3D interconnected networks of a ternary hierarchical carbon nanofiber/MnO₂/Ni(OH)₂ architecture as integrated electrodes for all-solid-state supercapacitors

Abstract

We demonstrate the design and fabrication of hierarchical Ni(OH)₂ nanosheets vertically grown on a porous carbon nanofiber/MnO₂ composite (CF/MnO₂) to form three dimensional interconnected networks via a facile hydrothermal process for supercapacitor applications. The obtained CF/MnO₂/Ni(OH)₂ electrode exhibits high specific capacitance (2079 F g⁻¹ at 0.5 A g⁻¹ in 6 M KOH aqueous solution), rendering its promising application as a potential electrode for supercapacitors. In order to increase the energy density, an asymmetric supercapacitor (ASC) has been successfully fabricated using CF/MnO₂/Ni(OH)₂ as the positive electrode and CFs as the negative electrode. The as-fabricated all-solid-state ASC device achieves a maximum energy density of 67.6 W h kg⁻¹, highly comparable with the previously reported Ni(OH)₂-based ASCs. The present hierarchical CF/MnO₂/Ni(OH)₂ ternary hybrid brings new opportunities to design and develop high-performance electrode materials for next-generation supercapacitors in flexible electronics.

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1. Introduction

Nowadays, electrochemical storage devices such as supercapacitors and Li-ion batteries have attracted considerable attention because of their broad application in portable electronics, power back-ups and hybrid electrical vehicles, which could efficiently solve the issue of the energy crisis caused by the environmental pollution and over usage of fossil-fuel resources.^1–3 Compared with other electrochemical energy storage devices, supercapacitors turn out to be more attractive attributed to their high energy density, fast charge–discharge rate and long cycling life. Generally, supercapacitors can be clarified into electric double-layer capacitors (EDLCs) and pseudocapacitors.^4,5 Although carbon-based electrodes possess excellent rate capability and superior cycling performance, they often suffer from fairly low specific capacitance and energy density. Due to their extremely high pseudocapacitance offered by rich redox reactions, transition metal oxides and hydroxides have drawn extensive interests recently among various supercapacitor electrode materials, such as RuO₂,^6,7 MnO₂,^8,9 Co₃O₄ ¹⁰ and Ni(OH)₂.^11–13 However, the metal oxide/hydroxide electrodes often exhibit poor rate capability and inferior cycling stability due to the swelling and shrinkage during their charging–discharging process, greatly hindering their further application in high power energy-storage devices.

To address these problems, one efficient way is to design and synthesize transitional metal oxide based hybrids by utilizing highly conductive materials like graphene, carbon nanotubes and carbon fibers as scaffolds to support nanostructured transitional metal oxide thin films to further improve the electrochemical performances.^14–17 On the other hand, tremendous efforts have been devoted to the rational synthesis of advanced core/shell heterostructures to achieve enhanced performances due to the synergistic effects between the two components,^16,18 such as metal oxide/metal oxide,^19–23 metal oxide/conductive polymer^24,25 and metal oxide/metal hydroxide.^26–28 For instance, Mai et al. reported the growth of Co₃O₄@CoMoO₄ nanowire arrays on nickel foam with an improved capacitance (1902 F g⁻¹ at 1 A g⁻¹) compared with those of pristine Co₃O₄ nanocone and CoMoO₄ nanosheet electrodes.²⁹ The obtained heterostructured nanocomposite can make full use of the advantages of each component, exhibiting enhanced electrochemical performances compared to the single component. Unfortunately, these metal oxides/hydroxides still could not meet the need for high performance electrodes especially excellent rate capability and cycling performance. Therefore, it is quite necessary to design and fabricate an electrode material that could not only achieve the superior capacitive performance but also possess satisfactory rate capability. In this end, it seems to be an efficient way to achieve this goal through efficiently utilizing the advantage of carbon materials, metal oxides and metal hydroxides.

In light of the above considerations, we present an alternative route to construct a novel ternary core/shell structure of Ni(OH)₂ nanosheet arrays on porous carbon fiber/MnO₂ hybrid (CF/MnO₂/Ni(OH)₂) for supercapacitor electrodes. This smart design of electrode offers several advantages as follows: on the one hand, the porous CFs with ultrahigh surface area not only provide a well conductive substrate for the shells but also shorten the ion transport path. On the other hand, the shell materials could contribute high pseudocapacitance to the overall capacitance to improve the capacitance. The fabricated electrode displays a high specific capacitance up to 2079 F g⁻¹ at 0.5 A g⁻¹ in 6 M KOH aqueous solution, which is much higher than those of CF/Ni(OH)₂ and CF/MnO₂. The assembled CFs//CF/MnO₂/Ni(OH)₂ asymmetric supercapacitor (ASC) device can achieve a high energy density of 67.6 W h kg⁻¹, highly comparable with the previously reported Ni(OH)₂-based ASCs.

