Design of three dimensional hybrid Co₃O₄@NiMoO₄ core/shell arrays grown on carbon cloth as high-performance supercapacitors

Abstract

Electrodes with rationally designed hybrid nanostructure composites can have superior electrochemical performance for supercapacitors to single structured materials. In this work, the uniform three dimensional (3D) hybrid Co₃O₄@NiMoO₄ nanowire/nanosheet arrays directly grown on carbon cloth were designed and synthesized via a two-step hydrothermal method. A series of characterizations of SEM, XRD, XPS, TEM and N₂ adsorption/desorption isotherms were used to verify the hierarchical core/shell hybrid nanostructure of as-prepared products, which combines the advantages of the good rate capability of Co₃O₄ nanowires and the high surface area of NiMoO₄ nanosheets. As a binder-free electrode, the fabricated 3D hybrid nanocomposite achieves a high areal capacitance of 3.61 F cm⁻² at a current density of 3 mA cm⁻² and a capacitance retention of 82% with the increase of current density from 3 to 15 mA cm⁻². Besides, the hybrid nanostructured product also exhibits lower bulk resistance and lower charge transfer resistance than single structured Co₃O₄ material. The outstanding electrochemical behaviour and the facile fabrication process suggest that this hybrid nanoarchitecture material has potential application in high-performance supercapacitors.

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1 Introduction

The ever-increasing demand for energy and power has driven the development of electrochemical energy storage devices. Supercapacitors (SCs), as a class of electrical energy storage device, are outstanding for their high power density, long cycle life and low maintenance cost.^1,2 Taking energy storage mechanisms into consideration, SCs are classified into two types: electrical double layer capacitors³ and pseudocapacitors.⁴ Compared to electrical double layer capacitors dominated by ion adsorption on the surface of active materials, pseudocapacitors can provide a much higher specific capacitance and energy density due to their fast and reversible redox reaction.⁵ In this regard, transition metal compounds, including oxides,^6,7 hydroxides,^8,9 and sulfides^10,11 are investigated as potential electrode materials for pseudocapacitors. Among these materials, binary metal oxides exhibit a higher performance than single-component metal oxides because of their feasible oxidation state, high electrical conductivity, potential low-cost and behaviour as environmental-friendly energy storage materials.^12,13 For instance, the metal molybdates, including NiMoO₄,^14,15 CoMoO₄ (ref. 16) and MnMoO₄,¹⁷ are promising potential electrode materials owing to their low-cost, abundant resources and environmental compatibility. In particular, NiMoO₄ has received increasing interest in the past several years for the high electrochemical activity of Ni ion.¹⁸ However, the reported electrode materials based on transition metal oxides are usually made by a traditional slurry-coating method, where the involved polymer binders and conducting additives can generate series resistance and deterioration of capacitance during redox reactions.^19,20 So the design and development of the new framework of metal oxide electrode materials are in progress.

Currently, the three dimensional hybrid core/shell nanostructures directly grown on conducting substrates as binder-free electrode have attracted more attention because they have higher surface/body ratios, larger surface areas, better permeabilities and more electroactive sites.^21–25 The synergistic effect caused by such hybrid core/shell nanostructures can facilitate the diffusion of active species and the transport of electrons.²⁶ For instance, Lou et al. synthesized hierarchical NiCo₂O₄@MnO₂ core/shell heterostructured nanowire arrays on Ni foam, which delivered a capacitance of 2.05 F cm⁻² at a current density of 20 mA cm⁻² with a cycling stability of 85% after 1000 cycles.²⁷ Wang et al. reported a facile two-step hydrothermal method to integrate two ternary metal oxides (NiCo₂O₄ and CoMoO₄) into core/shell nanowire arrays on Ni foam, and the resulting NiCo₂O₄@CoMoO₄ hybrid electrode exhibited areal capacitance of 14.67 F cm⁻² at a current density of 10 mA cm⁻², which are several times larger than the pristine NiCo₂O₄ electrode (1.42 F cm⁻²).²⁸ Zou et al. described the fabrication of 3D NiCo₂S₄@NiO₂ dendritic heterojunction arrays on Ni foam with a high capacitance of 4.28 F cm⁻² even at a current density of 50 mA cm⁻² and a high capacitance retention of 96.36% after 3000 cycles.²⁹ In this work, we present a facile strategy to prepare Co₃O₄ nanowire@NiMoO₄ nanosheet core/shell hybrid structures on carbon cloth for supercapacitor electrode application. The crystalline Co₃O₄ nanowires uniformly grown on carbon cloth current collector are used as the backbone to support and provide reliable electrical connection to the NiMoO₄ nanosheets coatings with mesoporous structure, enabling the full utilization of NiMoO₄ by offering more electroactive sites and faster electronic/ionic conductivity. Meanwhile, in contrast to Ni foam, carbon cloth as a flexible substrate also has many advantages of low-cost, good electrical conductivity, chemical stability, light weight, flexibility and high porosity. So such three dimensional hybrid Co₃O₄@NiMoO₄ core/shell nanocomposites are expected to exhibit good electrochemical performance as supercapacitor electrode materials.

