In situ hydrothermal fabrication of a MnO₂@CoMoO₄@Ni nanohybrid electrode and ultrahigh energy density of ASCs

Abstract

It still remains a big challenge to fabricate a high-energy-density supercapacitor (SC). Herein, an in situ hydrothermal method is developed to fabricate the high-performance MnO₂@CoMoO₄@Ni electrode, in which the high capacitance of MnO₂ and the high electrical conductivity of CoMoO₄ nanowires are fully utilized through the closely-contacted core@shell nanostructure. Amazingly, a flexible AC@Ni//MnO₂@CoMoO₄@Ni asymmetric supercapacitor (ASC) has been fabricated, which delivers an ultrahigh energy density (2.63 mW h cm⁻³) at a power density of 4 mW cm⁻³; after being charged for 10 s, the device assembled in series by two ASCs can efficiently power 15 light emitting diodes (LEDs, 5 mm-diameter red) for more than 5 minutes. Moreover, the ASC still retains 91.28% capacitance after 10 000 cycles. We hold that a hybrid nanostructure from a high-energy-density material with a high-electric-conductivity material is a promising strategy to acquire high-performance SCs.

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1. Introduction

In recent years, the increasing demands for power and energy have attracted tremendous attention. Thus it is desirable to explore sustainable and renewable energy sources.^1–6 To develop high-performance electrode materials is crucial for advanced energy storage and conversion devices.^5,7–9 Electrochemical capacitors (ECs) or supercapacitors (SCs) are one class of energy storage devices with a high power density, a cycle stability and a fast charge–discharge rate.^1–4,7,10 These remarkable properties endow SCs with a wide range of applications in hybrid electric vehicles, industry power and military devices.^2,11,12 Compared with batteries, however, the lower energy densities of presently existing SCs limit the actual applications.

Generally, energy density (E) of SC can be improved by maximizing specific capacitance and/or cell voltage. Specifically, E of SC is greatly affected by the operating voltage (V), according to the equation of E = 0.5CV²,^13,14 where C is the specific capacitance of SC. In order to increase voltage window, one of the most efficient ways is to develop asymmetric supercapacitors (ASCs), in which the advantages of two different electrode materials can be fully utilized to greatly increase V and C;^12,15–17 for example, Bi₂O₃//MnO₂,¹⁸ CoO@polypyrrole//active carbon (AC),¹⁹ MnO₂//Fe₂O₃,²⁰ MnO₂//AC,²¹ graphene–Ni(OH)₂//graphene,²² LiMn₂O₄//AC,¹⁶ carbon nanotube//MnO₂,¹⁵ Co₉S₈//Co₃O₄@RuO₂,¹⁴ H–TiO₂@MnO₂//H–TiO₂@C,²³ etc.

Besides, a few of ternary metal oxides have attracted significant interests because of variable oxidation state, high electrical conductivity, high stability and low cost, e.g., Zn₂SnO₄,²⁴ NiCo₂O₄,²⁵ NiMoO₄,²⁶ MnMoO₄,²⁷ CoMoO₄.^7–9,26,27 Compared with those with binary metal oxides, however, the ASCs built with ternary metal oxides have not been studied extensively. Thus, it still remains a big challenge to acquire the high-energy-density ASCs using the ternary metal oxides.

In addition, MnO₂ is one of the most promising electrochemical materials due to its high theoretical specific capacitance (1370 F g⁻¹), environmental friendliness, low cost and natural abundance.^28,29 Because of its poor electrical conductivity (10⁻⁵ to 10⁻⁶ S cm⁻¹), however, the experimentally achieved capacitance for MnO₂ is far lower than its theoretical capacitance.³⁰ It still remains a big challenge to improve the conductivity of MnO₂. In order to solve the problem, the addition of highly conductive materials, including carbon materials,^5,12,17 conducting polymers³¹ and the other binary metal oxides,^6,32 is usually adopted. However, the addition of ternary metal oxides has reported scarcely.²⁴

Herein, a facile, in situ hydrothermal method has been developed to fabricate the MnO₂@CoMoO₄@Ni nanohybrid electrode, in which the high capacitance of MnO₂ and the high conductivity of CoMoO₄ nanowires are fully utilized, by using the closely-contacted MnO₂@CoMoO₄ core@shell nanostructure. Moreover, the as-fabricated AC@Ni//MnO₂@CoMoO₄@Ni ASC delivers an ultrahigh energy density of 2.63 mW h cm⁻³ at a power density of 4 mW cm⁻³, which is higher than the reported ASCs in the exiting references.

