NiMoO₄ nanowire @ MnO₂ nanoflake core/shell hybrid structure aligned on carbon cloth for high-performance supercapacitors

Abstract

A novel hybrid structure of core/shell NiMoO₄@MnO₂ was directly synthesized on carbon cloth by a facile two-step hydrothermal route for supercapacitors. The smart combination of NiMoO₄ and MnO₂ shows a synergistic effect for capacitors with greatly enhanced performance. The NiMoO₄@MnO₂ electrode yields high-capacitance performance with a high areal capacitance of 3.90 F cm⁻² at a charge and discharge current density of 8 mA cm⁻² and 3.22 F cm⁻² at 24 mA cm⁻² with a desirable cycling ability (90.5% of the initial specific capacitance remains after 4000 cycles). Such core/shell hybrid nanoarchitectures exhibit remarkable electrochemical performance with high capacitance and excellent long-term cycling stability, which could be promising pseudocapacitive electrode materials for high-performance supercapacitors.

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1 Introduction

Supercapacitors (SCs) have attracted considerable attention in the field of energy storage owing to their high power density, long cycle life and fast charge and discharge rates.^1,2 The unique advantages of SCs are desirable for many applications that require high energy density and fast charge–discharge rate, such as portable electronics, hybrid electric vehicles and industrial equipment.^3–6 SCs can essentially be classified into two types: electric double layer capacitors (EDLCs) and pseudocapacitors. Pseudocapacitors usually offer much higher specific capacitance and energy density than SCs made of carbon based on double-layer charge storage. Pseudocapacitors fill the gap between the conventional batteries and capacitors in terms of electrochemical performance. More analogously to batteries, pseudocapacitors stored charges by means of absorption/desorption of ions (OH⁻, Li⁺, Na⁺, K⁺, etc.) and reversible surface/near-surface faradaic reactions.^7–11 Transition metal oxides and hydroxides are the most popular electrode materials of pseudocapacitors in view of their multiple oxidation states for pseudocapacitance generation.^12–18 However, the experimentally obtainable values are often much lower than the theoretical expectations because of inadequate use of entire pseudocapacitive materials and limited electrical conductivity of metal oxides at high rate. Thus, rational design and synthesis of electrode materials with advanced architectures and excellent properties for supercapacitors are a great challenge.

Numerous efforts have been devoted to the synthesis of hybrid electrode materials with the combination of two kinds of materials that enhanced the electrochemical performance. By combining unique properties of individual constituents, enhanced performance has been demonstrated.¹¹ However, due to the lack of well-defined micro-/nanostructures, the synergistic effect between individual constituents has not worked sufficiently and energy density in most cases is sacrificed.¹⁹ In addition, the limited electron transport can mask the true rate capability of the material.²⁰ Therefore, the charge storage ability of supercapacitors depends not only on the nature of electrode materials but also greatly on the design of electrode architecture. To improve the ion diffusion kinetics and electron transport at the electrode–electrolyte interface, core/shell hybrid nanostructures directly grown on substrates are well-suited architectures for high-performance supercapacitor electrodes. The “core” with high conductivity, would provide a direct path for the electrons transport and create channels for the effective transport of electrolyte. The “shell” is generally thin layer structure with enlarged surface area, which could shorten ion diffusion path and provide more efficient contacts between the electrolyte ions and active materials for Faradaic energy storage. The disordered nature of the coating material modifies the surface charge to facilitate the adsorption of ion from the electrolyte.^21–23 Meanwhile, the direct growth of nanostructures on conductive substrates avoids the use of polymer binder and conductive additives, substantially reduces the “dead volume” in electrode materials and facilitates the diffusion of active species and the transport of electrons.^24–29 As a result, reported core/shell hybrid structures on substrates have realized the full potential of the materials in terms of performance.^30–37 For example, hybrid nanostructures of MoO₂@Co(OH)₂,¹¹ Co₃O₄@MnO₂,¹⁹ Co₃O₄@NiCo₂O₄ (ref. 32) and Ni(OH)₂@Fe₂O₃ (ref. 36) have significantly enhanced both the capacitance and durability of electrodes. In these integrated smart architecture, structural features and electroactivities of each component are fully manifested. Despite these achievements, choosing novel suitable electrode materials and their assemblies in appropriate architecture to achieve better performance still remains a challenge due to the complicated synthesis processes.

