Lightweight mesoporous carbon fibers with interconnected graphitic walls for supports of form-stable phase change materials with enhanced thermal conductivity

Solar Energy Materials & Solar Cells 208 (2020) 110361

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Lightweight mesoporous carbon fibers with interconnected graphitic walls for supports of form-stable phase change materials with enhanced thermal conductivity Xiao Zhu a, **, Liang Han b, Fei Yang b, Jie Jiang b, Xilai Jia b, * a

College of Science, China Agricultural University, Beijing, 100083, PR China Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China

b

A R T I C L E I N F O

A B S T R A C T

Keywords: Porous carbon fiber Form-stable phase change material Thermal storage Paraffin

Thermal storage performance of form-stable phase change materials (PCMs) is often compromised by the heavy use of the supporting materials. Here, lightweight mesoporous carbon fibers (MCFs) with continuous graphitic walls are prepared by carbonization of polyvinylidene fluoride on magnesium oxide fibers for form-stable PCMs. The structure of the MCFs are comprehensively investigated to show their merits for supports of the PCMs. The MCFs are rich in relatively ordered mesopores, which can provide effective loading of paraffin and give high thermal storage capacities per unit weight. The paraffin loadings of the as-obtained MCF-700, MCF-800 and MCF-900 reach 93, 90, and 92 wt% with maintained shape stability, and deliver the thermal storage capacities of 176.54, 183.35, and 177.75 kJ kg 1, respectively. Due to the abundant pore networks and the improved interfacial property from the in-situ fluorine doping, the MCFs-supported PCMs exhibit high running stability and thermal storage efficiency in cycling. More importantly, the penetrating graphitic walls of the MCFs provide effective thermal-conductive pathways from outside to the interior for rapid thermal transfer. The results exhibit that the MCFs with interconnected graphitic walls can be an effective support of form-stable phase change materials for solar harvesting, etc.

1. Introduction Today, energy storage technologies play a more and more important role than ever before due to the huge energy consumption. Thermal energy storage technologies based on phase change materials (PCMs) exhibit wide potential applications such as in solar energy harvesting, smart buildings, and factory heat recovery [1–5]. The PCMs can be broadly divided into three classes, inorganic, organic, or composite, among which the organic solid-liquid PCMs are extensively studied due to their large thermal storage capacity, high chemical reliability ang low corrosiveness. However, they have poor shape stability and suffer from leakage [6], which result in unstable performance and decreased ther­ mal storage efficiency in use. Additionally, most of the pure PCMs exhibit low thermal conductivities, limiting the rapid thermal transfer in heating/cooling. Encapsulation techniques have been widely used to address the

leakage problems of the PCMs based on the barrier of the supporting materials and the interfacial interactions between them [7–9]. It is worth mentioning that a high loading of the PCMs is important to ach­ ieve a high thermal storage density, since a significant use of the sup­ ports always leads to decreased thermal storage capacities in unit mass. Moreover, a higher thermal conductivity of the composite PCMs is better for the heat transfer. Various kinds of porous materials have been developed as the supports of the composite PCMs, typically like the expand vermiculite [10–14] perlite [15–17], polymers [18–20], di­ atomites [21–25], and carbons [9]. The pore structure of the supporting materials plays an important role in the form-stable PCMs [26,27]. The macroporous supports may suffer from ineffective encapsulation of the PCMs and still encounter leakage problems, which usually need addi­ tional packaging materials. In addition, the large pores limit the heat transfer from the outside to in quickly, which is undesired in thermal storage. Reducing the pore size of the supports may increase the loading

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhu), [email protected] (X. Jia). https://doi.org/10.1016/j.solmat.2019.110361 Received 23 July 2019; Received in revised form 14 November 2019; Accepted 13 December 2019 0927-0248/© 2019 Published by Elsevier B.V.

