Elsevier

Journal of Power Sources

Volume 252, 15 April 2014, Pages 98-106
Journal of Power Sources

Short communication
Facile synthesis of hierarchical Co3O4@MnO2 core–shell arrays on Ni foam for asymmetric supercapacitors

https://doi.org/10.1016/j.jpowsour.2013.12.030Get rights and content

Highlights

  • Hierarchical Co3O4@MnO2 nanowire array was prepared by a facile hydrothermal method.

  • MnO2 coated on Co3O4 nanowire without carbon coating or electrochemical deposition.

  • The unique core–shell architecture exhibits a high capacitance of 560 F g−1.

  • Excellent cycling stability: 95% capacitance retention after 5000 cycles.

  • The asymmetric supercapacitor yielded a maximum power density of 158 kW kg−1.

Abstract

Hierarchical Co3O4@MnO2 core–shell arrays on Ni foam have been fabricated by a facile hydrothermal approach and further investigated as the electrode for high-performance supercapacitors. Owing to the high conductivity of the well-defined mesoporous Co3O4 nanowire arrays in combination with the large surface area provided by the ultrathin MnO2 nanosheets, the unique designed Co3O4@MnO2 core–shell arrays on Ni foam have exhibited a high specific capacitance (560 F g−1 at a current density of 0.2 A g−1), good rate capability, and excellent cycling stability (95% capacitance retention after 5000 cycles). An asymmetric supercapacitor with Co3O4@MnO2 core–shell nanostructure as the positive electrode and activated microwave exfoliated graphite oxide activated graphene (MEGO) as the negative electrode yielded an energy density of 17.7 Wh kg−1 and a maximum power density of 158 kW kg−1. The rational design of the unique core–shell array architectures demonstrated in this work provides a new and facile approach to fabricate high-performance electrode for supercapacitors.

Introduction

The increasing demand for sustainable and renewable power sources in modern electronic industries has stimulated intensive research efforts on the developments of high-performance, lightweight and environmental friendly energy storage devices [1], [2], [3]. Among various emerging energy storage technologies, supercapacitors (SCs), also known as electrochemical capacitors (ECs), are the ideal candidates for green energy storage because of their high power density, excellent pulse charge–discharge characteristics, super-high cycling life and safe operation [4], [5], [6]. Based on the mechanism of charge storage, supercapacitors can be classified as two kinds: electrical double-layer capacitors (EDLCs) that typically use carbon-active materials and pseudocapacitors that use redox-active materials [7], [8]. In particular, pseudocapacitors based on the transition metal oxides/hydroxides with variable valence exhibit a higher specific capacitance than those based on carbonaceous materials and conducting polymers as they can provide a variety of oxidation states for efficient redox charge transfer [9], [10], [11], [12]. Therefore, great efforts have been devoted to searching for inexpensive transition metal oxides with good capacitive characteristics, such as Co3O4 [13], NiO [14], MnO2 [15], Fe2O3 [16], V2O5 [17], and CuO [18].

Among these candidate materials, MnO2 exhibits many intriguing characteristics, such as low cost, environmental friendliness, natural abundance, high theoretical capacity (1370 F g−1), and wide operating potential window, suggesting it as the most promising electrode material for supercapacitors. However, low surface areas and poor electrical conductivity (10−5–10−6 S cm−1) of MnO2 limits the charge–discharge rate for high-performance supercapacitors [19], [20], [21]. In this regard, further efforts are focused on incorporating MnO2 nanostructures with carbon-based materials or conducting polymers [22], [23], [24], [25]. By combing unique properties of individual constituents, improved rate capability and cycling ability could be achieved in such an electrode. Another problem of MnO2-based materials arises from the low loading of active materials, which would lead to a low energy density. Therefore, it is still great challenge to boost the electrochemical utilization of the pseudocapacitance of MnO2 by rationally designing MnO2-based electrodes with novel structures and reliable electric connection. An emerging attractive concept is to directly grow smart integrated array architectures with the combination of multi-component structures on conducting substrates as binder-free electrodes for supercapacitors which can provide synergistic effects form all of their individual constituents, thus achieving high power and energy density, long cycle life, and high rate capability [26]. A challenge in this direction is to develop a desirable smart architecture, in which electrochemical activities of each component are fully manifested and the kinetics of ion/electron transport are guaranteed. Based on the above consideration, various MnO2-based nanocomposites with different geometrical attributes and morphological forms have been employed as electrodes for supercapacitors. For instance, Co3O4@MnO2, NiCo2O4@MnO2, Zn2SnO4@MnO2, Co3O4@Au@MnO2, Co3O4@Pt@MnO2, TiO2@MnO2, and Fe2O3@MnO2 nanocomposites have been developed with improved electrochemical performance [27], [28], [29], [30], [31], [32], [33], [34]. However, the synthetic procedures for these unique MnO2-based core–shell structures are still relatively complicated since they either need carbon coating or electrochemical deposition process. Thus, a novel but simple design and fabrication of multi-component hierarchical heterostructures with highly-accessible surface areas and fast ion diffusion for supercapacitors still remains a challenge.

