Hierarchical NiCo2O4@nickel-sulfide nanoplate arrays for high-performance supercapacitors

doi:10.1016/j.jpowsour.2014.11.015

Journal of Power Sources

Volume 276, 15 February 2015, Pages 19-25

https://doi.org/10.1016/j.jpowsour.2014.11.015 Get rights and content

Highlights

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Hierarchical NiCo₂O₄@ nickel-sulfide nanoplate arrays were prepared.
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The array electrode shows good integrity and porosity.
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The array electrode exhibits an improved areal capacitance of 1.85 F cm⁻² at 8 mA cm⁻².

Abstract

Electrodeposition of nickel sulfide (Ni–S) on NiCo₂O₄ (NiCo₂O₄@Ni–S) nanoplate arrays produces a new hierarchical core–shell functional-material. Weak crystalline Ni–S nanosheets have been uniformly coated on the NiCo₂O₄ nanoplate array obtained by hydrothermal growth. When tested as an electrode for supercapacitors, the NiCo₂O₄@Ni–S nanoplate arrays have been found to exhibit a significantly improved areal capacitance of 1.85 F cm⁻² at a current density of 8 mA cm⁻², good rate capability and cycling stability. In addition, the capacitance fading of the NiCo₂O₄@Ni–S arrays are attributed to the surface coarsening of the Ni–S nanosheets.

Graphical abstract

Nickel sulfide on NiCo₂O₄ core–shell nanoplate arrays are fabricated and show good energy density and cycling stability for supercapacitors.

Introduction

The environmental issues have pushed human being to find novel energy sources instead of fossil fuels to meet the increasing demands of energy consumptions. Currently, it is deeply urgent to develop reliable green energy storage technologies to achieve a secured and reliable energy supply. The high power performance of supercapacitors has made them important complement power sources of batteries to meet the increasing demands of energy storage and conversion [1], [2], [3]. In the two categories of supercapacitors (electrochemical double layer capacitor - EDLC and pseudocapacitor) with different charge storage mechanisms, the EDLC with traditional carbon materials has very limited specific energy. Researchers are trying to develop pseudocapacitive materials based on faradaic reactions with the specific capacitance an order of magnitude higher than that of carbon [4], [5]. To this end, metal oxides have been extensively studied as pseudocapacitor electrode materials, and it has been shown that the architecture design is very important to improve their supercapacitor performance [5], [6]. In particular, 3D array electrodes have drawn much attention because of their facile preparation and excellent performance [7], [8], [9]. On the other hand, recent research results show that metal sulfides can also be used as good active materials for pseudocapacitor applications [10], [11], [12], and especially, core–shell arrays of metal oxide and sulfide have shown excellent performance [12], [13], [14]. However, the syntheses of sulfides usually involve toxic gases as reactant or product, and therefore it is very urgent to develop a green strategy to construct the core–shell oxide-sulfide array architectures.

Nickel sulfides with various compositions, such as NiS, NiS₂, Ni₃S₂, Ni₆S₅, Ni₇S₆, and Ni₃S₄ [15], are an important class of semiconductor materials with great potential multifunctional applications in high-performance supercapacitors, lithium ion batteries, and the photocatalysis production of hydrogen [15], [16], [17], [18], [19], [20]. In particular, Ni₃S₂ is of interest because of its good performance as electrode materials for supercapacitors and lithium ion batteries [15], [21], [22], [23]. A number of Ni₃S₂ composites, such as one-dimensional hierarchical structures of Ni₃S₂ nanosheets grown on carbon nanotube backbone [15], Ni₃S₂/graphene [24], and Ni₃S₂ nanoflakes on a 3D porous nickel foam [25], have been prepared for enhanced performance. Recently, NiCo₂O₄ has drawn intensive attention because of its low cost, environmental friendliness, natural abundance and importantly, a high theoretical capacitance. In addition, the NiCo₂O₄ array electrodes can be facilely synthesized on various substrate with excellent cycling stability, making it a good substrate to grow other pseudocapacitive materials [26]. In this paper, we demonstrate the electrodeposition of Ni–S on NiCo₂O₄ arrays supported by nickel foam. The NiCo₂O₄@Ni–S arrays were further tested as an electrode material for supercapacitors and found to exhibit a high specific capacitance of 1.85 F cm⁻² at a current density of 8 mA cm⁻², good rate capability and cycling stability.

Section snippets

Reagents and materials

NiCl₂·6H₂O, urea, Co(NO₃)₂•6H₂O and the other chemicals were purchased from Aladdin Ltd. (Shanghai, China) and used as received without further purification. The water used throughout all experiments was purified through a Millipore system.

Growing NiCo₂O₄ array on nickel foam

In a typical synthesis, 1 mmol of Ni(NO₃)₂•6H₂O, 2 mmol of Co(NO₃)₂•6H₂O, 6 mmol of NH₄F and 15 mmol of urea were dissolved in 80 mL of deionized water under magnetic stirring for 30 min in air to form a clear pink solution. 12 mL of the above solution was

Results and discussion

The NiCo₂O₄@Ni–S nanoplate arrays were synthesized by a two-step process, as illustrated in Scheme 1. Firstly, NiCo₂O₄ nanoplate arrays were grown on nickel foam by a hydrothermal process in an 80 mL solution containing 1 mmol of Ni(NO₃)₂, 2 mmol of Co(NO₃)₂, 6 mmol of NH₄F and 15 mmol of urea [27]. Secondly, Ni–S nanostructures were electrodeposited on the pre-grown NiCo₂O₄ nanoplate arrays by CV in 10 mL solution containing 50 mM NiCl₂·6H₂O and 1 M thiourea [25]. The weak-crystalline nature

Conclusions

In conclusion, NiCo₂O₄@Ni–S hierarchical core–shell nanoplate arrays have been successfully fabricated by a two-step process combining hydrothermal synthesis and electrodeposition. Such NiCo₂O₄@Ni–S arrays can deliver high specific capacitances of 1.85 F cm⁻² and 1.15 F cm⁻² at current rates of 8 mA cm⁻² and 40 mA cm⁻², respectively, and also exhibit good cycling stability. The good stability of NiCo₂O₄ arrays, the good contact between nickel sulfides and NiCo₂O₄ arrays, and the pores present

Acknowledgments

X. Liu would like to acknwoledge the support from the National Natural Science Foundation of China (no. 21271082), and J. Chen AFOSR (no. FA9550-11-1-0135).

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Published by Elsevier B.V.

Hierarchical NiCo2O4@nickel-sulfide nanoplate arrays for high-performance supercapacitors

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Reagents and materials

Growing NiCo2O4 array on nickel foam

Results and discussion

Conclusions

Acknowledgments

J. Power Sources

J. Power Sources

Nano Energy

Electrochem. Comm.

J. Power Sources

J. Power Sources

Chem. Rev.

Nat. Mater.

Nanoscale

Adv. Mater.

Chem. Comm.

Chem. Comm.

Chem. Comm.

Nano Lett.

Chem. Comm.

J. Mater. Chem.

Nano Lett.

Hierarchical NiCo₂O₄@nickel-sulfide nanoplate arrays for high-performance supercapacitors

Growing NiCo₂O₄ array on nickel foam