2. Experimental section

2.1. Preparation of ternary CF/MnO₂/Ni(OH)₂ hybrid

The CFs was prepared through a facile electrospinning method previously reported by our group and subsequently the CF/MnO₂ hybrid was synthesized by immersing the CF substrate into a KMnO₄ solution (2 mg mL⁻¹) at 65 °C for 24 h.³⁰ After it was cooled to room temperature, the precipitate was collected, rinsed using distilled water and finally dried at 100 °C for 12 h in a vacuum oven. The above CF/MnO₂ hybrid was used as the substrate for subsequent growth of Ni(OH)₂ nanosheet shell through the hydrothermal method. Briefly, 8 mmol of Ni(NO₃)₂·6H₂O, 1.5 mmol of K₂S₂O₄ and 2 mL of NH₃·H₂O were dissolved into 50 mL of distilled water under vigorous stirring to form a homogeneous solution. The CF/MnO₂ substrate was immersed into the above solution at room temperature for 30 min. Then the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and kept at 120 °C for 2 h. After naturally cooling down to room temperature, the final product was collected and rinsed with distilled water several times, and dried in vacuum at 80 °C. CF/Ni(OH)₂ hybrid was also synthesized by the same method without the presence of MnO₂ for comparison.

2.2. Material characterizations

The nitrogen adsorption–desorption isotherms were measured using a Quantachrome NOVA 2000e sorption analyzer at 77 K. Specific surface areas and pore size distributions were calculated using the Brunauer–Emmett–Teller (BET) and density functional theory (DFT) or Barrett–Joyner–Halenda (BJH) models from the adsorption branches, respectively. Transmission electron microscopy (TEM) images were recorded on a JEOL 3010. Scanning Electron Microscopy (SEM) measurements were performed using a Hitachi S-4800 operating at an accelerating voltage of 15 kV. X-ray diffraction (XRD) data were collected on a Rigaku XDS 2000 diffractometer with Cu Kα (λ = 0.15406 nm). XPS was further characterized using a PHI 5000 Versa Probe to investigate the chemical structure of CF/MnO₂/Ni(OH)₂. Fourier transform infrared (FT-IR) spectra in KBr form were recorded on a Bruker VECTOR 22 FT-IR spectrometer. The thermal behavior of the materials was investigated by thermogravimetry-differential thermal analysis (TG-DTG) in Ar atmosphere.

2.3. Electrochemical measurements

Electrochemical measurements of the electrodes were firstly performed using a conventional three-electrode system in 6 M KOH electrolyte on a CHI 660D workstation. Platinum foil and Hg/HgO electrodes were used as the counter and reference electrodes, respectively. The working electrode was prepared by pressing the CF/MnO₂/Ni(OH)₂ hybrid (3.2 mg) between two pieces of Ni foam under 8 MPa. The electrochemical properties were tested by cyclic voltammetry (CV) and galvanostatic charge–discharge in the potential range of 0 to 0.5 V (vs. Hg/HgO). Electrochemical impedance spectroscopy (EIS) was measured in the frequency range between 0.01 Hz and 100 kHz. The specific capacitance of the electrodes was calculated based on the galvanostatic charge–discharge curves according to the following equation:

C_m = IΔt/mΔV (1)

where C_m is the specific capacitance, I is the constant discharge current, Δt is the discharge time, m is the total mass of the material on the electrodes and ΔV is the potential.

2.4. Fabrication of all-solid-state ASC device

To further characterize the electrochemical performance of the prepared hybrid, an all-solid-state ASC device was fabricated with the CFs and CF/MnO₂/Ni(OH)₂ hybrid as the negative and positive electrode, respectively. The polyvinylacetate (PVA)/KOH polymer gel was used as the electrolyte. Typically, 4 g of KOH and 4 g of PVA were mixed in 50 mL of deionized at 85 °C under vigorous stirring until the PVA/KOH gel became clear. Subsequently, the prepared CF/MnO₂/Ni(OH)₂, CF electrodes and the separator (cellulose acetate membrane) were immersed in the PVA/KOH gel for 2 h and the ASC device was fabricated and dried at room temperature for 24 h to evaporate the excess water. The mass ratio of the negative and positive electrodes was decided by the charge balance between the two electrodes in ASC device (q⁺ = q⁻) and the charge stored (q) by each electrode depends on the following equation:

q = mCΔV (2)

The specific capacitance of the ASC system can be calculated based on the CV curves according to the following equation:

where I is the response current density, V is the potential, v is the scan rate, and m is the total mass of the material on the electrodes.