Herein, we present a facile and scalable strategy to fabricate integrated NiMoO₄ nanowire @ MnO₂ nanoflake core/shell hybrid structure aligned on carbon cloth for supercapacitor electrode application. Till now, there is no study on electrochemical performance of the integrated electrodes combining merits of NiMoO₄ (ref. 38–42) and MnO₂,^43–49 although the individual capacitive property of both has been extensively investigated. This unique design has following advantages: first, both the core and shell materials are well-known pseudocapacitive metal oxides, which contribute to the overall electrochemical charge storage. Second, the NiMoO₄ nanowires (NWs) directly grown on conductive carbon cloth provide a scaffold for the MnO₂ nanoflakes growth, serve as fast electron access for charge storage and delivery, overcoming the limited electrical conductivity of MnO₂ itself to assure an effective electron transport. Third, ultrathin MnO₂ nanoflakes are well wrapped on NiMoO₄ NW surfaces, which would enable a fast reversible faradic reaction, reduce the charge–transfer resistance of NiMoO₄ and shorten ion diffusion path. Fourth, carbon cloth as a soft substrate manifests incomparable merits for assembled devices since they are low-cost, good electrical conductivity, chemical stability, light weight, flexibility and high porosity. Consequently, in the electrode design, all the desired functions of each constituent are efficiently utilized to realize a strong synergistic effect. The obtained NiMoO₄ nanowire @ MnO₂ nanoflake core/shell hybrid structure exhibit large areal capacitance (AC) of 3.90 F cm⁻² (2294 F g⁻¹) and desirable cycling stability in a 2 M KOH solution.

2 Experimental details

2.1 Synthesis of NiMoO₄@MnO₂ core/shell hybrid structure on carbon cloth

A commercially available carbon cloth (W0S1002) was used as the supporting material for NiMoO₄@MnO₂. All the reagents were analytical grade, the nickel nitrate and sodium molybdate were purchased from Tianjin Chemical Reagent Co., potassium permanganate was obtained from Sinopharm Chemical Reagent Co. and used without further purification. NiMoO₄ NWs were prepared by a facile hydrothermal synthesis method (150 °C 6 h).^38,41 To fabricate the NiMoO₄ NWs coated with thin MnO₂ nanoflakes, a piece of substrate covered with NiMoO₄ NWs was first immersed into a 5 mM aqueous KMnO₄ solution for 1 h, then placed standing against the wall of a Teflon-lined autoclave. The reaction solution was obtained by mixing 0.01 M of KMnO₄ and 0.01 M of Na₂SO₄ in 100 mL of distilled water under constant magnetic stirring and then transferred into Teflon-lined stainless steel autoclave liners. The liner was sealed in a stainless steel autoclave and maintained at 160 °C for 3 h and then cooled down to room temperature. After the reaction was over, the sample was removed and washed by ultrasonication in distilled water followed by acetone for a few minutes in order to remove the residual debris, then dried at 60 °C for 12 h to obtain a NiMoO₄@MnO₂ core/shell hybrid structure.

2.2 Characterization

The crystal structure of the samples was characterized with X-ray diffraction (XRD, Cu Kα irradiation; λ = 1.5418 Å) with a SIEMENS D5000 X-ray diffractometer. The morphology and microstructure of the synthesized sample were characterized by a scanning electron microscopy (SEM, Hitachi S4800) and a transmission electron microscope (TEM; JEOL-2010 with an accelerating voltage of 200 kV).

2.3 Electrochemical measurements

The electrochemical measurements were conducted using a three-electrode system in a 2 M KOH aqueous solution. The NiMoO₄@MnO₂ hybrid or pristine NiMoO₄ NWs (≈1 × 1 cm²; NiMoO₄ mass: ≈1.1 mg, MnO₂ mass: ≈0.6 mg) was directly used as the working electrode. A standard calomel electrode (SCE) was used as the reference electrode and a Pt foil as the counter electrode. The area specific capacitance of the electrode was calculated according to the following equations:where C is the areal capacitance of the electroactive materials, Δu is the potential (V), i is the discharging current density (A cm⁻² or A g⁻¹) and t is discharge time (s). All the electrochemical experiments were performed on a CHI660e electrochemical workstation (Chenhua, Shanghai). Electrochemical impedance spectroscopy (EIS) measurements were made with a superimposed 5 mV sinusoidal voltage in a frequency range from 0.01 Hz to 100 kHz at open circuit potential.

