eng 2015;164:55–62. [36] Shang M, Zhang S, Li N, Gua X, Li L, Wang Z. Di-functional nanocomposite films for efficient conversion and storage of solar energy. Sol Energ Mat Sol C
10
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Facile preparation of flexible eicosane/SWCNTs phase change films via colloid aggregation for thermal energy storage
T
Renjie Chena,c,d,1, Xinyu Huangb,1, Weibin Denga,c,d, Ruizhi Zhenga,c,d, Waseem Aftabb, ⁎ ⁎ ⁎ Jinmin Shib, Delong Xiea,c,d, , Ruqiang Zoub, , Yi Meia,c,d, a
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China College of Engineering, Peking University, Beijing 100871, China c Yunnan Provincial Key Laboratory of Energy Saving in Phosphorus Chemical Engineering and New Phosphorus Materials, China d Higher Educational Key Laboratory for Phosphorus Chemical Engineering of Yunnan Province, China b
H I GH L IG H T S
film yielded from the SWCNT hydrogel owned an excellent conductivity. • The films were obtained by volatilization of the eicosane-contained hydrogels. • The solidification temperature of eicosane was reduced when filled in the SWCNT film. • The sample displayed a latent heat and conductivity of 204.8 J/g and 620.3 S/m. • The • The phase change film exhibited high energy storage efficiency as 91.3%.
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change materials Single wall carbon nanotube Colloid aggregation Energy storage Electrothermal
Carbon nanotubes (CNTs) based phase change composites have been widely reported to be used in areas related to energy conversion and storage. However, these bulk-like materials are not convenient for the further applications. Herein, we introduce the preparation of electrically conductive CNT/polymer films into the fabrication of conductive PCM films. The stable single wall carbon nanotube (SWCNT) dispersion was prepared by ultrasonic treatment with the help of surfactant. The SWCNT hydrogel was prepared by the addition of CaCl2 (aq.) via colloid aggregation (diffusion-limited cluster aggregation). Eicosane was infiltrated into the SWCNT skeleton by diffusion after solvent replacement, and the flexible phase change film (PCMF) was easily obtained by volatilization synchronously. According to the SEM images, eicosane was encapsulated in the SWCNT scaffolding. And PCMF exhibited a promising conductivity, flexibility, thermal stability and reversibility during the heat storage and release processes. For example, when the SWCNT ratio was 27.1%, the latent heat and conductivity were 204.8 J/g and 620.3 S/m, respectively. The results of 200 repeated melting and freezing cycles revealed that the sample have stable latent heat performance. The eicosane/SWCNT films could store thermal energy at modulated input voltage, which could be applied by conventional batteries. And the highest energy storage efficiency of samples was up to 91.3%. These flexible electro-driven films have wide prospect in smart electronic device, small scale heater, infrared stealth, wearable devices, etc.
1. Introduction The energy that maintains the function of human society comes mostly from the consumption of fossil fuel [1]. While obtaining fossil fuel is facile and massive, its reserve is limited and regional. Therefore, the improvement of energy efficiency is necessary to secure energy
supply [2]. Energy conversion and storage can mitigate the fluctuation of energy supply [3,4]. It is regarded as an effective way to improve energy efficiency [5]. Among the energy storage materials, phase change material (PCM) plays a dominant part in the use of latent heat storage [6]. By storing/releasing heat during phase transition, PCMs can passively smooth the temperature difference, thus lowers the cost of
⁎
Corresponding authors at: Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China (Y. Mei). E-mail addresses: [email protected] (D. Xie), [email protected] (R. Zou), [email protected] (Y. Mei). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apenergy.2019.114320 Received 10 July 2019; Received in revised form 14 November 2019; Accepted 1 December 2019 0306-2619/ © 2019 Published by Elsevier Ltd.
Applied Energy 260 (2020) 114320
R. Chen, et al.
flexible PCM films were fabricated by volatilization synchronously. The interconnected carbon nanotubes held PCMs in position by nanoconfinement. The microstructure of the CNT film and composite were investigated by XRD, FTIR, SEM and EDS. The thermal properties were measured by DSC, TGA and DTA. The heat storage of the PCM composite could be triggered electrically by inputting a small voltage. The efficiency was valued and compared under different mass percentage and voltage.
