Generalized Conversion of Halogen-Containing Plastic Waste into

Apr 4, 2014 - Halogen-containing plastic materials have been converted into nanoporous carbon by a template carbonization method, using zinc powder as...
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Generalized Conversion of Halogen-Containing Plastic Waste into Nanoporous Carbon by a Template Carbonization Method Xiang Ying Chen,*,† Liang Xiao Cheng,† Xiao Deng,† Lei Zhang,† and Zhong Jie Zhang*,‡ †

School of Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China ‡ College of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei, Anhui 230039, P. R. China S Supporting Information *

ABSTRACT: Halogen-containing plastic materials have been converted into nanoporous carbon by a template carbonization method, using zinc powder as an efficient hard template. The mass ratio between plastics and zinc powder as well as carbonization temperature plays a crucial role in determining the carbon structures and resultant electrochemical performances. The PTFE-1:3-700 sample that is obtained by carbonizing polytetrafluoroethene and zinc powder (the mass ratio of 1:3) at 700 °C has a large BET surface area of 800.5 m2 g−1 and a high total pore volume of 1.59 cm3 g−1, also delivering excellent specific capacitance of 313.7 F g−1 at 0.5 A g−1. Moreover, it exhibits a superior cycling stability with high capacitance retention of 93.10% after cycling for 5000 times. More importantly, it can be extended to produce nanoporous carbon derived from other halogencontaining plastic materials such as poly(vinylidene fluoride) and poly(vinyl chloride), revealing the generality of the synthesis method.

1. INTRODUCTION Plastic materials are ubiquitous in our daily life and implemented in almost all domestic and industrial appliances. Especially with the industrial development in the last few decades, plastic consumption has drastically increased, primarily up to 400 million tons in 2016, rising by over 5% per year.1 As is well-known, plastic pollution incurred by the accumulation of plastic products in the environment can adversely affect the wildlife, wildlife habitat, or humans. Plastic recycling has been thus employed to recover scrap or waste plastic and reprocess the material into useful products. However, the recycling level is still very low, and in 2011, the overall plastic recycling rate was approximately 8% in the United States.2 From an environmental risk aspect, halogen-containing (such as fluorine, chlorine, bromine, iodine) plastic materials are thought to be more hazardous to humans and/or the environment owing to their primary use, initial manufacturing, disposal practice, and long-term potential for release of a toxic substance. To realize the purpose of dehalogenation from these plastics, diverse kinds of strategies have been put forward. For the defluorination of polytetrafluoroethene (PTFE) having a strong C−F bond (481 J mol−1), it can react with powder magnesium in supercritical carbon dioxide (Tc = 31.8 °C, Pc =7.4 MPa) at 650 °C for 6 h3 or with a refractory ceramic compound (silicon carbide) by a exothermic self-sustain reaction of combustion synthesis.4 For the dechlorination of poly(vinyl chloride) (PVC), it was performed in supercritical water (Tc = 374 °C, Pc = 22.1 MPa),5 by milling and heating with CaO and Ni(OH)26 or by grinding with CaO7 and La2O3.8 With respect to the defluorination of poly(vinylidene fluoride) (PVDF), it can be actualized by activation-free9 or NaOHactivation10 methods. However, most of the dehalogenation methods documented so far usually require complicated © 2014 American Chemical Society

apparatus and/or stern conditions, and especially the treated solids cannot be efficiently reused. Hence, how to felicitously dispose the halogen-plastics is still a big challenge. Besides, nanoporous carbon with high surface area, large pore volume, and hierarchical pore size distribution is believed to serve as fascinating electrode material for supercapacitors.11 The dehalogenation products of PTFE, PVDF, and PVC remain infinite carbon chains, which are therefore expected to form nanoporous carbon under certain circumstances. In this work, for the first time, we demonstrate a novel but efficient recycling method to convert PTFE, PVDF, and PVC plastic wastes into nanoporous carbons, using zinc powder as hard template. The as-produced carbon structures have been well characterized by means of XRD, Raman, BET, FESEM, and HRTEM, revealing their evident nanoporous features. Moreover, the present carbons especially derived from PTFE can deliver excellent electrochemical behaviors measured in a threeelectrode system using 6 mol L−1 KOH as electrolyte.

