UV-curable resin form-stable phase

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Graphene modified hydrate salt/UV-curable resin formstable phase change materials: continuously adjustable phase change temperature and ultrafast solar-to-thermal conversion Kunyang Yu, Yushi Liu, Fuzheng Sun, Minjie Jia, and Yingzi Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01165 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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Graphene modified hydrate salt/UV-curable resin form-stable phase change materials: continuously adjustable phase change temperature and ultrafast solar-to-thermal conversion Kunyang Yu1,a,b,c,d, Yushi Liu1,a,b,c,*, Fuzheng Suna, Minjie Jiaa, Yingzi Yanga,b,c,* aSchool bKey

of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China

Lab of Structures Dynamic Behavior and Control of the Ministry of Education,

Harbin Institute of Technology, Harbin 150090, China cKey

Lab of Smart Prevention and Mitigation of Civil Engineering Disasters of the

Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150090, China dKey

Laboratory of Earthquake Engineering and Engineering Vibration, Institute of

Engineering Mechanics, China Earthquake Administration, Harbin 150080, China

1These

authors are joint first authors.

*Corresponding author. Tel./fax: +86 45186281118. E-mail address: [email protected] (Y. Liu); [email protected] (Y. Yang).

Abstract Advanced phase change materials are of vital significance regarding utilizing thermal energy from solar power for controllable and effective thermal management under variable temperature environment. This study presents a graphene modified hydrate salt/UV-curable polyurethane acrylate resin copolymer form-stable phase change composite (graphene-EHS/UVPUA), wherein the phase change temperature

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can be continuously regulated through influence of graphene on entropy of system within the hydrate salt PCM. The results show that the phase change temperature decreases with the increasing graphene addition in range of tiny content level in the hydrate salt. The modified mechanism of adjustable phase change temperature through effect of graphene on hydrate salt are characterized by FTIR and Raman spectra based on chemical thermodynamics. Moreover, the solar-to-thermal conversion performance of the composite is investigated by Infrared Thermal Imager, and the result suggests that the time, from the presupposed initial temperature (20 ) to the terminal temperature (40 ), taken for graphene-EHS/UVPUA with the addition of only 0.02wt% graphene is reduced by 90.78%, compared with control sample under the designed solar-to-thermal energy conversion measurements. The obtained results indicate that the graphene-EHS/UVPUA composites are a promising route for achieving adjustable phase change temperature targets of the hydrate salt PCMs used for smart thermal management. Key words: form-stable phase change composite; hydrate salt; adjustable phase change temperature; graphene; solar-to-thermal conversion.

1. Introduction As an important way to minimize the waste of thermal energy, thermal energy storage system can greatly mediate the continuous global warming and energy crisis. Phase change materials(PCMs) are an effective bridge for application in thermal energy storage systems and heat management, being flexible enough to meet the mismatch of the energy supply and demand.1-3 As ideal solid-liquid phase change materials, inorganic hydrate salts are obtaining increasing attention in recent years, due to their high enthalpies of fusion and cost-effeictiveness.4-6 However, almost all of PCMs exhibit drawbacks such as the leakage problems of liquid-state and the difficulties in adjusting the phase transition behaviors.7-9 These limit the application of PCMs in practice for smart thermal management. The leakage problem during the phase transition process widely exists in solid-liquid inorganic hydrate salt PCMs, hindering the practical application. To deal ACS Paragon Plus Environment

