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Enhanced thermal performance of form-stable composite phase-change materials supported by novel porous carbon spheres for thermal energy storage
Journal of Energy Storage 27 (2020) 101134
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Enhanced thermal performance of form-stable composite phase-change materials supported by novel porous carbon spheres for thermal energy storage
T
Rong Jia, Sheng Weia,c, Yongpeng Xiaa,c, Chaowei Huanga, Yue Huanga, Huanzhi Zhanga,b, , ⁎ Fen Xua,b, Lixian Suna,b,c, , Xiangcheng Lina,b ⁎
a
School of Material Science & Engineering, Guilin University of Electronic Technology, Guilin 541004, China Guangxi Key Laboratory of Information Materials and Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, Guilin University of Electronic Technology, Guilin 541004, China c School of Mechanical & Electrical Engineering, Guilin University of Electronic Technology, Guilin 541004, China b
ARTICLE INFO
ABSTRACT
Keywords: Porous carbon microspheres n-Octadecane Phase change materials Thermal performance
n-Octadecane/novel porous carbon composite phase-change materials (PCMs) containing a novel porous carbon as the supporting material for n-octadecane were successfully fabricated by vacuum impregnation. The novel porous carbon was first prepared using functionalized polystyrene nanoparticles as a template and soluble starch as a carbon source. The porous carbon spheres can adsorb n-octadecane, preventing them from leaking during the phase-change procedure, and providing them with high thermal conductivity as well as good mechanical and thermal stability. Results show that the novel porous carbon possesses a multi-porous and regular spherical structure with a specific surface area of 1100 cm2/g. Fourier-transform infrared spectroscopy and scanningelectron-microscopy images prove that n-octadecane/novel porous carbon composite PCMs were successfully prepared and exhibited a regular spherical profile. Differential scanning calorimetry reveals that the composite PCMs have an excellent energy storage property, and the latent heat can reach 170.5 J/g, which is higher than the theoretical value. Furthermore, the composite PCMs exhibited an outstanding thermal stability after 100 repeated thermal cycles, and the thermal conductivity were improved up to 0.631 W/(m∙K), which enhanced by 266% compared with n-octadecane. The composite PCMs also displayed a good shape stability without any leakage during heating at 40 °C. These results confirm that the composite PCMs are promising for thermal energy-storage application systems.
1. Introduction Recently, owing to the growing consumption of fossil fuels, thermal energy storage has proved to be a promising technique for energy efficient utilization [1–3]. Phase change materials (PCMs), as good candidates for thermal energy storage, have large energy-storage capacity at a constant temperature [4–6]. Thus, PCMs are very popular for solarenergy utilization [7,8], building energy efficiency [9], heat management of electronics [10], waste-heat recovery [11], and so on. Among the widely studied PCMs, n-octadecane has attracted a great attention due to its high latent heat capacity, no subcooling, good chemical/thermal stability, excellent compatibility, no phase separation, no toxicity, and so on [12–14]. However, as a kind of organic solid-liquid PCM, it still has inherent defects, including low thermal
conductivity and easy leakage during the phase-change procedure [15,16], which seriously restrict their practical applications. As a result, it is popular to incorporate PCMs into porous materials to form novel shape-stabilized composite PCMs with high comprehensive properties. For instance, Yang et al. [17] combined graphene nanoplatelets with microcrystalline cellulose to prepare lightweight composite aerogels, the three-dimensional network of which was favourable to encapsulation of polyethylene glycol (PEG). Zhao et al. [18] prepared a biological porous carbon by a carbonization method to support PEG. Maleki et al. [19] fabricated a highly interconnected porous polystyrene-carbon nanotube foam as a PCM scaffold, and the composite displayed excellent thermal energy-storage properties and good shape stability. Xia et al. [20] demonstrated novel n-octadecane/graphene aerogel composite PCMs that exhibited good comprehensive properties.
⁎ Corresponding authors at: Guangxi Key Laboratory of Information Materials and Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, School of Material Science & Engineering, Guilin University of Electronic Technology, Guilin 541004, China E-mail addresses: [email protected] (H. Zhang), [email protected] (L. Sun).
https://doi.org/10.1016/j.est.2019.101134 Received 12 August 2019; Received in revised form 13 November 2019; Accepted 3 December 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 27 (2020) 101134
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Carbon materials with different morphology and structure have excellent physical and chemical properties [21,22]. As a kind of carbon material, porous carbon materials not only have large specific surface area and moderate adsorption heat, but also possess high thermal conductivity and good mechanical stability [23,24]. Thus, porous carbon materials have been intensively developed in many fields, such as lithium-ion batteries [25], water purification [26], gas adsorption [27], catalysis, and electrochemistry [28]. As a result, they have been proved to be a competitive candidate for PCMs as a supporting material, especially hollow carbon microspheres, which possess large specific surface area, low refractive index, and high thermostability [29]. Therefore, in this study, novel hollow carbon microspheres were chosen as the promising supporting materials for n-octadecane to prepare novel composite PCMs with excellent comprehensive properties. The hollow carbon microspheres can adsorb PCMs, preventing them from leaking during the phase-change procedure, and can provide them with high thermal conductivity as well as good mechanical and thermal stability. First, the novel hollow carbon microspheres were prepared through a template-assisted method by using functionalized polystyrene (PS) nanoparticles as a template and soluble starch as a carbon source. Then, n-octadecane was impregnated into the novel hollow carbon microspheres by vacuum impregnation to receive the novel composite PCMs. Additionally, microstructure, phase-change behaviours, and energy-storage properties of the novel hollow carbon microspheres and the composite PCMs were further investigated.
2.5. Characterization Microstructures of the samples were tested by Fourier-transform infrared (FTIR) spectrometry (FT-IR, Nicolet 6700, USA) on a KBr disk. Morphologies of the samples were characterized using scanning electron microscopy (SEM, JSM-6360LV, JEOL Ltd., Japan) at 20 kV, and the samples were covered with gold plating. Microstructures of the porous carbon were obtained from transmission electron microscopy (TEM, Hitachi JEM-1200EX, JEOL Ltd., Japan). The sample was well dispersed by sonication in ethanol, and then concentrated on a carboncoated copper grid. Microstructures of the porous carbon and composite PCMs were also tested by X-ray-diffraction (XRD, D8 Advance, Bruker, Germany) patterns at 40 kV using Cu Kα radiation in a 2θ range of 10°−90° N2 adsorption-desorption (Autosorb iQ2, Quantachrome sorptometer, USA) isotherms were measured at 77 K to evaluate the porous structure of the obtained carbon. Before measurement, the porous carbon was degassed in a vacuum at 160 °C for at least 6 h. The pore-size distributions and specific surface area of the novel porous carbon were evaluated separately by the Barrett-Joyner-Halenda (BJH) and Brunauer-Emmett-Teller (BET) methods. Thermal properties of the samples were determined via differential scanning calorimetry (DSC 250, TA, USA) from −30 °C to 110 °C at a scanning rate of 10 °C min−1 under N2 atmosphere. The thermal reliability was also investigated by DSC from −30 °C to 80 °C for 100 thermal cycles. Thermal stability of the composite PCMs were evaluated by thermal gravimetric analysis (TGA, Q600, TA, USA) from room temperature to 800 °C at a heating rate of 10 °C min−1 under N2 atmosphere. The thermal conductivity of the obtained composite PCMs was determined with a thermal conductivity analyser DR.-S from Shanghai Jiezhun Instrument Equipment CO. LTD at room temperature (ca. 25 °C), and the accuracy of the thermal conductivity is better than 5%. The leakage test was carried out as follows: the pure n-octadecane and the prepared samples were placed in a plate and then heated in a heating oven for 20 min at 40 °C. Thereafter, their thermo-behavior was captured using a digital camera.
2. Experimental 2.1. Materials Styrene with an analytical purity of 99%, sodium hydroxide (NaOH) with an analytical purity of 85%, sodium bicarbonate (NaHCO3) with an analytical purity of 99.5%, acrylic acid with a chemical purity of 98%, potassium persulfate (KPS) with an analytical purity of 99.5%, and soluble starch (analytical purity) were commercially supplied by Xilong Chemical Co. Ltd. (Shantou, China). n-Octadecane with an analytical purity of 90% was purchased from Tianjin Alfa Aesar Co. (Tianjin, China).
3. Results and discussion
2.2. Preparation of functionalized ps microspheres
3.1. Mechanism to prepare n-octadecane/porous carbon composite PCMs
First, styrene was purified by 5 wt% NaOH solution to remove the inhibitor. Second, amounts of purified styrene, acrylic acid, and NaHCO3 were mixed homogeneously in 80 mL of deionized water and heated at 80 °C for 1 h. Consequently, 1.6 wt% KPS was dropped into the solution to initiate reaction for 24 h. After that, the suspension of the functionalized PS microspheres was produced.