3 Results and discussion

The electrode of NiMoO₄@MnO₂ hybrid structure fabrication process is illustrated in Fig. 1. First, a high density of NiMoO₄ NWs grown vertically on carbon cloth is obtained via a facile hydrothermal method followed by a calcination process, according to previous work.⁴¹ Typical XRD patterns of the pristine NiMoO₄ NWs are distinctly demonstrated in Fig. S1.† In the second step, NiMoO₄ NWs on carbon cloth are immersed into the KMnO₄ solution by second hydrothermal reaction to grow thin MnO₂ nanoflakes. The growth of NiMoO₄@MnO₂ hybrid can be attributed to “oriented attachment” and “self assembly” processes involving a spontaneous self-organization between neighboring particles to share a common crystallographic orientation. In the process, bonding between the particles reduced overall energy by removing surface energy associated with unsatisfied bonds.^50,51 In the reaction, NiMoO₄ nanowires acted as the ‘backbone’ to guide the MnO₂ self-assembling growth in aqueous solution without surfactant and stabilizers. Then the ‘oriented attachment’ can guide the nanoparticles oriented growth. Because of the high surface energy and thermodynamics instability, nanoparticles can attach to the surface of NiMoO₄ 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. As shown in Fig. 2, the XRD pattern confirms the existence of the NiMoO₄ phrase (JCPDF card no. 86-0361) and MnO₂ phrase (JCPDF card no. 01-799). The XRD pattern also includes diffraction peaks corresponding to the carbon cloth substrate. Notably, no impurities have been detected. Fig. 3 shows the SEM and TEM image of NiMoO₄@MnO₂ core/shell hybrid structure aligned on carbon cloth. The low-magnification SEM images in Fig. 3a and b indicate the products with high density are uniformly distributed on the fibers of the carbon cloth substrate in a large scale. By contrast, the SEM images of NiMoO₄ NWs are displayed in Fig. S2.† As observed, the NiMoO₄ NWs have grown on the carbon cloth support to form net structure. A close SEM examination demonstrates the uniform coverage of MnO₂ nanoflakes on NiMoO₄ NW surface (Fig. 3c and d). The MnO₂ nanoflake shells are interconnected with each other and almost fully cover the entire NiMoO₄ core, forming a net surface morphology. Thus, the core/shell NiMoO₄@MnO₂ hybrids are highly accessible to electrolytes for energy storage due to the presence of convenient diffusion channels. Moreover, the robust mechanical adhesion of the hybrid structure is confirmed by an ultrasonication test which shows no material loss occurred after ultrasonication in solution for several minutes. To obtain the cross-section, we scraped off the active materials on the carbon cloth. The cross-sectional SEM image (Fig. S3†) of NiMoO₄@MnO₂ on a carbon fiber shows the thickness of the NiMoO₄@MnO₂ layer is about 1.5–2 μm.

Fig. 1 Illustration of the fabrication process of NiMoO₄@MnO₂ core/shell hybrid structure.

Fig. 2 XRD patterns of the NiMoO₄@MnO₂ hybrid on carbon cloth.

Fig. 3 (a–d) Low and high magnification SEM images of the core/shell NiMoO₄@MnO₂ hybrid structure on the carbon cloth. TEM micrographs of the samples; (e and f) TEM images of NiMoO₄@MnO₂ core/shell hybrid structure scratched from the carbon cloth with synthesized nanostructures; (g) HRTEM image of NiMoO₄ NWs; (h) HRTEM image of MnO₂ nanoflakes.