temperature manipulation [7,8]. Owing to this outstanding feature, PCMs are extensively applied in solar energy conversion and storage, electronics heat management, building energy conservation, waste heat utilization and off-peak electricity thermal storage systems [9,10]. Solid-liquid organic PCMs have been recognized as an ideal candidate because of their superior properties, including wide phase change temperatures for convenient use, excellent chemical stability, none toxicity, low corrosion, easy availability in natural resources, no phase segregation and negligible subcooling [11,12]. However, their utilizations have been hampered by several challenges. The liquefied PCM will leak if not properly contained [13,14]. And the insufficient thermal conductivities will lower the charging/discharging speed [15,16]. In order to solve these shortages of organic PCMs, two main methods are mainly used. Firstly, different additives have been blended in to prepare phase change composites, which can improve the performances of pure PCMs, such as thermal conductivity [17]. Secondly, form or shape stable technique is used to hold PCMs in position [18]. The skeletons can not only catch the PCMs from leakage, but also add their properties to composites [19–21]. Carbon nanotube (CNT) is one of the most common additives to prepare functional PCMs [22,23]. The cylindrical structure of carbon atoms provides the pathway for phonons and electrons through the composites. Therefore, it can be used as a thermal/electrical conductivity enhancer, which could improve the thermal storage efficiency [24,25]. By adding 5% CNTs, the thermal conductivity of the shapestabilized fatty acids was increased by 56.1% [26]. The thermal conductivity of PCM composite with metal organic framework was doubled by introducing CNTs [27]. The compatibility of CNTs enables the introduction into different forms of composites. CNT incorporated PCMs were used as the phase change slurry in the solar heat transfer utilization systems [28,29]. CNTs could also be the part shell of PCM microcapsules. When it was blended into the polymer based shell, the thermal stability, thermal transfer rate, hardness and Young's modulus of the microcapsules were improved [30]. And the conductive and stable PCM microcapsules could serve in versatile electro-thermal system [31]. CNTs have outstanding electrical conductivity comparable with metal. By fabricating interconnected CNTs framework, PCM composites can be applied in electro-thermal conversion [32,33]. The CNT sponge was used to encapsulate paraffin to fabricated composite that is electrically conductive. The heat storage could be driven by a low input voltage of 1.50–1.75 V. And the electro-to-heat storage efficiency is 40.6–52.5% [9]. When the CNT sponge was replaced by the CNT array, the electro-to-heat conversion efficiency of the sample was improved to 74.7% [32]. The results demonstrated that CNT-based bulk incorporation is an effective way to prepare functional PCMs composites. However, the electro-to-heat storage is limited from textile and foldable electronics. The PCM composite bulk is fragile, where folding could craze the bulk and disrupt energy transportation. To expand the application scenes, flexible PCM films have been designed by using polymer as support. Thermochromic membrane was fabricated by burying PCM microcapsules inside polyvinyl alcohol/polyurethane [34]. The membrane has temperature indication and the energy storage efficiency reached 67.5%. PCL/PCM fibers were electrospun onto polymer film to prepare smart food packaging [35]. The heat capacity was in the range of 88–119 J/g. Flexible PCM loaded polyvinyl alcohol (PVA) matrix with embedded Au nanoparticle was prepared for photothermal conversion [36]. The hydrogen-bonding keeps the PCM from leakage. PCM confined nanofiber aerogel films were prepared for infrared stealth [37]. The foldable films have a melting enthalpy as high as 179.1 J/g and can mimic the thermal radiance of the environment. Herein, we prepared a CNT/PCM film that combine flexibility and efficient heat conversion and storage. The method is based on colloid aggregation [38,39]. Single wall carbon nanotube (SWCNT) porous skeleton was obtained by adding CaCl2 (aq.) as dispersion agent. Eicosane was infiltrated into the SWCNT scaffold by diffusion. And the
2. Materials and methods 2.1. Materials Single wall carbon nanotubes (SWCNTs) were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Science. Sodium dodecyl sulfate, CaCl2, dichloromethane, acetonitrile, and absolute ethyl alcohol were provided by Tianjin Fengchuan Chemical Reagents Technologies Co., Ltd. Eicosane was purchased from Aladdin Industrial China. All dispersion experiments were carried out with deionized water. 2.2. Fabrication of SWCNT dispersion and eicosane/SWCNT film SWCNTs and SDS with the stoichiometric ratio were dispersed in the deionized water by a horn sonicator (SCIENTIZ-1500F, Ningbo Xinzhi Biotechnology Co., Ltd, China) with a cylindrical tip (20 mm end cap diameter). The output power was fixed at 525 W, the frequency was 20 kHz ± 200 Hz, and the duration of ultrasonic treatment was 45 min. The dispersion was stored hermetically at room temperature. The SWCNT hydrogel was obtained quickly by adding CaCl2 aqueous solution to the dispersion with same volume. After aging for 24 h at room temperature, absolute ethyl alcohol was used for replacing the water, and then alcohol was removed by dichloromethane again. Different contents of eicosane dissolved in the dichloromethane and diffused into the SWCNT-gels spontaneously at 30 °C for 6 h. The eicosane/SWCNT films were simply fabricated through volatilization at room temperature. 2.3. Characterizations UV–vis absorption spectra were recorded with a Shimadzu spectrophotometer UV-1800 operating between 200 and 500 nm. Samples were taken regularly during the dispersion (about 80 μl each, thus the solution volume under investigation is almost unchanged during the total experiment) and diluted by 50 times with distilled water, to attain the suitable SWCNTs contents for UV–vis measurements. The FT-IR Spectra were recorded on a Nicolet Nexus 870 from 400 to 4000 cm−1 with are solution of 2 cm−1 using KBr pellets. The SEM images of films were observed on QUANTA430 (Thermo Fisher Scientific Inc.) and energy dispersive spectroscopy (EDS) was obtained by EDAX-GENESIS (AMETEK, Inc.). Powder X-ray diffraction (PXRD) patterns of all the synthesized samples were recorded by a Rigaku SmartLab X-ray diffractometer operating at 45 kV and 200 mA with Cu Kα radiation (λ = 1.5406 Å). Temperature swing X-ray diffraction was performed on a Rigaku Smartlab-9kw. The temperature swing was programmed as 1 °C per minute and stayed constant for 5 min at 25, 30, 40, 50, 60 °C, respectively. The thermo-gravimetric analysis (TGA) was carried out at a STA (NETZSCH STA 449 F3 Jupiter) with a heating rate of 10 °C/min in N2 flow. The weight of samples was kept within 3–5 mg. The temperature was from 40 °C to 800 °C. The differential scanning calorimetry (DSC) data were collected with a differential scanning calorimeter (NETZSCH DSC 214 Polyma) with a heating/cooling rate of 10 °C/min under Ar. All the sample weights were about 5 mg sealed in an aluminum crucible. The conductivity of films was tested by automatic four-point test 2
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 1. Schematic of the formation steps of eicosane/SWCNT film.