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with this problem, various methods for preparing form-stable PCMs have been developed to prevent leakage. Usually shell-core structure is an excellent way to encapsulate PCMs, where shell serves as a kind of supporting materials and PCMs serve as the core for thermal storage.10 Zhang et al.11 synthesized the Na2SO4·10H2O@SiO2 solid nanobowls via synchronous hydrolysis reactions of TEOS and APTS, which showed excellent form-stable ability. Wang et al.12 prepared a novel shape-stabilized PCM via sol-gel process. The binary eutectic hydrate salt, Na2SO4·10H2O and Na2HPO4·12H2O in mass ratio of 1:1, was chosen as PCM, and the SiO2 obtained from the hydrolysis of Na2SiO3·9H2O was used as supporting materials. The prepared composite had good thermal reliability. Yuan et al.13 synthesized a polymer-coated CaCl2·6H2O/EG composite PCM by absorbing CaCl2·6H2O into EG, following by coating with the photo-cured polymer. The surfaces of the composite particles have been sealed by the polymer, which exhibited outstanding shape stability. However, the form-stable hydrate salt PCMs still face the challenge regarding encapsulation. To be specific, the supporting materials are likely to rupture and PCMs overflow from the supporting structure because of volume expansion of PCMs during the solid-liquid phase transition process. Fortunately, the use of UV-curable polyurethane acrylate resin to encapsulate hydrate salt PCMs can be developed into a feasible alternative approach to overcome the above problems. The polyurethane acrylate resin cured by ultraviolet radiation can form a strong and compact 3D network structure,14 holding a large number of hydrate salt. And unlike the conventional supporting materials, the UV-curable polyurethane acrylate resin with good strength, but also possesses excellent flexibility and toughness. This can help the supporting materials effectively relieves the stress from the volume expansion of PCMs, thus the risks that the overflow of PCMs from supporting materials and the rupture of shell materials can be eliminated. In order to achieve smart heat management, researchers have been working on adjusting phase transition behaviors of liquid-solid PCMs, e.g., the addition of 5wt% of Rubitherm® RT58 is found to completely prevent supercooling of RT21 microcapsules,15 the pore size of mesoporous SiO2 exhibits a prominent influence on ACS Paragon Plus Environment

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the enthalpy of octadecane,16 the heat absorption range is wider than pure paraffin waxes through intercalating paraffin waxes inside carbon nanotube,17 as well as our previous research that the latent heat of EHS/TiO2-P25 can be enhanced by the addition of TiO2-P25 nanoparticles,18 the endothermic single peak of EHS/PAAAM is replaced by two interconnected endothermic peaks through the interaction of GO and PAAAM with the hydrate salt.19 Although a great deal of work has been done, the difficulty of phase change temperature regulation is a widely accepted fact. For practical application of PCMs, the phase change temperature is of significance because it determines the application of PCMs occasion such as cooling application (up to 21 ), comfort application in building (between 22 and 28 ), hot water application (between 29 and 60 ).20 Therefore, there is an urgent need to develop effectively methods and theories to adjust the phase change temperature. So far, the inorganic and organic eutectic PCMs with different proportions of components forming eutectic mixtures have realized the adjustment of phase change temperature, e.g., the mixtures calcium chloride hexahydrate and quaternary ammonium salt in the mass ratio of 1:6-1:8 have a phase change temperature of 20.65 -23.05 ;21 the binary eutectic PCMs using 1-dodecanol and fatty acids are designed with melting temperatures ranging from 15

to 20 ;22 the melting points of CaCl2·6H2O with

0-15wt% fractions of Mg(NO3)2·6H2O range from 28.88

to 14.20 ,23 etc.

However, this method cannot guarantee a stable enthalpy value with the change of each component content in the mixtures, which could reduce heat storage capacity. Similarly, the confinement effect of PCMs impregnated into porous materials can also influence the phase change temperature. e.g., some literatures report that the phase change temperatures are increased to different degrees in the stearic/CMK-3 mesoporous assembly composite,24 paraffin wax/CNTs sponges composite25 and capric/expanded fly ash granules.26 In contrast, the polyethylene glycol/MOFs27 show the decreased phase change temperatures. However, most of their enthalpy values are much lower 40-75% than that of the pure PCMs component28 and the effect is also difficult in continuously adjusting the phase change temperature within a certain range, because it is difficult to accurately control the sizes, structure and uniformity of ACS Paragon Plus Environment

