Three-dimensional hierarchical self-supported NiCo₂O₄/carbon nanotube core–shell networks as high performance supercapacitor electrodes

Abstract

A three-dimensional (3D) hierarchical self-supported NiCo₂O₄/carbon nanotubes/nickel foam (NiCo₂O₄/CNT/NF) electrode has been developed by electrodepositing NiCo layered double hydroxides (LDHs) on a self-supported CNT layer grown on a macroporous NF substrate followed by a simple post-annealing process. The resulting 3D hierarchical self-supported NiCo₂O₄/CNT/NF electrode delivered high specific capacitances of 1533 F g⁻¹ and 1335 F g⁻¹ at current densities of 3 A g⁻¹ and 30 A g⁻¹, respectively, vastly superior to those of a NiCo₂O₄/NF electrode at the same current density. The NiCo₂O₄/CNT/NF electrode also had good cycling stability and showed 102% initial capacitance retention after 2500 cycles at progressively varying current densities. The performance of the NiCo₂O₄/CNT/NF electrode was further evaluated by a two-electrode asymmetric supercapacitor device. The asymmetric device delivered a high energy density of 48.3 W h kg⁻¹ at a power density of 799.9 W kg⁻¹ and still maintained an energy density of 17.1 W h kg⁻¹ as the power density increased up to 7995 W kg⁻¹. Remarkably, the asymmetric device also exhibited good cycling stability with capacitance retention of 78.9% after 2000 cycles at a current density of 2 A g⁻¹, indicating promising applications in supercapacitors. The results of this study also provide an alternative strategy for constructing high performance supercapacitor electrodes with high specific capacitance, good rate capability and cycling stability.

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Introduction

To address the vast consumption of traditional fossil fuels and causing serious environmental pollution issues, there is an urgent demand to develop sustainable and clean energy sources. Energy storage devices, a key component in the energy conversion-storage-delivery chain, have attracted much attention and increasing research interest in recent years.¹ Supercapacitors, also known as electrochemical capacitors, are regarded as one of the most promising candidates for use as power resources in various mobile electronic devices and hybrid electric vehicles owing to their high power density, long cycle lifespan and fast charge/discharge rates.² In general, two types of supercapacitors exist based on the underlying energy storage mechanisms: carbonaceous compound-based electrical double-layered capacitors (EDLCs), and transition metal oxides/hydroxides-based pseudocapacitors.³ Unlike that of EDLCs, where the electrostatic adsorption only occurred on the electrode surface, t7he pseudocapacitors stored energy mainly via fast and reversible redox reactions occurred not only on electrode surface but also in the bulk phase, thus possessing higher capacitance and energy density.³ Among the transition metal oxide-based supercapacitive materials, hydrous RuO₂ is renowned for its high capacitance of 1580 F g⁻¹ and good cycling stability, but the scarcity and toxic in nature seriously impede its commercial applications. Because of these reasons, numerous efforts are being devoted to seeking for alternative cost-effective and environmentally friendly pseudocapacitive materials, especially those who simultaneously possess the multiple redox states and highly reversible redox capability.^4–12

To date, several possible metal oxides and complex metal oxides, such as NiO,^13–15 Co₃O₄,^16,17 MnO₂,^18–20 Fe₃O₄,^21–23 and NiCo₂O₄,^24–28 have been explored for supercapacitor applications. Among them, spinel NiCo₂O₄ has been suggested as one of the most promising alternative supercapacitive materials with utilization of two metal oxides. It has been experimentally verified that NiCo₂O₄ inherits the advantages of both two metal oxides, and possesses a higher electrical conductivity (at least two orders magnitude) and a higher electrochemical activity than those of single ones.^29,30 Moreover, spinel NiCo₂O₄ is easy to form a mesoporous superstructure, which provides more electroactive sites for faradic reaction and simultaneously offers numerous effective electrolyte-accessible channels for ion transportation. Inspired by these advantageous features, several strategies, including sol–gel,^24,27 hydrothermal,^1,26,31 electrospun³² and potentiostatic deposition^3,33 have been adopted to synthesize various NiCo₂O₄ nanostructures, such as nanoparticle, microsphere, nanosheet, nanowire, nanotube, and NiCo₂O₄-based composite. However, in most cases, NiCo₂O₄ still suffers from either low capacitance or poor rate capability due to its low intrinsic electrical conductivity. Additionally, the non-conductive polymeric binders used during the electrode fabrication process are also unfavorable to the performance improvement of active materials. Therefore, for NiCo₂O₄-based supercapacitors, the simultaneous realization of high capacitance, excellent rate capability and cycling stability is still an ongoing challenge.