2. Experimental

2.1. Electrode fabrication

CoMoO₄@Ni electrode. This electrode was fabricated by an in situ hydrothermal approach. Typically, 1.5 mmol of Co(NO₃)₂·6H₂O was dissolved in 30 mL of distilled water, then the solution was magnetically stirred for 10 min at room temperature. Then, 1.5 mmol of the Na₂MoO₄·2H₂O was added to the solution above. After the mixture was stirred for 30 min, the solution was transferred to a 50 mL Teflon-lined autoclave, and then a piece of clean nickel foam (10 × 40 × 1.0 mm³) was aforehand immersed into the solution. The autoclave was heated at 150 °C for 5 h. After reaction, the autoclave was cooled naturally to room temperature; the sample was taken out, rinsed with distilled water for several times, and dried at 60 °C for 24 h.

MnO₂@CoMoO₄@Ni electrode. This electrode was also fabricated by an in situ hydrothermal approach. Typically, the 40 mL of precursor solution was prepared by mixing 0.1 M Na₂SO₄ and 0.1 M KMnO₄ solutions, and then was transferred to a 50 mL Teflon-lined autoclave, in which the as-prepared CoMoO₄@Ni above was immersed. Then, the autoclave was heated at 180 °C for 2 h. After reaction, the same procedures as above were employed to obtain the electrode.

2.2. ASC fabrication

To assemble ASC, an activated carbon (AC)@Ni was used as the negative electrode, and the MnO₂@CoMoO₄@Ni (or CoMoO₄@Ni) as the positive electrode.

The AC@Ni electrode was fabricated by a coating method. Typically, 80 wt% AC was mixed with 20 wt% PVA to form an homogeneous mixture, which was then cast onto a 1 × 4 cm² nickel foam. The AC@Ni negative electrode was combined with the MnO₂@CoMoO₄@Ni positive electrode (or CoMoO₄@Ni) to assemble the full cell, using 1 M KOH as the electrolyte. The mass ratio of positive and negative electrodes is based on the charge balance theory (Q₊ = Q₋). The specific capacitance, energy and power densities of the ASC were all calculated on base of the total mass of both negative and positive electrodes.

2.3. Characterization

The morphologies of the samples were characterized by scanning electron microscope (SEM, Hitachi, SU-1510) using an accelerating voltage of 15 kV. The fine surface structures of the samples were characterized by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F), equipped an electron diffraction (ED) attachment with an acceleration voltage of 200 kV and an energy dispersive X-ray spectrometer (EDS). The crystal phases of the samples were characterized by X-ray diffraction (XRD, Rigaku D/max-2550VB) with graphite monochromatized Cu Kα radiation (λ = 0.154 nm). X-ray photoelectron spectroscopy (XPS) measurements were done on a VG ESCALAB MKII XPS system with Mg Kα source and a charge neutralizer. Surface area was calculated by the Brunauer–Emmett–Teller (BET) method.

2.4. Electrochemical measurements

All the electrochemical measurements were carried out on a CHI 660D electrochemical working station at room temperature. A standard three-electrode cell was used, in which a 1 M KOH aqueous solution was used as electrolyte, the CoMoO₄@Ni (∼1 cm² area, ∼5 mg of CoMoO₄) or MnO₂@CoMoO₄@Ni (∼1 cm² area, ∼6 mg of MnO₂@CoMoO₄) was directly used as the working electrode, a Pt wire and a standard saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) and chronopotentiometry (CP) were conducted in a potential range of −0.1 to 0.4 V (vs. SCE). Electrochemical impedance spectroscopy (EIS) was performed from 0.1 Hz to 100 kHz at an open circuit potential with an alternating current (AC) voltage amplitude of 5 mV. Areal and specific capacitances were calculated using eqn (1) and (2) as follows.

