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polyurethane-based solid-solid phase change materials with graphene aerogel
Solar Energy Materials and Solar Cells 193 (2019) 13–21
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Multifunctional ZnO/polyurethane-based solid-solid phase change materials with graphene aerogel
T
Yan Zhoua,b, Xiaojian Wanga,b, Xiangdong Liua, Dekun Shenga, , Fance Jia,b, Li Donga,b, ⁎ Shaobin Xua,b, Haohao Wua,b, Yuming Yanga, ⁎
a
CAS Key Laboratory of High-Performance Synthetic Rubber and its Composite Materials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b University of Science and Technology of China, Hefei 230026, China
ARTICLE INFO
ABSTRACT
Keywords: Zinc oxide Graphene aerogel Electro-thermal conversion Light-thermal conversion
In this work, multifunctional polyurethane-based phase change materials with zinc oxide (ZnO) and graphene aerogel (GA) were prepared through facile impregnation. ZnO and GA improved the latent heat of the system from 86.8 J/g to 108.1 J/g for the heterogeneous nucleation and chemical mitigations. Furthermore, the photo-/ electro-thermal energy conversion efficiency of the system with ZnO and GA is 80.1%, 84.4%, respectively, for the efficient light absorption of ZnO and GA as well as the electrical conductivity of GA. The as-prepared materials with high latent heat, excellent form-stability, remarkable thermal stability as well as reliability, efficient light-/electro-thermal energy conversion, have great prospects for applying in energy harvesting, conversion and storage devices like solar heater, smart textile and greenhouse.
1. Introduction Nowadays, it is imperative to exploit and utilize green and sustainable energy resources due to the increasingly global grave energy and environmental crisis, which has threatened the survival of human [1–3]. Among these, the renewable and powerful solar energy has gained a lot of attention for its incomparable advantages like no geographical restrictions, inexhaustible, nontoxic, no pollutions and so on [1–4]. However, the fickleness of sunlight renders it necessary to storage and conversion of solar energy efficiently. Compared to high-cost photovoltaic technology with low efficiency, solar-thermal energy batteries are more cost-effective and efficient [2–4]. Phase change materials (PCMs) with high latent heat are ideal candidates for solar energy storage, which could store solar energy through melting and release thermal energy for use via solidification transition when the light is absent [5,6]. Recently, the applications of traditional PCMs such as polyethylene glycol (PEG), paraffin, are much limited by their traditional shortcomings like leakage during melting, no optical absorption, electrical insulation and etc. [6,7]. Extensive efforts have been dedicated to solving the leakage problem including vacuum impregnation [8–10], encapsulation [11], chemical grafting [12,13], blocking [14] as well as crosslinking [15,16]. In contrast to encapsulation with complicated
⁎
procedure and high-cost [7], vacuum impregnation is the most straightforward and simple way to realize the supporting as well as the functionalization of PCMs [9]. Howbeit, there is always desorption after multiple thermal cycles for the weak physical interaction between phase change parts and porous matrix. In this regard, chemical grafting or crosslinking is more reliable to obtain solid-solid phase change materials (SSPCMs) with better chemical and thermal stability [7]. Whereas, SSPCMs fabricated in this way are hard to get rid of these defects containing poor light absorption, electrical insulation [17,18]. For functionalize, tremendous fillers, such as organic dyes [17], metallic nanoparticles [4,19,20], graphene sheets [20–22] and carbon nanotubes [10,23], are usually integrated into the system. The ultralight-weight graphene aerogel (GA), with a three-dimensional (3D) interconnected porous network, has received increasing attention for its wide potential applications in environmental remediation, energy storage and conversion, biomedical applications and so on [24–26]. GA could not only accommodate and support PCMs because of the prominent capillary force and rich porous structure, but also endow multiple functions like electro-/light-thermal energy conversion with composites [27,28]. Zhang et al. fabricated light-driven paraffin/graphene foams composite through vacuum impregnation and the composite exhibited excellent form-stability at 100 °C with only ~3 wt% graphene [28]. Li and his group prepared a kind of GA-directed
Corresponding authors. E-mail addresses: [email protected] (D. Sheng), [email protected] (Y. Yang).
https://doi.org/10.1016/j.solmat.2018.12.041 Received 18 September 2018; Received in revised form 28 December 2018; Accepted 31 December 2018 0927-0248/ © 2019 Elsevier B.V. All rights reserved.
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PCMs smart fiber with multi-response to external stimulus like electrical, thermal and photonic signals [27]. Wu et al. obtained composites of polyurethane (PU) and pitch-based graphite foams with remarkable electro-heat conversion and substantially improved thermal conductivity [18]. Nano zinc oxide (ZnO) is a kind of cost-effective fillers, which has been widely employed in cosmetics, pharmaceutical, photocatalysis, ultraviolet (UV)-protector, sensors and reinforcement filler in ceramics and polymers, owing to its unique electrical and light absorption properties, as well as excellent biocompatibility [29–32]. Sahan et al. investigated the effects of tubular ZnO on the thermal properties of paraffin, and found that the latent heat of paraffin decreased by 7% when adding 10 wt% ZnO [33]. In our previous research [34], we fabricated more functional and stable PCMs using both chemical crosslinking and vacuum impregnation, where the air-dried halloysite nanotubes (HNTs)-GA serves as the supporting skeleton of PCMs as well as light-/electro-thermal energy conversion filler. The latent heat of sample increased by 16.5 J/g for incorporation 0.15 wt% HNTs and 1.02 wt% GA, which is far less than adding 1.02 wt% HNTs in our previous work [15]. The reason may be that the loaded nanoparticles on the skeleton of GA could not give full play to heterogeneous nucleation effects for the uneven dispersion in matrices. Here, we dispersed ZnO particles in the solution of PU precursor and impregnated that into air-dried GA to obtain the composite PCMs. The widespread ZnO particles with abundant hydroxyl could serve as the crosslinker and work synergistically with GA to consolidate the formstability of the system. Besides, ZnO particles with efficient light absorption and electrical conductivity are envisaged to contribute with GA to the optical- /electro-thermal energy conversion of the system. Therefore, the system with comprehensive properties is believed to have potential applications in solar energy harvesting, conversion and storage.
