The cathodic reduction of oxygen at microelectrodes

Sotiropoulos, Sotirios (1994) The cathodic reduction of oxygen at microelectrodes. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The cathodic reduction of oxygen at Pt, Au and Ag microelectrodes in aqueous solutions, including purified water with no added electrolyte, was studied with the aim of developing a simple two-electrode amperometric sensor. The degree of irreversibility of the oxygen reduction reaction increases as the microdisc radius decreases, i.e. as the mass transport rates towards the electrode increase. This results in a shift of the oxygen reduction voltammogram to more negative potentials with decreasing microelectrode size. In the case of Pt, the apparent numebr of electrons transferred during oxygen reduction in the diffusion controlled region, falls smoothly with increasing rate of mass transport, both for a Rotating Disc Electrode and for microelectrode experiments in neutral and base solutions, indicating the formation of diffusible intermediates (i.e. O₂^- or/and HO₂^-) involved in a slow chemical step within the oxygen reduction scheme.

H adsorption/desorption and oxide formation/stripping on Pt, proved to be dependent on the mass transport regime in unbuffered near-neutral solutions since in this case the local pH at the electrode surface is determined by the diffusible H⁺ and OH^- ions produced by the processes themselves.

The application of a potential program to a Pt microdisc (consisting of a measurement pulse in the diffusion limited region for oxygen reduction and of a cleaning pulse in the oxide formation region), resulted in an oxygen reduction signal that showed good linearity with the concentration of dissolved oxygen, high sensitivity (ca. 10^-4 A cm^-2 s^-1) but a low detection limit (ca. 1 ppm). The stability of the sensor was more than a few days in samples of purified water and in some samples of natural waters but less than a few hours in others. Ca⁺⁺ and Mg⁺⁺ ions do not affect the stability of the sensor and it is suspected that a trace metal cation is the cause of the decay in signal.

For oxygen determination in atmospheres, preliminary tests were carried out on two microelectrode-based, solid electrolyte devices. The first simply involved both the working microdisc (Pt or Ag) and the counter electrode (Ag, Sn, or Pt) covered with a thin film of solution-recast Nafion, Dupont. In the other one, two metal layers (Au or Pd) deposited on the same face of a Nafion membrane and separated by a strip of the polymer, were directly exposed to the gas sample. It appears that oxygen reduction occurs at the three-phase (metal/ionic polymer/gas) interface.

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