Optimized NiCo₂O₄/rGO hybrid nanostructures on carbon fiber as an electrode for asymmetric supercapacitors

Abstract

The NiCo₂O₄ nanowires and reduced graphene oxide (rGO) hybrid nanostructure has been constructed on carbon fibers (NiCo₂O₄/rGO/CF) via a hydrothermal method. The effects of graphene oxide (GO) concentration on the structure and performance of the NiCo₂O₄/rGO/CF were investigated in detail to obtain the optimized electrode. When the GO concentration was 0.4 mg ml⁻¹, the rGO/NiCo₂O₄/CF composite exhibited a maximum specific capacitance of 931.7 F g⁻¹ at 1 A g⁻¹, while that of NiCo₂O₄/CF was 704.9 F g⁻¹. Furthermore, the NiCo₂O₄/rGO/CF//AC asymmetric supercapacitor with a maximum specific capacitance of 61.2 F g⁻¹ at 1 A g⁻¹ was fabricated, which delivered a maximum energy density (24.6 W h kg⁻¹) and a maximum power density (8477.7 W kg⁻¹). Results suggested that the NiCo₂O₄/rGO/CF composite would be a desirable electrode for flexible supercapacitors.

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1. Introduction

Recently, much attention has been paid to developing flexible supercapacitors due to their potential applications in wearable electronic devices.^1–4 Conductive carbon fibers, metal foils and foams have served as platforms for depositing nanostructured active materials to construct flexible supercapacitor electrodes. In particular, continuous three-dimensional carbon materials including carbon fiber fabrics, textiles and cloths have received increasing attention owing to their advantages, such as high corrosion resistance, outstanding electrical conductivity, low cost and porous structure. To achieve high-performance flexible supercapacitors, they have been widely used as frameworks to construct hybrid nanostructures.^5–10 Especially, some metal oxides (e.g., Co₃O₄, MnO₂, NiCo₂O₄, Fe₂O₃, NiO, CoMoO₄, etc.) decorated on carbon textiles as flexible electrodes for supercapacitors have been developed.^5,11–20 Among various metal oxide electrode materials, NiCo₂O₄ has been widely researched owing to its high theoretical capacity.^21,22 However, the electrochemical performance of NiCo₂O₄-based flexible electrode is restricted because of low electrical conductivity of NiCo₂O₄. Recent reports have confirmed that the introduction of conductive materials, for example PPy, could enhance the electrochemical performance of NiCo₂O₄-based flexible electrodes.^23,24

Herein, in view of the good electrical conductivity of graphene, we have successfully constructed NiCo₂O₄ nanowires and reduced graphene oxide hybrid nanostructures on carbon fiber framework to obtain the novel NiCo₂O₄/rGO/CF composites, in which graphene sheets were decorated on the NiCo₂O₄ nanowires, forming the hierarchical structure. The as-obtained materials were used as supercapacitor electrodes and their electrochemical performance was investigated. The influences of GO on the composition, structure and electrochemical performance of NiCo₂O₄/rGO/CF electrodes were investigated in detail to obtain the optimized electrode materials.

2. Experimental

2.1. Synthesis of NiCo₂O₄/rGO hybrid nanostructures on CF (NiCo₂O₄/rGO/CF)

At first, NiCo₂O₄ nanowires were prepared via a hydrothermal method. 4 mmol Co(NO₃)₂·6H₂O, 2 mmol Ni(NO₃)₂·6H₂O and 24 mmol urea were poured into 50 ml water–ethanol mixed solution (volume ratio 1 : 1) and sonicated to form homogeneous solution. Next, the mixture was added into a 100 ml autoclave, and a slice of CF cloth (2 × 5 cm²) was put in. Then, the sample was heated to 120 °C and kept 8 h. At last, the as-prepared sample was washed with distilled water, and the NiCo-based precursor grown on CF cloth was obtained.

Urea was added to 50 ml 0.2–1.0 mg ml⁻¹ GO (the mass ratio of urea to GO was 15 : 1) aqueous dispersion and sonicated for 6 h. Then, the mixture was poured into the autoclave, and the CF loaded with NiCo-based precursor was immersed into the solution. Next, the autoclave was kept at 120 °C for 8 h. After that, the as-obtained samples were dried at 60 °C for 12 h, followed by calcination at 350 °C for 2 h to obtain NiCo₂O₄/rGO hybrid nanostructures on CF cloth.