The structure and morphology of the core/shell hybrid structure was further investigated by TEM. As illustrated in Fig. 3e, the pristine NiMoO₄ NWs with diameter of 30–60 nm (the TEM image of pristine NiMoO₄ NWs is shown in Fig. S4†) are uniformly covered with ultrathin MnO₂ nanoflakes (<15 nm). From Fig. 3f, it is evidently observed that the NiMoO₄ “nanocore” is tightly bonded with MnO₂ nanoflakes, forming a typical core/shell hybrid structure. The high-resolution TEM (HRTEM) image in Fig. 3g reveals the interplanar spacing of 0.206 nm, corresponding to the (330) plane of NiMoO₄ given in the standard files. The HRTEM measurement was also performed for the MnO₂ nanosheets in red rectangle (Fig. 3f). The HRTEM image in Fig. 3h shows the interplanar spacing of 0.22 nm and 0.24 nm, which are close to the (200) plane and (101) plane of MnO₂, respectively. Besides, the EDS analysis, shown in Fig. S5,† demonstrates the presence of Ni, Mo, Mn, O and C elements, arising from the carbon substrate.

To demonstrate the advantage of the hybrid electrode, the cyclicvoltammetry (CV), rate capability, cyclic stability studies and electrochemical impedance spectroscopy (EIS) were performed in a three-electrode electrochemical system using SCE as the reference and platinum foil as the counter-electrode. Fig. 4a displays the typical CV curves of the NiMoO₄@MnO₂ core/shell hybrid electrode (vs. SCE) at different scan rates of 5, 10, 20, 30 and 50 mV s⁻¹ in a potential range of −0.2–0.8 V. From the curves, two redox peaks at 0.18 and 0.52 V are clearly observed, which correspond to the reversible reactions of Ni²⁺/Ni³⁺ associated with the OH⁻ ions in the alkaline electrolyte. For comparison, the CV of pristine carbon cloth at 50 mV s⁻¹ is shown in the ESI, Fig. S6.† The capacitance and current of pristine carbon cloth are much lower than NiMoO₄@MnO₂ on carbon cloth, which indicated the contribution to capacity from carbon cloth substrate is particularly small. The CV curves of pristine NiMoO₄ NWs at 5 mV s⁻¹ and 50 mV s⁻¹ are also shown in the ESI, Fig. S7.† Obviously, a similar CV shape is still found for the core/shell hybrid, indicating the efficient utilization of the underlying NiMoO₄ NWs despite covered by the MnO₂ nanoflakes, while the CV of hybrid array expand obviously and the area integrated within the current–potential curves greatly increases for the core/shell hybrid, leading to a much larger pseudocapacitance. This is mainly due to the great contribution of ultrathin MnO₂ nanoflakes which introduce electrochemical redox reaction to boost the charge storage capability. The CV shape change should certainly be attributed to the presence of MnO₂, which can adsorb cations (K⁺) on the electrode surface from electrolyte and suffer from possible intercalation or deintercalation of K⁺.⁵² The peak current increases almost linearly with the scan rate, suggesting that the rates of electronic and ionic transportation were rapid enough with respect to the scan rates. The shape of the CV curves is not significantly influenced by increasing the scan rate, which indicates improved mass transportation and electron conduction in the host materials.

Fig. 4 Capacitance properties of the hybrid structure. (a) The CV curves of the NiMoO₄@MnO₂ on carbon cloth at different scan rates; (b) charge–discharge behavior of the hybrid structure on carbon cloth at different current densities; (c) ten consecutive charge–discharge curves of the hybrid structure at a current density of 24 mA cm⁻²; (d) areal capacitance for NiMoO₄@MnO₂ hybrid structure and NiMoO₄ NWs as a function of the current density.