It has been proven to be an effective strategy that three-dimensional (3D) hierarchical core–shell network structure, constructed by depositing electroactive material on highly conductive nanowires or nanotube networks, could make a great contribution to the optimization of electrochemical performance of electrode materials with enhanced specific capacitance, rate capability and cycling stability. Until now, this strategy has been successfully employed to fabricate various 3D core–shell network architectures, such as carbon nanotube (CNT)/MnO₂,^20,34 Ni_xCo_2x(OH)_6x/TiN,³⁵ CNT/Ni(OH)₂,³⁶ CNT/Co(OH)₂,³⁷ carbon nanofiber/Co₃O₄,³⁸ carbon nanofiber/MnO₂.^34,39 Among all conductive nanostructured substrates, CNT is a prominent substrate due to its excellent electrical conductivity, high specific surface area and good corrosion resistance in most chemical environments. However, the current research for CNT-supported supercapacitive materials mainly focuses on the simple individual or binary transition metal oxides^4,40–44 and/or hydroxides.^45–48 The investigation on fabrication of complex metal oxide/CNT core–shell composites and their supercapacitive performance exploration has been rarely reported.

Based on the aforementioned considerations, in this study, we construct a 3D hierarchical NiCo₂O₄/CNT/nickel foam (NiCo₂O₄/CNT/NF) core–shell network electrode via a potentialstatic deposition technique followed by a post-annealing process in air. In this typical binder-free electrode, the CNT networks grown on macroporous NFs are used as “core” to support the mesoporous NiCo₂O₄ nanoparticles, which could not only remarkably increase the number of electroactive sites but also notably shorten the electron transport pathway. Meanwhile, the big open interspaces between the CNT/NiCo₂O₄ core–shell structures and the nanosized pores in mesoporous NiCo₂O₄ superstructure can act as the double buffering reservoirs for electrolyte, ensuring more interfacial contact between the active material and electrolyte. With these advantageous features, the as-prepared 3D hierarchical NiCo₂O₄/CNT/NF core–shell network electrode shows excellent supercapacitive performance in both three-electrode cell system and asymmetric two-electrode cell system.

Experimental details

Growth of CNTs on NF substrate

All chemicals were of analytical grade and directly used as received. NFs (∼1.6 mm in thickness, Changsha lyrun Co. Ltd., China) were used as substrate for growth of self-supported CNT layer and also employed as the current collector of supercapacitor electrode. Prior to experiment, NFs were sequentially cleaned with 2% hydrochloric acid, acetone, and deionized water within an ultrasonic bath followed by drying with flowing N₂ gas. CNTs were grown on NFs via a thermal chemical vapor deposition (TCVD) process by using C₂H₂ as carbon source under H₂ atmosphere at a pressure of 20 kPa. The growth time depends on the desired length of CNTs and was set as 20 min in our experiment.

