INFLUENCE OF ANIONS ON OXYGEN/OZONE EVOLUTION ON PBO2/SPE AND PBO2/TI ELECTRODES IN NEUTRAL PH MEDIA

The formation of O-2/O-3 on PbO2 from the electrolysis of water in neutral solutions is shown here to present some analogies and some differences with respect to the same process in acid media. The electrolyte composition affects the current efficiency for O-3 formation (n) and the cell potential (E) of a Membrel type electrochemical assembly. An improvement in n and a decrease in E is observed upon addition of relatively low amounts of Na4SO4 or NaClO4 in pure water. We observe no effect of NaNO3 on either parameters. In agreement with literature data in acid solutions, F- causes an increase in both n and E. The results of electrochemical kinetic investigations with electrodes of PbO2 electrodeposited on Ti confirm the above data. Current-potential curves constructed from measurements in NaNO, show a region of Tafel linearity with slopes of 2RT/F and RT/F in the low and high currents range, respectively. Addition of Na2SO4 and NaClO4 to NaNO3 has an effect on the process at more positive potentials only: the RT/F slope decreases toward a value of RT/2F as the concentration of the ''foreign'' salt is increased. As an explanation of the observed behaviour, the possibility is advanced that a step following the discharge of water is rate determining at high positive potentials with the adsorption of intermediates described by Temkin conditions. The composition of the electrolyte is expected to influence the yield of ozone formation by affecting the coverage and free energy of adsorbed oxygen intermediates.