Elsevier

Electrochimica Acta

Volume 186, 20 December 2015, Pages 337-344
Electrochimica Acta

Highly conductive, porous RuO2/activated carbon nanofiber composites containing graphene for electrochemical capacitor electrodes

https://doi.org/10.1016/j.electacta.2015.11.003Get rights and content

Abstract

RuO2/activated carbon nanofiber (ACNF) composites containing graphene are prepared by a simple electrospinning method, followed by physical activated carbonation. We investigate the electrochemical properties as supercapacitor electrodes and the structural properties of these RuO2/ACNF materials as a function of the graphene concentration. The porous RuO2/ACNF composites exhibit an improved microstructure in terms of increased surface area, large mesopore volume fraction, and increased electrical conductivity with increasing graphene content. The RuO2/ACNFs with 3 wt% graphene are characterized as having a large specific surface area of up to 1552.2 m2g−1, mesopore volume fraction up to 53%, high electrical conductivity of over 0.59 Scm−1, gravimetric capacitance of 180.2 Fg−1 and energy density of 20.4–15.3 Whkg−1 over a power density range of 400–10,000 Wkg−1 in 6.0 M KOH electrolyte. These results suggest that RuO2/ACNF with graphene offers the benefits of low resistance for charge diffusion and a short pathway for ion transportation. Therefore, RuO2/ACNF with graphene shows good capacitive behavior when applied as an electrode material for supercapacitors in terms of high rate capability, large capacitance, and more efficient energy density.

Introduction

Electrochemical capacitors have attracted strong attention as an energy storage system for portable electronics, electric vehicles, and renewable energy systems, because of their great advantages such as high power density, low cost, and longer cycle life [1], [2], [3], [4], [5]. Electrochemical capacitors are separated into electrochemical double-layer capacitors (EDLCs) and pseudocapacitors (PCs) based on the mechanism of charge storage [6], [7]. Electrode materials based on EDLCs are porous carbon materials with high surface areas and suitable pore sizes, but their relatively low energy density limits energy-offering application. On the other hand, although PCs can achieve large energy density and high specific capacitance by the use of redox-active transition metal oxides such as RuO2 as faradic electrode materials, RuO2 suffers from a high raw material cost, low abundance, low porosity, and fast fading of capacitance [8], [9], [10], [11]. Therefore, combining amorphous RuO2 with carbon materials is expected to afford a promising electrode material for electrochemical capacitors in terms of long cycle-life and high specific capacitance, due to the optimization of both the faradaic capacitance of the RuO2 and the double layer capacitance of the carbon materials [12], [13], [14], [15]. Recently, Ting et al. reported that electrochemical capacitor electrodes of carbon nanofiber composites with 13 wt% RuO2 in 2.0 M H2SO4 solution showed a maximum specific capacitance of 155.1 F/g at a high sweep rate of 0.2 V s−1 [16]. Zhou et al. reported a single-walled carbon nanotube/RuO2 nanowire electrode that showed a gravimetric capacitance of 138.1 Fg−1, power density of 96.2 kWg−1, and energy density of 18.8 Whkg−1 in a H2SO4 electrolyte [17]. In addition to these applications of RuOx-based composites in electrochemical capacitor electrodes, Hagfeldt et al. reported a novel photoelectrochemical capacitor capable of simultaneous energy generation and storage by integrating all solid-state components such as a photorechargeable ruthenium oxide-based capacitor and a dye-sensitized solar cell [18].

Electrospinning is a unique method capable of producing nanoscale fibrous membranes with porosities of 30–90% and pore size in the range of sub-micrometers to a few micrometers. The polyporous structure in electrospun fiber membranes can provide abundant transport channels for ions and hence improve the interface compatibility of electrode and the electrolyte. Therefore, the electrospun membranes find various applications in dye-sensitized solar cells [19], batteries [20], and fuel cell [21]. In particular, electrospun activated carbon nanofibers (ACNFs) produced by electrospinning can affect the electrical properties of the supercapacitor significantly due to their thermal stability, high surface area, chemical resistance, and micropores that open directly to the outer surface [22], [23], [24], [25]. Although ACNFs have a high specific surface area, their numerous micropores, and low conductivity lead to energy and cycle stability losses as a result of the high internal resistance for charge diffusion in the electrolyte, which hinders the ionic accessibility for diffusion into the micropores during charge-discharge process. Furthermore, the addition of RuO2 in ACNFs has limited their applications in high power density supercapacitors due to the low cycle-life and slow kinetics of ion transport because the redox sites in the polymer backbone are not sufficiently stable for repeated redox processes [26], [27], [28], [29]. Recently, hierarchical porous RuO2/ACNF composites with well-developed mesoporous structure were prepared and electrochemically characterized by our group to solve this problem [30]. A large number of mesopores, which can provide low resistance for charge diffusion and a short pathway for ion transportation, are induced within a single CNF by poly(methyl methacrylate) (PMMA), which is a key factor affecting the formation of many hollow cores. However, hierarchical porous RuO2/ACNF composites showed a low rate capability and low capacitance in spite of their large mesopore volume fraction. This drawback can be overcome by the introduction of graphene into the carbon matrix to improve both the electric conductivity and the mechanical properties of the original carbon matrix [30], [31], [32], [33], [34]. Rao and Ruoff's group reported that graphene-based supercapacitors exhibited excellent performance with a specific capacitance of 75.0 Fg−1 and an energy density of 31.9 Whkg−1 in ionic liquid electrolytes [35], and a specific capacitance of 135.0 and 99.0 Fg−1 in aqueous and organic electrolytes, respectively [36]. Moreover, the presence of more ions and electronic tunnels will improve the electrochemical performance in the graphene-based hybrid materials, due to the high theoretical specific surface area, extraordinary electrical conductivity, and thermal and chemical stability of graphene [37], [38].