2 Experimental

2.1 Synthesis of Co₃O₄ nanowires

The Co₃O₄ nanowires array was grown on carbon cloth by a facile hydrothermal method.³⁰ Prior to the experiment, the carbon cloth substrates were carefully cleaned in 4 M HCl solution, ethanol and deionized water successively with the assistance of ultrasonication. In a typical synthesis, 1.5 mmol of Co(NO₃)₂·6H₂O, 6.0 mmol of NH₄F and 7.5 mmol of urea were dissolved in 15 mL deionized water, then the obtained homogeneous clear pink solution was transferred into a Teflon-lined stainless steel autoclave with a piece of pretreated carbon cloth (1 × 2 cm²) at 120 °C for 9 h. After that, the sample was collected and rinsed several times with deionized water and ethanol, followed by drying at 60 °C for 12 h. Finally, the carbon cloth with the as-prepared hydrate precursors was annealed at 350 °C for 2 h to obtain Co₃O₄ nanowires.

2.2 Synthesis of the hybrid Co₃O₄@NiMoO₄ core/shell nanocomposite

To obtain hybrid nanostructures of Co₃O₄@NiMoO₄, a secondary hydrothermal reaction was adopted to grow the NiMoO₄ nanosheets on the backbone of Co₃O₄ nanowires. For this, the obtained Co₃O₄ nanowires supported on carbon cloth substrate was directly immersed in an solution containing 1 mmol of Ni(CH₃COO)₂·4H₂O, 0.2 g of (NH₄)₂Mo₇O₂₄·4H₂O, 0.24 g of urea and 10 mL of deionized water in an autoclave at 160 °C for 10 h. Afterwards, the sample was collected and rinsed with deionized water and ethanol several times, followed by annealing at 400 °C in air for 2 h.

2.3 Material characterization

X-ray diffraction (XRD) patterns were recorded on a Bruker D8 advance diffractometer, using Cu-Kα radiation at 40 kV and 40 mA. The nanostructured samples were observed by using a field emission scanning electron microscope (FESEM, JEOL JSM-7001F) and a field emission transmission electron microscope (FETEM, FEI Tecnai G20). X-ray photoelectron spectroscopy (XPS) was carried out by using Thermo Scientific ESCALAB 250XI spectrometer. Nitrogen adsorption/desorption isotherms were measured on ASAP 2020 analyzer.

2.4 Electrochemical measurements

Electrochemical measurements were performed on the Autolab PGSTAT302N electrochemical workstation using a three-electrode test system in 2 M KOH aqueous solution. The Co₃O₄@NiMoO₄ core/shell arrays or pristine Co₃O₄ arrays on carbon cloth (≈1 × 1 cm²; hybrid Co₃O₄@NiMoO₄ mass: ≈2.0 mg, pristine Co₃O₄ mass: ≈1.2 mg) were directly used as the working electrode. A platinum electrode and a saturated calomel electrode (SCE) served as the counter electrode and the reference electrode respectively. The areal capacitance of the electrode was calculated according to the following equations:where C is the areal capacitance of the electroactive materials (F cm⁻² or F g⁻¹), i is the discharging current density (A cm⁻² or A g⁻¹), t is discharging time (s) and ΔV is the discharging potential (V). The electrochemical impedance spectroscopy (EIS) tests were carried out in the frequency range between 100 kHz and 0.01 Hz with a potential amplitude of 5 mV at open circuit potential.

