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Rapid production of few layer graphene for energy storage via dry exfoliation of expansible graphite
Composites Science and Technology 185 (2020) 107895
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Rapid production of few layer graphene for energy storage via dry exfoliation of expansible graphite Fukun Ma a, Liqiang Liu a, *, Xiaolin Wang a, Min Jing a, Wenjie Tan a, Xiaopeng Hao b a b
Shandong Jianzhu University, Jinan, 250101, China State Key Lab of Crystal Materials, Shandong University, Jinan, 250100, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Expansible graphite Graphene Dry exfoliation Phase change material Energy storage
Contrasted with the excellent properties of graphene, searching for an effective industrial production method is still necessary for progressing from the laboratory to commercial applications. This study presents a rapid and mass production method to get large-scale few layer graphene using the expansivity of expansible graphite. Under a suitable thermal treatment temperature (550 � C), the interlayer spacing of expansible graphite can in crease sharply, then assisted with simple mechanical crushing, large-scale few layer graphene are obtained. By this dry exfoliation method, we exfoliated expansible graphite into graphene with high quality and structural integrity. The method can be achieved with yield of 93% and the Atomic force microscopy (AFM) results show the nanosheets have a thickness about 2.3 nm. Using the availability of this method, we show the excellent application values of graphene in phase change material (PCM) field for energy storage. This work shows huge potential applications of graphene in many practical fields.
1. Introduction Because of increased environmental awareness and excessive nonrenewable power consumption, the conversion of solar energy into storable energy has attracted extensive attention [1–5]. As a promising technology that meet the demand for energy storage, phase change for heat storage is appealing [6,7]. Phase change materials (PCMs) can store and release a large amount of thermal energy in the form of latent heat during solid–liquid and liquid–solid phase changes [8–10]. By now, PCMs have found widely applications in many fields [11–14]. However, compared with the excellent storage capacity, the low thermal con ductivity is the weakness of most PCMs. To solve this problem, some materials with high thermal conductivities have been employed to improve the heat transfer of PCMs [15–18]. As the well-known two-di mensional (2D) material, the excellent mechanical property, thermal conductivity property and ultrathin nature of graphene make it a promising material in the field of energy storage [19–21]. For example, some kind of thermally annealed graphene has been used in the PCMs system to improve the thermal storage performances [22]; some PCM/graphene composite showed application prospect to reduce energy consumption in buildings. Unfortunately, the reported PCM/graphene composites applied only in laboratory due to the high production cost of
graphene [23]. How to reduce the graphene preparation cost effectively is an important issue for the practical application. Since the discovery in 2004, the outstanding properties of graphene exhibit extensive potential applications not only in the energy storage field, but also in the field of catalyst supports, Li-ion batteries, super capacitors, sensitive sensors, solar cells and so on [24–32]. Compared with the excellent application prospects of graphene, the developments of a high yield production method for practical application is still a big change [33,34]. In the last decade, enormous efforts have been done to get superior graphene. Using the typical bottom-up method, monolayer defect free graphene can be obtained by chemical vapor deposition (CVD) [35]. Compared with the high quality, the low quantity and high processing cost of this method cause troubles for the application of graphene. Correspondingly, some up-bottom methods such as mechan ical exfoliation, chemical exfoliation and hydrothermal reduction can exfoliate graphene from bulk graphite in large quantities [36–39]. Although some encouraging signs can be seen from those up-bottom methods, the application of graphene still suffers from some problems such as complex processes, advanced equipment, being time consuming etc [40,41]. Generally, ion intercalation is supposed to be an effect method to realize the exfoliation of layered materials. Some typical exfoliation
* Corresponding author. E-mail address: [email protected] (L. Liu). https://doi.org/10.1016/j.compscitech.2019.107895 Received 3 September 2019; Received in revised form 25 October 2019; Accepted 3 November 2019 Available online 5 November 2019 0266-3538/© 2019 Published by Elsevier Ltd.