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porous. Moreover, the phase change temperature of the majority of liquid-solid PCMs are outside the room temperature range. This also provides a challenge for PCMs used in human comfortable surroundings. Accordingly, a method to achieve continuous adjustment of phase change temperature and keep stable enthalpy values within room temperature range is urgently needed. Graphene, a single atom-thick sheet composed of sp2-hybridized, has unique and strong nonpolar structure, physicochemical properties and huge specific surface area. When graphene is dispersed into hydrate salt, graphene nanosheets inevitably happen to agglomeration, encapsulating a number of hydrate salt due to the nonpolar molecular structure of graphene. This process eventually forms a kind of multicomponent system containing graphene and hydrate salt. Through incorporation of graphene into hydrate salt, the interaction between graphene and hydrate salt could be induced through the special effect originated from graphene,51,52 causing the disturbance of the molecules arrangement within the hydrate salt. It is expected that this interaction can lead to an increase in the entropy of system within hydrate salt PCM, thus resulting in the adjustment of phase change temperature based on chemical thermodynamics. This provides significant insight into the adjustment of phase change temperature of hydrate salt PCMs. Solar energy as an eminent clean energy has attracted wide attention in recent decades. But the application of solar energy is susceptible to the influence of weather and day-night time, leading to the imbalance between energy supply and demand.29 Therefore, the main challenge is how to capture, convert and store solar energy effectively.30 Thermal energy storage system of PCMs with an efficient solar energy absorption can be expected to solve the above problem. Graphene has shown wide range of applications such as flat plate solar collectors,31 working fluid,32-34 water heater35 and water desalination36 for improving the performance of the solar-to-thermal conversion because of graphene possesses high light absorption ability. Hence, graphene nanosheets are added into the hydrate salt, which would be an effective route for improving solar-to-thermal performance of the thermal energy storage system of hydrate salt PCMs. ACS Paragon Plus Environment

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In this work, we prepared a EHS/UVPUA phase change materials composite with excellent

flexibility,

toughness

and

form-stable

ability.

Na2CO3·10H2O-Na2HPO4·12H2O eutectic hydrate salt (EHS) was adopted as the model hydrate salt PCM. The differential scanning calorimetry (DSC) method was used to measure the phase transition behaviors of the form-stable PCM composite. The mechanism of adjustable phase change temperature through effect of graphene on hydrate salt was characterized by FTIR and Raman spectra based on chemical thermodynamics. Scanning electron microscopy (SEM) was employed to observe the microstructure. Moreover, Solar-to-thermal performance was investigated by Infrared Thermal Imager. In addition, Thermal cycling stability and aging property were discussed as well.

2. Experimental section 2.1. Materials Sodium carbonate decahydrate (Na2CO3·10H2O, AR) and disodium hydrogen phosphate (Na2HPO4·12H2O, purity > 99%) were adopted as inorganic hydrate salt PCMs. The polycarboxylic acid served as surfactant for the dispersion of graphene. UV-curable polyurethane acrylate resin was supplied by HongYi chemical Co., Ltd. And graphene was provided by Tuling Jinhua Technology Co., Ltd. 2.2. Preparation of eutectic hydrate salt/UV-curable resin (EHS/UVPUA) form-stable phase change material The preparation of Na2CO3·10H2O-Na2HPO4·12H2O eutectic hydrate salt (EHS) was provided by Ref.37 Na2CO3·10H2O and Na2HPO4·12H2O were mixed according to the mass ratio of 2:3 in the beaker and kept in the thermostatic water bath of 55 . The mixed solution was stirred enough until it was uniform. Then the eutectic hydrate salts were cooled to completely crystallize and sealed preservation. The eutectic hydrate salt crystallizations were broken into particles with the diameters ranging from 200 to 500 M+2 In order to confirm the maximum content of the hydrate salt in the UVPUA supporting materials, the hydrate salt particles and UV-curable polyurethane acrylate resin were uniformly mixed with different mass ratios at room ACS Paragon Plus Environment

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Scheme 2 (a-e) The synthesis of graphene-EHS/UVPUA

2.4. Characterization Differential Scanning Calorimetry (DSC, NETZSCH, 200F3) was used to measure the phase change temperature and latent heat of samples. The scanning rate was 5K/min and the scanning temperature was from -5

to 60

. The masses of all

samples varied from 5 to 10 mg. The microstructures of samples were investigated by Fourier transform infrared (FT-IR, PerkinElmer, USA) with KBr pellets at a resolution

of

4cm-1.

The

wavenumber

range

was

400-4000

cm-1.

