Nitrogen-doped carbon nanotube supported double-shelled hollow composites for asymmetric supercapacitors

Abstract

In this work, a unique N–C@NiMoO₄ composite with a double-shelled hollow microtube structure has been developed through a simple and low-cost growth process. The composite contains a NiMoO₄ outer shell with high electrochemical activity and hollow nitrogen-doped carbon nanotubes (N–C NTs) with high conductivity, resulting in a high performance electrode material. The existence of N–C NTs effectively prevents the accumulation and agglomeration of the NiMoO₄ nanosheets during the formation of a uniform outer shell, providing a higher specific surface area (258.2 m² g⁻¹). The electrochemical study displays that the C@NiMoO₄ composite has a specific capacitance as high as 1242 F g⁻¹ at a current density of 1 A g⁻¹, and about 82.9% cycling stability after 2000 cycles. Moreover, the as-fabricated C@NiMoO₄//N–C asymmetric supercapacitor (ASC) can achieve a maximum high energy density of 44.6 W h kg⁻¹ at a power density of 250.4 W kg⁻¹. These results imply that the C@NiMoO₄ composite with a double-shelled hollow tube-structure is a promising candidate for high performance supercapacitors.

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1. Introduction

As a promising candidate for energy storage, supercapacitor devices have been acclaimed for their low-cost, high power density, long cycling lifespan, fast charge/discharge rates and wide operating temperature range.^1–5 According to the principle of electrical storage, supercapacitors are generally divided into electrical double-layer capacitors (EDLCs) and pseudocapacitors.^6–8 Notably, pseudocapacitors could offer substantially higher specific capacitance and energy density than typical EDLCs because of the fast and reversible Faradaic redox reactions at the surface of the electrode materials.^4,6 In this context, transition metal oxides, (RuO₂,⁹ MnO₂,¹⁰ MoO₃,¹¹ Co₃O₄,¹² NiO,¹³ NiCoO₂,¹⁴ NiCo₂O₄,¹⁵ NiMoO₄,¹⁶etc.), conducting polymers (polypyrrole,¹⁷ polyaniline¹⁸etc.), and their compounds have been used successfully as the electrode materials of supercapacitors. In particular, nickel molybdate with different structures as the electrode material for pseudocapacitors has aroused extensive attention due to its low-cost, rich resource reserves, environmentally friendly nature, high theoretical reversible capacity, rich polymorphism and high electrochemical performance.^19,20

However, metal oxides as the electrode materials of supercapacitors invariably suffer from disadvantages including low ionic conductivity, and poor electrochemical stability, which limit their application in supercapacitors.^14,21 One way to overcome these limitations is to prepare various nano/micro-structure NiMoO₄ materials with high surface area.^4,14 For example, Yin et al.²² synthesized hierarchical nanosheet-based NiMoO₄ nanotubes and demonstrated a specific capacitance of 864 F g⁻¹ at a current density of 1 A g⁻¹. Cai et al.²³ synthesized hierarchical NiMoO₄ nanospheres made from mesoporous nanosheets and drew the conclusion that the NiMoO₄ nanospheres possess a superior electrochemical performance compared to NiMoO₄ nanorods. This excellent electrochemical performance of such nanotube and nanosphere shaped electrode materials with nanosheets as the understructure may result from the high specific surface area, high electrical conductivity and stable structure.^24,25 Another method is to synthesize nano/micro-hybrids with carbon as a support material in order to improve the electrical conductivity and structural stability of the materials.^4,26 Liu et al.²⁷ synthesized a NiMoO₄–rGO composite via a microwave-solvothermal method and the composite exhibited superior electrochemical performance compared to pure NiMoO₄. As reported, taking carbon materials as conductive scaffolds can ideally achieve excellent electrical conductivity and superior cycling stability for composite electrodes. Moreover, carbon-based materials also have fast ion and electron transfer and enhanced capacitive performance due to their high porosity and enhanced wettability.^6,27,28 Therefore, the N–C NT based NiMoO₄ composite in this study is expected to provide improved electrochemical behaviour.