Fabrication and characterization of NiCo₂O₄/CNT/NF electrode

Before constructing NiCo₂O₄ on CNT backbone, the NF/CNT substrates were treated by air plasma at RF power of 50 W for 20 s to obtain a hydrophilic surface (Fig. S1a and b†). The NiCo₂O₄/CNT core–shell structure was fabricated by co-electrodepositing of NiCo layered double hydroxides (LDHs) on CNT backbone followed by a thermal annealing process. The co-electrodeposition was performed using a commercial CHI660E electrochemical workstation (Shanghai Chenhua Instrument Co., China) in a standard three-electrode cell configuration at a cathodic potential of −0.7 V with a saturated calomel electrode (SCE) as reference electrode, a platinum plate as counter electrode and a CNT/NF (1 × 1 cm²) substrate as working electrode. The electrolyte was an aqueous solution contained 5 mM Ni(NO₃)₂, 10 mM Co(NO₃)₂ and 3.75 mM NaNO₃. After deposition, the samples were rinsed with copious deionized water for several times followed by drying at 80 °C for 2 h and finally calcined at 300 °C for 2 h with a ramp rate of 1 °C min⁻¹ to transform the NiCo LDHs precursor into NiCo₂O₄ nanostructure. For comparison, NiCo₂O₄/NF electrode was also fabricated via the same electrochemical deposition procedure. The weight of NiCo₂O₄ on CNT/NF and NF substrate was calculated to be 0.78 mg cm⁻² and 0.21 mg cm⁻², respectively, by weighing the substrate before electrodeposition and after thermal annealing process. The as-prepared electrodes were characterized by an X-ray diffractometer (XRD, D/max-2400, Digaku, Japan) with radiation from a Cu target (Kα, λ = 0.1541 nm), a field-emission electron microscopy (FESEM; JSM-6701, JOEL) and a high resolution transmission electron microscopy (HRTEM; JEM 2010F, JOEL).

Electrochemical measurements

Cyclic voltammetry (CV) and galvanostatic charge/discharge (CDC) measurements of the as-prepared electrodes were performed on aforementioned electrochemical workstation within a three-electrode cell configuration containing 1 M KOH aqueous solution. The capacitances of the electrodes were calculated from corresponding CDC curves as follows:

C_s = I × Δt/(ΔV × m) (1)

where C_s (F g⁻¹) is the specific capacitance, I (A) is the discharge current, Δt (s) is the discharge time, ΔV (V) is the window potential during the discharge, and m (g) is the mass of the NiCo₂O₄ on NF or CNT/NF current collectors. Electrochemical impedance spectroscopy measurements were carried out at 0.24 V in an alternating current frequency ranging from 0.01 to 3 × 10⁴ Hz with an excitation signal of 5 mV.

Fabrication and electrochemical measurements of asymmetric supercapacitor device

An asymmetric supercapacitor device was assembled using as-prepared NiCo₂O₄/CNT/NF electrode as positive electrode, commercial activated carbon (AC, Shanghai Heda Carbon Co., Ltd, China) as negative electrode, commercial cellulose paper as separator and 1 M KOH solution as electrolyte, respectively. The capacitance of the as-assembled device is calculated from eqn (1) based on the total mass of active materials (NiCo₂O₄ and AC) on both two electrodes. The energy density and power density were calculated from following two equations:

P = E × 3600/t (3)

where E (W h kg⁻¹) and P (W kg⁻¹) are energy density and power density, respectively. The definitions of C, ΔV and t are the same as those in eqn (1).

Results and discussion

Construction of core–shell structure has been proven to be an efficient strategy to improve the electrochemical performance of supercapacitor electrode. Here this strategy is realized by depositing NiCo LDHs precursor on self-supported CNT networks grown on NF followed by a post-annealing process. Fig. 1a shows the FESEM image of CNTs grown on NFs. It was found that after the typical self-catalytic TCVD process a thin layer of hairy CNT film was uniformly grown on macroporous 3D NF skeleton along all direction and formed a 3D micro/nano hierarchical structure. The inset of Fig. 1a indicated that the grown hairy CNT layer consisted of randomly orientated, entangled CNTs with diameter ranging from 40–200 nm, forming a self-supported highly conductive 3D network structure via the underlying NF skeleton. Undoubtedly, the grown CNT layer greatly increases the surface area of NF. Considering its good electrical conductivity and excellent anti-corrosion properties, the grown CNT layer is expected to serve as a good skeleton to support NiCo LDHs supercapacitive materials and as a good current collector for supercapacitor electrode.

