Synthesis of nitrogen-doped plasma treated carbon nanofiber as an efficient electrode for symmetric supercapacitor
Introduction
There has been a large demand for energy storage devices with high performance owing to the rapid development of industry and global economy, and supercapacitors, one of the energy storage devices, have huge attention due to high rate capability, long cycling life, short charging time, simple operational mechanism, and high power density [1], [2], [3], [4], [5], [6], [7], [8], [9]. Recently, the supercapacitor market has increased further because of the demands in numerous devices such as hand phones, notebook, computerized cameras, cold starting of vehicles, emergency doors, vehicle ignition, and energy storage system from renewable energy. It is, therefore, desirable to increase the energy density for the effective use of supercapacitors.
Various carbonaceous materials, including activated carbon, carbon nanotubes, graphene, and carbon nanofibers (CNF), are good candidates for the supercapacitor electrodes because of their excellent physical and chemical properties [8], [9], [10], [11], [12]. Especially one dimensional CNF has large surface area, excellent flexibility, lightweight, and low cost, expecting good electrode material for the supercapacitor applications. One drawback of CNF for the applications is its hydrophobic property, resulting in high resistance between CNF surface and aqueous electrolyte. The accessibility of the electrolyte ions into CNF was obstructed. In addition, microporous structure and low graphitization degree of CNF are also limiting its application in supercapacitors [10,11]. Substantial effort has been needed to improve the electrochemical performance of CNF through modifying the surface structure by, for example, increasing hydrophilicity and interconnection, assisting the diffusion of electrolyte ions into the fibers and reimbursing proper contact between the electrolyte and electrode.
Plasma treatment is a simple and easy method to modify surface properties and structure of carbon material [12], [13], [14]. It could improve electrical conductivity, surface area, hydrophilicity, and even doping of heteroatoms [14], [15], [16], [17], [18]. One reported oxygen-plasma treatment onto CNF increased the specific surface area from 247 m2g−1 to 301 m2g−1 and oxygen to carbon ratio from 6.0% to 17.6% [16]. Ouyang et al. also reported that nitrogen-plasma applied to carbon cloth significantly increased the surface area and improved capacitance from 0.12 mFcm−2 to 391 mFcm−2 at 4 mAcm−2 [17]. Our group also has investigated the plasma effect on carbon materials [13,14,19]. It was concluded that the plasma treatment increased the electrochemical surface area, electrical conductivity, and hydrophilicity [13,19]. Additional functional groups which improved hydrophilic property and dispersion ability of materials were generated by the plasma treatment [14], and nitrogen-doping by the plasma boosted the charge transfer, especially, through the redox interaction [19]. However, the nitrogen doping was not enough only by using the plasma. It is necessary to use another method for the enhanced nitrogen doping.
Doping of heteroatoms is an interesting topic [18], [19], [20], [21], [22], [23], [24], and it can change the properties of carriers as donors or acceptors [24], [25], [26], [27]. Many researches for the heteroatom doping have been performed in order to increase the energy density of supercapacitors [19], [20], [21]. Nitrogen-doping is one of them, and its advantages for the high energy density have been demonstrated [21]. Generally, the nitrogen-doping could be obtained by the post treatment with nitrogen source, such as urea, amines, ammonia, melamine, polypyrrole, and polyaniline, and melamine has been known as one of good nitrogen source which provided very high nitrogen doping [21,22,[28], [29], [30]].
In this work, the CNF surface is modified by ambient plasma followed by nitrogen-doping with melamine. The CNF surface became more hydrophilic, and high nitrogen of 11 wt% could be doped. It was found that the plasma treatment increases the hydrophilic property, surface area, and conductivity of CNF, resulting that more nitrogen could be doped on the CNF surface as well as significantly enhanced energy density of the prepared electrode could be obtained.
Section snippets
Synthesis of materials
Carbon nanofiber (CNF, powder, diameter of 100 nm, length of 20 – 200 µm), melamine (99%), poly (sodium 4-styrene sulfonate) (PSS, powder, MW=18,000), sulfuric acid (98%), and hydrogen peroxide (30 wt% in water) were obtained from Merck and used directly without further purification.
200 mg of CNF and 1 g of melamine were ground together to make well mixed powder, and the mixture powder was put into a boat of the furnace (SH-TMFGC-100, Samheung energy, South Korea) followed by the heating at 500
Physiochemical characterization
The schematic of the sample preparation was shown in Fig. 1a. Briefly, the ambient plasma was applied directly to CNF to make hydrophilic P-CNF1, and then P-CNF1 was thermally treated with melamine to make nitrogen-doped P-CNF1 (N-P-CNF). The change of hydrophilic property of CNF after the plasma treatment was shown in Fig. 1b in which the sample after the plasma treatment (P-CNF1) was dispersed well with DI water, compared to the precursor CNF (left photo), demonstrating that the plasma
Conclusion
In this work, a facile and cost effective nitrogen-doped plasma treated carbon nanofiber (N-P-CNF) was prepared by using the ambient plasma treatment followed by the thermal treatment with melamine as a nitrogen precursor. The surface of CNF was modified by the plasma treatment, and thereafter more nitrogen could be doped. 11 wt% of nitrogen was doped in N-P-CNF, and the surface area and conductivity were also enhanced in N-P-CNF. Finally, high specific capacitance of 495 Fg−1 could be obtained
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF-2020R1I1A3A04037469).
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