Branched ultra-fine nickel oxide/manganese dioxide core–shell nanosheet arrays for electrochemical capacitors

Abstract

We demonstrate the fabrication of ultra-fine hierarchical NiO@MnO₂ core–shell nanosheet arrays on carbon fiber paper via a facile and two-step hydrothermal method as the electrode for supercapacitors. Owing to the unique hierarchical core–shell heterostructured nanosheet array configuration and synergetic contribution from the NiO core and MnO₂ shell, the as-prepared hybrid nanosheet arrays exhibited a high specific capacitance of 494.8 F g⁻¹ and good rate capability, which are better than those of the obtained individual component of the NiO nanosheets. More importantly, the asymmetric supercapacitor, composed of NiO@MnO₂ core–shell nanosheet arrays as the positive electrode and activated carbon as the negative electrode delivers a specific capacitance of 86.3 F g⁻¹ and reaches up to an energy density of 30.6 W h kg⁻¹ and power density of 400 W kg⁻¹. The improved performance of the NiO@MnO₂ core–shell nanosheet arrays demonstrates that rational design of electrode structure is an effective way to fabricate a high-performance electrode for supercapacitors.

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1. Introduction

Under the circumstances of the increasing exhaustion of non-renewable resources and worsening environmental pollution, energy conversion and storage is unquestionably becoming one of the great challenges. In order to exploit high efficiency energy storage devices, which are able to store large quantities of electrical energy and can be rapidly charged/discharged,^1–4 much effort is urgently devoted to the development of novel electrode materials. It is now well recognized that materials with multifunctional properties or a hybrid combination of various nanomaterials are expected to meet the on-board energy demands and are widely prepared due to their superior properties than each component.^5,6 Types of hybrid nanostructures include (i) segmented hybrid materials, (ii) core–shell hybrid materials, (iii) 0-D (zero-dimensionality) nanoparticle-decorated 1-D nanostructures.⁵ Particularly, ordered arrays of hybrid core–shell nanostructures can effectively improve the electrochemical reversibility and have advantage of efficient electron transfer between current collector and individual electrode material and fast ion transport/easy electrolyte access to electrode due to available void volume between adjacent nanostructured electrodes. Thus, enhanced properties or novel useful properties can be delivered when single-phase nanomaterial in becoming part of the hybrid combination.^6,7

Transition metal oxides have long been studied as potential electrode materials for lithium ion batteries and pseudocapacitors because of the ease of large-scale fabrication and their rich redox reactions involving different ions, which contributes to high specific capacitances, such as CoO_x,⁸ NiO,^9–11 MnO₂,^12–16 Fe₂O₃ (ref. 17 and 18) and V₂O₅.¹⁹ Among various candidate materials, NiO and MnO₂ are widely investigated as electrode materials for supercapacitor due to their considerable theoretical specific (2574 F g⁻¹ for NiO¹⁷ and 1370 F g⁻¹ for MnO₂ (ref. 11 and 12)) and their abundant resources. However, the poor capacity retention of NiO and low conductivity of MnO₂ (10⁻⁵ to 10⁻⁶ S cm⁻¹) hinder their practical application.^14,20,21 In this regard, one attractive concept is to incorporate MnO₂ or NiO into conductive carbon-based materials^22–26 (carbon nanotube, graphene and carbon aerogels) or conducting coating^11,16,27 (metals, carbon and polymers) to address this issue. For instance, Zhang et al.²⁵ prepared MnO₂ electrochemical reduced-GO electrodes which exhibited a high specific capacitance of 422.5 F g⁻¹. By taking advantages of graphene oxide (GO), manganese ions can transport more easily onto the surface of GO, which will effectively increase the active surface of MnO₂ and hence enhance the energy storage property. However, in this case, the energy density is generally sacrificed to achieve better rate capability and cycling stability. Another alternative way is to synthesize hybrid core–shell nanostructures aligning on current collectors as binder-free electrodes for electrochemical capacitors which can ensure the more efficient ion diffusion and faster electron transport.^7,28–34 Improved performance has been achieved in these hybrid materials due to its elegant synergy between individual component and the facility of efficient electron transfer between current collector and active materials.^29,35–37 Besides, the voids volume between neighboring nanostructured arrays can also promote fast ion transport and allow easy accessibility of electrolyte.^7,34 For example, MnO₂ nanostructure deposited onto 3-D nickel nanocone arrays with a high specific capacitance (632 F g⁻¹) and outstanding mechanical robustness was reported by Su's group.²⁷ In addition, the Ni@NiO core–shell electrode reported by Lu group shows ultrahigh capacitance of 2.0 F cm⁻² and super-long cyclic stability.³⁶ A key challenge in this direction is to build up an integrated ultra-fine smart architecture where structural features and electro-activities of individual component are fully manifested. In this regard, design of electrode materials with 2-D would be an ideal solution compared with widely studied 1-D structural configuration, which could offer more exposed active site than the latter.