In order to overcome the inherent defects of low thermal conductivity and leakage during the solid-liquid procedure for organic PCMs, porous carbon materials which possess large specific surface area and high thermal conductivity, were considered as an ideal supporting material for PCMs. Therefore, in this study, we designed a novel multiporous carbon as a novel supporting matrix for PCMs, the mechanism is schematically illustrated in Fig. 1. First, PS nanoparticles were prepared and functionalized by acrylic acid through emulsifier-free emulsion copolymerization. The carboxy groups were distributed homogeneously onto the surface of the resulted PS nanoparticles. The resulting PS nanoparticles were then dispersed evenly in aqueous solution as a template and soluble starch was then mixed with the PS particles. During this procedure, hydrogen bonds were formed between the carboxyl groups of PS and the hydroxyl groups of the soluble starch. Consequently, the soluble starch was gradually deposited onto the surface of the PS spheres, forming the spherical PS/soluble starch co-polymers. After calcination, the co-polymers were carbonized, keeping the spherical structure, and the PS spheres were eliminated, resulting in multiporous structure for the obtained carbon. As a result, the obtained multi-porous carbon spheres were successfully prepared. Furthermore, the n-octadecane/porous carbon composites were successfully prepared through vacuum immersion, and n-octadecane was combined with the multi-porous carbon spheres by physical adsorption. Hence, the multiporous carbon microspheres can supply the novel composite PCMs with high thermal conductivity, good mechanical and thermal stability, and excellent comprehensive properties.
2.3. Preparation of novel porous carbon First, the prepared PS suspension and soluble starch were mixed homogeneously in a beaker at 35 °C with stirring for 30 min. The mass ratio of PS suspension/soluble starch was 1:3. Then, the mixture was ultrasonicated for 50 min and poured into an autoclave at 180 °C for 12 h to accomplish hydrothermal synthesis. After being filtered and dried, the obtained polymer products were put into a tube furnace and calcinated at 550 °C under Nitrogen (N2) for 12 h with a heating rate of 5 °C/min. Consequently, the novel porous carbon was produced. 2.4. Preparation of n-octadecane/porous carbon composite PCMs The novel composite PCMs were prepared using a vacuum impregnation method. First, a certain amount of the obtained porous carbon was mixed with melted n-octadecane in a beaker. Then, the mixture was placed in a vacuum drying oven at 60 °C for 2 h. Finally, the novel n-octadecane/porous carbon composite PCMs were produced. The contents of n-octadecane are 50, 60, 70, and 80 wt%. 2
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Fig. 1. The schematic illustration of the novel porous carbon and the composite PCMs.
3.2. FT-IR analysis
the spectrum of the soluble starch, the bands at 1154, 1080, and 1020 cm−1 were assigned to the CeO stretching vibrations with the attachment of OH. The stretching and bending vibrations of the -OH were displayed at 3416 and 1637 cm−1, respectively. The observed band at 2927 cm−1 belonged to the asymmetric stretching vibration of CH2 [31,32]. Upon comparison with the spectrum of the PS/soluble starch copolymer, the peaks of PS microspheres and the soluble starch basically appeared in the spectrum, although some characteristics of PS microspheres and the soluble starch overlapped, such as the characteristic peaks of -COOH in PS microspheres and -OH in the soluble starch. It was clear that the band of 3405 cm−1 is attributed to free OH groups in the spectrum of the obtained multi-porous carbon. Apparently, the peaks at 500–950 cm−1 were almost resolved due to the calcination of the skeleton. In addition, there was less bound water presenting in the carbon spheres due to their multi-porous structure, which brought about the absorption peak at 1652 cm−1. Furthermore, the band at 2922 cm−1 was weakened, while the band at 1154 cm−1 disappeared. Nevertheless, the polymeric structure of soluble starch was basically retained for the obtained multi-porous carbon. Consequently, the multi-porous carbon spheres were successfully synthesized using soluble starch as the carbon source and styrene nanospheres as a template.
Fig. 2 shows the FTIR spectra of pure PS, the modified PS microspheres, soluble starch, PS/soluble starch co-polymer, and carbon microspheres. Compare with pure PS and the modified PS microspheres, it was evident that the new absorption band appearing at approximately 3423 cm−1 stands for -COOH, which was attributed to the co-polymerization of acrylic acid and styrene. The peaks at 3025, 3059, and 3082 cm−1 corresponded to the stretching vibration of the benzene ring. The peak at 2921 cm−1 represented the asymmetric stretching vibration of CH2. The typical peaks at 1601 and 1452 cm−1 were caused by the bending vibration of the C ] C bond in the benzene ring. The peaks at 756 and 697 cm−1 were caused by single substitution in the benzene ring. These are all the characteristic peaks of PS molecular chain [30]. In
3.3. Microimages and microstructure Fig. 3 shows SEM images of the functionalized PS microspheres, PS/ starch co-polymer, and porous carbon. It was significant that the PS microspheres were regular with a smooth surface and a uniform particle-size distribution. Their diameter was approximately 460 nm. The resulting carbon still presented a spherical structure with a diameter of 480 nm. However, the carbon spheres adhered together and showed some collapse in their structure. It was significant that the resultant PS/ soluble starch co-polymer microspheres presented an irregular surface due to the stacking of some starch on the surface of the microspheres, whose particle size is in a range of 3.47–4.38 µm. Nevertheless, they still possessed a uniform particle-size distribution, and, as seen in Fig. 3(d), the obtained carbon after calcination is comprised regular microspheres with a particle size in the range 3.61–5.23 µm.
Fig. 2. FT-IR spectra of (a) pure PS, (b) soluble starch, (c) modified PS microspheres, (d) PS/soluble starch copolymer and (e) the porous carbon microspheres. 3
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Fig. 3. SEM images of: (a) PS micro-spheres, (b) the carbon obtained from PS micro-spheres, (c) PS/soluble starch copolymer and (d) the porous carbon microspheres obtained from the copolymer.
Furthermore, the TEM image indicates that the obtained carbon microspheres exhibit a spherical structure and some light dots are homogeneously distributed in the microspheres as shown in Fig. 4, indicating a multi-porous structure of the obtained carbon. Also evidenced was a light core surrounded by a dark shell layer with a thickness of approximately 130 nm, identifying a hollow core-shell structure in the porous carbon microspheres. This result meant that the PS nanospheres were calcinated with little residual carbon, leading to the hollow core-shell structure. Some pores were also formed in the shell carbon due to a coarse accumulation of soluble starch. As a result, the regular multi-porous carbon microspheres were successfully prepared with a hollow core-shell structure and have a potential application in absorption systems. Additionally, the crystal structure of the obtained hollow carbon microspheres was further analyzed by XRD, and the results are shown in Fig. 5. It was significant that two wide peaks appeared in the range of 2θ of 21.7°, and 42.3°, indicating the typical peaks of amorphous carbon with non-crystalline structures [33]. Fig. 5. XRD pattern of the porous carbon microspheres.
Fig. 4. TEM image of the porous carbon microspheres. 4
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3.5. SEM images of n-octadecane/porous carbon composite PCMs According to the above results, we prepared n-octadecane/porous carbon composites by using the obtained porous carbon as a supporting material and n-octadecane as PCMs. In Fig. 7, it is obvious that the composites maintain the spherical structure of the carbon. However, the micro-morphologies of the composites are different due to the different contents of n-octadecane. Regarding the composite with 50 wt% n-octadecane, its surface was smooth and compact with a homogeneous particle-size distribution, which was consistent with that of porous carbon. The composites with 70 wt% n-octadecane still possessed a smooth and compact surface, and the particles were dispersed evenly. Furthermore, some of the composites were adhered together, resulting from the increase of n-octadecane, and some small particles also adhered to the surface of the composites due to the excessive amount of noctadecane. As the content of n-octadecane gradually increased to 80 wt%, most of the composites accumulated and were stacked together with a coarse surface. This implied that 70 wt% n-octadecane is the optimal content with which to prepare the n-octadecane/porous carbon composites, leading to a regular spherical structure for the composites. Fig. 6. N2 adsorption-desorption isotherms and pore size distribution (insert) of the porous carbon microspheres.
3.6. FTIR analysis of n-octadecane/porous carbon composite PCMs
3.4. N2 adsorption-desorption of porous carbon
The chemical compositions of n-octadecane and n-octadecane/ porous carbon composites were also evaluated by FTIR spectroscopy, and the results are shown in Fig. 8. It is evident that the spectra of all the composites were similar, although the intensity of the peaks was slightly different. The bending vibrations of -CH2 were observed at 1466 and 1378 cm−1. The intensive peaks at 2957, 2923, and 2853 cm−1 were assigned to the stretching vibrations of -CH3 and -CH2 in nalkyl. In addition, the in-plane rocking vibration of -CH2 appeared at 720 cm−1. It was noteworthy that these are the characteristic peaks of n-octadecane, and these peaks also could be seen in the FTIR spectra of all the composites. Furthermore, the intensive bands at 3424 and 1629 cm−1 were assigned to a small amount of bound water present in the carbon spheres due to their multi-porous structure. This result is also associated with the characteristic peaks of the porous carbon, and proves that n-octadecane was successfully encapsulated into the obtained novel porous carbon, forming the n-octadecane/porous carbon composites.
