Wood-based composite phase change materials with self-cleaning superhydrophobic surface for thermal energy storage

Wood-based composite phase change materials with self-cleaning superhydrophobic surface for thermal energy storage

Applied Energy 261 (2020) 114481 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Wood-b...

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Applied Energy 261 (2020) 114481

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Wood-based composite phase change materials with self-cleaning superhydrophobic surface for thermal energy storage

T

Haiyue Yang, Siyuan Wang, Xin Wang, Weixiang Chao, Nan Wang, Xiaolun Ding, Feng Liu, ⁎ ⁎ Qianqian Yu, Tinghan Yang, Zhaolin Yang, Jian Li, Chengyu Wang , Guoliang Li Key Laboratory of Bio-Based Material Science and Technology, Ministry of Education, Northeast Forestry University, Harbin 150040, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

composites own super• The hydrophobicity and thermal energy

• • •

storage capacity. The composites further prevent liquid leakage of TD in humid environment. The composites could improve energy storage capacity in humid environment. The composites with self-cleaning have efficient solar-to-thermal energy storage.

A R T I C LE I N FO

A B S T R A C T

Keywords: Phase change materials Delignified wood Shape-stability Superhydrophobicity Self-cleaning Thermal energy storage

Form-stable composite phase change materials, as thermal energy storage technology, show great promise for reducing energy consumption and relieving current energy shortage problems. However, porous supporting materials and most phase change materials are hydrophilic and hygroscopic, which cause crack-formation at the interfaces between supporting materials and phase change materials and decrease in thermal energy storage capacity of composite phase change material in wet or humid environment. There are almost no reports concerning this topic. Herein, form-stable and superhydrophobic composite phase change materials are fabricated by spraying superhydrophobic coating on the surface of composite phase change materials, in which delignified wood acts as a supporting material to protect against liquid leakage of 1-tetradecanol. The superhydrophobic composite phase change materials possess large water contact angle of 155° and superhydrophobic stability at 20–100 °C and pH 3–12, which prevents supporting materials and phase change materials from contacting with moisture in wet environment. In addition, the superhydrophobic composite phase change materials exhibit large latent heat of fusion (125.40 J/g), 29.58 J/g higher than that of composite phase change materials without superhydrophobic coating in wet environment. Moreover, the superhydrophobic composite phase change materials possess excellent thermal reliability and stability, efficient solar-to-thermal energy conversion and selfcleaning property, which are potential in the application of advanced energy-related devices and systems for thermal energy storage in wet or humid environment.



Corresponding authors. E-mail addresses: [email protected] (C. Wang), [email protected] (G. Li).

https://doi.org/10.1016/j.apenergy.2019.114481 Received 26 August 2019; Received in revised form 6 December 2019; Accepted 29 December 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic illustration of facile fabrication of superhydrophobic TD/DW composite PCMs. Pristine wood has anisotropic porous structure and its main components are lignin, hemicellulose and cellulose. After the delignification process, the unique porous structure was well-preserved. There is hydrogen bonding between cellulose framework and TD by vacuum-assisted impregnating PCM. After spraying the resin solution and modified SiO2 solution, the superhydrophobic TD/ DW composite PCMs were obtained for thermal energy storage and self-cleaning.

