Introduction

In the past decade, supercapacitors (SCs) have been extensively studied and recognized as the most potential candidate for promising and high-quality energy storage system because of their fascinating features which include excellent power density, good rate capability, long cycle life, and safe operation [1,2,3,4,5,6]. Up to now, supercapacitors have been mainly divided into two kinds due to their charge/discharge behavior and/or charge storage mechanisms: electric double-layer capacitors (EDLCs) and the pseudocapacitors [7,8,9]. EDLCs always use high surface area carbon-based materials (graphene, CNTs, porous carbon) which can achieve the charge storage at the interface between electrode and electrolyte. However, the low capacity and energy density of the EDLCs are yet to be improved, hindering the potential application for the high-performance energy storage. In contrast to EDLCs, pseudocapacitors can store the charge via redox reactions, intercalation and/or electrosorption, resulting in much higher electrochemical pseudocapacitance. Pseudocapacitor is mostly based on metal oxides/hydroxides due to their multiple oxidation chemical states which are favorable for larger charge storage in the charge/discharge process.

Up to now, numerous studies have been focused on various metal oxides for high-performance pseudocapacitor electrodes, such as RuO2, Co3O4, NiO, CuO and MnO2 [10,11,12,13,14]. Among these candidates for high-performance electrodes, MnO2 was believed to the most promising candidate for the fabrication of electrodes due to its high theoretical specific capacitance (close to 1400 F g−1), abundant resources, and environmental compatibility [15,16,17,18]. There have been lots of works devoted to explore various structures/morphologies and MnO2-based hybrid/composite materials for high-performance supercapacitors [19,20,21,22,23,24]. Nevertheless, a literature survey has indicated that the state-of-the-art capacitance of pure MnO2 electrodes is still far below the theoretical value. One potential strategy to further improve the capacity is to integrate two or more active materials with high redox electroactivity into one ordered nanostructure. Adopting this idea, more space or active surface/interface of the hybrid/composite structure would be created for the enhanced energy storage and the synergistic effect between different active materials can be utilized. It is noteworthy that NiO could be a promising candidate to integrate with MnO2 because of its excellent reversible redox activity, large theoretical capacitance (around 2573 F g−1), environment friendly nature and natural abundance [25].

In this paper, we reported the preparation of MnO2@NiO nanosheets@nanowires hierarchical structures and their use in supercapacitor electrodes. With the aid of electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM), we found that the NiO nanowires are uniformly deposited on the surface of MnO2 nanosheets to form a hierarchical nanosheet@nanowire structure. With the smart design for this integrated structure, a synergistic effect has been indicated: MnO2 nanosheet arrays provide a robust and porous scaffold for the further deposition of NiO nanowires to avoid the aggregation, while the further deposited NiO nanowires could highly increase the pseudocapacitance. When the MnO2@NiO was used as a binder-free electrode, it exhibits a large specific capacitance (374.6 F g−1 at a current density of 0.25 A g−1) and good cycling stability (92.7% capacitance retention with respect to the initial capacitance before cycling test). The strategy for engineering of MnO2@NiO nanosheets@nanowires hierarchical electrode with high performance can stimulate more interest in the exploration for hierarchical MnO2-based composites for use as multi-functional active materials in various fields.

Materials and methods

Synthesis of MnO2 nanosheets on nickel foam

MnO2 nanosheet on nickel foam was prepared through a one-step hydrothermal reaction. Before the hydrothermal reaction, Ni foam was cleaned in 3 M HCl aqueous solution to remove the surface NiO layer and ultrasound-treated with deionized water (DI water) and ethanol by ultrasonication for 20 min. The treated Ni foam was put into an autoclave followed by adding 30 mL of 0.05 M KMnO4 solution for hydrothermal reaction (the autoclave was kept at 160 °C for 24 h). Upon the cooling of the autoclave, the sample was taken out and cleaned by ultrasonication in DI water and ethanol for 30 min and finally dried in an oven at 80 °C. The loading mass of the MnO2 sheets on Ni foam is calculated to be around 1.8 mg cm−2.

Preparation of MnO2@NiO nanosheets@nanowires hierarchical structures

Before the deposition of NiO nanowires onto MnO2 nanosheets, a solution (35 mL) contains 1 mmol Ni(NO3)2·6H2O and 2 mmol CO(NH2)2 was prepared. And then the MnO2 deposited Ni foam was put into the as-prepared solution and transferred to the autoclave for a further hydrothermal reaction (kept in an oven set at 120 °C for 12 h). The sample was washed with DI water and ethanol and dried in an oven (set at 80 °C). After washing and drying, the sample was annealed at 300 °C for 2 h to get the MnO2@NiO nanosheets@nanowires hierarchical structures on Ni foam (the areal density of the MnO2@NiO composite is around 3.5 mg cm−2).

