Oxygen and ozone evolution at fluoride modified lead dioxide electrodes

This work examines the behaviour of fluorine modified beta-PbO2 electrodes in the processes of O-2 and O-3 evolution in sulphuric acid. The electroche mical kinetic analyses of these processes are based on quasi-steady-state p olarisation and impedance data. The good agreement between the two sets of measurements allows some basic conclusions to be drawn. In particular, the O-2 evolution process is always inhibited at F-doped PbO2 electrodes, and i mpedance results suggest possible changes in the mechanism, with electrodes orption of intermediates becoming more important as the concentration of th e doping element increases. The interpretation of the data for the less pos itive potentials region invokes the specific adsorption of SO2-4 as a facto r influencing the kinetics of O-2 evolution. The current efficiency for O-3 formation as a function of the amount of NaF added to the PbO2 growth solu tion reaches a maximum for a concentration of 0.01 mol dm(-3). A plausible cause for the decrease on the higher concentration side is the discharge of adsorbed SO2-4 (or HSO-4 eventually yielding persulphate. This reaction is known to be favoured in the presence of a relatively high amount of fluori de in the electrolyte. An analysis of the results of modified neglect of di atomic differential overlap (MNDO) calculations on Pb cluster models and of X-ray photoelectron spectroscopy (XPS) data suggests that the coverage by weakly adsorbed oxygen species (OH and H2O) is an important parameter that is influenced by F-doping. (C) 1999 Elsevier Science Ltd. All rights reserv ed.