vacuum impregnation method. Firstly, the calculated HDIB was dropped into the mixture of PEG and ZnO to form a homogeneous solution (Fig. 1b). Then, GA was dipped into the resultant under vacuum and kept at 80 °C for 48 h to get the final sample (Fig. 1c-d). The molar ratio of PEG/HDIB is 0.75 in all samples. For comparison, PU composed of PEG and HDIB, composite of ZnO/PU were also prepared in the same way and noted as PCM, ZPCM, respectively. The formula of samples is listed in Table 1. The synthesized GA and PCMs were analyzed using Fourier transform infrared spectrometer (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS), scanning electron microscopy (SEM), differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). For more details, please refer to Supporting Information. 2.4. Light-thermal conversion The light-thermal energy conversion test was carried out using Sun 2000 Solar Simulator (Abet Technologies, USA) equipped with an AM 1.5 filter and an optical power meter (Cel-NP 2000, Ceaulight, China). About 1.0 g sample was embedded in insulation foams sealed with a transparent glass. The temperature evolution inside sample was recorded by a recorder with a thermocouple. The schematic illustration of the experimental setup is shown in Fig. S1. 2.5. Electro-thermal conversion The electro-thermal energy conversion tests were implemented using a Keithley power supply (Fig. S2). Two sheets of copper foils were stuck on both ends of a brick of sample, acting as electrodes, with the assistance of conductive silver paste. The temperature evolution of samples under voltage supplying was recorded by a recorder equipped with a thermocouple. 3. Results and discussions
2. Experimental
3.1. Characterization of air-dried GA
2.1. Materials
The as-prepared GA is light weight (ρ~ 4.3 mg/cm3) with continuous 3D porous network, which could stand easily on the leaves of asparagus fern (Fig. 2a-b). Owing to the pore size of GA is in the order of hundreds of micrometers (100–700 µm), GA could be air-dried [36], which is of great importance to mass production and shorten the production period. The XPS spectra and XRD patterns (Fig. 2c-d) confirm that the GO has been well reduced in GA, ensuring the partial recovery of electrical conductivity and photo-thermal energy conversion ability. For GO, the intensity of C1s peak is apparently lower than that of O1s peak, due to the introduction of oxygen-containing groups. However, the intensity of C1s peak increases so sharply that the intensity of O1s is pretty lower than that of C1s peak in GA (Fig. 2c), as a result of the elimination of oxygen functional groups. The XRD patterns (Fig. 2d) reveal that the characteristic diffraction peak at 2θ = 10.7° in GO has vanished and a new diffraction peak of graphitization emerges at 2θ = 23.6° in GA, which further indicates the reduction of GO.
Graphene oxide (GO) was prepared through modified Hummer's method [16,21,35]. Polyethylene glycol (PEG, Mn=4000), purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China, was dried under vacuum at 110 °C for 3 h before use. N, N-Dimethylformamide (DMF), obtained from Beijing Chemical Works, China, was distilled under vacuum after refluxed and stirred overnight. L-ascorbic acid (LAA) and ZnO (30 ± 10 nm) were received from Aladdin and used directly. Hexamethylene Diisocyanate Biuret (HDIB, N75, with NCO% =15.6%) was purchased from Bayer (Germany) and used as received. 2.2. Preparation of air-dried GA The air-dried GA was fabricated from GO dispersion according to the literature [34,36], which is illustrated in Fig. 1a1-a2. Typically, the mixture of GO solution (4 mg/ml, 10 ml), dispersed with 80 mg L-AA, was sealed into a glass vial and heated for 45 min at 95 °C to get the partially reduced GO (pr-GO) hydrogel. Secondly, the hydrogel was cooled down to room temperature and then subjected to freeze-thaw process in fridge (-18 °C) and room temperature twice. Thirdly, the thawed hydrogel was heated again at 95 °C for 6 h to obtain the completely reduced GO hydrogel. And then, the hydrogel was dialyzed in water to remove unreacted reagent and subjected to drying at 70 °C to get the air-dried GA. Finally, GA was annealed at 200 °C for 2 h for further reduction.
3.2. Chemical structure and morphologies of PCMs Fig. 3 shows the FT-IR spectra of samples. In PCM, the characteristic absorption peaks of C-H bonds appear at 2888, 1468, 962 and 842 cm−1. Peak at 3344 cm−1 is ascribed to the associated symmetrical vibration of N-H group in PCM [37,38]. While the hydrogen-bonded C˭O stretching absorption peaks in –NH-COO- and –NH-CO-NH- group appear at 1716 and 1687 cm−1 [37,38], respectively, implying the successful synthesis of PU. In addition, the sharp peak at 1103 cm−1 belongs to the C-O-C stretching vibration. However, the absorption peak of N-H shifts from 3344 cm−1 to 3365 and 3379 cm−1 in ZPCM and GZPCM, respectively. What's more, the C˭O stretching absorption
2.3. Synthesis of composite PCM Here, the composite, noted as GZPCM, was prepared through facile 14
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Fig. 1. Schematic illustration of the fabrication procedure of GZPCM.
1111 cm−1 in ZPCM and GZPCM, respectively. The shifts of these characteristic peaks indicate the weakened hydrogen bonding in the system when combining with ZnO and GA [37,38]. That is maybe the results of disruption and barrier effects of dispersed ZnO as well as the 3D network structure of GA. As shown in Fig. 3, the peak at 3434 cm−1 is ascribed to characteristic absorption of –OH in ZnO, which could react with –NCO group [31]. Therefore, when ZnO nanoparticles were dispersed into the reaction system, HDIB would react randomly with –OH group on PEG and ZnO. At that time, ZnO serves as fixer and crosslinking agent that immobilizes the PU chains and disturbs the original structure of PCM. In like manner, the residual –OH group on
Table 1 Formula of composite PCM. samples
PCM wt%
GA wt%
ZnO wt%
PCM ZPCM GZPCM
100.00 98.99 97.71
0.00 0.00 1.32
0.00 1.01 0.97
peaks at 1716 and 1687 cm−1 shift to 1717, 1689 and 1718, 1690 cm−1 in ZPCM and GZPCM, respectively. Besides, the characteristic absorption peak of C-O-C bond changes from 1103 cm−1 to 1109 and
Fig. 2. (a) GA standing on asparagus fern, (b) SEM image of GA, XPS spectra (c) and XRD patterns of GO and GA. 15
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3.3. Phase change performances of PCMs Form-stability is of great importance for actual applications. PCM and ZPCM are white and opaque at room temperature, while GZPCM is black for the existence of GA (Fig. 5). When heated at 100 °C for 1 h, the sheets of PCM and ZPCM transform to transparent and elastic, implying the melting of rigid crystals. The appearance of GZPCM has slight change for the deep black. There is no trace of leakage. In actual, the remarkable solid-solid phase transition performance is attributed to the encapsulation of GA and the chemical crosslinking among system. The energy storage capacity is highly dependent on the phase change properties, which could be assessed by DSC and XRD. As shown in Fig. 6a, there are some differences among the DSC curves for samples. The phase change data are listed in Table 2. PCM possesses a melting enthalpy of 86.8 J/g and the peak temperature for melting and crystallization is 48.4 and 24.2 °C, respectively, which is much lower than that of pure PEG, whose melting and crystallization temperature is 56.9 °C, 37.1 °C, respectively and the latent heat is 182.7 J/g [15]. The latent heat, melting and crystallization temperature of ZPCM are 101.5 J/g, 55.3 and 32.1 °C, respectively. While infiltrating into GA, the phase change enthalpy, melting and crystallization temperature are 108.1 J/g, 57.1 and 34.6 °C, respectively. The difference among these PCMs in phase change properties is up to the crystallization ability of PEG segments in the system, which could be reflected by a parameter of releatively crystallization fraction Xc. The equation [15] is given by
Fig. 3. FT-IR spectra of samples.
