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Page 1 of 32 RSC Advances RSC Advances This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. This Accepted Manuscript will be replaced by the edited, formatted and paginated article as soon as this is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/advances Page 1 of 32 RSC Advances On the Large Capacitance of Nitrogen Doped Graphene Derived by a Facile Route M. Praveen Kumar 1, T. Kesavan 1, Golap Kalita 2, P. Ragupathy 1, ∗∗∗, Tharangattu N. Narayanan 1 and Deepak K. Pattanayak 1, ∗∗∗ 1CSIR - Central Electrochemical Research Institute, Karaikudi-630006, India. 2Nagoya Institute of Technology, Gokisho-cho, Nagoya-4668555, Japan. Manuscript * Corresponding Authors. E-mail: (D. K. P) [email protected]: (P. R) [email protected] Accepted Advances RSC 1 RSC Advances Page 2 of 32 Abstract Recent research activities on graphene identified doping of foreign atoms in to the honeycomb lattice as a facile route to tailor its bandgap. Moreover, the presence of foreign atoms can acts as defective centres in basal plane, and these centres can enhance the electrochemical activities of graphene surface. Here, we report a facile synthesis approach for the bulk synthesis of nitrogen doped graphene (N- Graphene) from graphene oxide by a hydrothermal process, with a large control over the amount of N-doping. The electrochemical activeness of N- Graphene (with 4.5 atomic % of nitrogen) is studied by conducting supercapacitor measurements. The N- Graphene exhibits remarkably high specific -1 capacitance of 459 Fg at a current density of 1 mA in 1 M H 2SO 4 electrolyte with a high Manuscript cycle stability compared to that of pristine graphene having a specific capacitance of 190 Fg - 1. The structure destabilisation of graphene in higher pH / high amount alkaline treatment is demonstrated, and hence optimization of amount reagents is necessary in developing graphene based high performance electronic or electrochemical devices. Accepted Keywords : Nitrogen doping: Supercapacitor: Graphene: Defects: Foreign atoms: EDLC: Alkaline reduction: Structure destabilisation. Advances RSC 2 Page 3 of 32 RSC Advances 1. Introduction Development of high power and energy density supercapacitors (SCs) has become one of the prime research areas in recent energy technology.1, 2 Based on the mode of charge energy storage mechanism, SCs are broadly classified into two categories (i) double layer capacitance (Electric Double Layer Capacitance (EDLC)) in which charge is stored between electrode / electrolyte interfaces, while (ii) faradaic process is responsible for charge storage in supercapacitors. However, the energy density of SCs (30 WhKg -1) is significantly lower than that of batteries (200 WhKg -1) and fuel cells. Relatively low specific capacitance of EDLC, in the range of 100-300 Fg -1 in aqueous electrolyte and 100-150 Fg -1 in non aqueous 3 electrolytes, is not able to satisfy the energy demand of many applications. Thus, it is of Manuscript great interest to build SCs from new materials with low cost, high capacitance and excellent cyclability. Out of various materials used for SC applications, carbon based materials place a unique role due to the availability of various crystalline forms of carbon with relatively high conductivity and surface area. Accepted Graphene, a 2-dimensional (2D) carbon nanostructure, has attracted great interest for applications in the fields of energy storage and conversion because of its unique physicochemical properties such as high surface area, excellent conductivity and mechanical strength. However, the reported capacitance values for graphene based SCs are much inferior 4 to the theoretical capacitance of single layer graphene. This can be due to the unavoidable Advances curling and bundling of graphene during the device fabrication, resulting in to a large reduction of its active surface area. Hence, improving the capacitance of graphene materials RSC is of great interest, many attempts have been underwent, including the introduction of dopants, and development of graphene / metal / metal oxides hybrids.5-9 3 RSC Advances Page 4 of 32 Doping foreign elements such as nitrogen (N) is accepted as a simple and effective way to tailor the local electronic structure of graphene,10, 11 and because of its atomic size and strong valence bonds similar to those of carbon atoms, nitrogen can be doped into the lattice of graphene – retaining its sp 2 functionalization, particularly N- doped graphene (N- Graphene) synthesized at very high temperatures (above 900 ºC) will