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Composites Science and Technology 185 (2020) 107895
method come and develop from the ion intercalation method such as liquid exfoliation, electrochemical exfoliation and chemical weathering exfoliation [42–44]. As a typical graphite intercalation compound, expansible graphite can often be used to prepare expanded graphite. Since the interlayer van der Waals forces in expanded graphite is very weak, there are articles to prepare graphene using expanded graphite [45,46]. However, most of the reported preparation methods using expanded graphite are just an extension of the existing methods. For example, graphene layer can be prepared form the expanded graphite using the liquid-phase exfoliation method [47]; graphene oxide can be prepared form the expanded graphite in reaction with the oxidant (KMnO4) [48]. Those methods still faced many limitations for the commercialized production. On the other hand, the worm-like structure did not bring its structural superiority into full play. In this research, the huge volume enlargement nature of expanded graphite has been used, and a novel dry exfoliation method is designed. Using this method, a mass of graphene can be obtained from bulk expansible graphite. The as-obtained graphene has large width and low thickness. Except for the high quality and quantity, the dry exfoliation method have other advantages such as rapid process, low cost and replicated on a mass production scale. This novel method make gra phene have practical application value in the field of energy storage.
thermostatic oil bath of 98 � C and placed in constant temperature water bath of 25 � C for cooling. 2.4. Characterization SEM was performed on a Hitachi S-4800 field-emission scanning electron microscope. TEM images were obtained using a Philips Tecnai 20U-Twin microscope at an acceleration voltage of 200 kV. AFM images were obtained with a Nanoscope Multi Mode V (Digital Instruments/ Bruker Systems), operating in Scan Asyst Air mode. The samples were prepared by depositing the graphene ethanol suspension on a Si/SiO2 substrate and dried in a vacuum oven for 12 h before AFM measurement. XRD patterns were obtained on a Bruker D8 advance X-ray diffractom eter with Cu–Kα radiation (λ ¼ 0.15418 nm). Thermal conductivity of the samples were tested by a DRL-II heat conduction coefficient detector (Xiangtan instruments, China). 3. Results and discussion The few layer graphene was exfoliated by a mechanical exfoliation method assisted with thermal expansion process. Fig. 1 shows the typical two-step “dry exfoliation” process. Firstly, the expansible graphite become expanded graphite after a thermal expansion process (Fig. 1a). Under a suitable temperature (550 � C), the volume of graphite can increase sharply. On the micro level, the buckling tendency of expanded graphite enlarge the interlayer spacing in some local regions. As shown in Fig. 1b, the corresponding van der Waals force between adjacent graphite sheets is overwhelmed and become very weak. Then, the sample is exfoliated using a high-speed pulverizer. During the shattering process (Fig. 1c), the interlayer van der Waals force is broken and most of the expanded graphite are fully exfoliated into individual layers. Lastly, graphene are obtained after the complete dry exfoliation process. The entire process was recorded by scanning electron microscopy (SEM) and optical microphotograph to obtain a detailed description (Fig. 2). Fig. 2a shows the images of bulk expansible graphite. Fig. 2b shows the images of expanded graphite after the thermal expansion process. Compared with Fig. 2a, there are large space between the adjacent layers, which indicates a weak van der Waals force in the expanded graphite sample. Fig. 2c shows the prepared sample after the shattering process for 5 min. The optical microphotograph shows the sample become extremely tiny particles. The SEM image shows that the tiny particles have already become thin two-dimensional (2D) sheets. Fig. 2d shows the typical image of as-obtained graphene after the dry exfoliation process. After the shattering process for 10 min, the sheets exhibit an even morphology, and are almost transparent, indicating their ultrathin nature [49]. Meanwhile, the sheets have large transverse sizes. The optical microphotograph shows graphene generate agglom eration, which also corresponding the ultrathin nature of the products. Oppositely, the agglomeration of the graphene has negative effects on the exfoliation process. A longer shattering time has little impact on the thickness of graphene. More details about the morphology progression of the exfoliation process have been shown in Supporting Information S1. The morphological characteristics of as-obtained graphene were further confirmed by transmission electron microscopy (TEM). The folded nanosheets in Fig. 3a show typical 2D characteristics and agglomerate state, similar to the about results in the SEM image. The corresponding lattice and selected area electron diffraction (SAED) of the products are displayed in Fig. 3b using high-resolution transmission electron microscopy (HRTEM). The intact lattice structure of graphene indicates few lattice imperfections after the exfoliation process. Mean while, only one set of six fold symmetric diffraction spots could be seen from the SAED image, indicating the extremely thin property of asobtained graphene. Atomic force microscopy (AFM) was conducted to further investigate