Herein, special nitrogen-doped carbon nanotubes which are made from polypyrrole (Ppy) nanotubes after a one-step carbonation process have been considered an ideal support material because of their high conductivity, highly porous nature and superior mechanical properties.^29,30 Benefiting from the highly porous nature of Ppy nanotubes and great graphitic texture with copious heteroatom functionalities, N–C nanotubes are more conducive to the introduction of metal oxides and then provide more stable metal oxide nanocrystals. It has been reported that the addition of graphite nitrogen into the carbon network enhances the conductivity of the carbon material and wettability of electrolyte electrode interfaces, and then provides more internal accessible surfaces for ions compared to CNTs.^29,31 Hence, the high conductivity of N–C materials tends to accelerate the transmission of electrons and ions, which will further facilitate the rapid charging–discharging of active materials. Moreover, the complex substance with desirable micro-/nanostructures of NiMoO₄ and N–C nanotubes can integrate the advantages between each component and mitigate their inherent shortcomings, providing a high-rate electrochemical behaviour.

Based on these considerations, we successfully synthesized N–C@NiMoO₄ microtubes with a double-shelled hollow structure through a simple hydrothermal process. The existence of N–C NTs in the hybrid material effectively prevented the accumulation and agglomeration of NiMoO₄ nanosheets, providing a specific surface area of up to 258.2 m² g⁻¹, high electrochemical activity and good pore structure. The results of electrochemical measurements indicated that the N–C@NiMoO₄ microtubes as an electrode material have a specific capacitance of 1242 F g⁻¹ at a current density of 1 A g⁻¹, and the cycling stability still remained as high as 82.9% even at a current density of 20 A g⁻¹ after 2000 cycles. Compared with pure NiMoO₄ microspheres, the N–C@NiMoO₄ composite possessed a superior current capacitive behaviour and a better rate capability when the current density is increased. Moreover, an asymmetric supercapacitor device based on N–C@NiMoO₄ microtubes as the positive electrode and N–C NTs as the negative one was fabricated. As expected, the as-assembled device delivered a maximum energy density of 44.6 W h kg⁻¹ at a power density of 250.4 W kg⁻¹ and excellent cycling stability of 104.1% retention after 2000 cycles.

2. Experimental

2.1 Reagents and materials

Ni(NO₃)₂·6H₂O, (NH₄)₆Mo₇O₂₄·4H₂O urea, isopropanol, hexadecyl trimethyl ammonium bromide (CTAB), tetraethoxysilane (TEOS), NH₃·H₂O, HCl, acetylene black, methyl orange (MO), FeCl₃·6H₂O, and pyrrole (py) were supplied by Sinopharm Chemical Reagent Co., Ltd. All the reactants in the experiments were of analytical grade and used without any further purification.

2.2 Synthesis of N–C@NiMoO₄ composites

The polypyrrole nanotubes (Ppy NTs) in the experiment were obtained from natural pyrrole (py) through a modified oxidation–reduction reaction based on a method reported previously.¹⁷ N–C nanotubes were prepared from Ppy nanotubes as precursors though a calcination process at 600 °C for 3 h (3 °C min⁻¹) under the protection of Ar.³⁰

To achieve enhanced surface activity during the growth of the N–C@NiMoO₄ composite, a layer of SiO₂ was coated on the surface of the N–C nanotubes in the following process.³² Briefly, 0.1 g of N–C nanotubes was evenly dispersed into a mixture containing 10 mL water and 40 mL isopropyl alcohol by ultrasound. This was followed by addition of 1 mL NH₃·H₂O and 0.3 g CTAB with stirring for 30 minutes, and then 0.16 mL TEOS was slowly dropped into the above solution. After stirring for a further 3 hours, the as-obtained N–C@SiO₂ was rinsed thoroughly with deionized water and ethanol.

The N–C@NiMoO₄ microtubes were prepared through an in situ growth process of NiMoO₄ nanosheets onto the N–C@SiO₂ tube surfaces, and then residual SiO₂ in the composites was eliminated through etching in 2 mol L⁻¹ NaOH at 80 °C.³³ The brief experimental operation is as follows: firstly, 0.01 g N–C@SiO₂ powder was dispersed in 10 mL deionized water by ultrasonication and vigorous stirring, meanwhile, 4 mL Ni(NO₃)₂·6H₂O (0.1 mol L⁻¹), 0.07 g (NH₄)₆Mo₇O₂₄·4H₂O and 0.12 g urea were homogeneously dispersed in 60 mL deionized water. Then after mixing the above two kinds of solution under vigorous stirring for 30 min, the mixture was subsequently placed in a Teflon-lined stainless steel autoclave and kept at 160 °C for 12 h. Thereafter, the resulting black products were separated by centrifugation, and collected after washing with deionized water and ethanol several times, then dried at 60 °C for 12 h under vacuum. Finally, the etching process of the composites was performed in 2 mol L⁻¹ NaOH, and the as-obtained product is defined as N–C@NiMoO₄ double-shelled hollow microtubes. For the purpose of comparison, NiMoO₄ microspheres without any addition of N–C tubes were synthesized using a similar process.

