Spontaneous Synthesis and Electrochemical Characterization of Nanostructured on Nitrogen-Incorporated Carbon Nanotubes

This paper investigated the layered manganese dioxide with hydrate () deposits onto nitrogen-containing carbon nanotube (CNxNTs) as a hierarchical electrode for an energy-storage device. The dense and entangled CNxNTs were directly grown by microwave plasma-enhanced chemical vapor deposition (MPECVD) on a carbon cloth (CC), and subsequently used as a current collector. By controlling the pH value of KMnO4 precursor solution, and incorporating nitrogen into CNTs as a reducing agent, the MnO2 thin layer was uniformly fabricated on the CNxNTs at room temperature by using a spontaneous reduction method. The role of incorporation nitrogen is not only capable of creating active sites on the CNT surface, but can also donate electrons to reduce to MnO2 spontaneously. From the measurements of cyclic voltammograms and galvanostatic charge/discharge, MnO2/CNxNTs/CC composite electrodes illustrated excellent specific capacitance of 589.1 Fg−1. The key factor for high performance could be attributed to the thin-layered MnO2 nanostructure, which resulted in the full utilization of MnO2 deposits. Hence, the hierarchically porous MnO2/CNxNTs/CC electrodes exhibited excellent capacitive behavior for electrochemical capacitor application.


Introduction
Electrochemical capacitors, commonly referred to as supercapacitors, combine the advantages of both conventional capacitors and rechargeable batteries.The high storage of electron energy and fast delivery of power within a short time make supercapacitors complementary charge-storage devices to renewable energy production devices, such as solar cells and fuel cells [1][2][3][4][5][6].Various studies have focused on supercapacitors because of their potential application as energy storage devices in the applications of hybrid electric vehicles and short-term power sources for portable and flexible electronic devices [4][5][6].
According to the energy storage mechanism, supercapacitors can be divided into two categories, as follows: electrochemical double-layer capacitors (EDLCs) and pseu-docapacitors.The EDLCs are chiefly composed of carbonbased materials with high specific surface area, such as activated carbon, carbon nanotubes, and graphene [7][8][9].The electron energy is reserved through the rapid ions adsorption/desorption at the interface between electrode and electrolyte, where the capacitance arises from electrostatic separation at this interface.The electrode materials for pseudocapacitors include electrochemically active materials with several redox states or structures, such as transition metal oxides (e.g., oxides of Ru, Ni, Sn, and Mn) and electronically conducting polymers [10][11][12][13][14][15].In pseudocapacitors, the electron storage mechanism involves reversible Faradaic reaction, which means that the bulk material can be used for energy stock.Therefore, the capacitive performance is expected to be higher than that of EDLCs.Among the materials for pseudocapacitors, hydrous manganese dioxide (MnO 2 • xH 2 O), as the electrode of a supercapacitor has recently attracted attention because of its high theoretical capacitance (∼1380 Fg −1 ), environmentally friendly nature, promising electrochemical performance, and the low cost of raw material [14][15][16][17][18][19].However, in the literature the specific capacitance of MnO 2 was typically restricted to 200∼300 Fg −1 [14][15][16][17] because of its intrinsically poor electronic conductivity and dense morphology.
To overcome these problems, recent investigations have highlighted the influence of the nanostructure of the electrode on the electrochemical performance for the effective utilization of active material [16][17][18][19].Some literatures have demonstrated the enhancement of specific capacitance by adopting the highly porous microstructure composite electrode [20].Therefore, in the design of a composite electrode, it is ideal to fabricate an extremely thin layer (∼nm) of nanostructured MnO 2 onto the current collector, controlling the coating layer thickness and uniform surface coverage to reach the high theoretical specific capacitance.To achieve this goal, the direct-growing nitrogen-incorporated carbon nanotubes (CNxNTs) on the carbon cloth (CC) function as a hierarchical current collector, which is used to improve electronic conductivity and increase specific surface area [21,22].Due to the incorporation of nitrogen, CNxNTs also inherently possess surface defects, creating a further increase of anchoring sites for uniform deposition of MnO 2 nanostructure, and facilitating the charge-transfer between active materials and CNxNTs.Hence, in this study, the entangled and direct-grown CNxNTs were fabricated via microwave plasma-enhanced chemical vapor deposition (MPECVD), and subsequent deposition of MnO 2 on CNxNTs surface by using the spontaneous reduction method was further constructed as the hierarchical electrode.The prepared MnO 2 /CNxNTs/CC composite, with a nanometerscale MnO 2 layer, is expected to exhibit excellent specific capacitance resulting from the large surface-to-volume ratio, high stability, and high-rate capability.