1. Introduction

have attracted considerable attention, by incorporating PCMs into porous supporting materials, such as carbon foams [10–12], aerogels [13,14], expanded graphite [15,16], metal–organic frameworks [17], and other porous minerals [18,19]. Notably wood has been widely applied as a supporting material for shape-stable composite PCMs, owing to its unique porous structure, low density, excellent mechanical strength, low cost, sustainability, and environmental-friendliness [20,21]. However, these researchers ignore one severe problem of shapestable composite PCMs in practical application, which is that porous supporting materials and phase change materials are hydrophilic, owing to their chemical groups and porous structure. The moisture absorption of these supporting materials can lead to cracking, dimensional instability, increased weight, bacterial infection, and decrease mechanical strength, which ultimately reduces their service life as supporting materials [22,23]. In addition, most phase change materials are hygroscopic (e.g., fatty alcohol, fatty acid) or water-soluble (e.g., polyethylene glycol), which decreases their thermal energy storage capacity after moisture absorption. Although novel shape-stable composite PCMs mitigate liquid leakage problem of pure PCMs, moisture absorption of the porous supporting materials and PCMs prevents their further practical applications in energy storage and thermal control engineering. Thus, waterproofing treatments for composite PCMs are urgently needed. Huang et al. hydrophobically modified silica aerogels to fabricate shape-stable composite PCMs, which own higher thermal energy storage capacity and stability than unmodified silica aerogels [24]. Li et al. prepared graphene-aerogel-directed phase-change smart fibers with a self-clean superhydrophobic surface [25]. Superhydrophobic coating is a simple way to prepare waterproof materials, which could maintain the original properties of supporting materials and PCMs. Furthermore, superhydrophobicity can endow the materials with self-cleaning, anti-fungal, anti-fouling, and anti-corrosive properties. In this work, superhydrophobic wood-based composite phase change materials (superhydrophobic TD/DW composite PCMs) are fabricated by impregnating TD into DW and spraying superhydrophobic

Thermal energy storage, as an environment-friendly energy-saving technology, shows great promise as a means of storing energy from renewable resource and reducing energy consumption [1]. Among thermal energy storage technology, phase change materials (PCMs) have potential in the fields of thermal management and thermal energy storage, owing to their ability to store and release latent heat during phase transition processes [2]. Generally, PCMs can be divided into inorganic (e.g., salts, salt hydrates, metal, and melt alloys) and organic PCMs (e.g., paraffin, polyethylene glycol, fatty acids, and polyhydric alcohols) based on their chemical constituents [3]. Organic PCMs have advantages over inorganic PCMs in terms of their low corrosion, no supercooling and no phase separation [4]. According to the type of phase change, PCMs can be classified into four categories, namely: liquid-gas, solid-gas, solidliquid, and solid-solid PCMs [5]. However, liquid-gas PCMs and solidgas PCMs are rarely used in practical applications owing to the large volume change during phase transformation process. In addition, solidsolid PCMs have the inherent disadvantage of low phase change enthalpy and high cost, despite existing the lowest volume change. Solidliquid PCMs have remarkable superiority with high latent heat storage capacity, and small volume change during the phase change process [6]. Solid-liquid PCMs also exhibit other advantages, such as good thermal reliability, low toxicities, and low cost. Among phase change materials, organic solid-liquid PCMs have been a key research direction for thermal energy storage [7]. An important organic solid-liquid PCM, 1-tetradecanol (TD), exhibits large storage capacity and suitable phase change temperature (approximately 37 °C and close to human body temperature) [8], which is widely applied in civil air-conditioning, solar heating, waste heat recovery, and building energy conservation [9]. At present, most researches on organic solid-liquid PCMs have mainly focused on addressing liquid leakage problem during the phase change process, due to the liquid leakage problem is harmful to surroundings. To address the problem, novel shape-stable composite PCMs 2

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tested. The superhydrophobic stability at different temperature was tested by keeping the composite in the oven at 20–100 °C for 10 min, respectively. The WCA at different pH was recorded after liquid droplets (5 μL) of pH 1–14 placed on the surface for 30 s. The wear resistant of superhydrophobic TD/DW composites was performed as follows: pushing the superhydrophobic TD/DW composites (bearing 50 g) to move on the sandpaper (grit no. 2000), and marking 10 cm of movement as one cycle (Fig. S5). The water jet impalement test was carried out to evaluate the mechanical stability of superhydrophobic TD/DW composites as follows: placing the samples under the current as the speed of 1500 mL/min with the distance of 10 cm (Fig. S8). Phase change temperature and phase change enthalpies were recorded with a differential scanning calorimeter (DSC, Q20, Ta, USA) from 25 to 60 °C at 2 °C/min under N2 atmosphere. Thermogravimetric analysis (TGA) was conducted on a Q20 Instruments (Ta, USA) at a heating rate of 10 °C/min from 25 to 800 °C under nitrogen. The solar-to-thermal energy conversion measurements were performed with simulated solar illumination (PLS-SXE300UV, Perfect Light, China) at 15 A, with a distance of 50 cm between the samples and the optical lens. The temperature was recorded by an infrared thermal camera (FLIR, E6, PerkinElmer, USA). The shape stabilities of TD, TD/DW composite and superhydrophobic TD/DW composite are measured as follows: these samples are placed on weigh paper and keep static at 60 °C for 15 min. By observing the weigh paper, it could be determined whether there is liquid leakage happening. The weight percentage gain (WPG) was obtained by immersing samples into deionized water and recording weight of all samples after different times. The calculation formula is as follows: WPG (%) = (Wafter im. − Wbefore im.)/Wbefore im.