Fig. 1 (a) FESEM image of hairy-like CNTs grown on NF. (b–d) FESEM images of as-prepared 3D NiCo₂O₄/CNT core–shell network structure with different magnifications.

The electrodeposition of NiCo LDHs on CNTs in our study mainly involved the reduction of NO₃⁻ to produce the OH⁻ ion in the vicinity of cathodic surface and the subsequent precipitation of NiCo LDHs precursor on CNT backbone due to the approximately equal solubility product constant (K_sp) of Co(OH)₂ (2.5 × 10⁻¹⁶) and Ni(OH)₂ (2.8 × 10⁻¹⁶).³⁰ The whole electrodeposition process can be described as following two equations:

NO⁻₃ + H₂O + 2e⁻ → NO⁻₂ + 2OH⁻ (4)

xNi²⁺ + 2xCo²⁺ + 6xOH⁻ → Ni_xCo_2x(OH)_6x (5)

The electrodeposited green NiCo LDHs precursor can be thermally converted into black NiCo₂O₄ loaded on CNT backbone via a typical oxidation reaction expressed as follows:

Fig. 1b–d shows the FESEM images of the as-prepared NiCo₂O₄/CNT/NF electrode. As demonstrated, after thermal treatment, the NF substrate was uniformly covered with 3D NiCo₂O₄/CNT network structure, roughly preserving the morphology of CNT networks before electrochemical deposition (Fig. 1b). High magnification FESEM images, as shown in Fig. 1c and d, indicated that NiCo₂O₄ nanoparticles were tightly adhered on the sidewall of CNTs, forming a core–shell network structure with open space size ranging from several hundred nanometers to several micrometers. By contrast, the NiCo₂O₄ directly grown on NFs via the same fabrication procedure, as shown in Fig. S2a and b,† was densely packed together and demonstrated a uniform distribution of intercrossed nanoflakes with pore size ranging from 100–300 nm, leading to a much reduced specific surface area. Fig. 2 shows the XRD pattern of NiCo₂O₄/CNT/NF electrode. As demonstrated, except for the peaks of CNTs and NF substrate, several well-defined diffraction peaks were clearly observed at 2θ values of 18.8°, 36.6°, 44.5°, 55.5°, 59.1°, and 64.9°, which can be indexed as the (111), (311), (400), (422), (511) and (440) plane reflections of the spinel NiCo₂O₄ crystalline structure (JCPDS: 20-0781), respectively. This indicates that the NiCo₂O₄ shell layer deposited on CNT core has the spinel crystalline structure with nickel located at octahedral sites and cobalt occupied both octahedral and tetrahedral sites;¹ see Fig. 2b. The broadening of the XRD peaks in Fig. 2a is correlated to the small crystallite size of the synthesized NiCo₂O₄, which is expected to have a higher electrochemical activity. According to Scherrer formula, the average crystallite size of the synthesized NiCo₂O₄ was calculated to be about 14.5 nm by analyzing the (311) peak in XRD pattern.

Fig. 2 XRD pattern (a) and crystal structure (b) of NiCo₂O₄ on CNT/NF substrate.

The detailed microstructure of the synthesized NiCo₂O₄ layer was characterized by using HRTEM. As shown in Fig. 3a, the NiCo₂O₄ nanoparticles observed in FESEM image actually consisted of randomly orientated mesoporous NiCo₂O₄ nanocrystals with size ranging from 5 to 20 nm and pores with size ranging from 2 to 8 nm. Further analysis results indicated that the interspacing distances between adjacent fingers in regions 1, 2 and 3 were 0.24, 0.47 and 0.29 nm, respectively, which are in a good agreement with the theoretical interplane spacing of (311), (111) and (220) planes of spinel NiCo₂O₄ (Fig. 3b–d), respectively. Selected-area electron diffraction (SAED) pattern, as shown in Fig. 3e, demonstrated the well-defined diffraction rings, suggesting the polycrystalline characteristics of NiCo₂O₄ nanoparticles. All above observations confirm the successful formation of nanosized mesoporous NiCo₂O₄ crystallites on 3D CNT backbone in our experiment.