2. Experimental section 2.1. Materials Expansible graphite was purchased from Qingdao Yanhai carbon materials Co. Ltd. Aluminum ammonium sulfate dodecahydrate (NH4Al (SO4)2∙12H2O) was of analytical reagent grade and purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai). All reagents were used as received. 2.2. Rapid production of graphene The few layer graphene was exfoliated from expanded graphite assisted with machine crushing process. Restricted by the laboratory conditions, a few hundred grammes of graphene can be obtained per day. In detail, 200 g of bulk expansible graphite was weighed with electronic balance. Then the expansible graphite was added into the bottom of an iron basin. To make sure the feasibility of thermal expansion process of graphite, the basin had no cover and had enough capacity to hold the obtained expanded graphite. The basin was put into a muffle furnace. Under atmospheric condition, the basin was heated to 550 � C at a controlled heating rate of 10 � C∙min 1 and maintained for 3 h to get expanded graphite. After the basin cooled naturally to room temperature, ~188 g of expanded graphite was obtained. Then, the asobtained expanded graphite was exfoliated in a high-speed pulverizer. To make sure the efficiency of the exfoliation process, the airtight cavity of pulverizer was full of dry expanded graphite. Under a revolving speed of 28000 rpm, the expanded graphite was shattered for 10 min. After the pulverizer shut down, the fluffy expanded graphite became conglobate graphene. At last, ~186 g of graphene with a 93% yield was obtained. 2.3. Preparation and performance of NH4Al(SO4)2∙12H2O/graphene composite The composite was prepared by solid-phase mixing milling. A certain weight of NH4Al(SO4)2∙12H2O samples were weighted by electronic scale. And the as-obtained graphene was added in the sample respec tively according to a certain additive amount (0 wt%, 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt% and 1 wt%). Then, the mixture was grinded into powder and putted into a plastic beaker for thermal cycling test of melting and solidification. A PT100 thermal resistor (temperature measurement error �0.1 � C) was inserted into the center of the beaker to test the temperature of the composite. The beaker was heated in the 2
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Composites Science and Technology 185 (2020) 107895
Fig. 1. Schematic of the processing steps involved in the exfoliation of graphene.
Fig. 2. Morphological characteristics of the as-prepared graphene. (a) SEM image of bulk expansible graphite. (b) SEM image of expanded graphite. (c) SEM image of the prepared intermediate products. (d) SEM image of as-prepared graphene. The insets show the corresponding optical microphotographs, and scale plates are 500 μm.
Fig. 3. (a) TEM image of graphene. (b) HRTEM image of graphene, the inset shows the SAED image.
the structural features of as-obtained graphene. The thickness and fine structure of the sample are shown in Fig. 4a. This image reveals the thickness of the nanosheets is about 2.31 nm, which was comparable to the reported thickness of graphene [50]. Meanwhile, the large size of the
sample is corresponding to the above-mentioned SEM image. The optical photograph shown in Fig. 4b reveals the change in the volume after the corresponding exfoliation process. As shown in this image, 500 mg of expansible graphite, expanded graphite and as-obtained graphene are 3
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Fig. 4. Structural characteristics of as-obtained graphene. (a) AFM image of as-obtained graphene. (b) Volume changes of expansible graphite, expanded graphite and graphene with the same weight (500 mg). (c) Raman spectra of expansible graphite, expanded graphite and as-obtained graphene. (d) XRD patterns of expansible graphite, expanded graphite and as-obtained graphene.
putted into the tubes loosely. The sharply increased volume of expanded graphite indicates the rapid depletion of interlayer van der Waals force. After the exfoliation process, the volume of as-obtained graphene return to the normal condition. The results are corresponding to the above optical microphotograph in Fig. 2. The quality of as-obtained products were further estimated by Raman spectroscopy. The Raman spectra of expansible graphite, expanded graphite and as-obtained graphene are shown in Fig. 4c. The bands located at ~1582 cm 1 are the G band of graphite, corresponding the in-plane vibrations of sp2 C atoms. The bands located at ~1356 cm 1 are the D band, originated from the defect of the structure [51]. Ac cording to the intensity ratio of the D band to the G band (ID/IG), the as-obtained graphene presents a low amount of defects as well as the raw graphite after the exfoliation process. The bands located at ~2700 cm 1 are the 2D band, which can be used to attest the number of graphene layers. Compared with the 2D peaks of expansible graphite and expanded graphite, the 2D peak of the as-obtained graphene shifts slightly to 2689 cm 1, indicating the few-layer graphene sheet [52]. Meanwhile, the 2D peak is not very strong, this could be due to the number of layers and disorder. X-ray diffraction (XRD) analysis was performed to study the structure of the as-obtained graphene. Fig. 4d reveals that the graphene exhibit a similar characteristic structure compared with expansible graphite. The intensity of the (002) peak of as-obtained graphene significantly decreases compared with that of bulk expansible graphite, demonstrating the ultrathin nature of the sheets [53]. By now, graphene and some analogues have been used in the field of energy storage [54]. The graphene obtained using oxidation reduction