The SWCNTs were dispersed in the water with various concentration of sodium dodecyl sulfate (SDS) by a horn sonicator. Fig. 2a showed the UV–vis spectra evolution of SWCNTs (0.05 wt%) /SDS (0.15 wt%) solution as a function of sonication time. There was an absorption peak at 269 nm where the dispersion behavior of SWNTs was monitored. The height was increased with the time of sonication. Therefore, we compared the absorbance of SWCNT dispersion (0.05 wt%) at different SDS concentration and sonication time, as displayed in Fig. 2b. The absorbance of the peak at 269 nm varied according to the sonication time and SDS content. It was obvious that the content of SDS influenced the duration of ultrasonic treatment. The dispersion effect was remarkably improved, when the ratio of SDS/SWNTs was changed from 2 up to 3. And it was not observably promoted when the ratio was increased. The extra SDS content would have negative influence on the performances of SWCNT dispersion and its further application [41]. Therefore, the ratio of SDS/SWNTs was 3 and the duration of ultrasonic treatment was 45 min.
system (Four-PointProbes-RTS8, Guangzhou 4 PROBES Tech. Co.) at room temperature. The electroheat storage performance was measured by electrochemical workstation (Zahnerennium/IM6). Films were loaded on the electrochemical work station with two copper wires stuck to each side as electrodes. A certain bias electric field of 3–7 V was Reversible applied to samples for about 100 s while recording the electric current flow. A thermal sensor (Pt-100) with an accuracy of ± 0.15 °C was planted into the target material to detect the temperatures plots during heating and cooling processes. The tensile stress-strain curves of all samples were recorded by using IBTC-300 in-situ dynamic material system at room temperature. The specimens were loaded at a constant rate of 1 mm min−1 until fracture or yield. 3. Results and discussions 3.1. Preparation of SWCNT dispersion
3.2. Preparation and characterization of SWCNT films
The preparation process of eicosane/SWCNT film was illustrated in Fig. 1. The stable SWCNT dispersion was the basis of the preparation of conductive phase change film. CNTs were easily prone to aggregation and precipitation in aqueous environment because of their high aspect ratio and hydrophobicity. When the surface of SWCNT was coated by surfactants, the re-aggregation of SWCNTs could be prohibited [40]. To obtain the stable SWCNT dispersion, SWCNTs could be exfoliated into the small agglomerates and single tubes after providing enough energy.
The SWCNT dispersion was stabilized by SDS and ultrasonic treatment. SDS could cover the surface of SWCNTs and prevent the agglomeration of them, thus the stable dispersion has obtained [41]. When the concentration of electrolyte was less than the critical coagulation concentration (CCC), the dispersion could keep stable because of the electrostatic repulsion among the SWCNTs. However, the
Fig. 2. (a) UV–vis spectra evolution of SWCNTs (0.05 wt%) /SDS (0.15 wt%) solution, (b) the absorbance of SWCNT aq. (0.05 wt%) with different SDS concentration and sonication time. 3
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 3. Photos of SWNTs dispersion with different CaCl2 solution (a), SWCNT film1 before solvent exchange (b), and SWCNT film2 after solvent exchange (c). Fig. 4. (a) The conductivity of SWCNT film1 and film2, (b) FTIR of SDS, SWCNT film1 and film2.
dispersions would agglomerate or even settle in the presence of electrolyte [42]. And the aggregation would happen due to the disappearance of electrostatic repulsion when the concentration of electrolyte exceeded the CCC according to Derjaguin-Landau-VerweyOverbeek (DLVO) theory [43–45]. Different valence ions could aggregate the stabilized SWCNTs to a certain coagulation concentration [39,40]. Here, the SWCNT hydrogel was prepared by the addition of CaCl2 (aq.) with different concentrations via diffusion limited cluster aggregation (DLCA) [46]. The influence of CaCl2 concentration on the SWCNTs dispersions are list in Table S1 and Fig. 3a. When the concentrations of CaCl2 were less than 6 mmol/L, the dispersions were stable (sample 1 and 2). An interconnected structure could be formed immediately at the proper concentration of CaCl2 solution (6–40 mmol/L, sample 3–6), owing to the compression of the electric double layer of SWCNTs surface [47]. However, excessive CaCl2 would destroy the structure of the hydrogel (sample 7), because the shield of surface charges of SWCNTs would lead to re-aggregation [43]. Therefore, the concentration of CaCl2 (aq.) was chose as 15 mmol/L. After homogenously dispersed in aqueous solution, SWCNT films could be formed by removing water from the hydrogel. Considering that the mass content of SDS was three times as much as SWCNTs, the influence of surfactants on the performance of SWCNT films could not be negligible. Two kinds of SWCNT films are prepared with different methods. The first one was simple evaporation to remove the water directly (film1, Fig. 3b). And the second was volatilization after replacement of water by organic solvents to eliminate the surfactant in solution (film2, Fig. 3c). Based on the results of four probe method (Fig. 4a), the conductivity of film2 was a few times higher than that of film1. The conductivity was improved from 3.152 × 104 S/m to 105 S/m. SDS, film1 and film2 were analyzed by Fourier Transform Infrared Spectroscopy (FTIR). According to the three curves in Fig. 4b, the FTIR spectrum of film2 differed
