A Simple Pyrolysis Route To Synthesize Carbon Nanofibers in Molten

Article pubs.acs.org/JPCC

A Simple Pyrolysis Route To Synthesize Carbon Nanofibers in Molten Zinc Chloride as an Anode Material for Li Ion Batteries Liangbiao Wang,*,†,§ Tao Mei,‡ Weiqiao Liu,† and Quanfa Zhou*,† †

Jiangsu Key Laboratory of Precious Metals Chemistry and Engineering, School of Chemistry and Environment Engineering, Jiangsu University of Technology, Changzhou 213001, P. R. China ‡ Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education, Key Laboratory for the Green Preparation and Application of Functional Materials, School of Materials Science and Engineering, Hubei University, Wuhan 430062, P. R. China § Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China ABSTRACT: In this study, carbon nanofibers (CNFs) have been synthesized by a simple pyrolysis of tetrahydrofuran using molten zinc chloride as a liquid environment at 600 °C for 10 h. Scanning electron microscope images reveal that the obtained CNFs have diameters of about 50 nm and lengths up to tens of micrometers. As an anode material for Li ion batteries, the obtained CNFs can deliver a reversible capacity of 310 mA h/g after 200 cycles at a current density of 100 mA/ g. When tetrahydrofuran is substituted by other carbon sources (ferrocene or lactic acid), CNFs can also be synthesized through this similar process; therefore, the pyrolysis method is a general way to prepare CNFs in molten zinc chloride.

1. INTRODUCTION In the past years, carbon nanomaterials have attracted considerable attention because of their unique mechanical, electronic, and chemical properties.1−3 Thus, nanostructured carbon with various morphologies, for instance, nanoparticles, nanorods, nanobelts, nanosheets, nanospheres, nanofibers, and nanotubes, has been produced and extensively investigated.4−11 Carbon nanofibers (CNFs) have been expected to be applied for a range of potential application, including electron field emission source, probe tip, and fuel cells.12,13 Meanwhile, CNFs can be used as electrode materials for lithium ion batteries for they have huge surface areas and present a relatively large amount of lithium ion insertion sites on their surface, which usual lead to good electrochemical properties.14,15 Up to now, a number of research groups have reported the preparation of CNFs, which have been produced by arc discharge, chemical vapor deposition (CVD), laser ablation, and so on.16−21 CNFs can also be produced through a cocatalyst deoxidization process.14 Recently, a molten salt route has been widely used in the synthesis of nanomaterials (for instance, Ge, Si, carbides, borides, and other materials) because the route is a rather green chemistry process.22−27 In this study, we report a novel approach to synthesize CNFs by a simple pyrolysis of tetrahydrofuran in molten zinc chloride. The reaction is carried out in an autoclave, in which molten zinc chloride can offer a liquid reaction environment for the formation of CNFs at the temperature of 600 °C. Scanning electron microscope images reveal that the obtained CNFs have diameters of about 50 nm and lengths up to tens of micrometers. As an anode material in lithium ion batteries, the obtained CNFs deliver high capacity and excellent cycling stability. When tetrahydrofuran is © XXXX American Chemical Society

substituted by other carbon sources (such as ferrocene or lactic acid), CNFs can also be produced in molten zinc chloride through similar processes.