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of the PCMs for higher thermal storage density; however, the phase-change properties of the PCMs may be suppressed in too small pores [28]. More and more efforts based on hierarchical materials with tunable porosity and high thermal conductivity have been developed as a more desirable candidate for composite PCMs. A carbon support that has neither too large nor too small pore net­ works should be very suitable for balancing the loading of the PCMs and the thermal conductivity. Among the carbon materials, expanded graphite [29,30], graphene [31–33], graphene oxide [34], carbon foam [27], carbon nanotube [35–37], and carbon fiber [38–40] have been prepared for form-stable PCMs due to the thermal-conductive property and high structural stability [41–44]. Despite the progresses achieved, lots of the carbons are consisting of disordered pores, which cannot be fully filled with the PCMs in high fractions. Meanwhile, the low graphitization of the disordered carbons often causes a disappointed thermal conductivity. It is aimed to provide a mesoporous carbon fiber with interconnected graphitic walls for supports of form-stable phase change materials with enhanced thermal conductivity. On the other hand, when the PCM loadings are high, there is often an instability of the composite PCMs due to the phase separation in the repeated charge/discharge thermal cycles. The uniform solidification nucleation of the PCMs on the supports becomes critical [45]. It is well known that the surfaces of the graphitic carbons are very hydrophobic, which is difficult for the uniform nucleation. Therefore, heteroatom doping of the carbonaceous surfaces has become an effective strategy to improve the surface properties. For example, the nitrogen-doped car­ bons have been reported to exhibit improved phase change properties [46,47]. However, most of the reported doping strategies based on post-treatment methods can hardly be uniform, which of course cannot fully induce uniform nucleation for stable running of the PCMs. Overall, the composite PCMs with high energy density, rapid thermal transfer, and excellent cycling stability are in high demand. Herein, a novel mesoporous carbon fiber (denoted as MCF) with continuous graphitic walls and in-situ fluorine doping has been prepared for formstable PCMs. It can balance the high loading of the PCMs and the ther­ mal conductivity. That is, the hierarchical porosity of the MCFs have strong absorption capability to ensure the high paraffin loading, which endows the composite PCMs with high storage capacities and excellent running stability. At the same time, the continuous graphitic walls of the MCFs can provide penetrating thermal transport pathways from the outside to internal, thereby delivering rapid thermal transfer properties.

MCF-700, MCF-800, and MCF-900, respectively. 2.3. Preparation of paraffin/MCF composite PCMs Paraffin/MCF composite PCMs were prepared by an impregnation method. Ethyl alcohol was added into the molten paraffin drop by drop under magnetic stirring. Then, the three kinds of MCFs with calculated percentages were soaked into the dispersed liquids by magnetic stirring. When the alcohol was evaporated completely, the black composite PCMs were collected, and denoted as paraffin/MCF-700, paraffin/MCF-800, and paraffin/MCF-900, respectively. 2.4. Material characterizations Scanning electron microscopy (SEM) was performed by a JEOL JSM6700 FE-SEM operated at 3.0 KV. Transmission electron microscopy (TEM) was performed by a JEM-2010 instrument operated at 120 KV. Xray diffraction (XRD) was performed on a BRUKER D8 ADVANCE X-ray powder diffractometer using Cu-Ka radiation (λ ¼ 1.54 Å). Nitrogen sorption isotherms were obtained at 77 K on the Micromeritics ASAP 2460 apparatus. The pore size distributions of MCFs were obtained from the adsorption branch of isotherms using the density functional theory (DFT) model. The surface compositions of MCFs were tested by X-ray photoelectron spectroscopy (XPS) with a PHI 3057 spectrometer using Mg–K X-rays at 1253.6 eV. Raman spectra were obtained on a Horiba JobinYvon Lab RAMHR800 Raman spectrometer with He–Ne laser excitation of 532 nm. Fourier transform infrared (FTIR) spectra of the samples were obtained with a Magna-IR 560 E S.P spectrometer by a KBr pellet technique. Specifically, the samples were first mixed and ground with KBr (A. R.) respectively. Then, the mixture was pressed into a transparent sheet using a tableting machine and then dried by an infrared lamp for 5 min. Finally, the sheet was placed in the infrared tester for testing. The leakage test of the paraffin/MCFs was carried out in a heated oven. Each of the sample on a filter paper was heated to 80 � C and maintained for 2 h. Thermogravimetric analysis (TGA) and deriv­ ative thermogravimetry (DTG) were experimented on NETZSCH STA449F3 analyser at 10 � C min 1 under a N2 flow of 50 ml min 1. The sample weight used for TGA was 10.0 mg. Differential scanning calo­ rimetry (DSC) was experimented on a scanning differential calorimeter (Pyris 6, PerkinElmer) in N2 atmosphere with the heating and cooling rate of 10 � C min 1 at 10–80 � C. The averaged weight of the samples used for DSC test was 5.6 mg. Thermal conductivity of the samples was measured on LFA 467 HyperFlash, Netzsch, Germany analyser based on a laser flash method. The samples were pressed into disks of 12.7 mm in diameter and 2.0 mm in thickness with a tablet press. To compare the temperature response of the paraffin/MCF compos­ ites and the pure paraffin in Fig. 11b, the paraffin/MCF composites or the pure paraffin was pressed into disks of 12.7 mm in diameter and 10.0 mm in thickness with a temperature probe is buried inside respectively. Then, the sample was placed into 45 � C oven to test the temperature response. After heating 300s, the power of the oven was cut off, and the temprature was also collected during the cooling time. Moreover, to get the thermal storage performance of the composites vas time in Fig. 12b, the compressed composites (12.7 mm in diameter and 10.0 mm in thickness) were placed into the oven at 60 � C, then the temperature response was collected.