2.3 Material characterization

The composition and structure of the composite materials were characterized by X-ray diffraction (XRD, PANalytical, Philip X’pert, Cu Kα, λ = 0.154056 nm). As an auxiliary test of XRD, X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250XI) was used to analyse the surface composition of the samples. The morphologies of the as-synthesized materials were observed by scanning electron microscopy (SEM, JEOL-JSM-6700F, and Japan Electronics) and transmission electron microscopy (TEM, JSM-2100F, Japan). N₂ adsorption/desorption tests (NOVA 4200e Surface Area and Pore Size Analyzer) via the Brunauer–Emmett–Teller (BET) method were used to obtain information on the specific surface areas, pore size and pore volume of the porous materials.

2.4 Electrochemical measurements

The electrochemical measurements that consist of cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out on an electrochemical workstation (CorrTest, Wuhan, China) through a three-electrode system in 2 mol L⁻¹ KOH aqueous solution. The preparation process of the working electrode was mainly similar to that in our previous work.²⁵ Typically, electroactive materials (N–C@NiMoO₄, NiMoO₄ and N–C NTs), acetylene black and polytetrafluoroethylene binder with a weight ratio of 80 : 15 : 5 were dispersed in ethanol to produce a homogeneous slurry; then the slurry was coated onto fresh foam nickel (1 cm ×1 cm) which had been cleaned with HCl (3 mol L⁻¹) and DI water/ethanol. Finally the as-prepared working electrodes were dried at 60 °C for 12 h in a vacuum oven.

The asymmetric device was equipped using N–C@NiMoO₄ double-shelled hollow microtubes as the positive electrode, N–C NTs as the negative one, and filter paper as the separator. To approach the maximum capacitance and operating potential window, the optimal mass ratio between the positive and negative electrodes was calculated according to charge balance theory (Q₊ = Q₋ = C_smΔV).³⁴

3. Results and discussion

In this work, N–C@NiMoO₄ double-shelled hollow composites taking N–C NTs as the inner shell and NiMoO₄ nanosheets as the outer shell were successfully synthesized as per the following steps (Scheme 1): firstly, conducting polymer Ppy nanotubes were translated into N–C NTs through a calcination procedure under the protection of Ar. Secondly, a layer of SiO₂ was coated on the surface of the N–C nanotubes through a surface treatment process. Lastly, NiMoO₄ nanosheets were in situ grown on the N–C NT surfaces through a hydrothermal process. With the assistance of calcination and etching treatment, a N–C@NiMoO₄ composite was obtained by removing possible residual SiO₂.

Scheme 1 The typical procedure for the synthesis of N–C@NiMoO₄ double-shelled hollow microtubes.

The typical powder XRD patterns of the N–C NTs, NiMoO₄ microspheres and C@NiMoO₄ double-shelled hollow microtubes are shown in Fig. 1. From Fig. 1A, a broad diffraction peak at around 24° can be observed in the XRD pattern, which corresponds to the graphitic texture, exhibiting the amorphous nature of the N–C NTs.³⁵ Except for a relatively flat peak at 22° that may be caused by the glass slice during the testing process, the XRD patterns of the NiMoO₄ microspheres (Fig. 1C) are in good accordance with the monoclinic structure of NiMoO₄ (JCPDS Card No. 45-0142).^36,37 In comparison, the XRD pattern of the C@NiMoO₄ microtubes (Fig. 1B) is similar to that of NiMoO₄ microspheres, which indicates that NiMoO₄ nanosheets were successfully grown on the surface of the N–C NTs, and the crystal structure did not change much during the formation of the hollow composite.

Fig. 1 XRD patterns of the N–C NTs (A), N–C@NiMoO₄ composite (B), and NiMoO₄ microspheres (C).