C_a = It/(ΔVS) (1)

C_sp = It/(ΔVm) (2)

where I is the constant discharge current, t is the discharging time, ΔV is the voltage drop upon discharging (excluding the IR drop), m is the mass of the active material, and S is the geometrical area of the electrode.

3. Results and discussion

3.1. MnO₂@CoMoO₄ core@shell nanostructure

Fig. 1a shows the charge storage and conversion mechanism, and Fig. 1b shows the typical preparation procedure of MnO₂@CoMoO₄@Ni. The first step is to grow the high-conductivity CoMoO₄ nanowires on nickel foam collector (CoMoO₄@Ni); and the second is to in situ coat MnO₂ nanoparticles on CoMoO₄@Ni. Fig. S1 (seeing ESI†) shows the typical X-ray diffraction (XRD) of CoMoO₄ nanowires and MnO₂@CoMoO₄ hybrid nanostructures, which are ripped off from nickel foam. All the diffraction peaks of the sample are in good agreement with the standard cards (CoMoO₄ JPCDS: 21-0868 and MnO₂ JPCDS: 53-0633).

Fig. 1 (a) Schematic charge storage and conversion mechanism; (b) illustration of CoMoO₄ nanowires and MnO₂@CoMoO₄ hybrid nanostructures.

To further gain the information on the chemical state and composition of MnO₂@CoMoO₄, we resort to X-ray photoelectron spectroscopy (XPS) measurement. The survey XPS spectrum of the sample demonstrates the presence of Mo, Co, Mn, C and O elements (Fig. 2a). The Co 2p XPS spectrum (Fig. 2b) presents two major peaks at a low combining energy of 780.83 eV and a high combining energy of 795.83 eV with a spin energy of 15 eV, corresponding to the Co 2p_3/2 and Co 2p_1/2 of CoMoO₄, respectively. The results agree well with the reported results.^33,34 The spin–orbit splitting value of Co 2p_3/2 and Co 2p_1/2 suggests the coexistence of Co²⁺ and Co³⁺.^35,36 In the Mo 3d XPS spectrum, we notice two peaks at 232.43 eV and 235.63 eV (Fig. 2c), corresponding to Mo 3d_5/2 and Mo 3d_3/2 of CoMoO₄, respectively. It reveals the existence of Mo⁶⁺.^37,38 The O 1s peaks at 530.28 eV and 532.03 eV (Fig. 2d) indicate the existence of O²⁻.¹⁸ The Mn 2p XPS spectrum (Fig. 2e) shows two obvious peaks at around 642.78 eV and 654.73 eV, corresponding to Mn 2p_3/2 and Mn 2p_1/2, respectively. Moreover, the spin energy (11.95 eV) of Mn 2p XPS peaks is well consistent with the previous reports on MnO₂,^36,39 confirming the existence of Mn⁴⁺.^30,36,39

Fig. 2 Typical XPS spectra of MnO₂@CoMoO₄ hybrid nanostructures: (a) survey spectra; (b) Co 2p; (c) Mo 3d; (d) O 1s; (e) Mn 2p.