obviously from the others. The absorption peaks of sulfate group could be found apparently at 1223 cm−1, 1078 cm−1 and 829 cm−1 from spectra of film1 and SDS. The stretching and bending vibration of long chain alkyl group of surfactant could be observed around 2918 cm−1, 2815 cm−1, 1467 cm−1 and 723 cm−1. These characteristic peaks disappeared or was weakened in the infrared spectrum, which indicated that most SDS was washed through solvent exchange. The same phenomenon could also be discovered in the XRD patterns as shown the Fig. S1. The X-ray relative diffraction peak intensity of SDS is weakened after solvent exchange. Further evidence to validate the two-step preparation of CNT films could be confirmed by SEM and EDS test. Three samples were tested. As showed in Fig. 5a, the as-received SWCNTs powders were formed by twisted carbon nanotubes. In Fig. 5b and c, SWCNT film1 was prepared by removing the water directly from the hydrogel, and SWCNT film2 was obtained after solvent exchange. Via ultrasonic treatment and colloid aggregation, the films were fabricated by interconnected individual carbon nanotubes. The interconnect network functioned as the pathway for phonons and electrons transport. Besides carbon, the characteristic elements of SDS and CaCl2 were detected in SWCNT film1 (sulfur, sodium, calcium and chlorine). In comparison, the weight content of carbon was raised and the weight contents of calcium and sodium were decreased in film2. The large amount of SDS and other inorganic salt on the surface would reduce the conductivity of SWCNT [38,41], and solvent exchange process could remove most of the salts, which restored the conductivity of SWCNT film. The tensile stress-strain curves of SWCNT films were tested with insitu dynamic material system at room temperature, as illustrated in Fig. 6. The tensile strength of SWCNT film1 and film2 is 24.10 and 8.13 MPa, respectively. The curves showed a linear elastic region before 4
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 5. SEM and EDS of raw SWCNT powder (a), SWCNT film1 (b) and film2 (c).
flexibility, which could be bent arbitrarily as shown in Fig. 7e and videos. These flexible films can be widely applied in smart electronic device, small scale heater, signal transmitter, etc. PCMFs were also measured by X-ray diffraction (XRD). The XRD pattern was shown in Fig. 8a. The peaks can be attributed separately to eicosane and SWCNT, and their XRD patterns were illustrated in Fig. S2.
break. The lower tensile strength of film2 was attributed to the larger free space between nanotubes. The loose skeleton provides enough room for PCM infiltration. 3.3. Configuration and morphology of eicosane/SWCNT films The schematic preparation of eicosane/SWCNT film was illustrated in Fig. 1. The twisted SWCNTs (Figs. 5a and 7b) were untangled and dispersed in water under pulsed tip-sonication with SDS. The hydrogel (Fig. 7c) was formed by carbon nanotube scaffolding when CaCl2 (aq.) was added to the dispersion. After solvent exchange, different contents of PCM dissolved in the dichloromethane and diffused into the SWCNTgels spontaneously at 30 °C. The phase change films (PCMFs) were obtained through volatilization (PCM content 95%, 90%, 85%, and 80%, named PCMF1, PCMF2, PCMF3 and PCMF4). In Fig. 7d, SWCNT film was a three-dimensional carbon nanotube scaffold, with open channel for PCM infiltration. The SEM image of eicosane/SWCNT film (Fig. 7f) shows that the surfaces of SWCNT were wrapped by eicosane, and the inter-tube pores is filled. Moreover, compared to the reported PCM/nano carbons composites [9,32], the PCMFs exhibited a reversible
3.4. Thermal stability and phase change behavior of eicosane/SWCNT films As depicted in Fig. 9a, the eicosane transformed to liquid quickly at 80 °C. Owing to the capillary adsorption effects of porous skeleton, liquid PCM was held by the various scale pores and surfaces of SWCNT scaffold (Fig. 7f), which avoided leakage during melting process. Even when the ambient temperature was above the melting point of eicosane, the phase change film maintained its shape showing promising stability. The shape-stabilized feature guarantees the long-term use of the composite. Thermogravimetric (TG) and derivative TG (DTG) analyses were used to determine the thermal stability of PCMFs. The TG curve of SWCNT was showed in Fig. S3. Fig. 9b presented that the eicosane 5
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 6. The tensile stress-strain curves of SWCNT films.
Fig. 8. XRD patterns of PCMFs (a) and PCMfilm2 before/after cycling (b).
temperatures of the maximum weight loss rate of PCMFs were also above the pure one. The results illustrated a better thermal stability. In addition, phase change behavior was measured by differential scanning calorimetry (DSC). Fig. 10a and 10b were the DSC curves of pure eicosane and PCMFs with PCM loadings from 66.6% to 78.1%, calculated by TGA. The onset melting points of PCMFs were below that of the eicosane, as listed in Table 1. Because the PCM was embedded into the SWCNT networks uniformly, the skeleton might influence the wrapped organic molecules, leading to the increased phase change point of PCMFs. The enthalpy of eicosane was 232.1 J/g. The enthalpies of PCMFs rose with the increasing contents of PCM, which all were lower than the pure. The reduction of enthalpy has been observed and reported previously in PCM/CNT composites [48,49]. However, the normalized enthalpies were also approaching to the eicosane with increasing content of PCM, which was increased to 204.8 J/g and 211.1 J/g when the contents of PCM were 72.9% and 78.1%, respectively. This phenomenon was influenced by the eicosane loading and the alkane-nanotube interaction [9]. The difference in the DSC patterns was attributed to the change of phase change behavior of eicosane. The phase change process of pure eicosane was presented as a single endothermic peak and a bimodal exothermal peak [50]. The liquid eicosane transferred to a metastable rotator phase before solidification. When eicosane was filled in the CNT films, the solidification temperature was reduced because of physical interactions and confinement. We also provided the XRD pattern of pure eicosane at different temperature and the comparison of eicosane
Fig. 7. The photos of SWCNT (a), SWCNT hydrogel (c) and eicosane/SWCNT film (e), and the SEM images of SWCNT (b), SWCNT film (d) and eicosane/ SWCNT film (f).
