Introduction

As energy storage devices with properties intermediate to those of batteries and electrostatic capacitors, electrochemical supercapacitors exhibit the desirable properties of high power density (ten times more than batteries), fast charging (with seconds), excellent cycling stability, small size and low mass, which make them one of the most promising candidates for next-generation power devices1,2,3,4,5,6,7. With characteristics complementary to those of rechargeable batteries and fuel cells, supercapacitors have been used in many applications, such as power back-up, pacemakers, air bags and electrical vehicles8. Currently, most commercial supercapacitors are made of high-surface-area carbonaceous materials2. However, these supercapacitors might not provide sufficient energy/power densities or efficiencies, and the specific capacitance severely decreases under high current. Therefore, much attention has been focused on improving the capacitance of supercapacitors, and their performance at high current densities, by constructing complex heterostructures with increased surface areas.

Three-dimensional (3D) hierarchical heterostructures with high surface/body ratios, large surface areas, better permeabilities and more surface active sites have potential in optical, catalytic and electrochemical applications9,10,11,12,13,14,15. Thus, the synthesis and application of 3D hierarchical heterostructures will be a focus of the nanomaterial research field16,17,18,19. Mai et al. have synthesized ultra-long hierarchical vanadium oxide nanowires by electrospinning, which exhibit much higher capacity in lithium ion batteries20. Meng et al. synthesized 3D Bi2WO6/TiO2 to enhance photocatalytic activity and found that the heterostructures exhibited much higher photodegradation efficiency than Bi2WO6 and TiO2 (ref. 19).

The molybdates MnMoO4 and CoMoO4 have excellent catalytic and electrochemical characteristics21,22,23. In this paper, hierarchical MnMoO4/CoMoO4 heterostructured nanowires were controllably synthesized using a facile refluxing method under mild conditions. MnMoO4 nanowires form the backbone material, and the CoMoO4 shell thickness could be tuned by changing the molar ratio of Mn source and Co source. The superior electrochemical performance of the structure was demonstrated.

Results

Characterization of MnMoO4/CoMoO4 heterostructured nanowires

To determine the phase structures of the products, the X-ray diffraction (XRD) measurements were conducted. The backbone material MnMoO4 nanowires (MnMoO4: JCPDS card No. 01-072-0285; MnMoO4·0.9H2O: JCPDS card No. 00-050-1286) were identified by the XRD pattern shown in Figure 1a (black line). The hierarchical MnMoO4/CoMoO4 heterostructured nanowires (open circles, open squares: JCPDS card No.00-021-0868) were identified in Figure 1a (red line). Because MnMoO4 (No.01-084-2102: a=10.469 Å, b=9.516 Å, c=7.143 Å) has a similar crystal lattice parameter as CoMoO4 (No.00-021-0868: a=10.210 Å, b=9.268 Å, c=7.022 Å), lattice matching between MnMoO4 and CoMoO4 can exist. Raman spectroscopy was applied to further investigate the structural change in bonding related to the hierarchical heterostructure (Fig. 1b). For those structures, the strong bands observed between 200 and 1,200 cm−1 can be assigned to bend and stretching frequencies of those materials. The band at 872 cm−1 in hierarchical MnMoO4/CoMoO4 heterostructured nanowires was not found in pure CoMoO4 nanorods or backbone MnMoO4 nanowires, which may be assigned to the new stretching vibration of Co–O–Mn.

Figure 1: Phase analysis.
figure 1

(a) XRD patterns of backbone MnMoO4 nanowires (black line) and hierarchical MnMoO4/CoMoO4 heterostructured nanowires (red line). (b) Raman spectra of hierarchical MnMoO4/CoMoO4 heterostructured nanowires, pure CoMoO4 nanorods and backbone MnMoO4 nanowires.

