▎ 摘　　要

In this work, a fast and easy synthesis of graphene sheets by mild tip-ultrasonic method was developed. N,N-Dimethylformamide (DMF) was used as dispersion media. Ultrasonic time effect on the graphene properties was investigated (30, 60 and 120 min). Additionally, graphene was doped by solvothermal treatment to obtain nitrogen-doped graphene metal-free electrocatalysts (NG). Raman spectra results indicated the formation of graphene-like carbon nanosheets composed by a mixture of few and multilayer sheets with a defective lattice. X-ray photoelectron spectroscopy (XPS) analysis show that nitrogen atoms were successfully doped into graphene structures with an atomic percentage (%oat.) in the range of 2.76-3.69. Electrochemical evaluation revealed that these materials are electroactive for the oxygen reduction reaction (ORR) in alkaline media. The NG60 metal-free electrocatalyst is of particular interest because it showed the best performance with a limit current of 1.1 mA cm(-2) and an on-set potential of 0.73 V vs. RHE. Therefore, the sonochemical synthesis is an attractive massively scalable method to obtain graphene to be applied in alternative energy systems. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.