weight ratios of films were from 66.6% to 78.1% as list in Table 1, which were lower than the designed values (from 80% to 95%). The loss results from the diffusion and volatilization process. Both TG curve and DTG curves exhibited that the thermal stability of PCM was improved by infiltrating into the SWCNT gel. The thermal gravity temperatures (the temperature of 5% weight loss, T-5%) of PCMFs were higher than eicosane’s. Comparing the values of maximum weight loss rate (Fig. 9c), the PCMFs were lower than that of eicosane. And the 6
Applied Energy 260 (2020) 114320
R. Chen, et al.
(Fig. S4a) and composites at 25 and 60 °C (Fig. S4b and S4c). The results demonstrated that the change of XRD patterns of composites corresponds to the change of the phase change of eicosane. Furthermore, the reversibility without decomposition was of importance for application of PCMs during the heat storage and release processes. The repeated test of PCMF2 was measured for 200 cycles at the same condition, which displayed a stable performance for each melting and freezing processes, as illustrated in Fig. 10c and 10d, and the results of the rest samples could be found in Fig. S5. The XRD patterns of PCMF2 before and after cycling were illustrated in Fig. 8b. The angles of peaks were identical, indicating the chemical structure stability of PCMF2 after repeatedly phase change cycles. Therefore, chemical stability and the good thermal storage/release cyclability of the films could reinforce their application for longer-term heat storage through frequent use. 3.5. Conductivity and electro-to-heat storage of eicosane/SWCNT films The result of four probe method showed that the SWCNT film has an outstanding conductivity of 100,000 S/m. According to the SEM images, eicosane not only covered the surface of SWCNTs, but also infiltrated into the pores of inter-tubes. These composite constructions would have an effect on the conductance of the PCMFs due to the coating of insulator. As illustrated in Fig. 11a, the conductivities of samples were 531.2 S/m, 620.3 S/m, 965.4 S/m and 3927.3 S/m respectively, which were increased with the increasing ratio of SWCNT. The resistances of different films could be calculated by their sizes and conductivities. And the resistance values of PCMFs were 73.3 Ω, 49.5 Ω, 43.4 Ω and 25.2 Ω respectively. Owing to the highly conductive SWCNT skeleton, electro-to-heat storage of our flexible phase change films could be triggered through applying an electric potential directly. According to the temperaturetime plots of PCMFs, when voltage was applied, the current past through the film could heat the nanotubes, where after joule heat was generated. When there was a voltage (3 V, 4 V or 5 V) applied on the sample at an environmental temperature of 26 °C, the temperature of the film would go up. The energy transferred from SWCNT skeleton to wrapped eicosane by heat conduction. The heat could be stored by phase transition process, which also could be released again. As shown in Fig. 11b, the temperature curves of PCMF1 rose gently during the initial time and reach a balance where the heat dissipation and the energy input achieved equilibrium state from the heated SWCNT skeleton to the circumstance. The highest temperatures of these three curves were below the melting point of eicosane, due to its lower electrical conductivity. If the input voltage was further enhanced to 6 V and 7 V, the temperature of sample could exceed the phase change point, which led to the heat storage process, as illustrated in Fig. S6. With the increasing of conductivity, the temperature-time plots of PCMF2 and PCMF3 differed from PCMF1, when the input voltages were 4 V and 5 V. The slopes gradually decreased until an inflection point was met, which indicated that the phase transition process occurred at the plateau region of temperature-time curve (Fig. 11c and d). The temperatures were between 33 °C and 36 °C, in accordance with the melting point of the same infiltrating composite. At the termination of phase change, film temperature continued to rise quickly, and met a balance again. However, when the voltage was 3 V, the temperatures curves were below the phase change point, revealing that input energy was insufficient to melt eicosane in the SWCNT film and the voltage threshold was higher. After switching off the applied voltages, the temperatures of all the curves decreased to the solidifying point rapidly (about 34 °C), and the temperatures maintained to release heat until the end of freezing. Besides, when the sample conductivity was further improved, the critical voltage of PCMF4 was reduced to 3 V, and the phase change process could be found in Fig. 11f. Here, the efficiency of electro-heat storage was calculated. In the calculation, all the infiltrate PCM is assumed to take part in the phase
Fig. 9. (a) Photos of eicosane and PCMFs, TGA (b) and DTG (c) curves of eicosane and PCMFs. Table 1 . TGA and DSC details of eicosane and PCMFs. Sample
T-5%oC
Residue (330°C) %
PCM %
T m oC
△Hm J/g
△Hn J/g
PCMF1 PCMF2 PCMF3 PCMF4 Eicosane
201.8 199.9 198.2 197.4 192.8
21.9 27.1 29.2 33.4 0.0
78.1 72.9 70.8 66.6 100.0
36.5 36.7 36.7 36.8 37.2
164.9 149.3 127.0 86.4 232.1
211.1 204.8 179.4 129.7 232.1
7
Applied Energy 260 (2020) 114320
R. Chen, et al.
Fig. 10. DSC curve of eicosane (a) and PCMFs (e), recycling test (c) and phase change stability (d) of PCMF2.