Characterization and electrochemical measurements

The detailed materials characterization and electrochemical tests of the as-prepared samples are shown in the experimental section in Supporting Information.

Results and discussion

The procedure for fabrication of MnO2@NiO nanosheets@nanowires hierarchical structure is shown in Fig. 1. In the first step, highly uniform MnO2 nanosheets were first grown on Ni foam by a simple hydrothermal reaction using KMnO4 as precursor. The bare Ni foam and MnO2 nanosheets-coated Ni foam can be easily distinguished from the SEM images shown in Figs. S1 and 2a, b. Typically, the bare Ni foam shows a macroporous structure with a smooth surface. The XRD pattern of the Ni foam only shows the XRD peaks from pure Ni after acid-treatment (inset in Fig. S1). After the hydrothermal reaction, a large arrays of MnO2 nanosheets were uniformly coated on the surface of Ni foam (Fig. 2a, b), resulting in a highly porous and interconnected structure, which can facilitate the fast transport of electrolyte ions during the electrochemical reactions. The thickness of the MnO2 nanosheets layer was found to be around 1.6 µm in Fig. S2b. The MnO2 nanosheets is found to be very robust and stable even after an ultrasonication treatment for more than 30 min before the SEM measurement, indicating its potential use for large-scale production. In the second step, NiO nanowires were obtained by a second hydrothermal reaction together with a further calcination process in air. SEM images of the MnO2@NiO nanosheets@nanowires are shown in Fig. 2c, d which indicates a highly uniform and porous structure. The wall of the MnO2 nanosheets was uniformly coated with many NiO nanowires, forming a feather-like structure. This feather-like porous structure was believed to be highly accessible to electrolyte ions and robust for use as electrodes. After deposition of NiO nanowires, the thickness of the MnO2@NiO nanosheets@nanowires composite was increased to around 2.3 µm (Fig. S3), indicating a greater loading mass of the active materials which would be benefit for larger output of the capacitor device.

Figure 1
figure 1

Schematic illustration of the synthesis of MnO2@NiO nanosheets@nanowires hierarchical structure

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Figure 2
figure 2

a, b SEM images of MnO2 sheets on Ni foam. c, d SEM images of MnO2@NiO nanosheets@nanowires hierarchical structure on Ni foam. e, f TEM images of MnO2@NiO nanosheets@nanowires hierarchical structure

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The microstructure of the MnO2@NiO nanosheets@nanowires was further verified using TEM. We first check the HRTEM image of the MnO2 nanosheets, and the result is shown in Fig. S4. It was clearly found that the average thickness of the MnO2 nanosheets is around 5–10 nm. A clear lattice spacing of 0.67 nm shown in Fig. S4b was corresponded to the interplanar spacing of the (001) plane of birnessite-type MnO2. TEM image of the hybrid MnO2@NiO nanosheets@nanowires structure is shown in Fig. 2e in which the wall of MnO2 nanosheets was uniformly coated with tiny and porous NiO nanowires. The diameter of the NiO nanowires was estimated to be around 15–20 nm from the HRTEM image (Fig. 2f). The two lattice spacing of 0.247 and 0.208 nm in the NiO nanowire were corresponded to the (111) and (200) interplanar spacing of bunsenite NiO (JCPDS card No. 47-1049). These TEM results further indicate the core–shell structure of the MnO2@NiO nanosheets@nanowires. The crystal phase of the MnO2 nanosheets and MnO2@NiO nanosheets@nanowires was determined by powder XRD (Fig. 3a). The diffraction peaks of the MnO2 nanosheets were well matched with the standard peaks from birnessite-type MnO2 (JCPDS card No. 80-1098) [26], except the three strong diffraction peaks from the Ni substrate. After the deposition of NiO nanowires, three new emerged peaks at 37.2°, 43.2°, and 62.8° were indexed to the (111), (200), and (220) planes of bunsenite NiO (JCPDS card No. 47-1049) [12]. Figure S5 shows the XRD pattern of the precursor coated on MnO2/Ni substrate (after second hydrothermal reaction but before the calcination process to obtain NiO). Except the peaks from MnO2 and Ni foam, the other diffractions peaks are in good agreement with the standard peaks from a nickel hydroxide carbonate (Ni2(OH)2CO3; JCPDS, No. 35-0501). In addition, the EDS mapping (Fig. 3b, c) of the MnO2@NiO nanosheets@nanowires was also performed to indicate the uniformity and purity (the EDS mapping of MnO2 nanosheets is shown in Fig. S6). The EDS mapping shows homogenous distribution of K, Mn, O, and Ni, further indicating the successful deposition of NiO on the MnO2 nanosheets to form the MnO2@NiO nanosheets@nanowires core–shell structure. N2 adsorption–desorption measurements were performed to study the surface area and porosity of the MnO2@NiO nanosheets@nanowires composite (Fig. S7). The results indicate that the MnO2@NiO has a BET surface area of 47.8 m2 g−1 with a pore volume of 0.21 cm3 g−1. The pore size distribution obtained from the desorption branch by the Barrett–Joyner–Halenda (BJH) method indicates an average pore size of ~ 15.8 nm, suggesting a mesoporous structure.

