Oxygen reduction on uncatalyzed carbon-PTFE gas diffusion cathode in alkaline medium

The reduction of O-2 to hydroperoxide ion HO2- on a commercial uncatalyzed carbon-polytetrafluoroethylene (PTFE) gas diffusion cathode has been studied in aqueous solutions with KOH concentrations ranging between 1.0 and 6.0 mol dm(-3). The O-2/HO2- couple behaves as reversible in open circuit and its apparent standard electrode potential, calculated from the Nernst equation, becomes less negative when raising the OH- concentration. Under linear sweep voltammetric conditions, the process is independent of both OH- and HO2- concentrations and a first-order reaction with respect to O-2 feed is found. In each alkaline medium the apparent activation energy for O-2 reduction at several potentials has been determined from Arrhenius-type plots at temperatures ranging between 5 and 45degreesC. While the Tafel slope is proportional to absolute temperature, the cathodic transfer coefficient is always close to unity. A reaction mechanism involving the initial reduction of O-2 on a located active site of the carbon black surface by a reversible one-electron transfer, followed by the irreversible protonation of the resulting adsorbed superoxide ion by water as the rate-determining step, is proposed. (C) 2002 The Electrochemical Society. [DOI: 10.1149/1.1432674] All rights reserved.