The X-ray photoelectron spectroscopy (XPS) spectrum of the N–C@NiMoO₄ double-shelled hollow composite was obtained in order to further analyse the composition of the as-prepared products. As expected, it can be seen that there are altogether five elements contained in the composite, namely C, N, O, Mo and Ni (Fig. 2). What's more, there is no Si signal in the XPS spectrum, indicating that SiO₂ was completely removed during the etching process. In addition, the complete elimination of SiO₂ can be further confirmed by the EDS analysis (Fig. S1G, ESI†). The XPS spectrum of C 1s (Fig. 2A) can be primarily fitted as three peaks centered at binding energies of 284.3, 285.6 and 287.7 eV, respectively. The main peak at the binding energy of 284.3 eV corresponds to the energy of sp² C=C bonds, which confirms the presence of graphitic carbon. Besides, there are also two small signals at higher binding energies, corresponding to C–O (285.6 eV) and C–N (287.7 eV) species, respectively.²⁹ In the N 1s XPS spectrum (Fig. 2B), two distinct peaks at 399.8 and 397.8 eV are clearly observed, which correspond to graphitic nitrogen species and hexagonal pyridinic nitrogen species, respectively. As expected, the existence of graphitic-N and pyridinic-N reveals the formation of N–C NTs.^29,30 In the O 1s XPS spectrum (Fig. 2C), the presence of two oxygen peaks suggests that there are two types of oxygen containing species in the N–C@NiMoO₄ double-shelled hollow microtubes. In detail, the large peak at 530.9 eV which corresponds to M–O–M, is typical of the lattice oxygen in N–C@NiMoO₄. The other peak at 532.7 eV can be considered to be the binding energy of C=O, which may be attributed to the oxygen groups remaining in the N–C tubes and the water molecule attached on the surface of the N–C@NiMoO₄ microtubes.²⁷ In the Mo 3d core level XPS spectrum (Fig. 2D), two remarkable peaks at the binding energies of 232.2 and 235.4 eV correspond to Mo 3d_5/2 and Mo 3d_3/2, respectively. It is worth mentioning that the energy separation is around 3.2 eV between Mo 3d_5/2 and Mo 3d_3/2, which is characteristic of the Mo⁶⁺ oxidation state in the molybdate ion.^38,39 In the Ni 2p XPS spectrum (Fig. 2E), two major peaks at 855.9 and 873.7 eV can be assigned to Ni 2p_3/2 and Ni 2p_1/2, and the energy separation between the two peaks is approximately 17.8 eV, which is consistent with the Ni²⁺ oxidation state.^39,40 From the results of XPS, it can be seen that there are Ni²⁺ and Mo⁶⁺ in the NiMoO₄ samples, confirming the successful synthesis of NiMoO₄ nanosheets on the N–C NTs surface.⁴¹

Fig. 2 XPS spectra in the C 1s (A), N 1s (B), O 1s (C), Mo 3d (D), and Ni 2p (E) regions for the N–C@NiMoO₄ double-shelled hollow microtubes.

The morphology and structure analyses of the as-obtained samples are shown in Fig. 3 and 4. The SEM and TEM images of Fig. 3A–C show a well dispersed state of Ppy nanotubes with a length in the microscale. As shown in Fig. 3B and C, the TEM images of the Ppy tubes illustrate that the Ppy tubes have an obvious hollow structure with a wall thickness of approximately 15 ± 3 nm and a diameter in the nanoscale. After the carbonization process (Fig. 3D), the as-prepared N-doped C NTs still possess a nanotube structure with a shortened length compared with the Ppy NTs, which may be caused by the rupture of the N–C NTs under high temperature. From closer inspection of the TEM images (Fig. 3E and F), decreased thickness of the N–C NTs can also be observed due to shrinkage during the high temperature treatment. The SEM image of N–C@SiO₂ is shown in Fig. 3G, and nanometric N–C@SiO₂ still possess a certain dispersion with a decreased length due to ultrasonic treatment. In Fig. 3H and I, the successful deposition of the SiO₂ layer on the surface of the N–C NTs can be clearly observed due to the obvious increase in wall thickness. It is worth mentioning that SiO₂ has the advantages of non-toxicity, excellent compatibility, good hydrophilicity, stability and it being easy to realize surface functionalization and so on. Deposition of a SiO₂ layer can improve the reaction inertia and surface wettability of N–C tubes prepared through a calcination treatment, which possibly provides favourable conditions for the in situ growth of NiMoO₄ on the surface of the N–C NTs. From longitudinal comparison between the SEM and TEM images of the three samples, it can be concluded that the samples still maintain a good tubular structure and uniform wall thickness after the calcination and deposition processes.