The

micromorphology of samples was observed by Scanning electron microscopy (SEM, PEI Co., Quanta 200). Infrared Thermal Imager (Testo 865, Germany) was employed to determine the surface temperature of samples in solar-to-thermal test. Raman spectra were recorded with a Renishaw in via Raman microscope (Renishaw Corporation, Britain) with an argon-ion laser (532.0nm) as the source. The thermal cycling stability was measured in a thermocycling experiment chamber by heating and cooling the samples from -5 to 55

repeatedly. The aging experiment was tested

in an aging test box containing ultraviolet aging lamp (300W).

3. Results and discussion 3.1. The determination of optimal load of EHS The cured UVPUA can form a dense 3D network structure carrying amount of EHS. In order to determine the maximum content of EHS maintained in the UVPUA network, five samples of different proportions were tested for form-stable ability with filter membranes. Meanwhile, considering evaporation of crystal water in EHS during the heating process, the weight of the samples was recorded, as well. As shown in Fig.1(a), the EHS/UVPUA composites were synthesized by the incorporation of 50wt%, 60wt%, 70wt%, 80wt% and 100wt% EHS. Each of the cured composites with different contents of EHS was cut into a square plate with side length of 1cm and thickness of 5mm, and then placed on filter membranes respectively. These samples were put in a thermostat at 60

for six hours, ensuring adequate melting of EHS.


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Table 1 The mass change of EHS/UVPUA composites with different EHS content after heating for 6h. (g)

Sample

(g)

O (%)

100% EHS

2.73

1.61

66.34%

80% EHS

1.54

1.40

18.35%

70% EHS

1.63

1.55

11.32%

60% EHS

1.59

1.57

3.38%

50% EHS

1.52

1.51

2.12%

: the weight of the samples before heating,

: the weight of the samples after

heating, and O5 the percentage of mass loss of crystal water in EHS/UVPUA composites with different EHS content.

3.2. Characterization of EHS/UVPUA and graphene-EHS/UVPUA SEM images shown in Fig. 2a-d illustrate that the morphology of the dried UVPUA and EHS/UVPUA. It can be seen that the surface of UVPUA is continuous, homogeneous and dense after curing, which indicates that UVPUA has been cured completely (Fig. 2a and 2b). From Fig. 2c and 2d, the needle-like hydrate salt can be clearly observed on the surface of UVPUA. The EDS energy spectrum (Fig. 2e) of the region marked by a red circle (Fig. 2c) show that the elemental composition are oxygen,

phosphorus

and

carbon,

corresponding

to

Na2CO3·10H2O

and

Na2HPO4·12H2O of EHS. As shown in Scheme 1e, owing to the outstanding chemical stability and mechanical property of UVPUA, UVPUA can form a crosslinked network holding a large amount of EHS, preventing from the occurrence of leakage in the liquid-state of EHS.


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Fig.4 FTIR spectra of (a): (1) pure graphene, (2) pure EHS, (3) pure UVPUA, and (4) EHS/UVPUA,

(5)

graphene-EHS/UVPUA

and

(b)

different

content

graphene-EHS/UVPUA.

As shown in Fig. 5, in the spectrum of UVPUA (Fig. 5c), the Raman band at 2932 cm-1 is attributed to the vibration of CH2, which can be considered as a characteristic peak of UVPUA. As for EHS, the peak at 1067cm-1 results from the symmetric stretching vibration in the CO32- of Na2CO3·10H2O,38, 39 the two bands observed at 981cm-1 and 868cm-1 correspond to the P-O stretching modes in the HPO42- of Na2HPO4·12H2O.40, 41 It is clear from Fig. 5d that the spectrum of the EHS/UVPUA composite maintains all the characteristic bands of EHS and UVPUA. Moreover, as displayed in Fig. 5a the three characteristic bands of graphene, the D-band, G-band and G’-band, where peak of D-band is almost vanished, indicating the high purity of graphene. Also, from Fig. 5e, the three characteristic bands of graphene are presented clearly in the graphene-EHS/UVPUA composite containing 0.02wt% graphene, which means the good combination of graphene and EHS/UVPUA. Meanwhile, from the Raman spectra of EHS/UVPUA (Fig. 5d) and graphene-EHS/UVPUA (Fig. 5e), the composites have maintained the characteristic peaks of graphene, Na2CO3·10H2O, Na2HPO4·12H2O, and UVPUA, and no new characteristic peaks, reflecting the presence of a newly generated substance, are observed. Similarly, as displayed in Fig. 4a(4) and (5), the FTIR spectra of the EHS/UVPUA and graphene-EHS/UVPUA


