Edgefunctionalized graphene nanoplatelet towards energy storage application
Authors: Mohapatra Lagnamayee , Sodiq Ahmed , Fasmin Fathima , Merzougui Belabbes A , Zaffou Rachid 
Published: June 2019

With the increased demand in energy resources, great efforts have been keen to develop advanced energy storage systems. A redox flow battery is a type of rechargeable battery where rechargablity provided by two chemical components dissolved in liquids and separated by a membrane. Energy is stored in the liquid electrolyte in external tanks, rather than in the battery cell. Flow batteries are technically similar to fuel cells are targeted at large-scale energy storage solutions. [1] Graphene based materials have attracted great attention for used as an alternative electrode materials for electrochemical energy storage system due to its unique properties of large surface area, chemical stability, super mechanical flexibility, high electric and thermal conductivities and an atomic thick two-dimensional sp2 hybridized carbon network. [2]To realize commercial use of graphene-based energy devices, it is highly desired to produce high-quality graphene at a low cost and large scale. Functionalization of graphene can further enhance its properties for efficient energy conversion and storage. Several innovative methods have been reported recently for functionalization of graphene, including mechanical exfoliation, surface polymerization vapor deposition (CVD), chemical exfoliation of graphite, sonication/intercalation,but these suffer from high manufacturing costs and technical difficulties.[3] Chemical doping of graphene with heteroatoms (e.g. Nitrogen) is one of the most feasible approaches to modulate its electronic properties. Traditionally, these methods often requires complicated processes and/or chemical reagents containing additional undesirable components in their structures. On the particular interest, mechanochemical ball milling process is a simple but efficient approach for producing edge-functionalized graphene nanoplatelets in large quantity and low cost. Previously, N-doped graphene materials generated by ball milling of graphite with N-containing gases like N2 and NH3 have been showed good electro catalytic activities for oxygen reduction reaction (ORR). [4,5,6] But,the amount of Nirogen doping is not sufficient in the above case. Therefore, we prepared N-doped graphene (N-GNP) by ball milling of graphene nanoplatelet (GNP) with melamine, which is nitrogen-rich solid organic compound and followed by pyrolysis. Both the temperature as well as the mass ratio between GNP and melamine affect the nitrogen content. In the ball milling process, the strong shear forces generated between high-speed rotating balls caused the mechanochemical cracking of the graphitic C– C bonds and spontaneous incorporation of Nitrogen from melamine at the broken edges of graphitic frameworks as well as the consequent exfoliation of graphene nanoplatelet. From the XRD, the (N-GNP) has peak broadening, indicates the occurrence of the ball-milling-induced edge doping of GNP. The XPS plot reveals the presence of three different nitrogen species in the (N-GNP), namely pyridinic, pyrolic and quaternary Nitrogen. The as fabricated materials used as a slurry electrode and their performances were investigated for VO2+/VO2+ redox couple for vanadium redox flow battery (VRB). The cyclic voltammetry (CV) results revealed that the Nitrogen functionalization of Graphene noplatelet allowed remarkable improvements in terms of both the reversibility and the current density than as received GNP. Electrical impedance spectroscopy (EIS) was used to further investigate that N-GNP is showing highest conductivity than GNP. Enhanced performance of N-GNP in terms of electrochemical activity and kinetic reversibility is owing to its electrical conductivity, surface area, graphitized surface and chemical stability of the electrodes affects the overall battery efficiency. References 1. P. Leung a, A.A. Shah, L. Sanz, C. Flox, J.R. Morante, Q. Xu, M.R. Mohamed,C. Ponce de Leon, F.C. Walsh, Journal of Power Sources, 360,2017, 243-283.2. H.-M. Tsai, S.-Y Yang, C.-C. M. Ma, X.F. Xie, Electro analysis, 2011, 23, 9, 2139 – 2143.3. RoshniYadav, C.K.Dixit, Journal of Science: Advanced Materials and Devices,2017, 2141-149.4. H. Wang, T. Maiyalagan, X.Wang, ACS Catal. 2012, 2, 781 − 794.5. W. Shi, K.-Hsu Wu, J. Xu, Q. Zhang, B.Zhang,D. S. Su, Chemistry of Materials, 2017, 29, 8670-8678.6. L. Dai, Y. Xue, L.Qu, H.-J. Cho, J.-B. Baek, Chem. Rev., 2015, 115, pp 4823–4892.