Figure 3
figure 3

a XRD patterns of MnO2 nanosheets and MnO2@NiO nanosheets@nanowires hierarchical structure on Ni foam. b, c SEM image and EDS mapping of MnO2@NiO nanosheets@nanowires hierarchical structure on Ni foam

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X-ray photoelectron spectroscopy (XPS) was used to further check the surface compositions and chemical state of the MnO2@NiO composite. As shown in Fig. 4a, the core-level Ni2p spectrum shows a multiplet-split Ni2p3/2 peak, a multiplet-split Ni2p1/2 peak and two corresponding shake-up-satellite peaks, suggesting the existence of NiO [27, 28]. The Mn2p spectrum (Fig. 4b) exhibits two main peaks located at 642.4 eV (Mn2p3/2) and 654.2 eV (Mn2p1/2) with a spin-energy separation of 11.8 eV, indicating the chemical state of Mn4+ in MnO2 [29]. In addition, the Mn3s spectrum (Fig. 4c) has two multiplet-split components which can be also used to distinguish Mn oxidation states. The spin-energy splitting (ΔE) of 4.7 eV further suggests the oxidation state of Mn4+ in MnO2@NiO composite [30]. The O1s spectrum shown in Fig. 4d exhibits two main peaks at 529.9 and 531.7 eV which are attributed to metal–oxygen bonds (Ni/Mn–O) and metal hydroxide (Ni/Mn–OH), respectively. The very tiny peak located at around 533.3 eV is due to the small amount of absorbed water on the surface of MnO2@NiO composite. The K2p spectrum is also shown in Fig. S8, which is matching with the EDS result. Based on the SEM, EDS, XRD, TEM and XPS results, the hierarchical MnO2@NiO nanosheets@nanowires composite that tightly connected with the Ni foam collector has been successfully achieved, which is crucial for fabricating highly conductive and high-performance binder-free electrodes.

Figure 4
figure 4

XPS spectra of a Ni2p, b Mn2p, c Mn3s and d O1s for MnO2@NiO nanosheets@nanowires composites

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To evaluate the capacitive property, the MnO2@NiO nanosheets@nanowires on nickel foam substrate was utilized as a binder-free electrode and tested. We first measured the CV curves of the MnO2@NiO electrode at different scan, and the result is shown in Fig. 5a. The asymmetrical redox peaks clearly indicate the pseudocapacitive nature of the MnO2@NiO electrode. It was found that the current changes significantly upon the increase in the scan rates, but the potentials of the reversible redox peaks show slight changes, suggesting an excellent reversibility of the electrodes. Thus, the charge storage mechanism of MnO2@NiO active material is mostly attributed to the pseudocapacitance from reversible redox (Faradaic) reactions. We further measure the charge/discharge response of the electrode, and the result is shown in Fig. 5b. The nonlinear GCD curves further indicate the pseudocapacitive nature of the MnO2@NiO electrode, which is matched with the results we observed in CV test. As calculated at a current density of 0.25 A g−1 from the GCD curve, the MnO2@NiO electrode possesses a large specific capacitance of 374.6 F g−1 (areal capacitance of 1.3 F cm−2). The specific capacitance of MnO2@NiO composite electrode is larger than that of MnO2@Ni foam (308.4 F g−1) and NiO@Ni foam electrodes (257.3 F g−1), suggesting a synergetic effect between these two components (detailed result of the electrochemical test of MnO2@Ni foam and NiO@Ni foam electrodes is shown in Fig. S9). In addition, we have measured the charge/discharge curves at various current densities and extracted the corresponding specific capacitances (Fig. 5c). It is clear that the specific capacitance of the MnO2@NiO electrode shows a retention of 64.8% with a 20-time increase in the current density (up to 5 A g−1), suggesting a good rate performance. The good electrochemical performance was due to the good electrical contact between MnO2@NiO nanosheets@nanowires with the Ni foam collector and also the large surface area of the porous nanowires-on-nanosheets structure. The cycling stability of the hierarchical MnO2@NiO electrode was further evaluated via charge/discharge tests for 5000 cycles. The capacitance retains approximately 92.7% of its initial value after cycling test, suggesting a good cycling performance. In addition, we did not observe any change of the GCD curves after 5000 cycles (inset in Fig. 5d), further indicating the high stability of our hierarchical MnO2@NiO electrode.