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Figure 2a–d shows the scanning electron microscopy (SEM) images of the MnMoO4 nanowires and obtained MnMoO4/CoMoO4 heterostructured nanowires. In Figure 2a, pure MnMoO4 nanowires with the length of 10 μm and the diameter of 500 nm were used as the backbone material. The hierarchical heterostructures of the MnMoO4/CoMoO4 sample are shown in Figure 2b–e. With the extension of reaction time, CoMoO4 nanorods with higher density grew aslant on the primary MnMoO4 nanowires (Supplementary Fig. S1). Further information about MnMoO4/CoMoO4 hierarchical heterostructure was obtained from transmission electron microscopy (TEM) (Fig. 2d). It is confirmed that CoMoO4 nanorods have diameters of about 50 nm. Close inspection at the heterojunction shows that the CoMoO4 nanorods have their roots inside the MnMoO4 nanowires, suggesting that the CoMoO4 nanorods are not just loosely attached to the nanowire surface. The high-resolution TEM investigation (Fig. 2e) demonstrates that the interlayer distances are consistent with the interplanar distances of CoMoO4 (0.335 nm) and MnMoO4 (0.288 nm). These results suggest that the prepared sample is considered to be a well-crystallized heterostructure with MnMoO4/CoMoO4 on nanoscale. Energy dispersive spectroscopy (EDS) microanalysis on selected areas was shown in Figure 2f. The 'shell' of the heterostructures consists of Co, Mo and O (the inset of Fig. 2f) elements, whereas the entire heterostructures consist of Mn, Co, Mo and O (Fig. 2f), indicating that the nanorods grown on the surface are mainly made of CoMoO4 and the parent hierarchical heterostructures are a mixture of MnMoO4 and CoMoO4, which agrees well with the results of XRD analysis. A MnMoO4/CoMoO4 nanocomposite without 3D hierarchical heterostructures was also synthesized by a simple refluxing method, as a control experiment (Supplementary Fig. S2).

Figure 2: Morphology characterization.
figure 2

(a) SEM image of the backbone MnMoO4 nanowires. (b, c) SEM images of hierarchical MnMoO4/CoMoO4 heterostructured nanowires. (d, e) TEM and high-resolution TEM images at the heterojunction of hierarchical MnMoO4/CoMoO4 heterostructured nanowires. (f) The EDS microanalysis on selected areas. Scale bars, 1 μm (a–c); 20 nm (d); 5 nm (e).

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Crystal growth mechanism

The crystal growth mechanism of 'self-assembly' and 'oriented attachment' were proposed to demonstrate the complicated nano-architecture process. The oriented attachment mechanism describes the spontaneous self-organization of adjacent particles, so that they share a common crystallographic orientation, followed by the joining of these particles at a planar interface. The process is particularly relevant in the nanocrystalline regime, where bonding between the particles reduces overall energy by removing surface energy associated with unsatisfied bonds24. In the reaction, MnMoO4 nanowires were used as the 'substrate', which can guide the CoMoO4 self-assembling growth in aqueous solution without surfactant and stabilizers. Then the 'oriented attachment' can guide the nanoparticles oriented growth. As shown in Figure 3, supersaturated solution with plenty of CoMoO4 small crystals were formed by adding Co resource. Because of the high surface energy and thermodynamics instability, nanoparticles can attach to the surface of MnMoO4 to decrease surface energy. The crystallographic orientation of the particles with respect to each other is determined by the minimization of the highest surface energy. Therefore, with a matching lattice, the lattice fringes' orientation and crystal growth direction are uniform to some extent. The 'substrate', MnMoO4, has similar lattice parameters and can control CoMoO4 nanoparticle self-assembly and oriented crystallization to form hierarchical MnMoO4/CoMoO4 heterostructured nanowires.

Figure 3: The construction of hierarchical MnMoO4/CoMoO4 nanowires.
figure 3

The green rod represents the backbone MnMoO4 nanowire, and orange rods the CoMoO4 nanorods. Red and blue balls are different ions dispersed in the aqueous solution.

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Electrochemical properties

Figure 4a,b displays the galvanostatic charge–discharge curves of hierarchical MnMoO4/CoMoO4 heterostructured nanowires and MnMoO4/CoMoO4 nanocomposite (No 3D), MnMoO4 nanowires, CoMoO4 in 2 M NaOH at room temperature. The specific capacitance has been calculated from equation (1):

Figure 4: Electrochemical characterizations in 2 M NaOH aqueous solution at room temperature.
figure 4

(a) Galvanostatic charge–discharge curves of hierarchical MnMoO4/CoMoO4 heterostructured nanowire electrodes at different current density. (b) Galvanostatic charge–discharge curves of MnMoO4, CoMoO4, MnMoO4/CoMoO4nanocomposite (No 3D), MnMoO4/CoMoO4 (3D) electrodes at current density of 1 A g−1. (c) Specific capacitance and energy density of different electrodes at different current density. (d) Charge–discharge cycling test of MnMoO4/CoMoO4 (3D) electrodes at the current density of 3 and 20 A g−1, showing 2% loss after 1,000 cycles; inset shows the galvanostatic charge–discharge cyclic curves of the first and last five cycles at 3 A g−1. (e) Cyclic voltammogram curves of MnMoO4/CoMoO4 (3D) electrodes. (f) AC impedance plots of MnMoO4/CoMoO4 electrodes.

