A catalytic activity of a mercury electrode towards dioxygen reduction in room-temperature ionic liquids

Oxygen (O2) reduction reaction (ORR) at a hanging mercury drop electrode (HMDE) and gold (Au) electrode in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) was studied. Cyclic voltammogram (CV) with a couple of well-defined cathodic and anodic peaks corresponding to the redox reaction of the O2/superoxide ion (O2-) couple was obtained at Au electrode. On the other hand, three cathodic peaks (at ca. −0.37, −0.53 (main peak) and −0.68 V) and one anodic peak (at ca. −0.32 V) were found on the CV for the ORR at HMDE. The main cathodic peak at HMDE was observed at more positive potential by ca. 0.23 V than that observed at Au electrode, and the ratios of the cathodic peak current and of the integrated amount of the cathodic current obtained at HMDE to those at Au electrode were found to be ca. 1.4 and 2, respectively. The obtained results suggest that a catalytic two-electron quasi-reversible ORR mainly takes place at HMDE in EMIBF4 and that the adsorption of 1-ethyl-3-methylimidazolium ion on the surface of HMDE plays a crucial role in the ORR.