Herein, we reported the design and synthesis of hierarchical NiO@MnO₂ core–shell nanosheets arrays on carbon fiber paper (CFP) as binder free-electrode for electrochemical capacitors. We choose ultra-fine NiO nanosheets as scaffold for the growth of branched MnO₂ shell. By elaborate design and rational combination of NiO and MnO₂ nanostructure, the NiO@MnO₂ nanosheets could deliver an areal specific capacitance of 1.0 F cm⁻² (494.8 F g⁻¹). In addition, the asymmetric supercapacitor composed of NiO@MnO₂ electrode and commercial activated carbon delivers an maximum energy density of 30.6 W h kg⁻¹ and power the density 6400 W kg⁻¹.

2. Experimental

2.1 Preparation of NiO nanosheets arrays

All the chemicals were analytical grade and were used without any further purification. The sample was prepared by a two-step hydrothermal method. Typically, a aqueous solution containing 0.05 M Ni(Ac)₂·4H₂O and 0.15 M hexamethylenetetramine (HMT) was transferred to a Teflon-lined stainless autoclave with CFP. After 5 h growth at 120 °C, the precursor were washed with ethanol and distilled water before dried. Finally, NiO nanosheet were obtained after annealed in air atmosphere at 350 °C for 2 h. The mass loading of NiO is ∼0.3 mg cm⁻².

2.2 Preparation of ultra-fine core–shell NiO/MnO₂ nanosheet arrays

For the synthesis of core–shell NiO/MnO₂ nanosheet, the obtained NiO was further put into a Teflon-lined stainless autoclave containing 0.04 M KMnO₄ aqueous solution which was kept at 160 °C for 30 minutes. Finally, core–shell NiO/MnO₂ nanosheet arrays were obtained after drying at room temperature. The mass loading of NiO@MnO₂ is 2.1 mg cm⁻².

2.3 Characterizations and electrochemical measurements

The morphologies and structures of samples were characterized by field-emission scanning electron microscopy (FESEM, JSM-7800F) equipped with an energy-dispersive X-ray spectroscopy detector (EDS), X-ray diffraction (XRD, shimadzu 2D-3AX, Cu Kα radiation, λ = 1.5418 Å) and transmission electron microscopy (TEM, Tecnai G2 F20 U-TWIN). The porous structures of materials were characterized by nitrogen adsorption–desorption measurements (Micromeritics ASAP 2020) along with the Brunauer–Emmett–Teller (BET) specific surface area and Barrett–Joyner–Halenda (BJH) pore size distribution.

Three-electrode electrochemical measurements were carried out at ambient temperature in 1 M KOH aqueous solution with active materials supported on CFP (1 × 1 cm²), a Pt foil and Ag/AgCl (saturated KCl) electrode used as working electrode, counter electrode and reference electrode, respectively. All potentials were referred to the reference electrode. For the tests in two-electrode system, both of the electrodes have the geometrical area of 1 cm² which were separated by a glass fiber filter paper with 1 M KOH aqueous solution as electrolyte. The NiO/MnO₂ nanosheets arrays were the positive electrode and the commercial activated carbon (AC) prepared by mixing active carbon, acetylene black and 60 wt% PTFE in a 80 : 15 : 5 mass ratio was negative electrode, which was then pressed onto nickel foam and dried overnight. The electrochemical performance of the samples were performed on a CHI66D (Chenhua, Shanghai) workstation for cyclic voltammetry (CV), chronopotentiometry (CP) tests and electrochemical impedance spectroscopy (EIS). The electrochemical impedance spectroscopy (EIS) tests were carried out at open circuit potential applying 5 mV ac voltage in the frequency range from 100 kHz to 0.01 Hz. The galvanostatic charge–discharge cycling tests were conducted using a LAND battery program-control test system (LAND, Wuhan).