The adsorption properties of multi-porous carbon mainly depend on its surface area, pore volume, and pore size. Therefore, N2 adsorptiondesorption isotherms and the resulting pore-size distribution are displayed in Fig. 6, to evaluate the absorption properties of the obtained carbon. It was clearly observed that the carbon spheres exhibited the type-IV isotherm. The initial adsorption at 0.0–0.2 relative pressure indicated the formation of micropores from the carbonization of soluble starch, which was consistent with the pore-size distribution of 0.5–0.75 nm. The adsorption at 0.2–0.9 relative pressure suggested a mesopore distribution of approximately 3 nm. This was because the PS microspheres were eliminated and the starch was cross-linked during carbonization. Additionally, according to the BET calculation, the surface area of the porous carbon reached 1100 m2/g, which further confirmed that the obtained carbon was a good absorption matrix and was favorable for the encapsulation of PCMs.
Fig. 7. SEM images of the n-octadecane/porous carbon composite PCMs with different content of n-octadecane: (a) 50 wt%, (b) 60 wt%, (c) 70 wt% and (d) 80 wt%. 5
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Fig. 10. DSC curves of (a) pristine n-octadecane, and the n-octadecane/porous carbon composite PCMs with different content of n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt%. The numbers of the curves are corresponding to the sample codes in Table 1.
Fig. 8. FT-IR spectrums of (a) pristine n-octadecane, and the n-octadecane/ porous carbon composite PCMs with different content of n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt%.
3.8. Thermal properties of n-octadecane/porous carbon composite PCMs The phase-change behavior was crucial for PCMs to evaluate their feasibility for thermal energy-storage applications. Therefore, DSC curves were employed to investigate the temperature change and energy-storage capability of the obtained composites, as seen in Fig. 10, the relevant data are displayed in Table 1. The n-octadecane changes its phase at a melting temperature of 29.9 °C and crystal temperature of 23.8 °C. The corresponding latent heat of n-octadecane is 236.4 and 235.4 J/g, respectively. This implied that n-octadecane is a proper PCM with a high latent heat for room-temperature applications in thermal energy-storage systems. As observed from Fig. 10, all the obtained composites obviously displayed a single strong melting/crystal peak, which are the same as in bulk n-octadecane, indicating that the composites have an obviously excellent phase-change property. This also confirmed that n-octadecane still maintained its good phase-change property and that porous carbon Table 1 Thermal characteristics of pristine n-octadecane and the prepared n-octadecane/porous carbon composite PCMs.
Fig. 9. XRD patterns of (a) pristine n-octadecane, and the n-octadecane/porous carbon composite PCMs with different content of n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt%.
3.7. XRD analysis of the n-octadecane/porous carbon composite PCMs The XRD patterns were further detected to evaluate the crystallinity of the n-octadecane/porous carbon composites, and are displayed in Fig. 9. The porous carbon was obviously amorphous, as was known from the XRD profile. Thus, the crystallinity of the porous-carbon-encapsulated n-octadecane was quite easy to test. It is significant that the intensive peaks at 2θ of 18.98°, 19.47°, 23.06°, 24.45°, 34.34°, and 39.29° were indexed as the (0 1 0), (0 1 1), (1 0 0), (1 1 1), (−1 1 0) and (0 −2 2) planes of the n-octadecane, respectively. These peaks also presented in the patterns of the composite PCMs, although the diffraction intensity was decreased after encapsulation. These results confirmed that the composite PCMs were crystalline, and the interior noctadecane well maintained its crystallinity, which ensured the good phase-change behavior of the novel n-octadecane/porous carbon composite PCMs.
Samples
Melting Tm ( °C)
△Hm (J∙g
(a) (b) (c) (d) (e)
29.9 29.3 29.5 29.8 28.7
236.4 130.1 140.6 170.5 188.0
Crystallization Tc ( °C) △Hc (J∙g
− 1
)
23.8 24.0 23.9 24.2 24.8
− 1
)
235.4 129.3 139.9 169.0 187.3
Tc: Peak temperature of the cooling curves, ΔHc: the latent heat during cooling process; Tm: Peak temperature of melting curves, ΔHm: the latent heat during melting process. Table 2 The thermal reliability of the n-octadecane/porous carbon composite PCMs with 70 wt% n-octadecane.
6
No. of cycles
Melting Tm ( °C)
△Hm (J∙g
1st 20th 50th 100th
31.5 30.7 31.3 30.7
173.3 169.4 164.1 163.0
− 1
)
Crystallization Tc ( °C) △Hc (J∙g 22.4 22.4 22.4 22.2
173.1 170.7 168.2 162.6
− 1
)
Journal of Energy Storage 27 (2020) 101134
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as a proper supporting matrix had little effect on its phase-change property. Furthermore, the melting temperature of all the composites was lower than that of n-octadecane, because the porous carbon with high thermal conductivity effected the thermal response rate of the composite PCMs. Hence, the composites could rapidly react to the temperature change and melt at a lower melting temperature compared with bulk n-octadecane. Moreover, the crystal temperatures of the composites were all higher, as they were derived from the fast thermal response rate. As a result, the supercooling of all the composites with different n-octadecane contents was lower than that of bulk n-octadecane. This result also confirmed that the porous carbon with high thermal conductivity enhanced the phase-change behavior of the interior n-octadecane. It further implied that porous carbon is a good supporting material with which to prepare composite PCMs, which not only keep the interior n-octadecane without leakage, but also provide the composite PCMs with excellent comprehensive thermal properties. The n-octadecane content had little influence on the phase-change temperatures of the composites. However, the latent heat of the composites increased significantly due to the increased n-octadecane content. In addition, the latent heat of the composite with 50 wt% n-octadecane was 130.1 J/g and 129.3 J/g during the melting and cooling processes, respectively, while for the composite with 60 wt% n-octadecane the melting and cooling latent heat values were slightly enhanced to 140.6 J/g and 139.9 J/g, respectively. As the content of noctadecane increased to 70 wt%, the corresponding latent heat values obviously increased to 170.5 J/g and 169.0 J/g, respectively which were higher than the respective theoretical values of 165.5 J/g and 164.8 J/g. Even in the composite with 80 wt% n-octadecane, which contained the largest content of n-octadecane among the four samples studied, its corresponding latent heat values were 188.0 J/g and 187.3 J/g, respectively, and lower than the respective ideal values of 189.1 J/g and 188.3 J/g. These results affirmed that 70 wt% n-octadecane was the optimal concentration with which to prepare the composite with excellent phase-change behavior and for which the porous carbon can fully absorb the n-octadecane. Meanwhile, the noctadecane was dispersed homogeneously into the porous structure of the porous carbon, and the carbon could form a continuous thermal conductive network that ensures that the interior n-octadecane fully changes its phase during its phase-change procedure. Consequently, this sample exhibited the best phase-change properties among all the samples. Once the n-octadecane content increased, excessive n-octadecane accumulated onto the surface of the composite, as can be seen from Fig. 3, which cannot fully change its phase with low thermal conductivity during the phase-change procedure, resulting in lower latent heat. This confirmed that the composite with 70 wt% n-octadecane is a good candidate for the preparation of composite PCMs with good properties.
Fig. 11. Thermal reliability of the n-octadecane/porous carbon composite PCMs with 70 wt% n-octadecane for 100 DSC thermal cycles. Inset shows the DSC curves of the sample. The numbers of the curves are corresponding to the sample codes in Table 2.
changes phase because of its good thermal conductivity. These results indicated that the durability of the composites is almost unchanged after 100 thermal cycles. Consequently, the obtained n-octadecane/ porous carbon composite PCMs possess outstanding thermal stability for thermal energy-storage systems. Furthermore, in order to identify whether the interior n-octadecane change its phase after 100 thermal cycles, XRD diffractions were performed on the sample again and shown in Fig. 12. It can be observed evidently that the characteristic diffraction peaks of n-octadecane are almost the same before and after 100 thermal cycles, and there is no new peak emerge or peak disappear after 100 thermal cycles. This result confirms that the phase of the interior n-octadecane has not changed after 100 cycles and the obtained n-octadecane/porous carbon composite PCMs display an excellent thermal stability and reliability. In addition, TGA curves were used to investigate whether there is any leakage of the interior n-octadecane during 100 thermal cycles. As clearly seen from Fig. 13, there is only a single degradation step, attributed to the evaporation of the interior n-octadecane. Evidently, the
3.9. Thermal reliability of n-octadecane/porous carbon composite PCMs The composite PCMs should have long operating cycle lives to ensure their thermal stability for practical applications. Therefore, the noctadecane/porous carbon composite PCMs with 70 wt% n-octadecane experienced 100 heating and cooling cycles to clarify their heat-storage durability. From Fig. 11, the phase-change temperatures of the composite PCM exhibited little change during 100 thermal cycles, and the small temperature changes are almost in the range of 1 °C. This slight discrepancy during 100 repeated thermal cycles is within the acceptable range of thermal energy-storage systems. Moreover, the latent heat of the composites showed a slight decrease after 100 thermal cycles. In addition, the loss ratio of the latent heat ranges from 2.25% to 5.95% after 100 thermal cycles. There was no obvious difference for the composites undergoing 100 thermal cycles. It further implied that the porous carbon can effectively control the interior n-octadecane in its porous structure during the phase-change procedure due to its good mechanical stability and ensure that the interior n-octadecane fully
Fig. 12. XRD diffractions of the composite PCMs before and after 100 thermal cycles. 7
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Fig. 13. TGA curves of the composite PCMs before and after 100 thermal cycles.