2. EXPERIMENTAL SECTION In a typical procedure, 1 mL of tetrahydrofuran and 8.50 g of zinc chloride are added into a stainless steel autoclave of 20 mL capacity. The autoclave is sealed and put into an electric stove. The temperature of the electric stove is raised from room temperature to 600 °C at a rate of 10 °C/min and maintained at 600 °C for 10 h. After the autoclave is cooled to room temperature naturally, the dark precipitate in the autoclave is collected and washed with hydrochloric acid, distilled water, and ethanol several times. The final sample is then dried under vacuum at 50 °C for 10 h and labeled as “sample 1” for further characterization. Produced by similar procedures, the products obtained by pyrolysis of other organic precursors [ferrocene (1 mL) and lactic acid (1 mL)] in molten zinc chloride are labeled as “sample 2” and “sample 3”, respectively. The X-ray powder diffraction (XRD) analysis is recorded with a Philips X’pert X-ray diffractometer equipped with Cu Kα radiation (λ = 1.541 78 Å). Raman spectrum is recorded on a LABRAM-HR micro-Raman spectrometer using a 514.5 nm Ar+ laser. The scanning electron microscope (SEM) images are taken on a JEOL-JSM-6700F field emission electron microscope. The TEM and HR-TEM images are taken on a Hitachi model H-800 transmission electron microscope and JEOLReceived: December 12, 2015 Revised: February 12, 2016

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DOI: 10.1021/acs.jpcc.5b12154 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C 2010 high-resolution transmission electron microscope, respectively. Electrochemical properties of the CNFs electrodes are determined by using coin-type (CR-2016) cells. A metallic Li sheet is used as counter electrode. The working electrode is fabricated by pasting a slurry of active material (CNFs) and polyvinylidene fluoride binder at a weight ratio of 9:1. The slurry is pasted on copper foil and dried at 110 °C for 12 h in a vacuum oven under vacuum before assembling into a coin cell in an argon-filled glovebox. The aqueous electrolyte consisted of a solution of 1 mol/L LiPF6 in an ethylene carbonate/ dimethyl carbonate/diethyl carbonate mixture (1:1:1, in wt %). Batteries are assembled in a glovebox filled with high-purity Ar gas. The galvanostatic charge−discharge test is performed using a LAND-CT2001A instrument between 0.001 and 3.00 V versus Li+/Li at different current densities at room temperature.

Figure 2. (a) A typical low-magnification SEM image. (b) A typical high-magnification SEM image. (c) TEM image. (d) HRTEM image of sample 1. The inset inpart d shows the related HRTEM image.

3. RESULTS AND DISCUSSION A typical XRD pattern of the sample 1 is shown in Figure 1a. The peak centered at 26.08° could be indexed as the (002)

Figure 1. (a) XRD pattern of the sample 1. (b) Raman spectrum of the sample 1.

diffraction planes of hexagonal graphite. The broad diffraction peak in Figure 1a indicates that low crystalline carbon is prepared in the process. A typical Raman spectrum of the sample 1 is shown in Figure 1b. Two broad peaks located at 1347 and 1587 cm−1 are observed. The peak at 1587 cm−1 (Gband) corresponds to the vibration of sp2-bonded carbon atoms in a 2D hexagonal lattice (E2g mode). The peak at 1347 cm−1 (D-band) corresponds to the vibration of carbon atoms with dangling bonds being in-plane terminations of disordered graphite.28,29 The relative intensity of the G band to the D band (IG/ID) of sample 1 is 1.10, implying that sample 1 possesses a lower graphitization degree and a mass of disorder. The result is consistent with its XRD pattern. The morphology of sample 1 is further analyzed by SEM and TEM. A typical low-magnification SEM image of sample 1 (shown in Figure 2a) reveals that sample 1 is composed of nanofibers and nanosheets. The yield of carbon nanofibers in the sample is estimated to be over 80% on the basis of the SEM view. Figure 2b shows a high-magnification SEM image, which reveals that the diameters of carbon nanofibers are about 50 nm and the lengths of carbon nanofibers are in the range from several micrometers to tens of micrometers. The TEM image further indicates that sample 1 is composed of nanofibers, as shown in Figure 2c. The HRTEM image of sample 1 is shown in Figure 2d. The interplanar spacing is about 0.34 nm (inset of Figure 2d), which corresponds to (002) spacing of hexagonal phase graphite. In order to evaluate the obtained CNFs as an anode material for Li ion batteries, coin-type cells with a Li sheet as a counter electrode are assembled. Figure 3 shows the 1st, 2nd, 10th,

Figure 3. Charge and discharge profiles of CNFs between 0.001 and 3.00 V for the 1st, 2nd, 10th, 100th, and 200th cycle at a current density of 100 mA h/g.

