Elsevier

Electrochimica Acta

Volume 45, Issues 4–5, 1 November 1999, Pages 713-720
Electrochimica Acta

Oxygen and ozone evolution at fluoride modified lead dioxide electrodes

https://doi.org/10.1016/S0013-4686(99)00250-9Get rights and content

Abstract

This work examines the behaviour of fluorine modified β-PbO2 electrodes in the processes of O2 and O3 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 O2 evolution process is always inhibited at F-doped PbO2 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 SO42− as a factor influencing the kinetics of O2 evolution. The current efficiency for O3 formation as a function of the amount of NaF added to the PbO2 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 SO42− (or HSO4) 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 H2O) is an important parameter that is influenced by F-doping.

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 FPbO2 (and FePbO2) 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 FPbO2 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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