Study of the oxygen electroreduction at nanostructured PtBi catalysts in alkaline medium

Nanostructured Pt1-xBix/C electrocatalysts (x from 0 to 0.3) are prepared via a microemulsion method. The electroactivity of the catalysts towards oxygen reduction reaction (orr) is investigated. The following order of catalysts for electrochemical activity towards orr is obtained: Pt0.8Bi0.2/C>Pt0.9Bi0.1/C>P/C>Pt0.7Bi0.3/C. RDE and RRDE experiments indicate a four-electron reduction reaction mechanism in the low overpotentials region with all Pt1-xBix/C electrocatalysts, whereas, at higher overpotentials a two-electron mechanism producing hydrogen peroxide co-exists with the four-electron mechanism producing water. Two Tafel slopes are obtained for all catalysts. In the low overpotentials region all Bi-containing catalysts display Tafel slopes close to -60 mV decade(-1), against -80 mV decade(-1) for pure platinum. In the high overpotentials region. the Tafel slopes are close to -120 mV decade(-1), except for the Pt0.7Bi0.3/C for which a Tafel slope close to -100 mV decade(-1) is observed. Results are explained in terms of protection of platinum surface from oxidation by the presence of more easily oxidizable species, leading to shift the reduction wave of 20-30 mV towards higher potentials, while the platinum surface coverage by bismuth oxides species leads to decrease the accessible platinum sites and further the orr kinetics. At last, the higher tolerance of a Pt0.8Bi0.2/C catalyst to the presence of ethylene glycol is demonstrate, as well as its higher performance as cathode catalyst under direct ethylene glycol solid alkaline membrane fuel cell working conditions. (C) 2007 Elsevier Ltd. All rights reserved.