2. EXPERIMENTAL SECTION A series of halogen-containing polymer wastes such as PTFE, PVDF, and PVC were converted into nanoporous carbon with the help of commercially available zinc powder through a template carbonization approach. In case of the system consisting of PTFE and zinc powder, we can acquire the PTFE-1:1-700, PTFE-1:2-700, and PTFE-1:3-700 samples when altering the mass ratios of 1:1, 1:2, and 1:3 at a designated carbonization temperature of 700 °C. Under the similar Received: Revised: Accepted: Published: 6990

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wt % polytetrafluoroethylene (PTFE) binder was fabricated using ethanol as a solvent. Slurry of the above mixture was subsequently pressed onto nickel foam under a pressure of 20 MPa, serving as the current collector. The prepared electrode was placed in a vacuum drying oven at 120 °C for 24 h. A three electrode experimental setup taking a 6 mol L−1 KOH aqueous solution as electrolyte was used in cyclic voltammetry and galvanostatic charge−discharge measurements on an electrochemical working station (CHI660D, ChenHua Instruments Co. Ltd., Shanghai). Here, the prepared electrode, platinum foil (6 cm2), and saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. Specific capacitances derived from galvanostatic tests can be calculated from the equation

reaction conditions, PVDF-1:3-700 and PVC-1:3-700 samples can also be obtained by simply substituting PTFE as PVDF or PVC. The whole schematic illustration of the production of nanoporous carbons can be depicted in Figure 1.

C=

I Δt mΔV

where C (F g−1) is the specific capacitance; I (A) is the discharge current; Δt (s) is the discharge time; ΔV (V) is the voltage window; and m (g) is the mass of active materials loaded in working electrode. Specific capacitances derived from cyclic voltammetry tests can be calculated from the equation

Figure 1. Schematic illustration for the production of nanoporous carbon by direct carbonization of mixture of halogen-containing polymer waste (PTFE, PVDF, or PVC) with zinc powder.

2.1. Typical Synthesis Procedure for the PTFE-1:3-700 Sample. PTFE powder (∼3 μm) and zinc powder (∼5 μm) with the mass ratio of 1:3 were ground and then transferred to a porcelain boat, flushed with Ar flow for 30 min, and further heated in a horizontal tube furnace up to 700 °C at a rate of 5 °C min−1 and maintained at this temperature for 2 h under Argon flow. The resultant product was immersed and ultrasonicated with dilute HCl solution to remove soluble/ insoluble substances and subsequently washed with adequate deionized water. Finally, the sample was dried under vacuum at 120 °C for 12 h to obtain the PTFE-1:3-700 sample. Similarly, PTFE-1:1-700 and PTFE-1:2-700 samples were obtained by adjusting the mass ratios of PTFE powder and zinc powder as 1:1 and 1:2, respectively. As an extension, PVDF-1:3-700 and PVC-1:3-700 samples were also achieved when heating the mixture of PVDF (or PVC) and zinc powder with the mass ratio of 1:3 at 700 °C. 2.2. Characterization. X-ray diffraction (XRD) patterns were obtained on a Rigaku D/MAX2500V with Cu Kα radiation. Raman spectra were recorded at ambient temperature on a Spex 1403 Raman spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. Field emission scanning electron microscopy (FESEM) images were taken with a Hitachi S-4800 scanning electron microscope. High-resolution transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) patterns were performed with a JEM-2100F unit. X-ray photoelectron spectra (XPS) were obtained on a VG ESCALAB MK II X-ray photoelectron spectrometer with an exciting source of Mg Kα (1253.6 eV). The specific surface area and pore structure of the carbon samples were determined by N2 adsorption−desorption isotherms at 77 K (Quantachrome Autosorb-iQ) after being vacuum-dried at 150 °C overnight. The specific surface areas were calculated by a BET (Brunauer−Emmett−Teller) method. Cumulative pore volume and pore size distribution were calculated by using a slit/cylindrical nonlocal density functional theory (NLDFT) model. 2.3. Electrochemical Measurements. A mixture of 80 wt % the carbon sample (∼4 mg), 15 wt % acetylene black, and 5

C=

1 mv(Vb

∫ − V) V a

Vb

I dV

a

where C (F g−1) is the specific capacitance; m (g) is the mass of active materials loaded in working electrode; v (V s−1) is the scan rate; I (A) is the discharge current; and Vb and Va (V) are high and low voltage limit of the CV tests.

