RRDE and Voltammetric Study of ORR on Pyrolyzed Fe/Polyaniline Catalyst. On the Origins of Variable Tafel Slopes

Catalytic activity of heat-treated iron and polyaniline-based oxygen reduction catalysts was studied in aqueous acidic media using the rotating ring disk (RRDE) technique and linear potential scan voltammetry employing stationary electrodes. The stationary voltammograms of the catalyst exhibit the presence of a reversible surface red-ox reaction at 0.647 V vs RHE. It is shown that molecular oxygen reversibly adsorbs on the catalyst surface at potentials more positive than the formal potential of the surface red-ox couple and that the adsorption occurs through either the oxidized form of this couple or an atom in its close proximity. The Tafel plots for oxygen reduction reaction (ORR) exhibit variable slopes ranging from 60 mV dec(-1) at the lowest overpotentials to more than 240 mV dec(-1) at high overpotentials. The kinetic data obtained from the RRDE experiments for various catalyst loadings and from the linear potential scan voltammetry of adsorbed oxygen demonstrate that the high Tafel slopes originate from intrinsic features of the reduction mechanism rather than incomplete catalyst utilization. It is postulated that the surface red-ox couple is Fe-III/Fe-II and that it takes an active part in ORR in the whole range of overpotentials. The proposed ORR mechanism involves a simple mediation by the Fe-III/Fe-II couple at low overpotentials and a concerted process of charge transfer and oxygen-oxygen bond splitting at high overpotentials.