100th, and 200th charge/discharge cycle of the obtained CNFs at 100 mA/g. It should also be noted that the first three cycles of the cells were tested at a low current density of 20 mA/g to activate CNFs sufficiently. The initial charge and discharge capacities are 356 and 686 mA h/g at a current density of 100 mA/g, respectively. We should note that the initial discharge capacity (686 mA h/g) for the CNFs is higher than the theoretical capacity of graphite of 372 mA h/g, implying that there is other lithium storage mechanisms apart from the classical graphite intercalation compound mechanism, which is consistent with the results of previous work.5 The charge and discharge capacities at a current density of 100 mA/g are 362 and 392 mA h/g for the 2nd cycle, 347 and 353 mA h/g for the 10th cycle, 324 and 326 mA h/g for the 100th cycle, and 308 and 310 mA h/g for the 200th cycle, respectively. There is a potential plateau near 0.5 V in the first discharge in Figure 3, and no clear plateau can be observed, which is ascribed to Li ion embedding into the graphite layers to form LiC6-class compounds after the second cycle. The curves of charge/ discharge capacities and Coulombic efficiencies of the CNFs electrode versus cycle number of the battery at a current density of 100 and 500 mA/g are shown in parts a and b of Figure 4, respectively. It can be observed that the discharge capacity decreased from 686 mA h/g (1st cycle) to 353 mA h/g (10th cycle) with a capacity loss rate of 37 mA h/g per cycle at 100 mA/g. The initial capacity fading could be attributed to the B

DOI: 10.1021/acs.jpcc.5b12154 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 5. SEM image of the obtained sample by pyrolysis of tetrahydrofuran at different temperatures: (a) 650 °C and (b) 700 °C. (c) The formation processes of carbon nanofibers and carbon nanospheres. Figure 4. Curves of charge/discharge capacities versus cycle number at different current densities: (a) 100 mA/g and (b) 500 mA/g. The insets in parts a and b show their Coulombic efficiency versus cycle number, respectively.

SEM observation (Figure 5b). Besides the reaction temperature, the effect of reaction time is also investigated. At the reaction temperature of 600 °C for 0.5 h, the obtained sample is composed of carbon nanofibers; however, the yield of carbon product is relatively low because the pyrolysis of tetrahydrofuran is not complete. As the reaction time is increased to 10 h, the yield of the carbon product reaches 80%, which is calculated from weights of the carbon product and tetrahydrofuran. A model similar to Gamaly and Ebbesen’s is used to discuss the growth of carbon nanofibers.31 In the model, carbon atoms attached to the surface of catalysts form graphite layers as a cap, which could be regarded as seed structures for the formation of a carbon nanostructure. The pyrolysis of tetrahydrofuran can be expressed as

formation of SEI and electrolyte decomposition. The carbon nanofibers electrode exhibits a reversible lithium capacity of about 310 mA h/g at 100 mA/g after 200 cycles. The inset of Figure 4a shows the corresponding Coulombic efficiency curve versus cycle number. The Coulombic efficiency quickly increases from 52.4% for first cycle to over 98% after several cycles. As the current density increases to 500 mA/g, the CNFs anode delivers a reversible lithium capacity of 173 mA h/g even over 200 cycles (shown in Figure 4b). The initial Coulombic efficiency is about 45.2%. The Coulombic efficiency increases to 95% after several cycles and stabilizes at about 100% in later cycles at the current density of 500 mA/g (inset of Figure 4b). The above results reveal that the obtained CNFs exhibit a high reversible capacity and stable cycle performance at a low current density. A series of experiments were carried out by altering experimental parameters of the process to study the formation of the carbon nanofibers. It is found that the ZnCl2 played a key role in the formation of the carbon nanofibers. Only carbon spheres are obtained by the pyrolysis of tetrahydrofuran in the similar process without adding ZnCl2.30 Meantime, as ZnCl2 is substituted by metallic zinc or metallic nickel, the bamboostructured carbon nanotubes and carbon fibrils formed by stacking graphite sheets of nanometer thickness can also been produced in a similar process, respecitvely.30,31 The reaction temperatures also play another critical role in the formation of carbon nanofibers. When the reaction temperature is below 500 °C, tetrahydrofuran cannot be completely decomposed to carbon. At the temperature of 550 °C for 10 h, the sample is composed of irregular particles and few carbon nanofibers. The yield of carbon nanofibers can reach a maximum of 80% in the sample at the temperature of 600 °C. SEM image of the sample obtained at the temperature of 650 °C is shown in Figure 5a, which reveals that the sample is composed of carbon spheres and carbon nanofibers. At the reaction temperature of 700 °C, most of carbon products are carbon nanospheres, as revealed by

