Synthesis of curly graphene nanoribbon/polyaniline/MnO₂ composite and its application in supercapacitor

Abstract

Curly graphene nanoribbon/polyaniline/MnO₂ (CGNR/PANI/MnO₂) nanocomposites with a unique structure is prepared. The formation mechanism of the CGNR/PANI/MnO₂ nanocomposite was proposed, and the morphology and structure were characterized by electron microscopy, X-ray diffraction, and Raman spectroscopy. The CGNR/PANI/MnO₂ nanocomposite was investigated for supercapacitor applications. The CGNR/PANI/MnO₂ electrode delivered a very high specific capacitance of 496 F g⁻¹, which was much higher than that of CGNR (131 F g⁻¹), PANI (301 F g⁻¹) and MnO₂ (33 F g⁻¹), whereas 81.1% of its initial capacitance was retained after 5000 cycles at a scan rate of 50 mV s⁻¹. The CGNR/PANI/MnO₂ electrode was also evaluated via a two-electrode configuration, and the supercapacitor delivered a specific capacitance of 103 F g⁻¹. The enhanced electrochemical performance of the CGNR/PANI/MnO₂ electrode was ascribed to the unique structure and the synergetic effect of the three components in the composite.

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Introduction

With the overconsumption of fossil fuels and the rapid deterioration of the environment, renewable and sustainable energy conversion technologies are urgently needed to address these problems.^1–3 Among the various energy storage and conversion devices, the supercapacitor (also termed as electrochemical capacitor) is one of the most prominent energy storage systems due to its high power density, high energy efficiency, and long cycle life.^4–7 As a result, supercapacitors have been studied extensively in hybrid electric vehicles, portable electronics, micro-autonomous robots and backup power systems.^8,9

Supercapacitors can be classified into two types on the basis of different charge storage mechanisms: electric double layer capacitors (EDLCs)¹⁰ and pseudocapacitors.¹¹ EDLCs store energy at the interface between the electrodes and electrolytes;¹² therefore, their capacitive performance relies strongly on the specific surface area and porosity of the electrode materials such as activated carbon, carbon nanotubes, and graphene. However, EDLCs normally display a relatively low capacitance, lying in the range 50–200 F g⁻¹, due to the limited accessible surface area and/or the stacking or aggregation of the carbon nanomaterials.¹³ For example, graphene, the hottest 2D carbon material, has a theoretical specific surface area of 2675 m² g⁻¹, which leads to a theoretical specific capacitance of 550 F g⁻¹.¹⁴ However, the realized surface area and capacitance of graphene are far below their theoretical values.^15–17 On the other hand, carbon nanotubes (CNTs), which are particularly interesting for supercapacitor electrodes because of their unique tubular porous structures and superior electrical conductivity, favor rapid ion and electron transfer.¹⁸ However, pristine CNTs are easily intertwined, causing a large decrease in surface area. Therefore, some creative approaches have been taken to address the problem. Sharma's group synthesized curly graphene nanoribbons (CGNRs) that were composed of a partial nanotube structure and partial graphene layered morphology by cutting the CNTs transversely and longitudinally, and the resulting CGNR showed a much higher specific surface area than the pristine CNTs. More importantly, these unoriented CGNRs could readily establish a 3D conductive framework, providing sufficient space to assemble other active materials.¹⁹

Pseudocapacitor electrode materials are mostly transition metal oxides and conducting polymers, storing energy through reversible redox reactions, which occur not only at the electrode surface, but also in the bulk close to the surface of the solid electrode.^20–22 Therefore, pseudocapacitor electrode materials normally show much larger theoretical capacitance.^23,24 However, their rate and cycling performance are moderate due to the relatively low electrical conductivity and structure destruction during charging and discharging. Among the various pseudocapacitive materials, MnO₂ stands out because of its low cost, environmentally friendliness and high theoretical specific capacitance (∼1300 F g⁻¹).^25,26 Nevertheless, the capacitance of MnO₂ is difficult to fully obtain due to its low intrinsic conductivity from 10⁻⁵ to 10⁻⁶ s cm⁻¹.²⁷ Therefore, MnO₂ with various structures and morphologies were synthesized to promote its electrochemical performance.^28–30 Polyaniline (PANI), as a typical conducting polymer, has attracted increasing interests owing to its high specific capacitance, good electrical conductivity, and readily synthetic method.^31–33 However, swelling and shrinkage during charging and discharging resulted in the poor cycling stability of PANI.^34,35

