Supercapacitive properties of porous carbon nanofibers via the electrospinning of metal alkoxide-graphene in polyacrylonitrile

Supercapacitive properties of porous carbon nanofibers via the electrospinning of metal alkoxide-graphene in polyacrylonitrile

Materials Letters 87 (2012) 157–161 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/m...

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Materials Letters 87 (2012) 157–161

Contents lists available at SciVerse ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Supercapacitive properties of porous carbon nanofibers via the electrospinning of metal alkoxide-graphene in polyacrylonitrile So Yeun Kim a, Bo-Hye Kim b,n, Kap Seung Yang a,b,c,nn, Kyoichi Oshida d a

Department of Advanced Chemicals and Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of Korea Alan G. MacDiarmid Energy Research Institute, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of Korea c Department of Polymer and Fiber System Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of Korea d Department of Electronics and Computer Science, Nagano National College of Technology, Nagano 381-8550, Japan b

a r t i c l e i n f o

abstract

Article history: Received 29 February 2012 Accepted 21 July 2012 Available online 31 July 2012

The polar group Si–O–C introduction was made into the carbon nanofibers, and the resulting carbon nanofiber composite (CNFC) electrodes were evaluated as an electrochemical capacitor. Simple thermal treatment of the electrospun nanofibers from the blend solution of tetraethyl orthosilicate and graphene with polyacrylonitrile introduced suitable micropores to accommodate many ions without a pore creation step such as oxidative activation. The supercapacitor electrode prepared with 3 wt% graphene showed high specific capacitance of 144.80 Fg  1, energy density of 18.49–10.83 Whkg  1 in the respective range of 400–30,000 Wkg  1 in 6 M KOH aqueous solution. The specific capacitance and energy density were 2 and 3 times higher, respectively, in comparison with pristine CNF. & 2012 Elsevier B.V. All rights reserved.

Keywords: Electrospinning Graphene Tetraethyl orthosilicate Carbon nanofiber composite Supercapacitor

1. Introduction Supercapacitors have been intensively investigated as primary and/or backup energy storage systems because they can sustain high power levels with long life cycles [1–7]. Various forms and textures of porous carbons have been examined as possible electrode materials for supercapacitors due to their stable physical and chemical properties, large specific surface area, controlled pore structure, and high conductivity [8–12]. In recent years, porous carbons with tunable structures, textures, and ordered porosity have been successfully obtained by the template such as mesoporous silica, porous coordination polymers (PCPs), and metal-organic frameworks (MOFs) [13–18]. The unique nature of nanoporous structures has exhibited promising electrochemical capacitive properties as supercapacitor electrode materials [19–24]. Researchers have explored graphene-based composite materials to improve the capacitance performance, due to superior electrical conductivities of graphene [25–27]. Electrospinning is a unique method capable of producing nanoscale fibers from both synthetic as well as natural polymers for numerous

n Corresponding author at: Alan G. MacDiarmid Energy Research Institute, Chonnam National University, Yong-Bong dong 300, 500-757 Gwangju, Republic of Korea. Tel.: þ82 62 530 0774; fax: þ82 62 530 1779. nn Corresponding author at: Department of Polymer & Fiber System Engineering, Chonnam National University, Yong-Bong dong 300, 500-757 Gwangju, Republic of Korea. Tel.: þ82 62 530 0774; fax: þ82 62 530 1779. E-mail addresses: [email protected] (B.-H. Kim), [email protected] (K.S. Yang).

0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.093

applications [28]. The nonwoven web obtained from the electrospinning [29,30] has a high specific surface area from shallow pores size, resulting in enhanced specific capacitance at elevated current density for energy storage devices. In this work, the highly spinnable polyacrylonitrile (PAN), graphene, and tetraethyl orthosilicate (TEOS) as pore generator were combined to prepare electrospun fiber webs. The resulting silicon-containing CNFCs were evaluated for electrode performances of a supercapacitor on the basis of pore characteristics and unique microstructural features.

2. Experimental The graphenes used in this study were xGNP-C750-grade materials produced by XG Science, USA. Elemental analysis shows that graphene composition was made as 88.68% carbon, 0.79% hydrogen, 1.11% nitrogen, and 7.65% oxygen using the Mettler method (Mettler-Toledo AG, Switzerland). Solution for electrospinning were prepared by dispersing the appropriate amount of graphene (3 and 5 wt% relative to PAN and TEOS) in TEOS/PAN with weight ratio of 1/9 in dimethylformamide (DMF). This solution was fed into a positively charged spinneret attached to an electrospinning apparatus. The electrospun fiber web was stabilized in air and then carbonized in N2 atmosphere at 800 1C. The samples were identified as TGP-3 and TGP-5, indicating concentrations of 3 and 5 wt% graphene. For the pristine samples, TG, GP-3, and CNF samples were prepared according to the flow diagrams in Fig. S1.