2. Experimental 2.1. Materials Paraffin wax (melting point: 56–58 � C) was bought from Beijing Chemical Works. Magnesium acetate, urea, polyvinylidene fluoride (PVDF), ethyl alcohol, and N-methyl pyrrolidone (NMP) were analytical reagent (A.R.) and purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received. 2.2. Preparation of MCFs First, Mg(OH)2 fibers used as the templates were synthesized by a hydrothermal method [48]. Second, 2 wt% PVDF-NMP solution was added into Mg(OH)2 template fibers drop by drop until a saturated adsorption was reached. Then, PVDF/Mg(OH)2 composite was obtained after removing NMP in a 80 � C vacuum oven for 3 h. Next, the dried product (0.5 g) was transferred into a combustion boat and carbonized at 700, 800, or 900 � C in a tube furnace for 2 h. The heating rate was 10 � C min 1 in Ar atmosphere (50 mL min 1). After that, the tubular furnace was naturally cooled to the room temperature. Then, black products were collected and washed with excessive 12 wt% hydro­ chloric acid and deionized water to remove the MgO templates. After freeze drying, the final products were obtained and denoted as

3. Results and discussions 3.1. Synthesis and structure of MCFs Fig. 1a shows the preparation processes of the MCFs and following loading of paraffin for the form-stable composite PCMs. We started with the synthesis of Mg(OH)2 template based on a hydrothermal method. The obtained Mg(OH)2 is in a well-defined fiber shape as observed under SEM (Fig. 1b). The as-prepared Mg(OH)2 fibers were saturated by a 2 wt 2

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% PVDF-NMP solution. After evaporation of NMP, PVDF was left and coated on the fiber templates. In Fig. 1c, obvious “ripple-like” layers are observed on the surface of the templates, which indicates that the Mg (OH)2 fibers were well coated by the PVDF. The coated fibers were then carbonized at 700, 800, and 900 � C under argon protection. In this process, the PVDF was carbonized into MCFs, while the Mg(OH)2 was converted into MgO. This process displayed a much lower temperature as compared with the reported high-temperature carbonization (2400 � C) for making the carbon supports of the PCMs [49]. Note that the MCFs maintain the fiber shape after the carbonation process (Fig. 1d), sug­ gesting a high structural stability of the templates. The lengths of the fibers were mostly in 50–60 μm, with the aspect ratio of ~68 (Fig. S1). Moreover, partial of the MCFs are cross-linked to form networks (as denoted by the yellow arrow) due to the high aspect ratios of the fibers. This cross-linked networks would be better for structural stability and promote the interfacial heat transfer. The microstructure of the MCFs prepared at different temperatures was first examined under electron microscopy characterizations. As shown in Fig. 2a, d and g, all the MCFs exhibit similar fiber shape under SEM observation. The diameter of the MCFs ranges from hundreds of nanometers to microns, comparable to that of the Mg(OH)2 fibers. Based on the TEM observation, uniform mesopores throughout the whole fi­ bers have been observed for MCF-700 (Fig. 2b and c), MCF-800 (Fig. 2e and f), and MCF-900 (Fig. 2h and i), indicating the successful conversion of PVDF to MCFs over 700 � C. The pore networks are penetrating, and

Fig. 1. (a) Schematic of the preparation of MCFs and paraffin/MCF composite PCMs. SEM images of (b) Mg(OH)2, (c) PVDF/Mg(OH)2, and (d) MCF-900.