Fig. 3 SEM (A) and TEM (B and C) images of Ppy NTs, SEM (D) and TEM (E and F) images of N–C NTs, and SEM (G) and TEM (H and I) images of N–C@SiO₂ NTs.

Fig. 4 SEM image (A) and TEM images (B and C) of NiMoO₄ microspheres; the low and high magnification SEM images (D–F), the TEM images (G–I) and HRTEM image (J) of N–C@NiMoO₄ double-shelled hollow microtubes.

For the purpose of contrast, fluffy NiMoO₄ microspheres were synthesized under the same conditions as the N–C@NiMoO₄ double-shelled hollow microtubes with the absence of N–C NTs. As shown in Fig. 4A, it can be clearly seen that these NiMoO₄ samples have a uniform diameter distribution with diameter of about 2 μm (Fig. 4B). On closer inspection of the TEM image in Fig. 4C, it can be concluded that the fluffy NiMoO₄ microspheres are composed of numerous closely packed NiMoO₄ nanosheets. From the SEM images of the N–C@NiMoO₄ composite under different magnifications (Fig. 4D–F), it can be roughly observed that the N–C@NiMoO₄ composite formed a hollow microtube-structure with numerous NiMoO₄ outer shells grown on the inner N–C NT shell. In Fig. 4F, a large number of spaces can be seen among these sheets, which is beneficial for ion transfer and infiltration of electrolyte. A closer observation of the TEM images in Fig. 4G–I reveals that the double-shelled hollow tube-structure was successfully synthesized containing NiMoO₄ nanosheets as the outer shell with high electrochemical activity and N–C NTs as the inner shell with high conductivity. The presence of N–C NTs efficiently avoids the stacking and aggregation of the NiMoO₄ nanosheets, which eventually leads to a unique hollow tube-structure with high specific surface area and makes the NiMoO₄ nanosheets exposed to the electrolyte with increased active sites. At the same time, due to the presence of N–C tubes, the N–C@NiMoO₄ composite provides a higher conductivity and a better ion channel for electron transfer during the electrochemical process. Based on the HRTEM image of the N–C@NiMoO₄ double-shelled hollow microtubes (Fig. 4J), an interplanar spacing of around 0.243 nm was measured, which corresponds to the (400) face of the monoclinic phase of NiMoO₄. Besides, the successful synthesis of the N–C@NiMoO₄ composite with numerous NiMoO₄ nanosheets grown on the inner N–C NT shell can be further elucidated by the elemental mapping images and EDS. (Fig. S1, ESI†).

In order to study the specific surface area and pore structure of the electrode materials, N₂ adsorption/desorption isotherms were acquired. As shown in Fig. 5, the N₂ adsorption/desorption isotherms of the N–C@NiMoO₄ double-shelled hollow microtubes, NiMoO₄ microspheres, N–C NTs and Ppy NTs are similar in shape. From comparison of the four adsorption/desorption isotherms, it can be seen that all of the samples have typical type IV isotherms with an H3-type hysteresis loop (p/p₀ > 0.4), which illustrates the existence of a mesoporous structure in the four kinds of materials. From the values shown in Table 1, the N–C@NiMoO₄ double-shelled hollow composite with numerous NiMoO₄ nanosheets grown on the N–C NT surfaces, has a larger specific surface area of 258.2 m² g⁻¹ and an appropriate pore volume of 1.084 cm³ g⁻¹ compared with the other three samples, which is in good agreement with SEM and TEM. In detail, the introduction of N–C NTs and the in-situ growth process of the NiMoO₄ nanosheets effectively prevent the aggregation of the outer shells and expose more NiMoO₄ nanosheets in the exterior, thereby increasing the specific surface area of the composite. Besides, the average pore size of the N–C@NiMoO₄ double-shelled hollow microtubes is about 9.164 nm, which is more narrow than that of the Ppy tubes (14.00 nm) and N–C tubes (12.33 nm), indicating a more stable pore structure of the composite.

Fig. 5 N₂ adsorption/desorption isotherms of N–C@NiMoO₄ double-shelled hollow microtubes, NiMoO₄ microspheres, N–C NTs and Ppy NTs.