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Where i (A) is the current density used for charge/discharge, Δt(s) is the time elapsed for the discharge cycle, m (g) is the weight of the active electrode and ΔV (V) is the voltage interval of the discharge. The specific capacitance and energy density calculated from each discharge curve are shown in Figure 4c. The hierarchical MnMoO4/CoMoO4 electrodes can reach to 204.1, 187.1, 163.4, 134.7 F g−1 and 28.4, 26.0, 22.7, 18.7 Wh kg−1 at 0.5, 1, 2, 3 A g−1. The capacitance for hierarchical MnMoO4/CoMoO4 heterostructured nanowires is significantly higher than that for pure one-dimensional nanorods (MnMoO4: 9.7 F g−1, 8.8 Wh kg−1; CoMoO4: 62.8 F g−1, 1.4 Wh kg−1; MnMoO4/CoMoO4 nanocomposite: 69.2 F g−1, 9.6 Wh kg−1 at a charge–discharge current density of 1 A g−1). The hierarchical MnMoO4/CoMoO4 heterostructured nanowire electrode exhibited good reversibility with cycling efficiency of 98% after 1,000 cycles shown in Figure 4d.

As reported, at low current densities, due to the low ohmic drop, the inner active sites or the pores of the electrode can be fully accessed; hence, high specific capacitance values can be achieved25. However, high current densities are of great importance in industrial applications. The measuring conditions of hierarchical MnMoO4/CoMoO4 heterostructured nanowires were implemented at higher current densities. The result reveals that the observed specific capacitance values 204.1 F g−1 for the hierarchical MnMoO4/CoMoO4 electrodes are higher than those reported, such as MoO2 (ref. 3), MnO2 (ref. 26) and Co3O4 (ref. 27), at similar current densities to a certain extent.

Discussion

To explain the electrochemical results, it is important to consider the hierarchical MnMoO4/CoMoO4 heterostructured nanowires that exert such a significant influence on charge storage. This consideration is closely related to the Brunauer–Emmet–Teller (BET) surface area (Supplementary Fig. S3). The BET surface area of MnMoO4 is 3.17 m2 g−1, MnMoO4/CoMoO4 nanocomposite is 28.0 m2 g−1 and MnMoO4/CoMoO4 (3D) after surface modification can reach to 54.06 m2 g−1, which is larger than that reported in the literature (MnMoO4 nanorods 15 m2 g−1 and CoMoO4 nanorods 25 m2 g−1 with smaller size28), so the inner active sites of the electrode can be fully accessed with large surface area. Besides, compared with self-aggregated short CoMoO4 and MnMoO4 nanorods synthesized by hydrothermal and micro-emulsion method, the hierarchical MnMoO4/CoMoO4 heterostructured nanowires exhibit much higher capacity. This is due to the fact that self-aggregation of the hierarchical MnMoO4/CoMoO4 heterostructured nanowires have been greatly reduced, which can keep the effective contact areas of active materials and electrolyte large and sufficient. Third, in comparison with pure CoMoO4 nanorods, the highly dense small CoMoO4 nanorods grow on the MnMoO4 'substrate' orderly but with low crystallinity. This structure can provide surface sites for redox reaction, and possess sufficient order to enable OH− access to the heterostructures facilely. It would seem, therefore, that the facile ability of OH− to be inserted is an extra account for the enhanced charge storage of hierarchical MnMoO4/CoMoO4 heterostructured nanowires. These factors are expected to contribute to the substantial level of capacitive ability for supercapacitors.