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composites contain all the characteristic peaks of each their respective components and no new peaks appeared. Therefore, it can be concluded that there is no chemical reaction between graphene and EHS and UVPUA.

Fig.5 Raman spectra of (a) graphene, (b) EHS, (c)UVPUA, (d)EHS/UVPUA and (e) graphene-EHS/UVPUA

3.3. Phase change properties of EHS/UVPUA and graphene-EHS/UVPUA The thermal behaviors of the composites were investigated by DSC. Fig. 6a shows the DSC curves of EHS/UVPUA (control), 0.005wt%, 0.01wt% and 0.02wt% graphene-EHS/UVPUA. In Fig. 6a, only one set of endothermic and exothermic peaks appears in the DSC curve of EHS/UVPUA, which indicates that Na2CO3·10H2O and Na2HPO4·12H2O have formed a uniform eutectic mixture, EHS and UVPUA have fully mixed. However, it is found that the phase change temperatures of graphene-EHS/UVPUA are obviously different from that of EHS/UVPUA. From Fig. 6b, the phase change temperatures of graphene-EHS/UVPUA regularly decrease with the increase of graphene content, compared to that of EHS/UVPUA. The phase change temperatures of EHS/UVPUA, 0.005wt%, 0.01wt% and 0.02wt% graphene-EHS/UVPUA are 30.8 , 26.2 , 24.9

and 23.5 , respectively, which

exhibits the tunable phase change temperature. Moreover, as shown in Fig. 6c the thermal energy storage capacity (melting enthalpy) of EHS/UVPUA (control), 0.005wt%, 0.01wt% and 0.02wt% graphene-EHS/UVPUA are 125.7, 123.9, 123.4 and 120.9 Jg-1, respectively. It can be seen that the melting enthalpies of ACS Paragon Plus Environment

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graphene-EHS/UVPUA are slightly lower than that of EHS/UVPUA. However, the graphene-EHS/UVPUA composites show an increased crystallization enthalpy. The crystallization enthalpies of EHS/UVPUA, 0.005wt%, 0.01wt% and 0.02wt% graphene-EHS/UVPUA are 76.5, 79.3, 80.7 and 82.9 Jg-1, respectively. Whether the melting enthalpies or the crystallization enthalpies of graphene-EHS/UVPUA, the maximum losses of them all do not exceed 10% of the control sample. Combining the results of the melting and crystallization enthalpies of EHS/UVPUA and graphene-EHS/UVPUA, these indicate that the adjustment of phase change temperature of graphene-EHS/UVPUA has little influence on the enthalpy values.

Fig.6 Thermal properties of the phase change composites. (a) The DSC curve for EHS/UVPUA, 0.005wt%,0.01wt%, and 0.02wt% graphene-EHS/UVPUA. (b) and (c) The details of phase transition behaviors of EHS/UVPUA, 0.005wt%, 0.01wt% and 0.02wt% graphene-EHS/UVPUA. Tm: phase change temperature, enthalpy and

c:

crystallization enthalpy. Note: the values of Tm,


m

m:

melting

and

c

are

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expressed to be mean ± SD of three times repeated tests. To determine the quantitative relationship between the content of graphene and the phase change temperature, graphene-EHS/UVPUA composites that contain various graphene additions have been measured by DSC. Each sample was tested 3 times and value of each data point in Fig. 7 was the average of the three tests results. Fig. 7 shows the influence of graphene content on the phase change temperature of graphene-EHS/UVPUA composites. It is clear that the phase change temperature of graphene-EHS/UVPUA decrease with increasing graphene content. when the content of graphene is limited to 0.02wt%. Interestingly, the phase change temperature of graphene-EHS/UVPUA which only contain 0.0025wt% graphene dramatically decreases by 3.5