give quaternary nitrogen.12 It has been widely accepted that the manipulated local electronic structure enhance the binding phenomenon with ions,13 and hence this feature can be utilized for making high performance storage devices from doped graphene.14 To the date, various methods have been reported for the synthesis of N-Graphene including chemical vapour deposition (CVD),15 nitrogen plasma process,16 electrochemical reactions,17 thermal annealing with urea,18 and Manuscript solvothermal route.19 However, vigorous reaction conditions and sophisticated instruments are the main obstacles to scale up the process and increase the N- content. On the other hand, hydrothermal methods along with reagents such as amine,20 pyrrole,21 urea,22 and hexamethylenetetramine 23 as the nitrogen sources have the advantage of mild reaction and Accepted easy to prepare in a large quantity of N- Graphene. Reported capacitance values for N- Graphene based SCs lie only in the range of 150 - 370 Fg -1 (please see Table 1). Thus, it is necessary to further increase the specific capacitance of N- Graphene by controlled nitrogen doping, if possible. Here, we report a very simple and low cost approach for the synthesis of N- Graphene Advances by a hydrothermal technique using ammonium hydroxide as nitrogen source, in the presence of hydrazine hydrate as a reducing agent. The electrochemical performance of N- Graphene is RSC investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge cycling. The influence of nitrogen doping on graphene and its electrochemical performance are systematically correlated and reported. 4 Page 5 of 32 RSC Advances 2. Experimental section 2.1. Synthesis of N-doped graphene Graphene Oxide (GO) has been synthesized by following the “Improved Method” reported by Tour et al for the synthesis of high quality GO from graphite powder.24 In this method, Graphite (2 g) is thoroughly mixed with the mixture of sulfuric acid and phosphoric acid (9 : 1 ratio) (Alfa aesar) at room temperature by magnetic stirring. Potassium permanganate (14 g) crystals are then slowly added to this mixture. The resultant mixture is stirred well by keeping the temperature at 90°C for 24 hrs. Then, 14 mL of 30 % H 2O2 (Alfa aesar) is added to this mixture, after keeping the mixture in an ice bath. The solution was subjected to vacuum filtration, and the residue was washed with water, 5 % HCl and ethanol Manuscript subsequently for 3 cycles. The resultant filtrate (GO) is soaked in ether and dried in a vacuum oven at 80°C for 24 hrs. The GO (0.1 g) is dispersed in deionised water (93 mL), and 7 mL of hydrazine hydrate (N 2H4) is added to this GO solution with a continuous stirring. After 2 hrs, Accepted ammonium hydroxide is added to this mixture (volume of ammonium hydroxide is varied from 3 - 10 mL in different samples to get different level of doping) and the mixture solution is kept in a teflon autoclave. Autoclave is kept in an oven at 180°C for 12 hrs. The residue is collected and dispersed in water and vacuum filtrated using a PTFE membrane filter paper (pore size = 0.22 µm). This residue is washed several times with DI water to make the pH ~ Advances 7. The filtrate is collected and allowed to dry at 50°C for 24 hrs in an oven. The resultant powder is collected and used for the further experiments. RSC 2.2. Physiochemical characterization: The structure, morphology and texture were characterized by powder X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). In order to confirm the crystal structure and phase purity of the 5 RSC Advances Page 6 of 32 sample, powder X-ray diffraction patterns were recorded on a Bruker D 8 Advance X-ray diffractometer with Cu Kα (λ = 1.5405 Å) radiation in the range of 5-65º (2 θ). Raman measurements were carried out using Renishaw (UK) In Via Raman microscope with 632.8 nm wavelength incident laser light. The morphology and structural properties of the as- synthesized graphene and N- Graphene were investigated using a FE-SEM (Carl Zesis, SUPRA 55VPFEI, Germany).TEM images of samples were taken using a FEI TECNAI G 220 with an accelerating voltage of 200 kV. TEM samples were prepared (mixing DMF and sample under sonication for 45 min) by placing a drop of solution on a carbon-coated copper grid and drying under UV light. Presence of functional groups in graphene as well as N- Graphene were observed using FTIR (BRUKER OPTIK GMBH, Germany; Model no Manuscript TENSOR 27). X-ray photoelectron spectroscopy (XPS) was carried out with PHI 5000 VersaProbe ULVAC instrument.
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