3. RESULTS AND DISCUSSION When heating a mixture of PTFE powder and zinc powder (the mass ratio of 1:1/1:2/1:3) at 700 °C in argon flow, we can obtain the PTFE-1:1/1:2/1:3-700 samples. All the resultant XRD patterns shown in Figure 2a possess broad and lowintensity diffraction peaks, which are approximately centered at 23.5° and 44.1°, assignable to (002) and (101) planes of standard graphite, respectively. This clearly indicates their amorphous natures with low degree of graphitization. Besides, no obvious peaks assignable to zinc powder or other zinc compounds can be detected, which also reveal the high purities on the composition. The possible reaction mechanism in the PTFE−zinc system for producing carbon material was investigated in brief. First, the addition of zinc powder is indispensable since no solid products appear by solely heating PTFE without zinc powder. The present phenomenon well coincides with the result of the pyrolysis of waste PTFE, which can form light ends tetrafluoroethylene, hexafluoropropylene, and octafluorocyclobutane and heavy ends (mainly 1- and 2octafluorobutylene).12 It is apparent that these kinds of pyrolysis gases will vanish along with the argon flow. Second, in order to make the zinc compound existent in the carbon powder clear, the PTFE-1:3-700 sample before being washed with HCl solution was tested by the XRD technique, as shown in Figure S1 (Supporting Information). As a result, the zinc compound is hexagonal phase ZnO (JCPDS Card No. 750576). Considering the melting points of PTFE and zinc powder as 327 and 420 °C, respectively, the present reaction mechanism probably occurs as follows: when heating the mixture of PTFE and zinc powder up to 327 °C, PTFE begins to melt and is covered on the surface with zinc powder. Next, 6991

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Figure 2. PTFE-1:1/1:2/1:3-700 samples: (a) XRD patterns; (b) Raman spectra; and (c−f) N2 adsorption−desorption isotherms and pore size distribution curves.

carbons.13 Differentiating from the individual 1582 cm−1 band of graphite, known as the G band, the PTFE-1:1/1:2/1:3-700 samples take on two distinct Raman peaks of 1343.8 cm−1 (D band) and 1589.6 cm−1 (G band). They usually arise from the K-point phonons of A1g symmetry and zone center phonons of E2g symmetry, respectively.14 The relative ratios of the D band to the G band (ID/IG) of the PTFE-1:1/1:2/1:3-700 samples are 0.90, 0.92, and 0.97, respectively, suggesting that all samples are not well developed and the local carbon structures consist of both graphitic and disordered carbon atoms.15 On the other hand, Tuinstra and Koenig have studied the relationship between the ID/IG ratio measured from the Raman spectrum and the graphitic in-plane microcrystallite size, La, achieved from X-ray diffraction, and it reveals a linear correlation between them: La = (4.35 nm)·(IG/ID).16 Thus, the parameters of La toward PTFE-1:1/1:2/1:3-700 samples can be estimated as 3.92, 4.01, and 4.22 nm, respectively. That is to say,

when beyond 420 °C, zinc powder melts to form a “liquid” droplet and the encapsulated PTFE decomposes into various C−F gases which then react with each other to form Zn−F compounds3,4 together with the generation of carbon materials. The freshly produced Zn−F compounds are metastable and further converted into substance ZnO due to the trace oxygen in argon flow. The compositions and chemical/electronic states of the elements that exist within the present carbons were detected by the XPS technique. Taking the PTFE-1:3-700 sample as an example, its XPS results including XPS survey and C 1s and O 1s spectra are displayed in Figure S2 (Supporting Information), revealing that only carbon and oxygen elements emerge with a high carbon content of 90.67%. Furthermore, the intrinsic structures of the carbons were determined by Raman spectroscopy since it is a standard nondestructive tool for the characterization of crystalline, nanocrystalline, and amorphous 6992

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sample has a large BET surface area of 800.5 m2 g−1 and a high total pore volume of 1.59 cm3 g−1. The present values are much higher than that of carbon material (430 m2 g−1) treated by combustion synthesis.4 As for the PTFE-1:1/1:2-700 samples, they exhibit BET surface areas of 362.7 and 571.2 m2 g−1, respectively, as well as total pore volume of 0.68 and 0.97 cm3 g−1, respectively. Thus, we can see that the mass ratio between PTFE powder and zinc powder has a significant impact on porosity control. Additionally, cumulative pore volumes and pore size distributions were also explored by using a slit/ cylindrical NLDFT model and the corresponding results are shown in Figure 2d−f. Explicitly, all of them exhibit multimodal features with several most probable distribution peaks, revealing the hierarchical pore size distributions within carbons. The PTFE-1:1/1:2/1:3-700 samples have the average pore widths of 16.6, 14.5, and 10.4 nm, respectively, which also closely correlate with the mass ratios of PTFE powder and zinc powder. The shape, size, and porosity nature of the PTFE-1:1/1:2/ 1:3-700 samples were further studied by FESEM and HRTEM techniques. Parts a-b, c-d, and e-f of Figure 3, respectively, display the FESEM images with different magnifications. In a panoramic manner, all carbons take on irregular blocks in