Fig. 15. TGA curves of (a) pristine n-octadecane, and the n-octadecane/porous carbon composite PCMs with different content of n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt%.
initial degradation temperature and the weight loss of the composite PCMs during the whole process have no change before or after the 100 thermal cycles. These results fully illustrate that the n-octadecane can be effectively encapsulated by the porous carbon without leakage during 100 thermal cycles and the composite PCMs have a great potential for long-term practical applications in thermal energy-storage systems.
the sample with 70 wt% n-octadecane also can achieve 0.631 W/(m∙K), which enhanced by 266% compared with that of n-octadecane. While the thermal conductivity of the sample with 50 wt% n-octadecane reached 0.6997 W/(m∙K), which improved by 306%. These results depict that the porous carbon formed a continuous thermal conductive network in the composite PCMs, which played a critical role in enhancing the thermal conductivity of the composite PCMs, and thus enhances the thermal response rate for thermal energy storage.
3.10. Thermal conductivity of n-octadecane/porous carbon composite PCMs
3.11. Thermal stability of n-octadecane/porous carbon composite PCMs
Thermal conductivity is a key factor to investigate the thermal response rate of PCMs to the storage and release of the latent heat. Therefore, enhancement in thermal conductivity is absolutely essential when designing novel composite PCMs. In this study, it is expected that the novel porous carbon with high thermal conductivity compared with PCMs can supply the composite PCMs with good thermal conductivity. As shown in Fig. 14, pure n-octadecane displayed a thermal conductivity as low as 0.1724 W/(m∙K). Evidently, all the obtained composite PCMs possess higher thermal conductivity than that of pure noctadecane. And the thermal conductivity of the composite PCMs significantly improved with the increasing weight amount of porous carbon, implying the presence of porous carbon endowed the composite PCMs with good thermal conductivity. And the thermal conductivity of
Thermal stability is an important factor for the application of PCMs in heat energy storage or thermal regulation. Therefore, TGA test was carried out to evaluate the thermal stability of the obtained composite PCMs, and the resulted TGA curves were shown in Fig. 15. All the sample significantly exhibited a single one weight loss step during the degradation process, which was ascribed to the molecular degradation of n-octadecane. And the weight loss percentage was in good agreement with the addition amount of n-octadecane. Furthermore, the onset degradation temperature for the composite PCM with 50 wt%, 60 wt%, 70 wt%, 80 wt% and pure n-octadecane was 166.06 °C, 170.82 °C, 178.31 °C, 199.7 °C and 215.5 °C, respectively, which increased with the increasing of the n-octadecane loading amount. This result indicated that the novel porous carbon was an appropriate supporting material for noctadecane, and the composites possess high thermal stabilities during their phase change temperature range of 23.9 °C~29.9 °C. Besides, the porous carbon enhanced the thermal conductivity of the obtained composite PCMs, which reduced the onset degradation temperature gradually of the composite PCMs. These results were in consistent with the results from the thermal conductivity tests. 3.12. Leakage test and shape stability of n-octadecane/porous carbon composite PCMs The shape stability is also an important parameter of the composite PCMs for thermal energy storage applications, which was investigated by a leakage test as shown in Fig. 16. As was shown, the pure n-octadecane would melt easily with obvious liquid leakage after heated 10 min, while the composite PCMs presented almost no change as seen from the morphology, indicating that the composite PCMs could well maintain their solid state confirming a good shape stability without any leakage. When heated for 20 min, the pure n-octadecane completely melt into liquid owing to its solid-to-liquid phase change procedure, the
Fig. 14. Thermal conductivity of pure n-octadecane and the obtained composite PCMs. 8
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2014GXNSFAA118401, and 2013GXNSFBA019244), the Science Research and Technology Development Program of Guangxi (AD17195073, AA17202030-1), Innovation Project of Guet Graduate Education (2019YCXS114, 2016YJCX21) and Program for Postgraduate Joint Training Base of GUET-CJYRE (No. 20160513-14-Z). References [1] R. Cao, S. Chen, Y. Wang, N. Han, H. Liu, X. Zhang, Functionalized carbon nanotubes as phase change materials with enhanced thermal, electrical conductivity, light-to-thermal, and electro-to-thermal performances, Carbon N Y 149 (2019) 263–272, https://doi.org/10.1016/j.carbon.2019.04.005. [2] Q. Sun, H. Zhang, J. Xue, X. Yu, Y. Yuan, X. Cao, Flexible phase change materials for thermal storage and temperature control, Chem. Eng. J. 353 (2018) 920–929, https://doi.org/10.1016/j.cej.2018.07.185. [3] K. Reddy, V. Mudgal, T. Mallick, Review of latent heat thermal energy storage for improved material stability and effective load management, J. 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Fig. 16. Photographs of (a) n-octadecane and the n-octadecane/porous carbon composite PCMs n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt% during heating process at 40 °C.
composite PCMs still retain their initial solid shape without any leakage although the surface of the sample with 80 wt% n-octadecane is slightly wet due to the excess n-octadecane sticking on the surface of the sample. These results reveal that the novel porous carbon would confine the movement of molten n-octadecane, that the n-octadecane can not leak out and the composite PCMs would keep solid state even the working temperature beyond its melting point. A conclusion could be drawn that the porous carbon played a key role in enhancing the shape stability of PCMs during phase change procedure and the obtained noctadecane/porous carbon composite PCMs possess excellent shape stability. 4. Conclusions To enhance the thermal conductivity and comprehensive properties of PCMs, novel porous carbon microspheres were designed using functionalized PS nanoparticles as a template and soluble starch as a carbon source. The n-octadecane/novel porous carbon composite PCMs using n-octadecane as PCMs and the novel porous carbon as the supporting material were successfully fabricated through vacuum impregnation. The porous carbon endows the composite PCMs with excellent comprehensive thermal performance. The N2 absorption-desorption curves, SEM images, and TEM analysis results show that the novel carbon possesses a multi-porous and regular spherical structure, and its ratio surface area reaches 1100 cm2/g with a pore-size distribution of approximately 1.76–6.15 nm. FTIR spectroscopy spectra and SEM images confirm that the n-octadecane/novel porous carbon composite PCMs were successfully prepared and exhibited a regular spherical profile. The composite PCMs have an excellent energystorage property, and the latent heat can reach 170.5 J/g, which is higher than the theoretical value. Furthermore, the composite PCMs exhibited outstanding thermal stability after 100 repeated thermal cycles. And the thermal conductivity of the sample with 70 wt% n-octadecane reached 0.631 W/(m∙K), which enhanced by 266% compared with n-octadecane. The composite PCMs also displayed a good shape stability without any leakage during heating at 40 °C. These results confirm that the composite PCMs are promising for thermal energy-storage application systems. Declaration of Competing Interest There are no conflict to declare. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51863005, 51462006, 51102230, 51871065, and U1501242), the Guangxi Natural Science Foundation (No. 2018GXNSFDA281051, 9
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carbon.2016.12.064. [28] X. Lu, L. Gu, J. Wang, J. Wu, P. Liao, G. Li, Bimetal-Organic framework derived cofe2o4/c porous hybrid nanorod arrays as high-performance electrocatalysts for oxygen evolution reaction, Advanced materials 29 (3) (2017) 1604437, , https:// doi.org/10.1002/adma.201604437. [29] S. Sami, S. Sadrameli, N. Etesami, Thermal properties optimization of microencapsulated a renewable and non-toxic phase change material with a polystyrene shell for thermal energy storage systems, Appl. Therm. Eng. 130 (2018) 1416–1424, https://doi.org/10.1016/j.applthermaleng.2017.11.119. [30] F. Ye, M. Miao, K. Lu, B. Jiang, X. Li, S. Cui, Structure and physicochemical properties for modified starch-based nanoparticle from different maize varieties, Food Hydrocoll. 67 (2017) 37–44, https://doi.org/10.1016/j.foodhyd.2016.12.041. [31] S. Ghaseminezhad, S. Shojaosadati, Evaluation of the antibacterial activity of ag/ fe3o4 nanocomposites synthesized using starch, Carbohydr. Polym. 144 (2016) 454–463, https://doi.org/10.1016/j.carbpol.2016.03.007. [32] Y. Men, X. Du, J. Shen, L. Wang, Z. Liu, Preparation of corn starch- g -polystyrene copolymer in ionic liquid: 1-Ethyl-3-methylimidazolium acetate, Carbohydr. Polym. 121 (2015) 348–354, https://doi.org/10.1016/j.carbpol.2014.12.068. [33] J. Shu, R. Ma, M. Shui, D. Wang, N. Long, Y. Ren, R. Zhang, J. Yao, X. Mao, W. Zheng, S. Gao, Facile controlled growth of silica on carbon spheres and their superior electrochemical properties, RSC Adv. 2 (13) (2012) 5806, https://doi.org/ 10.1039/c2ra20284k.