The energy density (E) and power density (P) of our fabricated ASC device were calculated from the CV curves according to the following equations:

E = 1/2CV² (4)

P = E/t (5)

where C is the specific capacitance of the assembled ASC device calculated based on the total weight of the materials in two electrodes according to the CV curves, V is the voltage range and t is the discharge time.

3. Results and discussion

The general fabrication protocol of the ternary hierarchical CF/MnO₂/Ni(OH)₂ hybrid is schematically illustrated in Fig. 1a. Firstly, the interconnected CFs are prepared via a facile electrospinning process according to the previous work. As observed, the as-spun CFs possess a one-dimensional (1D) nanostructure with an average diameter of around 1 μm interconnected with each other (Fig. S1a†). Additionally, there are numerous pores existed in the CF substrates, which can be seen from the TEM image (Fig. S1b†). The N₂ adsorption/desorption isotherm and pore size distribution further confirm the porous structure of CFs (Fig. S1c†). Type IV isotherm curve with H3-type hysteresis loop can be clearly seen from N₂ adsorption–desorption isotherm. The pore size calculated by DFT method assuming a slit geometry for micropores (1.1 and 1.5 nm) and a cylindrical pore geometry for the mesopores (3.0 nm) reveals the existence of well-defined micro- and mesoporous structure of the CF substrate (Fig. S1c†). After that, the CFs is served as an interfacial reactive template for the subsequent growth of MnO₂ nanosheets. Compared with that of pristine CFs, a great number of uniform 1D nanostructures with a hierarchical architecture can be found in Fig. 1b. Besides, the surface of the CF becomes much rougher and is totally covered with large amounts of interconnected homogeneous MnO₂ nanosheets (Fig. 1b). As shown in the high-resolution SEM image (Fig. 1c), uniform and ultrathin MnO₂ nanosheets radially grow on the surface of CFs to form a porous coaxial core/shell structure with several nanometer in thickness. From the TEM image of CF/MnO₂ hybrid (Fig. 1d), we can obviously find that the coated MnO₂ layer on CFs is very thin with ∼12 nm in thickness, keeping a highly porous configuration. Subsequently, the Ni(OH)₂ shell is grown on the CF/MnO₂ through a facile hydrothermal approach to form a 3D interconnected ternary hybrid. Obviously, the CF/MnO₂/Ni(OH)₂ ternary composite has a uniform diameter of ∼1.3 μm (Fig. 1e and f), much larger than pristine CFs (1 μm) and CF/MnO₂ (∼1 μm), indicating that CF/MnO₂ surfaces are homogeneously covered by uniform, dense and ultrathin Ni(OH)₂ nanosheets. These 1D architectures are composed of numerous ultrathin sheet-like subunits, which are uniformly and vertically grown on the surfaces of CF/MnO₂, forming a highly porous surface morphology. High magnification SEM image (Fig. 1g) presents that the Ni(OH)₂ nanosheets are 150–260 nm in size and 8–12 nm in thickness. The spatially dispersed Ni(OH)₂ nanosheets interconnect with each other to form considerable nanovoids on the surface of 1D nanofibers, which in turn facilitates the transport/diffusion of electrolyte ions. Moreover, the Ni(OH)₂ nanosheets are compactly connected with CF/MnO₂, allowing rapid and efficient electron transfer. The CF/Ni(OH)₂ hybrid has been also synthesized for comparison without the growth of MnO₂ on the CFs and the SEM images are presented in Fig. S1d and e.† Similarly, the CF/Ni(OH)₂ also possesses the morphology of hierarchical nanostructure with numerous Ni(OH)₂ nanosheets coating on the surfaces of CF substrate.

Fig. 1 (a) Schematic illustration of the fabrication processes of the CF/MnO₂/Ni(OH)₂ ternary hybrid architecture. (b and c) SEM and (d) TEM images of CF/MnO₂. (e–g) SEM images of CF/MnO₂/Ni(OH)₂ hybrid at different magnification.