method is expensive, so some researchers used the graphene oxide for improving thermal conductivity [55]. The result was encouraging, however the cost was still expensive which restricted its large-scale application. The benefit of this exfoliation method lies in its high pro duction and simplicity. As show in Fig. 5, the as-obtained graphene is of practical application value in the field of energy storage. NH4Al (SO4)2∙12H2O is selected as the raw PCM, and different contents of graphene are added in the pure NH4Al(SO4)2∙12H2O in order to form the PCM/graphene composites. Fig. 5a shows the heating process of the samples. The as-obtained graphene can reduce the heat transfer time of PCM effectively. In the first half-hour of the heating process, the tem perature of all samples reach the melt point of NH4Al(SO4)2∙12H2O. Compared with curve of pure PCM, the composites have shorter heating time. This phenomenon is much clearer in the process of the samples from solid state to liquid state. Pure NH4Al(SO4)2∙12H2O exhibit typical heating curve of PCM and it take 2.5 h to reach 98 � C. The heating rate of the composites increases with the increase of the amount of graphene. When the amount of graphene is 1 wt%, the PCM/graphene composite have the fast heating rate and it only take 1.5 h to reach the setting temperature. The cooling process of the samples is shown in Fig. 5b. Except for the faster cooling rate, the as-obtained graphene can reduce the supercooling degree of NH4Al(SO4)2 effectively. The pure NH4Al (SO4)2∙12H2O has large supercooling degree (41.5 � C), associated with the solidification nature of hydrated salt. The supercooling degrees of the PCM/graphene composites reduce with the increase of the content of graphene. The PCM/1 wt% graphene composite has the supercooling degree of 23.1 � C. This could be due to the contribution of the defects of graphene, which can be the nucleation points of heterogeneous 4
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Fig. 5. Thermophysical property of PCM/graphene composites. (a) Heating curves of the samples. (b) Cooling process of the samples. (c) The thermal conductivity of NH4Al(SO4)2∙12H2O and NH4Al(SO4)2∙12H2O/graphene composites. (d) DSC curves of NH4Al(SO4)2∙12H2O and NH4Al(SO4)2∙12H2O/1 wt% graphene composite.
nucleation process [56]. More details about the supercooling degree of the composites have been shown in Supporting Information S2–S3. Fig. 5c shows the thermal conductivity of pure NH4Al(SO4)2∙12H2O and corresponding graphene composites. The thermal conductivity of sample is enlarged continually with the increase of graphene content. The sample with 1 wt% graphene has the largest thermal conductivity (4.063 W m 1 K 1) while the pure NH4Al(SO4)2∙12H2O sample has the smallest one (0.542 W m 1 K 1). In the beginning, the growth rate of thermal conductivity is slow when a small amount of graphene is added. This can be due to the small content of graphene separate from each other and the thermal conducting paths are deterred. With the increase of graphene content, the thermal conducting paths form progressively, and the growth rate of thermal conductivity become stable. Except for the graphene oxide, as a suitable analogue at the right price, graphene nanoplatelets have also been used in the PCM system to improve the performance. However, the graphene nanoplatelets are very thick which show limited performance. Compared with the reported work, the asobtained graphene can improve the thermal conductivity effectively [57]. The DSC curves of NH4Al(SO4)2∙12H2O and NH4Al(SO4)2∙12H2O/ 1 wt% graphene composite are shown in Fig. 5d. The initial fusion temperature of pure PCM is 93.70 � C, corresponding to melting point of NH4Al(SO4)2∙12H2O [58]. Meanwhile, the phase change enthalpy of NH4Al(SO4)2∙12H2O is 267.8 J/g. When the graphene is added in the system, the melting point and phase change enthalpy of the composite have slight changes. The melting point of the composite is 93.28 � C, and phase change enthalpy reduce to 262.4 J/g. This result indicate the NH4Al(SO4)2∙12H2O/graphene composite still keep the excellent heat storage performance as the pure NH4Al(SO4)2∙12H2O. In a word, this work shows the excellent application values of graphene in PCM field.
4. Conclusions This study presents a novel dry exfoliation method for preparing graphene. The proposed method features rapid process, high quality, low cost and replicated on a mass production scale. Under laboratory conditions, the yield can reach 93%. The thermal expansion and high speed shattering play key roles in the exfoliation process. The asobtained graphene presents a small value for thickness and a large size for width. This low cost exfoliation method make graphene have practical value in the thermal storage field. The as-prepared PCM/gra phene composites show excellent thermophysical properties. It is believed that this approach will be feasible and attractive for producing graphene for commercial applications. 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 This work was sponsored by Taishan Scholars Project Funding. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compscitech.2019.107895.