Fig. 2. SEM and TEM images of the (a–c) MCF-700, (d–f) MCF-800, and (g–i) MCF-900. (j–l) Elemental mapping images of the MCF-900, showing that the uniform distribution of the C, F, and O elements. 3

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the diameters of the pores are distributed around several nanometers. Such 3D porosity is expected for high loadings of PCMs for higher gravimetric thermal storage density, which is different from the disor­ dered or closed pores that cannot be fully utilized [50–52]. Moreover, the skeletons of the MCFs display ultrathin-graphitic walls, with the thickness being less than 5 nm. This is different from the thick walls of the traditional carbon foams. Such ultrathin graphitic networks can improve the thermal transfer of the PCMs to the interior. Additionally, the elemental mapping of the MCFs (Fig. 2j-l) also supports that the fluorine element exhibits a homogeneous distribution without any locally aggregated phase in the carbon skeleton (Fig. 2k), which can facilitate uniform condensation nucleation during repeated charge/di­ scharge thermal cycles. The porosity of the MCFs was further characterized using N2-sorption measurement. Fig. 3a shows that the MCFs exhibit the type-Ⅳ isotherms. They all display significant uptake from medium pressure and arising of adsorption/desorption hysteresis loops, which indicate the existence of mesopores. The hysteresis loops also support the strong capillary force of the pores of the MCFs. The Brunauer-Emmett-Teller (BET) specific sur­ face areas of the MCF-700, MCF-800, and MCF-900 are calculated to be 627, 798, and 937 m2 g 1, respectively, much larger than traditional metal or carbon foams. The pore size distributions of the MCFs were calculated based on the DFT model. As shown in Fig. 3b, all the MCFs display similar hierarchical porosity, consisting of mesopores around 4.0–5.0 nm and micropores near 1.3 nm. It is well known that the microstructure of the supporting materials has important effect on the loading of the PCMs. Here, the hierarchical pores of the MCFs are ex­ pected to be more suitable for PCM encapsulation, since they have stronger capillary force than the macropores (>50 nm), yet display much limit negative influence on phase change behavior of PCMs. And this will be discussed in the thermal-storage measurements of the composite PCMs. Similar to most porous carbons [53], XRD patterns of the MCFs display two broad peaks at 23.7 and 41.9� (Fig. 4a), corresponding to the (002) and (100) crystal faces. The broad feature should be ascribed to the porous structure, which has highly decreased the crystalline struc­ ture. As increased the synthesis temperature, the d(002) peak intensity of the MCFs becomes more noteworthy, suggesting the increased graphi­ tization, which is further characterized by Raman spectroscopy. In Fig. 4b, all the MCFs exhibit the typical G and D bands of carbon in the Raman spectra. The G band (1591 cm 1) corresponds to the zone center E2g symmetry related to phonon vibrations in-plane sp2 carbon, while the D band (1320 cm 1) is related with A1g symmetry due to the disordered carbon and edge defects [54]. According to the Gaussian fitting [55,56], the intensity ratios of the D to G band (ID/IG) of the MCF-700 and MCF-800 were close, while further increasing the carbonization temperature to 900 � C, the ratio was decreased due to the