PCM3 was 42.3% under 4 V and 83.8% under 5 V. When the applied voltage is increased, the Joule heat is increased as well. However, the heat dissipation from the sample to the atmosphere is limited. Therefore, the charging time is remarkably shortened, which improved the efficiency in turn [9,13]. When used in a real circumstance, the flexible film will cover the substance, whose thermal conductivity is higher than the air. And the efficiency will be lower than it is shown in the present
transition, so electro-heat storage efficiency (η) was defined as mΔHR η = 2 , where m was the mass of sample, △H was the apparent enU t thalpy, R was the resistance, U was input voltage, and t was time of the phase change process obtained by the tangential method (Fig. S7). The efficiencies of various samples were summarized in Table S2 and Fig. 11f. Comparing the three samples, the energy storage efficiency was much higher at 5 V applied bias. For example, the efficiency of
Fig. 11. (a) Conductivity of four PCMFs, (b) Temperature evolution of PCMF1, (c) Temperature evolution of PCMF2, (d) Temperature evolution of PCMF3, (e) Temperature evolution of PCMF4, (f) energy storage efficiencies of three samples. 8
Applied Energy 260 (2020) 114320
R. Chen, et al.
interests or personal relationships that could have appeared to influence the work reported in this paper.
case. As revealed in Fig. 11f, the efficiency of the composite decreased as the increase of CNT percentage under the same voltage. The reason is that the energy capacity m·ΔH and the resistance R are improved as PCM percentage increased. However, the charging time is also lengthened. As the PCM percentage improves, the efficiency would meet a point when the negative effect of charging time appears. The effect should also be considered when designing the application device. The efficiencies of PCMF2, PCMF3 and PCMF4 were 91.3%, 83.8% and 51.1% under 5 V input voltage, and 80.1%, 42.3% and 30.2% under 4 V input voltage, respectively. It was known that PCM content and sample conductivity assumed inverse ratio connection. Too much eicosane covered on the surface of nanotubes could greatly increase the resistance of film, which needs a higher input voltage for electro-driven thermal storage. A suitable coating rate of eicosane of SWCNT network could minimize the convection heat loss from heated SWCNT to air under smaller voltage. Among all the conditions, the maximum value was 91.3% of PCMF2 at 5 V. The electro-driven heating property enabled the film to be used as wearable devices for heat conservation. In a recent model, PCM was embedded in soft polymer to provide thermal protection of biological tissues [51]. Similarly, the SWCNT-incorporated composite can be fabricated as a wearable device to keep human-body warm. Besides, the flexible electro-driven films also have wide possibility in smart electronic device, small scale heater, infrared stealth, etc.
Acknowledgments The work has been financially supported by the Natural Science Foundation of China (No. 51603096, and No. 21663015). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apenergy.2019.114320. References [1] Waseem A, Huang X, Wu W, Liang Z, Asif M, Zou R. Nanoconfined phase change materials for thermal energy applications. Energ Environ Sci 2018;11:1392–424. [2] Shchukina EM, Graham M, Zheng Z, Shchukin DG. Nanoencapsulation of phase change materials for advanced thermal energy storage systems. Chem Soc Rev 2018;47:4156. [3] Zhang L, Zhou K, Wei Q, Ma L, Ye W, Li H, et al. Thermal conductivity enhancement of phase change materials with 3D porous diamond foam for thermal energy storage. Appl Energy 2019;233–234:208–19. [4] Gasiaa J, Maldonadoa JM, Galatib F, Simoneb MD, Cabezaa LF. Experimental evaluation of the use of fins and metal wool as heat transfer enhancement techniques in a latent heat thermal energy storage system. Energ Convers Manage 2019;184:530–8. [5] Zheng Z, Chang Z, Xu G, McBride F, Ho A, Zhuola Z, et al. Microencapsulated phase change materials in colar-thermal conversion systems: understanding geometrydependent heating efficiency and system reliability. ACS Nano 2017;11:721–9. [6] Sheng N, Zhu R, Dong K, Nomura T, Zhu C, Aoki Y, et al. Vertically aligned carbon fibers as supporting scaffolds for phase change composites with anisotropic thermal conductivity and good shape stability. J Mater Chem A 2019;7:4934–40. [7] Merlin K, Soto J, Delaunay D, Traonvouez L. Industrial waste heat recovery using an enhanced conductivity latent heat thermal energy storage. Appl Energy 2016;183:491–503. [8] Gasiaa J, Graciaa A, Zsembinszki G, Cabeza LF. Influence of the storage period between charge and discharge in a latent heat thermal energy storage system working under partial load operating conditions. Appl Energy 2019;235:1389–99. [9] Chen L, Zou R, Xia W, Liu Z, Shang Y, Zhu J, et al. Electro- and photodriven phase change composites based on wax-infiltrated carbon