An effective way to solve the intrinsic problems of these unitary electrode materials is to fabricate new nanocomposites that combine different types of supercapacitor electrode materials together.^36–38 A variety of nanocomposite electrode materials have been prepared to improve the supercapacitor performance by combining EDLCs electrode materials with pseudocapacitive materials. Yuan et al. reported that MWCNTs coated PANI displayed a high specific capacitance of up to 560 F g⁻¹ at 1 mV s⁻¹ and good cycling stability.³⁹ Xu et al. reported novel and free-standing hierarchical carbon nanofiber/graphene oxide/PANI films showing a specific capacitance of up to 450.2 F g⁻¹ at a scan rate of 10 mV s⁻¹.⁴⁰ Shen et al. showed that the PANI/graphene/CNTs had good cycling performance of more than 91% capacitance retained after 5000 cycles.⁴¹ These studies indicated that the electrochemical performance was enhanced when the composite materials were used as electrodes.

In this study, we prepared a curly graphene nanoribbon/polyaniline/MnO₂ ternary nanocomposite, in which the curly graphene nanoribbon (CGNR) was used as a 3D frame to enable rapid electron transfer, and aniline monomers were then polymerized in situ in the CGNR frame to fill the space among CGNR, and finally MnO₂ nanoneedles were interspersed on the CGNR/PANI composite to pack the gap between CGNR and PANI. The results show that the CGNR/PANI/MnO₂ nanocomposite displays outstanding electrochemical performance due to the remarkable synergic effects of CGNR, PANI and MnO₂.

Experimental section

Synthesis of OCGNR/PANI binary composite

Oxidized curly graphene nanoribbon (OCGNR) was prepared using a modified Hummers' method.⁴² Briefly, MWCNTs (1 g) and sodium nitrate (0.5 g) were added in 46 mL 98% H₂SO₄ and the suspension was then stirred for 1 h in an ice bath. Under vigorous stirring, 5 g of KMnO₄ was added to the suspension at a temperature below 10 °C. Furthermore, the temperature was increased to 35 °C and the mixture was stirred for 4 h. Subsequently, 100 mL of ultrapure water was added slowly with continual agitation, and the mixture was kept stirring for another 0.5 h. After cooling to room temperature, H₂O₂ (5 mL) and ultrapure water (180 mL) were added to the suspension. The suspension was washed with HCl (10 vol%), ethanol and ultrapure water until the pH of the solution was neutral and dried at about 60 °C. For the preparation of OCGNR/PANI, 50 mg of OCGNR was added to 25 mL of 1 M HCl + 2.15 mM aniline solution (mass ratio of aniline to OCGNR was 8 : 2). The mixture was sonicated for 1 h to obtain a uniform suspension. Thereafter, a designed amount of ammonium peroxydisulfate (APS) was added rapidly to the abovementioned mixture under vigorous agitation, and the solution was kept stirring overnight. The final product OCGNR/PANI was obtained by centrifugation, and then washed with ultrapure water, absolute ethanol, and hexanes. The product was dried at 60 °C under vacuum for further use.

Synthesis of CGNR/PANI/MnO₂ ternary composite

The CGNR/PANI/MnO₂ nanocomposite was prepared hydrothermally. Briefly, 80 mg of the previously prepared OCGNR/PANI and 30 mg of MnSO₄·H₂O were added to 40 mL H₂O. After ultrasonic treatment, concentrated sulfuric acid was added dropwise under constant stirring to adjust the pH. Subsequently, 15 mg of KMnO₄ dissolved in 20 mL H₂O was added to the previous solution dropwise with continuous stirring. The mixture was transferred immediately to a 100 mL Teflon-lined autoclave and heated to 120 °C for 8 h. After the reaction, the autoclave was allowed to cool to room temperature. The solid was separated and washed several times with ultrapure water and dried at 60 °C in a vacuum oven. The obtained composite was labeled as OCGNR/PANI/MnO₂. The as-prepared product was then reduced by hydrazine monohydrate to achieve the final product CGNR/PANI/MnO₂.

For comparison, CGNR/PANI was also prepared. 100 mg of the previously prepared OCGNR/PANI was dispersed and sonicated in 50 mL H₂O and 0.1 mL hydrazine monohydrate was added. Afterwards, the temperature was gradually increased to 95 °C and the mixture was refluxed for 2 h. The reduced materials were filtered and washed several times with ultrapure water. The obtained product was named as CGNR/PANI.