higher graphitization. Moreover, the preparation method could form in-situ fluorine doping on the MCFs, which was investigated by XPS analysis. In the survey curves of the MCFs, characteristic peaks of C, O, and F are observed at 284.78, 532.75, and 689.70 eV, respectively (Fig. S2). The results are consistent with the elemental mapping images of the MCFs, where C, O, and F elements are distributed uniformly. The elemental compositions of the three MCFs were obtained from the XPS data and were listed in Table 1. It is no surprise that C is the main element in the MCFs, but note that the F content of MCF-800 is the highest among the three. Further investigation of C1s spectrum confirms that at least seven kinds of C exist in the MCFs: sp2-C (284.8 eV), sp3-C (285.4 eV), C–O and C–F – O (287.5 eV), –CF2 (289.0 eV), and –COO–(290.1 eV), (286.3 eV), –C– respectively (Fig. 4c). It needs to point out that the sp2-C is the main fraction, consistent with the graphitic structure. The graphitic structure can provide a better thermal transfer for the PCMs. The high-resolution XPS peaks of F (C–F, 686.7 eV; CF2, 689.7 eV) are shown in Fig. 4d, which confirm the successful in-situ doping of the MCFs. The small fraction of the fluorine doping of the MCFs can improve the surface properties of the graphitic carbon for facilitating uniform solidification of the PCMs, important for stable running during repeated charge/ discharge thermal cycles. 3.2. Structure of paraffin/MCFs Based on the pore structure of MCFs, melted paraffin were easily absorbed into the samples based on a simple impregnation method. This was confirmed by the N2 adsorption characterization of the MCFs before and after being loaded with paraffin (Fig. S3). The disappearance of the pore networks of MCFs indicated that the paraffin was mostly absorbed into the pores of the MCFs rather than being coated on the surface. The macroscopic morphology of the paraffin/MCF composite PCMs are black powders and could be easily compressed into a bulk. After being com­ pressed into a disc shape, leakage tests were performed at a heated temperature: 80 � C for 2 h (Fig. S4). It shows that the pure paraffin is quickly melted after placing in 80 � C for 2 h (Fig. 5a), while the com­ posite PCMs with the paraffin weight over 90 wt% can maintain the shape (Fig. 5b–d), supporting the good form-stable property. SEM observation shows that the pores of the MCFs are completely filled with paraffin at such high paraffin fractions (Fig. 5e–g). It can be seen that the paraffin is mainly encapsulated within the internal pores, with little coated on the surfaces. The graphitic walls of the MCFs form effective space barriers to prevent the leakage. The thermal stability of the paraffin/MCF composites was further investigated by TGA (Fig. 6a). The results show that paraffin/MCF composite PCMs exhibit nearly no mass loss below 210 � C, indicating the high thermal stability of the materials. The weight loss of the composite

Fig. 3. (a) N2 sorption isotherms and (b) DFT pore size distributions of the MCF-700, MCF-800, and MCF-900. 4

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Fig. 4. (a) XRD patterns, (b) Raman spectra of the MCFs. Divided XPS spectra of the (c) C and (d) F elements of the MCF-900. Table 1 Structural properties of the MCF-700, MCF-800, and MCF-900. Sample

Surface area (m2 g 1)

Pore volume (cm3 g

MCF-700 MCF-800 MCF-900

627 798 937

0.73 0.78 0.87

a

1

)

Pore size (nm)

Porosity (%)

Raman ID/IG

4.37 4.08 4.42

93.3 94.1 94.5

1.05 1.10 0.78

Compositiona (wt.%) C

O

F

92.66 94.60 95.93

5.09 2.10 3.59

2.25 3.30 0.48

The elemental weight percentage is obtained from XPS data.

PCMs is 92, 90, and 93 wt%, respectively, supporting a high loading of the paraffin in the form-stable composite PCMs. The DTG curve of the pure paraffin exhibits two obvious weight losses at 251 and 304 � C. By comparison, the weight loss of the composites is largely averaged over 304 � C, supporting the improved thermal stability. Moreover, the onset weight loss temperature of the paraffin/MCF composite PCMs has been increased by nearly 40 � C as compared with that of the pure paraffin, suggesting the enhanced thermal stability due to the introduction of MCFs. The chemical compatibility of the paraffin and MCFs was investi­ gated by XRD and FTIR characterizations. In Fig. 7a, the XRD pattern of the paraffin displays clear characteristic peaks at 21.5� and 24.1� , sug­ gesting paraffin is partially crystallized. Those characteristic peaks have also been detected in the as-prepared paraffin/MCF composite PCMs, while the broad peaks of the MCF are not obvious after loading paraffin due to its weak intensity. The results indicate that the introduction of MCFs still keep the crystallization of paraffin, which is vital for ensuring the phase change of the PCMs for thermal storage. Moreover, FTIR curves of the composite PCMs and pure paraffin are compared to