nanotube sponges. ACS Nano 2012;6:10884–92. [10] Wu W, Huang X, Li K, Yao R, Chen R, Zou R. A functional form-stable phase change composite with high efficiency electro-to-thermal energy conversion. Appl Energy 2017;3:474–80. [11] Yu D, He Z. Shape-remodeled macrocapsule of phase change materials for thermal energy storage and thermal management. Appl Energy 2019;247:503–16. [12] Ye S, Zhang Q, Hu D, Feng J. Core–shell-like structured graphene aerogel encapsulating paraffin: shape-stable phase change material for thermal energy storage. J Mater Chem A 2015;3:4018–25. [13] Chen R, Yao R, Xia W, Zou R. Electro/photo to heat conversion system based on polyurethane embedded graphite foam. Appl Energy 2015;152:183–8. [14] Long L, Ye H, Gao Y, Zou R. Performance demonstration and evaluation of the synergetic application of vanadium dioxide glazing and phase change material in passive buildings. Appl Energy 2014;136:89–97. [15] Chen R, Cui Y, Tian H, Liu Z, Shu Y, Li C, et al. Controllable thermal rectification realized in binary phase change composites. Sci Rep 2015;5:8884. [16] Geng X, Li W, Wang Y, Lu J, Wang J, Wang N, et al. Reversible thermochromic microencapsulated phase change materials for thermal energy storage application in thermal protective clothing. Appl Energy 2018;217:281–94. [17] Sari A, Bicer A, Al-Sulaiman F, Karaipekli A, Tyagi V. Diatomite/CNTs/PEG composite PCMs with shape-stabilized and improved thermal conductivity: preparation and thermal energy storage properties. Energ Build 2018;164:166–75. [18] Li C, Yu H, Song Y, Wang M, Liu Z. A n-octadecane/hierarchically porous TiO2 form-stable PCM for thermal energy storage. Renew Energ 2020,;145:1465–73. [19] Yang J, Jia Y, Bing N, Wang L, Xie H, Yu W. Reduced graphene oxide and zirconium carbide co-modified melamine sponge/paraffin wax composites as new form-stable phase change materials for photothermal energy conversion and storage. Appl Therm Eng 2019;163:114412. [20] Wu H, Li S, Shao Y, Jin X, Qi X, Yang J, et al. Melamine foam/reduced graphene oxide supported form-stable phase change materials with simultaneous shape memory property and light-tothermal energy storage capability. Chem Eng J 2020,;379:122373. [21] Chen T, Liu C, Mu P, Sun H, Zhu Z, Liang W, et al. Fatty amines/graphene sponge form-stable phase change material composites with exceptionally high loading rates and energy density for thermal energy storage. Chem Eng J 2020,;382:122831. [22] Li M, Chen M, Wu Z, Liu J. Carbon nanotube grafted with polyalcohol and its influence on the thermal conductivity of phase change material. Energ Convers Manage 2014;83:325–9. [23] Li B, Nie S, Hao Y, Liu T, Zhu J, Yan S. Stearic-acid/carbon-nanotube composites with tailored shape-stabilized phase transitions and light–heat conversion for
4. Conclusion In conclusion, a novel, facile, and efficient strategy has been introduced to fabricate electrical conductive and flexible phase change films via colloidal aggregation. The SWCNT dispersion was prepared by ultrasonic treatment with SDS. The SWCNT hydrogel was fabricated by the addition of CaCl2 (aq.). Eicosane was diffused into the porous SWCNT scaffolds spontaneously at 30 °C, after the water of hydrogel was replaced by organic solvent. The phase change films were obtained through volatilization. Due to the capillarity, the composites displayed excellent form stability against leakage, when the ambient temperature was higher than the phase transition point of eicosane. Moreover, the phase change films showed a promising heat storage density (204.8 J/g for PCMF2 with a SWCNT mass ratio of 27.1% and 211.1 J/g for PCMF1 with a SWCNT mass ratio of 21.9%). In addition, these films presented the nice thermal stability and reversibility during the long-term heat storage and release processes. According to results by the four probe method, the electrical conductivities of phase change films were 531.2 S/m, 620.3 S/m, 965.4 S/m and 3927.3 S/m respectively, so they could store thermal energy at modulated input voltage. And the highest energy storage efficiency of the samples was up to 91.3% (PCMF2 under 5 V input voltage). Owing to the flexibility of samples and unique performances of SWCNTs, our flexible electro-driven films have wide possibility in smart electronic device, small scale heater, infrared stealth, wearable devices etc. CRediT authorship contribution statement Renjie Chen: Conceptualization, Methodology, Validation, Data curation, Writing - original draft, Visualization. Xinyu Huang: Conceptualization, Methodology, Validation, Data curation, Writing review & editing, Visualization. Weibin Deng: Investigation, Writing original draft. Ruizhi Zheng: Investigation. Waseem Aftab: Investigation. Jinmin Shi: Investigation. Delong Xie: Writing - review & editing, Supervision, Funding acquisition. Ruqiang Zou: Supervision, Funding acquisition. Yi Mei: Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial 9