coating on the surface of TD/DW composite PCMs (Fig. 1). The DW preserves the unique porous structure and prevents liquid leakage of the TD during the phase change process as supporting materials. Superhydrophobic coating could prevent supporting materials and phase change materials from contacting with moisture, which could further prevent liquid leakage of TD, improve energy storage capacity and expand the scope of application in wet environment. In addition, the microstructure, wettability, thermal properties, thermal stability, shape stability, solar-to-thermal energy conversion and storage of the composites are investigated. The multifunctional composites with superhydrophobicity and thermal energy storage capacity hold promise for energy conservation and energy storage applications in humid environment. 2. Experimental section 2.1. Materials Basswood slices (20 mm × 20 mm × 5 mm) were used in this study. Sodium chlorite (NaClO2), hydrophilic SiO2 (7–40 nm), perfluorodecyltriethoxysilane (PFDS) and epoxy resin were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. 1Tetradecanol (TD, ≥99%) was obtained from Tianjin Guangfu Fine Chemical Research Institute. Glacial acetic acid, formic acid and acetone were supplied by Tianjin Kaitong Chemical Reagent Co., Ltd. Deionized water was prepared in our laboratory. 2.2. Preparation of delignified wood

3. Results and discussion

The basswood slices were treated with 1 wt% NaClO2 with glacial acetic acid solution (pH = 4.6) at 80 °C for 12 h, then rinsed with deionized water for three times to remove most of the chemicals. The treated wood slices were freeze-dried for 24 h, and delignified wood (DW) was obtained.

3.1. Microstructures of the superhydrophobic TD/DW composite PCMs Fig. 2 illustrates the unique structure of the wood-based samples. Pristine wood has unique anisotropic porous structure (Fig. 2a and e). After a facile delignification process, most lignin and hemicellulose could be removed while maintaining the inherent mesoporous structure (Fig. 2b and f) and the delignified wood is mainly composed of cellulose nanofibers. After vacuum-assisted infiltration of TD, the lumens of the DW were completely filled with TD (Fig. 2c). The results of FT-IR and XRD analyses (Figs. S1 and S2) show that no chemical interaction occurs between TD and DW. There is no obvious interface observed between the vessels and TD, owing to formation of strong hydrogen bonds between the cellulose framework and TD (Fig. 2g). Notably, some gaps appear between the DW and TD after immersing the TD/DW composites into water. The reason for these gaps is that cellulose nanofibers are hydrophilic and swell when they meet water. The microcracks between the TD and DW propagate in soggy TD/DW composites with prolonged times and changing temperatures. Fig. 2i and Fig. S3 show the roughened surface composed of the epoxy resin/modified SiO2 particles of superhydrophobic TD/DW composite PCMs, in which epoxy resin acts as an adhesive to affix the modified SiO2 particles. Roughened surfaces with micro-nano structure are known to be essential for generating superhydrophobicity. The EDX mapping images of the elemental distribution of F and Si on the coating surface are shown in Fig. 2j and k, confirming that the SiO2 particles are modified with PFDS, which provides the necessary low surface energy materials for the surface. The EDX spectrums in Fig. 2n and Fig. S4 show the major chemical composition of the coating surface of superhydrophobic TD/DW composite PCMs: carbon, oxygen, silicon and fluorine.