Fig. 3 shows the scanning electron microscopy (SEM) images of CoMoO₄ nanowires and MnO₂@CoMoO₄ hybrids, which are ripped off from nickel foam. The CoMoO₄ nanowires have the length of ca. 5 μm and the diameters of 200–500 nm with the aspect ratios of 10–25 (Fig. 3a). The MnO₂@CoMoO₄ hybrids are shown in Fig. 3b and c. It is obvious that the MnO₂ nanoparticles (<50 nm) have densely grown on the CoMoO₄ nanowires. Further, Fig. 4 presents the high-resolution transmission electron microscopy (HRTEM) images of MnO₂@CoMoO₄ sample. Obviously, the typical core–shell hybrid structure has formed (Fig. S2, seeing ESI†), in which every CoMoO₄ nanowires have been closely coated by numerous uniform MnO₂ nanoparticles (Fig. 4a). The lattice spacing of 0.22 nm in Fig. 4b and c corresponds to the (300) crystal planes of CoMoO₄, indicating a high crystallinity of CoMoO₄; whereas no lattice spacing of MnO₂ can be identified, indicating the formation of amorphous MnO₂. The energy dispersive X-ray spectrometry (EDS) mapping analysis (Fig. 4d) confirms the presence of Co, Mo, O Mn, K and Cu. The Cu and K signals come from Cu grid and the inserted ones in the interlayer of MnO₂ during the preparation process, respectively.^6,40 As shown in Fig. S3 (seeing ESI†), the N₂ sorption isotherms show that the MnO₂@CoMoO₄ sample has a higher BET area (38.2 m² g⁻¹) than the CoMoO₄ nanowires (7.0 m² g⁻¹), which is mainly attributed to the formation of MnO₂ nanoparticles. The large BET area is advantageous to accumulate more charges and increase the charge/discharge rate.

Fig. 3 SEM images of CoMoO₄ nanowires and MnO₂@CoMoO₄ hybrid nanostructures: (a) CoMoO₄; (b and c) MnO₂@CoMoO₄.

Fig. 4 HRTEM images and EDS spectrum of MnO₂@CoMoO₄ hybrid nanostructures ripped off from nickel foam: (a) TEM; (b and c) lattice fringe images; (d) EDS.

3.2. MnO₂@CoMoO₄@Ni electrodes

Cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) were measured to investigate the electrochemical properties of both CoMoO₄@Ni and MnO₂@CoMoO₄@Ni electrodes. In a conventional three-electrode system, Pt wire and a standard saturated calomel electrode (SCE) were used as a counter electrode and a reference electrode, respectively.

The CV curves were recorded in 1 M KOH aqueous solution. Fig. 5a presents the CV curves of both electrodes at a scanning rate of 2 mV s⁻¹, and Fig. S4 (seeing ESI†) presents the CV curve of Ni foam. Compared with CoMoO₄@Ni, the MnO₂@CoMoO₄@Ni electrode has an obviously higher capacitance on base of integral areas. For MnO₂@CoMoO₄@Ni electrode, a pair of redox peaks can be obviously observed in the CV curves, which correspond to the anodic and catholic peaks, respectively; namely, the anodic peak at about 0.35 V is relative to the oxidation process and the catholic one at 0.18 V is attributed to the reduction process. The results suggest a typical pseudo-capacitive characteristic. The proximately rectangular CV shape should result from MnO₂. In KOH aqueous solution, the K⁺ ions can be adsorbed by MnO₂ shell, which could be expressed as eqn (3) as follows.^6,24,30

MnO₂ + K⁺ + e⁻ ↔ (MnO₂⁻)(K⁺) (3)

Fig. 5 Electrochemistry properties of MnO₂@CoMoO₄@Ni and CoMoO₄@Ni electrodes: (a) cyclic voltammetry (CV); (b) galvanostatic charge–discharge curves; (c) specific capacitances and areal capacitances at different current densities; (d) cycle performances; (e) the 1st and the 2000th cycles for the MnO₂@CoMoO₄@Ni; (f) Nyquist plot of the EIS spectra.

The CoMoO₄ core also has a typical pseudo-capacitive behavior according to the faradic reactions ((4) and (5)) as follows.^7,27

3[Co(OH)₃]⁻ ↔ Co₃O₄ + 4H₂O + OH⁻ + 2e⁻ (4)

Co₃O₄ + H₂O + OH⁻ ↔ 3CoOOH + e⁻ (5)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (6)

These quasi-reversible electron transfer processes involve in the redox reaction of Co²⁺/Co³⁺, which is associated with OH⁻ anions in the alkaline electrolyte.²⁷ The result is in good agreement with the reports on CoMoO₄.⁷ These features are very different from those of electric double-layer capacitors with an ideal rectangular shape. Furthermore, the MnO₂@CoMoO₄@Ni electrode shows a better capacitive behavior and a higher rate capability at different scanning rates, compared with CoMoO₄@Ni one (Fig. S5a and b, seeing ESI†).

