Abstract
Rotating ring disk (RRDE) methods are used to quantify the poisoning of the oxygen reduction reaction (ORR) at Pt/VC catalysts by adsorbed sulfur species. Coulometric analysis indicates that only S(0) is adsorbed on the Pt/VC surface. RRDE measurements show that the ORR activity decreases significantly as the sulfur coverage increases. Sulur coverage {greater than or equal to} 0.37, drastically changes both the activity and the reaction pathway of the ORR on the Pt/VC catalyst from a 4-electron to 2-electron process reaction. At high current densities, the Tafel slopes of the sulfur-poisoned electrodes increase, and the electrodes are no longer efficient for the ORR, as evident by the loss 95% mass activity with initial sulfur coverage of only 0.14. The decrease in activity due to the adsorbed sulfur is accompanied by an enhanced formation of hydrogen peroxide (up to 70% for initial sulfur coverage {greater than or equal to} 0.37).
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