3 Results and discussion

As shown in Fig. 1, the fabrication of hybrid Co₃O₄@NiMoO₄ nanostructure as a binder-free electrode involves two key steps of hydrothermal reactions. First, the aligned pristine Co₃O₄ nanowires were grown on the fibers of carbon cloth substrate by a hydrothermal and post-annealing process. Second, interconnected NiMoO₄ nanosheets with different lateral sizes were controllably grown on the backbone of Co₃O₄ nanowires through a secondary hydrothermal process.

Fig. 1 Schematic illustration of the fabrication process for 3D hybrid Co₃O₄@NiMoO₄ core/shell nanowires/nanosheets array on carbon cloth.

The SEM images of the as-synthesized pristine Co₃O₄ nanowires and hybrid Co₃O₄@NiMoO₄ core/shell nanostructures grown on carbon cloth are shown in Fig. 2. From Fig. 2a–c, it can be observed that the free standing Co₃O₄ nanowires are densely and uniformly distributed on the fibers of the carbon cloth substrate with an average diameter of ca. 50–80 nm and length up to ca. 5 μm. And the high magnification SEM image of the sample reveals the rather smooth surface and sharp tip of the Co₃O₄ nanowires with diameter gradually decreasing from the bottom to the tip in Fig. 2c. Moreover, the slim Co₃O₄ nanowires are also observed to be homogenously aligned and separated apart adequately, which can serve as the ideal conductive scaffolds for the subsequent growth of the NiMoO₄ nanosheets shell. From the SEM images of the hybrid sample shown in Fig. 2d–f, it is clearly observed that the Co₃O₄ “branches” are tightly bonded and totally covered with “leaves” of NiMoO₄ nanosheets after the secondary hydrothermal reaction. In addition, the NiMoO₄ nanosheets are typically interconnected with each other to construct a highly porous and rough morphology on the surface of Co₃O₄ nanowires, resulting in the creation of three dimensional hybrid core/shell nanostructures on a large scale on the carbon cloth. These NiMoO₄ nanosheets not only have a robust adhesion and electrical contact to the Co₃O₄ nanowires, but also connect to each other and finally form a network like structure. Moreover, the interconnected nanosheets with a large lateral size produce abundant space and electroactive sites, making them fully accessible to electrolyte.^31,32

Fig. 2 SEM images of (a–c) the Co₃O₄ nanowires array on the carbon cloth, (d–f) the hybrid Co₃O₄@NiMoO₄ core/shell nanostructure array on carbon cloth.

The phase structure of the as-synthesized pristine Co₃O₄ nanowires and hybrid Co₃O₄@NiMoO₄ core/shell nanostructures on carbon cloth are displayed in Fig. 3a. Apparently, the main reflection peaks at a 2θ value of 31.9°, 36.9°, 44.8°, 59.4° and 65.2° can be well indexed to the (220), (311), (400), (511) and (440) planes of Co₃O₄ (JCPDS no. 42-1467).^33,34 Unfortunately, no new peaks are observed in the XRD pattern of the Co₃O₄@NiMoO₄ hybrid nanocomposite, indicating the low content and poor crystallinity of the NiMoO₄ nanosheets compared with Co₃O₄ nanowires.³⁵ However, the surface chemical composition of the hybrid sample can be verified by XPS, as shown in Fig. 3b–d. The Co 2p core level spectrum shows two peaks at 781 and 796 eV, corresponding to the Co 2p3/2 and Co 2p1/2 for Co₃O₄ respectively.³⁶ The Ni 2p core level spectrum reveals two major peaks at 857.2 and 874.9 eV, coupling with two satellite peaks at 862.9 and 881.1 eV, which correspond to Ni 2p3/2 and Ni 2p1/2 respectively. The main binding energy peaks of Ni 2p3/2 and Ni 2p1/2 are separated by 17.7 eV, which is a signature of the Ni²⁺ oxidation state.^34,37 Two binding energy peaks located at 232.6 and 235.7 eV with splitting width 3.1 eV can be observed in the region of Mo 3d, which are characteristics of the Mo⁶⁺ oxidation state.³⁸ The XPS results confirm that the valence of Ni and Mo elements are +2 and +6 respectively, which is in accordance with the phase structure of NiMoO₄.