confirm the chemical compatibility of the paraffin and MCFs (Fig. 7b). Typical absorption peaks of paraffin are observed at 2923, 2826, 1466, and 725 cm 1, corresponding to –CH2- stretching vibration, –CH3stretching vibration, –CH2- in-plane bending vibration, and -(CH2)nrocking vibration, respectively. There are no obvious differences as comparing the peaks of paraffin with those of paraffin/MCF composite PCMs, except a weak and broad peak of 3435 cm 1 due to surface groups of the MCFs (FTIR curves of the MCFs, Fig. S5). This supports that the chemical structure of the paraffin was not changed during the prepa­ ration of the composite PCMs, since the MCFs could not react with the paraffin. 3.3. Thermal storage properties of paraffin/MCFs Thermal storage properties of the composite PCMs were measured by DSC and compared with the pure paraffin at 10–80 � C. Fig. 8a and b displays the DSC heating and freezing curves, respectively. Two peaks corresponding to the solid-solid and solid-liquid phase change of paraffin are observed on both heating and cooling curves. The exact 5

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Fig. 5. (a–d) Optical photographs of the paraffin and the composite PCMs after in 80 � C for 2 h, showing the high shape stability of the composite PCMs. (e–g) SEM images of the paraffin/MCF composite PCMs.

Fig. 6. (a) TGA and (b) DTG curves of the paraffin and as-prepared paraffin/MCF composite PCMs.

Fig. 7. (a) XRD patterns and (b) FTIR curves of the paraffin and as-prepared paraffin/MCF composite PCMs.

phase transition temperatures of the composites are similar to that of paraffin (Table 2). It shows no obvious changes for the phase change behavior, since the main interactions between the paraffin and the graphitic walls of MCFs are mainly physical barrier. The results are in good agreement with the structural and compositional analysis as mentioned above. From the DSC curves, the melting enthalpy of the pure paraffin is calculated to be 196.54 J g 1. Because the MCFs do not melt/solidify in

the temperature range (Fig. S6), the phase change enthalpies of the composite PCMs are usually lower than that of the pure paraffin. Based on the total weight, the melting enthalpy of the MCF-700-93%, MCF800-90%, and MCF-900-92% PCMs is 176.54, 183.35, and 177.75 J g 1, respectively. The actual storage capacity of MCF-800-90% is a little higher than the expected value based on the mixing rule, while the other two are comparable to the expected value (Fig. 8c). The little increase of the thermal storage capacity might be ascribed to the enhanced 6

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Fig. 8. (a) Endothermic and (b) exothermic curves of the paraffin/MCF-700, paraffin/MCF-800, and paraffin/MCF-900 composite PCMs. (c) Calculated thermal storage capacities (form the DSC curves) of the composite PCMs compared with the expected theoretical value based on the mixing rule (the expected value ¼ the exact fraction of paraffin � the melting enthalpy of the pure paraffin).

and discharge enthalpy are 177.75 and 174.00 J g 1, respectively, corresponding to an efficiency (defined as the discharging/charging capacities) of 98% for this cycle. It needs to point out that the efficiency maintains steady during the following cycles. That is, the absorbed heat of the composite PCMs in melting process could be almost released during the cooling process, suggesting high reversibility. What’s more, 200 charge/discharge thermal cycles were tested on paraffin/MCF-800 sample as the example to show the long-term thermal reliability. As shown in Fig. 9g, the capacity and thermal conversion efficiency of the PCMs has almost no attenuation after the first 50 cycles, and only a reduction of 1.57% is observed for the melting enthalpy after the 200 charge/discharge thermal cycles. To understand the reason for the high stability, the paraffin/MCF900 composite PCM after 50 cycles was observed under SEM. Obvi­ ously, it still retains the fiber morphology after 50 endothermic/ exothermic cycles (Fig. 10a). FTIR spectra of the paraffin/MCF-900 composite PCM before and after 50 cycles also exhibit no obvious changes of the functional groups (Fig. 10b). After further removal of the paraffin, the SEM and TEM images of the cycled MCF-900 presents the porous structure (Fig. 10c and d), with no obvious changes as compared with its pristine morphology. All those results support the robust structure of the MCFs and the high structural stability of them as sup­ ports of the composite PCMs. Again, the reason for the high stability can be ascribed to the effective encapsulation of melted paraffin in the hi­ erarchical porosity of the MCFs. That is, when MCFs are used as the supporting materials, paraffin could be effectively restricted within the MCFs based on the strong capillary force and the interfacial interactions even in melting state [62,63]. The thermal conductivities of the paraffin and paraffin/MCF com­ posite PCMs are shown in Fig. 11a. The pure paraffin exhibits a thermal conductivity of 0.27 W m 1 K 1, while the thermal conductivities of the paraffin/MCF-700, paraffin/MCF-800, and paraffin/MCF-900 PCMs are 0.47, 0.53 and 0.51 W m 1 K 1, respectively. The increased thermal conductivities of the composite PCMs can be ascribed to the continuous thermal conductive networks formed by the graphitic walls of the MCFs. Besides, the fiber supports of high aspect ratios can provide longer-range heat transfer pathways as compared with the powder additives, thereby offering better thermal conductivities for composite PCMs [9]. The 3D thermal conductive pathways of MCFs can transfer heat quickly into the interior of the composite PCMs. In addition, the thermal conductivities of the paraffin/MCF composites were related with the intrinsic thermal conductivity of themselves, and the matching of the two, which is related with the interfacial contacts between them [64,65]. Paraffin/MCF-800 displayed the highest thermal conductivity in the three composites due to the moderate temperature treatment and the stronger interfacial contacts from the higher fluorine doping. Fig. 11b exhibits that the temperature response of the paraffin/MCF composite