GA (peak at 3436 cm−1) also could participate in the construction of the composite PCM [21]. Moreover, the continuous network of GA with reduced GO sheets hampers the interaction of PU chains for little oxygen-containing groups on the wall cell. In this case, the initial structure of PCM is disrupted more severely. As a result, the hydrogen bonding in PU would be weakened to a certain extent. Fig. 4 displays the morphologies of fracture surface and the energy dispersive spectra (EDS) of samples. The cross-section of PCM exhibits irregular wrinkles due to the crystallization of PEG segments (Fig. 4a). While the fractured surface of ZPCM becomes rougher with numerous ZnO particles distributed homogeneously (Fig. 4b). The EDS spectrum and the Zn element mapping image shown in Fig. 4e further confirm that the ZnO dispersed in PU uniformly. The cross-section of GZPCM performs a little difference from ZPCM, where network lines distribute on the rough surface (Fig. 4c), which maybe the skeleton of GA. The element mapping and EDS spectrum indicate that ZnO particles dispersed homogeneously in GZPCM (Figs. 4f).
Xc =
Hm × 100% x· Hm0
(1)
ΔHm, ΔHm0 and x are the latent heats of the composite, pure PCM and the mass fraction of PCM in the composite system, respectively. Providing that the Xc of PCM is 100.0%, the Xc of ZPCM and GZPCM is 118.1% and 127.5%, respectively, which has a 18.1% and 27.5% increase, respectively. In Fig. 6b, two sharp diffraction peaks at 2θ= 19.1 and 23.2°, which correspond to the PEG lattice plane of (120）and (13¯2) [15], respectively, appear in all samples, implying the well crystallization of PEG. When incorporation of nano ZnO particles, the intensity of diffraction peaks increases obviously, and three new peaks at 2θ = 32.4, 34.5 and 36.1° emerge in ZPCM, ascribed to the (100), (002) and (101) crystal planes of ZnO [32], respectively, which suggests the
Fig. 4. Morphologies of fracture surface of (a) PCM, (b) ZPCM, (c) GZPCM, and EDS spectra of (d) PCM, (e) ZPCM, (f) GZPCM (the embedded is Zn element mapping image).
16
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Fig. 5. Appearances of (a) PCM, (b) ZPCM, (c) GZPCM at room temperature and 100 °C for 1 h (a´-c´).
Fig. 6. (a) DSC curves and XRD patterns of PCMs.
as heterogeneous nucleation agent [32], which provide numerous nucleation sites for the confined PEG segments. Thus, there is no more self-nucleation under lower temperature for these segments could crystallize easily on the surface of ZnO at a higher temperature. Secondly, the -OH group on ZnO could consume a part of -NCO group [31], which results in the reduced -NCO group reacting directly with PEG segments. Consequently, the straight chemical confinement of PEG is relived. What's more, ZnO disrupts hydrogen bonding among PU chains (Fig. 3), which further reduces the restrictions of PEG segments. Hence, the parts that could crystallize increase for the reducing of confinements of PEG, resulting in more crystals with less defects and enhanced latent heat. The more complete the crystals are, the more energy they need to melt, which leads in higher melting temperature. In the same way, when GA immersed in the mixture of ZnO, PEG and HDIB, the residual -OH group on GA walls also takes part in the reaction with -NCO group. Based on this, the -NCO group left to PEG segments are bound to reduce. Hence, the chemical constraints of PEG get further alleviated. Besides, the 3D continuous network of GA with
Table 2 Phase change data of PCMs. samples
PCM wt%
Tm(°C)
ΔHm(J/g)
Tc(°C)
ΔHc(J/g)
Xc(%)
PCM ZPCM GZPCM
100.00 99.00 97.71
48.4 55.3 57.1
86.8 101.5 108.1
24.2 32.1 34.6
85.8 99.9 106.7
100.0 118.1 127.5
existence of ZnO. While after impregnating into GA, the intensity of characteristic peaks of PEG increases further and the peaks of ZnO become too vigue to be figured out. The enhanced latent heat, phase change temperature as well as the diffraction intensity all reveal that the crystallization ability of PEG segments in system has been improved to a certain extent for compounding with ZnO and GA. This prominent elevation of phase change properties gives the credit to the heterogeneous nucleation and the mitigation of confinement from ZnO and GA. Firstly, the uniformly dispersed ZnO particles work 17
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Table 3 Phase change data and efficiency for light-/electro-thermal energy conversion and storage. Ref.
Composition
ΔHm of PEG (J/g)
ΔHm of PCMs (J/g)
ηl-t (%)
ηe-t (%)
[17] [18] [20] [21] [27] [34] Present work
MPEG/Polyamine/Dye R PEG/HDI/27 wt% PGF PEG/TDI/1–4 wt% Fe3O4-GNS PEG/HDIB/9.1 wt% RGO PEG/16 wt% ASF PEG/HDIB/1.17 wt% HNTs-GA PEG/HDIB/1.32 wt%GA/0.97 wt% ZnO
132.1 175.6 184.5 182.7 188.4 182.7 182.7
98.9 60.3 100.9 107.2 124 103.3 108.1
94.1 unknown 72.7–92.3 78.7 unknown 78.4 80.1
unknown 85 unknown unknown unknown 66.3 84.4
little oxygen-containing groups imbedded in the composite matrix, has little interaction with PU segments and could serve as natural barrier hindering the closely interaction among PU chains [27,34], which could also be proved by the weaken hydrogen bonding interaction in GZPCM (Fig. 3). Besides, the wrinkle surface of GA wall cell could also provide abundant nucleation sites for PEG segments, elevating the crystallization temperature of the system. PEG segments in GZPCM suffer less chemical and physical constraints and accordingly could arrange and crystallize more easily to form better crystals. In turn, owing to the enhanced crystallization properties, more energy is needed to destroy more crystals with less defects, resulting in higher melting temperature. Table 3 lists the phase change data of PEG-based multifunctional composites in literature. Here, ηl-t and ηe-t are the efficiency for lightthermal and electro-thermal conversion and storage, respectively. It can be noted that the phase change enthalpy in present work is better than that in most of study, especially PU-based system, suggesting the synthesized PCMs have excellent thermal energy storage capacity.