Fig. 5b displays the galvanostatic charge–discharge curves of both electrodes at a current density of 1 mA cm⁻². The galvanostatic charge–discharge curves also demonstrate a faradaic process. In comparison, the MnO₂@CoMoO₄@Ni electrode has a much longer discharging time than CoMoO₄@Ni one. Their galvanostatic charge–discharge measurements were also carried out at various current densities ranging from 1 to 50 mA cm⁻² (Fig. S5c and d, seeing ESI†). Compared with CoMoO₄@Ni electrode, the charge–discharge curves of MnO₂@CoMoO₄@Ni are approximately symmetric, indicating that the latter one has a superior reversibility and a higher coulombic efficiency; and the CoMoO₄@Ni electrode has a high potential drop, because the CoMoO₄@Ni electrode has a large internal resistance (Fig. 5f and S6, seeing ESI†).

Fig. 5c shows the specific capacitances and areal capacitances of both electrodes at various current densities. The MnO₂@CoMoO₄@Ni electrode has areal capacitances of 6.968, 4.404, 3.294, 2.612, and 1.620 F cm⁻² at of 2, 5, 10, 20 and 50 mA cm⁻², respectively; and the MnO₂@CoMoO₄@Ni displays an areal discharge capacitance of 8.056 F cm⁻² at 1 mA cm⁻², which is 21.6 times higher than that (0.356 F cm⁻²) of CoMoO₄@Ni. It is obvious that the areal capacitance of MnO₂@CoMoO₄@Ni is greatly higher than that of CoMoO₄@Ni. It is found that our results are much better than those of the other electrodes in the existing reports, such as CoO@PPy (4.43 F cm⁻² at 1 mA cm⁻²),¹⁹ Co_xNi_1−x(OH)₂/NiCo₂S₄ (2.86 F cm⁻² at 4 mA cm⁻²),³⁶ Co₃O₄@MnO₂ (∼0.75 F cm⁻² at 3 mA cm⁻²),⁶ Co_0.67Ni_0.33 DHs/NiCo₂O₄ (∼1.64 F cm⁻² at 2 mA cm⁻²).²⁵ Furthermore, a high specific capacitance of 734 F g⁻¹ is achieved at 1 A g⁻¹ for the MnO₂@CoMoO₄@Ni electrode (calculated by total mass of MnO₂@CoMoO₄), 68.2 times higher than that (10.6 F g⁻¹) of CoMoO₄@Ni. The specific capacitance of MnO₂@CoMoO₄@Ni electrode is also much higher than those of the other reports, for example, Zn₂SnO₄/MnO₂ (642.4 F g⁻¹ at 1 A g⁻¹).²⁴ Herein, we hold that two important factors are responsible for the improved specific and areal capacitances as follows: first, the shell materials have a high energy density (e.g., MnO₂, Co₃O₄, Fe₂O₃, NiO, etc.), and the core materials have a high electrical conductivity (e.g., ZnCo₂O₄, Zn₂SnO₄, CoMoO₄, NiCo₂O₄, etc.); second, an efficient ion passageway is needed, to which the ions of electrolyte solution can freely approach.

The cycle lifetime is another important requirement. The cycle lifetimes of CoMoO₄@Ni and MnO₂@CoMoO₄@Ni were investigated at 10 and 20 mA cm⁻² in the potential window of −0.1 to 0.4 V, respectively (Fig. 5d). After 2000 cycles, the CoMoO₄@Ni shows a poor cycle stability (∼20% capacitance loss) at 10 mA cm⁻². However, the MnO₂@CoMoO₄@Ni electrode still retains nearly 100% capacity at a high current density of 20 mA cm⁻². The high cycle stability of MnO₂@CoMoO₄@Ni is also proved by the almost overlapped charge–discharge curves at the first and the 2000th cycles (Fig. 5e). All results demonstrate the high cycle lifetime and Coulomb efficiency (CE) of MnO₂@CoMoO₄@Ni electrode.

