Photodriven Shape-Stabilized Phase Change Materials with

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Photodriven Shape-Stabilized Phase Change Materials with Optimized Thermal Conductivity by Tailoring the Microstructure of Hierarchically Ordered Hybrid Porous Scaffolds Jie Yang, Li-Sheng Tang, Lu Bai, Rui-Ying Bao, Zhengying Liu, Bang-Hu Xie, Ming-Bo Yang, and Wei Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00565 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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ACS Sustainable Chemistry & Engineering

Photodriven Shape-Stabilized Phase Change Materials with Optimized Thermal Conductivity by Tailoring the Microstructure of Hierarchically Ordered Hybrid Porous Scaffolds Jie Yang, Li-Sheng Tang, Lu Bai, Rui-Ying Bao, Zhengying Liu, Bang-Hu Xie, Ming-Bo Yang, Wei Yang* College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, No. 24 South Section 1, Yihuan Road, Chengdu, 610065, Sichuan, People’s Republic of China. E-mail addresses: [email protected] (W Yang)

ABSTRACT Graphene oxide (GO)/boron nitride (BN) hybrid porous scaffolds (HPSs) with the optimally three-dimensional (3D) aligned network structure are fabricated using an unidirectional ice-templated strategy. A variety of HPSs with multilayer structure are obtained due to the various temperature gradients generated by tuning freezing temperature, in which GO and BN are expelled from the ice growth front to assemble between the oriented ice crystals. We demonstrate that the obtained HPSs have a significant influence on the properties of the resulting composite phase change materials (PCMs), including thermal conductivity and shape-stability. Various thermally conductive pathways are formed by tuning freezing temperature, which contributes to further understanding the relationship between thermal conductivity of composites and their internal network structure. Upon increasing the freezing temperature from -120 to -30 °C, the thermal conductivity of the composite PCMs increases and reaches a maximum value at -50 °C and then decreases with a further increase in freezing temperature to -30 °C. The resultant composite PCMs exhibit a high thermal conductivity (3.18 W m-1 K-1) at a relatively low BN loading of ca. 28.7 wt%, maintaining high package capacity (ca. 72 wt%) and energy storage density. Furthermore, the composite PCMs also present excellent thermal reliability and realize an efficient solar energy conversion. This strategy provides an insight for the design of high-performance composite PCMs with potential to be used in advanced thermal management and energy conversion systems. KEYWORDS: Composite phase change materials, Hierarchically ordered porous scaffolds, Tuning freezing temperature, Optimized thermal conductivity, Energy conversion

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INTRODUCTION Latent heat storage of phase change materials (PCMs) is one of the most promising techniques and systems to store thermal energy coming from solar irradiation, industrial waste heat and surplus heat,1-3 which in turn relieves the crises of energy and environment. Consequently, organic solid-liquid PCMs, such as polyethylene glycol (PEG) and paraffin wax (PW), have been studied extensively because of their high energy storage density, isothermal characteristics, low or negligible supercooling degree and desirable thermal stability.1, 4-7 Although PCMs possess great potential in improving the utilization efficiency of heat energy, practical applications of organic PCMs have been hindered by their poor solar energy conversion ability, low thermal conductivity and the leakage of melt during phase transition.4, 7-12 One prospective way in driving solar energy conversion is the introduction of carbonaceous materials due to their excellent photoabsorption.1, 8, 13-16 Strenuous efforts have been made to enhance the thermal properties and to explore encapsulated methods for the organic PCMs. Recently, three-dimensional (3D) multifunctional materials have been used to simultaneously form thermally conductive pathway and supporting network, leading to the improvement of thermal conductivity and shape-stability for the organic PCMs at the same time.17-23 It is noteworthy that 3D structural materials like aerogel or foam discussed here are macroscopically and different from the porous materials with meso or nanopores,5,

10, 24-30

and the assembly process and packing structure play a

significant role in the porous properties of the 3D scaffolds, especially the macropore size and pore wall thickness,19, 20, 31-33 thus further affecting the final performance of the composites. On the other hand, the geometrical structure of thermally conductive fillers, mainly including number of layers, size and aspect ratio, plays an important role in improving the thermal conductivity of composites.34,