Fig. 3 (a) XRD patterns of the as-synthesized Co₃O₄ nanowires and hybrid Co₃O₄@NiMoO₄ core/shell array on carbon cloth. (b–d) XPS spectra of Co 2p, Ni 2p and Mo 3d for hybrid Co₃O₄@NiMoO₄.

More information about the morphological and structural features of the hybrid Co₃O₄@NiMoO₄ nanocomposite was obtained by TEM and high-resolution TEM (HRTEM). As illustrated in Fig. 4a and b, the pristine Co₃O₄ “nanocore” is tightly bonded with ultrathin NiMoO₄ nanosheets, forming a typical core/shell structure, which is in good accordance with the SEM observation. Moreover, the TEM results clearly indicate that the interconnected NiMoO₄ nanosheets around Co₃O₄ nanowires are highly porous and ultrathin with a few nanometers in thickness. Interestingly, the ultrathin NiMoO₄ nanosheets exhibit graphene-like morphology with transparent feature, further suggesting the ultrathin nature (Fig. 4b). HRTEM measurements of the shell materials (Fig. 4c) typically display a set of clear lattice fringe with an interplanar space of ca. 0.214 nm, corresponding to the (121) planes of NiMoO₄ nanosheets in the hybrid structure.²⁴ The corresponding selected area electron diffraction (SAED) pattern (as shown in the inset of Fig. 4c) also indicates the polycrystalline nature of the ultrathin nanosheets, which is consistent with the XPS results.

Fig. 4 (a and b) TEM images of Co₃O₄@NiMoO₄ core/shell nanostructure array scratched from the carbon cloth; (c) HRTEM image and the inset corresponding to SAED pattern taken from the shell region of the Co₃O₄@NiMoO₄ hybrid array.

As shown in Fig. 5, the N₂ adsorption/desorption isotherms of pristine Co₃O₄ nanowires and Co₃O₄@NiMoO₄ nanocomposite are similar in shape. Both can be observed a type-IV isotherms with a type-H3 hysteresis loop in the relative pressure range of 0.5–1.0 P/P₀, which is ascribed to the presence of mesoporous structure in the samples. The mesoporous feature can be further confirmed by the pore size distribution analysis in the inset of Fig. 5. The sharp peaks in the pore size range of 2–4 nm and 7–15 nm are all attributed to mesoporous channels in the sample architectures, which is in good accordance with the TEM observation. Besides, it can be found that there are also many micropores in the samples, indicating the meso–micro dual porous structure in the as-prepared products, especially the hybrid Co₃O₄@NiMoO₄ nanocomposite. Moreover, in contrast to the Co₃O₄ nanowires, the hybrid Co₃O₄@NiMoO₄ nanocomposite show a more distinct hysteresis loop and more meso–micro porous volume, which can be attributed to the presence of ultrathin NiMoO₄ nanosheets grown on Co₃O₄ nanowires. These ultrathin NiMoO₄ nanosheets create voids between adjacent particles that increase the surface area.⁷ So the specific surface area of Co₃O₄@NiMoO₄ nanocomposite was calculated to be 251.2 m² g⁻¹, which was much higher than that of pristine Co₃O₄ nanowires (23.5 m² g⁻¹). This meso–micro dual porous structure with high surface area of composites can play an important role in providing more electroactive sites and shorter diffusion paths for electrochemical reactions on the electrode surface.^39,40

Fig. 5 Nitrogen adsorption/desorption isotherms plots and BJH pore size distribution of (a) Co₃O₃ nanowires and (b) Co₃O₄@NiMoO₄ hybrid nanocomposite.