For comparison, a piece of NiCo₂O₄ nanowires on CF (NiCo₂O₄/CF) was prepared using the same procedure as NiCo₂O₄/rGO/CF.

2.2. Characterization

The microstructure of samples was characterized via a scanning electron microscopy (SEM, JEOL JSM-7001F) and a transmission electron microscopy (TEM, JEOL JEM-2010F). The crystal structure was analyzed via a X-ray diffraction (XRD, Bruker D8 ADVANCE).

The electrochemical characterizations, containing cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) tests and electrochemical impedance spectroscopy (EIS), were conducted with a CHI660D electrochemical workstation. A three-electrode and two-electrode system were used to evaluate the electrochemical performance of the single electrodes and asymmetric supercapacitor (ACS) cell, respectively.

3. Results and discussion

3.1. Characterization of composition and microstructure

Fig. 1 shows the XRD patterns of NiCo₂O₄/rGO hybrids scratched down from CF with different concentrations of GO. It can be seen that there were similar diffraction peaks for all samples. Characteristic diffraction peak corresponding to graphene sheets (2θ ≈ 25.1°) had not been observed,^25,26 which may be due to its low diffraction intensity or highly disorderly structure.²⁷ The dominant peaks at 2θ values of 19.0°, 31.4°, 36.9°,44.9°, 59.6° and 65.5° were ascribed to the (111), (220), (311), (400), (511) and (440) planes of NiCo₂O₄ (JCPDS 01-073-1702), which confirmed the formation of NiCo₂O₄. According to the patterns, it could be concluded that the concentration of GO had no obvious influence on the crystal structure of NiCo₂O₄ nanowires.

Fig. 1 XRD patterns of NiCo₂O₄/rGO with different GO concentrations.

The SEM images of carbon fiber and NiCo₂O₄/rGO/CF composites are shown in Fig. 2. It can be observed that much surface defects were observed on the surface of fiber (Fig. 2a), which is helpful for the growth of NiCo₂O₄ nanowires. From Fig. 2b–d, it is observed that the NiCo₂O₄ nanowires were uniformly and densely grown over the surface of carbon fibers. Among NiCo₂O₄ nanowires, graphene sheets or aggregates were observed, the smaller rGO sheets were embedded in the NiCo₂O₄ nanowires, while the larger rGO sheets were supported by the top of NiCo₂O₄ nanowires, forming the hierarchical structure. Especially, when the concentration of GO was 0.4 mg ml⁻¹, no rGO aggregates were observed, rGO and NiCo₂O₄ nanowires distributed uniformly on the surface of CF, forming the hierarchical porous nanostructure, in which rGO would act as good conductive pathway for fast electron transfer, which would be helpful for improving the electrochemical properties of electrode materials. However, with increasing GO concentration, graphene sheets aggregated and thus the NiCo₂O₄ nanowires were damaged (Fig. 2e and f). Especially when the concentration of GO was 1.0 mg ml⁻¹, NiCo₂O₄ nanowires collapsed seriously (Fig. 2f), resulting in the destruction of the hierarchical porous nanostructure. Subsequently, the migration of ions would be hindered due to massive graphene aggregation, and the electrochemical properties of electrode material would be influenced.

Fig. 2 SEM images of (a) CF and NiCo₂O₄/rGO/CF samples with different GO concentrations: (b) 0.2 mg ml⁻¹; (c) 0.4 mg ml⁻¹; (d) 0.6 mg ml⁻¹; (e) 0.8 mg ml⁻¹ and (f) 1.0 mg ml⁻¹.

The TEM and HRTEM images of individual NiCo₂O₄ nanowire are shown in Fig. 3. It can be seen that the average diameter of NiCo₂O₄ nanowire was approximately 40 nm (Fig. 3a). Moreover, as seen in the high-magnification TEM image (Fig. 3b), the NiCo₂O₄ nanowires were composed of nanocrystallites with the size of 5–10 nm, which was consistent with XRD results. Among nanocrystallites lots of nanopores were observed, forming highly porous structure.

Fig. 3 (a) TEM and (b) HRTEM images of individual NiCo₂O₄ nanowire.