2.3. Preparation of TD/DW composite PCMs The TD/DW composite PCMs were fabricated by a vacuum-assisted infiltration method. DW was immersed into melted TD at 70 °C for 4 h in a vacuum oven. Followed by removal of the liquid TD on the surface of the DW with filter paper, TD/DW composite PCMs were obtained. 2.4. Preparation of superhydrophobic TD/DW composite PCMs First, modified SiO2 solution was prepared by mixing 2 g of hydrophilic SiO2, 2 mL of PFDS, 2 mL of formic acid and 100 mL of acetone under magnetic stirring for 2 h at room temperature. Then, the TD/DW composite PCMs were sprayed with epoxy resin/acetone solution (5%, m/v). Subsequently, the pre-coated samples were further deposited with modified SiO2 solution. The superhydrophobic TD/DW composite PCMs were obtained after curing for 12 h at room temperature. A schematic diagram of the preparation of superhydrophobic TD/DW composite PCMs is illustrated in Fig. 1. 2.5. Characterization Scanning electron microscope (SEM) images and energy dispersive spectroscopy (EDX) spectra were collected with a TM3030 tabletop microscope (Hitachi, Japan). The chemical composition and crystal structure were measured with a Fourier Transform Infrared Spectrometer (FTIR, Thermo Fisher Scientific Nicolet 6700, USA) in the range of 400–4000 cm−1 and X-ray diffractometer (XRD, D/max 2200 VPC, Rigaku, Japan) from 5° to 80° at a scanning speed of 4°/min, respectively. Water contact angle (WCA) images of water droplets (5 μL) were collected by an OCA20 contact angle system (Data-physics, Germany). At least five different positions on all the samples were

3.2. Wettability and self-cleaning properties of the superhydrophobic TD/ DW composite PCMs To investigate the wettability of the DW, pure TD, TD/DW composites and superhydrophobic TD/DW composites, water contact 3

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Fig. 2. (a and b) SEM images of cross-sectional PW and DW, respectively. (e and f) SEM images of longitudinal-sectional PW and DW, respectively. These show that the morphology is well-preserved after lignin removal. SEM images of (c) TD/DW composites and (g) high magnification images of the TD/DW composites, showing tight hydrogen bonding between TD and DW. SEM images of (d) soggy TD/DW composites and h) high magnification images of soggy TD/DW composites, revealing gaps between TD and DW. (i) SEM image of the coating surface of the superhydrophobic TD/DW composites. (j and k) EDX mapping images of the elemental distribution of F and Si in the coating surface. (n) EDX elemental content of the coating surface of superhydrophobic TD/DW composites. Fig. 3. (a) Optical images of static water droplets (dyed with methylene blue) on different surfaces. (i) DW; (ii) pure TD; (iii) TD/DW composites; (iv) superhydrophobic TD/DW composites. Insets show photographs of water contact angle. These show DW, pure TD, TD/DW composites are hydrophilic and superhydrophobic TD/DW composites are superhydrophobic. (b) Photos of (I and ii) superhydrophobic TD/ DW composites in water after 0 s and 24 h; and TD/DW composites in water after 0 s and 30 s, respectively. (c) WPG of DW, pure TD, TD/DW composites and superhydrophobic TD/DW composites in water immersion experiments at different times. (d) Self-cleaning test of superhydrophobic TD/DW composites.

and the absorption time is 18 s (Video S3), which is less than that of pure TD, due to the presence of DW. In contrast, after spraying the epoxy resin / modified SiO2 particles, the WCA is 155° and the surface exhibits superhydrophobicity (WCA > 150°). When the superhydrophobic TD/DW composites are placed in water, they continue to float on the water surface after 24 h. However, the TD/DW composites

measurements were performed at room temperature. As shown in Fig. 3a and Video S1 (Supporting Information), the dyed water droplet is absorbed within 2 s by DW with a WCA of 0°, indicating that DW is hydrophilic. In addition, the pure TD and TD/DW composites are also hydrophilic. The WCA of pure TD is 19.9°, and the water droplet spread out within 145 s (Video S2). The WCA of the TD/DW composites is 0° 4