3. Results and discussion

3.1 Morphology and structure characterizations

Fig. 1a illustrates the XRD patterns of NiO and hierarchical NiO@MnO₂ core–shell arrays. Apart from the peaks located at 26.6° and 54.5° for substrate, the peaks located at 37.3°, 43.4° and 62.8° can be indexed to the of NiO (JCPDS 44-1159). For NiO@MnO₂ core–shell arrays, two new peaks appear, which can be indexed to (002) and (020) or (−312) peaks for MnO₂ (JCPDS 80-1098). Besides, the peak located at 37.3° became broad and shifted to a lower degree direction, which may be due to the overlap of (101) plane NiO and (−111) plane of MnO₂. It is worth noting that due to the second growth of MnO₂ on NiO, the (012) and (110) peaks of NiO became weak and almost disappeared. The XRD results indicate the coexistence of NiO and MnO₂. The successful growth of MnO₂ is also evidenced by EDS in Fig. 1b, which is in accordance with XRD results. It shows that the sample contains Ni, Mn and O with and a trace presence of K, arising from the residual K in the interlayer of MnO₂.

Fig. 1 (a) XRD pattern of NiO and NiO@MnO₂. (b) EDS spectra of NiO@MnO₂.

The SEM images of the as prepared samples with different magnifications are presented in Fig. 2. It can be observed that NiO with ultra-fine nanosheets are grown uniformly on CFP, forming a 3D hierarchical structure. Further after the reaction in KMnO₄, NiO nanosheet was covered with uniform and ultra-thin MnO₂ nanosheets, which are thought to have developed naturally as a result of the crystallographic habit of birnessite-type MnO₂, building a 2D nanosheet@nanosheet core–shell unique architecture. As can be seen from Fig. 2b, the NiO ultrathin sheets are converted to thick walls and the MnO₂ layer on the surface of NiO is also constructed by small ultrathin sheets. The detailed structural information of hierarchical NiO@MnO₂ core–shell was further investigated by TEM. Fig. 2c and d show a typical transmission electron microscopy (TEM) image at different magnification. The results confirm the uniform growth of MnO₂ on NiO and their sheet-like morphology. The high-resolution TEM (HRTEM) image shown in Fig. 2e reveals the interplanar spacing of 0.212, 0.252 and 0.253 nm for two curling nanosheets, corresponding to the (−112), (200) and (−201) plane of birnessite-type MnO₂. Besides, the typical diffraction rings in SAED pattern shows (113) and (024) plane of NiO and (114) and (201) plane of MnO₂. SAED results together with the HRTEM images, demonstrates a polycrystalline nature of NiO@MnO₂.

Fig. 2 SEM images of (a) NiO and (b) NiO@MnO₂ with different magnifications. Inset is the locally enlarged image. (c) and (d) TEM images of a NiO@MnO₂ core–shell nanosheet scratched off the CFP with different magnifications. (e) HRTEM image and (f) the corresponding SAED pattern of NiO@MnO₂ nanosheet.

To further investigate the surface and porous properties of the hierarchical NiO@MnO₂ core–shell arrays, the N₂ adsorption–desorption isotherm pores were measured. Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods are employed to calculate the surface area value and pore size distribution of the samples, respectively. As shown in Fig. 3, it can be seen that the N₂ adsorption–desorption curves of NiO@MnO₂ core–shell arrays has a hysteresis loop in a relative pressure (P/P₀) range of 0.5–1.0, characteristic of type-IV isotherms.^32,33 The N₂ adsorption–desorption isotherm results indicate that NiO@MnO₂ core–shell arrays electrode exhibit a BET surface area of 40.3 m² g⁻¹. Barrett–Joyner–Halenda (BJH) pore size distribution further demonstrates that the average pore size was ∼3.3 nm.

Fig. 3 (a) Nitrogen adsorption and desorption isotherms of NiO@MnO₂ nanosheet and (b) the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution.

