Self-supported phase-pure Ni3S2 sheet-on-rod nanoarrays with enhanced pseudocapacitive properties and high energy density
Graphical abstract
A facile hydrothermal method was developed to synthesize self-supported phase-pure Ni3S2 sheet-on-rod nanoarray on nickel foam. The as-prepared sample demonstrated enhanced pseudocapactive properties with superior energy densities when assembled into an asymmetric supercapacitor as cathode with activated carbon as the anode.
Introduction
Recent developments in nanoscience and nanotechnology have provided us with new solutions to tackle the ever increasing energy demand [1], [2]. Rationally designed nanomaterials with unique structures and advanced chemical compositions have demonstrated promising performance in renewable energy systems, such as lithium-ion batteries (LIBs) [3], [4], electrochemical supercapacitors (ESCs) [5], [6], [7], [8], and devices for oxygen/hydrogen evolution [9]. Among the different categories of nanostructures, self-supported three-dimensional (3D) nanomaterials have attracted an enormous amount of attention for their ability to serve as electrode materials for energy storage devices, such as LIBs and ESCs [10], owing to the following distinct advantages [2]:
- a)
Because the electrode material is “self-supported” (defined as the active material being directly grown on the conductive substrate) [2], it creates a highly efficient electron/ion transfer pathway. This means that the material can be directly applied in the device assembly, eliminating the use of binder/conductive agents as commonly practised in the fabrication of present electrode materials, thus avoiding a costly and labour-intensive preparation process;
- b)
The growth of the electroactive materials from the 2D substrate to 3D space significantly increases the active surface area, which is essential for the two types of energy storage devices mentioned above.
Several previous reports have discussed how hierarchical structures assembled from 2D nanosheet demonstrate promising performance in both energy storage and photocatalysis applications [11], [12], [13], [14]. It is thus highly desirable to introduce secondary structures into these self-supported nanomaterials in order to further extend their potential applications. As previously summarized by Ellis et al. [2], the commonly practised methods to fabricate 3D nanoarrays of self-supported 1D nanostructured materials, i.e. nanorods or nanotubes, include templating methods and direct anodization. Although these approaches are relatively straightforward and can provide facile control over the formation of the targeted nanotubes or nanorods, they grant little possibility of creating hierarchical structures with secondary subunits. Likewise, some works have used solution synthesis systems to demonstrate the successful growth of 2D nanosheets on the surface of 1D structures that constitute the 3D nanoarray [15], [16], [17], [18], [19], but these techniques generally contained complicated multiple steps and the obtained products usually involved inhomogeneous chemical phases. For example, Zhou et al. has achieved a similar structure with complex chemical compositions by coating Ni(OH)2 on Ni3S2 [15].
The difficulty in finding a one-pot synthesis of phase-pure 1D hierarchical nanomaterials probably lies in the anisotropic assembly of the 2D building blocks. In order to overcome this challenge, we herein report a facile hydrothermal method to grow self-supported phase-pure Ni3S2 nanorod array on a Ni foam substrate. These Ni3S2 nanorods possess an interesting secondary structure in that their surface is composed of ultrathin Ni3S2 nanosheets. When such a 3D electrode material was applied for pseudocapacitor, it demonstrated a very high and stable capacitance of 2.7 F cm−2 at a current density of 5 mA cm−2 for prolonged charge-discharge tests of up to 5000 cycles. Furthermore, when the as-prepared Ni3S2 nanoarray was assembled into an asymmetric electrochemical capacitor as the cathode, with an activated carbon anode, the device exhibited a capacitance as high as 140 F g−1 for 3000 cycles and superior energy densities of 115 and 102 Wh kg−1 at power densities of 506 and 1094 W kg−1, respectively.
Section snippets
Results and discussion
Fig. 1 shows the morphology of the as-prepared Ni3S2 nanoarray as viewed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is apparent that the sample contains rod-like nanostructures grown on the surface of the Ni foam substrate (Fig. 1A). At a higher magnification (Fig. 1B), it can be observed that the nanorods are about 4–5 μm in length, and the array does not have a condensed packing of the nanorods, as each one of them is surrounded by a large vacant
Conclusion
In this work, a facile hydrothermal method for the growth of phase-pure Ni3S2 nanorod arrays on Ni foam was developed. These nanorods have a unique secondary structure: their surface is covered with very fine and well-defined nanosheets. When such an interesting nanoarray was applied as the electrode material in a pseudocapacitor, it was found that it initially demonstrates a high areal capacitance of 2.0 F cm−2, which increases during the first few hundred cycles to a value of 2.7 F cm−2 at
Experimental section
Materials synthesis. The Ni3S2 nanorod array was synthesized by a facile hydrothermal method. Briefly, a 30 ml aqueous solution containing 0.1 g NiSO4·6H2O and 0.1 g thioacetamide was transferred into a polytetrafluorethylene-lined stainless steel autoclave, then a 2 × 2 cm2 piece of Ni foam was placed into the solution. The autoclave was heated at 180 °C in an electric oven for 20 h, and then allowed to cool naturally to room temperature. The resulting Ni foam was carefully taken out and
Acknowledgements
This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Programme (CRP Award No. NRF-CRP 10-2012-6).
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