Abstract
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.
1. 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 [1–6]. 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 [4–6].
According to the energy storage mechanism, supercapacitors can be divided into two categories, as follows: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs are chiefly composed of carbon-based materials with high specific surface area, such as activated carbon, carbon nanotubes, and graphene [7–9]. 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 [10–15]. 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 (), 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 [14–19]. However, in the literature the specific capacitance of MnO2 was typically restricted to 200~300 Fg−1 [14–17] 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 [16–19]. 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 MnO2 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 MnO2 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 MnO2 on CNxNTs surface by using the spontaneous reduction method was further constructed as the hierarchical electrode. The prepared MnO2/CNxNTs/CC composite, with a nanometer-scale MnO2 layer, is expected to exhibit excellent specific capacitance resulting from the large surface-to-volume ratio, high stability, and high-rate capability.
2. Experimental Section
2.1. 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/m2; 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 H2SO4 + 0.1 M NiSO4 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 (CH4/H2/N2 = 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.
2.2. Preparation of MnO2/CNxNTs/CC Composite Electrode
The spontaneous reduction method was used to fabricate the composite electrode [24]. The prepared CNxNTs/CC electrode was immersed into the precursor of 0.1 M KMnO4 solution to deposit the MnO2 for different reaction times, from 50 min to 250 min. The pH of 0.1 M KMnO4 solution was adjusted to neutral by 0.01 M H2SO4 solution. The MnO2 was directly deposited onto the CNxNTs surface via a spontaneous reduction between the CNxNTs and . 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 MnO2 deposition (H2SO4, KMnO4) were of analytical-reagent grade from Aldrich. Double-distilled water was used throughout the process.
2.3. Characterization of MnO2/CNxNTs/CC Composite Electrode
The morphologic and structural evolution, with different reaction times of composite electrode, was 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 Na2SO4 aqueous solution as the electrolyte by using a conventional three-electrode system. The MnO2/CNxNTs/CC composite, with a definite area of 1 × 1 cm2, 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).
3. Results and Discussion
3.1. Morphology
The preparation procedure for MnO2/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 KMnO4 aqueous solution as a precursor, was subsequently employed to deposit thin-layered MnO2 nanostructures on CNxNTs. This MnO2/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 d-spacing was determined to be 0.342 nm, which is the spacing between the (002) crystalline planes of graphite.
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The conformal deposition of MnO2 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 MnO2/CNxNTs/CC composite at different deposition times from 50 min to 250 min, respectively. After MnO2 deposition, the CNxNTs diameter grew slightly thicker, and the pore volume between each tube gradually decreased with increasing deposition time. The MnO2/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 MnO2, which would block the insertion of cations, indicates inferior capacitive behavior. At a deposition time below 250 min, uniform and floccule-like MnO2 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 MnO2/CNxNTs hybrid electrode, compared to the pristine CNTs. In consideration to the diffusion length of cations, the thickness of MnO2 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 specific capacitance. TEM micrographs and corresponding selected area electron diffraction pattern (SAED), as shown in Figure 3(f), were taken from the MnO2/CNxNTs/CC composite. According to the TEM image, the layered structure was composed of several petal-shaped thin nanosheets. Continuous ring pattern of SAED also confirmed their nanocrystalline nature. The d-spacings of (200), (110), (111), and (201), measured from the SAED pattern, are consistent with Birnessite-type MnO2 (JCPDS 42–1317). ICP-MS was utilized to determine the loading amount of MnO2 on the CNxNTs/CC current collector, as illustrated in Figure 4. The loading amount of MnO2 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.
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3.2. 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 E2g 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 N-doping process. The intensity ratio of ID/IG increased with N decoration, which also exhibits raised defect density [27]. XPS was applied for reaching deeper into the electronic 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, 28–30]. 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 MnO2/CNxNTs/CC composited electrode, which was synthesized by using the spontaneous reduction method. Three major features for birnessite-type MnO2 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 MnO6 groups. The band located at 575 cm−1 is usually attributed to the (Mn–O) stretching vibration in the basal plane of the MnO6 sheet. All the nanocomposites have the similar Raman spectra of the birnessite-type MnO2, which confirms that MnO2 successfully precipitated onto the CNxNTs.
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3.3. 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 MnO2/CNTs, while CNTs acted as a reducing agent in the solution containing 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 pyridine-like 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 MnO2 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 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 ions can significantly enhance 70% of the reduction power. These features of CNxNTs enable us to raise the kinetic reaction of MnO2 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.
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3.4. Capacitive Properties
The electrochemical behavior of MnO2/CNxNTs/CC composite electrodes, with different MnO2 deposition times, was characterized by CV in 0.1 M Na2SO4 aqueous solutions at a scan rate of 10 mVs−1 under a potential range from 0 to 0.9 V, as shown in Figure 7(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 MnO2/CNxNTs/CC composite electrodes are always higher than that of CNxNTs/CC electrode, implying the origination of capacitive contribution from the MnO2 layer. All the CV curves of MnO2/CNxNTs/CC composite electrodes have a rectangular shape and symmetric 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 MnO2 deposits. However, when the deposition time exceeds 150 min, a further increase of MnO2 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 density of the MnO2/CNxNTs/CC composite electrode was attributed to the uniform covering of nanostructured MnO2 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 MnO2 further shortened the ionic transfer path, resulting in the full utilization of MnO2 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 MnO2/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).
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Figure 7(c) displays the galvanostatic charge/discharge plots of MnO2/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 (Fg−1) is the specific capacitance of the electrode; (s) is the discharge time; ΔV (V) is the potential window where the operation processes; I (A g−1) is the applied current density based on the total electrode material. The specific capacitance values of MnO2/CNxNTs/CC composite electrode evaluated from the discharge curves are 583.1, 510.7, 520, 410, and 270.6 Fg−1 at the deposition times of 50, 100, 150, 200, and 250 min, respectively. A specific capacitance as high as 589.1 Fg−1 implies that the capacitive characteristics of the MnO2/CNxNTs/CC composite electrodes are a promising material for supercapacitors. The charge/discharge behavior of MnO2/CNxNTs/CC composite electrode at different current densities was examined, as shown in Figure 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 MnO2/CNxNTs/CC composite electrode was also investigated, at a current density of 0.84 A g−1 in 0.1 M Na2SO4 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 MnO2/CNxNTs/CC electrode can be attributed to the conformal coverage of nanostructured MnO2, which facilitates the fast penetration of the electrolytes and the extraordinary electronic conductivity of CNxNTs to allow rapid electron-transfer for charge storage and delivery.
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4. Conclusions
Uniform and conformal coverage of the MnO2 thin layer successfully deposited onto hierarchical current collector of direct-grown CNxNTs on CC by using a simple and cost-effective 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 to MnO2. The hierarchical electrode of MnO2/CNxNTs/CC benefits the effective utilization of MnO2 thin layer and the superior conductivity of CNxNTs for electron path. The specific capacitance of MnO2/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 MnO2/CNxNTs/CC as a promising active material for a large-scale, flexible, and electrochemically stable supercapacitor.
Acknowledgments
This paper was financially supported by the Ministry of Education, Asian Office of Aerospace Research and Development under AFOSR, National Science Council, National Taiwan University, and Academia Sinica, Taiwan.