Characterization

X-ray powder diffraction (XRD) of the samples was conducted using a Rigaku D/Max Ultima II diffractometer with Cu-Kα radiation (λ = 0.15418 nm). The morphology of the samples was characterized by scanning electron microscopy (SEM, Hitachi S-4800 Japan) and transmission electron microscopy (TEM, JEM-2100F, Japan). Raman spectroscopy was carried out using a Labram-010 (France) with 632 nm laser excitation. X-ray photoelectron spectroscopy (XPS, K-Alpha 1063) was carried out using focused monochromatized Al Kα radiation.

Electrochemical measurements

Electrochemical testing of the materials was measured in a three-electrode system using a Pt sheet as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode with a CHI760C potentiostat system. The working electrodes were prepared as follows: 3 mg of the sample was dispersed in H₂O to make a suspension with a concentration of 1 mg mL⁻¹, and 20 μL of the suspension was dropped onto a glassy carbon electrode with a diameter of 5 mm. After several hours of drying at room temperature, the electrode was covered with a Nafion solution (0.5 wt%, 5 μL) and dried in air again. The electrochemical properties of the CGNR/PANI/MnO₂, MnO₂, PANI and CGNR were tested in 1 M H₂SO₄ by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge. In a two-electrode system, the symmetric CGNR/PANI/MnO₂ supercapacitor was assembled using modified glassy carbon electrodes as the positive and negative electrodes.

Results and discussion

Fig. 1 presents a schematic of the formation of the CGNR/PANI/MnO₂ ternary nanocomposite. First, the raw MWCNTs were cut longitudinally and unzipped transversely into oxidized curly graphene nanoribbon (OCGNR) using the modified Hummers' method. The hydrophilic OCGNR was dispersed uniformly in water, which facilitated further processing. Aniline was added to the OCGNR suspension to be adsorbed on the surface of the OCGNR. Subsequently, the adsorbed aniline monomers were oxidized in situ and polymerized with the aid of APS, resulting in the formation of the OCGNR/PANI nanocomposite. During the hydrothermal reaction, the positively charged Mn²⁺ ions interacted strongly with the negatively charged carboxyl and hydroxyl groups on the OCGNR surface due to electrostatic interactions or covalent chemical bonding. Second, the added KMnO₄ reacted with the bonded Mn²⁺, leading to the formation of MnO₂. At the initial stage, the nucleation occurred accompanying the redox reaction on the OCGNR surface, and the oxygen-containing functional groups acted as anchor sites for the growth of MnO₂ crystal. The OCGNR/PANI could control the growth of MnO₂, limiting the expansion of MnO₂ crystal. Finally, the OCGNR/PANI/MnO₂ was reduced by hydrazine to produce the final product CGNR/PANI/MnO₂.

Fig. 1 Schematic of the preparation of the CGNR/PANI/MnO₂ composite.

Morphology and structure

Fig. 2 shows SEM images of the MWCNTs, CGNR, CGNR/PANI and CGNR/PANI/MnO₂. Fig. 2a shows the pristine MWCNTs with a diameter of 60–100 nm and a length of 5–15 μm. Fig. 2b shows the SEM image of the CGNR, and it clearly displayed that the MWCNTs were opened and unzipped into curly graphene structure. This transformation caused an increase in diameter from 100 nm to 200 nm, and this opened and wider morphology was beneficial for ions to access. Fig. 2c shows an SEM image of the CGNR/PANI. The in situ polymerized PANI with a nanoflake structure was grown on the interweaved CGNR, and the nano-PANI flakes were filled in the space among the individual curly graphene nanoribbons to avoid the stacking of CGNR. Fig. 2d shows the morphology of the CGNR/PANI/MnO₂, and it was observed that the MnO₂ nanoneedles were dispersed uniformly over the surfaces of the CGNR and PANI. The MnO₂ nanoneedles could further restrain the stacking of the CGNR, but the CGNR also functioned as stable substrates to support the MnO₂ nanoneedles to maintain their physical structure and accelerate electron transfer, which could maximize the capacitance of MnO₂.

Fig. 2 SEM images of the (a) MWCNT, (b) CGNR, (c) CGNR/PANI, and (d) CGNR/PANI/MnO₂.