More detailed structural information and morphological evolution of the CF/MnO₂/Ni(OH)₂ are investigated by TEM. As shown in low-magnification TEM image (Fig. 2a), the CF/MnO₂/Ni(OH)₂ have uniform morphology with diameters of ∼1.3 μm, and the Ni(OH)₂ nanosheets are uniformly and vertically grown on the surface of the CF/MnO₂ to form a porous structure, which is consistent with the SEM observations (Fig. 1f and g). An enlarged view of a particular region clearly confirms the core–double-shell structure consisting of a CF core inside, a MnO₂ layer (about 12 nm in thickness) in the middle and a Ni(OH)₂ layer (about 150 nm in thickness) on the outside. The Ni(OH)₂ nanosheets are interconnected with each other and distributed vertically on the CF/MnO₂ surface, forming a intriguing 3D hierarchical porous nanostructure. The lattice distances of 0.24 and 0.29 nm can be clearly measured in Fig. 2c, which correspond to the (100) and (011) planes of hexagonal Ni(OH)₂ (a = b = 0.312 nm, c = 0.46 nm). Notably, we can clearly find many micropores (<2 nm) from the low contrast region in the high resolution TEM image (Fig. 2c), which are favorable for the rapid transport and diffusion of electrolyte ions during the charge/discharge process. The selected-area electron diffraction (SAED) pattern taken from the edge of nanosheets shows a set of well-defined diffraction rings (Fig. 2d), confirming the polycrystalline characteristic of Ni(OH)₂ nanosheets. Additionally, the energy dispersive X-ray spectrometry (EDS) analysis was also conducted to confirm the composition of CF/MnO₂/Ni(OH)₂ hybrid (Fig. 2e), demonstrating the presence of Ni, Mn, O, and C elements (Cu from the TEM copper grid). Finally, such unique ternary core–double-shell nanostructure is also confirmed by cross sectional compositional line profiles (inset of Fig. 2f) and selected area elemental mapping (Fig. 2g). Noteworthy, the shell is a binary hybrid, in which MnO₂ is located in the middle layer while Ni(OH)₂ is in the outside layer.

Fig. 2 (a–c) TEM images, (d) corresponding SAED pattern and (e) EDS spectrum of the CF/MnO₂/Ni(OH)₂ hybrid. (f) Cross sectional compositional line profiles and (g) selected area elemental mapping of the highlighted region in (f).

The porous characteristic of the CF/MnO₂/Ni(OH)₂ hybrid was further investigated by N₂ adsorption/desorption measurement. As presented in Fig. 3a the CF/MnO₂/Ni(OH)₂ composite shows type IV isotherms with a notable H3 hysteresis loops, indicating the presence of well-defined mesopores. The BET surface area of the CF/MnO₂/Ni(OH)₂ hybrid is calculated to be 262.8 m² g⁻¹ and the pore size distribution is centered at about 2.4, 4, 7 and 11 nm as measured by the BJH method. The existing numerous mesopores and void space between the stacked Ni(OH)₂ nanosheets are beneficial to the penetration of the electrolyte ions, and the crosslinked and interconnected structures ensure easy access of ions to the electrode/electrolyte interface, which is vital for the surface redox reactions. The phase and crystallographic structures of the CF/MnO₂, CF/Ni(OH)₂ and CF/MnO₂/Ni(OH)₂ hybrid were further verified by XRD. As shown in Fig. 3b, two broad peaks observed at 25° and 44° can be attributed to the (002) and (100) crystal plane of the disordered carbon.³¹ The peaks at around 12°, 37° and 66° in the XRD pattern of CF/MnO₂ are corresponding to the (001), (110) and (021) planes of birnessite-type MnO₂ (JCPDS card no. 42-1317), respectively.^30,32 After the deposition of Ni(OH)₂, the (002) and (100) peaks of CFs become much weaker, indicating that the surface of CFs is mostly covered by Ni(OH)₂. The broad diffraction peaks located at about 2θ = 33°, 38°, 59° can be well indexed with (100), (101) and (110) plane reflections of β-Ni(OH)₂ (JCPDS card no. 14-0117), respectively.³³ With regard to the CF/MnO₂/Ni(OH)₂ hybrid, the XRD pattern is similar to CF/MnO₂ and CF/Ni(OH)₂, indicating that the CF/MnO₂/Ni(OH)₂ hybrid has been well synthesized. To investigate the surface property of the hybrid, we conducted FT-IR spectrum analysis (Fig. 3c). The bands centered at 3439, 1628 and 1391 cm⁻¹ can be observed for the spectrum of CFs, which are assigned to the –OH, C=C and C–O functional groups in CFs. There is a strong peak observed at 1391 cm⁻¹ in the spectrum of due to the stretching of C–O in C–OH and C–O–C groups.³⁴ Additionally, the spectra of CF/Ni(OH)₂ and CF/MnO₂/Ni(OH)₂ hybrids have many vibration bands in common as labeled with the dotted lines. The sharp and narrow band at 3645 cm⁻¹ is attributed to ν_O–H stretching vibration of α-Ni(OH)₂.³⁵ The absorption peak centered at 650 cm⁻¹ can be assigned to an in-plane Ni–O stretching vibration, which is the typical absorption peak of Ni(OH)₂.^36,37

Fig. 3 (a) Nitrogen adsorption/desorption isotherm and pore size distribution curve for CF/MnO₂/Ni(OH)₂. (b) XRD patterns and (c) FT-IR spectra of the CFs, CF/MnO₂, CF/Ni(OH)₂ and CF/MnO₂/Ni(OH)₂ samples.