Herein, we demonstrate a facile and cost-effective approach to design and fabricate hierarchical Co3O4@MnO2 core–shell arrays on Ni foam as a binder-free electrode for high-performance supercapacitors, in which the mesoporous Co3O4 arrays served as the “core” and the ultrathin branch MnO2 nanosheets are the “shell” layer. By virtue of the synergetic contribution from individual constituents and the sophisticated configuration, the resulting Co3O4@MnO2 core–shell arrays exhibit a much higher capacitance (560 F g−1) and excellent cycling ability (95% retention after 5000 cycles) with respect to pristine Co3O4 arrays. In addition, an asymmetric supercapacitor with hierarchical Co3O4@MnO2 core–shell arrays as the positive electrode and activated microwave exfoliated graphite oxide activated graphene (MEGO) as the negative electrode manifests an energy density of 17.7 Wh kg−1 with a maximum power density of 158 kW kg−1. Undoubtedly, the facile design of hierarchical architecture and control over the multi-composition demonstrated in this work offers a promising strategy for the fabrication of high-performance electrodes for supercapacitors.

Section snippets

Materials

All the chemical reagents were purchased from Alfa Aesar, which were of analytical purity and used without any further purification.

Synthesis of Co3O4 nanowire arrays

The Co3O4 nanowire arrays were fabricated by a modified hydrothermal method according to the previous work [35]. In a typical synthesis, Co(NO3)2·6H2O (1–4 mmol), NH4F (2–8 mmol), CO(NH2)2 (5–20 mmol) were dissolved in 50 mL of deionized water and stirred for 10 min to form a clear solution (the different mixture solutions were used to obtain different morphologies

Structure and morphology

Fig. 1 presents the composition and crystallite phase purity of the Co3O4 nanowire arrays and hierarchical Co3O4@MnO2 core–shell arrays. As shown in Fig. 1a, except for the peaks originating from the Ni foam, the diffraction peaks of Co3O4 arrays are observed of 19.0°, 31.3°, 36.9°, 59.4°, and 65.2°, which could be assigned to the (111), (220), (311), (511), and (440) planes of the cubic Co3O4 (JCPDS card no. 42-1467). It can be observed in the XRD pattern of the hierarchical Co3O4@MnO2

Conclusions

In summary, we have developed a facile and cost-effective strategy to fabricate hierarchical Co3O4@MnO2 core–shell array architectures with high electrochemical performance for supercapacitors. Owing to the synergistic effect of the mesoporous Co3O4 nanowires core and the ultrathin MnO2 nanosheets shell, the resulting Co3O4@MnO2 core–shell nanocomposite electrode enables facilitated ion/electron transport and accordingly exhibits a large specific capacitance, good rate capability and excellent

Acknowledgment

The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (Grant no. 51104194), Doctoral Fund of Ministry of Education of China (20110191120014), No. 43 Scientific Research Foundation for the Returned Overseas Chinese Scholars, National Key laboratory of Fundamental Science of Micro/Nano-device and System Technology (2013MS06, Chongqing University), State Education Ministry and Fundamental Research Funds for the Central Universities

References (51)

  • Y. Liu et al.

    J. Power Sources

    (2013)
  • P. Lu et al.

    Electrochim. Acta

    (2012)
  • Y.Q. Zhang et al.

    J. Power Sources

    (2012)
  • Z. Wang et al.

    J. Alloys Compd.

    (2013)
  • D.P. Dubal et al.

    J. Power Sources

    (2013)
  • Y.X. Zhang et al.

    J. Power Sources

    (2014)
  • G. Wang et al.

    J. Power Sources

    (2013)
  • Z.-C. Yang et al.

    J. Power Sources

    (2013)
  • M. Nakayama et al.

    J. Power Sources

    (2008)
  • X. Zhang et al.

    Electrochim. Acta

    (2013)
  • L. Deng et al.

    J. Power Sources

    (2011)
  • Q. Cheng et al.

    Carbon

    (2011)
  • J.R. Miller et al.

    Science

    (2008)
  • P. Simon et al.

    Nat. Mater.

    (2008)
  • C. Liu et al.

    Adv. Mater.

    (2010)
  • L.L. Zhang et al.

    Chem. Soc. Rev.

    (2009)
  • G. Wang et al.

    Chem. Soc. Rev.

    (2012)
  • W. Wei et al.

    Chem. Soc. Rev.

    (2011)
  • W. Chen et al.

    Nano Lett.

    (2011)
  • W. Zhou et al.

    Energy Environ. Sci.

    (2013)
  • P. Yang et al.

    ACS Nano

    (2013)
  • Y. Zhang et al.

    CrystEngComm

    (2012)
  • X.-H. Xia et al.

    Chem. Commun.

    (2011)
  • X. Zhao et al.

    ACS Nano

    (2012)
  • Q. Qu et al.

    Adv. Energy Mater.

    (2012)
  • Cited by (0)

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