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

The electrocatalytic activity for H2 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 H2 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 2π* 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 H2 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”.