Fig. 3a shows TEM images of the CGNR, showing that the CNTs had been unzipped in to CGNR. Fig. 3b presents a TEM image of the CGNR/PANI/MnO₂. MnO₂ nanoneedles (as pointed by the arrows) with a diameter of ∼10 nm and length of ∼100 nm dispersed uniformly on the surfaces of the CGNR and PANI were clearly observed. The EDS result in Fig. 3c showed that C, N, O, and Mn were presented in the CGNR/PANI/MnO₂, which confirmed the existence of PANI and MnO₂ in the composite. The amount of manganese dioxide in the nanocomposite was measured by EDS, and the result showed that the weight fraction of Mn was 11.29%, representing 16.01% of MnO₂ in the composite, which was the optimal amount of MnO₂ in the CGNR/PANI/MnO₂ composite (details in the ESI†).

Fig. 3 (a) TEM image of CGNR, and (b) TEM and (c) EDS images of the CGNR/PANI/MnO₂ composite.

Typical X-ray diffraction (XRD) patterns of the CGNR, PANI, MnO₂ and CGNR/PANI/MnO₂ are shown in Fig. 4a. The XRD pattern of the CGNR exhibited a broad peak at 2θ = 25.2° corresponding to the (002), indicating the reduction of the OCGNR and formation of CGNR. The XRD pattern of the PANI showed peaks at 14.9° (011), 20.6° (021) and 25.8° (200) of the emeraldine salt form of PANI. The XRD pattern of MnO₂ displayed diffraction peaks at 12.7° (110), 18.1° (200), 28.8° (310), 37.4° (211), 49.8° (411), and 60.2° (521), which were indexed to a pure tetragonal phase of α-MnO₂ (JCPDS 44-0141).⁴³ The XRD pattern of the CGNR/PANI/MnO₂ showed all the characteristic peaks of CGNR, PANI and MnO₂, confirming the successful synthesis of the CGNR/PANI/MnO₂ composite.

Fig. 4 (a) XRD patterns of the CGNR, PANI, MnO₂, and CGNR/PANI/MnO₂, and (b) Raman spectra of the CGNR, PANI, MnO₂, and CGNR/PANI/MnO₂.

Fig. 4b presents the Raman spectra of the CGNR, PANI, MnO₂, and CGNR/PANI/MnO₂. The CGNR displayed two typical peaks at 1335 cm⁻¹ and 1594 cm⁻¹ corresponding to the in-plane bond stretching motion of C sp² atoms (G band) and the benzenoid rings of CGNR (D-band). The PANI displayed several peaks at 518, 1176, 1329, 1482, and 1592 cm⁻¹, which were attributed to the out-of-plane C–N–C torsion, imine deformation, in-plane ring deformation, C–N⁺ stretching, C=N stretching of quinoid, and C–C stretching of benzenoid.¹⁹ MnO₂ displayed a sharp peak at 630 cm⁻¹ corresponding to Mn–O lattice vibration in MnO₂ nanoneedles. The CGNR/PANI/MnO₂ composite displayed all the characteristic peaks mentioned above, further confirming the formation of the ternary nanocomposite.

The CGNR/PANI/MnO₂ composite was also characterized by XPS. Fig. 5a shows the XPS survey scan spectra; Mn (2p_1/2, 2p_3/2), N 1s, O 1s and C 1s were observed. Fig. 5b shows details from the XPS for the binding energy range of the relevant Mn species. There were two main peaks between 635 and 660 eV that were resolved to 642.1 eV and 653.8 eV for Mn 2p_3/2 and Mn 2p_1/2, respectively, which were consistent with the well-characterized MnO₂ XPS spectra in the literature.²⁷

Fig. 5 (a) XPS survey spectra of the CGNR/PANI/MnO₂ composite and (b) the details from the XPS for the binding energy range of the relevant Mn species.