Table 2 Thermal characteristics of the paraffin/MCF composites in melting/freezing. PCM

Paraffin Paraffin-MCF700-93% Paraffin-MCF800-90% Paraffin-MCF900-92%

Melting process

Freezing process

Ts (� C)

Tp (� C)

Hm (J/ g)

Ts (� C)

Tp (� C)

Hf (J/ g)

46.94 47.31

55.21 56.65

196.54 176.54

52.93 52.97

48.65 47.84

194.79 172.36

47.38

56.67

183.35

53.71

48.76

179.06

46.77

55.96

177.75

51.68

46.30

174.00

Ts, starting temperature; Tp, peak temperature.

interactions between paraffin molecules and the fluorine doping of the MCFs. The thermal storage capacities of the composite PCMs are compared with the typical form-stable PCMs made from paraffin and graphene [6], carbon nanotube [36,57,58], graphene oxide [59], or other porous supports [9,60,61], etc. (Table S1). Obviously, this light­ weight MCFs achieve high PCM loadings and obtain good thermal storage capacities. Also, the composite PCMs can provide balanced thermal conductivities for heat transfer as later discussed. Since it is important to obtain a total thermal storage performance for the composite PCMs, encapsulation ratio (R) and encapsulation ef­ ficiency (E) are defined and investigated based on following expressions respectively: R¼

Hm;comp � 100% Hm;PCM

(1)

E¼

Hm;comp þ Hf;comp � 100% Hm;PCM þ Hf;PCM

(2)

where, Hm,comp and Hf,comp are the melting and freezing enthalpy of the composite PCMs, while Hm,PCM and Hf,PCM are the melting and freezing enthalpy of pure paraffin, respectively. The R and E value of the com­ posite PCMs are 89.8/89.2%, 93.3/92.6%, and 90.4/90.0%, respec­ tively, comparable to the composite fractions. Moreover, both the encapsulation ratio and efficiency of the MCF-800 is the best in the three composite PCMs. It may be ascribed to the higher content of fluorine element of the MCF-800, which leads to more interfacial interactions. Except for the high thermal storage capacities, the paraffin/MCF composite PCMs also deliver excellent running stability. Fig. 9a–f shows the DSC curves of the paraffin/MCF-700, paraffin/MCF-800, and paraffin/MCF-900 composite PCMs in 50 cycles, with the curves almost overlapped during the cycling. Charge/discharge thermal capacities were calculated based on the DSC curves, suggesting very limit capacity fading. Taking the paraffin/MCF-900 as an example, the initial charge 7

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Fig. 9. DSC curves and the charge/discharge thermal capacities for the (a, b) paraffin/MCF-700, (c, d) paraffin/MCF-800, and (e, f) paraffin/MCF-900 composite PCMs in 50 cycles. (g) Long-term cycling stability of paraffin/MCF-800 in 200 cycles, with inset showing the overlap of the DSC curves.