Preparation of CNxNTs/CC Current Collector.
To further extend our previous result, the nitrogen-containing carbon nanotubes (CNxNTs) were directly grown on a commercially available CC (E-TEK, USA), with specifications as follows: B-1 Designation A (weave = plain; weight = 116 g/m 2 ; thickness = 0.35 mm; 0 wt.% wet proofing), and the entire composite was used as the current collector.The detailed procedure for direct growth of CNxNTs can be found elsewhere [23], but is described here briefly.The CNxNTs/CC composite was prepared by nickel-catalyst-assisted MPECVD technique.First, nickel, as catalysts for CNxNTs growth, were electrochemically deposited onto CC in an electrolyte solution of 0.1 M H 2 SO 4 + 0.1 M NiSO 4 under a galvanostatic condition of 0.5 mA cm −2 for durations of 500 s.Prior to the growth of CNxNTs, the nickel-coated CC was subjected to hydrogen-plasma treatment of the catalyst at a microwave power of 1 kW, under chamber pressure 28 torr for 10 min.Subsequently, the synthesis of CNxNTs was conducted in a mixture of precursors (CH 4 /H 2 /N 2 = 20 : 80 : 80) at a microwave power of 2 kW, under a chamber pressure of 40 torr and a substrate temperature of 900 • C for a growth duration of 10 min. .Once the deposition process was completed, the CNxNTs/CC electrode with the deposits was rinsed with distilled water, and subsequently dried at room temperature for further analysis.All the chemicals used for MnO 2 deposition (H 2 SO 4 , KMnO 4 ) were of analytical-reagent grade from Aldrich.Double-distilled water was used throughout the process.investigated by field-emission scanning electron microscopy (FESEM, JEOL-6700), high-resolution transmission electron microscope (HRTEM, JEOL-400 EX), X-ray photoemission spectroscopy (Microlab 350 XPS), and Raman spectroscopy (Jobin-Yvon LabRAM HR800).Electrochemical measurements were conducted using Solartron electrochemical test system (1470E) at ambient temperature.The capacitive properties of the composite electrode were investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge method, in 0.1 M Na 2 SO 4 aqueous solution as the electrolyte by using a conventional three-electrode system.The MnO 2 /CNxNTs/CC composite, with a definite area of 1 × 1 cm 2 , was used as the working electrode.A platinum foil served as the counter electrode, and an Ag/AgCl (3 M KCl, 0.207 V versus SHE) reference electrode was used to control the potential of the working electrode.All electrochemical experiments were conducted at room temperature, and all potential values refer to an Ag/AgCl reference electrode.The mass per unit area of the active materials was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, PerkinElmer ICP-OES Optima 125 3000).

Results and Discussion
3.1.Morphology.The preparation procedure for MnO 2 / CNxNTs/CC composite electrode is illustrated schematically in Figure 1.First, the entangled network of CNxNTs with high surface area, which facilitates the transportation of ions from bulk solution to electrode surface, was fabricated onto a carbon fiber surface via MPECVD [25].The spontaneous reduction method, using KMnO 4 aqueous solution as a precursor, was subsequently employed to deposit thin-layered MnO 2 nanostructures on CNxNTs.This MnO 2 /CNxNTs/CC nanocomposite electrode, with binder-free and rapid electron transfer, benefits capacitive characteristics.Figure 2(a) displays the SEM image of the as-grown CNxNTs/CC composite as the current collector.Each of the carbon fibers was uniformly covered by the dense and entangled CNxNTs with narrow diameter distribution ranging from 40 to 60 nm (48 nm in average).This hierarchical nanostructure enabled us to further increase the surface area compared to naked carbon cloth, as well as the pore distribution, which facilitated the access of the ion.According to the image of HRTEM, as shown in Figure 2(b), all the CNxNTs are multiwalled and have typical "bamboo-like" compartments.The substitutional nitrogen doping can drastically modify the morphology of CNxNTs by introducing a number of pentagons and heptagons into the graphitic layers.The dspacing was determined to be 0.342 nm, which is the spacing between the (002) crystalline planes of graphite.The conformal deposition of MnO 2 was performed by using the spontaneous reduction method at room temperature, by controlling the deposition time for adjusting layer thickness.Figures 3(a)-3(d) shows the SEM images of the MnO 2 /CNxNTs/CC composite at different deposition times from 50 min to 250 min, respectively.After MnO 2 deposition, the CNxNTs diameter grew slightly thicker, and the pore volume between each tube gradually decreased with increasing deposition time.The MnO 2 /CNxNTs/CC, with a deposition time of 50 min, as shown in Figure 3(a), formed a core-shell structure, with a diameter of approximately 70 nm while an aggregation appeared at the deposition time of 250 min as illustrated in Figure 3(d).The aggregation of MnO 2 , which would block the insertion of cations, indicates inferior capacitive behavior.At a deposition time below 250 min, uniform and floccule-like MnO 2 deposits can be observed on the CNxNTs in the high-magnification SEM image of Figure 3(e), which results in an apparent roughness of the MnO 2 /CNxNTs hybrid electrode, compared to the pristine CNTs.In consideration to the diffusion length of cations, the thickness of MnO 2 film should be controlled to less than 150 nm (Figure 3(e)), which utilized ion insertion/desertion more efficiently, and could result in a higher ICP-MS was utilized to determine the loading amount of MnO 2 on the CNxNTs/CC current collector, as illustrated in Figure 4.The loading amount of MnO 2 linearly increased with extending deposition time.After the deposition time exceeds 150 min, the saturation of the loading amount, accompanied by apparent aggregation, can be observed, which is consistent with the SEM findings.When the loading mass reaches 0.65 mg cm −2 , there are no sufficient surface sites for deposition, and therefore, results in accumulation.
This implied that the limitation of the loading amount was significantly dependent on the surface area of the carbon support.The loading level of the active materials was found to be approximately 0.2-0.7 mg cm −2 .