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Rapid production of few layer graphene for energy storage via dry exfoliation of expansible graphite Fukun Ma a, Liqiang Liu a, *, Xiaolin Wang a, Min Jing a, Wenjie Tan a, Xiaopeng Hao b a b
Shandong Jianzhu University, Jinan, 250101, China State Key Lab of Crystal Materials, Shandong University, Jinan, 250100, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Expansible graphite Graphene Dry exfoliation Phase change material Energy storage
Contrasted with the excellent properties of graphene, searching for an effective industrial production method is still necessary for progressing from the laboratory to commercial applications. This study presents a rapid and mass production method to get large-scale few layer graphene using the expansivity of expansible graphite. Under a suitable thermal treatment temperature (550 � C), the interlayer spacing of expansible graphite can in crease sharply, then assisted with simple mechanical crushing, large-scale few layer graphene are obtained. By this dry exfoliation method, we exfoliated expansible graphite into graphene with high quality and structural integrity. The method can be achieved with yield of 93% and the Atomic force microscopy (AFM) results show the nanosheets have a thickness about 2.3 nm. Using the availability of this method, we show the excellent application values of graphene in phase change material (PCM) field for energy storage. This work shows huge potential applications of graphene in many practical fields.
1. Introduction Because of increased environmental awareness and excessive nonrenewable power consumption, the conversion of solar energy into storable energy has attracted extensive attention [1–5]. As a promising technology that meet the demand for energy storage, phase change for heat storage is appealing [6,7]. Phase change materials (PCMs) can store and release a large amount of thermal energy in the form of latent heat during solid–liquid and liquid–solid phase changes [8–10]. By now, PCMs have found widely applications in many fields [11–14]. However, compared with the excellent storage capacity, the low thermal con ductivity is the weakness of most PCMs. To solve this problem, some materials with high thermal conductivities have been employed to improve the heat transfer of PCMs [15–18]. As the well-known two-di mensional (2D) material, the excellent mechanical property, thermal conductivity property and ultrathin nature of graphene make it a promising material in the field of energy storage [19–21]. For example, some kind of thermally annealed graphene has been used in the PCMs system to improve the thermal storage performances [22]; some PCM/graphene composite showed application prospect to reduce energy consumption in buildings. Unfortunately, the reported PCM/graphene composites applied only in laboratory due to the high production cost of
graphene [23]. How to reduce the graphene preparation cost effectively is an important issue for the practical application. Since the discovery in 2004, the outstanding properties of graphene exhibit extensive potential applications not only in the energy storage field, but also in the field of catalyst supports, Li-ion batteries, super capacitors, sensitive sensors, solar cells and so on [24–32]. Compared with the excellent application prospects of graphene, the developments of a high yield production method for practical application is still a big change [33,34]. In the last decade, enormous efforts have been done to get superior graphene. Using the typical bottom-up method, monolayer defect free graphene can be obtained by chemical vapor deposition (CVD) [35]. Compared with the high quality, the low quantity and high processing cost of this method cause troubles for the application of graphene. Correspondingly, some up-bottom methods such as mechan ical exfoliation, chemical exfoliation and hydrothermal reduction can exfoliate graphene from bulk graphite in large quantities [36–39]. Although some encouraging signs can be seen from those up-bottom methods, the application of graphene still suffers from some problems such as complex processes, advanced equipment, being time consuming etc [40,41]. Generally, ion intercalation is supposed to be an effect method to realize the exfoliation of layered materials. Some typical exfoliation
* Corresponding author. E-mail address: [email protected] (L. Liu). https://doi.org/10.1016/j.compscitech.2019.107895 Received 3 September 2019; Received in revised form 25 October 2019; Accepted 3 November 2019 Available online 5 November 2019 0266-3538/© 2019 Published by Elsevier Ltd.