Fig. 3 (a–d) HRTEM images and (e) SAED of NiCo₂O₄ nanoparticles on CNT/NF substrate. Images of (b–d) are the enlargement of regions 1, 2 and 3 in panel a, respectively.

XPS technique was used to investigate the chemical state of the as-prepared NiCo₂O₄/CNT/NF electrode in our study. As shown in Fig. 4a, the survey spectrum indicated the presence of Ni, Co, O, and C element from CNTs, as well as the absence of other impurities. Fig. 4b and c gives the Ni 2p and Co 2p emission spectra, respectively. By using the Gaussian fitting method, the Ni 2p emission spectrum can be fitted as two spin–orbit doublets and two shakeup satellites (indicated as “Sat.”). The deconvoluted Ni 2p_3/2 spectrum showed a peak at 854.6 eV corresponding to the Ni²⁺ ions located in the octahedral sites and a peak at 855.5 eV corresponding to Ni³⁺ ions located in tetrahedral sites, while the shakeup satellite peak centered at 860.8 eV was related to both Ni²⁺ and Ni³⁺ ions. Similarly, the Co 2p emission spectrum also can be fitted into two spin–orbit doublets, characteristics of Co²⁺ and Co³⁺, and two shakeup satellites. All these facts imply that the chemical composition of as-prepared NiCo₂O₄ contains Ni²⁺, Ni³⁺, Co²⁺, and Co³⁺, which is consistent with the results in literature.^1,27,49

Fig. 4 XPS spectra of NiCo₂O₄: (a) survey scan, (b) Ni 2p and (c) Co 2p core levels.

CV and galvanostatic CDC measurements were employed to characterize the electrocapacitive performance of the as-prepared two electrodes. Fig. 5a and b shows the CV curves of NiCo₂O₄/NF electrode and NiCo₂O₄/CNT/NF electrode at various sweep rates, respectively. It can be observed that all CV curves of NiCo₂O₄/NF electrode at various sweep rates were characterized by a pair of well-defined redox peaks derived from the faradic reaction expressed as follows:

NiCo₂O₄ + OH⁻ + H₂O ⇔ NiOOH + 2CoOOH + e⁻ (7)

while for the CV curves of NiCo₂O₄/CNT/NF electrode, besides the obvious redox peaks between 0.2–0.6 V, the typical rectangular shaped regions were also clearly observed within potential range of 0–0.2 V. According to previous reports,^50,51 the rectangular regions were closely related with the electrical double layer between the NiCo₂O₄ and hydroxyl ions, which can be expressed as following equation:

NiCo₂O₄ + OH⁻ ⇔ NiCo₂O₄∥OH⁻ (8)

where NiCo₂O₄∥OH⁻ represents the electrical double layer formed on electrode/electrolyte interface. These suggest that both EDLCs and faradic reactions are involved during the energy conversion process of NiCo₂O₄/CNT/NF electrode. To evaluate the contribution of EDLCs from CNTs, we compared the CV curves of NiCo₂O₄/CNT/NF electrode and NF/CNT substrate at sweep rate of 10 mV s⁻¹. As demonstrated in Fig. 5c, CV curve of CNT/NF substrate, in contrast to that of NiCo₂O₄/CNT/NF electrode, was nearly a horizontal line along the potential-axis, indicating an extremely low EDLCs value. Therefore, the remarkable EDLCs behavior of NiCo₂O₄/CNT/NF electrode is mainly resulted from the reversible adsorption of electrolyte ions on surface of mesoporous NiCo₂O₄ rather than from CNT/NF substrate. Although the CNT layer has the negligible contribution to capacitance of NiCo₂O₄/CNT/NF electrode, it provides a highly conductive skeleton for depositing mesoporous NiCo₂O₄ and enables the formation of NiCo₂O₄/CNT core–shell networks. Additionally, the peak potential of NiCo₂O₄/CNT/NF electrode shifted only ∼50 mV for a five-time increase in sweep rate range, suggesting the superfast electronic transport rate and good rate capability of the NiCo₂O₄/CNT/NF electrode.