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sink within 30 s (Fig. 3b). This result is attributed to the TD/DW composites being hydrophilic, and becoming heavier after absorbing water. Fig. 3c shows the WPG of DW, pure TD, TD/DW composites and superhydrophobic TD/DW composites. The DW, pure TD, TD/DW composites follow the pattern of moisture absorption with a linear increasing at the beginning and keeping stable over prolonged time. The WPGmax values of the DW, pure TD, and TD/DW composites are 187.4%, 14.1%, and 30.21%, respectively. The reason is that DW, pure TD, and TD/DW composites are all hydrophilic. However, the WPGmax of the superhydrophobic TD/DW composites is only 1.2%, indicating that the composites are superhydrophobic and exhibit low moisture absorption. Furthermore, the superhydrophobic TD/DW composites also present excellent self-cleaning properties (Fig. 3d). When the surface of the superhydrophobic TD/DW composites is contaminated by powders, the powders are washed away by continual dropping of water, which cleaned the surface. These results indicate that superhydrophobic TD/DW composites exhibit superhydrophobic and selfcleaning properties, which could prevent TD and DW directly contact with water and protect their original nature unaffected in wet environment.

Table 1 Thermal properties of pure TD, soggy TD, TD/DW composites, soggy TD/DW composites, superhydrophobic TD/DW composites and superhydrophobic TD/ DW100 composites. Samples

Tm (°C)

Tc (°C)

Tt (°C)

Δ Hm (J/g)

Δ Hc+t (J/g)

W (%)

pure TD soggy TD TD/DW soggy TD/DW superhydrophobic TD/DW superhydrophobic TD/DW100

36.76 37.72 36.24 36.27 35.66

36.56 36.56 36.58 36.39 36.54

33.08 33.0 32.18 32.41 31.91

209.2 196.00 125.60 95.82 125.40

204.50 196.10 124.50 93.17 124.60

100 93.69 60.04 45.80 59.94

36.52

36.61

31.72

124.40

121.90

59.46

Note: Tm: The melting temperature; Tc: The liquid-metastable solid phase change temperature; Tt: the metastable solid-solid phase change temperature; Δ Hm: the latent heat of fusion; Δ Hc+t: the latent heat of solidification; W: the ΔH mass fraction of TD loading, which can be calculated by Eq. (1): W = ΔHCPCM s PCM

where W is the mass fraction of TD; ΔHCPCMs is the latent heat of prepared composite phase change materials; ΔHPCM represents the latent heat of pure TD.

phase change, and the peaks at lower temperature are corresponded to the metastable solid-solid phase change. However, only one endothermic peak appears during the melting process. This is because the temperature of the solid-metastable solid and metastable solid-liquid transition is close and indistinguishable. The melting temperature (Tm) of pure TD is 36.76 °C, similar to human body temperature. The liquidmetastable solid phase change temperature (Tc) and metastable solidsolid phase change temperature (Tt) are 36.56 and 33.08 °C, respectively. The latent heat of fusion (Δ Hm) of pure TD is 209.20 J/g.

3.3. Thermal properties of superhydrophobic TD/DW composite PCMs The thermal properties, including phase change temperature and latent heat of fusion, are measured by DSC and the results are shown in Fig. 4a, b and Table 1. There are two clear exothermic peaks during crystallization process because of the existence of metastable intermediate solid phase [9,26]. For the exothermic peaks, the peaks at higher temperature are corresponded to the liquid-metastable solid

Fig. 4. (a) DSC curves of pure TD, soggy TD, TD/DW composites, soggy TD/DW composites and superhydrophobic TD/DW composites. (b) DSC curves of superhydrophobic TD/DW composites before and after 100 heating-cooling cycles, which verifies that superhydrophobic TD/DW composites own excellent thermal reliability. (c) TGA curves of pure TD, soggy TD, TD/DW composites, soggy TD/DW composites and DW. (d) DTG curves of pure TD, soggy TD, TD/DW composites, soggy TD/DW composites and DW. 5

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cellulose) and TD. However, it is obviously observed that soggy TD/DW composites show some liquid leakage at 60 °C, owing to the presence of microcracks between DW and TD (shown in the SEM images of Fig. 2h). Therefore, to ensure the normal use of TD/DW composites in a humid environment, superhydrophobic TD/DW composites are urgently needed. Furthermore, there is no liquid leakage of superhydrophobic TD/DW composites at 60 °C.

