Preparation and electrochemical performance of nitrogen-doped graphene nanoplatelets

Abstract

The highly desirable properties of nitrogen-doped graphene nanomaterial, such as high surface area, good hydrophilicity, and enhanced electrocatalytic activity and charge-transfer property, make it an ideal candidate for electrode materials used in the field of energy conversion and storage. Up to now, methods of synthesizing nitrogen-doped graphene nanomaterials mainly include chemical vapor deposition, thermal annealing graphite oxide with NH3, and graphene treated with nitrogen plasma. However, these methods of producing the nitrogen-doped graphene nanomaterials are either costly for practical applications or involving environmently hazardous reagents, and the full potentials of nitrogen-doped graphene materials are hard to achieve without scalable production at low cost. Therefore, a simple and cost-effective method of producing the nitrogen-doped graphene nanomaterial is desirable. In this paper, nitrogen-doped graphene nanoplatelets are prepared by a simple and eco-friendly mechanochemical pin-grinding process under N2 atmosphere through using natural graphite flake as the precursor at room temperature. The as-prepared nitrogen-doped graphene sample is characterized by X-ray photoelectron spectroscopy, Raman spectra, nitrogen adsorption, SEM, and TEM. The images of SEM and BET (Brunauer-Emmett-Teller) surface area measurements demonstrate an effective and spontaneous delamination of the starting graphite into small graphene nanoplatelets even in the solid state by pin-grinding process. The cleavage of graphitic C-C bonds by pin grinding creates numerous active carbon species, which can directly react with nitrogen. X-ray photoelectron spectroscopy measurements indicate that the active carbon species react with nitrogen to form the aromatic C-N in pyrazole and pyridazine rings at the fresh broken edges of the graphitic frameworks. Both pyrrolic nitrogen and pyridinic nitrogen are at the edge of carbon framework, which can provide chemically active sites to improve the electrochemical performance of carbon material. Electrochemical impedance spectroscopy indicvates that nitrogen-doped graphene nanoplatelets possess excellent electrocatalytic activity for the redox reaction between iodide and triiodide ions, used in dye-sensitized solar cells. The charge-transfer resistance of nitrogen-doped graphene nanoplatelet electrode is 1.1 cm2, which is comparable to that of Pt electrode. The capacitance properties of the as-prepared nitrogen-doped graphene nanoplatelets are also investigated. Cyclic voltammetry and galvanostatic charge-discharge curves show that nitrogen-doped graphene nanoplatelets have good capacitive performance. At a current density of 0.3 A/cm2, the specific capacitance of nitrogen-doped graphene nanoplatelets is 202.8 F/g. The good electrochemical performance of nitrogen-doped graphene nanolplatelet can be attributed to its high surface area and doping nitrogen at the edge. The simple and eco-friendly preparation procedure, low cost, and good electrochemical performance allow the as-prepared nitrogen-doped graphene nanoplatelets to be a promising candidate for the electrode materials in dye-sensitized solar cells and supercapacitors.