Synthesis and characterization of Nitrogen-doped &CaCO3-decorated reduced graphene oxide nanocomposite for electrochemical supercapacitors
Introduction
Expected depletion of the natural resources and serious environmental condition are driving the researchers to identify alternative environmentally friendly sources of energy [1], [2], [3]. Supercapacitors, also known as electrochemical capacitors, are one of the auspicious electrochemical energy storage devices due to their high power characteristics compared to batteries, high energy density compared to conventional capacitors, long cycling life and short charging time [3], [4], [5], [6], [7].
There are two energy storage mechanisms involved in the supercapacitors. The first one is related to separation of electronic and ionic charges at electrode/electrolyte interface and it is called the electrochemical double-layer capacitors (EDLC). Various carbon-based materials such as carbon nanotubes (CNTs) and activated carbon (AC) have been considered for use as the electrodes material in EDLC capacitors [3], [8], [9], [10], [11], [12]. While,the second one involves absorption or insertion of electro-active species into solid phase accompanied by charge transfer processes, this kind is called pseudocapacitors; numerous metal oxides such as SnO2, MoO3, RuO2, and MnO2, and electronically conducting polymers have been used to enhance specific capacitance via pseudocapacitive redox reactions [3], [13], [14], [15], [16], [17], [18], [19]. As aforementioned, most of the invoked metals were utilized in the oxide form which can be more sensitive to the chemicals compared to some inorganic groups such as carbonate.
Although, alkaline earth metal-based compounds (e.g. CaCO3) are abundant and cheap materials, exploiting of these materials in supercapacitors application is very rare. Calcium carbonate has three different anhydrous polymorphs forms such as hexagonal β-CaCO3 (calcite), orthorhombic λ-CaCO3 (aragonite), andμ-CaCO3 (vaterite) [20]. Thermodynamically, the moststable form of CaCO3 under normal conditions is hexagonal β-CaCO3 (calcite), aragonite and vaterite are not stable, but they transform into the stable calcite [21].
Carbon-based supercapacitors revealed outstanding capacitance performance due to their high surface area and high electrical conductivity [22]. Single- and few- layer graphenes have gained much prominence in the recent years due to the heightened awareness of their potential applications in lithium secondary batteries, nanoelectronics and supercapacitors [23], [24], [25]. However, most of the electronic applications are handicapped by the absence of a bandgap in the inherent material [26], [27]. It becomes necessary to tune the band gap in graphene, in this regard numerous methodologies have been developed to enhance the semiconducting properties demonstrated by creating confined geometries of quantum dots [28], nanomesh [29], nanoribbons [30], but one of the most feasible methods to control the semiconducting properties of graphene is by doping with elements such as nitrogen and boron, which is a process deliberately used to tailor the electronic and electrochemical properties of graphene. Nitrogen atom can modify the electronic band structure of graphene significantly by open up an electronic bandgap between valence and conduction bands. There are different approaches of doping of graphene with heteroatoms such as solvothermal synthesis [31], pyrolysis of graphene oxide with melamine [32], gas phase synthesis [33], plasma reaction [34], wet chemical reaction of graphene oxide with dicyandiamide [35] and CVD based synthesis [34], [36].
Thus the objective of this research is to combine the merits of nitrogen-doping and CaCO3-incoporation for the reduced graphene oxide in order to introduce a beneficial use of the prepared composite in supercapacitors applications. In this article, we report a promising and low cost synthesis of nitrogen/CaCO3/reduced graphene oxide nanocomposite by simple heat reflux technique; moreover the synthesis technique was efficient, green and cost effective with nitrogen doped graphene and CaCO3 nano composite formed in one step.
The electrochemical measurement was carried out in aqueous KOH, because the KOH electrolyte has low resistivity and high stability compared to H2SO4 electrolyte. Moreover, the KOH solution also shows good performance at low and high temperature. Interestingly, electrochemical tests showed that the introduced nanocomposite electrode exhibited a high specific capacitance and an excellent rate discharge performance in 1 M KOH solution.
Section snippets
Materials
Natural graphite powder, Calcium acetate monohydrate (CaAc, 99 %) and Hydrazine monohydrate (98%) were bought from Sigma-Aldrich, USA. Urea (Ur, 99 %), Sulfuric acid (99%), hydrochloric acid, hydrogen peroxide, were purchased from Samchun Pure Chemical Co., Ltd. (Korea). Potassium permanganate (Junsei Chemical Co., Ltd., Japan) was used as an oxidizing agent. All the chemicals were used without any further purification.
Preparation of Graphene Oxide (GO)
The modified Hummers method [37] was used to oxidize natural graphite powder
Results and Discussion
Among the various analytical techniques, X-ray diffraction is one of the most important characterization tools used in solid state chemistry and materials science to investigate the fingerprint characterization of the crystalline materials and the determination of their structure. The representative XRD pattern of the N-rGO/CaCO3 nanocomposite powder is presented in Fig. 1. As shown, the strong diffraction peaks at 2θ values of 20.9°, 25.9°, 42.6° and 77.7° corresponding to (0 1 1), (1 1 1), (0 2 2)
Conclusion
In summary, a novel composite consisted of N-rGO/CaCO3 can be synthesized by a facile heat reflux method. N-rGO/CaCO3 exhibits a large specific capacitance of 214 Fg−1 in 1 M KOH electrolyte at a scan rate of 5 mV s−1 and capacitive retention shows remarkable stability as high as 88.7% after 10,000 cycles. Moreover, current study reveal that CaCO3 appear to be even more suited for supercapacitor applications. Overall, the introduced study opens new avenue for cheap and effective alkaline earth
Acknowledgments
This Research was financially supported by National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2014R1A4A1008140) and National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSET) (No. 2012R1A2A2A01046086).
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