Compared with pure TD, the Tm, Tc, and Tt values of soggy TD are close to that of pure TD. However, the Δ Hm of the soggy TD is 196.10 J/g, which is 13.10 J/g less than that of the pure TD. This result is attributed to TD being hydrophilic and hygroscopic. In addition, the phase change behavior and phase change temperature of the TD/DW composites are similar to those of pure TD. The TD/DW composites have high latent heat of fusion (125.60 J/g), which confirms that the TD loading reached 60.04%. When the TD/DW composites were immersed in water, the latent heat of fusion of the soggy TD/DW composites is only 95.82 J/g. The TD/DW composites absorb more water than pure TD, thus reducing Δ Hm values of the soggy TD/DW composites. After spraying the epoxy resin/ modified SiO2 particles, the phase change temperature and the latent heat of fusion (125.40 J/g) are similar to those of the TD/DW composites. Furthermore, when the superhydrophobic TD/DW composites are placed in water, the Δ Hm is unchanged, indicating that the superhydrophobic TD/DW composites absorb no water and exhibit good superhydrophobicity. In addition, the large latent heat of fusion (125.40 J/g) of the superhydrophobic TD/DW composites shows that the composites own high energy storage capacity, giving a broaden prospect for practical applications for thermal energy storage. To investigate the thermal reliability of superhydrophobic TD/DW composites, the 100 heating-cooling thermal cycling tests are performed. Fig. 4b shows DSC curves of the superhydrophobic TD/DW composites before and after 100 cycles, and there is no obvious change on phase change temperature and latent heat of fusion (other than a slight decrease by less than 1 J/g). Therefore, the superhydrophobic TD/DW composites show excellent thermal reliability after many thermal cycles, which means the composites possess long service life.

3.6. Chemical and mechanical stability of the superhydrophobic TD/DW composite PCMs In order to evaluate the superhydrophobicity of the prepared TD/ DW composites at different temperatures and under harsh conditions. Fig. 5c shows the WCAs of the superhydrophobic TD/DW composites at different temperatures. There is no obvious change of the WCAs from 20 to 100 °C and the WCAs remain above 150°, indicating that the superhydrophobicity of the prepared TD/DW composites is not influenced at 20–100 °C. In addition, as shown in Fig. 5d, the WCAs remain above 150° at pH values ranging from 3 to 12. The WCAs are only slightly lower than 150° under strong acid conditions at pH 1 and 2 (144.3° ± 0.5° and 145.8° ± 0.3°, respectively), and strong basic conditions at pH 13 and 14 (149.1° ± 0.9° and 146.7° ± 0.3°, respectively), while the prepared TD/DW composites remain hydrophobic under these conditions. These results demonstrate that the prepared TD/DW composites are superhydrophobic over a wide pH range. Hence, the superhydrophobic TD/DW composites still keep the superhydrophobic property and prevent liquid leakage problem of TD during phase change process in wet or humid environment over wide ranges of temperature and pH. To evaluate the wear resistant of superhydrophobic TD/DW composites, the sandpaper fraction test is performed (the device as shown in Fig. S5), the surface topography and water contact angle after fraction cycles are shown in Figs. S6 and S7. The surface of superhydrophobic coating still keep rough structure after 30 fraction cycles, and the WCAs all remain above 150° before 30 fraction cycles, indicating that the superhydrophobic TD/DW composites exhibit excellent wear resistant, which is favorable for practical application. In addition, the water jet impalement test is carried out to evaluate the mechanical stability (Video S4). After 5 min water blast as the speed of 1500 mL/min (Fig. S8), the surface is still rough (Fig. S9), and the WCA of surface coating still keep 155°, which demonstrates the superhydrophobic coating possesses outstanding mechanical stability to ensure long-term use.