Table 1 Specific surface area and pore size of the Ppy NTs, N–C NTs, NiMoO₄ microspheres and N–C@NiMoO₄ double-shelled hollow microtubes

Material	Specific surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
Ppy NTs	63.24	0.2389	14.00
N–C NTs	165.3	0.4074	12.33
NiMoO4 microspheres	199.8	0.5477	7.021
C@NiMoO4 microtubes	258.2	1.084	9.164

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The electrochemical properties of the as-prepared electrode materials were mainly studied by cyclic voltammetry (CV), and galvanostatic charge–discharge (GCD) in order to analyze their potential applications for supercapacitors. The CV measurement of the N–C@NiMoO₄ double-shelled hollow microtubes, NiMoO₄ microspheres and N–C NTs was executed at a 5 mV s⁻¹ scan rate in 2 mol L⁻¹ KOH electrolytes, and the curves are shown in Fig. 6A. It can be seen that the CV curves of the N–C@NiMoO₄ double-shelled hollow microtubes and NiMoO₄ microspheres have a similar redox peak shape, which corresponds to the electrochemical reaction of Ni²⁺/Ni³⁺. But the position of the two peaks and the current density responses are slightly different, which may be attributed to the microtube structure and the existence of N–C NTs in the N–C@NiMoO₄ composite during the electrochemical process.³⁶ It is worth noting that the redox reaction of the Mo atoms does not occur. The contribution of Mo atoms to the capacitance is negligible, while the Mo atom plays an important role in improving the conductivity to enhance the electrochemical performance of the composite.^42–44 In addition, it is well known that the specific capacitance is proportional to the area surrounded by the CV curve at the same scan rate. It is reasonable to speculate that the N–C@NiMoO₄ composite has a higher specific capacitance due to its higher redox current intensity and larger integral area among the three electrode materials.⁴⁵ These conclusions can also be further summarized from the following GCD measurements. The high specific capacitance of the double-shelled hollow N–C@NiMoO₄ microtubes may be because of a large number of active sites on the surface of the composite materials and the existence of a porous structure, which is beneficial to ion transport during the electrochemical process. The CV curves of the N–C@NiMoO₄ microtubes in the voltage range of 0–0.65 V at different scan rates (5–50 mV s⁻¹) are shown in Fig. 6B. It is pretty obvious that the CV curves have a similar redox peak at different scan rates. The cathodic and anodic peak currents increase with an increase in scan rate, and the position of the cathodic and anodic peaks respectively shifts toward a high potential and low one, respectively, which is due to the polarization effect of the electrode.⁴⁶ The unchanged shape of the CV curve as the scan rate increases from 5 to 50 mV s⁻¹ indicates the high rate capability and good reversibility.^47,48Fig. 6C contains the discharge curves of the N–C@NiMoO₄ double-shelled hollow microtubes as electrode materials at different current densities in the range of 1–20 A g⁻¹. It is well known that the discharge time is proportional to the specific capacitance value during the process of charge–discharge test according to the following equation [eqn (1)],²⁵ where C_sp is the specific capacity of the electrode materials (F g⁻¹), I represents the constant current of the charge–discharge test (A), Δt is the discharge time (s), m is the mass of active material on the electrode (g), and ΔV means the voltage range of the constant current charge–discharge test (V).

C_sp = IΔt/mΔV (1)

According to eqn (1), the specific capacitance of the N–C@NiMoO₄ composites is 1242, 1123, 1031, 928, 822, 774, 670 and 581 F g⁻¹ at a current density of 1, 2, 3, 5, 8, 10, 15 and 20 A g⁻¹, respectively. And the corresponding specific capacitances of the N–C@NiMoO₄ microtubes, NiMoO₄ microspheres and N–C NTs at different current densities are shown in Fig. 6D. The specific capacitance retention even at the high current density of 20 A g⁻¹ is 46.8% for the hollow N–C@NiMoO₄ composite. In comparison, the specific capacitance for the NiMoO₄ microspheres was only 639 F g⁻¹ at a current density of 1 A g⁻¹ and only 45.5% of the capacitance was retained under 20 A g⁻¹. As presented in Fig. 6D, the N–C@NiMoO₄ composite exhibits the highest specific capacitance and rate capability compared to the other two electrodes. Such enhanced electrochemical performance may be due to the synergistic effect from the inner and outer shell materials, higher electronic conductivity for improved charge transfer and larger specific surface area for increased active sites compared to the NiMoO₄ microspheres.