The chemical compositions and metal oxidation states of the composite are investigated by XPS. Fig. 4 presents the XPS spectra of the CF/MnO₂/Ni(OH)₂ hybrid. The XPS full-survey-scan spectrum in Fig. 4a revealed that the CF/MnO₂/Ni(OH)₂ hybrid is primarily composed of C, O, Ni and Mn elements. High-resolution C 1s spectrum shown in Fig. 4b can be deconvoluted into three peaks located at 284.9, 286.5 and 289.1 eV, which are assigned to C–C, C–OH and COOH species, respectively.^38,39 The Mn 2p XPS spectrum presents two distinct characteristic peaks located at 642.3 and 654.1 eV, corresponding to the Mn 2p_3/2 and Mn 2p_1/2 spin–orbit peaks, respectively. And the spin-energy separation of 11.8 eV further confirms the presence of MnO₂ in the hybrid.^40,41 In the Ni 2p XPS spectrum, there are two shakeup satellites (indicated as “Sat”) close to two sharp spin–orbit doublets centered at 855.3 and 873.0 eV, which are related to the Ni 2p_3/2 and Ni 2p_1/2 signals of Ni(OH)₂, respectively (Fig. 4d).⁴² The spin-energy separation between the two Ni 2p peaks is about 17.4 eV, matching well with previously reported values of Ni(OH)₂.^14,43 Additionally, the high resolution spectrum of O 1s (Fig. 4e) could be deconvoluted into one main peak at 531.2 eV from O atoms in Ni–O and Mn–O bonds and another peak at 533.7 eV from O atoms in carboxyl groups.¹⁴ The mass content of Ni(OH)₂ in the CF/MnO₂/Ni(OH)₂ hybrid can be determined by the TG. Fig. S2a† shows the representative TG and DTG curves of CF/MnO₂/Ni(OH)₂ measured in Ar atmosphere. This sample shows a slight weight loss of 4 wt% below 240 °C, which is ascribed to the evaporation of adsorbed water molecules. The evident weight loss at the temperature range of 240–400 °C is attributed to the decomposition of Ni(OH)₂ to NiO.⁴⁴ Based on the residual weight of CF/MnO₂/Ni(OH)₂, the mass content of Ni(OH)₂ in the CF/MnO₂/Ni(OH)₂ hybrid is 77.3 wt%. In Fig. S2b,† the mass content of MnO₂ in the CF/MnO₂ hybrid is 6.6 wt%. Thus, we can draw a conclusion that the mass content of MnO₂ in the CF/MnO₂/Ni(OH)₂ hybrid is 1.5 wt%.

Fig. 4 XPS spectra of the as-prepared CF/MnO₂/Ni(OH)₂ hybrid: (a) survey spectrum, (b) C 1s, (c) Mn 2p, (d) Ni 2p and (e) O 1s.

With the aim of evaluating of this unique architecture electrode for real application, the electrochemical performances were firstly measured using a three-electrode system in 1 M KOH aqueous electrolyte. Fig. 5a shows the typical CV curves of the CF/MnO₂/Ni(OH)₂ electrode ranging from 1 to 20 mV s⁻¹ between a potential window of 0–0.5 V in 1 M KOH aqueous electrolyte. The shape of the CV curves clearly demonstrates the pseudocapacitive characteristic of the electrode, which is distinguishable from those of EDLCs. Specially, a pair of well-defined redox peaks can be clearly observed at 0.3/0.45 V (vs. Hg/HgO) for all the CV curves at various scan rates, which mainly corresponds to the faradic redox reactions of M–O/M–O–OH associated with anion OH⁻,²⁷ where M refers to Ni or Mn. Obviously, the current densities increase with the increase of scan rates from 1 to 20 mV s⁻¹ and the shapes of the CV curves show almost no significant change, implying rapid transport of electrolyte ions, a good electrochemical reversibility and small equivalent series resistance of the CF/MnO₂/Ni(OH)₂ electrode. The position of the redox peaks shifts slightly with the increase of scan rates due to internal resistance of the electrode. In Fig. 5a, the integrated area of the Ni foam is much smaller compared with CF/MnO₂/Ni(OH)₂ at 1 mV s⁻¹, suggesting that the capacitance contribution from the Ni foam is negligible.