, compared with that of EHS/UVPUA composite. This suggests

that only a very small amount of graphene is needed to have a significant effect on the phase change temperature. Moreover, along with the addition of graphene increasing, the change of the phase change temperature of graphene-EHS/UVPUA gradually decreases. When the content of graphene is greater than 0.02wt%, the phase change temperature of graphene-EHS/UVPUA maintains at around 23.5 . For this phenomenon, the reason is that surfactants may reach the limit of its loading capacity and cannot disperse more graphene in EHS solution, which prevents graphene from further disturbing the change of phase change temperature of graphene-EHS/UVPUA. In

addition,

the

experimental

data

of

phase

change

temperature

of

graphene-EHS/UVPUA is fitted and the result is shown in Fig. 8. The relation is expressed in the following equation: = 23.47623 + Where

{

( + 0.0000656321) 0.00579

}

is the phase change materials of graphene-EHS/UVPUA and

content of the graphene. The correlation coefficient of eqn. (1) is 0.99.


(1) is the

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Fig.7 Phase change temperature of graphene-EHS/UVPUA with different contents graphene

3.4. Adjustment effects of graphene on phase change temperature It is well known that the phase change temperature of thermal energy storage based on phase change materials is of significant because it determines the application occasion. Now, graphene-EHS/UVPUA composites have achieved adjustable phase change temperature through change in graphene content. This significant phenomenon can be explained by theory of chemical thermodynamics.42 For a system of constant chemical composition (e.g. EHS), according to the fundamental equation of chemical thermodynamics, 0

In which,

0,

0,

0,

=

0

0

!0 "0

(1)

!0 and "0 are the Gibbs free energy, volume, pressure,

entropy and temperature of the system, respectively. The notation “d” represents differential law. When a relatively small third component W (e.g. graphene nanosheets) is added to the system. For a multicomponent system containing a third component W, it can be concluded that, 1

Where,

1,

1,

1,

=

1

1

!1 "1 + #$ %$

(2)

!1 and "1 are the Gibbs free energy, volume, pressure,

entropy and temperature of the multicomponent system, respectively. #$ represents


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the chemical potential of W and %$ is the amounts of substance of W. Based on the theory of the Gibbs free energy change, the whole process of phase transformation is in thermodynamic equilibrium state, while at equilibrium

0=

1

=0. Therefore, Eqs. (1) - (2), rewriting, 1

1

!1 "1 + #$ %$ =

0

(3)

!0 "0

0

In consideration of the fact, the content of the third component is very little for the whole system. Thus, it can be concluded that

1= 0,

1= 0.

Eq. (3) can be further

simplified, #$%$ = !1 "1

(4)

! 0 "0

Generally, for the constant chemical component system and the multicomponent system, it fits the following equations, &0 = &1 =

0

1

0 1

Where, the symbolic meanings of

(5)

+ "0 ! 0

(6)

+ "1 !1 + #$%$ 0,

0,

"0, !0,

1,

1,

"1, !1, #$ and %$

are the same as Eqs. (1) and (2). &0 and &1 respectively denoted the enthalpy of the two systems mentioned above. Eqs. (5) – (6) and it can be further arranged as, &0 d&1 = "0 !0

"1 !1

#$ %$

(7)

Combining Eqs. (4) and (7), it can be concluded that, &0 d&1 = "0 !0

"1 ! 1

! 1 "1 + !0 "0

(8)

Because the whole system is in the phase transformation process, the temperatures of the constant chemical system and multicomponent system are both constant. Thus, "0 and

"1 can be considered as infinitesimal quantities, i.e.,

"0= "1=0. Eq.(8)

can be further reduced to, &0

&1 = " 0 ! 0

"1 ! 1

(9)

EHS/UVPUA and graphene-EHS/UVPUA composites respectively represent constant chemical system and multicomponent system. As shown in Fig. 4b, the peak in the shadows is composed of C=O of -COOH, C=O of -CONH2 originated from the polyurethane acrylic and O-H bending vibration of crystal water from the EHS, respectively. It is clear that the peak shifts to lower wavenumbers with increasing content of graphene of the graphene-EHS/UVPUA composites. However, the amount ACS Paragon Plus Environment

