Abstract
The behavior of the redox couple at equilibrium on a commercial uncatalyzed carbon-polytetrafluoroethylene (PTFE) oxygen-diffusion electrode, fed with partial pressures between 0.21 and 1.0 atm, has been studied by electrochemical impedance spectroscopy. Measurements have been made in the open-circuit potential using aqueous solutions with KOH concentrations in the range 1.0-6.0 mol dm−3 and concentrations up 50 mmol dm−3 at 25.0°C. Under these conditions, the system is controlled by activation, the charge-transfer resistance being much higher than ohmic, adsorption, and diffusion resistive elements. The double-layer capacities show that the wetted electroactive areas, much smaller than the total area, depend on the KOH concentration used for activation. True exchange current densities of about 1 μA cm−2 are obtained, while their apparent values are two orders of magnitude greater, since the sluggish reaction is compensated by the high electroactive area. The cathodic process is a first-order reaction with respect to the feed, and independent of and concentrations. For the anodic one, a zero order for and a first order for and are calculated. These results agree with the previously proposed mechanism for the reduction to on the same electrode from voltammetric studies. Indirect evidence on a weak adsorption of is found from the impedance diagrams. © 2002 The Electrochemical Society. All rights reserved.
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