3.2 Electrochemistry measurements

The electrochemical performance of as-prepared hierarchical NiO@MnO₂ core–shell nanosheets structure is evaluated as binder-free electrodes for supercapacitors by cyclic voltammetry (CV) and galvanostatic charge–discharge measurements carried out in 1 M KOH. Fig. 4a shows the representative CV curves of NiO@MnO₂ core–shell nanosheets and pristine NiO nanosheets arrays at the scan rate of 10 mV s⁻¹. The CV curve of NiO nanosheets shows a pair of redox peaks, which correspond to the conversion between different nickel oxidation states.^11,12 For NiO@MnO₂ core–shell nanosheets, a pair of redox peaks with a larger enclosed rectangle shaped CV curve can also be observed, revealing the pseudocapacitive characteristics and its higher specific capacitance.^31,34 CV curves of NiO@MnO₂ with various sweep rates ranging from 5 to 50 mV s⁻¹ are plotted in Fig. 4b. With the increase of sweep rates, the current density increases and the positions of anodic and cathodic peaks shift to a more anodic and cathodic direction, respectively, indicating the fast redox reactions occur at electro-active material/electrolyte interface. Fig. 4c show the galvanostatic charge–discharge profiles tested at different current density. The observation of nearly symmetric potential-time curves at all current densities implies the high charge–discharge coulombic efficiency and low polarization of the unique electrode. The specific capacitance retention of NiO@MnO₂ nanosheets and pristine NiO nanosheets arrays calculated from the galvanostatic charge–discharge curves as a function of scan rate is shown in Fig. 4d. It is obvious that the specific capacitances decrease with increasing scan rate which is due to less number of ions lodged in the electrode at higher scan rate that leads to low utilization of active materials. The SCs of the NiO@MnO₂ electrode calculated based on the charge–discharge curves is much higher than that of pristine NiO nanosheets under the same current density. In addition, NiO demonstrated a better rate capability compared with NiO@MnO₂ when the current density varies from 2 to 8 mA cm⁻², which might be due to a low utilization of active materials for NiO@MnO₂ with the increasing of current density. Furthermore, to verify the results, electrochemical impedance spectroscopy (EIS) tests were carried out at open circuit potential applying 5 mV ac voltage in the frequency range from 100 kHz to 0.01 Hz. The Nyquist plots of the NiO and NiO@MnO₂ electrodes are illustrated in Fig. 4e. It is can be seen that NiO@MnO₂ demonstrates a smaller internal resistance than that of NiO nanosheets. In addition, there are two semicircles at high frequency region, corresponding to the charge-transfer resistance (R_ct) at the NiO core/MnO₂ shell and MnO₂/electrolyte interface, resulting in higher charge-transfer resistance than that of NiO. That is why the rate capability of NiO is better than that of NiO@MnO₂ when the discharge current density changes from 2 mA cm⁻² to 8 mA cm⁻² as exhibited in Fig. 4d. Cycling performance is a key factor in determining the supercapacitors for many practical applications. In this study, a long-term repeated charge–discharge cycling tests of the as-synthesized NiO@MnO₂ conducted at a constant current density of 20 mA cm⁻² were employed. The results and charge–discharge curves for the last 17 cycles are shown in Fig. 4f. It can be seen that the specific capacitance retention of the electrode materials increases around 400^th cycle and then gradually decreases, indicating the presence of an activation process of the electrodes at the beginning of galvanostatic charge–discharge test.²⁸ During this process, the electrode will be fully activated through the intercalation and de-intercalation of ions, resulting in the increase of active points inside the electrode materials, hence enhancing the specific capacitance. The hybrid core/shell nanowires array exhibits capacitance retention of 78% after 10 000 cycles. The electrochemical properties of NiO@MnO₂ here is better than those similar reported materials, such as TiO₂ NW@MnO₂ NS (120 F g⁻¹, 0.1 A g⁻¹),¹⁴ Co₃O₄ NW@MnO₂ NS (384.3 F g⁻¹, 1 A g⁻¹),³³ Ti@MnO₂ (467.8 F g⁻¹, A g⁻¹),³⁰ and Ni@MnO₂ (304 F g⁻¹, 2 A g⁻¹).³⁸ The improved performance is mainly attributed to the unique core–shell structure of NiO@MnO₂ composed of two dimensional nanosheets.