Fig. 5 CV curves of (a) NiCo₂O₄/NF electrode and (b) NiCo₂O₄/CNT/NF electrode at sweep rates ranging from 10–50 mV s⁻¹. (c) Comparison of CV curves of NiCo₂O₄/CNT/NF electrode and CNT/NF substrate at sweep rate of 10 mV s⁻¹.

Series galvanostatic CDC measurements were performed on the two as-prepared NiCo₂O₄ electrodes in 1 M KOH electrolyte within the potential window of 0–0.5 V at current densities ranging from 3 to 30 A g⁻¹. As shown in Fig. 6a and b, the shape of CDC curves of NiCo₂O₄/NF electrode and NiCo₂O₄/CNT/NF electrode had a notable difference. The CDC curves of NiCo₂O₄/NF electrode at various current densities were similar to those in previous reports and demonstrated the characteristics of purely faradic capacitive behavior (Fig. 6a). While for the CDC curves of NiCo₂O₄/CNT/NF electrode, as shown in Fig. 6b, they can be divided into two distinct regions: a linear region between 0–0.2 V and a voltage plateaus between 0.2–0.5 V. The capacitance within the linear region mainly originated from the EDLCs formed on interface of mesoporous NiCo₂O₄/electrolyte, while the capacitance within the voltage region mainly resulted from the interfacial faradic redox couples of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺. Undoubtedly, the pseudocapacitance still plays the dominant role in the electrochemical capacitance of NiCo₂O₄/CNT/NF electrode because the discharging time of the plateau region is much longer than that of the linear region. The capacitance values of two electrodes at various current densities were calculated based on their corresponding CDC curves and are plotted in Fig. 6c. Encouragingly, the NiCo₂O₄/CNT/NF electrode delivered the high capacitance of 1533, 1492, 1446, 1384 and 1335 F g⁻¹ at current densities of 3, 5, 10, 20 and 30 A g⁻¹, respectively. This indicates that about 87% of capacitance is still remained as the current density increases from 3 to 30 A g⁻¹, suggesting the superb rate capability. In contrast, although the NiCo₂O₄/NF electrode also delivered the good rate capability, its specific capacitances were very low and only reached about one half of those of NiCo₂O₄/CNT/NF electrode at the same current density. The capacitance and rate capability of such integrated binder-free 3D hierarchical NiCo₂O₄/CNT/NF core–shell structured electrode are also superior to those of most of slurry-derived NiCo₂O₄-based electrode (such as nanowire,²⁴ hollow microspheres,³⁰ flower-like⁵² and NiCo₂O₄/graphene composite⁵³) and self-standing NiCo₂O₄ nanostructure-based electrode^50,54–56 with roughly the same mass loading level (see Table S1†). All these further confirm the advantages of the NiCo₂O₄/CNT core–shell network structure in achieving both high specific capacitance and superb rate capability.

Fig. 6 CDC curves of (a) NiCo₂O₄/NF electrode and (b) NiCo₂O₄/CNT/NF electrode at various current densities. (c) Comparison of the capacitance of two electrodes at various current densities. (d) Cycling stability of the NiCo₂O₄/CNT/NF electrode at various current densities in 1 M KOH solution.