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Enhanced thermal performance of form-stable composite phase-change materials supported by novel porous carbon spheres for thermal energy storage
T
Rong Jia, Sheng Weia,c, Yongpeng Xiaa,c, Chaowei Huanga, Yue Huanga, Huanzhi Zhanga,b, , ⁎ Fen Xua,b, Lixian Suna,b,c, , Xiangcheng Lina,b ⁎
a
School of Material Science & Engineering, Guilin University of Electronic Technology, Guilin 541004, China Guangxi Key Laboratory of Information Materials and Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, Guilin University of Electronic Technology, Guilin 541004, China c School of Mechanical & Electrical Engineering, Guilin University of Electronic Technology, Guilin 541004, China b
ARTICLE INFO
ABSTRACT
Keywords: Porous carbon microspheres n-Octadecane Phase change materials Thermal performance
n-Octadecane/novel porous carbon composite phase-change materials (PCMs) containing a novel porous carbon as the supporting material for n-octadecane were successfully fabricated by vacuum impregnation. The novel porous carbon was first prepared using functionalized polystyrene nanoparticles as a template and soluble starch as a carbon source. The porous carbon spheres can adsorb n-octadecane, preventing them from leaking during the phase-change procedure, and providing them with high thermal conductivity as well as good mechanical and thermal stability. Results show that the novel porous carbon possesses a multi-porous and regular spherical structure with a specific surface area of 1100 cm2/g. Fourier-transform infrared spectroscopy and scanningelectron-microscopy images prove that n-octadecane/novel porous carbon composite PCMs were successfully prepared and exhibited a regular spherical profile. Differential scanning calorimetry reveals that the composite PCMs have an excellent energy storage property, and the latent heat can reach 170.5 J/g, which is higher than the theoretical value. Furthermore, the composite PCMs exhibited an outstanding thermal stability after 100 repeated thermal cycles, and the thermal conductivity were improved up to 0.631 W/(m∙K), which enhanced by 266% compared with n-octadecane. The composite PCMs also displayed a good shape stability without any leakage during heating at 40 °C. These results confirm that the composite PCMs are promising for thermal energy-storage application systems.
1. Introduction Recently, owing to the growing consumption of fossil fuels, thermal energy storage has proved to be a promising technique for energy efficient utilization [1–3]. Phase change materials (PCMs), as good candidates for thermal energy storage, have large energy-storage capacity at a constant temperature [4–6]. Thus, PCMs are very popular for solarenergy utilization [7,8], building energy efficiency [9], heat management of electronics [10], waste-heat recovery [11], and so on. Among the widely studied PCMs, n-octadecane has attracted a great attention due to its high latent heat capacity, no subcooling, good chemical/thermal stability, excellent compatibility, no phase separation, no toxicity, and so on [12–14]. However, as a kind of organic solid-liquid PCM, it still has inherent defects, including low thermal
conductivity and easy leakage during the phase-change procedure [15,16], which seriously restrict their practical applications. As a result, it is popular to incorporate PCMs into porous materials to form novel shape-stabilized composite PCMs with high comprehensive properties. For instance, Yang et al. [17] combined graphene nanoplatelets with microcrystalline cellulose to prepare lightweight composite aerogels, the three-dimensional network of which was favourable to encapsulation of polyethylene glycol (PEG). Zhao et al. [18] prepared a biological porous carbon by a carbonization method to support PEG. Maleki et al. [19] fabricated a highly interconnected porous polystyrene-carbon nanotube foam as a PCM scaffold, and the composite displayed excellent thermal energy-storage properties and good shape stability. Xia et al. [20] demonstrated novel n-octadecane/graphene aerogel composite PCMs that exhibited good comprehensive properties.
⁎ Corresponding authors at: Guangxi Key Laboratory of Information Materials and Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, School of Material Science & Engineering, Guilin University of Electronic Technology, Guilin 541004, China E-mail addresses: [email protected] (H. Zhang), [email protected] (L. Sun).
https://doi.org/10.1016/j.est.2019.101134 Received 12 August 2019; Received in revised form 13 November 2019; Accepted 3 December 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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Carbon materials with different morphology and structure have excellent physical and chemical properties [21,22]. As a kind of carbon material, porous carbon materials not only have large specific surface area and moderate adsorption heat, but also possess high thermal conductivity and good mechanical stability [23,24]. Thus, porous carbon materials have been intensively developed in many fields, such as lithium-ion batteries [25], water purification [26], gas adsorption [27], catalysis, and electrochemistry [28]. As a result, they have been proved to be a competitive candidate for PCMs as a supporting material, especially hollow carbon microspheres, which possess large specific surface area, low refractive index, and high thermostability [29]. Therefore, in this study, novel hollow carbon microspheres were chosen as the promising supporting materials for n-octadecane to prepare novel composite PCMs with excellent comprehensive properties. The hollow carbon microspheres can adsorb PCMs, preventing them from leaking during the phase-change procedure, and can provide them with high thermal conductivity as well as good mechanical and thermal stability. First, the novel hollow carbon microspheres were prepared through a template-assisted method by using functionalized polystyrene (PS) nanoparticles as a template and soluble starch as a carbon source. Then, n-octadecane was impregnated into the novel hollow carbon microspheres by vacuum impregnation to receive the novel composite PCMs. Additionally, microstructure, phase-change behaviours, and energy-storage properties of the novel hollow carbon microspheres and the composite PCMs were further investigated.
2.5. Characterization Microstructures of the samples were tested by Fourier-transform infrared (FTIR) spectrometry (FT-IR, Nicolet 6700, USA) on a KBr disk. Morphologies of the samples were characterized using scanning electron microscopy (SEM, JSM-6360LV, JEOL Ltd., Japan) at 20 kV, and the samples were covered with gold plating. Microstructures of the porous carbon were obtained from transmission electron microscopy (TEM, Hitachi JEM-1200EX, JEOL Ltd., Japan). The sample was well dispersed by sonication in ethanol, and then concentrated on a carboncoated copper grid. Microstructures of the porous carbon and composite PCMs were also tested by X-ray-diffraction (XRD, D8 Advance, Bruker, Germany) patterns at 40 kV using Cu Kα radiation in a 2θ range of 10°−90° N2 adsorption-desorption (Autosorb iQ2, Quantachrome sorptometer, USA) isotherms were measured at 77 K to evaluate the porous structure of the obtained carbon. Before measurement, the porous carbon was degassed in a vacuum at 160 °C for at least 6 h. The pore-size distributions and specific surface area of the novel porous carbon were evaluated separately by the Barrett-Joyner-Halenda (BJH) and Brunauer-Emmett-Teller (BET) methods. Thermal properties of the samples were determined via differential scanning calorimetry (DSC 250, TA, USA) from −30 °C to 110 °C at a scanning rate of 10 °C min−1 under N2 atmosphere. The thermal reliability was also investigated by DSC from −30 °C to 80 °C for 100 thermal cycles. Thermal stability of the composite PCMs were evaluated by thermal gravimetric analysis (TGA, Q600, TA, USA) from room temperature to 800 °C at a heating rate of 10 °C min−1 under N2 atmosphere. The thermal conductivity of the obtained composite PCMs was determined with a thermal conductivity analyser DR.-S from Shanghai Jiezhun Instrument Equipment CO. LTD at room temperature (ca. 25 °C), and the accuracy of the thermal conductivity is better than 5%. The leakage test was carried out as follows: the pure n-octadecane and the prepared samples were placed in a plate and then heated in a heating oven for 20 min at 40 °C. Thereafter, their thermo-behavior was captured using a digital camera.
