Elsevier

Electrochimica Acta

Volume 306, 20 May 2019, Pages 549-557
Electrochimica Acta

High-performance supercapacitors based on reduced graphene oxide -wrapped carbon nanoflower with efficient transport pathway of electrons and electrolyte ions

https://doi.org/10.1016/j.electacta.2019.03.155Get rights and content

Highlights

  • A novel three-dimension hybrid carbon material consisting of RGO and N-doped carbon nanoflower was fabricated.

  • As-prepared hybrid carbon delivers good capacitive performance both in the KOH and ionic liquid electrolyte.

  • The symmetric supercapacitor exhibits integrated large energy and power density.

  • The enhancement on capacitive performance is ascribed to the efficient transport pathway of electrons and ions.

Abstract

Despite that a variety of carbon materials have been explored for electrochemical energy storage, rationally design the carbon structure for efficient electrons transfer and ions diffusion to further improve its performance is still a challenge. In this report, a novel three-dimension hybrid carbon material (NCNF/RGO) prepared from reduced graphene oxide (RGO) and N-doped carbon nanoflower (NCNF) consisting of carbon nanosheets has been fabricated by a hydrothermal treatment and freeze-drying method, and demonstrated as supercapacitor's electrode. Benefiting from such structure design with thin porous nanosheets, interworking mesoporous channel and RGO wrapping for efficient electrolyte ions diffusion and electrons transfer, the resulting hybrid electrode displays high specific capacitance of 344 F g−1 at a current density of 0.5 A g−1 and 179 F g−1 even at 50 A g−1 in the KOH electrolyte, and 152 F g−1 at 1 A g−1 in the ionic liquid electrolyte with a wide voltage range. Significantly, the assembled symmetric supercapacitor using the NCNF/RGO as electrode materials reaches a large energy density of 84.2 Wh Kg−1 at a power density of 1.0 kW kg−1, indicating its great potential application.

Introduction

Due to its high power density and long cycle life, supercapacitor has attracted great attention as a promising energy storage device for wide range of applications in portable electronics, plus techniques and hybrid electrical vehicles etc [[1], [2], [3]]. Improving the energy density of supercapacitor is still a major challenge in this field [[4], [5], [6]]. According to the calculated equation of energy density (E = CsV2/2), it is dependent on the two crucial parameters - specific capacitance (Cs) and working voltage (V), which are relied on the characteristics of electrode materials and electrolyte.

As one supercapacitor's electrode material, the various carbons are the primary candidate because of their intrinsically high conductivity, excellent chemical stability and large specific surface area for gathering ions via electric double layers capacitance (EDLC) mechanism [[7], [8], [9], [10], [11]]. On the basis of the theory by Gogotsi et al., micropores of the carbon electrodes are proved to be optimal for achieving a high capacitance [12]. However, there is serious kinetics limitation in electrolyte ions transport in micro-porosity systems especially at high current rate, thus inducing the unsatisfactory capacitive performance of the corresponding electrodes. Consequently, optimization the carbon materials' structure including morphology, size and porosity etc has been focused to increase high accessible surface area for collecting more charge and shorten the ion transport length for accelerating the diffusion of electrolyte ions. Among various carbon materials, porous carbon nanosheets with 2D open morphology would exhibit intrinsic advantage for fast charge storage since the ion transport length is significantly shortened in the thin dimension [[13], [14], [15], [16], [17], [18], [19], [20], [21]]. For some instances, Ling et al. reported that biomass-derived B/N co-doped thin carbon nanosheets can achieve good capacitive performance [13]. Multi-heteroatom doped ultrathin porous carbon nanosheets have also been fabricated via a potassium compound-assistant method, and demonstrated high capacitance performance [14]. Nevertheless, the randomly stacked nanosheets between individual units tend to agglomerate, which hinders the penetration of electrolyte ions into the carbon surface, thus reducing the charge storage capacity of electrode materials. Some reports in recent years suggest that the flower-like superstructure consisting of the nanosheets is an ideal methodology to offer high accessible surface areas and efficient pathways for ionic transport [[22], [23], [24]]. Furthermore, one can envision that flower-like carbon interconnected by high conductive materials will demonstrate intrinsic advantage for fast electronic and ionic transport, and the exposure of charge storage interfaces, thus provide the high capacitive performance.

On other hand, supercapacitor using room temperature ionic liquid (IL) as electrolyte offers great promise for next-generation high-performance energy storage devices in the view of the ultrahigh energy density, which is attributed to their wide stable work voltage [[25], [26], [27], [28], [29], [30], [31], [32], [33]]. However, ILs with large ionic size usually exhibits high viscosity and low conductivity, seriously restricting the diffusion of ions into porous carbon materials, and thus comprising the electrochemical performance especially at high current rate as compared with those in the aqueous electrolytes. Therefore, it is extremely important to design new nanostructured porous carbons with 3D interworking channels for shortening the diffusion path length of large IL ion and high accessible specific surface area for more charge storage, and meanwhile, good conductivity to ensure rapid charge migration.

Herein, we rationally fabricate a novel three-dimension (3D) hybrid carbon material (NCNF/RGO) prepared from reduced graphene oxide (RGO) and N-doped carbon nanoflower (NCNF) consisting of carbon nanosheets by a hydrothermal treatment and freeze-drying as supercapacitor's electrode. Benefiting from such structure design with thin porous nanosheets, interworking mesoporous channel and RGO wrapping for high accessible surface area, efficient electrolyte ion diffusion and charge transfer, the resulting hybrid carbon delivers good capacitive performance both in the KOH electrolyte and ionic liquid electrolyte. Significantly, the assembled symmetric supercapacitor using the NCNF/RGO as electrode materials exhibits integrated large energy and power density.

Section snippets

Preparation of hybrid carbon materials

Synthesis of NCNF: 3.0 g glucose and 1.5 g urchin-like silicon nanospheres (Usingle bondSiO2) prepared according with our modified method reported previously [34] were added into a beaker with 75 ml H2O, and well-distributed in water to form a mixture with the help of an ultrasonic instrument. Then the mixture was transferred to a Teflon lined autoclave to react at 180 °C for 12 h. After cooled, the resultant was filtered, washed and dried, successively, to yield the ochre powder. Then, the powder was

Materials characterization

The synthetic strategy of hybrid carbon materials is schematically illustrated in Fig. 1. Briefly, N-doped carbon nanoflower (NCNF) was firstly fabricated using urchin-like silicon nanosphere as template and glucose as carbon source, successively followed by annealing under the NH3 atmosphere and removal of the template. Secondly, the 3D NCNF/RGO hybrid nanomaterial was successfully generated by simply mixing NCNF and graphene oxide, and then via the hydrothermal reduction and freeze-drying

Conclusion

In summary, we have presented a rational design and fabrication of a novel 3D hybrid carbon material (NCNF/RGO) consisting of reduced graphene oxide (RGO) and N-doped carbon nanoflower (NCNF), and demonstrated its capacitive performance as supercapacitor's electrode. Benefiting from such structure design with thin porous nanosheets, interworking mesoporous channel and RGO wrapping providing efficient transport pathway of electrons and electrolyte ions, the as-prepared hybrid carbon achieves a

Acknowledgements

The work was supported by grants from the National Natural Science Foundation of China (Grant Nos. 51872208, 51672193 and 51402217).

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