Surprising dependence of the current density of bromate electroreduction on the microelectrode radius as manifestation of the autocatalytic redox-cycle (EC `') reaction mechanism

Bromate reduction from strongly acidic solutions under steady-state conditions in the presence of a very small amount of bromine has been studied voltammetrically at disk microelectrodes of various radii. In conformity with theoretical predictions the intensity of the average current density depends on the electrode size in a nonmonotonous manner, passing through a maximum for a certain radius. This behavior is a direct consequence of the autocatalytic character of this process where the non-electroactive bromate anion is reduced owing to the catalytic cycle based on the bromine/bromide redox-mediator couple. The experimentally observed dependence of the maximal current density, j(max), on the inverse disc radius, 1/r(0), for electrodes of larger sizes approaches a straight line corresponding to the ``strong current limit'', which exceeds the diffusion-limited current density for bromate ion.