increasing the mass ratio of PTFE powder and zinc powder can improve the crystallite size within carbon. The authentic porosities of the PTFE-1:1/1:2/1:3-700 samples were investigated by N2 adsorption−desorption technique. Figure 2c indicates the overall N2 adsorption− desorption isotherms with a relative pressure (P/P0) of 0−1.0. First of all, these isotherms can be ascribed to type-IV species according to their profiles, revealing the coexistence of micro-/ meso-/macropores within carbons. Moreover, it is concluded from these isotherms that they have low micropore contents but large meso-/macropore ones.17 Second, on account of the sequences of quantities adsorbed (labeled in longitudinal coordinates), we can discern that the PTFE-1:3-700 sample exhibits the largest surface area while the PTFE-1:1-700 sample has the lowest one. As listed in Table 1, the PTFE-1:3-700 Table 1. Surface Areas and Pore Structures of the Carbon Samples sample

BET surface area (m2 g−1)

total pore volume (cm3 g−1)

average pore width (nm)

PTFE-1:1-700 PTFE-1:2-700 PTFE-1:3-700

362.7 571.2 800.5

0.68 0.97 1.59

16.6 14.5 10.4

Figure 3. FESEM images: (a-b) PTFE-1:1-700; (c-d) PTFE-1:2-700; and (e-f) PTFE-1:3-700; (g-i) HRTEM images as well as the magnified HRTEM image and SAED pattern of the PTFE-1:3-700. 6993

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Figure 4. PTFE-1:1/1:2/1:3-700 samples measured in a three-electrode system using 6 mol L−1 KOH as electrolyte: (a) GCD curves at a current density of 5 A g−1 and (b) specific capacitances calculated from GCD curves. PTFE-1:3-700 sample: (c) cycling durability curve (the inset is the CV curves before/after cycling 5000 times); (d) Nyquist plots before/after cycling 5000 times (the inset is the magnified ones).

discharge (abbr. GCD) curves at a current density of 5 A g−1. All curves are nearly triangular with good linearity in a voltage window of −1.0−0 V, well complying with the electrical double-layer capacitor (EDLC) features. That is to say, there is little or no contribution from pseudo-supercapacitors. The results of specific capacitances as a function of current densities, usually named as rate capability, are depicted in Figure 4b. It can be clearly seen that the PTFE-1:3-700 sample delivers the best electrochemical behavior while the PTFE-1:1-700 sample has the lowest. In detail, at a designated current density of 0.5 A g−1, the PTFE-1:3-700 sample has the largest specific capacitance of 313.7 F g−1, whereas the PTFE-1:2-700 sample and the PTFE-1:1-700 sample are of 224.1 and 147.4 F g−1, respectively. These specific capacitances are much larger than most of those ever reported, revealing their favorable potentials for supercapacitors.18 It should be pointed out that the specific capacitances decreasing with the enhancement of current densities primarily derives from the increasing diffusion limitation between electrolyte and electrode carbon material.19,20 Figure S3 (Supporting Information) shows the cyclic voltammetry (abbr. CV) and GCD curves of the carbons with various mass ratios and carbonization temperatures. Jointly judged by Figure 4 and Figure S3, it is readily distinguished that the PTFE-1:3-700 sample exhibits the best electrochemical performance among these carbons. Besides, in order to determine the cycling durability of the PTFE-1:3-700 sample, it was charged/discharged for 5000 times when the current density of 20 A g−1 was designated. As shown in Figure 4c, high

shapes. It is also clear to us that there exists a wealth of visible pores covered with the surfaces of the carbons. From the viewpoint of the supercapacitor application, most of the electrical double-layer capacitor contribution comes from micropores/mesopores, instead of macropores. Therefore, the pores observed by the FESEM technique usually are useless for electrochemical performance. To gain the insight of the structural nature of carbons, HRTEM and SAED techniques have been carried out. Taking the PTFE-1:3-700 sample as example, its HRTEM images are shown in Figure 3 g-h. Interestingly, the carbon sample is almost entirely composed of pores of tens of nanometers, located in the scope of mesopores. Of course, a certain amount of micropores also occurs within the carbon. In addition, Figure 3i depicts a typical magnified HRTEM image, whose lattice fringes are considerably disordered, suggesting the amorphous nature. What is more, the SAED pattern with somber diffraction rings, as shown in the inset of Figure 3i, also indicates the amorphous state of the PTFE-1:3-700 sample. On the whole, the results of the HRTEM image and SAED pattern in Figure 3i well accord with that of XRD pattern shown in Figure 2a. Note that few reports have been documented so far on the supercapacitor application of carbons directly derived from the degradation of PTFE waste. Herein, considering the PTFE1:1/1:2/1:3-700 samples possessing large BET surface areas and high porosities, as illustrated in Figures 2 and 3, these carbons were investigated serving as supercapacitor electrode materials in a three-electrode system using 6 mol L−1 KOH as electrolyte. Figure 4a displays the galvanostatic charge− 6994

