Oxygen and ozone evolution at fluoride modified lead dioxide electrodes
Introduction
The behaviour of anodes at high potentials is interesting because of the electrochemical synthesis of important oxidants used in the chemical industry such as perchlorate, peroxysulphate and ozone, and in studies on the abatement of recalcitrant pollutants by electrochemical methods. Lead dioxide electrodes are still widely used for these purposes as the material is cheap and relatively stable under the high positive potentials required. Extensive literature data have shown that the electrochemical activity of these electrodes depends considerably on the composition of the electrolyte. In particular, the nature of anions is known to have a marked influence on ozone and persulphate production. Thus, for example, an enhancement of these processes by F− added to the electrolyte has long been known [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].
The electrocatalytic activity of PbO2 electrodes, as well as their stability, can often be considerably enhanced by the incorporation of some foreign ions added to the electrodeposition solution. Among these, Bi3+, Fe3+ and F− gave very good PbO2 electrodes for oxygen transfer reactions, including O3 formation [13], [14], [15], [16], [17], [18], [19].
In a recent work, we studied the influence of F− on the electrodeposition of PbO2 on platinum [20]. We observed a rise in the Pb(II) oxidation rate in the presence of fluoride, which we explained on the basis of a mechanism where the effect of fluoride is that of increasing the surface concentration of oxygen species that are more strongly bound to the electrode surface. In that paper we also anticipated some results on the electrocatalytic activity of FPbO2 (and FePbO2) doped electrodes in the processes of O2 and O3 evolution.
Herein we report on the results of a deeper investigation into the processes of O2 and O3 electrogeneration at PbO2 and FPbO2 anodes, using steady-state and impedance techniques. We also discuss the results of modified neglect of diatomic differential overlap (MNDO) calculations using cluster models containing or not containing fluorine.
Section snippets
Experimental
Electrochemical kinetic and impedance experiments were performed on an EG&G model 273A potentiostat/galvanostat, EG&G model 5210 lock-in amplifier using EG&G software. Simulation calculations of impedance data were done using B.A. Boukamp’s equivalent circuit simulation program. Measurements were carried out in 1 M H2SO4 prepared from ultrapure sulphuric acid (Merck) and Millipore water, using a conventional three compartment cell. The counter electrode was a large Pt flag, or a cylindrical Pt
Electrocatalytic activity of F-doped PbO2
Incorporation of fluoride into lead dioxide leads to a change in its electrocatalytic activity which is clearly seen in both the processes of oxygen evolution and O3 production. The current efficiency for O3 formation at F-doped PbO2 anodes was found to be a function of the NaF concentration used in the electrodeposition bath (Fig. 1). These data were obtained using a solid polymer electrolyte (spe) membrane cell (Section 2) at 25°C but the trend was the same as that observed using a
Conclusions
In this research work we examined the electrocatalytic behaviour of fluorine-doped PbO2 electrodes in the processes of O2 and O3 evolution. By comparison with unmodified PbO2, F-doping shifts the O2 evolution process to higher potentials for a given current, for the higher F-doping levels, however, this increase is seen mainly at lower currents. On the other hand, as the concentration of F− in the PbO2 growth solution is increased, the current efficiency of O3 formation first rises and then
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