35

For example, it has been proved that

compared with the composites made from monolayer or few-layer graphene sheets with similar aspect ratios, thick graphite nanoplatelets exhibit the highest efficiency in improving thermal conductivity of composites based on combined multiscale modeling and experimental approach.36 In addition, two-dimensional (2D) plateletshaped nanomaterials with a high aspect ratio, such as graphite (or graphene) and boron nitride (BN), have been identified to have superior performance in enhancing the effective thermal conductivity of composites.37 It is worth noting that 2D graphite


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and BN show a highly anisotropic thermal property: the in-plane thermal conductivity is far higher than the through-plane thermal conductivity.38-40 Therefore, to make full use of the unique anisotropies in shape and thermal conductivity, the aligned conductive structures have been designed to fabricate the composites with high thermal conductivity.41-46 Owing to its simplicity, controllability and flexibility, the ice-templated assembly strategy has attracted a great deal of interest in constructing wide varieties of hierarchical or oriented structures for the fabrication of multifunctional composites, including electrically conductive materials,47-49 thermally conductive materials,31, 43, 44, 50

electromagnetic shielding materials,51,

52

dielectric materials,53 adsorption

materials,54 sensors,32, 55 energy materials,19, 56 etc. To our best knowledge, although BN filled composite PCMs or polymer composites with enhanced thermal conductivity via an ice-templated assembly have been reported previously, only one or two temperatures are used to construct the structure.19, 31, 43, 50, 57, 58 It is ambiguous how the hierarchically ordered structures controlled by the certain temperature gradients determine the properties of composite PCMs. In this work, we demonstrate that an ice-templated assembly strategy with the certain temperature gradients from 120 °C to -30 °C can be adopted to tailor the microstructure of hybrid porous scaffolds (HPSs) for the optimized properties of the composite PCMs, especially thermal

conductivity. We establish

the

relationship between the

various

microstructures of the HPSs and the thermal conductivity of corresponding composite PCMs, which enables the optimization of the properties of the high-performance PCMs based energy conversion materials and devices.

EXPERIMENTAL SECTION Materials. The original graphite (purity > 99.9%) and BN (~30 um, purity > 99%) were purchased from Laixi Nanshu Fada Graphite Company (Qingdao, China) and Qinhuangdao Eno High-Tech Material Development CO., LTD, respectively. PEG (Mn = 10 000) was provided by Aladdin Reagent (Shanghai, China). Potassium persulfate (K2S2O8), potassium permanganate (KMnO4), concentrated sulfuric acid (H2SO4), phosphorus pentoxide (P2O5), hydrogen peroxide (H2O2) and hydrochloric acid (HCl) were obtained from Haihong Chemical Reagents Company (Chengdu, China).



Preparation of HPSs. Graphene oxide (GO) was synthesized by an improved Hummers’ method.59 The GO/BN hybrid slurries were first prepared by dispersing various loadings (300 and 500 mg mL-1) of BN into 10 mg mL-1 GO deionized water mixture, followed by ultrasonic bath treatment (KQ-400KDB, 40 kHz, 400 W, Kun Shan Ultrasonic Instruments Co., Ltd, China) and magnetic stirring to mix the hybrid slurries well, and the BN/GO ratio in the resulting mixture was 30:1 and 50:1 respectively. It is noteworthy that the concentration of BN does not exceed 500 mg mL-1 in order to keep a relatively high energy storage density for the final composite PCMs and ensure the strength of prepared porous scaffolds for further utilization. After that the slurries were then poured into a columniform polymer mold placed on top of heat sink whose temperature was controlled and recorded by a paperless recorder. To investigate the effects of the freezing temperature, the hybrid slurries were frozen at different temperatures (i.e., -120, -80, -50 and -30 °C). After freezedrying for 24h, the HPSs with different BN loadings were finally obtained and denoted as a1, a2, a3, a4 and b1, b2, b3, b4 according to the BN loading and freezing temperature, where a and b respectively represented the mixture with 300 and 500 mg mL-1 BN, and the number 1, 2, 3, 4 represented the freezing temperature at -120, -80, -50 and -30 °C. Preparation of PEG/HPSs composite PCMs. The PEG/HPSs composite PCMs were fabricated by using a vacuum-assisted infiltration method. The HPSs were immersed in the degassed PEG melt and then transferred into a vacuum oven at 90 °C for 4h. The obtained composite PCMs were labelled as A1, A2, A3, A4 and B1, B2, B3, B4, which corresponded with a1, a2, a3, a4 and b1, b2, b3, b4, respectively. Figure 1 detailedly illustrated the whole process of preparation of HPSs and PEG/HPSs composite PCMs. Characterization. The morphology of HPSs and fracture surface of PEG/HPSs composite PCMs were characterized using a field-emission scanning electron microscope (SEM, JEOL JSM-5900LV, Japan). Thrmogravimetric analysis (TGA) was conducted on a NETZSCH STA 409 instrument at a heating rate of 10 °C min-1 from 30 °C to 700 °C in air atmosphere. A differential scanning calorimetry (DSC, TA Q20 instrument, USA) was used to explore phase change temperatures and phase change enthalpies of pure PEG and the composite PCMs at a heating/cooling rate of 10 °C min-1 in a highly purified nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR, 4000–400 cm-1) and X-ray diffraction (XRD, 2θ = 5–60°) ACS Paragon Plus Environment