Structural Characterization and Composition Analysis.
Figure 5(a) demonstrates the effect of Raman spectra of N dopant on CNxNTs.The 1350 cm −1 peak (D band) corresponds to the disorder-induced character because of the finite particle size effect or lattice distortion while the 1580 cm −1 peak (G band) corresponds to the in-plane stretching vibration mode E 2g of single crystal graphite [26].These two characteristic peaks show some blue shift toward a higher wave number after the N incorporation.The factor could be ascribed to the tensile stress caused by the Ndoping process.The intensity ratio of I D /I G increased with N decoration, which also exhibits raised defect density [27].XPS was applied for reaching deeper into the electronic International Journal of Electrochemistry structures of CNxNTs, as illustrated in Figure 5(b).The N 1s XPS spectrum shows two characteristic peaks after N incorporation, compared to that of undoped CNTs, with no evident peaks.The peak located at 398.1 eV is assigned to tetrahedral nitrogen bonding within a substitutional pyridine-like dopant structure, and another peak, located at 400.8 eV, corresponds to trigonal nitrogen bonding within a graphene-like dopant structure [21,23,[28][29][30].The graphene and pyridine-like defects in the CNxNTs are shown in Figure 5(c).Hence, the resultant XPS spectra indicate that the N atoms were successfully doped into CNTs to form a chemical bonding.Figure 5(d) shows the Raman spectrum of MnO 2 /CNxNTs/CC composited electrode, which was synthesized by using the spontaneous reduction method.Three major features for birnessite-type MnO 2 can be observed at 506, 575, and 640 cm −1 [31].The Raman band at 640 cm −1 can be considered the symmetric stretching vibration (Mn-O) of the MnO 6 groups.The band located at 575 cm −1 is usually attributed to the (Mn-O) stretching vibration in the basal plane of the MnO 6 sheet.All the nanocomposites have the similar Raman spectra of the birnessite-type MnO 2 , which confirms that MnO 2 successfully precipitated onto the CNxNTs.