Electrochemical impedance spectroscopy (EIS) is an important parameter of SCs. Fig. 5f shows the Nyquist plots of CoMoO₄@Ni and MnO₂@CoMoO₄@Ni electrodes. The EIS data is further fitted to a simulated equivalent electric circuit (the inset of Fig. S6, seeing ESI†), which consists of an electrolyte solution resistance (R_s), a constant phase element (CPE) to account for the electrical double-layer capacitance, a charge transfer resistance (R_ct) and a pseudocapacitive element (C_p) from the redox processes. The R_s values are measured to be 2.71 Ω and 2.78 Ω for CoMoO₄@Ni and MnO₂@CoMoO₄@Ni, respectively. However, the R_ct values of CoMoO₄@Ni and MnO₂@CoMoO₄@Ni electrodes are calculated to be 2.84 Ω and 0.52 Ω, respectively. It is well known that Faraday resistance is a crucial limiting factor for the power of SC.^1,24 Hence, the results firmly demonstrate that a low Faraday resistance leads to a high areal power of our MnO₂@CoMoO₄@Ni electrode.

3.3. ASC

In order to evaluate the application performances of MnO₂@CoMoO₄@Ni electrode, an AC@Ni//MnO₂@CoMoO₄@Ni ASC has been fabricated by using MnO₂@CoMoO₄@Ni as the cathode, AC@Ni as the anode (charge storing mechanism: C + KOHCK⁺ + OH⁻ (ref. 41)), 1 M KOH solution as the electrolyte, and one piece of cellulose paper as the separator (Fig. 6a and b). The areal of our ASC is determined to be 4 cm². For AC@Ni, 10 mg of AC was employed so as to keep charge balance for two electrodes.^6,7 The mass ratio of cathode and anode is accorded to the charge balance theory (Q₊ = Q₋), which are dependent on the specific capacitance (C), the potential range (ΔV) and the mass (m) of active material on the electrodes, namely, Q = Cm. In order to ensure Q₊ = Q₋, the mass balance of active materials follows eqn (7):

Fig. 6 Electrochemical performances of AC@Ni//MnO₂@CoMoO₄@Ni ASC: (a and b) configurations; (c) CV curves; (d) galvanostatic discharge curves at different current densities; (e) photograph of 15 light-emitting diodes (LEDs) powered by ASC; (f) energy density vs. power density; (g) cycle stability after 10 000 cycles.

Fig. 6c presents the CV curves of AC@Ni//MnO₂@CoMoO₄@Ni ASC at various scanning rates (2, 5, 10 20, 50, 100 mV s⁻¹), and Fig. S7 (seeing ESI†) shows the CV curves of positive and negative electrodes, which are tested in a three-electrode cell. From the CV curve of the MnO₂@CoMoO₄@Ni electrode, a typical faradic pseudocapacitive behavior (battery type) is observed;^12,22,42 however, the CV curve of AC@Ni electrode presents an electrochemical double layer behavior (capacitor type).^12,22,42 The as-fabricated ASCs can make full use of both capacitive behaviors to enhance the operation voltage window. The presented redox peaks demonstrate a typical pseudocapacitive behavior of ASC. The operating cell voltage (1.6 V) is much higher than those (0.8–1.0 V) of conventional AC-based symmetric capacitors. It should noted that water splitting can be avoided, although aqueous solution has a narrow theoretical operation voltage window (∼1.23 V versus reversible hydrogen electrode (RHE)).⁴² This is because an overpotential exist over the CoMoO₄ nanowires and MnO₂ nanoparticles.

Fig. 6d and S9a (seeing ESI†) illustrates the typical galvanostatic discharge curves of our AC@Ni//MnO₂@CoMoO₄@Ni ASC at various current densities (1, 2, 5, 10, 20, 50, 100, 200 mA cm⁻³). Fig. S9b (seeing ESI†) shows that the Coulombic efficiency of ASC first declines, and then increases due to an “activation process”. After being charged for 10 seconds, one ASC device can efficiently power one red round light-emitting diode (LED, 3 mm in diameter) for more than 3 minutes (seeing Fig. S8, ESI†). When two ASCs are assembled in series, the device can efficiently power 15 red round LEDs (5 mm in diameter) for more than 5 minutes after being charged for 10 s (Fig. 6e); after that, this device can further power one 3 mm-diameter LED for more than 7 minutes. The results demonstrate an excellent application performance of ASC.