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method come and develop from the ion intercalation method such as liquid exfoliation, electrochemical exfoliation and chemical weathering exfoliation [42–44]. As a typical graphite intercalation compound, expansible graphite can often be used to prepare expanded graphite. Since the interlayer van der Waals forces in expanded graphite is very weak, there are articles to prepare graphene using expanded graphite [45,46]. However, most of the reported preparation methods using expanded graphite are just an extension of the existing methods. For example, graphene layer can be prepared form the expanded graphite using the liquid-phase exfoliation method [47]; graphene oxide can be prepared form the expanded graphite in reaction with the oxidant (KMnO4) [48]. Those methods still faced many limitations for the commercialized production. On the other hand, the worm-like structure did not bring its structural superiority into full play. In this research, the huge volume enlargement nature of expanded graphite has been used, and a novel dry exfoliation method is designed. Using this method, a mass of graphene can be obtained from bulk expansible graphite. The as-obtained graphene has large width and low thickness. Except for the high quality and quantity, the dry exfoliation method have other advantages such as rapid process, low cost and replicated on a mass production scale. This novel method make gra phene have practical application value in the field of energy storage.
thermostatic oil bath of 98 � C and placed in constant temperature water bath of 25 � C for cooling. 2.4. Characterization SEM was performed on a Hitachi S-4800 field-emission scanning electron microscope. TEM images were obtained using a Philips Tecnai 20U-Twin microscope at an acceleration voltage of 200 kV. AFM images were obtained with a Nanoscope Multi Mode V (Digital Instruments/ Bruker Systems), operating in Scan Asyst Air mode. The samples were prepared by depositing the graphene ethanol suspension on a Si/SiO2 substrate and dried in a vacuum oven for 12 h before AFM measurement. XRD patterns were obtained on a Bruker D8 advance X-ray diffractom eter with Cu–Kα radiation (λ ¼ 0.15418 nm). Thermal conductivity of the samples were tested by a DRL-II heat conduction coefficient detector (Xiangtan instruments, China). 3. Results and discussion The few layer graphene was exfoliated by a mechanical exfoliation method assisted with thermal expansion process. Fig. 1 shows the typical two-step “dry exfoliation” process. Firstly, the expansible graphite become expanded graphite after a thermal expansion process (Fig. 1a). Under a suitable temperature (550 � C), the volume of graphite can increase sharply. On the micro level, the buckling tendency of expanded graphite enlarge the interlayer spacing in some local regions. As shown in Fig. 1b, the corresponding van der Waals force between adjacent graphite sheets is overwhelmed and become very weak. Then, the sample is exfoliated using a high-speed pulverizer. During the shattering process (Fig. 1c), the interlayer van der Waals force is broken and most of the expanded graphite are fully exfoliated into individual layers. Lastly, graphene are obtained after the complete dry exfoliation process. The entire process was recorded by scanning electron microscopy (SEM) and optical microphotograph to obtain a detailed description (Fig. 2). Fig. 2a shows the images of bulk expansible graphite. Fig. 2b shows the images of expanded graphite after the thermal expansion process. Compared with Fig. 2a, there are large space between the adjacent layers, which indicates a weak van der Waals force in the expanded graphite sample. Fig. 2c shows the prepared sample after the shattering process for 5 min. The optical microphotograph shows the sample become extremely tiny particles. The SEM image shows that the tiny particles have already become thin two-dimensional (2D) sheets. Fig. 2d shows the typical image of as-obtained graphene after the dry exfoliation process. After the shattering process for 10 min, the sheets exhibit an even morphology, and are almost transparent, indicating their ultrathin nature [49]. Meanwhile, the sheets have large transverse sizes. The optical microphotograph shows graphene generate agglom eration, which also corresponding the ultrathin nature of the products. Oppositely, the agglomeration of the graphene has negative effects on the exfoliation process. A longer shattering time has little impact on the thickness of graphene. More details about the morphology progression of the exfoliation process have been shown in Supporting Information S1. The morphological characteristics of as-obtained graphene were further confirmed by transmission electron microscopy (TEM). The folded nanosheets in Fig. 3a show typical 2D characteristics and agglomerate state, similar to the about results in the SEM image. The corresponding lattice and selected area electron diffraction (SAED) of the products are displayed in Fig. 3b using high-resolution transmission electron microscopy (HRTEM). The intact lattice structure of graphene indicates few lattice imperfections after the exfoliation process. Mean while, only one set of six fold symmetric diffraction spots could be seen from the SAED image, indicating the extremely thin property of asobtained graphene. Atomic force microscopy (AFM) was conducted to further investigate