The cyclic voltammogram (CV) is performed to identify the charge storage mechanism in MnMoO4/CoMoO4. Figure 4e shows the CVs of MnMoO4/CoMoO4. Comparing to the MnMoO4 and CoMoO4 electrodes (Supplementary Fig. S4), one more pair of redox peaks can be observed in the CV curves of MnMoO4/CoMoO4, which indicate that the pseudocapacitance mainly comes from the Faradic redox reaction of both Mn and Co. Comparing to the Pourbaix diagram of Mn, Co and Mo (Supplementary Fig. S5)29, Faradic redox reactions are as follows:

Supplementary Figure S4 and Figure 4e show that with the increase of the scan rate, the redox current increased, the anodic peak shifted towards positive potential and the cathodic peak shifted towards negative potential. The peak current increases nearly linearly with the scan rate confirming the pseudocapacitive behaviour. The increase of the current response with the scan rate also indicates that the kinetics of interfacial Faradic redox reactions and the rates of electronic and ionic transport are even rapid enough at scan rates as high as 100 mV s−1.

Figure 4f displays the Nyquist plots of MnMoO4/CoMoO4 at different voltage. The semicircle in the high-frequency range corresponds to the charge-transfer resistance caused by the Faradic reaction, which was correlated with the intercalation and deintercalation of cations. Between 0.1–0.4 V, the plots in the low-frequency range correspond to the diffusion-limited mechanism, which confirms that the main pseudocapacitive behaviour occurred between 0.1–0.4 V. The result is concordant with the two pairs of redox peaks of CVs.

In summary, a mild facile refluxing method was used to fabricate hierarchical MnMoO4/CoMoO4 heterostructured nanowires possessing a specific capacitance of 187.1 F g−1 at a high current density of 1 A g−1 and a good reversibility with cycling efficiency of 98% after 1,000 cycles. This demonstrates that constructing hierarchical MnMoO4/CoMoO4 nanowire heterostructures by combining 'oriented attachment' and 'self-assembly' is a significant and simple route to improve electrochemical properties. The hierarchical MnMoO4/CoMoO4 nanowire heterostructure described in this paper may have potential applications in energy storage and other electrochemical devices.

Methods

Synthesis of MnMoO4/CoMoO4 heterostructured nanowires

MnMoO4 nanowires, the backbone material, were prepared by simple micro-emulsion method with Na2MoO4 and MnCl2. The synthesis of hierarchical MnMoO4/CoMoO4 heterostructured nanowires was performed in a flask with mild magnetic stirring for different periods. Typically, MnMoO4 nanowires were dispersed in 40 ml water and the dispersion was refluxed at 60 °C. Then, CoCl2 and the same amount of Na2MoO4 were added to the mother solution discontinuously. To follow the crystal growth process, the samples were taken at different time intervals. Then the sample was washed with deionized water and ethanol, and dried for further characterization. The CoMoO4 shell thickness could be tuned by changing the molar ratio of Mn source and Co source.

Characterization and electrochemical measurement

The crystal phase and purity of the product were characterized by X-ray powder diffraction. XRD patterns of the products were recorded on a Rigaku D/MAX-III diffractometer (Rigaku) with monochromatized Cu Ka radiation (k=1.5406 Å). SEM performed on JSM-5610LV was used to characterize the morphology of the synthesized nanomaterials. TEM, high-resolution TEM were recorded by using a JEOL JEM-2010 FEF microscope (JEOL) at an accelerating voltage of 200 kV. EDS was performed on a JEM 2100F STEM/EDS and the X-ray energy resolution is 132 eV. Laser Raman spectrometer was recorded using the INVIA, Renishaw. BET surface areas were measured using Gemini 2360 instrument by adsorption of nitrogen at −209 °C. A three electrodes method consisting of an nickel foam as working electrode (0.07 cm2), Pt wire and Ag/AgCl (saturated KCl) electrodes as counter and reference electrodes were used. Cyclic voltammetry and galvanostatic charge–discharge studies were performed using an Autolab Potentiostat Galvanostat. NaOH (2.0 M) was used as the electrolyte. The working electrode consisted of 60 wt% of the active material (for example, MnMoO4/CoMoO4), 35.5 wt% of conductivity agent (carbon black, Super-P-Li) and 4.5 wt% of binder (polytetrafluoroethylene).

Additional information

How to cite this article: Mai, L.-Q. et al. Hierarchical MnMoO4/CoMoO4 heterostructured nanowires with enhanced supercapacitor performance. Nat. Commun. 2:381 doi: 10.1038/ncomms1387 (2011).