An impedance study of the O2|HO2- system in equilibrium on a gas diffusion electrode

The behavior of the O-2\HO2- redox couple at equilibrium on a commercial uncatalyzed carbon-polytetrafluoroethylene (PTFE) oxygen-diffusion electrode, fed with O-2 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 HO2- concentrations up 50 mmol dm(-3) at 25.0degreesC. 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 muA cm 22 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 O-2 feed, and independent of HO2- and OH- concentrations. For the anodic one, a zero order for O-2 and a first order for HO2- and OH- are calculated. These results agree with the previously proposed mechanism for the O-2 reduction to HO2- on the same electrode from voltammetric studies. Indirect evidence on a weak adsorption of HO2- is found from the impedance diagrams. (C) 2002 The Electrochemical Society.