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of the polyurethane acrylic resin in the composites is unchanged and graphene does not react with the polyurethane acrylic resin. According to previous reports on the interaction of graphene with water molecules, confinement of graphene can affect the arrangement of water molecules and change the bond angle.43, 44 Graphene nanosheets inevitably happen to agglomeration during the dispersion in EHS solution, encapsulating a number of EHS due to the nonpolar molecular structure of graphene. This process leads to the incorporation of graphene into EHS, which further induces the interaction between graphene and EHS. Therefore, the cause of peak shift could be the interaction of graphene and crystal water. Similarly, as represented in Fig. 5, the three bands observed at 1067cm-1, 981cm-1 and 868cm-1 from the CO32- of the Na2CO3·10H2O and the HPO42- of Na2HPO4·12H2O, shifting to the 1078cm-1, 999cm-1 and 860cm-1, respectively. Confinement effect of carbon materials can cause unit cell structure changes of crystal.45 The shift results of Raman spectra could affected by the interaction of graphene and EHS, leading to the change of unit cell structure. At the beginning of process of phase transformation, the EHS in EHS/UVPUA and graphene-EHS/UVPUA composites are in crystalline state with the ordered arrangement and the entropy values of these two systems can be approximated to be the same. However, at the end of the phase transition process, the EHS in EHS/UVPUA can be regarded as a single chemical system with a relative sample phase transition process. But the multicomponent system of graphene-EHS/UVPUA has more molecular states than single chemical system due to the confinement effect of graphene on EHS, leading to that the entropy of multicomponent is larger than that of the constant chemical system. It ultimately leads to

!1 >

!0. Moreover, from

Fig. 6c, the melting enthalpy of graphene-EHS/UVPUA is slightly lower than that of EHS/UVPUA. Hence

&0 >

&1, i.e.,

&0

&1 > 0, the left side of the Eq. (9) is

a positive number. In order to balance the Eq. (9), clearly, "1 should be less than "0. This result has successfully indicated that graphene-EHS/UVPUA can effectively adjust the phase change temperature based on thermodynamics.

3.5. The solar-to-thermal of graphene-EHS/UVPUA ACS Paragon Plus Environment

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Due to excellent light capturing ability of graphene and the black appearance, the graphene-EHS/UVPUA composite is regarded as an ideal light absorbent. The solar-to-thermal energy conversion measurements were illustrated in Fig. 8. The (1) EHS/UVPUA, (2) 0.005wt%, (3) 0.01wt% and (4) 0.02wt% graphene-EHS/UVPUA composites were cut into the same square plate with side length of 4 cm and thickness of 5 mm. They were put in the thermostatic chamber of 20

for 2 hours, ensuring

that the temperature of each sample reaches the same designed initial value. Then all the samples of EHS/UVPUA and different content graphene-EHS/UVPUA composites from Fig. 8a immediately placed under the natural sunlight, and the thermal images were recorded by Infrared Thermal Imager every 30 seconds or so. When the temperature of each sample was 20 , it was regarded as the beginning of the initial measurement time, i.e., t=0. The measurements would be terminated when the temperature of each sample reached 40 ± 0.5

and meanwhile the terminational

time were recorded. As shown in Fig. 8b, it can be seen that the terminational time for different tiny graphene-EHS/UVPUA samples reached 40 ± 0.5

is greatly

shortened, compared with EHS/UVPUA. Notably, the terminational time of 322 seconds that 0.02wt% graphene-EHS/UVPUA sample spent, while EHS/UVPUA sample had not crossed its phase change temperature at that time, was dramatically reduced by 90.78% comparing with that of 3494 seconds of EHS/UVPUA sample. And the time spent on 0.005wt% and 0.01wt% graphene-EHS/UVPUA samples were reduced by 66.28% and 78.36%, respectively, which exhibits remarkable solar-to-thermal conversion ability of the graphene-EHS/UVPUA composites, due to excellent capacity of graphene to absorb solar radiation. Furthermore, the uniform appearance of the composites from Fig. 8c suggest that graphene, EHS and UVPUA have mixed well. Meanwhile, because graphene nanosheets possess high specific surface area and thermal conductivity, the graphene composites can adequately receive solar radiation and rapidly spread heat around. Benefiting from these, as displayed in Fig. 8b, the uniform color of the infrared image of each sample indicate that graphene-EHS/UVPUA composites can uniformly absorb solar radiation and prevent thermal stress damage due to local overheating. ACS Paragon Plus Environment