3.2. Electrochemical performance of NiCo₂O₄/rGO/CF composites

The electrochemical performance of the as-obtained NiCo₂O₄/rGO/CF composites were investigated via a three-electrode system with the electrolyte of 3 M KOH solution, in which the NiCo₂O₄/rGO/CF, Ag/AgCl and Pt acted as the working electrode, reference electrode and counter electrode, respectively. Fig. 4a shows the CV curves of NiCo₂O₄/rGO/CF electrodes with different GO concentrations at 20 mv s⁻¹ under the potential window of −0.2 to 0.6 V. It shows that a couple of redox peaks were obviously displayed for all the NiCo₂O₄/rGO/CF electrodes, suggesting the faradaic behaviors.²⁸ The redox mechanism of NiCo₂O₄ in KOH, and the possible faradaic reactions are represented as follows:

NiCo₂O₄ + OH⁻ + H₂O ↔ NiOOH + 2CoOOH + e⁻ (1)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (2)

Fig. 4 (a) CV curves at 20 mV s⁻¹, (b) GCD curves at 1.0 A g⁻¹, (c) the specific capacitance and (d) EIS Nyquist spectra (the inset shows the enlarged plot) for NiCo₂O₄/rGO/CF electrodes.

Fig. 4b presents GCD curves of NiCo₂O₄/rGO/CF electrodes at 1.0 A g⁻¹ at potential window ranging from 0 to 0.42 V. Evidently, the GCD curves displayed nonlinear and poor symmetrical shapes, indicating the combination of electric double layer and pseudo-capacitance characteristic. According to the GCD curves, the specific capacitance value can be calculated using the following equation:²⁹

where C (F g⁻¹), m (g), ΔV (V), I (A) and Δt (s) stand for the specific capacitance, the mass of the active material, the potential window, the current and the time, respectively.

According to Fig. 4a and b, the CV and GCD curves of NiCo₂O₄/rGO/CF electrodes were apparently effected by the GO concentration. The specific capacitance of NiCo₂O₄/rGO/CF electrodes with different GO concentrations are presented in Fig. 4c. When the GO concentration was 0.4 mg ml⁻¹, benefiting from the hierarchical porous structure and the synergistic effects between reduced graphene oxides and NiCo₂O₄ nanowires, the maximum specific capacitance of 931.7 F g⁻¹ was obtained, while that of NiCo₂O₄/CF electrode was 704.9 F g⁻¹. However, when the GO concentration reached 1.0 mg ml⁻¹, the specific capacitance of NiCo₂O₄/rGO/CF decreased to 707.2 F g⁻¹. As mentioned above, the hierarchical porous nanostructure is helpful for improving the electrochemical performance. Massive graphene aggregation could result in the collapse of NiCo₂O₄ nanowires, leading to the destruction of the hierarchical porous nanostructure and the decrease in the specific capacitance, which was in conformity with the SEM results.

To gain a better comparison of the capacitive behaviors of the samples, EIS was conducted in a frequency range of 0.1–100 kHz with an amplitude of 5 mV. Generally, the Nyquist plot contains two parts, one part is a semicircle arc in high-frequency region, in which the intercept at the real axis (Z′) could reflect the solution resistance and the diameter of the semicircle could reflect the interfacial charge-transfer resistance (R_ct); the other part is a line in low-frequency region, and the slope of the straight line could reflect the electrolyte diffusion in electrode materials. According to Fig. 4d, there was no significant difference in the high frequency region while the intercept increased gradually and the slope of the straight line decreased with increasing GO concentration. The NiCo₂O₄/rGO/CF electrode with GO concentration of 0.4 mg ml⁻¹ showed a steep slope shape in low-frequency region, indicating rapid ion diffusion in electrolyte and lower diffusion resistance,³⁰ which can be explained by the abundant pores and good conductive pathways between NiCo₂O₄ nanowires and rGO sheets. When the GO concentration increased, the slope of the straight lines decreased, indicating that the ion diffusion decreased, which might be due to the destruction of the hierarchical nanostructure caused by massive rGO aggregates. Results showed that GO concentration had remarkable effect on the structure and electrochemical performance of NiCo₂O₄/rGO/CF electrodes. It is important to construct the hierarchical porous nanostructure by controlling the GO content to obtain high-performance flexible supercapacitor electrodes.