Fig. 5 Three-electrode electrochemical measurements of all the electrodes in 6 M KOH aqueous solution: (a) CV curves of the CF/MnO₂/Ni(OH)₂ electrode at various scan rates. (b) Galvanostatic charge–discharge curves of the CF/MnO₂/Ni(OH)₂ electrode at different current densities from 0.5 to 10 A g⁻¹. (c) Galvanostatic charge–discharge curves of CF/MnO₂, CF/Ni(OH)₂ and CF/MnO₂/Ni(OH)₂ electrodes at the current densities of 0.5 A g⁻¹. (d) Variation of specific capacitance of the samples at various current density. (e) Nyquist plot and (f) cycling performance of the CF/MnO₂/Ni(OH)₂ electrode measured at 5 A g⁻¹ over 3000 cycles.

Fig. 5b represents the galvanostatic charge–discharge profiles of the CF/MnO₂/Ni(OH)₂ hybrid electrode between 0 and 0.5 V (vs. Hg/HgO) at various current densities ranging from 0.5 to 10 A g⁻¹. It can be clearly observed that the charge/discharge curves are nonlinear and deviate from the typical triangular shape of EDLCs, indicating the Faradic pseudocapacitance characteristics of the charge storage. There are voltage plateaus at around 0.45 V (vs. Hg/HgO) during charge and 0.30 V (vs. Hg/HgO) during discharge, implying the faradic redox reactions and this result is highly consistent with the peaks observed in the CV curves. Negligible voltage drop of the CF/MnO₂/Ni(OH)₂ hybrid electrode can be found (0.02 V) even at high current density of 10 A g⁻¹, suggesting superior rate capability and low internal resistance of the electrode which is crucial for energy-storage devices. Additionally, the CF/MnO₂/Ni(OH)₂ hybrid electrode shows obvious longer charge–discharge time compared with the CF/MnO₂ and CF/Ni(OH)₂ electrodes at the same current density (Fig. 5c), indicating the largest specific capacitance of the CF/MnO₂/Ni(OH)₂ hybrid electrode. The specific capacitance of all the electrodes at various current densities is calculated according to the galvanostatic charge–discharge curves. Notably, the specific capacitance of the CF/MnO₂/Ni(OH)₂ hybrid electrode is as high as 2079 F g⁻¹ at 0.5 A g⁻¹ (Fig. 5d), much higher than those of CF/MnO₂ (252 F g⁻¹) and CF/Ni(OH)₂ (1418 F g⁻¹) electrodes and highly comparable with the previously reported Ni(OH)₂ based electrodes (Table 1).^{12,27,28,33,42,45–55} Significantly, the CF/MnO₂/Ni(OH)₂ electrode still deliver high specific capacitance of 1233 F g⁻¹ even at a relatively high current density of 20 A g⁻¹, indicating an outstanding rate capability. Furthermore, the electrochemical performance of the CF/MnO₂/Ni(OH)₂ electrode was further investigated by EIS. The Nyquist plot of the electrode presents low equivalent series resistance (0.2 Ω) with negligible charge transfer resistance (Fig. 5e), demonstrating superior interface contact between electrolyte and electrode. In order to further demonstrate the superiority of the synergistic effect of the constructed ternary core–double-shell nanostructure, the cycling performance of the CF/MnO₂/Ni(OH)₂ was evaluated at progressively increased current densities since it is another crucial requirement for energy-storage devices in practical applications. As shown in Fig. 5f, after 3000 times of continuous cycling at 5 A g⁻¹, 90.1% of the initial capacitance is still maintained, highlighting the remarkable long-term stability. The remarkable cycling performance further indicates the robustness of the ternary core–double-shell hierarchical nanostructures and the strong integration between the CF core and MnO₂/Ni(OH)₂ nanosheet shells, which can easily withstand long-term charge/discharge cycling.

Table 1 Summary of electrochemical data of previously reported Ni(OH)₂ based electrodes for comparison