Fig. 6a shows the N₂ adsorption/desorption isotherms of CGNR and CGNR/PANI/MnO₂, both adsorption/desorption isotherms showed type IV isotherms with a hysteresis loop at a relative pressure (P/P₀) of 0.45–1, indicating the mesoporous structure of the two samples.⁴⁴ This showed that the integration of PANI and MnO₂ did not ruin the mesoporous structure of the CGNR, and these mesopores were essentially important to contribute to the capacitance. From the adsorption/desorption isotherms, the BET surface area of CGNR was estimated to be 229 m² g⁻¹, which was much higher than that of the pristine MWCNTs.⁴⁵ The BET surface area of the CGNR/PANI/MnO₂ was estimated to be 263 m² g⁻¹, which was larger than that of the CGNR, which may be because PANI and MnO₂ promoted the specific surface area through restraining stacking and agglomeration of the CGNR. Fig. 6b shows the pore size distribution curves estimated by Barrett–Joyner–Halenda analysis; the curves showed an intensive pore size distribution of ∼3.8 nm. In addition, the adsorption average pore width of the CGNR/PANI/MnO₂ was 5.8 nm, which was smaller than that of the CGNR (6.2 nm) and the pore volume of the CGNR (0.339 cm³ g⁻¹) was smaller than that of the CGNR/PANI/MnO₂ (0.356 cm³ g⁻¹), which were beneficial for the ion access.

Fig. 6 (a) N₂ adsorption/desorption isotherms of CGNR and CGNR/PANI/MnO₂ and (b) pore size distribution curves of the CGNR and CGNR/PANI/MnO₂.

Electrochemical performance

The electrochemical performance of the CGNR, PANI, MnO₂, and CGNR/PANI/MnO₂ was evaluated in 1 M H₂SO₄ by cyclic voltammetry (CV) and galvanostatic charge–discharge testing. Fig. 7a shows the CV curves of the CGNR, PANI, MnO₂, and CGNR/PANI/MnO₂ composite at a scan rate of 50 mV s⁻¹. The CV curves of the CGNR, MnO₂, and CGNR/PANI/MnO₂ all displayed a rectangle shape, indicating good supercapacitive characteristic,⁴⁶ whereas the CV curve of the PANI displayed two pairs of redox peaks, indicating the redox transition of PANI from a semiconducting state (leucoemeraldine form) to the conducting state (polaronic emeraldine form).³² In particular, CGNR/PANI/MnO₂ showed the highest current density, representing the largest specific capacitance. Fig. 7b shows the CV curves of the CGNR/PANI/MnO₂ at different scan rates and all the CV curves showed a good rectangular shape, demonstrating good rate performance of the electrode material.

Fig. 7 (a) Cyclic voltammograms of the CGNR, PANI, MnO₂, and CGNR/PANI/MnO₂ at a scan rate of 50 mV s⁻¹, and (b) cyclic voltammograms of the CGNR/PANI/MnO₂ at different scan rates.

Galvanostatic charge/discharge testing was carried out in 1 M H₂SO₄. Fig. 8a shows the charge/discharge curves of the CGNR, PANI, MnO₂, and CGNR/PANI/MnO₂ electrodes at a current density of 1 A g⁻¹. All curves displayed linear responses, showing good agreement with the supercapacitor characteristics.⁴⁷ The specific capacitance C was calculated using the following equation:⁴⁸

where I is the constant charge/discharge current (A), t is the discharge time (s), U is the potential window (V), and m is the mass of the electrode material (g). The specific capacitances of the CGNR, PANI, MnO₂ and CGNR/PANI/MnO₂ were calculated to be 131, 301, 33 and 496 F g⁻¹, respectively. The CGNR/PANI/MnO₂ delivered the largest specific capacitance, indicating that the addition of an appropriate amount of MnO₂ and PANI greatly promoted the capacitance.

Fig. 8 Galvanostatic charge/discharge curves of (a) the CGNR, PANI, MnO₂, CGNR/PANI/MnO₂ at 1 A g⁻¹ and (b) the CGNR/PANI/MnO₂ at different current densities.

Fig. 8b shows the specific capacitance of the CGNR/PANI/MnO₂ at different current densities from 1 to 10 A g⁻¹. Fig. 9 shows the specific capacitance of the CGNR/PANI/MnO₂ as a function of the current density, and the corresponding data of the CGNR, PANI, MnO₂ are also shown for comparison. As shown, the specific capacitance decreased with increasing current density, and the CGNR/PANI/MnO₂ electrode delivered the largest capacitance at all the discharge current densities. At a very high current density of 10 A g⁻¹, the CGNR/PANI/MnO₂ electrode delivered a remarkable specific capacitance of 348 F g⁻¹, which was among the highest value in the literature.^25,41,42

Fig. 9 Specific capacitance of the CGNR, PANI, MnO₂ and CGNR/PANI/MnO₂ at different current densities from 1 to 10 A g⁻¹.

