Elsevier

Electrochimica Acta

Volume 68, 30 April 2012, Pages 146-152
Electrochimica Acta

Nanographene derived from carbon nanofiber and its application to electric double-layer capacitors

https://doi.org/10.1016/j.electacta.2012.02.034Get rights and content

Abstract

The fascinating properties of graphene are attracting considerable attention in engineering fields such as electronics, optics, and energy engineering. These properties can be controlled by controlling graphene's structure, e.g., the number of layers and the sheet size. In this study, we synthesized nanosized graphene from a platelet-type carbon nanofiber. The thickness and size of nanographene oxide are around 1 nm and 60 nm and we obtained nanographene by hydrazine reduction of nanographene oxide. We applied the nanographene to an ionic-liquid electric double-layer capacitor (EDLC), which exhibited a much larger capacitance per specific surface area than an EDLC using conventional activated carbon. Furthermore, the capacitance increased significantly with increasing cycle time. After 30th cycle, the capacitance was achieved 130 F g−1, though the surface area was only 240 m2 g−1. These results suggest that nanographene structure induce the capacitance enhancement.

Introduction

Graphene is a two-dimensional flexible sheet of carbon atoms. It has exceptional electronic properties: an electron mobility of 200,000 cm2 V−1 [1], [2] and a large specific surface area (2600 m2 g−1). Therefore, graphene can be regarded as a promising electrode material for energy storage devices such as Li-ion batteries, fuel cells, and electric double-layer capacitors (EDLCs) [3], [4], [5], [6], [7], [8].

Numerous recent studies reported that graphene's characteristics can be controlled by changing its structure, such as the layer number, defect density, edge state, and sheet size. For example, the metallicity and band gap depend strongly on the sheet size and edge state, e.g., armchair or zigzag edges [9], [10], [11]. Thus, a suitable structure should be selected in order to apply graphene to a device.

The energy density of an EDLC obeys the equation E = 1/2CV2 (E: energy density, C: capacitance, V: applied voltage). Improving the applied voltage increases the energy density. Ionic liquids show promise as electrolytes owing to their desirable characteristics, such as good flame retardant properties and stability over a wide temperature range and potential window [12]. However, their viscosity is much higher than that of conventional aqueous/organic electrolytes, resulting in an inferior ion diffusion coefficient [13]. In nanosized graphene, it is anticipated that ions will efficiently intercalate/de-intercalate into graphene layers because the diffusion length can be reduced. Thus, the use of nanographene is expected to improve the energy density of EDLCs.

To prepare nanographene, we chose platelet-type carbon nanofiber (CNF), rather than conventional graphite powder, as the starting material. The CNF consisted of graphite platelets stacked in a direction perpendicular to the fiber axis [14], [15], [16]. Therefore, its structure is more suitable for producing nanographene by exfoliation than that of at least micrometer-sized graphite powder. Fig. 1 shows schematic diagrams of graphene and nanographene. To exfoliate the platelet-type CNF, we applied the modified Hummers method, which is a convenient, inexpensive technique for high-yield synthesis of graphene nanosheets [17]. This method is widely used for many energy storage devices such as lithium-ion secondary batteries, catalytic fuel cell supports, and EDLC electrodes.

Section snippets

Graphene synthesis

Platelet-type CNF was provided by Mitsubishi Materials Electronic Chemicals Co., Ltd. The modified Hummers method [18], [19] was applied as follows. First, CNF was added to H2SO4 (98%) at room temperature, and the mixture was stirred for 5 min before the addition of KMnO4 in ice water. Next, the mixture was heated to 35 °C and stirred for 30 min. H2O was added, and the mixture was stirred at 100 °C for 1 h. Finally, H2O2 and H2O were added in order to stop oxidation. Subsequently, this solution was

Results and discussion

Fig. 2 shows a SEM image and the diameter distribution of the platelet-type CNF. The diameter ranges from 40 to 380 nm, with a peak at 80 nm in this histogram. Fig. 3 shows a graphene oxide (GO) synthesized from platelet CNF and its thickness and size distributions by using AFM. After 2 h of sonication, the thickness and size were reduced to approximately 1 nm and approximately 60 nm, respectively. The thickness of monolayer GO can be regarded as 1 nm because the minimum thickness obtained in this

Conclusion

In conclusion, we successfully prepared monolayer GO in nanosized sheets from CNF. After reduction, the obtained nanographene exhibited the following electrical double-layer properties.

  • (1)

    Despite its small surface area of 240 m2 g−1, the capacitance was 60 F g−1 under cyclic voltammetry with a potential window from −1.75 to 0.75 V.

  • (2)

    The capacitance was 100 F g−1 under cyclic voltammetry with a potential window from −2.5 to 1.5 V.

  • (3)

    The capacitance continued to increase per cycle until the 30th cycle.

References (26)

  • Y. Zhang et al.

    Nature

    (2005)
  • X. Du et al.

    Electrochim. Acta

    (2010)
  • Y. Chen et al.

    Carbon

    (2011)
  • A. Tanaka et al.

    Carbon

    (2004)
  • S.-H. Yoon et al.

    Carbon

    (2005)
  • S. Stankovich et al.

    Carbon

    (2007)
  • J.-H. Zhou et al.

    Carbon

    (2007)
  • S. Mitani et al.

    Carbon

    (2005)
  • M. Toyoda et al.

    Carbon

    (2004)
  • K.S. Novoselov et al.

    Nature

    (2005)
  • E. Yoo et al.

    Nano Lett.

    (2008)
  • X. Zhou et al.

    J. Mater. Chem.

    (2011)
  • E. Yoo et al.

    Nano Lett.

    (2009)
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