Fig. 6 CV curves of N–C@NiMoO₄ double-shelled hollow microtubes, NiMoO₄ microspheres and N–C NTs at the scan rate of 5 mV s⁻¹ (A); CV curves of the N–C@NiMoO₄ double-shelled hollow composite at various scan rates (B); the discharging curves of N–C@NiMoO₄ microtubes at different current densities from 1 to 20 A g⁻¹ (C); the specific capacitance of N–C@NiMoO₄ microtubes, NiMoO₄ microspheres and N–C NTs at different current densities (D); the cycling stability of N–C@NiMoO₄ and NiMoO₄ at 20 A g⁻¹ (E); the Nyquist plots of the N–C@NiMoO₄ double-shelled hollow microtubes, NiMoO₄ microspheres and N–C NTs (F).

As an important parameter to evaluate the practical application potential for supercapacitors, the electrochemical stability of the N–C@NiMoO₄ double-shelled hollow microtubes was investigated by means of cyclic charge–discharge operation.^49,50Fig. 6E reveals the cycling performance of the N–C@NiMoO₄ composites and NiMoO₄ micropheres at a current density of 20 A g⁻¹. It displays that the double-shelled hollow N–C@NiMoO₄ microtubes present a superior cycling stability of 82.9% specific capacitance after 2000 cycles. At the same operation, the capacitance retention was only 55.7% for the NiMoO₄ micropheres. The loss of specific capacitance may be due to the destruction of the structure and the reduction of surface active sites, resulting in an increase in resistance during the charge–discharge process. The improvement in cycling performance of the N–C@NiMoO₄ double-shelled hollow microtubes may be due to the introduction of N–C NTs as a substrate, which effectively prevents the NiMoO₄ nanosheets from aggregating and increases the number of active sites exposed to the electrolyte; these changes are beneficial to the Faraday reaction. Meanwhile, the unique double-shelled hollow structure with high surface area and good pore size distribution of the N–C@NiMoO₄ composite could accommodate the volume change and prevent the pulverization of the electrode material during the cycling process. What's more, the Nyquist plots of the three electrode materials (Fig. 6F) were studied in order to obtain more information on the superior electrochemical behavior. The semicircle at high frequencies is usually linked to the ionic charge-transfer resistance (R_ct).⁵¹ The smaller diameter corresponds to the lower R_ct. The lower R_ct presents a larger electroactive surface area of the electrode materials, which is possibly because of the large specific surface area and high electrical conductivity of the N–C NTs in the composite.^52,53 At low frequencies, a straight line represents the diffusive resistance of the electrode. As shown in Fig. 6F, the straight line of the double-shelled hollow N–C@NiMoO₄ microtubes is nearly perpendicular to the real axis, indicating high capacitive behavior.^46,54 These results suggest that NiMoO₄ nanosheets grown on N–C NTs possess a lower resistance than NiMoO₄ microspheres during the electrochemical process.

To further confirm the excellent electrochemical performance of the double-shelled hollow N–C@NiMoO₄ composite electrode material in practical applications, an asymmetric supercapacitor (ASC) device was assembled by configuring the N–C@NiMoO₄ composite material as the positive electrode and N–C as the negative one in 2 mol L⁻¹ KOH electrolyte with a piece of filter paper as the separator (Fig. 7). As shown in Fig. S2 (ESI†), the voltage window of the N–C electrode is from −1.0 to 0 V with representative EDLC behaviour. Combined with the 0–0.65 V voltage window of the N–C@NiMoO₄ microtube electrode in a three-electrode system, the total cell voltage of the ASC device can be increased up to 1.65 V. Fig. 7A shows the CV curves of the N–C@NiMoO₄//N–C ASC device in different voltage windows from 1.25 V to 1.85 V at the same scan rate of 10 mV s⁻¹. Considering the influence of voltage window on the energy density of supercapacitors, the electrochemical windows of the ASC device can be extended to 1.65 V as expected. As shown in Fig. 7B, the CV peak current of the N–C@NiMoO₄//N–C ASC in the voltage window of 0–1.65 V increases with an increase in scan rate from 10 mV s⁻¹ to 50 mV s⁻¹, and the position of the redox peaks is shifted to high and low potential, respectively. Galvanostatic charge–discharge curves of the N–C@NiMoO₄//N–C ASC device at a series of current densities were acquired in the range of 0–1.65 V and presented in Fig. 7C. According to these curves, an increase in specific capacitance with a decrease in current density was calculated and shown in Fig. 7D. It can be seen that the specific capacitance of the N–C@NiMoO₄//N–C ASC device is 118, 105.1, 91.56, 75.11, 67.09 and 61.76 F g⁻¹ at a current density of 0.3, 0.5, 1, 2, 3 and 5 A g⁻¹, respectively. The energy density and power density of the ASC can be estimated according to eqn (2) and (3) based on the results of galvanostatic charging–discharging (Fig. 7E).⁵⁵