Materials	Electrolyte	Capacitance	Rate capability	Cycle life	Ref.
CNT@Ni(OH)2	1 M KOH	1136 F g^−1 (2 A g^−1)	34% (20 A g^−1)	92% (1000)	45
Ni(OH)2 nanosheet	2 M KOH	2384 F g^−1 (1 A g^−1)	—	75% (3000)	46
Ni3S2@Ni(OH)2/3DGN	3 M KOH	1277 F g^−1 (2 mV s^−1)	56% (20 mV s^−1)	99% (2000)	33
RGO/Ni(OH)2	6 M KOH	1404 F g^−1 (2 A g^−1)	72% (20 A g^−1)	90% (1000)	47
Ni(OH)2/CNF	2 M KOH	2523 F g^−1 (5 mV s^−1, based on the mass of Ni(OH)2)	68% (100 mV s^−1)	83% (1000)	48
		701 F g^−1 (5 mV s^−1, based on the total mass)
Ni(OH)2/UGF	6 M KOH	1560 F g^−1 (0.5 A g^−1)	70% (10 A g^−1)	65% (1000)	49
Ni(OH)2–MnO2	1 M KOH	2628 F g^−1 (3 A g^−1)	51% (20 A g^−1)	—	27
Ni(OH)2/graphene	6 M KOH	2194 F g^−1 (2 mV s^−1)	41% (20 mV s^−1)	—	50
Ni(OH)2/MWCNT film	2 M KOH	1487 F g^−1 (5 mV s^−1)	67% (5 mV s^−1)	96% (1000)	51
RGO/α-Ni(OH)2	1 M KOH	1672 F g^−1 (1 A g^−1)	75% (20 A g^−1)	81% (2000)	12
Ni(OH)2/fibrous carbon fabric	1 M KOH	1416 F g^−1 (1 A g^−1)	71% (10 A g^−1)	62% (10 000)	52
Ni(OH)2/RGO/Ni	1 M KOH	3328 F g^−1 (1.5 A g^−1)	45% (4.2 A g^−1)	95% (3000)	53
Ni(OH)2/CoO/RGO	1 M NaOH	1510 F g^−1 (1 mV s^−1)	43% (20 mV s^−1)	85% (2000)	54
Ni(OH)2–CFG	2 M NaOH	2276 F g^−1 (1 A g^−1)	31% (100 A g^−1)	100% (1000)	55
MnO2/RGO/Ni(OH)2	1 M KOH	3296 F g^−1 (1.3 A g^−1)	∼49% (3.7 A g^−1)	90% (5000)	42
CF/MnO2/Ni(OH)2	1 M KOH	2079 F g^−1 (0.5 A g^−1)	59% (20 A g^−1)	91.3% (3000)	This work

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The outstanding pseudocapacitive performance of the CF/MnO₂/Ni(OH)₂ electrode is mainly attributed to its unique porous core–double-shell structure with the synergistic contribution from ultrathin and porous Ni(OH)₂ nanosheets, ultrathin MnO₂ nanosheets and the conductive CF core. Firstly, the CFs provide a high surface core for the growth of ultrathin MnO₂ and Ni(OH)₂ nanosheets, effectively inhibiting their aggregation and enabling them to be well separated from each other, which could make them fully accessible to electrolyte ions and accelerate electron transfer for the faradic reaction due to the intrinsically excellent electrical conductivity of CFs. This will be beneficial to the enhancement of the electrochemical kinetics and rate capability. Secondly, ultrathin Ni(OH)₂ nanosheets construct a highly porous structure on MnO₂ nanosheets. In such case, although the MnO₂ nanosheets are completely covered by large amount of Ni(OH)₂ nanosheets, the porous structure of MnO₂ could further increase the contact area of the electrolyte-material and enhance ion diffusion, which is of significant importance to high rate capability of supercapacitors. Thirdly, the MnO₂ could also contribute some pseudocapacitance to the overall capacitance of the composite through faradaic redox reactions. Finally, the direct growth of MnO₂ and Ni(OH)₂ nanosheets on CFs could guarantee robust mechanical adhesion and excellent electrical connection, precluding the use of nonconductive polymer binders and additional conducting additives, which may generally give rise to extra contact resistance or weight of electrochemical inactive materials.