C4 H8O → 4C + H 2O + 3H 2

In our experiment, zinc oxide acted as a catalyst in the formation of nanofibers, which originates from the reaction of ZnCl2 and H2O (coming from the pyrolysis of tetrahydrofuran). Furthermore, as the melting point of the zinc chloride is 290 °C, the zinc chloride is in the molten state at the temperature of 600 °C. The molten zinc chloride in the autoclave, as a stable liquid reaction medium, could facilitate the formation of CNFs. In the reaction process, the tetrahydrofuran is pyrolyzed into carbon atoms, and the carbon atoms form a homogeneous system in the autoclave. Then, the carbon atoms are distributed on the surface of the ZnO. The nanostructures of carbon product are closely related to the velocities of the carbon atoms. Finally, carbon nanofibers are obtained if and only if carbon atoms are continually provided with proper velocity. The growth process of the carbon nanofibers is quite similar to the synthesis of carbon nanorods through pyrolysis of organic presursors, as reported by Zuo and his co-workers. The carbon atom’s moving velocity is closely related to the reaction temperature. At the reaction temperature of 700 °C, the velocity of carbon atoms is high, and the graphite layers form carbon nanospheres. The formation processes of the carbon nanofibers and nanospheres are shown in Figure 5c. Similar reaction results are mentioned in the literature.31,32 In the contrast experiments, when ferrocene or lactic acid is separately used instead of tetrahydrofuran as the carbon source, C

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carbon nanofibers can also be obtained with different yield. SEM images of sample 2 (shown in Figure 6a,b) reveal that

Figure 6. (a and b) SEM images of sample 2. (c and d) SEM images of sample 3.

sample 2 consisted of carbon nanofibers, nanospheres, and nanosheets, and the yield of the carbon nanofibers is about 70%, based on the SEM view. The SEM images of sample 3 (shown in Figure 6c,d) reveal that sample 3 is composed of nanofibers and nanospheres, the yield of the CNFs in sample 3 is about 50%, and the length of CNFs is about 10 μm. On the basis of the above experiment results, the CNFs with different yields can be obtained by pyrolysis of organic precursors. The yields of the CNFs in the samples relate to the thermal stabilities of the organic precursors and the moving velocities of carbon atoms in the formation process.

4. CONCLUSIONS In summary, CNFs has been prepared by a simple pyrolysis of tetrahydrofuran in molten zinc chloride. The yield of CNFs in the obtained sample is about 80%. The sample as electrode shows a capacity of 310 mA h/g at a constant current density of 100 mA/g. CNFs can also be obtained by pyrolysis of other organic precursors in molten zinc chloride; therefore, the pyrolysis method is a general method for the preparation of CNFs.

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AUTHOR INFORMATION

Corresponding Authors

*L.W.: e-mail, [email protected]; tel, 86-519-6999823, fax: +86-519-6999516. *Q.Z.: e-mail, [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. U1404505 and 21401049). REFERENCES

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