The enhanced electrochemical performance of NiCo₂O₄/CNT/NF electrode is intrinsically associated with its special core–shell structure and can be confirmed by EIS measurements (Fig. S3†). The EIS spectrum of NiCo₂O₄/CNT/NF electrode demonstrated lower charge-transfer interfacial resistance (R_ct) at high-/medium frequency region (the semicircle with smaller diameter) and lower diffusion resistance at low frequency region (the straight line with stiffer slope). The lower charge transfer resistance suggests the faster faradic response and greater distribution of electric double layer at electrode interface, two of which closely depend on the large electroactive surface area of mesoporous NiCo₂O₄ nanoparticles deposited on 3D self-supported CNT network. Additionally, the formation of NiCo₂O₄/CNT core–shell network structure is also favorable for lowering of R_ct by providing large amount of highly conductive channels for charge transportation and simultaneously shortening the charge transportation pathway from active material to current collector. All these results are in good agreement with the their CV and CDC behaviors, which clearly demonstrate that NiCo₂O₄/CNT/NF electrode can deliver higher faradic capacitance and more pronounced EDLCs as compared to those of NiCo₂O₄/NF electrode.

Long-term cycling performance, especially at progressively increasing current densities, which can more precisely reflect the stability of supercapacitor, was also investigated in this study. Fig. 6d shows the cycling performance of the as-prepared NiCo₂O₄/CNT/NF electrode subjected to continuous charging/discharging for 2500 cycles at various current densities. As demonstrated, the capacitance of NiCo₂O₄/CNT/NF electrode gradually increased up to 1759 F g⁻¹ during the initial 500 cycles at a current density of 3 A g⁻¹, which can be attributed to the full activation of the electrode. After that, the capacitance gradually decreased and still delivered a high capacitance of 1728 F g⁻¹ at 1000^th cycles. As the current density was progressively increased up to 5 and 10 A g⁻¹, the NiCo₂O₄/CNT/NF electrode also exhibited the good capacitance stability with more than 94% retention at each 500 cycles. After continuous cycling test for 2000 cycles at progressively varying current densities, the current density was turned back to 3 A g⁻¹. Surprisingly, a high capacitance of 1628 F g⁻¹ still can be delivered and maintained another 500 cycles with a low capacitance loss of 4%. By and large, the as-prepared NiCo₂O₄/CNT/NF electrode demonstrated the excellent cycling stability and delivered about 102% initial capacitance retention (or 88% retention of maximum capacitance) after 2500 cycles at various current densities, superior to that of NiCo₂O₄/NF electrode in this work and those of most NiCo₂O₄-based electrode in literature (see Table S1†). The excellent cycling performance of NiCo₂O₄/CNT/NF electrode is mainly attributed to the advanced self-supported CNT/NiCo₂O₄ core–shell network structure and the good adhesion between the NiCo₂O₄ nanoparticles and CNT skeleton. This advanced electrode design concept is also can be easily extended to construct other hierarchical 3D metal oxide or metal hydroxide/CNT supercapacitive electrodes.