3.4. Thermal stability of superhydrophobic TD/DW composite PCMs Fig. 4c and d show TGA curves and the corresponding DTG curves of pure TD, soggy TD, TD/DW composites, soggy TD/DW composites and DW. The one-step degradation of pure TD starts at approximately 110 °C and ends at 197 °C. Thus, only one peak appears in the corresponding DTG curve, and the maximum weight loss occurs at 197.97 °C. However, the soggy TD shows a two-step degradation, and there is 18.86% weight loss below 60 °C owing to evaporation of water. This is because TD is hydrophilic and hygroscopic. But the existence of water did not affect the intrinsic characteristic of the TD. The TGA curve of the TD/DW composites shows two-stage decomposition processes, including first stage of weight loss attributed to degradation of TD and second stage of weight loss between 200 and 340 °C owing to the evaporation of DW. In addition, it is clearly observed that the TD filling content is 56.55% in the TD/DW composites. Therefore, two peaks appear in the corresponding DTG curves at 168.90 and 323.21 °C, respectively. The TGA curve of soggy TD/DW composites features three thermal degradation steps, including 28.81% weight loss of water from 25 to 100 °C, 44.89% decomposition of TD from 100 to 210 °C, and 20% degradation of DW from 210 to 360 °C. The corresponding three peaks appear in the DTG curve. Thus, the superhydrophobic TD/DW composites possess good thermal stability below 110 °C, which is an appropriate in low-temperature range for practical application in thermal energy storage.

3.7. Solar-to-thermal energy conversion and storage Solar energy, as renewable and pollution-free ideal energy resource, has attracted much attention in terms of its effective utilization and storage. Therefore, investigations of the solar-to-thermal energy conversion and storage of the superhydrophobic TD/DW composites are of great interest. The samples are tested by placing them under simulated solar illumination and recording their temperature variation with an infrared thermal camera. The temperature variation curves with time and corresponding infrared images of the DW and the superhydrophobic TD/DW composites are shown in Fig. 6. When the DW and superhydrophobic TD/DW composites are irradiated under light, they absorb the solar energy and their temperatures increase. Interestingly, there is a platform on the curve of the superhydrophobic TD/DW composites from 37 to 40 °C, corresponding to the solid-liquid phase change of the TD and storage of thermal energy in the form of latent heat. However, no platform emerges in the curve of DW, owing to no effective phase-change working substance. The solar-to- thermal energy storage efficiency (ƞ) determined from the ratio of the stored heat and the received energy in the solar radiation could be calculated with Eq. (1) as:

3.5. Shape stability of the superhydrophobic TD/DW composite PCMs Shape stability is a very important parameter of phase change materials for practical applications. The liquid leakage test is carried out at 20 °C and 60 °C to verify the shape stability of the pure TD, TD/DW composites, soggy TD/DW composites, and superhydrophobic TD/DW composites, as shown in Fig. 5a. When the temperature is 60 °C, above the melting temperature, the solid pure TD melts and does not maintain its shape. However, the TD/DW composites retain their shape without any liquid leakage. This is because of hydrogen bonding and capillary force acting between the DW (the main component of which is 6

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Fig. 5. Liquid leakage test of pure TD, TD/ DW composites, soggy TD/DW composites and superhydrophobic TD/DW composites at (a) 20 °C and (b) 60 °C. It shows the superhydrophobic TD/DW composites possess excellent form-stable property. The diameter of pure TD is 15 mm. The dimensions of these composites are all 20 mm × 20 mm × 3 mm. The water contact angle curves of superhydrophobic TD/ DW composites at different (c) temperature and (d) pH of water droplet, which indicates superhydrophobic TD/DW composites still keep the superhydrophobic property over wide ranges of temperature and pH.