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patterns were collected by Nicolet 6700 FTIR spectrometer (Nicolet Instrument Company, USA) with a resolution of 4 cm−1 in the transmission mode and a Rigaku UltimaIV diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 0.15406 nm) at a scanning speed of 10 ° min-1, respectively. The thermal conductivity of the hybrid porous scaffolds and the composite PCMs was measured using a Hot Disk Thermal Constant Analyzer (TPS 2500, Hot Disk AB Company, Sweden) at room temperature (ca. 25 °C). A CEL-HXUV300 xenon lamp with an AM 1.5 filter and a commercial Seebeck thermoelectric device were assembled manually to perform a solar-to-electric energy conversion system under a simulated sunlight irradiation of 800 mW cm-2. The output current (I) was recorded using a Keithley 2400 electrometer. The cold end of the thermoelectric device was attached to a heat sink under tap water (~ 30 °C) or ice water (~ 0 °C) in this work.

Figure 1. Schematic diagram of the preparation of the HPSs and composite PCMs.

RESULTS AND DISCUSSION Tailoring the microstructures of the HPSs and the composite PCMs. Figure 2 shows the 3D macroporous architectures for the HPSs. Two main features of the hierarchically ordered structure are observed. First, the interlayer space and the thickness of layer increase gradually from ca. 28 um to ca. 73 um and from ca. 17 um to ca. 60 um, respectively, as the freezing temperature increases from -120 °C to 30 °C at the same BN loading, owing to that the microstructures are shaped by the ice crystals during freeze-drying, and the details is shown in Figure S1. The smaller supercooling degree (the higher freezing temperature) is, the relatively slower crystallization rate is, resulting in the crystals with a larger size. Moreover, the freezing with volume expansion drives BN to assemble into hierarchically ordered structures during the growth of crystals.32, 49 Second, some bridges stacked from BN



connect the adjacent lamellar walls and gradually reduce. Note that a part of the bridges is broken by the large-size ice crystals with the elevation of the freezing temperature. This is because the ice crystals can grow very large under a high freezing temperature, which destroy the bridge already formed in the initial stage. After HPSs are filled with PEG melt, the 3D BN hierarchically ordered networks marked with red arrows are maintained well, as shown in Figure 3. The features would have an significant effect on the thermal conductivity of the PEG/HPSs composite PCMs.

Figure 2. The perpendicular cross-sectional SEM images of (a) and (e) a1, (b) and (f) a2, (c) and (g) a3, (d) and (h) a4.


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Figure 3. SEM images of fractured surfaces of (a) and (b) A2, (c) and (d) A4.