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The morphology of the composites was characterized using an FE-SEM (Hitachi, S-4700, Japan) equipped with EDX spectroscopy. The surface functionality of the CNFCs was examined by FTIR spectroscopy (Nicolet 200). Backscattering Raman measurement was carried out with a Renishaw in Via-Reflex at room temperature. A He–Ne laser was used giving a monochromatic red light of 633 nm wavelength at a power of approximately 15 mW on sample surface. XRD was obtained from ground-up samples of the fibers, using a D-Max-2400 diffractometer and CuKa radiation (l ¼0.15418 nm). From the position of the (002) peak (2y) in XRD, the interplanar spacing d(002) was determined using the Bragg equation [31]. From the position and the full-width at half-maximum intensity of the (002) peak, an attempt was made to estimate the crystalline height Lc using the Scherrer equation [31]. The specific surface area was analyzed by the BET method using an ASAP 2020 Physisorption Analyzer (Micromeritics, USA). The full range of pore sizes over a continuous scale was confirmed through density functional theory calculations (DFT PLUS for Windows). The cell performances of the supercapacitor were tested with two 2.25 cm2 CNF electrodes in a

6 M KOH aqueous solution. The capacitance of the electrodes was galvanostatically measured with a WBCS 3000 battery cycler system in the potential range of 0–1 V and at a current density of 1– 20 mA cm  2.

3. Results and discussion SEM images obtained at low and high magnification of the various CNF composite webs are presented in Fig. 1. All of the samples exhibited long and continuous cylindrical morphologies with 100 300 nm of average diameter. The SEM images of GP-3 and TGP-3, shown in Fig. 1(a) and (c) revealed a smooth surface, whereas the TGP-5 in Fig. 1(d) showed a bent shape with node or joint due to the agglomeration of the graphene at high graphene concentration. At high magnification, the morphologies of the TGP became rougher and displayed a more porous appearance with 10  20 nm pores (Fig. 1(b)). The elemental mapping (Fig. 1(e)) represents the silicon element dispersion on the surface of the

Fig. 1. FE-SEM images showing (a) GP-3, (b) TP, (c) TGP-3, and (d) TGP-5, (e) highly magnified SEM image and the corresponding elemental mapping, and (f) EDX data of individual fiber.

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fibers. The corresponding EDX spectrum indicates that the distribution of C, O, and Si elements in the fiber (Fig. 1(f)), highlighting the homogeneous distributions of amorphous Si–O–C and/or O–Si–O in the carbon matrix. The IR spectra of CNF, TGP-3, and TGP-5 are shown in Fig. 2(a). Three main features are observed in agreement with the literature: (i) in case of TGP-3, and TGP-5, the main bond of Si–O–C in the range of 930 1215 cm  1 is composed of the C–O bond and Si–O cross-link bond compared with CNFs [32]. (ii) the band at 1600 cm  1 is attributed to the CQC vibrations and (iii) the broad

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band at ca. 3490 cm  1 is due to hydrogen bonded O–H stretching vibrations. From the XRD patterns (Fig. 2(b)) of TGP-3, TGP-5, and TP, a broad peak located at 201–301 is assigned as the d(002) layers, representing the presence of disordered carbon structure and the amorphous Si–O–C/Si–O–Si and disordered carbon phases [33]. The Raman spectra (Fig. 2(c)) of the TGP-3, TGP-5, and TP typically show peaks in the range of 300–500 and 1300–1600 cm  1, respectively. In these Raman spectra there are two large peaks, one is near 1338 cm  1 which is D peak from amorphous structures of carbon, another is near 1605 cm  1 which is G peak from

Fig. 2. (a) FTIR spectra of the TGP-3, TGP-5, and CNF, (b) XRD peaks, (c) Raman spectra of the TGP-3, TGP-5, and TP, (d) a plot for crystalline trend.

Fig. 3. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions determined by DFT calculations of TGP-3 and CNF.