2. Experimental section 2.1. Materials Expansible graphite was purchased from Qingdao Yanhai carbon materials Co. Ltd. Aluminum ammonium sulfate dodecahydrate (NH4Al (SO4)2∙12H2O) was of analytical reagent grade and purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai). All reagents were used as received. 2.2. Rapid production of graphene The few layer graphene was exfoliated from expanded graphite assisted with machine crushing process. Restricted by the laboratory conditions, a few hundred grammes of graphene can be obtained per day. In detail, 200 g of bulk expansible graphite was weighed with electronic balance. Then the expansible graphite was added into the bottom of an iron basin. To make sure the feasibility of thermal expansion process of graphite, the basin had no cover and had enough capacity to hold the obtained expanded graphite. The basin was put into a muffle furnace. Under atmospheric condition, the basin was heated to 550 � C at a controlled heating rate of 10 � C∙min 1 and maintained for 3 h to get expanded graphite. After the basin cooled naturally to room temperature, ~188 g of expanded graphite was obtained. Then, the asobtained expanded graphite was exfoliated in a high-speed pulverizer. To make sure the efficiency of the exfoliation process, the airtight cavity of pulverizer was full of dry expanded graphite. Under a revolving speed of 28000 rpm, the expanded graphite was shattered for 10 min. After the pulverizer shut down, the fluffy expanded graphite became conglobate graphene. At last, ~186 g of graphene with a 93% yield was obtained. 2.3. Preparation and performance of NH4Al(SO4)2∙12H2O/graphene composite The composite was prepared by solid-phase mixing milling. A certain weight of NH4Al(SO4)2∙12H2O samples were weighted by electronic scale. And the as-obtained graphene was added in the sample respec tively according to a certain additive amount (0 wt%, 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt% and 1 wt%). Then, the mixture was grinded into powder and putted into a plastic beaker for thermal cycling test of melting and solidification. A PT100 thermal resistor (temperature measurement error �0.1 � C) was inserted into the center of the beaker to test the temperature of the composite. The beaker was heated in the 2
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Fig. 1. Schematic of the processing steps involved in the exfoliation of graphene.
Fig. 2. Morphological characteristics of the as-prepared graphene. (a) SEM image of bulk expansible graphite. (b) SEM image of expanded graphite. (c) SEM image of the prepared intermediate products. (d) SEM image of as-prepared graphene. The insets show the corresponding optical microphotographs, and scale plates are 500 μm.
Fig. 3. (a) TEM image of graphene. (b) HRTEM image of graphene, the inset shows the SAED image.
the structural features of as-obtained graphene. The thickness and fine structure of the sample are shown in Fig. 4a. This image reveals the thickness of the nanosheets is about 2.31 nm, which was comparable to the reported thickness of graphene [50]. Meanwhile, the large size of the
sample is corresponding to the above-mentioned SEM image. The optical photograph shown in Fig. 4b reveals the change in the volume after the corresponding exfoliation process. As shown in this image, 500 mg of expansible graphite, expanded graphite and as-obtained graphene are 3
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Fig. 4. Structural characteristics of as-obtained graphene. (a) AFM image of as-obtained graphene. (b) Volume changes of expansible graphite, expanded graphite and graphene with the same weight (500 mg). (c) Raman spectra of expansible graphite, expanded graphite and as-obtained graphene. (d) XRD patterns of expansible graphite, expanded graphite and as-obtained graphene.
putted into the tubes loosely. The sharply increased volume of expanded graphite indicates the rapid depletion of interlayer van der Waals force. After the exfoliation process, the volume of as-obtained graphene return to the normal condition. The results are corresponding to the above optical microphotograph in Fig. 2. The quality of as-obtained products were further estimated by Raman spectroscopy. The Raman spectra of expansible graphite, expanded graphite and as-obtained graphene are shown in Fig. 4c. The bands located at ~1582 cm 1 are the G band of graphite, corresponding the in-plane vibrations of sp2 C atoms. The bands located at ~1356 cm 1 are the D band, originated from the defect of the structure [51]. Ac cording to the intensity ratio of the D band to the G band (ID/IG), the as-obtained graphene presents a low amount of defects as well as the raw graphite after the exfoliation process. The bands located at ~2700 cm 1 are the 2D band, which can be used to attest the number of graphene layers. Compared with the 2D peaks of expansible graphite and expanded graphite, the 2D peak of the as-obtained graphene shifts slightly to 2689 cm 1, indicating the few-layer graphene sheet [52]. Meanwhile, the 2D peak is not very strong, this could be due to the number of layers and disorder. X-ray diffraction (XRD) analysis was performed to study the structure of the as-obtained graphene. Fig. 4d reveals that the graphene exhibit a similar characteristic structure compared with expansible graphite. The intensity of the (002) peak of as-obtained graphene significantly decreases compared with that of bulk expansible graphite, demonstrating the ultrathin nature of the sheets [53]. By now, graphene and some analogues have been used in the field of energy storage [54]. The graphene obtained using oxidation reduction