Thermophysical properties of the composite PCMs. Our previous results unraveled that GO was completely burnt off in the air atmosphere when heated to 700 °C, while there was a negligible loss of mass for BN due to its remarkable stability.19 Therefore, the content of the fillers is 20.1 and 28.7 wt% in A and B series composite PCMs, respectively, which is determined by TGA curves shown in Figure S2. The phase change temperatures, peak melting/crystallization temperature (Tmp/Tcp), and phase change enthalpies, melting/crystallization enthalpy (∆Hmc/∆Hcc), are investigated using DSC. Figure 4a shows the DSC curves of pure PEG and the representative composite PCMs, A3 and B3. Compared with pure PEG, a negligible change of phase change temperatures for the composite PCMs is found, indicating that the introduction of the HPSs has no notable effects on Tmp and Tcp of PEG. As expected, the phase change enthalpies decrease as the filler loading increases, which results from the fact that a part of working substance is replaced by the fillers. But in this study, the composite PCMs, A3 and B3, maintain a relatively melting enthalpy, up to 157.7 and 143.6 J g-1 which reach 79.8 % and 72.6 % of pure PEG, respectively, as shown in Figure 4b. The above results are also listed in Table 1 in details. Additionally, it is proved that the HPSs obtained from the different freezing temperatures have little influence on the thermophysical properties of the same series composite PCMs with various internal microstructures, as shown in Figure S3, Table S1 and S2, reflecting that the content of fillers in each series composites is nearly the same as what we design. The XRD patterns shown in Figure S4 for pure PEG and the composite PCMs are performed to figure out the crystallization behaviors. Similar to the neat PEG, the composite PCMs also exhibit two main diffraction peaks appearing at ca. 19.1° and ca. 23.5° corresponding to the (120) and (032) planes of the PEG crystal respectively, indicating that the introduction of HPSs has no effect on the crystallization of PEG. Furthermore, almost no reduction in phase change temperature and heat of fusion is observed over the 100 heating and cooling cycles for the composite PCMs according to the DSC cyclic curves shown in Figure S5a for A3 and Figure 4c for B3. Also, Figure S5b and Figure 4d show that the FTIR spectra of the composite PCMs are virtually identical after 100 thermal cycles, demonstrating a stable chemical structure of the composite PCMs. These results disclose that the composite PCMs possess an excellent thermal repeatability and stability.



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Figure 4. (a) DSC curves and (b) phase change enthalpies (energy storage densities) of PEG, A3 and B3. (c) The mass specific heat of fusion normalized by the first heating cycle (inset shows the DSC curves of B3 with 100 thermal cycles) and (d) FTIR spectra of B3 before and after 100 thermal cycles. Table 1. Thermophysical properties of PEG, A3 and B3. Samples

Tcp (°C)

∆Hcc (J g-1)

Tmp (°C)

∆Hmc (J g-1)

PEG

40.8

187.8

64.5

197.7

A3

42.1

150.6

65.5

157.7

B3

42.6

136.0

64.8

143.6

Thermal conductivity of the composite PCMs. Figure S6 and 5a show the thermal conductivity of the two series porous scaffolds obtained from different freezing temperatures and the corresponding composite PCMs, respectively. It is noteworthy that the thermal conductivity is altered by changing the freezing temperature. For example, the thermal conductivity is 1.72 W m-1 K-1 for A series composite PCMs consisting of the HPSs prepared at a freezing temperature of 120 °C, and it increases to 1.89 W m-1 K-1 as the freezing temperature is raised to 50 °C. However, a further increase in the freezing temperature to -30 °C results in a reduction in thermal conductivity to 1.67 W m-1 K-1. Similarly, the thermal conductivity for the porous scaffolds and B series composite PCMs reaches a maximum value at -50 °C. The pure PEG, like most organic PCMs, exhibits an inherently low thermal conductivity (0.33 W m−1 K−1). However, B4 with a relatively


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low BN loading (28.7 wt%) shows a high thermal conductivity of 3.18 W m−1 K−1, which is enhanced by 864% compared with that of pure PEG, as shown in Figure 5b. It is worth noting that a very low content of GO has little effect on the thermal conductivity.57 In order to clarify the superiority of the 3D hierarchically ordered structure in enhancing the thermal conductivity of composite PCMs, Figure 5c compares the thermal conductivity in this work and the previously reported organic composite PCMs.1, 5, 17, 19, 23, 60-62 Undeniably, it is important for the composite PCMs to control the filler loading, which directly affects their final latent heat or energy storage density. Considering that the working substance is replaced by the filler, the energy storage density (phase change enthalpy) of the final composites decreases as the filler loading increases. Hence, the phase change enthalpy of the composite PCMs is well-advised to be taken into account in the comparison, with the parameter, relative melting enthalpy (η), defined as the ratio of melting enthalpy between the composites and pure matrix. Excitingly, the composite PCMs exhibit a superior thermal conductivity in this work, simultaneously maintaining a relatively high energy storage density, which is difficult for the fabrication of high-performance composite PCMs, especially the BN based composite PCMs. The relationship between the structure and the thermal conductivity is illustrated in Figure 6 to reveal the thermal conductive mechanism of the composite PCMs, where the red 3D network represents the thermally conductive pathway composed of BN and an extremely low amount of GO in the matrix. It is universally acknowledged that the thermally conductive pathway in the composites determines the final thermal conductivity. Based on the morphologies of the HPSs and the composite PCMs in Figure 2 and 3, the freezing temperature plays a significant role in the formation of the thermally conductive network. It is interesting to note that the thermally conductive channel becomes wider gradually from ca. 17 um to ca. 60 um with the increase of the freezing temperature, which depends on the process of ice growth. Thus, the reduction of the thermally conductive interface makes the heat transfer more effective when the freezing temperature increases from -120 °C to -50 °C. The HPSs provide continuous pathways for phonon transport with a small thermal resistance. Unexpectedly, with the further increase of freezing temperature from -50 °C to -30 °C, a well formed network structure (including the wall and bridge) is destroyed owing to the growth of the large-size ice crystals driven by the high freezing temperature, leading to the phonon scattering at the defect. Moreover, a large thermal resistance ACS Paragon Plus Environment