Figure 5
figure 5

a Cyclic voltammograms of MnO2@NiO nanosheets@nanowires electrode in 1 M KOH aqueous electrolyte. b Charge/discharge curves at different current densities. c Specific capacitance measured at different current densities. d Cycling performance of the electrode. The inset shows the charge/discharge curves of the last 10 cycles of the electrode

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To show the potential use of the MnO2@NiO nanosheets@nanowires composite as electrode material for high-performance capacitor device, we have fabricated an asymmetric supercapacitor (ASC) device using MnO2@NiO and activated microwave exfoliated graphite oxide (MEGO) as the positive and negative electrodes, respectively. CV curves of the ASC were first recorded at various pre-set voltage windows to explore the optimal potential windows four device (Fig. 6a). It is clear that the ASC could reach an optimal window up to 1.6 V without significant polarization. This large potential window will be benefit for increasing the energy density at a given specific capacitance. We further measured the charge/discharge response of the electrode (Fig. 6b) at several pre-set current densities. All the charge/discharge curves show similar shapes and behaviors, indicating a good rate capability which is crucial for high-power supercapacitors. We have thus calculated the specific capacitance of the total device (considering the whole weight of the cathode and anode (i.e., mass of MnO2@NiO + MEGO)) from the discharge branch. The calculated capacitance of the ASC is 43.4 F g−1 at a pre-set current density (0.25 A g−1). In addition, the specific capacitances of the ASC at a series of current densities are shown in Fig. 6c. The good rate capability can be ascribed to the fast and reversible electrochemical reactions during the charge/discharge process. As is known to all that the high-quality supercapacitors should own both high energy density and power density, we have thus recorded the Ragone plot of the MnO2@NiO//MEGO capacitor (Fig. 6d). The ASC device shows a good gravimetric energy density (15.4 W kg−1) and a large power density (9360 W kg−1), which are superior to those of MnO2-based and NiO-based ASC devices [26, 29, 31,32,33,34]. These fascinating capacitive performances might be due to the hierarchical and porous nanowires-on-nanosheets structure and the synergistic effect of these two components. The results indicate that the as-prepared MnO2@NiO nanosheets@nanowires composite on Ni foam could find use in bind-free electrode materials for high-performance pseudocapacitors.

Figure 6
figure 6

a Cyclic voltammograms of MnO2@NiO//MEGO ASC device measured at different potential windows at a scan rate of 50 mV s−1. b Galvanostatic charge/discharge curves recorded at different current densities. c Specific capacitances as a function of current densities. d Ragone plot of the assembled ASC device. The inset shows that a red-light-emitting diode (LED) was lighted by the charged ASC device

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Conclusions

We have synthesized uniform MnO2@NiO nanosheets@nanowires hierarchical structures and used them as binder-free electrodes for supercapacitors. Various measurements were conducted to investigate the hierarchical core–shell nanostructure, and it was found to be of very high uniformity. With the rational design of the hierarchical and porous structure and the synergistic effect of different components, the as-prepared MnO2@NiO shows a good specific capacitance (374.6 F g−1 at a current density of 0.25 A g−1; areal capacitance of 1.3 F cm−2) with an outstanding cycling performance (a 92.7% capacitance retention after 5000 charge/discharge cycles). In addition, a large-potential-window ASC device with successfully fabricated using MnO2@NiO as cathode material and MEGO as anode material shows promising energy and power densities. Our work here proposed a simple but effective strategy for the low-cost and massive preparation of hierarchical and porous composites for use as high-quality and stable supercapacitor electrodes.