EIS is an efficient way to measure the electrochemical properties of electrode materials. The impedance spectra were obtained over the frequency range from 10⁵ Hz to 0.01 Hz at the open circuit potential. Fig. 10 shows a Nyquist plot of the CGNR/PANI/MnO₂ composite, and there was an approximate semi-circle in the high frequency region, followed by a near-linear section in the low frequency region. In the high frequency region, the equivalent series resistance (ESR, intersection on the real axis at high frequency) and charge transfer resistance (R_ct, the diameter of the semicircle) were estimated to be 3.6 Ω and 0.8 Ω, respectively.⁴⁹ In the low frequency range, a nearly vertical line represented the very good capacitive characteristics of the electrode.⁵⁰ The low internal resistance demonstrated that the unique structure of the CGNR/PANI/MnO₂ electrode facilitated ion and electron transfer during charging and discharging.

Fig. 10 Nyquist plot of the CGNR/PANI/MnO₂ electrode.

The electrochemical stability of the electrodes was investigated at a scan rate of 50 mV s⁻¹ for 5000 cycles. Fig. 11 displays the specific capacitance retention as a function of the cycle number. The specific capacitance of PANI and MnO₂ decreased sharply over cycling, and only retained 18% and 15% of its initial capacitance after 5000 cycles, whereas the CGNR had nearly no capacitance loss after cycling. The specific capacitance of the CGNR/PANI/MnO₂ electrode decreased slightly, retained 81.1% of the initial capacitance after 5000 cycles, demonstrating enhanced cycling stability.

Fig. 11 Cycling stability of the CGNR, PANI, MnO₂ and CGNR/PANI/MnO₂ electrodes.

Overall, the enhanced capacitive performance was ascribed to the unique structure and the synergic effect of the three components. The CGNR with a porous structure and good electric conductivity was functioned as supports to facilitate ion and electron transfer and to avoid the aggregation of PANI and MnO₂. On the other hand, the PANI nanoflakes and MnO₂ nanoneedles alleviated the stacking of the CGNR, maintaining a high surface area and mesoporous structure.

The CGNR/PANI/MnO₂ supercapacitor was also fabricated and investigated by CV and galvanostatic charge/discharge via a symmetric two-electrode configuration. Fig. 12a displays the CV curves of the symmetric supercapacitor device at scan rates ranging from 5 mV s⁻¹ to 100 mV s⁻¹, and all the CV curves displayed a very good rectangular shape in the voltage window of 1 V, representing remarkable supercapacitor characteristics. Fig. 12b shows the charge/discharge curves at various scan rates, and again linear charge/discharge curves were observed. The specific capacitance of the symmetric supercapacitor was 103 F g⁻¹ (based on the total mass in the supercapacitor) at 0.5 A g⁻¹, which decreased to 81 F g⁻¹ with increasing current density to 5 A g⁻¹.

Fig. 12 (a) CV curves of the CGNR/PANI/MnO₂ in a two-electrode configuration at scan rates from 5 to 100 mV s⁻¹ and (b) galvanostatic charge/discharge curves of two-electrode supercapacitor at different current densities.

Energy density and power density are two important performance indicators for supercapacitors. The energy density and power density were estimated using the equations:⁵¹

where E is the energy density of the supercapacitor (W h kg⁻¹), P is the power density of the supercapacitor (W kg⁻¹), C is the specific capacitance of the supercapacitor, and t is the discharge time. A Ragone plot using the data in Fig. 12b was constructed, and the maximum energy density was 14.3 W h kg⁻¹.

Fig. 13 shows the Ragone plot, which also contains the comparative recent MnO₂ or PANI based supercapacitor data re-plotted from the literature,^15,52,53 indicating a much better energy density and power density combination.

Fig. 13 A Ragone plot constructed from data in Fig. 12b, together with some comparative re-plotted data of ternary material-based supercapacitors.

