Oxygen reduction on carbon supported Pt and PtRu catalysts in alkaline solutions

Abstract

In this paper, carbon supported Pt and PtRu were employed as oxygen reduction catalysts in 0.1 M NaOH solutions. Onset potentials for oxygen reduction reactions (ORRs) for both catalysts are similar (around 0.05 V vs. Hg/HgO/OH −). At an ORR overpotential of 0.29 V, the values of kinetic ORR current ( I k) on the PtRu/C catalyst, normalized to geometric area and active surface area, are 3 and 14 times lower, respectively, than those measured on the Pt/C catalyst. Rotating ring-disk electrode (RRDE) measurements show that the H 2O 2 yields on both catalysts are negligible at potentials higher than −0.3 V, and the yields are less than 4% even at higher overpotentials, indicating that the ORR mechanism on the PtRu/C is the same as that on the Pt/C, i.e., the 4e − transfer is the predominant pathway. The values of activation energy of the ORR on the Pt/C and PtRu/C are similar at low current densities – about 40 and 35 kJ mol −1, respectively. The simultaneous electro-oxidation of ethanol (EOR) and electro-reduction of O 2 was studied in O 2-saturated 0.1 M NaOH solutions. For the Pt/C, the ORR is suppressed by ethanol adsorbates at potentials lower than −0.05 V vs. Hg/HgO/OH − when both ethanol and oxygen exist in the solution. At potentials higher than −0.05 V, the EOR is suppressed by the ORR. The high EOR oxidation current on the Pt/C cathode would produce mixed potentials and degrade fuel cell performance. For the PtRu/C, the EOR has little influence on the ORR. The stability tests show that PtRu/C is stable under fuel cell cathode working conditions.