Oxygen and ozone evolution at fluoride modified lead dioxide electrodes

Abstract

This work examines the behaviour of fluorine modified β-PbO 2 electrodes in the processes of O 2 and O 3 evolution in sulphuric acid. The electrochemical kinetic analyses of these processes are based on quasi-steady-state polarisation 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 PbO 2 electrodes, and impedance results suggest possible changes in the mechanism, with electrodesorption of intermediates becoming more important as the concentration of the doping element increases. The interpretation of the data for the less positive potentials region invokes the specific adsorption of SO 4 2− as a factor 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 PbO 2 growth solution 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 SO 4 2− (or HSO 4 −) eventually yielding persulphate. This reaction is known to be favoured in the presence of a relatively high amount of fluoride in the electrolyte. An analysis of the results of modified neglect of diatomic 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 H 2O) is an important parameter that is influenced by F-doping.