A composite electrode consisting of nickel hydroxide, carbon nanotubes, and reduced graphene oxide with an ultrahigh electrocapacitance
Graphical abstract
The preparation of a composite electrode material consisting of Ni(OH)2 nanoparticles embedded in carbon nanotubes as pillars for graphene sheets is demonstrated and used as supercapacitor electrode. The composite displays a specific capacitance as high as 1235 F g−1 at a current density of 1 A g−1 and good cycle stability, representing a new carbon architecture for high-performance supercapacitors.
Highlights
► Ni(OH)2 embedded in carbon nanotubes as pillars for graphene sheets was prepared. ► The composite showed a high specific capacitance and good cycle stability. ► High performance is due to the unique 3D nanostructure of the capacitor electrode.
Introduction
Nickel hydroxide, Ni(OH)2, has been widely used as a positive electrode in commercial alkaline rechargeable batteries [1], [2], [3]. Recent research results have shown that it is also a promising candidate for pseudocapacitors owing to its well-defined electrochemical redox activity, high specific capacitance, low cost, and the availability of various morphologies [4], [5], [6], [7], [8], [9]. The major issues associated with Ni(OH)2 when used as an electrode are its poor electrical conductivity and volume expansion or swelling during charge/discharge cycles, especially for high-rate applications [10], [11]. Therefore, considerable effort has been made to improve the electrochemical performance of Ni(OH)2 by tuning its morphology at nanoscale [10], [12], [13], [14], [15] and modifying it to make composite materials [8], [9], [16], [17], [18]. The highest specific capacitance observed on a Ni(OH)2-nickel foam composite is 3152 F g−1 at a current density of 4 A g−1 [9]. However, the composite displayed a significant capacitance loss – about 50% of initial capacitance loss after 300 cycles. In addition, the specific capacitance was dropped to 280 F g−1 at a current density of 16 A g−1. Wang and co-workers [8], [19] described a two-step method to grow Ni(OH)2 nanocrystals on graphene. The authors observed that highly crystalline Ni(OH)2 nanoplates grown on high-quality graphene sheets possessed a specific capacitance of about 935 F g−1.
Three-dimensional (3D) nanoporous structures have been shown to have advantages in many technological applications, such as hydrogen storage [20] and supercapacitors [21], [22], [23]. Recently, we have demonstrated an approach to the preparation of porous conducting-polymer-pillared graphene oxide (GO) and reduced graphene oxide (RGO) sheets [24], [25]. We have also reported a method for the preparation of 3D porous carbon-nanotube-pillared GO and RGO nanostructures [26]. Excellent electrical conductivity, efficient electron transfer and high specific surface area were observed from the carbon-nanotube-pillared RGO composite.
On the basis of our previous studies [24], [25], [26], we here demonstrate an approach to preparing graphene-based composite materials consisting of RGO sheets, Ni(OH)2 nanoparticles and carbon nanotubes (CNTs). The composite materials displayed an excellent electrochemical performance as a pseudocapacitor electrode with specific capacitances of 1235 F g−1 at a current density of 1 A g−1 and 780 F g−1 at a higher current density of 20 A g−1, as well as a good cycle stability. The superior electrochemical performance of the composite materials is attributed to the desirable 3D conducting architecture and the well-defined redox properties of the Ni(OH)2 nanoparticles. To the best of our knowledge, this is the first work demonstrating the preparation of 3D CNT-pillared RGO sheets with embedded Ni(OH)2 nanoparticles for pseudocapacitors.
Section snippets
Preparation of GO and RGO
The GO dispersion was prepared by sonication of graphite oxide, which was prepared from natural graphite (crystalline graphite flakes, particle size ranging from 45 to 500 μm, carbon content was 99%) using a modified Hummers method [25], [27]. 5 g of graphite and 2.5 g of NaNO3 were mixed with 120 mL of H2SO4 (95 wt%) in a 500 mL flask. The mixture was stirred for 30 min in an ice bath. While maintaining vigorous stirring, 15 g of KMnO4 was added. The rate of addition was carefully controlled
Results and discussion
The experimental steps for the preparation of samples are illustrated in Scheme 1. The details for the preparation of CNT-pillared RGO (RGOCNT) can be found elsewhere [26]. The Ni(OH)2-embedded RGOCNT was obtained through electrochemical conversion of the nickel nanoparticles in an alkaline solution using the cyclic voltammetry technique (see details in the experimental section). Fig. 1a shows the scanning electron microscopy (SEM) image of the restacked RGO platelets after impregnation with
Conclusions
In summary, we have demonstrated the preparation of 3D Ni(OH)2-embedded-RGOCNT composite materials with excellent electrocapacitive properties. The symmetric supercapacitor based on electrode RGOCNT-15-4 exhibited a high specific capacitance (1235 F g−1 at a current density of 1 A g−1) with an excellent rate capability and a good cycle stability. The unique advantages of such 3D nanostructures as a supercapacitor electrode include fast ion and electron transfer throughout the electrode matrix,
Acknowledgments
Financial support from The Australian Research Council (ARC) under the ARC Future Fellow Program (FT100100879) is acknowledged.
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