The electrochemical properties of the as-prepared pristine Co₃O₄ nanowires and hybrid Co₃O₄@NiMoO₄ nanocomposite as electrodes investigated in detail are as follows. The typical cyclic voltammetry (CV) curves of Co₃O₄ nanowires and Co₃O₄@NiMoO₄ nanocomposite electrodes within the potential window of −0.2 to 0.8 V (vs. SCE) at various scan rates are shown in Fig. 6a and c. Both the CV curves are consisted of obvious redox peaks, indicating that the specific capacitance characteristics are mainly attributed to the faradaic redox reactions of Co³⁺/Co⁴⁺ and/or Ni²⁺/Ni³⁺.^15,41,42 Besides, compared with the CV curves of Co₃O₄ nanowires, the Co₃O₄@NiMoO₄ nanocomposite has more distinct redox peaks and larger enclosed CV curve area at the same scan rate (as seen in Fig. 7a), reflecting that the hybrid Co₃O₄@NiMoO₄ electrode has higher specific capacitance. This is in good accordance with the results obtained by the galvanostatic charge–discharge measurement in Fig. 6b and d. The areal capacitances of the electrodes can be calculated from the discharge curves and the corresponding results are plotted in Fig. 7b. Encouragingly, the areal capacitances of hybrid Co₃O₄@NiMoO₄ nanocomposite electrode are 3.61 F cm⁻² (1805 F g⁻¹) at low current density of 3 mA cm⁻² (1.5 A g⁻¹) and 2.96 F cm⁻² (1480 F g⁻¹) at high current density of 15 mA cm⁻² (7.5 A g⁻¹), and at least 82% level can be maintained, whereas the maximum areal capacitance of the pristine Co₃O₄ nanowires is only 0.40 F cm⁻² (333 F g⁻¹). Impressively, these results are remarkable as compared to many of the previously reported Co₃O₄ and NiMoO₄ based materials. For example, the ZnO@Co₃O₄ core/shell hetero-structures on Ni foam is 1.52 F cm⁻² at 5 mA cm⁻²;¹⁹ the Co₃O₄@NiMoO₄ nanosheets array on nickel foam is 2.67 F cm⁻² at 3 mA cm⁻²;³⁴ the NiMoO₄ nanowires on carbon cloth is 1.27 F cm⁻² at 5 mA cm⁻².⁴³ Such enhanced pseudocapacitive performance further proves the great advantages of the present two-step hydrothermal method, which would make the Co₃O₄ nanowires more suitable as backbone for the subsequent construction of 3D hybrid Co₃O₄@NiMoO₄ nanocomposite electrode for supercapacitor applications.

Fig. 6 Electrochemical performance of the pristine Co₃O₄ nanowires (a and b) and the Co₃O₄@NiMoO₄ nanocomposite (c and d) electrodes measured in 2 M KOH solution. (a and c) CV curves of the pristine Co₃O₄ nanowires and the Co₃O₄@NiMoO₄ nanocomposite electrodes at different scan rates; (b and d) galvanostatic discharge curves of the pristine Co₃O₄ nanowires and the Co₃O₄@NiMoO₄ nanocomposite electrodes at different current densities.

Fig. 7 (a) CV curves of the pristine Co₃O₄ nanowires, hybrid Co₃O₄@NiMoO₄ nanocomposite and bare carbon cloth electrodes at a scan rate of 5 mV s⁻¹; (b) area capacitance of the pristine Co₃O₄ nanowires and hybrid Co₃O₄@NiMoO₄ nanocomposite electrodes as a function of current densities.