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Page 22 of 29

cycles. The results of thermal cycle test indicate that the minor changes of the latent heat and phase change temperature in graphene-EHS/UVPUA, suggesting the excellent stability of the prepared composite. Furthermore, the form-stable PCMs should be thermally stable for long-term irradiation. In consideration of ageing resistance of the composite PCMs, an aging experiment was carried out through the graphene-EHS/UVPUA samples irradiated by ultraviolet aging lamp (300W) for 100 hours. As displayed in Fig. 9, the DSC curve of graphene-EHS/UVPUA after aging test was extremely similar to that of the as-prepared one. In addition, from Table 2, the melting and crystallization enthalpies of graphene-EHS/UVPUA were measured to be 117.2 Jg-1 and 80.3 Jg-1, decreased by only 3.3 Jg-1 and 2.5 Jg-1 compared with those of as-prepared graphene-EHS/UVPUA. Meanwhile, the phase change temperature of graphene-EHS/UVPUA with aging test merely increased by 0.1

as

presented in Table 2. The obtained results indicate that graphene-EHS/UVPUA possesses outstanding stability and ageing resistance performance for a long period of time.

Fig.9 DSC curves of 0.02wt% graphene-EHS/UVPUA before and after aging and 500 thermal cycling test.

Table 2 Thermal properties of graphene-EHS/UVPUA before and after 500 thermal cycles and aging tests.


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0.02wt% graphene

X&((J/g)

X&)(J/g)

"(( )

after 1st cycle

120.9

82.9

23.5

after 500th cycle

114.6

78.6

23.8

after aging test

117.2

80.3

23.6

X&(: melting enthalpy, X&): crystallization enthalpy and "(: phase change temperature. 4. Conclusions In summary, a kind of graphene modified hydrate salt/UV-curable polyurethane acrylate

resin

copolymer

form-stable

phase

change

composite

(graphene-EHS/UVPUA) was successfully prepared with tunable phase change temperature through confinement effect of graphene, outstanding flexibility, form-stable ability and ultrafast solar-to-thermal conversion. (1) The form-stable ability test indicates that EHS/UVPUA composite has excellent form-stable ability preventing the leakage problem during the phase transition process. Meanwhile, benefiting from outstanding flexibility and toughness of UV-curable polyurethane acrylate resin, the stress due to the change of volume during the phase change process can effectively relieve. (2) The DSC test results demonstrate that the phase change temperature of graphene-EHS/UVPUA regularly decreases with the increase of graphene addition in a tiny content range, compared to that of EHS/UVPUA. The adjustable range of the phase change temperature of the graphene-EHS/UVPUA is from 30.8

to

23.5 . (3) The modified mechanism of adjustable phase change temperature of the graphene-EHS/UVPUA is confirmed by FTIR and Raman spectra through revealing the effect of graphene on hydrate salts based on chemical thermodynamics. (4) The solar-to-thermal energy conversion measurements results indicate that the graphene-EHS/UVPUA

composite

possesses


excellent

solar-to-thermal

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conversion ability. (5) The prepared graphene-EHS/UVPUA composite exhibits that outstanding stability of latent heat and phase change temperature after the thermal stability and aging tests. Overall, our work suggests a novel approach to adjust the phase change temperature of hydrate salt PCMs and provides a promising candidate form-stable PCM for solar-to-thermal energy storage.

Acknowledgments The financial support from National Key R&D Program of China (No. 2017YFB0309901) and its Provincial funding (No.GX18A025) as well as China Postdoctoral Science Foundation (2018M631936) for current research is gratefully acknowledged.

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UV-curable resin form-stable phase

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