To further evaluate its capacitive performance for practical applications, asymmetric device was assembled using as-prepared 3D hierarchical NiCo₂O₄/CNT/NF electrode as positive electrode, commercial AC as negative electrode, commercial cellulose paper as separator and 1 M KOH solution as electrolyte, respectively. Based on the capacitance values of positive NiCo₂O₄/CNT/NF electrode (Fig. 6b) and negative AC electrode (Fig. S4†) as well as their corresponding potential windows, the mass ratio of NiCo₂O₄ and AC was set as 0.31 to balance the charges stored in both two electrodes. Fig. 7a shows the CV curve of the assembled device within the different potential windows. As demonstrated, the assembled device exhibited highly reversible energy storage behavior within potential window of 0–1.6 V and an obvious oxygen evolution reaction-induced irreversible process within potential window of 1.6–1.8 V. Therefore, the potential window of 0–1.6 V was chosen for further investigation of the overall electrochemical performance of assembled asymmetric device. Fig. 7b shows the CV curves of the assembled device at various scan rates from 10–30 mV s⁻¹ within the potential window of 0–1.6 V. Obviously, all the CV curves of the device behave similarly in shape and demonstrate a profile close to the rectangular shape with a pair of light redox peaks located at 1.3/0.95 V, different from those of individual AC and NiCo₂O₄/CNT/NF electrode. The possible reason is that the hybrid capacitor device integrates the performance of both two electrodes into a mutual complementarity system. It also can be observed that the current density of the device increases approximately linearly with the square root of the scan rate (see Fig. S5†), indicating the diffusion-controlled energy storage process. Based on the CDC curves (inset of Fig. 7c) and total mass of active materials on both two electrodes, the capacitances of the assembled device were calculated via the eqn (1) and are plotted in Fig. 7c. As demonstrated, the assembled device delivered a high cell capacitance of 135.8 F g⁻¹ at a current density of 1 A g⁻¹, and still remain a moderate capacitance of 48.1 F g⁻¹ as current density increase up to 10 A g⁻¹. Energy density and corresponding power density of the device were calculated on the CDC curves and the results are demonstrated as a Ragone plot in Fig. 7d. The assembled device delivered a high energy density of 48.3 W h kg⁻¹ at a power density of 799.9 W kg⁻¹. Encouragingly, the assembled device still can remain as high as 17.1 W h kg⁻¹ even at a high power density of 7995 W kg⁻¹ with energy retention of 35.4% for the power density increase nearly 10 times, suggesting good practical performance. It should be noted that the commercial AC with moderate capacitance value of 275 F g⁻¹ at current density of 1 A g⁻¹ was used as negative electrode, seriously limiting the performance of the assembled device. In the future, if coupled with negative electrode with higher capacitive performance, such as N-doped activated carbon which has the high capacitance of 300–500 F g⁻¹,⁵⁷ higher cell energy density would be expected for NiCo₂O₄/CNT-based asymmetric supercapacitor device. The repeated CDC measurement at current density of 2 A g⁻¹ was used to evaluate the durability of the as-assembled device. As shown in Fig. 8, the device can remain about 78.9% of its original capacitance even after 2000 cycles, further confirming its promising application in supercapacitors.

Fig. 7 Electrochemical evaluation of the asymmetric supercapacitor device: (a) CV curves at various potential windows with a scan rate of 10 mV s⁻¹. (b) CV curves at various scan rates within a potential window of 0–1.6 V. (c) Capacitance of at different current densities (inset image is corresponding CDC curves). (d) Ragone plot related to energy density and power density.

Fig. 8 Cycling stability of the asymmetric device measured at a current density of 2 A g⁻¹.

Conclusions

We have successfully developed a 3D hierarchical NiCo₂O₄/CNT/NF electrode via a facile and scalable three-step fabrication route. The resulting NiCo₂O₄/CNT/NF electrode displayed a typical hierarchical core–shell network structure with mesoporous spinel NiCo₂O₄ loaded on self-supported CNT layer. Owing to its special core–shell network structure, the NiCo₂O₄/CNT/NF electrode delivered the high specific capacitance, good rate capability and excellent cycling stability at progressively varying current densities. With the as-prepared NiCo₂O₄/CNT/NF electrode as positive electrode and commercial AC as negative, the assembled asymmetric supercapacitor device delivered high power density and energy density as well as good cycling stability, suggesting the promising applications in supercapacitors. In the future, if coupled with negative electrode material with higher capacitance, the energy density and power density of the device can be further greatly improved. Predictably, the developed synthetic methodology and strategy for constructing NiCo₂O₄/CNT/NF electrode can be easily extended to prepare series 3D hierarchical metal oxide or metal hydroxide/CNT architectures as highly active materials for fuel cells, batteries and more specifically for applications in rechargeable supercapacitors.

Acknowledgements

Dr X. Zhao thanks the financial support offered by the National Natural Science Foundation of China (Grant no. 21306072).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09048a

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