thermal energy (purple curve with arrow) to heat the PU matrix. Thus, the temperature of samples rises rapidly. The converted light energy is stored in the form of thermal energy though the melting of the crystals in system, which suffer from a consecutive flow of heat for a period time. The electro-thermal conversion and storage could be realized owing to the continuous 3D conductive network of GA. The temperature-time curves of ZPCM and GZPCM under 15 V supply, are displayed in Fig. 7b. The temperature of ZPCM has no change when loading power supply for the huge resistivity of sample because the conductive pathway does not form with only 1 wt% ZnO. The temperature of GZPCM rises rapidly when the power is switched on. Immediately, a temperature plateau during 55–60 °C appears, suggesting the electro-thermal energy conversion and storage. When crystals have melted, the temperature rises again. Once turning off the power, the temperature drops sharply until a plateau emerges, which means the solidification of PEG segments proceeding. The electro-thermal conversion efficiency (η) could be calculated by Eq. (3) [10]
=
3.4. Light- and electro-thermal conversions
m H S (tt t f )
(3)
ΔH and m are the latent heat obtained by DSC and mass of the sample, respectively. U is the voltage applied to sample, and I is the electric current. tt and tf are the starting and terminating time before and after the phase transition, respectively. The calculated η for electrothermal energy conversion of GZPCM under 15 V is 84.4%. The mechanism of electro-thermal energy conversion and storage is shown in Fig. 7c. When the power supply is loaded on the system, the resistance of ZPCM is so large that the current is close to zero. Hence, little Joule heat is produced. In GZPCM, the current could flow from one electrode to another through the 3D continuous skeleton of GA, composed of graphene wall cells like shown in Fig. 2b. Meanwhile a lot of Joule heat (purple curve) is produced and transported to the ambient PU matrix, resulting in the climb of temperature until it reaches the melting temperature of crystals, when massive heat is absorbed and stored through the melting of crystals. Further being charged with electricity, the temperature of the system keeps increasing. Although the η for light-thermal energy conversion and storage is much lower than that reported in lots of literature in Table 3. However, GZPCM realizes both η that more than 80% in light-/electro-thermal energy conversion with only 1.32 wt% GA and 0.97 wt% ZnO. This kind of materials would be potential to be used in smart devices like agricultural greenhouse, textile, solar heater and etc., which could be charged by sunlight or electricity. During daylight, the system could store the surplus solar energy through melting. At night, the stored energy could be released for work. If there is a rainy season, the devices could be charged by electricity to work.
The light-thermal conversion test was carried out under simulated solar illumination (AM 1.5) at a constant intensity of 100 mW/cm2, and the temperature evolution inside the sample was recorded by a data recorder equipped with thermocouple [21]. Under solar irradiation, the light is immediately absorbed by ZnO and GA and converted to heat simultaneously, which results in the temperature rise of ZPCM and GZPCM, as shown in Fig. 7a. The temperature of ZPCM and GZPCM evolves in a similar trend. Under the illumination of light, the temperature of samples first rises rapidly and then slows down to reach a plateau for melting of crystals and following rises quickly. Once light off, the temperature descends hastily and then decreases gently to reach a plateau, which corresponds to the solidification of PCM. Howbeit, the time to reach the same temperature for ZPCM always lags behind that of GZPCM. For instance, it takes 1725 s for GZPCM to reach 71 °C under light irradiation, but the temperature of ZPCM is only 55.8 °C at that time. Furthermore, it requires 2093 s′ irradiation for ZPCM to reach 65.5 °C. The light-thermal conversion and storage efficiency (η) can be calculated by the Eq. (2) [21] as following.
=
m H UI (tt t f )
(2)
Here, m is the mass of the sample, ΔH is the latent heat obtained by DSC, ρ is the intensity of light irradiation of the simulated solar light source (100 mW/cm2 @AM 1.5), S is the surface area of sample, tt and tf are the starting and terminating time before and after the phase transition, respectively. The calculated η of ZPCM and GZPCM is 46.9%, 80.1%, respectively. The higher η for GZPCM is attributed to the whole band light absorption of GA [24,25], and the synergistic effect of ZnO, which could absorb UV light efficiently [31]. The process of lightthermal energy conversion and storage is illustrated in Fig. 7c. When exposed to simulated solar light, the dispersed ZnO nanoparticles and GA with connected 3D network could absorb light and convert it into
3.5. Thermal stability and reliability Fig. 8 displays the TGA curves and DSC curves before and after 100 thermal cycling. The first slight thermal decomposition of PCM appears at around 286 °C, which corresponds to the temperature at 5% weight 18
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Fig. 7. Temperature-time curves of PCMs under (a) sunlight radiation at 100 mW/cm2 and (b) under a power supply of 15 V, (c) schematic illustration of the mechanism of light- and electro-thermal conversion and storage.
loss, due to the decomposition of urethane and biuret groups. The second degradation stage for PCM emerges at 410 °C (temperature at maximum weight loss rate), which is consistent with the decomposition of PEG. While the temperature at 5% weight loss of ZPCM and GZPCM is 290 and 330 °C, which is 4 and 48 °C higher than that of PCM. Besides, the temperature at 10% weight loss for PCM increase by 2 and 18 °C from 341 °C, in ZPCM and GZPCM, respectively. Although the temperature at 5% and 10% weight loss of ZPCM and GZPCM is a higher than that of PCM, the temperature at maximum weight loss rate exhibits almost no viriation. Whereas, ZPCM degrades the fastest at the temperature lower than 260 °C and between 350 and 400 °C. In comparision, GZPCM performs better all the time in thermal stability than ZPCM and PCM. The reasons for this phenomenon are the compititive effects of thermal barrier and formation of free oxygen and oxygen vancancies of ZnO under thermal exicitation [39]. When the former dominates, the thermal stability will be improved. While the latter dominates, the thermal stability will decrease. The 3D continuous inflammable network of GA with large wall sheets serves as thermal barrier inhibiting the emission of the decomposition products. Hence, the thermal stability of GZPCM is always better than that of PCM and ZPCM.
Fig. 8b-d displays the DSC curves of the samples after 100 phase change cycling. There are only very slight variations in phase transition temperature and latent heat of PCMs after 100 thermal cycles, which infers the good thermal reliability of products. 4. Conclusions Polyurethane-based composite phase change materials incorporation of ZnO and GA were prepared through facile impregnation. The latent heat, phase transition temperatures of the system increased from 86.8 J/g, 24.2 °C to 101.5 J/g and 32.1 °C, 108.1 J/g and 34.6 °C for incorporation of ZnO, both ZnO and GA, respectively. Besides, the light-/electro-thermal energy conversion efficiency of the system with ZnO and GA is 80.1%, 84.4%, respectively, for the efficient light absorption of ZnO and GA as well as the electricity conductive pathways of GA. This system with high latent heat, form-stablility, excellent thermal stability and thermal reliability, efficient light-/electro-thermal conversion and storage, has great potentials to be applied in solar energy conversion and storage, smart textile and other energy storage devices. 19
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Fig. 8. (a) TGA curves of PCMs and DSC curves of (b) PCM (c) ZPCM (d) GZPCM before and after 100 thermal cycle.