To further evaluate the performances of the electrode materials, the galvanostatic charge–discharge measurements were performed in the voltage range between 0 and 0.5 V at various current densities. Fig. 4b shows the galvanostatic discharge curves at current densities ranging from 8 to 32 mA cm⁻². The discharge areal capacitance of NiMoO₄@MnO₂ hybrid at 20 mA cm⁻² was measured to be 3.36 F cm⁻², which is higher than the capacitance of pristine NiMoO₄ (0.91 F cm⁻², shown in the ESI, Fig. S8†). This result directly reveals the advantage of a core/shell hybrid structure pseudocapacitive material for capacitance improvement. The AC of NiMoO₄@MnO₂ electrode was calculated from discharge time according to the eqn (1). Fig. 4c exhibits ten consecutive charge–discharge curves of the hybrid structure at a current density of 24 mA cm⁻². Fig. 4d further illustrates the current density dependence of the areal capacitance of both the hybrid and NiMoO₄ NWs. Within the current density from 8 to 32 mA cm⁻², the hybrid always delivers a much higher areal capacitance than the NiMoO₄ NWs. The NiMoO₄@MnO₂ core/shell hybrid electrode delivered high AC values of 3.90, 3.62, 3.49, 3.36, 3.22, 3.19 and 3.07 F cm⁻² (2294 F g⁻¹, 2053 F g⁻¹, 1976 F g⁻¹, 1894 F g⁻¹, 1876 F g⁻¹, 1806 F g⁻¹) at discharge current densities of 8, 12, 16, 20, 24, 28 and 32 mA cm⁻² (4.7 A g⁻¹, 7.1 A g⁻¹, 9.4 A g⁻¹, 11.8 A g⁻¹, 14.1 A g⁻¹, 16.5 A g⁻¹ and 18.8 A g⁻¹), respectively. The AC gradually decreased with the increase of current density due to the incremental voltage drop and insufficient active material involved in redox reaction at higher current densities. Strikingly, at the highest current density of 32 mA cm⁻², the NiMoO₄@MnO₂ still showed a high capacitance of 3.07 F cm⁻². Such high AC at large current densities further proves the great advantages of the NiMoO₄@MnO₂ hybrid (shown in Fig. 5a). The open space between NiMoO₄ NWs allows for easy diffusion of the electrolyte into the inner region of electrodes, and the carbon cloth can provide numerous fast electronic transfer channels to improve the electrochemical performance. Furthermore, the directly grown hybrid can ensure good mechanical adhesion and electrical connection to the current collector, avoiding the use of polymer binders and conducting additives, which generally increase the series resistance and the deterioration of capacitance during redox reactions. Compared with single NiMoO₄ component, NiMoO₄@MnO₂ core/shell hybrid presents many competitive advantages such as rich accessible electroactive sites, short ion transport pathways, superior electron collection efficiency, and even fascinating synergetic properties or multifunctionalities of components. By coating of each NiMoO₄ NW with flaky MnO₂, the space between the NiMoO₄ NWs is utilized abundantly, which would increase the energy and power per unit area. Moreover, the hybrid nanostructure can effectively decrease the surface energy of the active nanomaterials which may lead to a high level of reversibility. As a result, the AC of NiMoO₄@MnO₂ is higher than previously reported core/shell hybrid materials, such as NiO–TiO₂ nanotube arrays (3 F cm⁻² at 0.4 mA cm⁻²),⁵³ Co₃O₄@MnO₂ core/shell nanorod arrays (0.56 F cm⁻² at 1.25 mA cm⁻²)¹⁹ and NiCo₂O₄@MnO₂ core/shell NW arrays (2 F cm⁻² at 10 mA cm⁻²).³³ The results imply that NiMoO₄@MnO₂ core/shell hybrid could be a promising electrode material for high-performance supercapacitors.

Fig. 5 (a) Schematic illustration showing the charge storage advantage of NiMoO₄@MnO₂ core/shell hybrid structure, in which both NiMoO₄ and MnO₂ provide channels for electron transport. (b) Cycling performance of the NiMoO₄@MnO₂ hybrid structure and NiMoO₄ NWs; (c) impedance Nyquist plots of the NiMoO₄@MnO₂ hybrid structure and NiMoO₄ NWs on carbon cloth at open circuit potential.