Thus, our research objective in the present study is to design highly conductive, porous RuO2/ACNF composites containing graphene for high-performance electrochemical capacitor applications. Graphene is used as a minor additive to the electrode to improve the electrical conductivity owing to its high surface area and electrical conductivity, high flexibility, and mechanical strength. The electrospun RuO2/ACNF composite paper with a small quantity of graphene has suitable characteristics for enhancing the electrochemical performance, due to the enhanced electrical conductivity and formation of a good charge-transfer complex through the addition of graphene. In this structure, the graphene provides a pathway for rapid ion transport and low resistance for charge diffusion in the electrolyte due to the high electrical conductivity, which enhances the enhanced power density and rate capability. Additionally, the energy density and specific capacitance can be increased because graphene is based on the double electrical layer capacitance mechanism [8], [9], [10], [39], [40], [41], [42]. Herein, RuO2/ACNF composites containing graphene are morphologically and electrochemically characterized to evaluate the viability of their electrochemical application in aqueous electrolytes.

Section snippets

Materials and Fabrication

Polyacrylonitrile (PAN), PMMA, ruthenium(III) acetylacetonate, and dimethylformamide (DMF) were purchased from Aldrich Chemical Co. (USA) and used as received. The graphene used in this study was xGNP-C750-grade material produced by XG Science, USA. Graphene was characterized by elementary analysis using the Mettler method (Metler-Toledo AG, Switzerland) as 88.68% carbon, 0.79% hydrogen, 1.11% nitrogen, and 7.65% oxygen. Graphene with a functional group can be easily dispersed in organic

Results and discussion

Fig. 1 shows the overall photographs of electrospun fiber webs derived from ACNF, RuPM-ACNF, and RuPMG(1)-ACNF webs. Electrospun fiber web was successfully prepared as a white material from ACNF, compared to pink for RuPM-ACNF and gray for RuPMG(1)-ACNF, corresponding to adding ruthenium(III) acetylacetonate and graphene, respectively. Upon closer inspection, all of the electrospun fibers had a smooth outer surface with homogeneously distributed diameters ranging from 200 to 250 nm.

Fig. 2a–b

Conclusions

Highly conductive, porous RuO2/ACNF composites containing graphene were prepared by a simple electrospinning method, followed by physical activation process for electrochemical capacitor electrodes. The RuPMG(3)-ACNF electrode with a graphene composition in the composite fibers of 3 wt% showed the best performance in electrochemical tests in both specific capacitance (180.0 Fg−1) and energy density (20.4 Whkg−1). The introduction of graphene into RuO2/ACNF provided a pathway for rapid ion

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A3053053).

References (51)

  • M. Skunik-Nuckowska et al.

    Integration of solid-state dye-sensitized solar cell with metal oxide charge storage material into photoelectrochemical capacitor

    Journal of Power Sources

    (2013)
  • Z. Zhang et al.

    Radiation-crosslinked nanofiber membranes with well-designed core-shell structure for high performance of gel polymer electrolytes

    Journal of Membrane Science

    (2015)
  • K.-P. Wang et al.

    The performance of electric double layer capacitors using particulate porous carbons derived from PAN fiber and phenol-formaldehyde resin

    Carbon

    (2006)
  • Z. Ryu et al.

    Porous structure of PAN-based activated carbon fibers

    Carbon

    (1998)
  • Y. Zhang et al.

    Progress of electrochemical capacitor electrode materials: A review

    International Journal of Hydrogen Energy

    (2009)
  • J.H. Jang et al.

    Electrochemical capacitor performance of hydrous ruthenium oxide/mesoporous carbon composite electrodes

    Journal of Power Sources

    (2003)
  • F. Pico et al.

    Ruthenium oxide/carbon composites with microporous or mesoporous carbon as support and prepared by two procedures. A comparative study as supercapacitor electrodes

    Electrochimica Acta

    (2009)
  • B.-H. Kim et al.

    Highly conductive, mesoporous carbon nanofiber web as electrode material for high-performance supercapacitors

    Electrochimica Acta

    (2012)
  • K.S. Yang et al.

    Preparation and electrochemical properties of RuO2-containing activated carbon nanofiber composites with hollow cores

    Electrochimica Acta

    (2015)
  • Z. Tai et al.

    Enhancement of capacitance performance of flexible carbon nanofiber paper by adding graphene nanosheets

    Journal of Power Sources

    (2012)
  • Z.-S. Wu et al.

    Graphene/metal oxide composite electrode materials for energy storage

    Nano Energy

    (2012)
  • R.B. Rakhi et al.

    Enhancement of the energy storage properties of supercapacitors using graphene nanosheets dispersed with metal oxide-loaded carbon nanotubes

    Journal Power Sources

    (2011)
  • D.-W. Wang et al.

    Fabrication of Graphene/Polyaniline Composite Paper via In Situ Anodic Electropolymerization for High-Performance Flexible Electrode

    ACS Nano

    (2009)
  • J. Shen et al.

    One-pot polyelectrolyte assisted hydrothermal synthesis of RuO2-reduced graphene oxide nanocomposite

    Electrochimica Acta

    (2013)
  • D.W. Kim et al.

    Highly conductive polymer electrolytes supported by microporous membrane

    Solid State Ionics

    (2001)
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