Solution growth of NiO nanosheets supported on Ni foam as high-performance electrodes for supercapacitors
- Hailong Yan†1, 2,
- Deyang Zhang†1, 2,
- Jinyou Xu1, 2,
- Yang Lu1, 2,
- Yunxin Liu3,
- Kangwen Qiu†1, 2,
- Yihe Zhang4 and
- Yongsong Luo1, 2Email author
© Yan et al.; licensee Springer. 2014
Received: 20 June 2014
Accepted: 9 August 2014
Published: 22 August 2014
Well-aligned nickel oxide (NiO) nanosheets with the thickness of a few nanometers supported on a flexible substrate (Ni foam) have been fabricated by a hydrothermal approach together with a post-annealing treatment. The three-dimensional NiO nanosheets were further used as electrode materials to fabricate supercapacitors, with high specific capacitance of 943.5, 791.2, 613.5, 480, and 457.5 F g-1 at current densities of 5, 10, 15, 20, and 25 A g-1, respectively. The NiO nanosheets combined well with the substrate. When the electrode material was bended, it can still retain 91.1% of the initial capacitance after 1,200 charging/discharging cycles. Compared with Co3O4 and NiO nanostructures, the specific capacitance of NiO nanosheets is much better. These characteristics suggest that NiO nanosheet electrodes are promising for energy storage application with high power demands.
KeywordsNiO nanosheets Hydrothermal approach Long cycle life Flexible supercapacitors
Supercapacitors, also called electrochemical capacitors, are the most promising energy storage and power output technologies for digital communication devices, hybrid electric vehicles, and other high-power energy sources, which are attributed to the advantages of high power density, short charging time, high cycle efficiency, and long cycle life [1–6]. However, due to the low energy density of current supercapacitor products, nowadays, developing novel electrode materials with enhanced energy density, while maintaining a high power density, good specific capacitance, and cycling stability for supercapacitors, has become a primary research focus. Unfortunately, the practical applications of supercapacitors are largely hindered due to the lack of high-performance electrode materials at a reasonable cost [7–9]. Carbon-based materials and many transition metal oxides have been widely investigated as electrode materials for supercapacitors with notable improvements achieved [10–13]. However, the relatively low specific and volumetric capacitances of carbon-based materials and extremely high cost of the state-of-art RuO2 materials have seriously limited their practical application in supercapacitors. The development of nanomaterials, especially metal oxides, will undoubtedly provide a promising solution to enhance the capacitive performance because of their high surface area, and ion transport pathways. Several promising materials, including nickel oxide, cobalt oxide, and manganese oxide, have been intensively studied as advanced electrode materials for supercapacitors [14–16]. Nickel oxide (NiO) has been intensively studied as supercapacitors for its high theoretical specific capacitance of 2,584 F g-1[17, 18]. In other words, lightweight and flexibility have become one of the most important development trends of portable electronics in these years [19, 20]. Whether flexible portable electronics become popular depends on the improvements of the technology, especially by developing the flexible high-performance energy storage devices.
A facile solution method is developed to grow NiO nanonsheets directly on Ni foam, which possess an enhanced electrochemical performance for supercapacitors. Our optimized supercapacitor shows a specific capacitance of 943.5 F g-1 at a current density of 5 A g-1. It is found that the specific capacitance of NiO nanonsheets is higher than those of Co3O4 and NiO nanostructures fabricated by the same method. It can be concluded that vertical NiO nanosheets would be particularly suited to the high-performance electrodes for supercapacitors.
Synthesis of NiO nanosheets on Ni foam
The morphology of the synthesized product was examined using field emission scanning electron microscopy (S4800, Hitachi, Chiyoda-ku, Japan). The chemical composition of the product was characterized by X-ray diffraction (XRD; D8 Advance X-ray Diffractometer, Cu Kα, λ = 1.5406 Å, Bruker, Saarbrucken Germany). Raman spectra were recorded on an INVIA Raman microprobe (Renishaw Instruments, Wotton-under-Edge, England) with a 532-nm laser excitation. The thermogravimetric analysis (TGA) curve was performed using a SDT Q600 TA with 100 ml min-1 of air flow from 20°C to 600°C at a heating rate of 10°C min-1.
The capacitive performance of the samples was tested on a CHI 660E electrochemical workstation (CH Instruments, Chenhua, Shanghai, YP, China) with cyclic voltammetry and chronopotentiometry functions using a three-electrode cell where Pt foil serves as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The mass loading of unit area on the Ni foam can be calculated to be 1.78 mg cm-2. The electrolyte used was a 3 M KOH aqueous solution.
Results and discussion
where C m (F g-1) is the specific capacitance, I (A) is the discharge current, Δt (s) is the charging/discharging time, ΔV (V) is the voltage window for discharge, and m (g) is the mass of the active NiO material in the electrode. Thus, the specific capacitance can be calculated to be 943.5, 791.2, 613.5, 480, and 457.5 F g-1 at the scan rates of 5, 10, 15, 20, and 25 A g-1, respectively (Figure 6d). The specific capacitance of NiO nanosheets is much higher than that of NiO nanobelts, nanorods, and nanosheets reported previously [22–26]. To evaluate the important role of NiO nanosheets for high-performance electrodes, the specific capacitances of Co3O4 nanoneedles and NiO powders are also tested at the scan rates of 5, 10, 15, 20, and 25 A g-1, respectively. The specific capacitances of NiO nanosheets win out over those of Co3O4 nanoneedles and NiO powders (Additional file 1: Figure S2).
The improved electrochemical performance could be related to the following structural features. Firstly, the aligned NiO nanosheets with a high surface area facilitate ion diffusion from the electrolyte to each nanosheets, making full use of the active materials . Secondly, the vertical NiO nanosheets could ensure good mechanical adhesion, and more importantly, vertical nanosheets can build up a shortcut and high-speed bridge between the current collector and active materials (Additional file 1: Figure S3). Thirdly, Ni foam as the platform for sustaining nanosheets can withstand strain relaxation and mechanical deformation, preventing the electrode materials from seriously swelling and shrinking during the insertion-deinsertion process.
Well-aligned NiO nanosheets are fabricated by a hydrothermal approach, and it is used as binder-free electrodes for supercapacitors. The ultrathin NiO nanosheets supported on the nickel foam is able to deliver areal capacitances of 1.98 and 1.68 F cm-2 at current densities of 11.8 and 23.5 mA cm-2, respectively. The vertical NiO nanosheets on the substrate can withstand strain relaxation and mechanical deformation. When the electrode material is bent, it can still retain 91.1% of the initial capacitance after 1,200 charging/discharging cycles. The specific capacitance of NiO nanosheets is much higher than those of Co3O4 and NiO nanostructures. Such highly integrated binder- and additive-free electrodes made by electroactive NiO nanosheets might hold some potential for the fabrication of high-performance flexible energy storage devices.
This work was financially supported by the National Natural Science Foundation of China (Nos. U1304108, U1204501, and 11272274), the Science and Technology Key Projects of Education Department Henan Province (No. 13A430758), the Natural Scientific Foundation of Hunan Province (No. 13JJ4080), the Young Backbone Teacher of Xinyang Normal University (No. 2013GGJS-18), and the Innovative Research Team (in Science and Technology) in the University of Henan Province (No. 13IRTSTHN018).
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