Conclusions

We successfully prepared CGNR/PANI/MnO₂ nanocomposites, in which MWCNTs were tailored into CGNR and formed a conductive 3D frame, aniline was polymerized in situ in the CGNR, and MnO₂ nanoneedles were interspersed uniformly on the CGNR and PANI to form a three-dimensional architecture. The CGNR/PANI/MnO₂ nanocomposite displayed a high BET surface area of 263 m² g⁻¹ and the morphology and structure were studied carefully. The CGNR/PANI/MnO₂ electrode delivered a high specific capacitance of 496 F g⁻¹ at a current density of 1 A g⁻¹, reducing to 348 F g⁻¹ at 10 A g⁻¹, and the capacitance retained 81.1% of its initial value after 5000 cycles. The two-electrode configuration testing showed that the CGNR/PANI/MnO₂ had a better energy density and power density combination than some MnO₂ or PANI based composites reported in the literature. The enhanced electrochemical performance was attributed to the unique structure of the composite, in which PANI and MnO₂ were supported by the 3D CGNR frame with a high surface area and high conductivity, which was beneficial for ion and electron transfer.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 21271069, J1210040, 51238002, J1103312), the Science and Technology Program of Hunan Province (Grant No. 2015JC3049), and the Fundamental Research Funds for the Central Universities.