exists in the interlayer matrix resulting from the large interlayer distance from ca. 28 um to ca. 73 um as the freezing temperature increases from -120 °C to -30 °C. This phenomenon also emerges in other work that graphite foams fabricated from the template-assisted chemical vapor deposition (CVD) are introduced into organic PCMs to prepare composite PCMs with high thermal conductivity.9 In order to reduce the thermal resistance for the heat transfer from the PCM inside the large pore, an improved CVD method is developed to grow carbon nanotubes and graphene inside the pores.20, 33 In addition, the ice templating method is an effective way to construct 3D aligned structure.44, 50 The XRD patterns of the composite PCMs shown in Figure S4 provides the evidence of the alignment of BN microplatelets,39, 40, 63 and the higher the freezing temperature is, the better the degree of orientation is, reaching a superior level at -50 °C. Therefore, the HPSs obtained from the freezing temperature at -50 °C endow the composite PCMs with the highest thermal conductivity.

Figure 5. Thermal conductivity of (a) the composite PCMs and (b) PEG, A3 and B3. (c) Comparison of the thermal conductivity of B3 in this work and the previously reported organic PCMs at a similar relative melting enthalpy (η).


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Figure 6. Schematic diagram of the structure and mechanism of thermal conductivity enhancement for the composite PCMs.

Shape-stability of the composite PCMs. Besides the thermal conductivity and energy storage density, the shape-stability is another important parameter for thermal energy storage materials. Leakage tests are conducted to investigate the shapestabilized property of the as-prepared composite PCMs with various microstructures. Figure 7 shows photographs of PEG and A series composite PCMs with the increase of temperature from 30 °C to 90 °C. After being heated to 70 °C, above its melting temperature, PEG starts to melt with obvious liquid leakage, while the composite PCMs remain defined shape without any leakage. When heated to 90 °C, PEG completely melt into liquid owing to the solid-to-liquid phase transition. By contrast, no obvious leakage phenomenon is observed for the composite PCMs, which is attributed to the capillary and surface tension forces between PEG and HPSs. There are intermolecular hydrogen bonding interactions between GO with massive oxygencontaining functional groups in HPSs and PEG containing C-O and O-H groups, which also contributes to improving the shape-stability of the composite PCMs.64 Note that there is a trend that the leakage becomes slightly serious for the composite PCMs with the increase of freezing temperature, which is consistent with the previous results.20 The SEM images of the surface of A2 and A4 composite PCMs shown in Figure 7b and c give additional evidence of the trend. Nevertheless, only a small amount of PEG on the surface for A series composite PCMs leak.



Figure 7. (a) Shape-stability of PEG and A series composite PCMs with the increase of temperature. SEM images of (b) A2 and (c) A4 after the leakage test.

Another leakage test is carried out to study the effect of BN loading on the shapestability, as shown in Figure 8a and b. As expected, A3 and B3 exhibit no leakage at 90 °C compared with pure PEG. When compressed by a weight of 50 g, their original shapes and hierarchically ordered structures are well-preserved, indicating the molten PEG is retained by the HPSs. A closer inspection reveals that a small amount of PEG in A3 is released under the pressure, while almost no PEG leaks obviously in B3 from the SEM images shown in Figure 8b and d. Additionally, Figure 8e shows the DSC curve of B3 after the compression by 50 grams force. The thermophysical properties of B3 are extraordinarily stable before and after the test, maintaining a high phase change enthalpy up to 141.5 J g-1, which is ca. 99 % of the original enthalpy.


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Figure 8. Shape-stability of PEG, A3 and B3 (a) with the increase of temperature and (b) bearing the compression by 50 grams force at 90 °C. SEM images of (c) A3 and (d) B3 after the compression by 50 grams force for several minutes. (e) DSC curve of B3 after the compression by 50 grams force.

