Impact of Sulfur Dioxide on the Oxygen Reduction Reaction at Pt/Vulcan Carbon Electrocatalysts

The poisoning of the oxygen-reduction reaction (ORR) by adsorbed sulfur-containing species was quantified for platinum fuel-cell materials using rotating ring disk electrode methodology. Electrodes of Pt on Vulcan carbon (Pt/VC) were contaminated by submersion in -containing solutions. The initial sulfur coverage of the Pt was determined from the total charge consumed as the sulfur was oxidized from at (vs a reversible hydrogen electrode) to water-soluble sulfate at . Electrodes were then evaluated for their ORR activity. Significant (33%) loss in Pt mass activity was measured when approximately 1.2% of the Pt surface had adsorbed the sulfur-containing species. Sulfur coverage of 14% caused a 95% loss in mass activity. When 37% of the Pt surface was covered with sulfur, the reaction pathway of the ORR on the Pt/VC catalyst changed from a 4-electron to 2-electron process reaction for peroxide, a reagent which can aggressively attack Nafion. We conclude that adsorbed sulfur is not removed under typical steady-state operating conditions of a proton exchange membrane fuel cell, so it will affect operation by decreasing mass activity of the catalysts and by enhancing formation of the deleterious by-product.