The Ragone plots of AC@Ni//MnO₂@CoMoO₄@Ni ASC describe the relationship between volumetric energy density and volumetric power density (Fig. 6f and 7). The volumetric energy density (E) and power density (P) are calculated using eqn (8) and (9) as follows:

E = ∫(IΔv(t)dt)/(m) or E = ∫(IΔv(t)dt)/(V) (8)

P = E/Δt (9)

where I is the discharging current, Δv(t) is discharging voltage excluding the IR drop, dt is time differential, m is the total mass of active materials in two electrodes, V is the volume of device (1 × 4 × 0.2 cm³), and Δt is the discharging time. Surprisingly, our AC@Ni//MnO₂@CoMoO₄@Ni ASC displays a high volumetric energy density of 2.63 mW h cm⁻³ at a volumetric power density of 4 mW cm⁻³. Even at a high volumetric power density of 400 mW cm⁻³, a volumetric energy density of 0.88 mW h cm⁻³ is still achieved. Furthermore, Fig. S10 (seeing ESI†) presents that a high energy density of 30.94 W h kg⁻¹ is achieved at a power density of 47.06 W kg⁻¹; and a high energy density of 10.33 W h kg⁻¹ is achieved at a power density of 4707.34 W kg⁻¹. It is noted that our ASC has an obviously higher volumetric energy density than the reports by the others, for example, graphene-based SC (0.06 mW h cm⁻³ at 0.2 W cm⁻³, PVA/H₃PO₄),⁴³ TiN SC (0.05 mW h cm⁻³ at 0.1 W cm⁻³, PVA/KOH),⁴⁴ H–TiO₂@MnO₂//H–TiO₂@C-based ASCs (0.3 mW h cm⁻³ at 0.2 W cm⁻³, 5 M LiCl),²³ MnO₂ NWs//Fe₂O₃ NTs (0.55 mW h cm⁻³ at 0.02 W cm⁻³, PVA/LiCl),²⁰ Co₉S₈ nanorod//Co₃O₄@RuO₂ nanosheet array ASC (1.44 mW h cm⁻³ at 0.89 W cm⁻³, PVA/KOH).^14,45

Fig. 7 Specific capacitance and volumetric capacitance of AC@Ni//MnO₂@CoMoO₄@Ni ASC.

After 10 000 cycles, amazingly, an outstandingly long lifetime is reached for our ASC (Fig. 6g), which has been rarely reported for the aqueous electrolyte-based ASC. In the 10 000 cycles, the first and the last ten cycles of ASC are also provided in Fig. S11 (seeing ESI†). After 10 000 cycles, 91.28% of areal capacitance is still remained, which demonstrates a high cycle stability of our ASC.

4. Conclusions

The high-performance MnO₂@CoMoO₄@Ni electrode can be fabricated by an in situ hydrothermal method, in which the high capacitance of MnO₂ and the high electrical conductivity of CoMoO₄ nanowires are fully utilized due to the closely-contacted core@shell nanostructure. Amazingly, the AC@Ni//MnO₂@CoMoO₄@Ni ASC can deliver a high energy density (2.63 mW h cm⁻³) at a power density (4 mW h cm⁻³) and present an outstandingly long cycle stability after 10 000 cycles. The ASC is expected to be the next-generation energy storage device.

Acknowledgements

This work is financially supported by National Science Foundation of China (21377060), Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry (20121707), Six Talent Climax Foundation of Jiangsu (20100292), the Key Project of Environmental Protection Program of Jiangsu (2013005), the Project Funded by the Science and Technology Infrastructure Program of Jiangsu (BM201380277), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) sponsored by SRF for ROCS, SEM (2013S002).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05790j

‡ The first and second authors equally contribute to this study.

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