Solar-to-electric energy conversion. Solar energy, as a lasting renewable and pollution-free energy source, has attracted a great deal of interest in developing burgeoning energy conversion materials and technologies. In this work, with the combined solar energy and thermal energy storage materials, the PEG/HPSs composite PCMs are employed to serve as the heat source for solar thermoelectric generators. A solar-to-electric energy conversion system is designed to generate electricity (Figure 9a). The HPSs with GO can act as an effective photon captor and molecular heater to endow the composite PCMs with the improved photoabsorption.8 The samples are placed on the hot end of a commercially available square Seebeck thermoelectric conversion module with a 4 cm side length and then are exposed to the simulated solar illumination (AM 1.5) controlled intensity of 800 mW cm-2, and the cold end of the module is attached to a heat sink under tap water (ca. 30 °C) or ice water (ca. 0 °C). Figure 9b shows I-t curves recorded by a Keithley electrometer for PEG, b3, A3 and B3. When A3 and B3 are exposed to the simulant light at 800 mW cm-2, the generated electrical current I increases rapidly until a platform



corresponding to the solid–liquid phase change appears, storing thermal energy in the form of latent heat. The current I drops rapidly and then reaches another platform corresponding to the liquid-solid phase change after turning off the light, releasing stored heat from the composite PCMs during the crystallization of PEG. For the control samples (neat PEG and b3 representing the b series porous scaffold obtained at -50 °C), there are no phase change platforms emerging for them in the whole process owing to the poor photoabsorption and no working substance, respectively. First, the composite PCMs realize an efficient light-to-thermal energy conversion shown in Figure S7. After that the converted heat energy can be utilized to generate electricity using a thermoelectric device, leading to the Seebeck effect. The difference in temperature is controlled by changing cold source to tuning the intensity of I, as shown in Figure 9c. When the cold source is changed from tap water (ca. 30 °C) to ice water (ca. 0 °C), the intensity of steady-state I during the cooling process is improved by almost twice for the same sample because of the change of three times difference in temperature for thermoelectric conversion module, which provides a feasible route to regulate the heat generated I.

Figure 9. (a) Experimental setup for the solar-to-electric energy conversion. I-t curves of (b) PEG, b3, A3 and B3 and (c) A3 with various cold sources under sunlight irradiation of 800 mW cm-2.


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CONCLUSIONS In the present work, we rationally devised and fabricated photodriven shape-stabilized composite PCMs with highly thermal conductivity up to 3.18 W m-1 K-1 at a relatively low BN loading (28.7 wt%). HPSs with different microstructures have been produced by changing the freezing temperature from -120 to -30 °C. We demonstrate that the interlayer spacing and the thickness of layer increase with the increase of the freezing temperature, while the bridges connecting the adjacent lamellar walls gradually reduce and are destroyed. These structural differences significantly affect the properties of the final composite PCMs, especially the thermal conductivity. The optimal freezing temperature is around -50 °C for the fabrication of thermally conductive porous scaffolds and corresponding composite PCMs. Furthermore, an adjustable intensity of heat generated I has been realized for the combined solar energy and thermal energy storage materials systems during the solar-to-electric energy conversion. This understanding of the microstructure of the HPSs in PEG/ HPSs composite PCMs provides a new insight in fabricating high-performance organic composite PCMs applied in advanced energy conversion fields.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Synthesis of GO, Sizes of the interlayer space (d) and the thickness of layer (t) of the a series scaffolds, TGA curves of PEG and composite PCMs, DSC curves of PEG and composite PCMs, Thermophysical properties of PEG and composite PCMs, XRD patterns of PEG and A series composite PCMs, DSC curves and FTIR spectra of A3 with 100 thermal cycles, Thermal conductivity of the original porous scaffolds, Lightto-thermal conversion of PEG and composite PCMs.

AUTHOR INFORMATION Corresponding Authors * Telephone: + 86 28 8546 0130; Fax: + 86 28 8546 0130; E-mail: [email protected]. ORCID Wei Yang: 0000-0003-0198-1632 Notes



The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (NNSFC Grant No. 51422305 and 51721091) and State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2014-2-02) are gratefully appreciated.

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How do the hierarchically ordered structures determine the properties of composite PCMs, especially thermal conductivity?


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Photodriven Shape-Stabilized Phase Change Materials with

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