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graphitic structures of carbon [34]. The broad band appeared between 300 and 500 cm  1 might be due to a contribution from the amorphous Si–O–C-related structure [35]. Fig. 2(d) summarizes the interlayer spacing d002, crystalline height Lc from XRD and the crystalline width La (La ¼ 4.4/R, R is the ratio of the integrated intensity of D peak to G peak, nm) from the Raman spectra [36]. With adding the graphene, a decrease in d002 and increase in Lc and La are also observed, indicative of the formation of more ordered carbons [33]. It is expected that the incorporation of the graphene can influence the electrochemical performance by the p–p interaction between carbon layers, and graphene can reduce the energy loss during charge–discharge. Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves by the DFT method of the samples are shown in Fig. 3. The adsorption isotherms of TPG3 and CNF show typical type I behavior indicating high microporosity, and the adsorption of nitrogen was nearly complete at a low relative pressure, P/P0 o0.1 (Fig. 3(a)). The pore size distributions of all carbon materials were determined by density functional theory (DFT) and are shown in Fig. 3(b). According to the pore size distributions, 60  80% of the micropores in TGP-3 were ultramicropores with diameters less than 0.6 nm. The narrow micropores and mesopores are developed for the TGP-3 materials. Alternatively, CNF presented a broader distribution of micropore sizes, and an average diameter of 1.4 nm was observed. These results suggested that the presence of ultramicropores with diameters less than 0.6 nm leads to selective ion adsorption [18,37] and well-dispersed mesopores can provide pathway for fast transportation of the electrolyte ions [11,38]. The pore characteristics of various CNFCs were investigated by nitrogen adsorption measurements at 77 K, as shown in Table S1. The specific surface area decreased from 437.64 m2/g for TPG-3 to 313.16 m2/g for TGP-5. At higher graphene concentration, the reduction of the specific surface area could be resulted from the aggregation of graphenes and less effective to contribution to create pores. Furthermore, the surface area of TP is higher than that of GP-3, because TEOS acts as a catalyst for the creation of micropores on the outer surface of CNFCs. TEOS trapped in electrospun nanofibers can be transformed into its hydrated form (i.e., Si(OEt)4  nH2O). During the stabilization process, silanol (Si(OH)x) is produced by the hydrolysis of Si–OEt bonds into Si– OH bonds. Extensive condensation of SiOH leads to the formation of amorphous O–Si–O and/or Si–O–C with gas evolution (e.g., CO, CO2, H2, CH4, H2O, etc.), which leads to a porous structure [39]. An increase in electrical conductivity was observed, from 0.24 S/cm for CNF to 0.35 S/cm for TP to 0.38 S/cm for GP-3, with adding graphene in Table S1. The porous CNFC electrodes were cut into rectangles and were attached to nickel foam to evaluate the capacitance of the material in a two-electrode system and 6 M KOH (aq) electrolyte. Cyclic voltammograms (CVs) obtained at a scan rate of 25 mVs  1 are presented in Fig. 4(a). The TGP-3 electrode shows a much larger quasi-rectangular shape than the other samples, whereas the GP-3 electrode shows a rectangular and symmetric shape. This result represents the expansion of the electrical double-layer region because TGP-3 has heteroatoms (nitrogen, oxygen and silicon), a large surface area with moderate porosity, and good electrical conductivity. The symmetric cell assembled with the CNFCs electrode was subjected to measure specific capacitances in the voltage range of 0–1.0 V with varying discharge current densities from 1 to 20 mA cm  2 in aqueous electrolytes (Fig. 4(b)). The capacitance of the cells decreased as the current densities increased, due to the internal resistance of the electrodes. The TGP-3 web electrode had a greater specific capacitance at all current densities than the other electrodes, which was attributed to the higher electrical conductivity and the larger specific

Fig. 4. Electrochemical test of Si–O–C based CNFCs in KOH (aq) electrolyte; (a) CVs at a scan rate of 25 mVs  1, (b) specific capacitances as a function of a various current densities, (c) Ragone plots.

surface area of TGP-3. This same trend of power and energy performance, which indicates a good behavior in specific capacitance, was also evaluated with a Ragone plot. Fig. 4(c) shows Ragone plots of the CNFC web electrodes. The TGP-3 web electrode exhibits high energy density of 18.49–10.83 Whkg  1 in the power density range of 400–30,000 W/kg; in comparison of

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the energy density of 2.0–6.0 Whkg  1 for the pristine CNF in the same power density range.

4. Conclusion We successfully prepared electrodes in combining the advantageous properties of PAN, TEOS and graphene precursors in the electrospinning process, followed by suitable stabilization and carbonization, to form CNFC electrodes for supercapacitors with enhanced energy and power densities. The TGP-3 web electrode showed the best performance in electrochemical tests for EDLC. The electrodes of the EDLC improved not only in the specific capacitance but also in the energy density; from the specific capacitance of 60.0 F/g, energy density of 6.0 Whkg  1 of the CNF, to 144.80 F/g and 18.49 Whkg  1, respectively, at the composite fibers of 3 wt% of graphene. The introduction of TEOS and graphene into the PAN solution modified the morphological structure, resulting in increases in specific surface area electrical conductivity leading to the enhancement of the electrochemical performances of CNF composites.

Acknowledgments This research was supported the National Research Foundation of Korea(NRF) Grant (NRF-2010-616-D00018) and the Ministry of Education, Science and Technology (MEST) (K2090100172510E0100-09700).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.matlet.2012.07.093.

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