Appendix A. Supporting information [14]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2018.12.041.
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Multifunctional ZnO/polyurethane-based solid-solid phase change materials with graphene aerogel
T
Yan Zhoua,b, Xiaojian Wanga,b, Xiangdong Liua, Dekun Shenga, , Fance Jia,b, Li Donga,b, ⁎ Shaobin Xua,b, Haohao Wua,b, Yuming Yanga, ⁎
a
CAS Key Laboratory of High-Performance Synthetic Rubber and its Composite Materials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b University of Science and Technology of China, Hefei 230026, China
ARTICLE INFO
ABSTRACT
Keywords: Zinc oxide Graphene aerogel Electro-thermal conversion Light-thermal conversion
In this work, multifunctional polyurethane-based phase change materials with zinc oxide (ZnO) and graphene aerogel (GA) were prepared through facile impregnation. ZnO and GA improved the latent heat of the system from 86.8 J/g to 108.1 J/g for the heterogeneous nucleation and chemical mitigations. Furthermore, the photo-/ electro-thermal energy conversion efficiency of the system with ZnO and GA is 80.1%, 84.4%, respectively, for the efficient light absorption of ZnO and GA as well as the electrical conductivity of GA. The as-prepared materials with high latent heat, excellent form-stability, remarkable thermal stability as well as reliability, efficient light-/electro-thermal energy conversion, have great prospects for applying in energy harvesting, conversion and storage devices like solar heater, smart textile and greenhouse.
1. Introduction Nowadays, it is imperative to exploit and utilize green and sustainable energy resources due to the increasingly global grave energy and environmental crisis, which has threatened the survival of human [1–3]. Among these, the renewable and powerful solar energy has gained a lot of attention for its incomparable advantages like no geographical restrictions, inexhaustible, nontoxic, no pollutions and so on [1–4]. However, the fickleness of sunlight renders it necessary to storage and conversion of solar energy efficiently. Compared to high-cost photovoltaic technology with low efficiency, solar-thermal energy batteries are more cost-effective and efficient [2–4]. Phase change materials (PCMs) with high latent heat are ideal candidates for solar energy storage, which could store solar energy through melting and release thermal energy for use via solidification transition when the light is absent [5,6]. Recently, the applications of traditional PCMs such as polyethylene glycol (PEG), paraffin, are much limited by their traditional shortcomings like leakage during melting, no optical absorption, electrical insulation and etc. [6,7]. Extensive efforts have been dedicated to solving the leakage problem including vacuum impregnation [8–10], encapsulation [11], chemical grafting [12,13], blocking [14] as well as crosslinking [15,16]. In contrast to encapsulation with complicated
⁎
procedure and high-cost [7], vacuum impregnation is the most straightforward and simple way to realize the supporting as well as the functionalization of PCMs [9]. Howbeit, there is always desorption after multiple thermal cycles for the weak physical interaction between phase change parts and porous matrix. In this regard, chemical grafting or crosslinking is more reliable to obtain solid-solid phase change materials (SSPCMs) with better chemical and thermal stability [7]. Whereas, SSPCMs fabricated in this way are hard to get rid of these defects containing poor light absorption, electrical insulation [17,18]. For functionalize, tremendous fillers, such as organic dyes [17], metallic nanoparticles [4,19,20], graphene sheets [20–22] and carbon nanotubes [10,23], are usually integrated into the system. The ultralight-weight graphene aerogel (GA), with a three-dimensional (3D) interconnected porous network, has received increasing attention for its wide potential applications in environmental remediation, energy storage and conversion, biomedical applications and so on [24–26]. GA could not only accommodate and support PCMs because of the prominent capillary force and rich porous structure, but also endow multiple functions like electro-/light-thermal energy conversion with composites [27,28]. Zhang et al. fabricated light-driven paraffin/graphene foams composite through vacuum impregnation and the composite exhibited excellent form-stability at 100 °C with only ~3 wt% graphene [28]. Li and his group prepared a kind of GA-directed
Corresponding authors. E-mail addresses: [email protected] (D. Sheng), [email protected] (Y. Yang).
https://doi.org/10.1016/j.solmat.2018.12.041 Received 18 September 2018; Received in revised form 28 December 2018; Accepted 31 December 2018 0927-0248/ © 2019 Elsevier B.V. All rights reserved.
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PCMs smart fiber with multi-response to external stimulus like electrical, thermal and photonic signals [27]. Wu et al. obtained composites of polyurethane (PU) and pitch-based graphite foams with remarkable electro-heat conversion and substantially improved thermal conductivity [18]. Nano zinc oxide (ZnO) is a kind of cost-effective fillers, which has been widely employed in cosmetics, pharmaceutical, photocatalysis, ultraviolet (UV)-protector, sensors and reinforcement filler in ceramics and polymers, owing to its unique electrical and light absorption properties, as well as excellent biocompatibility [29–32]. Sahan et al. investigated the effects of tubular ZnO on the thermal properties of paraffin, and found that the latent heat of paraffin decreased by 7% when adding 10 wt% ZnO [33]. In our previous research [34], we fabricated more functional and stable PCMs using both chemical crosslinking and vacuum impregnation, where the air-dried halloysite nanotubes (HNTs)-GA serves as the supporting skeleton of PCMs as well as light-/electro-thermal energy conversion filler. The latent heat of sample increased by 16.5 J/g for incorporation 0.15 wt% HNTs and 1.02 wt% GA, which is far less than adding 1.02 wt% HNTs in our previous work [15]. The reason may be that the loaded nanoparticles on the skeleton of GA could not give full play to heterogeneous nucleation effects for the uneven dispersion in matrices. Here, we dispersed ZnO particles in the solution of PU precursor and impregnated that into air-dried GA to obtain the composite PCMs. The widespread ZnO particles with abundant hydroxyl could serve as the crosslinker and work synergistically with GA to consolidate the formstability of the system. Besides, ZnO particles with efficient light absorption and electrical conductivity are envisaged to contribute with GA to the optical- /electro-thermal energy conversion of the system. Therefore, the system with comprehensive properties is believed to have potential applications in solar energy harvesting, conversion and storage.