PCMs is much rapider than the pure paraffin, as also reflected in the cooling process, further confirming the enhanced thermal conductivity of the composite PCMs. To demonstrate the potential applications, a strategy has been

proposed by using the paraffin/MCF composites to collect solar energy from sunlight to provide domestic hot water (Fig. 12a). The prepared composite PCM can be directly packed into a heat-storage device, with the water pipe wrapped in the composite bulk. The collected sunlight 8

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Fig. 10. (a) SEM image of the paraffin/MCF-900 composite PCM after 50 cycles. (b) FTIR spectra patterns of the paraffin/MCF-900 composite PCM before and after 50 cycles. (c) SEM (inset is the enlarged morphology) and (d) TEM images of the cycled MCF-900 after removal of the paraffin.

Fig. 11. (a) The thermal conductivities of the paraffin and paraffin/MCF composite PCMs. (b) Comparison of the temperature response of the paraffin and paraffin/ MCF PCMs below the melting temperature.

can be tuned to reach the composite PCMs at controlled temperature (e. g. 60–100 � C). To test the effect, the composite PCM bulks were directly exposed to the temperature of 60 � C to investigate the thermal proper­ ties. The temperature response of the module made by the paraffin/MCF PCMs presents obvious phase change in 48–50 � C (Fig. 12b), with obvious temperature plateaus for all the three composite PCMs. This is related with the tangible thermal absorption. In accordance with the thermal conductivity, paraffin/MCF-800 composite first reached the phase change plateau due the higher thermal conductivity; while the other two (paraffin/MCF-700 and paraffin/MCF-900) are later and comparable. From this aspect, paraffin/MCF-800 was more suitable for the application among the three composites, since it provided the highest thermal storage capacity and could give the best thermal con­ ductivity. Moreover, the preparation temperature of it was moderate.

The high capacity of the composite PCM can increase the thermalstorage density, important for saving the space of the proposed appli­ cations. Then, the pore structure of MCFs could accommodate the vol­ ume changes during solid-to-liquid transitions and form effective barriers to prevent the leakage of the melted paraffin (as shown in inset of Fig. 12b), thereby offering the high cycling stability. Moreover, the porous structure and the black body of the MCFs are suitable for ab­ sorption of the solar energy [66]. 4. Conclusions In summary, we have demonstrated the synthesis of a lightweight porous carbon fibers with ultrathin graphitic walls from a MgOtemplated method for form-stable PCMs. Different from the carbons 9

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Fig. 12. (a) A proposed strategy for the paraffin/MCF composites to collect solar energy from sunlight to provide domestic hot water. (b) Temperature response of the paraffin/MCF composite bulks based on the phase change process.

with disordered or closed porosity, the MCFs are rich in mesopores and exhibit high capability for loading of paraffin (>90 wt%). The thermal storage capacities of the paraffin/MCF-700, paraffin/MCF-800, and paraffin/MCF-900 reach 176.54, 183.35, and 177.75 J g 1, respectively. Along with the hierarchical porosity, in-situ fluorine doping of MCFs can improve the interfacial property of the MCFs, which render the com­ posite PCMs with uniform solidification during the repeated charge/ discharge thermal cycles. Therefore, the composite PCMs exhibit excellent characteristics in shape stability and maintain high cycling stability and storage efficiency in cycling. Moreover, the graphitic walls of the MCFs render the composite PCM with an enhanced thermal conductivity, which can deliver rapid thermal transfer to interior. All the experimental results support that the as-prepared MCFs can be used for form-stable PCMs for thermal storage.

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Author contribution Xiao Zhu: Conceptualization, Methodology, Experimental, Writing. Liang Han: Experimental, Data Analysis. Fei Yang: Experimental, Visualization. Jie Jiang: Data collection. Xilai Jia: Data Analysis, Writing- Reviewing and Editing, Fund Support. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are thankful to the Fundamental Research Funds for the Central Universities (FRF-AS-17-007), the National Natural Science Foundation of China (No. 51502347), the research funds of the Uni­ versity of Science and Technology Beijing (No. 06500045), and Petro­ China Innovation Foundation (No. 2017D-5007-0507) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110361.

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