2. Experimental 2.1. Materials Styrene with an analytical purity of 99%, sodium hydroxide (NaOH) with an analytical purity of 85%, sodium bicarbonate (NaHCO3) with an analytical purity of 99.5%, acrylic acid with a chemical purity of 98%, potassium persulfate (KPS) with an analytical purity of 99.5%, and soluble starch (analytical purity) were commercially supplied by Xilong Chemical Co. Ltd. (Shantou, China). n-Octadecane with an analytical purity of 90% was purchased from Tianjin Alfa Aesar Co. (Tianjin, China).
3. Results and discussion
2.2. Preparation of functionalized ps microspheres
3.1. Mechanism to prepare n-octadecane/porous carbon composite PCMs
First, styrene was purified by 5 wt% NaOH solution to remove the inhibitor. Second, amounts of purified styrene, acrylic acid, and NaHCO3 were mixed homogeneously in 80 mL of deionized water and heated at 80 °C for 1 h. Consequently, 1.6 wt% KPS was dropped into the solution to initiate reaction for 24 h. After that, the suspension of the functionalized PS microspheres was produced.
In order to overcome the inherent defects of low thermal conductivity and leakage during the solid-liquid procedure for organic PCMs, porous carbon materials which possess large specific surface area and high thermal conductivity, were considered as an ideal supporting material for PCMs. Therefore, in this study, we designed a novel multiporous carbon as a novel supporting matrix for PCMs, the mechanism is schematically illustrated in Fig. 1. First, PS nanoparticles were prepared and functionalized by acrylic acid through emulsifier-free emulsion copolymerization. The carboxy groups were distributed homogeneously onto the surface of the resulted PS nanoparticles. The resulting PS nanoparticles were then dispersed evenly in aqueous solution as a template and soluble starch was then mixed with the PS particles. During this procedure, hydrogen bonds were formed between the carboxyl groups of PS and the hydroxyl groups of the soluble starch. Consequently, the soluble starch was gradually deposited onto the surface of the PS spheres, forming the spherical PS/soluble starch co-polymers. After calcination, the co-polymers were carbonized, keeping the spherical structure, and the PS spheres were eliminated, resulting in multiporous structure for the obtained carbon. As a result, the obtained multi-porous carbon spheres were successfully prepared. Furthermore, the n-octadecane/porous carbon composites were successfully prepared through vacuum immersion, and n-octadecane was combined with the multi-porous carbon spheres by physical adsorption. Hence, the multiporous carbon microspheres can supply the novel composite PCMs with high thermal conductivity, good mechanical and thermal stability, and excellent comprehensive properties.
2.3. Preparation of novel porous carbon First, the prepared PS suspension and soluble starch were mixed homogeneously in a beaker at 35 °C with stirring for 30 min. The mass ratio of PS suspension/soluble starch was 1:3. Then, the mixture was ultrasonicated for 50 min and poured into an autoclave at 180 °C for 12 h to accomplish hydrothermal synthesis. After being filtered and dried, the obtained polymer products were put into a tube furnace and calcinated at 550 °C under Nitrogen (N2) for 12 h with a heating rate of 5 °C/min. Consequently, the novel porous carbon was produced. 2.4. Preparation of n-octadecane/porous carbon composite PCMs The novel composite PCMs were prepared using a vacuum impregnation method. First, a certain amount of the obtained porous carbon was mixed with melted n-octadecane in a beaker. Then, the mixture was placed in a vacuum drying oven at 60 °C for 2 h. Finally, the novel n-octadecane/porous carbon composite PCMs were produced. The contents of n-octadecane are 50, 60, 70, and 80 wt%. 2
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Fig. 1. The schematic illustration of the novel porous carbon and the composite PCMs.
3.2. FT-IR analysis
the spectrum of the soluble starch, the bands at 1154, 1080, and 1020 cm−1 were assigned to the CeO stretching vibrations with the attachment of OH. The stretching and bending vibrations of the -OH were displayed at 3416 and 1637 cm−1, respectively. The observed band at 2927 cm−1 belonged to the asymmetric stretching vibration of CH2 [31,32]. Upon comparison with the spectrum of the PS/soluble starch copolymer, the peaks of PS microspheres and the soluble starch basically appeared in the spectrum, although some characteristics of PS microspheres and the soluble starch overlapped, such as the characteristic peaks of -COOH in PS microspheres and -OH in the soluble starch. It was clear that the band of 3405 cm−1 is attributed to free OH groups in the spectrum of the obtained multi-porous carbon. Apparently, the peaks at 500–950 cm−1 were almost resolved due to the calcination of the skeleton. In addition, there was less bound water presenting in the carbon spheres due to their multi-porous structure, which brought about the absorption peak at 1652 cm−1. Furthermore, the band at 2922 cm−1 was weakened, while the band at 1154 cm−1 disappeared. Nevertheless, the polymeric structure of soluble starch was basically retained for the obtained multi-porous carbon. Consequently, the multi-porous carbon spheres were successfully synthesized using soluble starch as the carbon source and styrene nanospheres as a template.
Fig. 2 shows the FTIR spectra of pure PS, the modified PS microspheres, soluble starch, PS/soluble starch co-polymer, and carbon microspheres. Compare with pure PS and the modified PS microspheres, it was evident that the new absorption band appearing at approximately 3423 cm−1 stands for -COOH, which was attributed to the co-polymerization of acrylic acid and styrene. The peaks at 3025, 3059, and 3082 cm−1 corresponded to the stretching vibration of the benzene ring. The peak at 2921 cm−1 represented the asymmetric stretching vibration of CH2. The typical peaks at 1601 and 1452 cm−1 were caused by the bending vibration of the C ] C bond in the benzene ring. The peaks at 756 and 697 cm−1 were caused by single substitution in the benzene ring. These are all the characteristic peaks of PS molecular chain [30]. In
3.3. Microimages and microstructure Fig. 3 shows SEM images of the functionalized PS microspheres, PS/ starch co-polymer, and porous carbon. It was significant that the PS microspheres were regular with a smooth surface and a uniform particle-size distribution. Their diameter was approximately 460 nm. The resulting carbon still presented a spherical structure with a diameter of 480 nm. However, the carbon spheres adhered together and showed some collapse in their structure. It was significant that the resultant PS/ soluble starch co-polymer microspheres presented an irregular surface due to the stacking of some starch on the surface of the microspheres, whose particle size is in a range of 3.47–4.38 µm. Nevertheless, they still possessed a uniform particle-size distribution, and, as seen in Fig. 3(d), the obtained carbon after calcination is comprised regular microspheres with a particle size in the range 3.61–5.23 µm.
Fig. 2. FT-IR spectra of (a) pure PS, (b) soluble starch, (c) modified PS microspheres, (d) PS/soluble starch copolymer and (e) the porous carbon microspheres. 3
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Fig. 3. SEM images of: (a) PS micro-spheres, (b) the carbon obtained from PS micro-spheres, (c) PS/soluble starch copolymer and (d) the porous carbon microspheres obtained from the copolymer.
Furthermore, the TEM image indicates that the obtained carbon microspheres exhibit a spherical structure and some light dots are homogeneously distributed in the microspheres as shown in Fig. 4, indicating a multi-porous structure of the obtained carbon. Also evidenced was a light core surrounded by a dark shell layer with a thickness of approximately 130 nm, identifying a hollow core-shell structure in the porous carbon microspheres. This result meant that the PS nanospheres were calcinated with little residual carbon, leading to the hollow core-shell structure. Some pores were also formed in the shell carbon due to a coarse accumulation of soluble starch. As a result, the regular multi-porous carbon microspheres were successfully prepared with a hollow core-shell structure and have a potential application in absorption systems. Additionally, the crystal structure of the obtained hollow carbon microspheres was further analyzed by XRD, and the results are shown in Fig. 5. It was significant that two wide peaks appeared in the range of 2θ of 21.7°, and 42.3°, indicating the typical peaks of amorphous carbon with non-crystalline structures [33]. Fig. 5. XRD pattern of the porous carbon microspheres.
Fig. 4. TEM image of the porous carbon microspheres. 4
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3.5. SEM images of n-octadecane/porous carbon composite PCMs According to the above results, we prepared n-octadecane/porous carbon composites by using the obtained porous carbon as a supporting material and n-octadecane as PCMs. In Fig. 7, it is obvious that the composites maintain the spherical structure of the carbon. However, the micro-morphologies of the composites are different due to the different contents of n-octadecane. Regarding the composite with 50 wt% n-octadecane, its surface was smooth and compact with a homogeneous particle-size distribution, which was consistent with that of porous carbon. The composites with 70 wt% n-octadecane still possessed a smooth and compact surface, and the particles were dispersed evenly. Furthermore, some of the composites were adhered together, resulting from the increase of n-octadecane, and some small particles also adhered to the surface of the composites due to the excessive amount of noctadecane. As the content of n-octadecane gradually increased to 80 wt%, most of the composites accumulated and were stacked together with a coarse surface. This implied that 70 wt% n-octadecane is the optimal content with which to prepare the n-octadecane/porous carbon composites, leading to a regular spherical structure for the composites. Fig. 6. N2 adsorption-desorption isotherms and pore size distribution (insert) of the porous carbon microspheres.
