Highly conductive, porous RuO2/activated carbon nanofiber composites containing graphene for electrochemical capacitor electrodes
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)
Ultracapacitors: why, how, and where is the technology
Journal of Power Sources
(2000)- et al.
Hierarchical porous carbons with controlled micropores and mesopores for supercapacitor electrode materials
Carbon
(2008) - et al.
One-pot hydrothermal synthesis of ruthenium oxide nanodots on reduced graphene oxide sheets for supercapacitors
Journal of Alloys and Compounds
(2012) - et al.
RuO2/graphene hybrid material for high performance electrochemical Capacitor
Journal of Power Sources
(2014) - et al.
Hydrothermal synthesis of hydrous ruthenium oxide/graphene sheets for high-performance supercapacitors
Electrochimica Acta
(2013) - et al.
Hydrous RuO2/carbon nanowalls hierarchical structures for all-solid-state ultrahigh-energy-density micro-supercapacitors
Nano Energy
(2014) - et al.
Exfoliated graphite-ruthenium oxide composite electrodes for electrochemical Supercapacitors
Journal of Power Sources
(2008) - et al.
Flexible ruthenium oxide-activated carbon cloth composites prepared by simple electro deposition methods
Energy
(2013) - et al.
Understanding RuO2·xH2O/carbon nanofibre composites as supercapacitor electrode
Journal of Power Sources
(2008) - et al.
Hydrothermally synthesized RuO2/Carbon nanofibers composites for use in high-rate supercapacitor electrodes
Composites Science and Technology
(2012)
Integration of solid-state dye-sensitized solar cell with metal oxide charge storage material into photoelectrochemical capacitor
Journal of Power Sources
Radiation-crosslinked nanofiber membranes with well-designed core-shell structure for high performance of gel polymer electrolytes
Journal of Membrane Science
The performance of electric double layer capacitors using particulate porous carbons derived from PAN fiber and phenol-formaldehyde resin
Carbon
Porous structure of PAN-based activated carbon fibers
Carbon
Progress of electrochemical capacitor electrode materials: A review
International Journal of Hydrogen Energy
Electrochemical capacitor performance of hydrous ruthenium oxide/mesoporous carbon composite electrodes
Journal of Power Sources
Ruthenium oxide/carbon composites with microporous or mesoporous carbon as support and prepared by two procedures. A comparative study as supercapacitor electrodes
Electrochimica Acta
Highly conductive, mesoporous carbon nanofiber web as electrode material for high-performance supercapacitors
Electrochimica Acta
Preparation and electrochemical properties of RuO2-containing activated carbon nanofiber composites with hollow cores
Electrochimica Acta
Enhancement of capacitance performance of flexible carbon nanofiber paper by adding graphene nanosheets
Journal of Power Sources
Graphene/metal oxide composite electrode materials for energy storage
Nano Energy
Enhancement of the energy storage properties of supercapacitors using graphene nanosheets dispersed with metal oxide-loaded carbon nanotubes
Journal Power Sources
Fabrication of Graphene/Polyaniline Composite Paper via In Situ Anodic Electropolymerization for High-Performance Flexible Electrode
ACS Nano
One-pot polyelectrolyte assisted hydrothermal synthesis of RuO2-reduced graphene oxide nanocomposite
Electrochimica Acta
Highly conductive polymer electrolytes supported by microporous membrane
Solid State Ionics
Cited by (45)
Synthesis of binder-free nanofibers ZnS/MoS<inf>2</inf>/NiF electrode material for asymmetric supercapacitor applications
2024, Journal of Energy StorageUltra-high supercapacitor performance of NiSRu@NiO nanocomposites on nickel foam electrodes
2024, Journal of Energy StorageTailored functional group vitalization on mesoporous carbon nanofibers for ultrafast electrochemical capacitors
2023, Applied Surface ScienceCore-sheath heterostructure of MnCo<inf>2</inf>O<inf>4</inf> nanowires wrapped by NiCo-layered double hydroxide as cathode material for high-performance quasi-solid-state asymmetric supercapacitors
2022, Journal of Alloys and CompoundsCitation Excerpt :They have been placed great expectation in paving the gap between conventional dielectric capacitors and chemical batteries [6–9]. In comparison with the electric double-layer capacitors (EDLCs) which store energy by the reversible sorption of charges at the interface between electrode and electrolyte, pseudo-capacitors which reserve energy based on Faraday redox reaction have attracted increasing attention due to their higher charge storage capability [10–12]. However, the energy densities of the typical supercapacitors are roughly an order of magnitude lower than those of batteries, which restricts their application in some cases.