Nickel nanoparticles prepared by hydrazine hydrate reduction and their application in supercapacitor
Graphical abstract
The surface nickel atoms of the as-prepared nickel nanoparticles can react easily with O2 and water in the atmosphere to form nickel oxide/hydroxide species, which can generate substantial psudocapacitance.
Highlights
► Nickel nanoparticles as supercapacitor electrode material. ► The size of nickel nanoparticles is about 12 nm. ► The specific capacitance of nickel nanoparticles reaches 416.6 F/g.
Introduction
As an energy storage device, supercapacitor bridges the gap between conventional capacitor and battery, and has been extensively studied in recent years, especially for the electrode materials. The electrode materials can be classified into three categories, i.e. carbon materials [1], [2], [3], [4], metal oxides/hydroxides [5], [6], [7], [8] and conducting polymers [9], [10], [11]. As a substitute for carbon electrode materials, metal oxides/hydroxides commonly show higher specific capacitance than carbon materials and receives great attentions in recent years [12], [13]. Among metal oxides/hydroxides, nickel oxide/hydroxide is attractive to researchers for its good capacitive performances and low cost compared to other metal oxides/hydroxides such as Ru oxide/hydroxide [14]. For instance, Ranga Rao's group studied various nanostructures of NiO to enhance the capacitance values, where a capacitance value of about 400 F g− 1 has been achieved [15], [16]. Up to date, there are numerous reports about preparing nickel oxides/hydroxides with different morphologies such as film [17], [18], [19], [20], [21], nanoplatelet/nanoflake [22], [23], [24], microsphere [25], nanotube [26], nanowall arrays [27], mesoporous particle [28], which exhibit fascinating capacitive performance. It is well accepted that the electrochemical energy storage of nickel oxide/hydroxide is attributed to the following surface redox reactions [29], [30], [31].
Our previous studies [32] have shown that the nickel oxide/hydroxide species naturally forming on the surface of nickel metal in the atmosphere can also undergo the above-mentioned redox reactions and exhibited electrochemically capacitive activity. If the specific surface area of nickel metal is enlarged by reducing the size of the nickel metal, the amount of nickel oxide/hydroxide species that naturally forms on the surface of nickel metal will be increased as well, which could bring about large pseudocapacitance to the nickel metal. If the size of the nickel metal is reduced to nanometer scale, the pseudocapacitance derived from its surface nickel oxide/hydroxide species could be substantial enough to make nickel nanoparticles a possible candidate for supercapacitor electrode material. Another advantage of nickel nanoparticles is that this material has good electrical conductivity because the bulk material of these nanoparticles is metallic nickel.
Herein, we prepared nickel nanoparticles through reducing nickel chloride by hydrazine hydrate. The chemical composition and electrochemical capacitive properties of nickel oxide/hydroxide species on the surface of nickel nanoparticles were investigated intensively in this work. To the best of our knowledge, this is the first report exploring the feasibility of using nickel nanoparticles as supercapacitor electrode material.
Section snippets
Material preparation
All the reagents are AR grade. The nickel nanoparticles were prepared through reducing nickel chloride by hydrazine hydrate according to modified Wu's method [33]. The reaction mechanism can be described by the following reaction equation,
In a typical experiment, 0.952 g nickel chloride and 5.0 g hydrazine hydrate were dissolved in 395.0 ml ethylene glycol. Then, 4.0 ml 1.0 M sodium hydroxide solution was added and stirred in a capped flask for 1 h at 60° C. The obtained
Results and discussion
Fig. 1 shows the micrographs of the as-prepared product. It can be seen obviously from the SEM image that the products are ball-shape aggregates of nanoparticles. In order to observe the size of nanoparticles, TEM observation was also conducted. The TEM image shown in the inset of Fig. 1 presents that the size of nanoparticles is uniform, about 10 nm, which indicates that nanoscale particles were prepared successfully.
In order to reveal the crystal structure of the as-prepared nanoparticles,
Conclusions
Nickel nanoparticles were prepared successfully through reducing nickel chloride by hydrazine hydrate. According to TEM observations and XRD analysis, the size of nickel nanoparticles is about 12 nm. XPS analyses reveal that some surface nickel atoms on the nickel nanoparticles chemically react with O2 and water in the atmosphere to form nickel oxide/hydroxide species, which accounts for pseudocapacitive capacitance of the nickel nanoparticles indicated by cyclic voltammetry and galvanostatic
Acknowledgments
This work was financially supported by Natural Science Foundation of China (51107076), Key Sci-Tech Research Project of Education Department of China (210125), China Postdoctoral Science Foundation (20110491570), Outstanding Young Scientist Foundation of Shandong Province (BS2009NJ014), Key Sci-Tech Development Project of Shandong Province (2009GG10007006), and Young Teacher Supporting scheme of Shandong University of Technology.
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