3.6. FTIR analysis of n-octadecane/porous carbon composite PCMs
3.4. N2 adsorption-desorption of porous carbon
The chemical compositions of n-octadecane and n-octadecane/ porous carbon composites were also evaluated by FTIR spectroscopy, and the results are shown in Fig. 8. It is evident that the spectra of all the composites were similar, although the intensity of the peaks was slightly different. The bending vibrations of -CH2 were observed at 1466 and 1378 cm−1. The intensive peaks at 2957, 2923, and 2853 cm−1 were assigned to the stretching vibrations of -CH3 and -CH2 in nalkyl. In addition, the in-plane rocking vibration of -CH2 appeared at 720 cm−1. It was noteworthy that these are the characteristic peaks of n-octadecane, and these peaks also could be seen in the FTIR spectra of all the composites. Furthermore, the intensive bands at 3424 and 1629 cm−1 were assigned to a small amount of bound water present in the carbon spheres due to their multi-porous structure. This result is also associated with the characteristic peaks of the porous carbon, and proves that n-octadecane was successfully encapsulated into the obtained novel porous carbon, forming the n-octadecane/porous carbon composites.
The adsorption properties of multi-porous carbon mainly depend on its surface area, pore volume, and pore size. Therefore, N2 adsorptiondesorption isotherms and the resulting pore-size distribution are displayed in Fig. 6, to evaluate the absorption properties of the obtained carbon. It was clearly observed that the carbon spheres exhibited the type-IV isotherm. The initial adsorption at 0.0–0.2 relative pressure indicated the formation of micropores from the carbonization of soluble starch, which was consistent with the pore-size distribution of 0.5–0.75 nm. The adsorption at 0.2–0.9 relative pressure suggested a mesopore distribution of approximately 3 nm. This was because the PS microspheres were eliminated and the starch was cross-linked during carbonization. Additionally, according to the BET calculation, the surface area of the porous carbon reached 1100 m2/g, which further confirmed that the obtained carbon was a good absorption matrix and was favorable for the encapsulation of PCMs.
Fig. 7. SEM images of the n-octadecane/porous carbon composite PCMs with different content of n-octadecane: (a) 50 wt%, (b) 60 wt%, (c) 70 wt% and (d) 80 wt%. 5
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Fig. 10. DSC curves of (a) pristine n-octadecane, and the n-octadecane/porous carbon composite PCMs with different content of n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt%. The numbers of the curves are corresponding to the sample codes in Table 1.
Fig. 8. FT-IR spectrums of (a) pristine n-octadecane, and the n-octadecane/ porous carbon composite PCMs with different content of n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt%.
3.8. Thermal properties of n-octadecane/porous carbon composite PCMs The phase-change behavior was crucial for PCMs to evaluate their feasibility for thermal energy-storage applications. Therefore, DSC curves were employed to investigate the temperature change and energy-storage capability of the obtained composites, as seen in Fig. 10, the relevant data are displayed in Table 1. The n-octadecane changes its phase at a melting temperature of 29.9 °C and crystal temperature of 23.8 °C. The corresponding latent heat of n-octadecane is 236.4 and 235.4 J/g, respectively. This implied that n-octadecane is a proper PCM with a high latent heat for room-temperature applications in thermal energy-storage systems. As observed from Fig. 10, all the obtained composites obviously displayed a single strong melting/crystal peak, which are the same as in bulk n-octadecane, indicating that the composites have an obviously excellent phase-change property. This also confirmed that n-octadecane still maintained its good phase-change property and that porous carbon Table 1 Thermal characteristics of pristine n-octadecane and the prepared n-octadecane/porous carbon composite PCMs.
Fig. 9. XRD patterns of (a) pristine n-octadecane, and the n-octadecane/porous carbon composite PCMs with different content of n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt%.
3.7. XRD analysis of the n-octadecane/porous carbon composite PCMs The XRD patterns were further detected to evaluate the crystallinity of the n-octadecane/porous carbon composites, and are displayed in Fig. 9. The porous carbon was obviously amorphous, as was known from the XRD profile. Thus, the crystallinity of the porous-carbon-encapsulated n-octadecane was quite easy to test. It is significant that the intensive peaks at 2θ of 18.98°, 19.47°, 23.06°, 24.45°, 34.34°, and 39.29° were indexed as the (0 1 0), (0 1 1), (1 0 0), (1 1 1), (−1 1 0) and (0 −2 2) planes of the n-octadecane, respectively. These peaks also presented in the patterns of the composite PCMs, although the diffraction intensity was decreased after encapsulation. These results confirmed that the composite PCMs were crystalline, and the interior noctadecane well maintained its crystallinity, which ensured the good phase-change behavior of the novel n-octadecane/porous carbon composite PCMs.
Samples
Melting Tm ( °C)
△Hm (J∙g
(a) (b) (c) (d) (e)
29.9 29.3 29.5 29.8 28.7
236.4 130.1 140.6 170.5 188.0
Crystallization Tc ( °C) △Hc (J∙g
− 1
)
23.8 24.0 23.9 24.2 24.8
− 1
)
235.4 129.3 139.9 169.0 187.3
Tc: Peak temperature of the cooling curves, ΔHc: the latent heat during cooling process; Tm: Peak temperature of melting curves, ΔHm: the latent heat during melting process. Table 2 The thermal reliability of the n-octadecane/porous carbon composite PCMs with 70 wt% n-octadecane.
6
No. of cycles
Melting Tm ( °C)
△Hm (J∙g
1st 20th 50th 100th
31.5 30.7 31.3 30.7
173.3 169.4 164.1 163.0
− 1
)
Crystallization Tc ( °C) △Hc (J∙g 22.4 22.4 22.4 22.2
173.1 170.7 168.2 162.6
− 1
)
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as a proper supporting matrix had little effect on its phase-change property. Furthermore, the melting temperature of all the composites was lower than that of n-octadecane, because the porous carbon with high thermal conductivity effected the thermal response rate of the composite PCMs. Hence, the composites could rapidly react to the temperature change and melt at a lower melting temperature compared with bulk n-octadecane. Moreover, the crystal temperatures of the composites were all higher, as they were derived from the fast thermal response rate. As a result, the supercooling of all the composites with different n-octadecane contents was lower than that of bulk n-octadecane. This result also confirmed that the porous carbon with high thermal conductivity enhanced the phase-change behavior of the interior n-octadecane. It further implied that porous carbon is a good supporting material with which to prepare composite PCMs, which not only keep the interior n-octadecane without leakage, but also provide the composite PCMs with excellent comprehensive thermal properties. The n-octadecane content had little influence on the phase-change temperatures of the composites. However, the latent heat of the composites increased significantly due to the increased n-octadecane content. In addition, the latent heat of the composite with 50 wt% n-octadecane was 130.1 J/g and 129.3 J/g during the melting and cooling processes, respectively, while for the composite with 60 wt% n-octadecane the melting and cooling latent heat values were slightly enhanced to 140.6 J/g and 139.9 J/g, respectively. As the content of noctadecane increased to 70 wt%, the corresponding latent heat values obviously increased to 170.5 J/g and 169.0 J/g, respectively which were higher than the respective theoretical values of 165.5 J/g and 164.8 J/g. Even in the composite with 80 wt% n-octadecane, which contained the largest content of n-octadecane among the four samples studied, its corresponding latent heat values were 188.0 J/g and 187.3 J/g, respectively, and lower than the respective ideal values of 189.1 J/g and 188.3 J/g. These results affirmed that 70 wt% n-octadecane was the optimal concentration with which to prepare the composite with excellent phase-change behavior and for which the porous carbon can fully absorb the n-octadecane. Meanwhile, the noctadecane was dispersed homogeneously into the porous structure of the porous carbon, and the carbon could form a continuous thermal conductive network that ensures that the interior n-octadecane fully changes its phase during its phase-change procedure. Consequently, this sample exhibited the best phase-change properties among all the samples. Once the n-octadecane content increased, excessive n-octadecane accumulated onto the surface of the composite, as can be seen from Fig. 3, which cannot fully change its phase with low thermal conductivity during the phase-change procedure, resulting in lower latent heat. This confirmed that the composite with 70 wt% n-octadecane is a good candidate for the preparation of composite PCMs with good properties.