η=

mΔH ρS (tt − t f )

superhydrophobic TD/DW composites is 49.84%, and it could be mentioned that the real energy storage efficiency should be even higher because the sample is exposed to its surroundings without insulation. When the light is turned off, the temperature of the DW and superhydrophobic TD/DW composites decrease rapidly. The other platform appears on the curve of superhydrophobic TD/DW composites, indicating that the liquid-solid phase change takes place and the stored latent heat is released. It takes only 545 s from 80 °C to room temperature for DW, while 1630 s for the superhydrophobic TD/DW

(1)

where m is the quality of the composite PCMs, ΔH is the fusion enthalpy of the composite PCMs, ρ is the intensity of the light irradiation of the simulated light source (400 mW cm−2), S is the surface area of the composite PCMs, tt and tf are the starting and ending points of the phase change process obtained by the tangential method (inset of Fig. 6c). Thus, the calculated energy storage efficiency (ƞ) of the

Fig. 6. Solar-to–thermal energy conversation: (c) temperature-time curves of DW and superhydrophobic TD/DW composites. There are two platforms at temperaturerise and temperature-fall periods on the curve of superhydrophobic TD/DW. (a and b) corresponding infrared thermal images of DW and superhydrophobic TD/DW composites at temperature-rise period and temperature-fall period. 7

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editing the English text of a draft of this manuscript.

composites. Hence, the superhydrophobic TD/DW composites own better thermal insulation performance than DW. In addition, the charging and discharging process could be observed by the temperaturetime curve of the superhydrophobic TD/DW composites (Fig. 6c). There are three stages of the heating and charging process: first, the temperature of the superhydrophobic TD/DW composites increase to 37 °C from room temperature, which shows sensible heat energy storage of solid TD in superhydrophobic TD/DW composites; Second, the temperature remains steady from 37 to 40 °C, indicating the latent heat energy storage of solid TD in superhydrophobic TD/DW composites; Third, the temperature increases rapidly from 40 °C, displaying that sensible heat energy storage of liquid TD, respectively. Likewise, the cooling and discharging processes have three stages, and these stages show the same variations as the curves of the charging process. It only takes 300 s on the charging process. However, it takes 5.4 times as long as the charging process on the discharging process. This fast charging and slow discharging are advantageous for practical applications. All results show that superhydrophobic TD/DW composites own efficient solar-to-thermal energy conversion and storage, which has potential for applications in making effective use of solar energy and saving energy consumption for regulating temperature.

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4. Conclusions In this work, shape-stable composite phase change materials coated with superhydrophobic surface are prepared. The superhydrophobic composite phase change materials not only further prevent liquid leakage and improve thermal energy capacity in humid environments, but also endows the composites with self-cleaning functionality. The superhydrophobic composite phase change materials possess large water contact angle of 155°, excellent wear resistant and superhydrophobic stability at 20–100 °C and pH 3–12. Furthermore, the superhydrophobic composite phase change materials have suitable phase change temperature at 35.66 °C, large energy storage capacity (125.4 J/ g), good thermal reliability after 100 heating-cooling cycles, favorable thermal stability below 110 °C and efficient solar-to-thermal energy conversion. Therefore, superhydrophobic composite phase change materials show great potential in outdoor applications for thermal energy storage. CRediT authorship contribution statement Haiyue Yang: Conceptualization, Writing - original draft. Siyuan Wang: Investigation, Validation. Xin Wang: Writing - review & editing. Weixiang Chao: Validation. Nan Wang: Investigation. Xiaolun Ding: Investigation. Feng Liu: Validation. Qianqian Yu: Resources. Tinghan Yang: Investigation. Zhaolin Yang: Resources. Jian Li: Supervision, Data curation. Chengyu Wang: Supervision, Data curation. Guoliang Li: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was supported by the Fundamental Research Funds for the Central Universities (Grant no.2572019AB14), the National Natural Science Foundation of China (Grant no. 31770605, 31822008), The National Key Research and Development Program of China (Grant no. 2017YFD0600204) and the Outstanding Youth Foundation of Heilongjiang (Grant no. JC2018006). We thank Andrew Jackson, PhD, from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for

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