vacuum impregnation method. Firstly, the calculated HDIB was dropped into the mixture of PEG and ZnO to form a homogeneous solution (Fig. 1b). Then, GA was dipped into the resultant under vacuum and kept at 80 °C for 48 h to get the final sample (Fig. 1c-d). The molar ratio of PEG/HDIB is 0.75 in all samples. For comparison, PU composed of PEG and HDIB, composite of ZnO/PU were also prepared in the same way and noted as PCM, ZPCM, respectively. The formula of samples is listed in Table 1. The synthesized GA and PCMs were analyzed using Fourier transform infrared spectrometer (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS), scanning electron microscopy (SEM), differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). For more details, please refer to Supporting Information. 2.4. Light-thermal conversion The light-thermal energy conversion test was carried out using Sun 2000 Solar Simulator (Abet Technologies, USA) equipped with an AM 1.5 filter and an optical power meter (Cel-NP 2000, Ceaulight, China). About 1.0 g sample was embedded in insulation foams sealed with a transparent glass. The temperature evolution inside sample was recorded by a recorder with a thermocouple. The schematic illustration of the experimental setup is shown in Fig. S1. 2.5. Electro-thermal conversion The electro-thermal energy conversion tests were implemented using a Keithley power supply (Fig. S2). Two sheets of copper foils were stuck on both ends of a brick of sample, acting as electrodes, with the assistance of conductive silver paste. The temperature evolution of samples under voltage supplying was recorded by a recorder equipped with a thermocouple. 3. Results and discussions
2. Experimental
3.1. Characterization of air-dried GA
2.1. Materials
The as-prepared GA is light weight (ρ~ 4.3 mg/cm3) with continuous 3D porous network, which could stand easily on the leaves of asparagus fern (Fig. 2a-b). Owing to the pore size of GA is in the order of hundreds of micrometers (100–700 µm), GA could be air-dried [36], which is of great importance to mass production and shorten the production period. The XPS spectra and XRD patterns (Fig. 2c-d) confirm that the GO has been well reduced in GA, ensuring the partial recovery of electrical conductivity and photo-thermal energy conversion ability. For GO, the intensity of C1s peak is apparently lower than that of O1s peak, due to the introduction of oxygen-containing groups. However, the intensity of C1s peak increases so sharply that the intensity of O1s is pretty lower than that of C1s peak in GA (Fig. 2c), as a result of the elimination of oxygen functional groups. The XRD patterns (Fig. 2d) reveal that the characteristic diffraction peak at 2θ = 10.7° in GO has vanished and a new diffraction peak of graphitization emerges at 2θ = 23.6° in GA, which further indicates the reduction of GO.
Graphene oxide (GO) was prepared through modified Hummer's method [16,21,35]. Polyethylene glycol (PEG, Mn=4000), purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China, was dried under vacuum at 110 °C for 3 h before use. N, N-Dimethylformamide (DMF), obtained from Beijing Chemical Works, China, was distilled under vacuum after refluxed and stirred overnight. L-ascorbic acid (LAA) and ZnO (30 ± 10 nm) were received from Aladdin and used directly. Hexamethylene Diisocyanate Biuret (HDIB, N75, with NCO% =15.6%) was purchased from Bayer (Germany) and used as received. 2.2. Preparation of air-dried GA The air-dried GA was fabricated from GO dispersion according to the literature [34,36], which is illustrated in Fig. 1a1-a2. Typically, the mixture of GO solution (4 mg/ml, 10 ml), dispersed with 80 mg L-AA, was sealed into a glass vial and heated for 45 min at 95 °C to get the partially reduced GO (pr-GO) hydrogel. Secondly, the hydrogel was cooled down to room temperature and then subjected to freeze-thaw process in fridge (-18 °C) and room temperature twice. Thirdly, the thawed hydrogel was heated again at 95 °C for 6 h to obtain the completely reduced GO hydrogel. And then, the hydrogel was dialyzed in water to remove unreacted reagent and subjected to drying at 70 °C to get the air-dried GA. Finally, GA was annealed at 200 °C for 2 h for further reduction.
3.2. Chemical structure and morphologies of PCMs Fig. 3 shows the FT-IR spectra of samples. In PCM, the characteristic absorption peaks of C-H bonds appear at 2888, 1468, 962 and 842 cm−1. Peak at 3344 cm−1 is ascribed to the associated symmetrical vibration of N-H group in PCM [37,38]. While the hydrogen-bonded C˭O stretching absorption peaks in –NH-COO- and –NH-CO-NH- group appear at 1716 and 1687 cm−1 [37,38], respectively, implying the successful synthesis of PU. In addition, the sharp peak at 1103 cm−1 belongs to the C-O-C stretching vibration. However, the absorption peak of N-H shifts from 3344 cm−1 to 3365 and 3379 cm−1 in ZPCM and GZPCM, respectively. What's more, the C˭O stretching absorption
2.3. Synthesis of composite PCM Here, the composite, noted as GZPCM, was prepared through facile 14
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Fig. 1. Schematic illustration of the fabrication procedure of GZPCM.
1111 cm−1 in ZPCM and GZPCM, respectively. The shifts of these characteristic peaks indicate the weakened hydrogen bonding in the system when combining with ZnO and GA [37,38]. That is maybe the results of disruption and barrier effects of dispersed ZnO as well as the 3D network structure of GA. As shown in Fig. 3, the peak at 3434 cm−1 is ascribed to characteristic absorption of –OH in ZnO, which could react with –NCO group [31]. Therefore, when ZnO nanoparticles were dispersed into the reaction system, HDIB would react randomly with –OH group on PEG and ZnO. At that time, ZnO serves as fixer and crosslinking agent that immobilizes the PU chains and disturbs the original structure of PCM. In like manner, the residual –OH group on
Table 1 Formula of composite PCM. samples
PCM wt%
GA wt%
ZnO wt%
PCM ZPCM GZPCM
100.00 98.99 97.71
0.00 0.00 1.32
0.00 1.01 0.97
peaks at 1716 and 1687 cm−1 shift to 1717, 1689 and 1718, 1690 cm−1 in ZPCM and GZPCM, respectively. Besides, the characteristic absorption peak of C-O-C bond changes from 1103 cm−1 to 1109 and
Fig. 2. (a) GA standing on asparagus fern, (b) SEM image of GA, XPS spectra (c) and XRD patterns of GO and GA. 15
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3.3. Phase change performances of PCMs Form-stability is of great importance for actual applications. PCM and ZPCM are white and opaque at room temperature, while GZPCM is black for the existence of GA (Fig. 5). When heated at 100 °C for 1 h, the sheets of PCM and ZPCM transform to transparent and elastic, implying the melting of rigid crystals. The appearance of GZPCM has slight change for the deep black. There is no trace of leakage. In actual, the remarkable solid-solid phase transition performance is attributed to the encapsulation of GA and the chemical crosslinking among system. The energy storage capacity is highly dependent on the phase change properties, which could be assessed by DSC and XRD. As shown in Fig. 6a, there are some differences among the DSC curves for samples. The phase change data are listed in Table 2. PCM possesses a melting enthalpy of 86.8 J/g and the peak temperature for melting and crystallization is 48.4 and 24.2 °C, respectively, which is much lower than that of pure PEG, whose melting and crystallization temperature is 56.9 °C, 37.1 °C, respectively and the latent heat is 182.7 J/g [15]. The latent heat, melting and crystallization temperature of ZPCM are 101.5 J/g, 55.3 and 32.1 °C, respectively. While infiltrating into GA, the phase change enthalpy, melting and crystallization temperature are 108.1 J/g, 57.1 and 34.6 °C, respectively. The difference among these PCMs in phase change properties is up to the crystallization ability of PEG segments in the system, which could be reflected by a parameter of releatively crystallization fraction Xc. The equation [15] is given by
Fig. 3. FT-IR spectra of samples.
