An efficient process to prepare basic imidazolium ionic liquids (ILs) solution free of halide has been developed based on the rapid ion exchange between alkyl sulfate anion (ROSO3-, R = alkyl) and OH- in ethanol. Eight halide-free ILs with a flexible range of cations and anions were prepared by neutralizing the basic IL solution with corresponding acids. The viscosity, ionic conductivity or melting points of the ILs obtained by present method were examined and discussed. The results implied that the whole process is green since only ethanol and water were used as solvents and by-products were only solid KROSO3 and KY (Y = desired anion). Furthermore, a typical imidazolium ionic liquid, [Emim]HSO4, was synthesized by different precursors with various amounts of Br impurity, and its effect oxygen reduction reaction (ORR) of Pt/C catalyst was systematically investigated. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.