method is expensive, so some researchers used the graphene oxide for improving thermal conductivity [55]. The result was encouraging, however the cost was still expensive which restricted its large-scale application. The benefit of this exfoliation method lies in its high pro duction and simplicity. As show in Fig. 5, the as-obtained graphene is of practical application value in the field of energy storage. NH4Al (SO4)2∙12H2O is selected as the raw PCM, and different contents of graphene are added in the pure NH4Al(SO4)2∙12H2O in order to form the PCM/graphene composites. Fig. 5a shows the heating process of the samples. The as-obtained graphene can reduce the heat transfer time of PCM effectively. In the first half-hour of the heating process, the tem perature of all samples reach the melt point of NH4Al(SO4)2∙12H2O. Compared with curve of pure PCM, the composites have shorter heating time. This phenomenon is much clearer in the process of the samples from solid state to liquid state. Pure NH4Al(SO4)2∙12H2O exhibit typical heating curve of PCM and it take 2.5 h to reach 98 � C. The heating rate of the composites increases with the increase of the amount of graphene. When the amount of graphene is 1 wt%, the PCM/graphene composite have the fast heating rate and it only take 1.5 h to reach the setting temperature. The cooling process of the samples is shown in Fig. 5b. Except for the faster cooling rate, the as-obtained graphene can reduce the supercooling degree of NH4Al(SO4)2 effectively. The pure NH4Al (SO4)2∙12H2O has large supercooling degree (41.5 � C), associated with the solidification nature of hydrated salt. The supercooling degrees of the PCM/graphene composites reduce with the increase of the content of graphene. The PCM/1 wt% graphene composite has the supercooling degree of 23.1 � C. This could be due to the contribution of the defects of graphene, which can be the nucleation points of heterogeneous 4
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Fig. 5. Thermophysical property of PCM/graphene composites. (a) Heating curves of the samples. (b) Cooling process of the samples. (c) The thermal conductivity of NH4Al(SO4)2∙12H2O and NH4Al(SO4)2∙12H2O/graphene composites. (d) DSC curves of NH4Al(SO4)2∙12H2O and NH4Al(SO4)2∙12H2O/1 wt% graphene composite.
nucleation process [56]. More details about the supercooling degree of the composites have been shown in Supporting Information S2–S3. Fig. 5c shows the thermal conductivity of pure NH4Al(SO4)2∙12H2O and corresponding graphene composites. The thermal conductivity of sample is enlarged continually with the increase of graphene content. The sample with 1 wt% graphene has the largest thermal conductivity (4.063 W m 1 K 1) while the pure NH4Al(SO4)2∙12H2O sample has the smallest one (0.542 W m 1 K 1). In the beginning, the growth rate of thermal conductivity is slow when a small amount of graphene is added. This can be due to the small content of graphene separate from each other and the thermal conducting paths are deterred. With the increase of graphene content, the thermal conducting paths form progressively, and the growth rate of thermal conductivity become stable. Except for the graphene oxide, as a suitable analogue at the right price, graphene nanoplatelets have also been used in the PCM system to improve the performance. However, the graphene nanoplatelets are very thick which show limited performance. Compared with the reported work, the asobtained graphene can improve the thermal conductivity effectively [57]. The DSC curves of NH4Al(SO4)2∙12H2O and NH4Al(SO4)2∙12H2O/ 1 wt% graphene composite are shown in Fig. 5d. The initial fusion temperature of pure PCM is 93.70 � C, corresponding to melting point of NH4Al(SO4)2∙12H2O [58]. Meanwhile, the phase change enthalpy of NH4Al(SO4)2∙12H2O is 267.8 J/g. When the graphene is added in the system, the melting point and phase change enthalpy of the composite have slight changes. The melting point of the composite is 93.28 � C, and phase change enthalpy reduce to 262.4 J/g. This result indicate the NH4Al(SO4)2∙12H2O/graphene composite still keep the excellent heat storage performance as the pure NH4Al(SO4)2∙12H2O. In a word, this work shows the excellent application values of graphene in PCM field.
4. Conclusions This study presents a novel dry exfoliation method for preparing graphene. The proposed method features rapid process, high quality, low cost and replicated on a mass production scale. Under laboratory conditions, the yield can reach 93%. The thermal expansion and high speed shattering play key roles in the exfoliation process. The asobtained graphene presents a small value for thickness and a large size for width. This low cost exfoliation method make graphene have practical value in the thermal storage field. The as-prepared PCM/gra phene composites show excellent thermophysical properties. It is believed that this approach will be feasible and attractive for producing graphene for commercial applications. 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 This work was sponsored by Taishan Scholars Project Funding. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compscitech.2019.107895.
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