References

1. S. Li, J. Niu, Y. C. Zhao, K. P. So, C. Wang, C. A. Wang and J. Li, Nat. Commun., 2015, 6, 7872.
2. Q. Liu, O. Nayfeh, M. H. Nayfeh and S. T. Yau, Nano Energy, 2013, 2, 133–137.
3. X. Xiao, W. Zhou, Y. Kim, I. Ryu, M. Gu, C. Wang, G. Liu, Z. Liu and H. Gao, Adv. Funct. Mater., 2015, 25, 1426–1433.
4. X. Cao, Y. Shi, W. Shi, G. Lu, X. Huang, Q. Yan, Q. Zhang and H. Zhang, Small, 2011, 7, 3163–3168.
5. Z. Lei, F. Shi and L. Lu, ACS Appl. Mater. Interfaces, 2012, 4, 1058–1064.
6. G. A. Snook, P. Kao and A. S. Best, J. Power Sources, 2011, 196, 1–12.
7. C. Xiang, M. Li, M. Zhi, A. Manivannan and N. Wu, J. Power Sources, 2013, 226, 65–70.
8. Y. Huang, J. Liang and Y. Chen, Small, 2012, 8, 1805–1834.
9. P. Tang, L. Han and L. Zhang, ACS Appl. Mater. Interfaces, 2014, 6, 10506–10515.
10. L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 2009, 38, 2520–2531.
11. G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41, 797–828.
12. D. Yu, Q. Qian, L. Wei, W. Jiang, K. Goh, J. Wei, J. Zhang and Y. Chen, Chem. Soc. Rev., 2015, 44, 647–662.
13. H. Gao, F. Xiao, C. B. Ching and H. Duan, ACS Appl. Mater. Interfaces, 2012, 4, 2801–2810.
14. C. Liu, Z. Yu, D. Neff, A. Zhamu and B. Z. Jang, Nano Lett., 2010, 10, 4863–4868.
15. Y. Jin, H. Chen, M. Chen, N. Liu and Q. Li, ACS Appl. Mater. Interfaces, 2013, 5, 3408–3416.
16. J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang and F. Wei, Carbon, 2010, 48, 3825–3833.
17. J. Yan, J. Liu, Z. Fan, T. Wei and L. Zhang, Carbon, 2012, 50, 2179–2188.
18. Z. Yang, M. Liu, C. Zhang, W. W. Tjiu, T. Liu and H. Peng, Angew. Chem., Int. Ed., 2013, 52, 3996–3999.
19. S. Grover, S. Goel, V. Sahu, G. Singh and R. K. Sharma, ACS Sustainable Chem. Eng., 2015, 3, 1460–1469.
20. A. Burke, J. Power Sources, 2000, 91, 37–50.
21. J. W. Lee, A. S. Hall, J. D. Kim and T. E. Mallouk, Chem. Mater., 2012, 24, 1158–1164.
22. Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li and L. Zhang, Int. J. Hydrogen Energy, 2009, 34, 4889–4899.
23. Z. Gao, F. Wang, J. Chang, D. Wu, X. Wang, X. Wang, F. Xu, S. Gao and K. Jiang, Electrochim. Acta, 2014, 133, 325–334.
24. M. Toupin, T. Brousse and D. Bélanger, Chem. Mater., 2002, 14, 3946–3952.
25. M. Toupin, T. Brousse and D. Bélanger, Chem. Mater., 2004, 16, 3184–3190.
26. W. Wei, X. Cui, W. Chen and D. G. Ivey, J. Phys. Chem. C, 2008, 112, 15075–15083.
27. M. Liu, W. W. Tjiu, J. Pan, C. Zhang, W. Gao and T. Liu, Nanoscale, 2014, 6, 4233–4242.
28. S. Chen, J. Zhu, X. Wu, Q. Han and X. Wang, ACS Nano, 2010, 4, 2822–2830.
29. Y. Hou, Y. Cheng, T. Hobson and J. Liu, Nano Lett., 2010, 10, 2727–2733.
30. Y. F. Li, S. S. Li, D. L. Zhou, A. J. Wang, P. P. Zhang, C. G. Li and J. J. Feng, J. Solid State Electrochem., 2014, 18, 2521–2527.
31. J. Li, H. Xie, Y. Li, J. Liu and Z. Li, J. Power Sources, 2011, 196, 10775–10781.
32. J. Xu, K. Wang, S. Z. Zu, B. H. Han and Z. Wei, ACS Nano, 2010, 4, 5019–5026.
33. K. Zhang, L. L. Zhang, X. S. Zhao and J. Wu, Chem. Mater., 2010, 22, 1392–1401.
34. R. B. Rakhi, W. Chen, D. Cha and H. N. Alshareef, J. Mater. Chem., 2011, 21, 16197–16204.
35. M. Liu, Y. E. Miao, C. Zhang, W. W. Tjiu, Z. Yang, H. Peng and T. Liu, Nanoscale, 2013, 5, 7312–7320.
36. L. L. Jiang, X. Lu, C. Xie, G. Wan, H. Zhang and T. Youhong, J. Phys. Chem. C, 2015, 119, 3903–3910.
37. P. Sekar, B. Anothumakkool and S. Kurungot, ACS Appl. Mater. Interfaces, 2015, 7, 7661–7669.
38. C. Yuan, L. Su, B. Gao and X. Zhang, Electrochim. Acta, 2008, 53, 7039–7047.
39. Y. Zhou, Z. Y. Qin, L. Li, Y. Zhang, Y. L. Wei, L. F. Wang and M. F. Zhu, Electrochim. Acta, 2010, 55, 3904–3908.
40. D. Xu, Q. Xu, K. Wang, J. Chen and Z. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 200–209.
41. J. Shen, C. Yang, X. Li and G. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 8467–8476.
42. X. Yu, K. Li, H. Zhou, S. Wei, C. Zhang, X. Li and Y. Kuang, RSC Adv., 2015, 5, 88950–88957.
43. L. Chen, Z. Song, G. Liu, J. Qiu, C. Yu, J. Qin, L. Ma, F. Tian and W. Liu, J. Phys. Chem. Solids, 2013, 74, 360–365.
44. W. Ma, S. Chen, S. Yang, W. Chen, Y. Cheng, Y. Guo, S. Peng, S. Ramakrishna and M. Zhu, J. Power Sources, 2016, 306, 481–488.
45. Y. Nie, X. Xie, S. Chen, W. Ding, X. Qi, Y. Wang, J. Wang, W. Li, Z. Wei and M. Shao, Electrochim. Acta, 2016, 187, 153–160.
46. Y. Sun, W. Zhang, D. Li, L. Gao, C. Hou, Y. Zhang and Y. Liu, Electrochim. Acta, 2015, 178, 823–828.
47. Q. Zhang, L. Li, Y. Wang, Y. Chen, F. He, S. Gai and P. Yang, Electrochim. Acta, 2015, 176, 542–547.
48. B. Mu, W. Zhang, S. Shao and A. Wang, Phys. Chem. Chem. Phys., 2014, 16, 7872–7880.
49. C. Fu, A. Mahadevegowda and P. S. Grant, J. Mater. Chem. A, 2015, 3, 14245–14253.
50. Z. Wang, D. O. Carlsson, P. Tammela, K. Hua, P. Zhang, L. Nyholm and M. Strømme, ACS Nano, 2015, 9, 7563–7571.
51. D. Zha, P. Xiong and X. Wang, Electrochim. Acta, 2015, 185, 218–228.
52. P. Xiong, C. Hu, Y. Fan, W. Zhang, J. Zhu and X. Wang, J. Power Sources, 2014, 266, 384–392.
53. Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao and E. Xie, ACS Nano, 2013, 7, 174–182.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02777f

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