CO Tolerance of Pt alloy electrocatalysts for polymer electrolyte fuel cells and the detoxification mechanism

The electrocatalytic activity for H-2 oxidation in the presence of 100 ppm CO has been investigated on a series of binary Pt alloy electrocatalysts with non-precious metals of various compositions. Regardless of the composition, Pt-Fe, Pt-Ni, Pt-Co and Pt-Mo alloys have been found to exhibit excellent CO tolerance in H-2 oxidation, similar to that of the Pt-Ru alloy. At these CO-tolerant electrodes, the equilibrium coverage of CO was suppressed to values less than ca. 0.6. Based on X-ray photoelectron spectroscopy (XPS) data, it was found that the surfaces of all non-precious metal alloys are composed of a thin Pt layer with an electronic structure different from that of pure Pt, indicating an increased 5d vacancy of Pt in the layers of the CO-tolerant alloys. The CO coverage, particularly with multi-bonding, was lowered due to decreased electron donation from the Pt band to the 2 pi* orbital of CO. A weakening of bond strength between the Pt skin layer and CO was also indicated by in situ FTIR, suggesting that the H-2 oxidation sites are not blocked by CO due to its enhanced mobility. Thus, the mechanism of CO tolerance described above at the Pt skin on alloy surfaces is proposed as a "detoxification mechanism''.