Applied Energy 260 (2020) 114320
R. Chen, et al.
2017;164:188–92. [37] Lyu J, Liu Z, Wu X, Li G, Fang D, Zhang X. Nanofibrous kevlar aerogel films and their phase change composites for highly efficient infrared stealth. ACS Nano 2019;13:2236–45. [38] Zhang C, Wen B, Gao L, Chen Y, Yang M. Fabrication of surfactant-removed polymer composites with single-walled carbon nanotube networks. J Appl Polym Sci 2011;119:155–61. [39] Wu Q, Hu J. Waterborne polyurethane based thermoelectric composites and their application potential in wearable thermoelectric textiles. Compos B 2016;107:59–66. [40] Islam M, Rojas E, Bergey D, Johnson A, Yodh A. High weight fraction surfactant solubilization of single-wall carbon nanotubes in water. Nano Lett 2003;3:269–73. [41] Yu J, Lu K, Souty E, Grossiord N, Koning C, Loos J. Characterization of conductive multiwall carbon nanotube/polystyrene composites prepared by latex technology. Carbon 2007;45:2897–903. [42] Li M, Huang C. Stability of oxidized single-walled carbon nanotubes in the presence of simple electrolytes and humic acid. Carbon 2010;48:4527–34. [43] Smith B, Wepasnick K, Schrote K, Cho H, Ball H, Fairbrother D. Influence of surface oxides on the colloidal stability of multi-walled carbon nanotubes: a structureproperty relationship. Langmuir ACS J Surf Colloids 2009;25:9767–76. [44] Amal R, Raper J, Waite T. Effect of fulvic acid adsorption on the aggregation kinetics and structure of hematite particles. J Colloid Interf Sci 1992;151:244–57. [45] Ding P, Pacek A. Effect of pH on deagglomeration and rheology/morphology of aqueous suspensions of goethite nanopowder. J Colloid Interf Sci 2008;325:165–72. [46] Xie D, Wu H, Zaccone A, Braun L, Chen H, Morbidelli M. Criticality for shear-induced gelation of charge-stabilized colloids. Soft Matt 2010;6:2692–8. [47] Lin D, Liu N, Yang K, Zhu L, Xu Y, Xing B. The effect of ionic strength and pH on the stability of tannic acid-facilitated carbon nanotube suspensions. Carbon 2009;47:2875–82. [48] Zeng J, Cao Z, Yang D, Xu F, Sun L, Zhang X, et al. Effects of MWNTS on phase change enthalpy and thermal conductivity of a solid-liquid organic PCM.J. Therm Anal Calori. 2009;95:507–12. [49] Bayramoglu E. Thermal Properties and Stability of N-octadecane based composites containing multiwalled carbon nanotubes. Polym Compos 2011;904–909. [50] Liu H, Niu J, Wang X, Wu D. Design and construction of mesoporous silica/n-eicosane phasechange nanocomposites for supercooling depression and heat transfer enhancement. Energy 2019;188:116075. [51] Shi Y, Wang C, Yin Y, Li Y, Xing Y, Song J. Functional soft composites as thermal protecting substrates for wearable electronics. Adv Funct Mater 2019;29:1905470.
thermal energy storage. Energ Convers Manage 2015;98:314–21. [24] Wang Y, Tang B, Zhang S. Single-walled carbon nanotube/phase change material composites: sunlight-driven, reversible, form-stable phase transitions for solar thermal energy storage. Adv Funct Mater 2013;23:4354–60. [25] Tang Y, Alva G, Huang X, Su D, Liu L, Fang G. Thermal properties and morphologies of MA–SA eutectics/CNTs as composite PCMs in thermal energy storage. Energ Build 2016;127:603–10. [26] San A, Bicer A, Al-Ahmed A, Al-Sulaiman F, Zahir M, Mohamed S. Silica fume/ capric acid-palmitic acid composite phase change material doped with CNTs for thermal energy storage. Sol Energ Mat Sol C 2018;179:353–61. [27] Wang J, Huang X, Gao H, Li A, Wang C. Construction of CNT@Cr-MIL-101-NH2 hybrid composite for shape stabilized phase change materials with enhanced thermal conductivity. Chem Eng J 2018;350:164–72. [28] Chena Y, Zhang Q, Wend X, Yin H, Liu J. A novel CNT encapsulated phase change material with enhanced thermal conductivity and photo-thermal conversion performance. Sol Energ Mat Sol C 2018;184:82–90. [29] Nie C, Tong X, Wu S, Gong S, Peng D. Paraffin confined in carbon nanotubes as nano-encapsulated phase change materials: experimental and molecular dynamics studies. RSC Adv 2015;5:92812. [30] Huang Y, Zhang H, Wan X, Chen D, Chen X, Ye X, et al. Carbon nanotube-enhanced double-walled phase-change microcapsules for thermal energy storage. J Mater Chem A 2017;5:7482–93. [31] Zheng Z, Jin J, Xu G, Zou J, Wais U, Beckett A, et al. Highly stable and conductive microcapsules for enhancement of joule heating performance. ACS Nano 2016;10:4695–703. [32] Liu Z, Zou R, Lin Z, Gui X, Chen R, Lin J, et al. Tailoring carbon nanotube density for modulating electro-to-heat conversion in Phase change composites. Nano Lett 2013;13:4028–35. [33] Guo X, Liu C, Li N, Zhang S, Wang Z. Electrothermal conversion phase change composites: the case of polyethylene glycol infiltrated graphene oxide/carbon nanotube networks. Ind Eng Chem Res 2018;57:15697–702. [34] He Y, Li W, Han N, Wang J, Zhang X. Facile flexible reversible thermochromic membranes based on micro/nanoencapsulated phase change materials for wearable temperature sensor. Appl Energy 2019;247:615–29. [35] Chalco-Sandoval W, Fabra MJ, López-Rubio A, Lagaron JM. Development of polystyrene-based films with temperature buffering capacity for smart food packaging. J Food Eng 2015;164:55–62. [36] Shang M, Zhang S, Li N, Gua X, Li L, Wang Z. Di-functional nanocomposite films for efficient conversion and storage of solar energy. Sol Energ Mat Sol C
10