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Figure 5. PVDF-1:3-700 sample: (a) FESEM image and (b) Raman spectrum; PVC-1:3-700 sample: (c) FESEM image and (d) Raman spectrum.

the PVDF-1:3-700 and the PVC-1:3-700 samples are 0.94, and 1.15, respectively, revealing the coexistence of graphitic and disordered carbon atoms within carbons, compared with a single 1582 cm−1 band of graphite. Figure 6a,c displays the GCD curves of the PVDF-1:3-700 and PVC-1:3-700 samples at current density of 0.5−20 A g−1 when measured in a three-electrode system using 6 mol L−1 KOH as electrolyte. All of the GCD curves exhibit triangular shapes in a voltage window of −1.0−0 V, indicating their energy storage mostly derived from EDLC mode. The specific capacitances calculated from GCD curves are shown in Figure 6b,d, respectively. At a current density of 0.5 A g−1, the PVDF1:3-700 sample has the specific capacitance of 127.6 F g−1, while that of the PVC-1:3-700 sample is 41.9 F g−1. And both of them are much lower than that of the PTFE-1:3-700 sample (313.7 F g−1). The discrepancies between them are believed to correlate with their pore structures and surface functionalities.

capacitance retention of 93.10% remains even after cycling for 5000 times, distinctly indicating its excellent cycling stability. And the corresponding CV curves before/after 5000 cycles, as depicted in the inset of Figure 4c, also evince this result of high retention. Besides, Figure 4d presents the Nyquist plots before/ after cycling 5000 times of the PTFE-1:3-700 sample. As is well-known, Nyquist plots show the frequency response of the carbon electrode/electrolyte system and are a plot of the imaginary component (Z″) of the impedance against the real component (Z′). The cell corresponds more closely to an ideal capacitor the more vertical the curve at low frequency.21 It is therefore seen from Figure 4d that the present supercapacitor system slightly deteriorates after 5000 cycles. This point is also confirmed by the enlarged inconspicuous arc, representative of the electronic resistance,22 in the high frequency region. Apart from the PTFE−zinc powder system, it is also successfully extended to PVDF−zinc powder and PVC−zinc powder systems, implying the generality of halogen-containing plastic waste conversion into nanoporous carbon. Herein, the PVDF-1:3-700 and PVC-1:3-700 samples, achieved by heating the mixture of PVDF (or PVC) and zinc powder with the mass ratio of 1:3 at 700 °C, will be investigated in depth. Figure 5a,c presents the typical FESEM images of the PVDF-1:3-700 and the PVC-1:3-700 samples, respectively, and both of them consist of a large numbers of irregular blocks. The resultant Raman spectra displayed in Figure 5b,d display the similar positions of D band and G band. The relative ratios of ID/IG of

4. CONCLUSIONS A simple template carbonization method, using zinc powder as hard template, has been utilized to convert halogen-containing plastic waste materials into nanoporous carbon. The structures and electrochemical applications in supercapacitors of the asproduced carbons are investigated in detail. It is believed that some superior advantages exist in this work and can be summarized as follows: 6995

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Figure 6. PVDF-1:3-700 and PVC-1:3-700 samples measured in a three-electrode system using 6 mol L−1 KOH as electrolyte: (a, c) GCD curves at different current densities; (b, d) specific capacitances calculated from GCD curves.

Notes

(1) PTFE, PVDF, and PVC wastes as well as zinc powder are commercially available and inexpensive, making the future industrial production scalable, reproducible, and economic; (2) the carbon materials especially stemmed from PTFE can exhibit large BET surface areas and large pore volumes as well as superior electrochemical performance as supercapacitor electrode materials; (3) the present template carbonization method is expected to rationally treat with other kinds of halogen-containing plastic materials, thus decreasing the resultant environmental pollution and making our world more green and harmonious.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21101052), China Postdoctoral Science Foundation (20100480045), and the University Natural Science Research Project of Anhui Province (KJ2013B209).

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures including XRD pattern, XPS survey, C 1s, O 1s, and carbon/oxygen contents of PTFE-1:3-700, CV and GCD curves, and specific capacitance. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*Tel./Fax: +86-551-62901450. E-mail: [email protected] (X.Y.C.). *E-mail: [email protected] (Z.J.Z.). 6996

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