To further evaluated the practical application of the as-prepared CF/MnO₂/Ni(OH)₂ hybrid, an all-solid-state ASC device was fabricated with the as-obtained CF/MnO₂/Ni(OH)₂ hybrid as the positive electrode and the CFs as the negative electrode with PVA/KOH gel electrolyte. Fig. 6a shows the CV curves of the positive and negative electrodes at a scan rate of 2 mV s⁻¹ in 6 M KOH aqueous electrolyte. By expressing the total cell voltage as the sum of the potential ranges of the positive and negative electrodes, the as-fabricated all-solid-state ASC could be operated up to 1.6 V. From the CV curves of the all-solid-state ASC at different operation windows in 6 M KOH aqueous electrolyte (Fig. S3a†), it can be found that the stable working voltage windows of the fabricated ASC could be extended up to 1.6 V with no obvious polarization curves as a consequence of perfect combination of the stabilities of the positive/negative electrodes in different potential ranges. Fig. 6b exhibits the CV curves of the optimized all-solid-state ASC at different scan rates ranging from 5 to 200 mV s⁻¹ between 0 and 1.6 V. Notably, the double contribution from electric double-layer capacitance and pseudocapacitance can be clearly confirmed at all scan rates. The calculated specific capacitance as a function of the scan rates is plotted in Fig. 6c (based on the total mass of active materials from two electrodes). The specific capacitance of the all-solid-state ASC device can achieve 190 F g⁻¹ at a scan rate of 2 mV s⁻¹ and remains 85 F g⁻¹ at 100 mV s⁻¹. Fig. S3b† displays the cycling performance of the all-solid-state ASC device measured at a scan rate of 50 mV s⁻¹ up to 3000 cycles. It is worth noting that the specific capacitance could retain 93% of its initial capacitance even after 3000 cycles, indicating outstanding cycling stability of the fabricated ASC device. As well all know, the power and energy densities are two key parameters for supercapacitors in practical application. The Ragone plot of the CFs//CF/MnO₂/Ni(OH)₂ all-solid-state ASC derived from the CV curves are given in Fig. 6d. The all-solid-state ASC device could deliver a high energy density of 67.6 W h kg⁻¹ at a power density of 304.1 W kg⁻¹ and still maintains 30.1 W h kg⁻¹ at a power density of 6.8 kW kg⁻¹. More significantly, the obtained maximum energy density of our fabricated all-solid-state ASC is highly comparable with the recently reported Ni(OH)₂ based ASC devices (Fig. 6d), such as AC//Ni(OH)₂/CNT/NF (50.6 W h kg⁻¹),⁵⁶ AC//nanoporous Ni(OH)₂ (68 W h kg⁻¹),⁵⁷ reduced graphene oxide//CNT/Ni(OH)₂ (35 W h kg⁻¹).⁵⁸

Fig. 6 (a) CV curves of CFs and CF/MnO₂/Ni(OH)₂ electrodes performed in three-electrode cells in 6 M KOH aqueous solution at a scan rate of 2 mV s⁻¹. (b) CV curves and (c) specific capacitance of CFs//CF/MnO₂/Ni(OH)₂ all-solid-state ASC device measured at different scan rates. (d) Ragone plot of the CFs//CF/MnO₂/Ni(OH)₂ all-solid-state ASC device and the previously reported values for Ni(OH)₂ based ASCs, such as AC//Ni(OH)₂/CNT/NF,⁵⁶ AC//nanoporous Ni(OH)₂,⁵⁷ reduced graphene oxide//CNT/Ni(OH)₂,⁵⁸ AC//Ni(OH)₂,⁴³ RGO/CNT//MWCNT/Ni(OH)₂/PEDOT,¹⁴ porous graphene//Ni(OH)₂/graphene,⁵⁹ 3DGN//CNT–Ni(OH)₂.⁴⁵ (e) Galvanostatic charge/discharge curves of a single all-solid-state ASC and two ASCs connected in series. (f) Two assembled all-solid-state ASC devices connected in series to simultaneously light up a white commercial LED indicator.

In order to expand the operation voltage windows, we can connect multiple all-solid-state ASCs in series. Fig. 6e presents the galvanostatic charge/discharge curves of a single device and the two devices connected in series. The operating voltage window of the device connected in series could be extended to 3.2 V with similar discharge times. To highlight that the fabricated ASC could be employed as efficient power sources in practical applications, the two assembled all-solid-state ASC devices connected in series are capable of lighting up a white commercial light-emitting diode (LED) indicator (Fig. 6f), demonstrating that the ASC device holds substantial promise to be used as an energy-storage devices for practical applications.

4. Conclusions

In summary, ternary hierarchical CF/MnO₂/Ni(OH)₂ nanoflake core–double-shell architecture has been synthesized by an electrospinning method combined with hydrothermal approach. Such an architecture is directly used as binder- and conductive agent-free integrate electrodes for supercapacitors, which exhibits superior electrochemical performances, such as ultrahigh specific capacitance (2079 F g⁻¹ at 0.5 A g⁻¹) and excellent rate capability. Additionally, the fabricated all-solid-state ASC device achieves a high energy density of 67.6 W h kg⁻¹ at 304.1 W kg⁻¹. Moreover, two ASC devices in the series could light up a commercial white LED indicator, making the prepared CF/MnO₂/Ni(OH)₂ hybrid a highly promising candidate for high performance energy devices in future practical applications. The rational design concept and synthesis strategy could be easily extended to other metal oxides, which could pave a new way for next-generation energy storage devices with ultrahigh energy densities.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (21471041), Natural Science Foundation of Heilongjiang Province of China (ZD201214).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13902g

‡ Both authors contributed equally to this work.

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