GA (peak at 3436 cm−1) also could participate in the construction of the composite PCM [21]. Moreover, the continuous network of GA with reduced GO sheets hampers the interaction of PU chains for little oxygen-containing groups on the wall cell. In this case, the initial structure of PCM is disrupted more severely. As a result, the hydrogen bonding in PU would be weakened to a certain extent. Fig. 4 displays the morphologies of fracture surface and the energy dispersive spectra (EDS) of samples. The cross-section of PCM exhibits irregular wrinkles due to the crystallization of PEG segments (Fig. 4a). While the fractured surface of ZPCM becomes rougher with numerous ZnO particles distributed homogeneously (Fig. 4b). The EDS spectrum and the Zn element mapping image shown in Fig. 4e further confirm that the ZnO dispersed in PU uniformly. The cross-section of GZPCM performs a little difference from ZPCM, where network lines distribute on the rough surface (Fig. 4c), which maybe the skeleton of GA. The element mapping and EDS spectrum indicate that ZnO particles dispersed homogeneously in GZPCM (Figs. 4f).
Xc =
Hm × 100% x· Hm0
(1)
ΔHm, ΔHm0 and x are the latent heats of the composite, pure PCM and the mass fraction of PCM in the composite system, respectively. Providing that the Xc of PCM is 100.0%, the Xc of ZPCM and GZPCM is 118.1% and 127.5%, respectively, which has a 18.1% and 27.5% increase, respectively. In Fig. 6b, two sharp diffraction peaks at 2θ= 19.1 and 23.2°, which correspond to the PEG lattice plane of (120）and (13¯2) [15], respectively, appear in all samples, implying the well crystallization of PEG. When incorporation of nano ZnO particles, the intensity of diffraction peaks increases obviously, and three new peaks at 2θ = 32.4, 34.5 and 36.1° emerge in ZPCM, ascribed to the (100), (002) and (101) crystal planes of ZnO [32], respectively, which suggests the
Fig. 4. Morphologies of fracture surface of (a) PCM, (b) ZPCM, (c) GZPCM, and EDS spectra of (d) PCM, (e) ZPCM, (f) GZPCM (the embedded is Zn element mapping image).
16
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Fig. 5. Appearances of (a) PCM, (b) ZPCM, (c) GZPCM at room temperature and 100 °C for 1 h (a´-c´).
Fig. 6. (a) DSC curves and XRD patterns of PCMs.
as heterogeneous nucleation agent [32], which provide numerous nucleation sites for the confined PEG segments. Thus, there is no more self-nucleation under lower temperature for these segments could crystallize easily on the surface of ZnO at a higher temperature. Secondly, the -OH group on ZnO could consume a part of -NCO group [31], which results in the reduced -NCO group reacting directly with PEG segments. Consequently, the straight chemical confinement of PEG is relived. What's more, ZnO disrupts hydrogen bonding among PU chains (Fig. 3), which further reduces the restrictions of PEG segments. Hence, the parts that could crystallize increase for the reducing of confinements of PEG, resulting in more crystals with less defects and enhanced latent heat. The more complete the crystals are, the more energy they need to melt, which leads in higher melting temperature. In the same way, when GA immersed in the mixture of ZnO, PEG and HDIB, the residual -OH group on GA walls also takes part in the reaction with -NCO group. Based on this, the -NCO group left to PEG segments are bound to reduce. Hence, the chemical constraints of PEG get further alleviated. Besides, the 3D continuous network of GA with
Table 2 Phase change data of PCMs. samples
PCM wt%
Tm(°C)
ΔHm(J/g)
Tc(°C)
ΔHc(J/g)
Xc(%)
PCM ZPCM GZPCM
100.00 99.00 97.71
48.4 55.3 57.1
86.8 101.5 108.1
24.2 32.1 34.6
85.8 99.9 106.7
100.0 118.1 127.5
existence of ZnO. While after impregnating into GA, the intensity of characteristic peaks of PEG increases further and the peaks of ZnO become too vigue to be figured out. The enhanced latent heat, phase change temperature as well as the diffraction intensity all reveal that the crystallization ability of PEG segments in system has been improved to a certain extent for compounding with ZnO and GA. This prominent elevation of phase change properties gives the credit to the heterogeneous nucleation and the mitigation of confinement from ZnO and GA. Firstly, the uniformly dispersed ZnO particles work 17
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Table 3 Phase change data and efficiency for light-/electro-thermal energy conversion and storage. Ref.
Composition
ΔHm of PEG (J/g)
ΔHm of PCMs (J/g)
ηl-t (%)
ηe-t (%)
[17] [18] [20] [21] [27] [34] Present work
MPEG/Polyamine/Dye R PEG/HDI/27 wt% PGF PEG/TDI/1–4 wt% Fe3O4-GNS PEG/HDIB/9.1 wt% RGO PEG/16 wt% ASF PEG/HDIB/1.17 wt% HNTs-GA PEG/HDIB/1.32 wt%GA/0.97 wt% ZnO
132.1 175.6 184.5 182.7 188.4 182.7 182.7
98.9 60.3 100.9 107.2 124 103.3 108.1
94.1 unknown 72.7–92.3 78.7 unknown 78.4 80.1
unknown 85 unknown unknown unknown 66.3 84.4
little oxygen-containing groups imbedded in the composite matrix, has little interaction with PU segments and could serve as natural barrier hindering the closely interaction among PU chains [27,34], which could also be proved by the weaken hydrogen bonding interaction in GZPCM (Fig. 3). Besides, the wrinkle surface of GA wall cell could also provide abundant nucleation sites for PEG segments, elevating the crystallization temperature of the system. PEG segments in GZPCM suffer less chemical and physical constraints and accordingly could arrange and crystallize more easily to form better crystals. In turn, owing to the enhanced crystallization properties, more energy is needed to destroy more crystals with less defects, resulting in higher melting temperature. Table 3 lists the phase change data of PEG-based multifunctional composites in literature. Here, ηl-t and ηe-t are the efficiency for lightthermal and electro-thermal conversion and storage, respectively. It can be noted that the phase change enthalpy in present work is better than that in most of study, especially PU-based system, suggesting the synthesized PCMs have excellent thermal energy storage capacity.