The long-term cycling stability of the as-fabricated NiMoO₄@MnO₂ core/shell hybrid and pristine NiMoO₄ NWs were examined by repeated charge–discharge processes, as shown in Fig. 5b. It is clear that both the AC and cycling stability are largely enhanced in the core/shell hybrid electrode. The AC degradation of the core/shell hybrid is estimated to be from 3.22 to 2.92 F cm⁻² even at a high current density of 24 mA cm⁻². The capacitance loss for NiMoO₄@MnO₂ core/shell hybrid after 4000 cycles is only 9.5%. By contrast, only 76.9% capacitance was retained (23.1% loss) for a pristine NiMoO₄ NWs at current density of 10 mA cm⁻². The MnO₂ nanoflakes coated on surface of NiMoO₄ NWs improve the electrochemical stability for long cycle life applications at high current densities. Besides, the superior cycling stability can be in part explained by the structural stability of the electrode. After long-term cycling at high current densities, the structural integrity and basic morphology of the NiMoO₄@MnO₂ hybrid were overall well preserved with little structural deformation as shown in the ESI, Fig. S9.† In this respect, MnO₂ is proposed to serve as a protecting layer to maintain the NiMoO₄ structural integrity during the bulk redox reaction, which would otherwise weaken the nanowire due to the harsh and frequent phase variation. In order to further understand the fundamental behavior of supercapacitor electrodes, EIS analysis was measured and the corresponding Nyquist plots of the two electrodes are shown in Fig. 5c. From the plots, we can see that the MnO₂ coating leads to a relatively smaller bulk resistance (R_b), R_b decreased from 1.12 Ω to 0.77 Ω, suggesting the electron conductivity of NiMoO₄ NW electrode is improved. Moreover, the semicircle (which corresponds to double layer capacitance C_dl and charge-transfer resistance R_ct) displays the charge-transfer process at the working electrode–electrolyte interface. The NiMoO₄@MnO₂ hybrid also displays lower charge-transfer resistance than pristine NiMoO₄ NWs, which is beneficial to the rate capability of the hybrid. The reduced Faraday resistance rendered by the intriguing material combination and the core/shell structure leads to enhanced electrochemical reaction, contributing greatly to high AC of NiMoO₄@MnO₂ hybrid. On the other hand, the decreased Warburg impedance (W) represents lower diffusion resistance in NiMoO₄@MnO₂ than pristine NiMoO₄, which can be attributed to the large surface area of ultrathin MnO₂ nanoflakes which facilitated the diffusion of OH⁻ to the entrance of NiMoO₄ nanocore. The SC performance of the hybrid structure of NiMoO₄@MnO₂ and NiMoO₄ NWs have been summarized by a radar plot in Fig. 6, we can comprehensively evaluate the metrics of as-prepared supercapacitor electrodes, including cycle life, internal resistance, capacitance, etc.⁵⁴

Fig. 6 Radar plots to compare the supercapacitor performance of NiMoO₄@MnO₂ hybrid structure (red curves) and NiMoO₄ NWs (blue curves) on carbon cloth.

4 Conclusions

In summary, this study demonstrated a facile and scalable strategy to construct NiMoO₄ nanowire @ MnO₂ nanoflake core/shell hybrid structure aligned on carbon cloth for high-performance supercapacitors. The as-fabricated NiMoO₄@MnO₂ core/shell hybrid structure electrode exhibited excellent supercapacitor performance with a high capacitance of 3.07 F cm⁻² even at 32 mA cm⁻² and desirable rate performance. Such intriguing capacitive behavior is attributed to the unique core/shell hybrid configuration and the synergistic effects of the combined pseudocapacitive contributions from the NiMoO₄ NW core and the ultrathin MnO₂ shell layer. Our work opens up the possibility of constructing high-performance core/shell hybrid materials by self assembly method. The electrode design concept can be readily generalized to other materials and again confirms the feasibility of rational design of 3D hybrid nanostructure electrodes for high-performance supercapacitors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 21003041, 61376073), Hunan Provincial Natural Science Foundation of China (Grant no. 11JJ7004), Hunan Provincial Innovation Foundation For Postgraduate (Grant no. 521293014) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20120161110016).

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Footnote

† Electronic supplementary information (ESI) available: The XRD patterns, SEM and TEM images of NiMoO₄ NWs; the EDS patterns of NiMoO₄@MnO₂ hybrid; CV curves and galvanostatic discharge curves of NiMoO₄ NWs; SEM image of the NiMoO₄@MnO₂ hybrid electrode after 4000 cycles. See DOI: 10.1039/c4ra14442b

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