Electrochemical impedance spectroscopy (EIS) was carried out to further explore the electrochemical behaviors of the Co₃O₄@NiMoO₄ hybrid electrode. The Nyquist plots of the pristine Co₃O₄ nanowires and hybrid Co₃O₄@NiMoO₄ nanocomposite electrodes in the frequency range of 0.01 Hz to 100 kHz are shown in Fig. 8. The two plots all exhibit a semicircle loop followed by a rising along imaginary impedance axis. In the low frequency region, the slope of the curves shows the Warburg resistance, which represents the ion diffusion of the electrolyte into the inner surface of electrode materials.^44,45 As compared with the pristine Co₃O₄ electrode, the hybrid Co₃O₄@NiMoO₄ electrode shows a slightly less vertical straight line along the imaginary axis (as shown in Fig. 8), indicating a bit higher ion diffusion resistance. This can be attributed to the meso–micro dual porous structure of hybrid Co₃O₄@NiMoO₄ nanocomposite, in which the microporous channels are less favorable for the electrolyte penetration and OH⁻ ion diffusion into the inner of electrode materials than mesoporous channels. In the high frequency region, the intersection of the curves at the real impedance axis indicates the bulk resistance of the electrochemical system, including a series resistance of the electrolyte inside the pore and the interfacial resistance at the electroactive material/current collector and at the electrode/electrolyte interface.^46,47 In addition, the diameter of the depressed semicircle at high frequency region corresponds to the charge transfer resistance on the electrode surface.^48,49 As shown in the inset of Fig. 8, the hybrid electrodes are observed to exhibit much lower bulk resistance and lower charge transfer resistance than the pristine Co₃O₄ electrode, since the interconnected neighboring NiMoO₄ nanosheets can contact with each other and provide more electroactive sites and additional convenient pathways for electron transport in the electrochemical system.

Fig. 8 EIS spectra of the pristine Co₃O₄ nanowires and hybrid Co₃O₄@NiMoO₄ electrodes.

Finally, as one of the key issue for supercapacitor electrode materials, the cycling stability has been tested for the pristine Co₃O₄ nanowires and hybrid Co₃O₄@NiMoO₄ composite, as shown in Fig. 9. For the pristine Co₃O₄ electrode, the areal capacitance is about 0.32 F cm⁻² after 2500 cycles at the discharge current density of 10 mA cm⁻² (about 95.6% retention). Although, the hybrid Co₃O₄@NiMoO₄ electrode displays a declining behaviour during the long-term cycling tests (about 77.4% retention after 3000 cycles), the final capacitance value is still much better than that of pristine Co₃O₄ even at a higher discharge current density of 15 mA cm⁻². In addition, for Co₃O₄@NiMoO₄ electrode, the capacitance loss (16.6%) was mainly during the first 600 cycles, which can be generally explained by the irreversible of the Faraday reactions or the dissolution of active materials from the hybrid nanostructures at the beginning of the cycle test.^34,50 However, it still retains 92.7% after the rest of 2400 cycles, exhibiting a relatively good stability.

Fig. 9 Cycling performance of the pristine Co₃O₄ nanowires and hybrid Co₃O₄@NiMoO₄ electrodes at different current densities.

4 Conclusions

In summary, the uniform three dimensional hybrid Co₃O₄@NiMoO₄ core/shell nanocomposite directly grown on carbon cloth are successfully synthesized by a facile two-step hydrothermal method. As a binder-free electrode, the constructed 3D hybrid nanocomposite delivers impressive pseudocapacitive performances (a high areal capacitance of 3.61 F cm⁻² at current density of 3 mA cm⁻² and a capacitance retention of 82%), in contrast to the pristine Co₃O₄ nanowires. Such intriguing capacitive behavior is attributed to the 3D nanocomposite configuration and the synergistic effect of the combined pseudocapacitive contributions from the Co₃O₄ nanowires and the ultrathin NiMoO₄ nanosheets. This work suggests that the aforementioned hybrid core/shell nanoarchitecture is very suitable and promising as advanced electrode materials for high-performance supercapacitors.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21576054), Pearl River Nova Program of Guangzhou (Grant No. 2011J2200041), Guangdong Province Science and Technology Project (Grant No. 2012A030600006), Science and Technology Achievements Transformation Projects of Guangdong Higher Education Institutes (Grant No. cgzhzd1104).

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