thermal energy (purple curve with arrow) to heat the PU matrix. Thus, the temperature of samples rises rapidly. The converted light energy is stored in the form of thermal energy though the melting of the crystals in system, which suffer from a consecutive flow of heat for a period time. The electro-thermal conversion and storage could be realized owing to the continuous 3D conductive network of GA. The temperature-time curves of ZPCM and GZPCM under 15 V supply, are displayed in Fig. 7b. The temperature of ZPCM has no change when loading power supply for the huge resistivity of sample because the conductive pathway does not form with only 1 wt% ZnO. The temperature of GZPCM rises rapidly when the power is switched on. Immediately, a temperature plateau during 55–60 °C appears, suggesting the electro-thermal energy conversion and storage. When crystals have melted, the temperature rises again. Once turning off the power, the temperature drops sharply until a plateau emerges, which means the solidification of PEG segments proceeding. The electro-thermal conversion efficiency (η) could be calculated by Eq. (3) [10]
=
3.4. Light- and electro-thermal conversions
m H S (tt t f )
(3)
ΔH and m are the latent heat obtained by DSC and mass of the sample, respectively. U is the voltage applied to sample, and I is the electric current. tt and tf are the starting and terminating time before and after the phase transition, respectively. The calculated η for electrothermal energy conversion of GZPCM under 15 V is 84.4%. The mechanism of electro-thermal energy conversion and storage is shown in Fig. 7c. When the power supply is loaded on the system, the resistance of ZPCM is so large that the current is close to zero. Hence, little Joule heat is produced. In GZPCM, the current could flow from one electrode to another through the 3D continuous skeleton of GA, composed of graphene wall cells like shown in Fig. 2b. Meanwhile a lot of Joule heat (purple curve) is produced and transported to the ambient PU matrix, resulting in the climb of temperature until it reaches the melting temperature of crystals, when massive heat is absorbed and stored through the melting of crystals. Further being charged with electricity, the temperature of the system keeps increasing. Although the η for light-thermal energy conversion and storage is much lower than that reported in lots of literature in Table 3. However, GZPCM realizes both η that more than 80% in light-/electro-thermal energy conversion with only 1.32 wt% GA and 0.97 wt% ZnO. This kind of materials would be potential to be used in smart devices like agricultural greenhouse, textile, solar heater and etc., which could be charged by sunlight or electricity. During daylight, the system could store the surplus solar energy through melting. At night, the stored energy could be released for work. If there is a rainy season, the devices could be charged by electricity to work.
The light-thermal conversion test was carried out under simulated solar illumination (AM 1.5) at a constant intensity of 100 mW/cm2, and the temperature evolution inside the sample was recorded by a data recorder equipped with thermocouple [21]. Under solar irradiation, the light is immediately absorbed by ZnO and GA and converted to heat simultaneously, which results in the temperature rise of ZPCM and GZPCM, as shown in Fig. 7a. The temperature of ZPCM and GZPCM evolves in a similar trend. Under the illumination of light, the temperature of samples first rises rapidly and then slows down to reach a plateau for melting of crystals and following rises quickly. Once light off, the temperature descends hastily and then decreases gently to reach a plateau, which corresponds to the solidification of PCM. Howbeit, the time to reach the same temperature for ZPCM always lags behind that of GZPCM. For instance, it takes 1725 s for GZPCM to reach 71 °C under light irradiation, but the temperature of ZPCM is only 55.8 °C at that time. Furthermore, it requires 2093 s′ irradiation for ZPCM to reach 65.5 °C. The light-thermal conversion and storage efficiency (η) can be calculated by the Eq. (2) [21] as following.
=
m H UI (tt t f )
(2)
Here, m is the mass of the sample, ΔH is the latent heat obtained by DSC, ρ is the intensity of light irradiation of the simulated solar light source (100 mW/cm2 @AM 1.5), S is the surface area of sample, tt and tf are the starting and terminating time before and after the phase transition, respectively. The calculated η of ZPCM and GZPCM is 46.9%, 80.1%, respectively. The higher η for GZPCM is attributed to the whole band light absorption of GA [24,25], and the synergistic effect of ZnO, which could absorb UV light efficiently [31]. The process of lightthermal energy conversion and storage is illustrated in Fig. 7c. When exposed to simulated solar light, the dispersed ZnO nanoparticles and GA with connected 3D network could absorb light and convert it into
3.5. Thermal stability and reliability Fig. 8 displays the TGA curves and DSC curves before and after 100 thermal cycling. The first slight thermal decomposition of PCM appears at around 286 °C, which corresponds to the temperature at 5% weight 18
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Fig. 7. Temperature-time curves of PCMs under (a) sunlight radiation at 100 mW/cm2 and (b) under a power supply of 15 V, (c) schematic illustration of the mechanism of light- and electro-thermal conversion and storage.
loss, due to the decomposition of urethane and biuret groups. The second degradation stage for PCM emerges at 410 °C (temperature at maximum weight loss rate), which is consistent with the decomposition of PEG. While the temperature at 5% weight loss of ZPCM and GZPCM is 290 and 330 °C, which is 4 and 48 °C higher than that of PCM. Besides, the temperature at 10% weight loss for PCM increase by 2 and 18 °C from 341 °C, in ZPCM and GZPCM, respectively. Although the temperature at 5% and 10% weight loss of ZPCM and GZPCM is a higher than that of PCM, the temperature at maximum weight loss rate exhibits almost no viriation. Whereas, ZPCM degrades the fastest at the temperature lower than 260 °C and between 350 and 400 °C. In comparision, GZPCM performs better all the time in thermal stability than ZPCM and PCM. The reasons for this phenomenon are the compititive effects of thermal barrier and formation of free oxygen and oxygen vancancies of ZnO under thermal exicitation [39]. When the former dominates, the thermal stability will be improved. While the latter dominates, the thermal stability will decrease. The 3D continuous inflammable network of GA with large wall sheets serves as thermal barrier inhibiting the emission of the decomposition products. Hence, the thermal stability of GZPCM is always better than that of PCM and ZPCM.
Fig. 8b-d displays the DSC curves of the samples after 100 phase change cycling. There are only very slight variations in phase transition temperature and latent heat of PCMs after 100 thermal cycles, which infers the good thermal reliability of products. 4. Conclusions Polyurethane-based composite phase change materials incorporation of ZnO and GA were prepared through facile impregnation. The latent heat, phase transition temperatures of the system increased from 86.8 J/g, 24.2 °C to 101.5 J/g and 32.1 °C, 108.1 J/g and 34.6 °C for incorporation of ZnO, both ZnO and GA, respectively. Besides, the light-/electro-thermal energy conversion efficiency of the system with ZnO and GA is 80.1%, 84.4%, respectively, for the efficient light absorption of ZnO and GA as well as the electricity conductive pathways of GA. This system with high latent heat, form-stablility, excellent thermal stability and thermal reliability, efficient light-/electro-thermal conversion and storage, has great potentials to be applied in solar energy conversion and storage, smart textile and other energy storage devices. 19
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Fig. 8. (a) TGA curves of PCMs and DSC curves of (b) PCM (c) ZPCM (d) GZPCM before and after 100 thermal cycle.
Appendix A. Supporting information [14]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2018.12.041.
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