Spontaneous Reduction Mechanism.
The utilization of spontaneous reduction method for anchoring heteroatom on CNTs sidewall principally dominates by the difference between reduction potential of ion and work function of CNTs [32].Using this technique, Ma et al. [24] in 2007 demonstrated the fabrication of MnO 2 /CNTs, while CNTs acted as a reducing agent in the solution containing MnO 4 − ions.However, the slow kinetic reaction resulted in the requirement of a long reaction time and a high reaction temperature.To improve the slow kinetics, the N-doping technique can create a number of graphene and pyridinelike defects as anchoring sites on the surface of CNxNTs.These preferential defect sites also generate the hydrophilic interfaces on the sidewalls of CNxNTs, resulting in the rapid reaction between CNxNTs and MnO 2 deposit.In addition, CNxNTs exhibited a metallic behavior because of the N dopant as the electron donor.Therefore, the work function of CNxNTs can be further reduced in comparison to that of pure CNTs, which provides an even higher energy difference to raise the reduction rate.A detailed energy diagram, as shown in Figure 6, was demonstrated to explain the reaction mechanism.The left-hand side of the diagram is the work function (eV) of CNT, and the right-hand side is the reaction potential (V versus NHE) of MnO 4 − precursor solution, Typically, the work function of CNTs can be reduced from ∼5 eV to ∼4.5 eV after N incorporation [33].The potential difference between the work function of CNTs and the reduction potential of MnO 4 − ions can significantly enhance 70% of the reduction power.These features of CNxNTs enable us to raise the kinetic reaction of MnO 2 deposit during the spontaneous reduction process.Furthermore, this enhancement of reduction power can rapidly force the reaction at room temperature.This spontaneous reduction technique, using novel properties of the N-doped CNTs, yields a promising route for fabricating the nanocomposite electrode in various applications.a potential range from 0 to 0.9 V, as shown in Figure 7  density of the MnO 2 /CNxNTs/CC composite electrode was attributed to the uniform covering of nanostructured MnO 2 and the high surface area of CNxNTs.Such a hierarchical composite electrode with high surface area can provide more superficial redox reaction within the interface between the electrode and electrolyte, which contribute to the high capacitance of the composite electrode [34].Besides, the thin-layered MnO 2 further shortened the ionic transfer path, resulting in the full utilization of MnO 2 deposits.The CNxNTs with high electric conductivity also serve as a pathway for electron transfer, which advances the capacitive performance of the composite electrode.Consequently, the MnO 2 /CNxNTs composite electrode can reasonably sustain the shape from higher to lower scan rate which indicates that this hybrid electrode has significant charge and discharge efficiency, as shown in Figure 7(b).
Figure 7(c) displays the galvanostatic charge/discharge plots of MnO 2 /CNxNTs/CC composite electrodes at different deposition times.The symmetric anodic charging and cathodic discharging current reveal excellent electrochemical reversibility, which is consistent with CV observations.The specific capacitance of the electrode was calculated from the charge/discharge profile by using the following equation: where C s (Fg −1 ) is the specific capacitance of the electrode; t (s) is the discharge time; ΔV (V) is the potential window where the operation processes; I (A g  8(a).From the charge and discharge process, the linear and symmetrical curves, at various operated current densities, illustrated the features of high columbic efficiency and lower equivalent series resistance.The cycling lifetime of the MnO 2 /CNxNTs/CC composite electrode was also investigated, at a current density of 0.84 A g −1 in 0.1 M Na 2 SO 4 medium.As shown in Figure 8(b), the gravimetric capacitance decreased by 7% during the first 250 cycles, and remained almost constant during the entire 2,000 cycles.The possible reason for the decrease of capacitance may be because of the equilibration of the electrode potential.The coulomb efficiency, which is the ratio of charge capacitance and discharge capacitance, demonstrates retention of more than 98% during the cycling process, as shown in Figure 8(b).Hence, these capacitive natures of MnO 2 /CNxNTs/CC electrode can be attributed to the conformal coverage of nanostructured MnO 2, which facilitates the fast penetration of the electrolytes and the extraordinary electronic conductivity of CNxNTs to allow rapid electrontransfer for charge storage and delivery.

Conclusions
Uniform and conformal coverage of the MnO 2 thin layer successfully deposited onto hierarchical current collector of direct-grown CNxNTs on CC by using a simple and costeffective spontaneous reduction method at room temperature.The N incorporation in CNTs significantly affects not only the creation of surface defects, which act as anchoring sites, but also the function of electron donors, to further reduce the work function of CNxNTs in facilitating the spontaneous reduction of MnO 4 − to MnO 2 .The hierarchical electrode of MnO 2 /CNxNTs/CC benefits the effective utilization of MnO 2 thin layer and the superior conductivity of CNxNTs for electron path.The specific capacitance of MnO 2 /CNxNTs/CC composite electrode can be as high as 589.1 Fg −1 , which is higher than those reported in prior studies.The excellent cycle lifetime of over 2,000 cycles of galvanostatic charge/discharge process demonstrates the MnO 2 /CNxNTs/CC as a promising active material for a large-scale, flexible, and electrochemically stable supercapacitor.

Figure 6 :
Figure 6: Energy diagram between the (a) Undoped CNTs and (b) N-doped CNTs and the KMnO 4 solution.

)Figure 8 :
Figure 8: (a) Variation in gravimetric capacitance of MnO 2 /CNxNTs/CC hybrid electrode (deposition time of 150 min) with different current density and the inset is the corresponding charge-discharge curves.(b) Cycle life of MnO 2 /CNxNTs/CC supercapacitor.
(a).For comparison, the black-dot CV curve presents a typical double-layer capacitive behavior of CNxNTs/CC electrode, of which the current density is approximately ∼0.4 mA cm −2 .The current densities of MnO 2 /CNxNTs/CC composite electrodes are always higher than that of CNxNTs/CC electrode, implying the origination of capacitive contribution from the MnO 2 layer.All the CV curves of MnO 2 /CNxNTs/CC composite electrodes have a rectangular shape and sym-metric feature, which indicate an ideal capacitive behavior.The curves with increasing deposition time show a tendency toward a higher current density in CV which can be attributed to the increase of MnO 2 deposits.However, when the deposition time exceeds 150 min, a further increase of MnO 2 deposit shows an inverse tendency toward less current density.The reason resulted from a thicker film and even the formation of aggregation, which was confirmed by SEM results.At a deposition time of 150 min, the high current