3.3. Electrochemical performance of ACS device

To further assess the potential of NiCo₂O₄/rGO/CF electrodes in practical application, an asymmetric supercapacitor (NiCo₂O₄/rGO/CF//AC) was assembled, in which the NiCo₂O₄/rGO/CF and active carbon (AC) acted as the positive and negative electrodes, respectively.

The electrochemical properties of the NiCo₂O₄/rGO/CF//AC were illustrated in Fig. 5. The CV curves of the NiCo₂O₄/rGO/CF//AC supercapacitor within 0–1.7 V at different scan rates from 10 to 150 mV s⁻¹ were depicted in Fig. 5a. As expected, the shapes of all CV curves maintained the same even at 150 mV s⁻¹, implying outstanding rate capability of the ASC device. From the GCD curves (Fig. 5b), it demonstrated that the capacitive behavior consisted of electric double layer and pseudo-capacitance characteristic. The specific capacitance values of the supercapacitor were illustrated in Fig. 5c. It showed that the maximum specific capacitance of 61.2 F g⁻¹ at 1 A g⁻¹ was achieved. When the current density was 10 A g⁻¹, the specific capacitance decreased to 41.8 F g⁻¹, and the capacitance retention was 68.3%. For supercapacitor, the energy density (E) and power density (P) are two main parameters, which can be calculated according to the following formulas:³¹

here C, V and Δt represent the specific capacitance of the device, the potential range, and the discharge time, respectively. The relationship between E and P can be described by the Ragone plot, as displayed in Fig. 5d. The as-obtained supercapacitor achieved a maximum energy density of 24.6 W h kg⁻¹ with a power density of 850.3 W kg⁻¹, and a maximum power density of 8477.7 W kg⁻¹ with an energy density of 16.7 W h kg⁻¹, which was superior to the reported MS/NCO//AC (18.4 W h kg⁻¹ with a power density of 1200.2 W kg⁻¹),³² NiCo₂O₄ NSs@HMRAs//AC (15.4 W h kg⁻¹ with a power density of 700 W kg⁻¹),³³ and FeSe₂//NiCo₂O₄ (10.4 W h kg⁻¹ with a power density of 0.2 kW kg⁻¹)³⁴ devices. Moreover, the cycling life was evaluated by charge–discharge cycling at 2 A g⁻¹, as depicted in Fig. 5e, which exhibited 83.8% capacitance retention after 5000 cycles.

Fig. 5 Electrochemical properties of the NiCo₂O₄/rGO/CF//AC device: (a) CV curves at various scan rates, (b) GCD curves at different current densities, (c) the specific capacitance at various current densities, (d) Ragone plot and (e) cycling test.

4. Conclusions

In summary, we have prepared the NiCo₂O₄/rGO/CF composites, in which rGO and NiCo₂O₄ nanowires were constructed on carbon fibers, forming hierarchical hybrid nanostructures. The effects of graphene oxide content on the structure and performance of the NiCo₂O₄/rGO/CF were in detail investigated. Results showed that the microstructure and electrochemical performance of the NiCo₂O₄/rGO/CF composites were affected significantly by GO content. As compared to NiCo₂O₄/CF with the specific capacitance of 704.9 F g⁻¹, benefiting from the hierarchical porous structure and the synergistic effects between reduced graphene oxide and NiCo₂O₄ nanowires, the NiCo₂O₄/rGO/CF composite with 0.4 mg ml⁻¹ GO possessed a maximum specific capacitance of 931.7 F g⁻¹ at 1 A g⁻¹. In addition, the NiCo₂O₄/rGO/CF//AC supercapacitor with a potential window of 1.7 V was assembled, which exhibited a maximum energy density of 24.6 W h kg⁻¹ (at a power density of 850.3 W kg⁻¹) and a maximum power density of 8477.7 W kg⁻¹ (at an energy density of 16.7 W h kg⁻¹), and delivered 83.8% capacitance retention over 5000 cycles. Results further demonstrated that the introduction of reduced graphene oxide could effectively improve the electrochemical performance of NiCo₂O₄/CF electrodes. However, the novel NiCo₂O₄/rGO/CF electrodes with controllable structure and excellent performance need to be further investigated to satisfy the needs of high-performance supercapacitors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Jiangsu University for Senior Intellectuals (No. 15JDG177).

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