E = (C_m × ΔV²)/7.2 (2)

P = (E × 3600)/Δt (3)

where E (W h kg⁻¹) is the energy density of the ASC, C_m (F g⁻¹) represents the specific capacitance of the ASC device, ΔV (V) is the voltage window of the device; P (W kg⁻¹) represents the power density of the cell, and Δt (s) is the time in the discharge process. The energy density of the N–C@NiMoO₄//N–C ASC device is up to 44.6 W h kg⁻¹ at a power density of 250.4 W kg⁻¹, and it still remains at 23.3 W h kg⁻¹ when the power density is increased to 4081 W kg⁻¹. It is noted that the energy density of the N–C@NiMoO₄//N–C ASC device is higher than the reported values of AC@Ni//MnO₂@CoMoO₄@Ni (30.94 W h kg⁻¹ at 47.06 W kg⁻¹);⁵⁶ AC//NiMoO₄–CoMoO₄·xH₂O (28 W h kg⁻¹ at 100 W kg⁻¹);⁵⁷ AC//NiMoO₄ nanospheres (26.1 W h kg⁻¹ at 175 W kg⁻¹);⁵⁸ AC//CoMoO₄@NiMoO₄ (28.7 W h kg⁻¹ at 267 W kg⁻¹);⁵⁹ and AC//NiCo₂O₄@NiMoO₄ (21.7 W h kg⁻¹ at 157 W kg⁻¹).⁶⁰Fig. 7F reveals the superior cycling stability of the ASC device upon continuous galvanostatic charge–discharge cycling at a constant current density of 5 A g⁻¹. It can be seen that the N–C@NiMoO₄ ASC device retains 104.1% of its initial capacitance after 2000 cycles, which demonstrates the excellent electrochemical stability of the N–C@NiMoO₄ double-shelled hollow microtube material.

Fig. 7 Electrochemical performance of the N–C@NiMoO₄//N–C asymmetric supercapacitor. CV curves at different voltage windows (A); CV curves at different scan rates (B); galvanostatic charge–discharge curves at different current densities (C); specific capacitance of the ASC device at different current densities (D); Ragone plots (energy density vs. power density) (E); cycling performance (F).

4. Conclusions

In summary, a N–C@NiMoO₄ composite with a tube-structure has been successfully synthesized via a simple and environmentally friendly hydrothermal method. It is remarkable that the brilliant double-shelled hollow composite with NiMoO₄ nanosheets as the outer shell and N–C nanotubes as the inner shell, effectively integrates NiMoO₄ materials (high specific capacitance) with N–C NTs (excellent conductive properties), and exhibits a series of prominent features including high specific surface area, incremental electrical conductivity, short ion diffusion paths, more active sites and flexiblity. N–C@NiMoO₄ double-shelled hollow microtubes as an electrode material have a specific capacitance of up to 1242 F g⁻¹ at 1 A g⁻¹, superior current capacitive behaviour and better rate capability compared with bare NiMoO₄ microspheres and N–C NTs. Furthermore, the as-designed N–C@NiMoO₄//N–C asymmetric supercapacitor device with a high operation voltage delivers a high energy density of 44.6 W h kg⁻¹ at a power density of 250.4 W kg⁻¹ and provides an excellent cycling stability of 104.1% after 2000 cycles.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The present work is financially supported by the Natural Science Foundation under the grants of NSFC (51602289), the Outstanding Young Talent Research Fund of Zhengzhou University (32210461), and the Startup Research Fund of Zhengzhou University (51090104).

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Footnote

† Electronic supplementary information (ESI) available: STEM image, EDS mapping results and EDS spectrum of the double-shelled hollow N–C@NiMoO₄ composite; CV curve of the N–C NT electrode in a three-electrode system. See DOI: 10.1039/c7nj03422a

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