Electrochemical reduction of oxygen in some hydrophobic room-temperature molten salt systems

The electrochemical reduction of oxygen was investigated in some hydrophobic room-temperature molten salt systems (ionic liquids) consisting of bis(trifluoromethanesulfone) imide (TFSI(-)) anion with trimethyl-n-hexylammonium (TMHA(+)), 1-butyl-1-methylpyrrolidinium (BMP(+)), 1-ethyl-3-methylimidazolium (EMI(+)) or 1,2-dimethyl-3-propylimidazolium (DMPI(+)) cation. The oxygen dissolved in these melts was reduced to the superoxide ion on a gold electrode. The superoxide ion was stable against the aliphatic and alicyclic organic cations (TMHA(+) and BMP(+)) but reacted with the aromatic ones (EMI(+) and DMPI(+)), suggesting that nucleophilic reagents attack the melts consisting of imidazolium cations. The acceptor number of these TFSI(-)-based molten salt systems was estimated to be comparable to those of acetonitrile and dimethyl sulfoxide from the comparison of the redox potential of O(2)/O(2)(-). The superoxide ion was further reduced, presumably to the peroxide, on a gold electrode in the melts consisting of the aliphatic or alicyclic organic cation. (C) 2003 The Electrochemical Society.