Abstract: Pd/Pt bimetallic electrodes are prepared using under-potential deposition (UPD) of Cu following with galvanic displacement of Cu by Pt(denoted as Pd/Ptx, here x denoted the cycle No. of UPD Cu and the subsequent Pd-Cu displacement processes). The atomic ratios of Pt:Pd in the surface region can be tuned by holding the low potential limit for Cu UPD at 0.34 V, and tuning the cycle No. of UPD Cu and Pd-Cu displacement processes. The surface composition of Pd/Pt bimetallic electrodes were examined by x-ray photon-electron spectroscopy (XPS) and electrochemical infrared spectroscopy (EC-IRS) using CO as a probe. And the activity of Pd/Ptx bimetallic electrodes toward oxygen reduction reaction (ORR) have been studied using cyclic voltammetry (CV) in a flow cell. XPS and EC-IRS demonstrate that the atomic ratios of Pt:Pd increase monotonically with the cycle No. of Cu-upd and Pt-Cu displacement. For Pd/Pt4 electrode, the atomic ratios of Pt:Pd in the region of ca. 2~3 nm from the surface is 1:4, while Pt:Pd ratio at the surface is ca. 5:1. With the increase in Pt ratio at the surface, Pd/Ptx electrodes become more and more oxidation resistant. The ORR activity increase with x up to 4 and it remains unchanged at x?4. For Pd/Ptx electrode (with x>3), its ORR activity is higher than that of polycrystalline(pc) Pt. Under otherwise identical condition, the half wave potential for ORR at Pd/Pt4 electrode is about 25 mV more positive than that at pc-Pt electrode. The enhancement in ORR activity of Pd/Ptx bimetallic electrodes is attributed to the reduction of adsorption energy of O containing species on Pt atoms induced by adjacent Pd atoms at the surface or by Pd atoms under the surface Pt layer.

Key words: Pd electrode, Ultra-low Pt loading, Oxygen reduction, Cathode, Fuel cells