Fig. 11. Thermal reliability of the n-octadecane/porous carbon composite PCMs with 70 wt% n-octadecane for 100 DSC thermal cycles. Inset shows the DSC curves of the sample. The numbers of the curves are corresponding to the sample codes in Table 2.
changes phase because of its good thermal conductivity. These results indicated that the durability of the composites is almost unchanged after 100 thermal cycles. Consequently, the obtained n-octadecane/ porous carbon composite PCMs possess outstanding thermal stability for thermal energy-storage systems. Furthermore, in order to identify whether the interior n-octadecane change its phase after 100 thermal cycles, XRD diffractions were performed on the sample again and shown in Fig. 12. It can be observed evidently that the characteristic diffraction peaks of n-octadecane are almost the same before and after 100 thermal cycles, and there is no new peak emerge or peak disappear after 100 thermal cycles. This result confirms that the phase of the interior n-octadecane has not changed after 100 cycles and the obtained n-octadecane/porous carbon composite PCMs display an excellent thermal stability and reliability. In addition, TGA curves were used to investigate whether there is any leakage of the interior n-octadecane during 100 thermal cycles. As clearly seen from Fig. 13, there is only a single degradation step, attributed to the evaporation of the interior n-octadecane. Evidently, the
3.9. Thermal reliability of n-octadecane/porous carbon composite PCMs The composite PCMs should have long operating cycle lives to ensure their thermal stability for practical applications. Therefore, the noctadecane/porous carbon composite PCMs with 70 wt% n-octadecane experienced 100 heating and cooling cycles to clarify their heat-storage durability. From Fig. 11, the phase-change temperatures of the composite PCM exhibited little change during 100 thermal cycles, and the small temperature changes are almost in the range of 1 °C. This slight discrepancy during 100 repeated thermal cycles is within the acceptable range of thermal energy-storage systems. Moreover, the latent heat of the composites showed a slight decrease after 100 thermal cycles. In addition, the loss ratio of the latent heat ranges from 2.25% to 5.95% after 100 thermal cycles. There was no obvious difference for the composites undergoing 100 thermal cycles. It further implied that the porous carbon can effectively control the interior n-octadecane in its porous structure during the phase-change procedure due to its good mechanical stability and ensure that the interior n-octadecane fully
Fig. 12. XRD diffractions of the composite PCMs before and after 100 thermal cycles. 7
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Fig. 13. TGA curves of the composite PCMs before and after 100 thermal cycles.
Fig. 15. TGA curves of (a) pristine n-octadecane, and the n-octadecane/porous carbon composite PCMs with different content of n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt%.
initial degradation temperature and the weight loss of the composite PCMs during the whole process have no change before or after the 100 thermal cycles. These results fully illustrate that the n-octadecane can be effectively encapsulated by the porous carbon without leakage during 100 thermal cycles and the composite PCMs have a great potential for long-term practical applications in thermal energy-storage systems.
the sample with 70 wt% n-octadecane also can achieve 0.631 W/(m∙K), which enhanced by 266% compared with that of n-octadecane. While the thermal conductivity of the sample with 50 wt% n-octadecane reached 0.6997 W/(m∙K), which improved by 306%. These results depict that the porous carbon formed a continuous thermal conductive network in the composite PCMs, which played a critical role in enhancing the thermal conductivity of the composite PCMs, and thus enhances the thermal response rate for thermal energy storage.
3.10. Thermal conductivity of n-octadecane/porous carbon composite PCMs
3.11. Thermal stability of n-octadecane/porous carbon composite PCMs
Thermal conductivity is a key factor to investigate the thermal response rate of PCMs to the storage and release of the latent heat. Therefore, enhancement in thermal conductivity is absolutely essential when designing novel composite PCMs. In this study, it is expected that the novel porous carbon with high thermal conductivity compared with PCMs can supply the composite PCMs with good thermal conductivity. As shown in Fig. 14, pure n-octadecane displayed a thermal conductivity as low as 0.1724 W/(m∙K). Evidently, all the obtained composite PCMs possess higher thermal conductivity than that of pure noctadecane. And the thermal conductivity of the composite PCMs significantly improved with the increasing weight amount of porous carbon, implying the presence of porous carbon endowed the composite PCMs with good thermal conductivity. And the thermal conductivity of
Thermal stability is an important factor for the application of PCMs in heat energy storage or thermal regulation. Therefore, TGA test was carried out to evaluate the thermal stability of the obtained composite PCMs, and the resulted TGA curves were shown in Fig. 15. All the sample significantly exhibited a single one weight loss step during the degradation process, which was ascribed to the molecular degradation of n-octadecane. And the weight loss percentage was in good agreement with the addition amount of n-octadecane. Furthermore, the onset degradation temperature for the composite PCM with 50 wt%, 60 wt%, 70 wt%, 80 wt% and pure n-octadecane was 166.06 °C, 170.82 °C, 178.31 °C, 199.7 °C and 215.5 °C, respectively, which increased with the increasing of the n-octadecane loading amount. This result indicated that the novel porous carbon was an appropriate supporting material for noctadecane, and the composites possess high thermal stabilities during their phase change temperature range of 23.9 °C~29.9 °C. Besides, the porous carbon enhanced the thermal conductivity of the obtained composite PCMs, which reduced the onset degradation temperature gradually of the composite PCMs. These results were in consistent with the results from the thermal conductivity tests. 3.12. Leakage test and shape stability of n-octadecane/porous carbon composite PCMs The shape stability is also an important parameter of the composite PCMs for thermal energy storage applications, which was investigated by a leakage test as shown in Fig. 16. As was shown, the pure n-octadecane would melt easily with obvious liquid leakage after heated 10 min, while the composite PCMs presented almost no change as seen from the morphology, indicating that the composite PCMs could well maintain their solid state confirming a good shape stability without any leakage. When heated for 20 min, the pure n-octadecane completely melt into liquid owing to its solid-to-liquid phase change procedure, the
Fig. 14. Thermal conductivity of pure n-octadecane and the obtained composite PCMs. 8
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2014GXNSFAA118401, and 2013GXNSFBA019244), the Science Research and Technology Development Program of Guangxi (AD17195073, AA17202030-1), Innovation Project of Guet Graduate Education (2019YCXS114, 2016YJCX21) and Program for Postgraduate Joint Training Base of GUET-CJYRE (No. 20160513-14-Z). References [1] R. Cao, S. Chen, Y. Wang, N. Han, H. Liu, X. Zhang, Functionalized carbon nanotubes as phase change materials with enhanced thermal, electrical conductivity, light-to-thermal, and electro-to-thermal performances, Carbon N Y 149 (2019) 263–272, https://doi.org/10.1016/j.carbon.2019.04.005. [2] Q. Sun, H. Zhang, J. Xue, X. Yu, Y. Yuan, X. Cao, Flexible phase change materials for thermal storage and temperature control, Chem. Eng. J. 353 (2018) 920–929, https://doi.org/10.1016/j.cej.2018.07.185. [3] K. Reddy, V. Mudgal, T. Mallick, Review of latent heat thermal energy storage for improved material stability and effective load management, J. 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Fig. 16. Photographs of (a) n-octadecane and the n-octadecane/porous carbon composite PCMs n-octadecane: (b) 50 wt%, (c) 60 wt%, (d) 70 wt%, (e) 80 wt% during heating process at 40 °C.
composite PCMs still retain their initial solid shape without any leakage although the surface of the sample with 80 wt% n-octadecane is slightly wet due to the excess n-octadecane sticking on the surface of the sample. These results reveal that the novel porous carbon would confine the movement of molten n-octadecane, that the n-octadecane can not leak out and the composite PCMs would keep solid state even the working temperature beyond its melting point. A conclusion could be drawn that the porous carbon played a key role in enhancing the shape stability of PCMs during phase change procedure and the obtained noctadecane/porous carbon composite PCMs possess excellent shape stability. 4. Conclusions To enhance the thermal conductivity and comprehensive properties of PCMs, novel porous carbon microspheres were designed using functionalized PS nanoparticles as a template and soluble starch as a carbon source. The n-octadecane/novel porous carbon composite PCMs using n-octadecane as PCMs and the novel porous carbon as the supporting material were successfully fabricated through vacuum impregnation. The porous carbon endows the composite PCMs with excellent comprehensive thermal performance. The N2 absorption-desorption curves, SEM images, and TEM analysis results show that the novel carbon possesses a multi-porous and regular spherical structure, and its ratio surface area reaches 1100 cm2/g with a pore-size distribution of approximately 1.76–6.15 nm. FTIR spectroscopy spectra and SEM images confirm that the n-octadecane/novel porous carbon composite PCMs were successfully prepared and exhibited a regular spherical profile. The composite PCMs have an excellent energystorage property, and the latent heat can reach 170.5 J/g, which is higher than the theoretical value. Furthermore, the composite PCMs exhibited outstanding thermal stability after 100 repeated thermal cycles. And the thermal conductivity of the sample with 70 wt% n-octadecane reached 0.631 W/(m∙K), which enhanced by 266% compared with n-octadecane. The composite PCMs also displayed a good shape stability without any leakage during heating at 40 °C. These results confirm that the composite PCMs are promising for thermal energy-storage application systems. Declaration of Competing Interest There are no conflict to declare. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51863005, 51462006, 51102230, 51871065, and U1501242), the Guangxi Natural Science Foundation (No. 2018GXNSFDA281051, 9
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