Fig. 4 (a) CV curves of NiO and NiO@MnO₂ nanosheet at the scan rate of 10 mV s⁻¹. (b) CV curves and (c) galvanostatic charge–discharge plots of NiO@MnO₂ core–shell nanosheet at different scan rates and current densities, respectively. (d) The corresponding specific and areal capacitance of NiO and NiO@MnO₂ nanosheet as a function of current density. (e) EIS Nyquist plots of NiO and NiO@MnO₂ nanosheet. The insets are the partial enlarged Nyquist plots. (f) Cycling performance of NiO@MnO₂ nanosheet electrode at current density of 20 mA cm⁻². The inset is the galvanostatic charge–discharge curves of last 17 cycles.

To further assess the application value of the NiO@MnO₂ core–shell NSAs, we fabricated an asymmetric supercapacitor, consisting of a battery-type faradic electrode (positive electrode) as the energy source and a capacitor-type electrode (negative electrode) as the power source, which can exhibit higher energy density than that of symmetric ones due to the extended operating voltage.¹⁰ NiO@MnO₂ and commercial activated carbon were employed as positive electrode and negative electrode, respectively, with 1 M KOH solution as the electrolyte and one piece of filter paper as the separator. In order to obtain the maximum performance of the device, the charge balance between the two electrodes was considered based on the three-electrode electrochemical measurements of both NiO@MnO₂ and AC electrodes. The whole mass loading of the asymmetric capacitor is ∼4.8 mg. The electrochemistry test results are presented in Fig. 5. As expected, CV curves of the device at different potential window (Fig. 5b) shows that the cell voltage could be extended to 1.6 V. Fig. 5b displays the CV curves of the device at different scan rates. All the curves exhibit relatively rectangular shape, indicating its good charge–discharge properties and rate capability. Galvanostatic charge–discharge tests (Fig. 5c) were employed to calculate the energy density and power density of the supercapacitor and Ragone plot at different current densities was obtained (Fig. 5e). A specific capacitance of 86.3 F g⁻¹ is obtained based on the total mass active materials (NiO@MnO₂ and active carbon). An energy density of 30.6 W h kg⁻¹ is achieved at the power density of 400 W kg⁻¹ and still remains 18.4 W h kg⁻¹ at high power density of 6.4 kW kg⁻¹, which demonstrates its superiority in retention of energy density. Fig. 5f reveals the cycling life and Coulomb efficiency of the asymmetric supercapacitor at the current density of 4 A g⁻¹. It retains 98.3% of its initial capacitance after 10 000 cycles, which highlights the capability of the hierarchical NiO@MnO₂ core–shell NSAs in real application.

Fig. 5 (a) CV curves of the asymmetric supercapacitor at the scan rate of 50 mV s⁻¹ in different applied potential windows. (b) CV curves and (c) galvanostatic discharge curves of the asymmetric supercapacitor at different scan rates and current densities, respectively. (d) The specific capacitance of the asymmetric supercapacitor as a function of current density (e) Ragone plot of our supercapacitor device compared with other data. (f) Cycling performance and Coulomb efficiency of the asymmetric supercapacitor at the current density of 4 A g⁻¹.

4. Conclusion

In summary, we demonstrated the rational design and fabrication of hierarchical hybrid NiO@MnO₂ core–shell NSAs on CFP by a facile and two-step hydrothermal approach as electrode for supercapacitor applications. The NiO nanosheets function as scaffold to anchor ultra-thin MnO₂ nanosheets, leading to a well-designed hybrid architecture. The as-prepared hierarchical nanostructure exhibits improved electrochemical performance than the individual parts, which can be ascribed to the elaborate NiO nanosheets scaffold and the high specific surface area provided by branched ultra-thin MnO₂ shell as well as synergetic contribution from the porous NiO nanosheets core and the ultrathin MnO₂ shell. The as-prepared electrode delivers high capacitance of 494.8 F g⁻¹ and desirable rate performance and electrochemical stability (78% retention after 10 000 cycling test). In addition, a maximum energy density of 30.6 W h kg⁻¹ and power density of 6.4 kW kg⁻¹ is achieved in a asymmetric supercapacitor where NiO@MnO₂ and active carbon was employed as positive and negative electrodes, respectively.

Acknowledgements

This work was supported by the fundamental research funds for the Central Universities (Project No. 106112015CDJZR305501) and the sharing of Chongqing University's large-scale equipment.

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