Elsevier

Journal of Power Sources

Volume 402, 31 October 2018, Pages 413-421
Journal of Power Sources

Cotton fabric and zeolitic imidazolate framework (ZIF-8) derived hierarchical nitrogen-doped porous carbon nanotubes/carbon fabric electrodes for all-solid-state supercapacitors

https://doi.org/10.1016/j.jpowsour.2018.09.052Get rights and content

Highlights

  • A 3D porous nitrogen-doped carbon cloth composite electrode was prepared.

  • Porous nitrogen-doped carbon skeleton promotes the electrochemical properties.

  • The composite electrode shows a specific capacitance of 390 F/g at 2 mV/s.

  • The flexible all-solid-state supercapacitor delivers high energy density.

Abstract

In this work, a facile self-sacrifice template approach is developed to fabricate a nitrogen-doped carbon nanotubes/carbon fabric electrode with a hierarchically porous structure from core-shell ZnO@ZIF-8 nanorod array layers on the cotton fiber surface inside cotton fabrics. Numerous pores are also generated on the carbon nanotube walls. The nitrogen-doped porous carbon nanotube array layer provides large surface area, proper porosity and heteroatom-doped carbon skeleton, leading to an enhanced electrochemical performance. This flexible electrode exhibits a high specific capacitance of 390 F/g at a scan rate of 2 mV/s. This electrode also shows good long-term cycling stability and stable electrochemical performance under mechanical deformations. The assembled all-solid-state symmetric supercapacitor exhibits a high energy density of 23.4 Wh/kg at a power density of 91.4 W/kg.

Introduction

Flexible supercapacitors are regarded as one of the most promising energy storage devices for powering wearable electronics due to their light weight, flexibility, high power density, long cycling life, rapid charge/discharge rate, safety, easy maintenance and environmental friendliness [[1], [2], [3], [4], [5], [6]]. However, the low capacitance and energy density are the main issues blocking their practical application. Therefore, developing high-performance flexible supercapacitor electrode materials is the key to achieve the satisfied electrochemical performance [[6], [7], [8], [9]].

Among various active materials (i.e., carbon materials, metal oxides and conducting polymers), 2D thin-film carbon materials, especially the carbon fabric, can storage energy through the electric double-layer capacitance (EDLC) mechanism, which have aroused the general attention due to their high conductivity, good chemical stability, and excellent flexibility [[10], [11], [12], [13]]. Several reported literatures reveal that the cotton fabric weaved by cellulose fibers is an inexpensive natural product, which has great potential in the fabrication of flexible carbon electrode materials through thermally treatment in inert atmosphere [[14], [15], [16], [17]]. The resulting carbon fabrics possess high electrical conductivity and good mechanical properties. Meanwhile, this strategy can be easily scaled up due to the commercially available cotton fabric. According to the energy storage mechanism of EDLC, carbon supercapacitor electrodes storage energy through the physical adsorption of electrolyte ions on the interface between the electrode material and electrolyte [[18], [19], [20]]. The electrochemical capacitance is proportional to the accessible surface area of the carbon electrode [21]. Hence enlarging the surface area of carbon fabric is an effective approach to enhance its electrochemical performance. Apart from that, to further increase electrochemical capacitance, proper doping heteroatoms into carbon framework is desirable for enhancing the conductivity, surface wettability as well as capacitance by inducing additional pesudocapacitance. To date, several strategies have been adopted to prepare high-performance cotton fabric derived carbon fabric electrodes. Xue et al. prepared a flexible carbonized cotton fabric supercapacitor electrode, which exhibited stable electrochemical performance under the mechanical folding condition [22]. However, its specific capacitance was only ∼14 F/g at a current density of 0.22 A/g using 1 mol/L Na2SO4 as the electrolyte. Li et al. also observed the similar low electrochemical performance of 1.5 F/g at 1.0 A/g for a pure cotton derived carbon fabric electrode using 1 mol/L H2SO4 as the electrolyte [23]. The unsatisfied energy storage capacity of cotton derived electrodes severely limits their practical application. Although using the activating agent (KOH and NH3) during the thermal treatment of cotton fabric could effectively increase surface area and form N-doping skeleton of the carbon fabric to increase the electrochemical performance, however, these approaches significantly degraded the mechanical strength and flexibility of the carbon fabric electrodes, and arose safety concerns [23,24]. Therefore, designing and tuning the electrode structure of cotton fabric derived carbon fabric electrodes with the key characteristics including large surface area, accessible porosity, proper heteroatom doping and robust electrode structure is still a great challenge in flexible supercapacitors.

In this work, a facile self-sacrifice template approach is developed to construct a nitrogen-doped porous carbon nanotube array layer on the carbon fiber surface inside cotton derived carbon fabrics through carbonizing the core-shell ZnO@ZIF-8 nanorod array layers. The nitrogen-doped carbon nanotubes with microporous walls provide large surface area, proper porosity and heteroatom-doped carbon skeleton, leading to the significant enhancement of electrochemical performance. The strong interaction between carbon nanotube array layers and fiber surfaces, and robust electrode structure guarantee a good electrochemical and mechanical stability when it was tested in a long-term cycling process and under the mechanical deformation condition. The assembled all-solid-state systematical supercapacitor provides an efficient, flexible energy storage device which shows high energy density and good flexibility.

Section snippets

Materials

Cotton fabric is a commercial product purchased from a local market. Zinc acetate dihydrate, triethylamine, sodium hydroxide, zinc nitrate hexahydrate, 2-methylimidazole, and hexamethylenetetramine were obtained from Sinopharm Chemical Reagent Co., Ltd. Isopropyl alcohol, ammonium chloride, N,N-Dimethylformamide, and ammonium chloride were purchased from Shanghai Lingfeng Chemical reagent Co., Ltd. All chemicals were analytical grade and used without further purification.

Synthesis of ZnO seed solution

ZnO seed solution was

Results and discussion

The synthesis strategy shown in Fig. 1 illustrates the design and fabrication of NC/CT fabric electrode. The ZnO nanorod array layers first grew on cotton fiber surface inside the cotton fabric by a seed-assisted hypothermal method. The ZnO nanorods slowly dissolved when immersing the ZnO nanorod/cotton fabric in 2-methylimidazole solution. The released Zn2+ gradually reacted with 2-methylimidazole, forming a layer of ZIF-8 on the surface of ZnO nanorods. After carbonizing the ZnO

Conclusions

In summary, we developed a facile self-sacrifice template approach to fabricate a cotton derived carbon fabric electrode, N-doped porous carbon nanotubes/carbon fabric electrode, with a hierarchically porous structure from core-shell ZnO@ZIF-8 nanorod array layers on the cotton fabric. The 3D hierarchical porous NC/CT electrode material with large surface area, accessible porosity and heteroatom doped carbon surface structure facilitates to achieve high electrochemical performance. This

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51402048), the Fundamental Research Funds for the Central Universities and DHU Distinguished Young Professor Program.

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