Effects of Pt Coverage in Pt@PdCu5/C Core-Shell Electrocatalysts on the Oxygen Reduction Reaction and Methanol Tolerance

Carbon-supported Pt@PdCu5 core-shell nanoparticles (Pt@PdCu5/C) with Pt contents from 14 to 49 atom % have been synthesized by first preparing the PdCu5/C cores by polyol reduction followed by an aqueous galvanic displacement reaction to obtain the Pt-rich shell. The effect of Pt coverage on oxygen reduction reaction and methanol tolerance was investigated. Pt surface enrichment has been confirmed by comparing the near surface to bulk metal composition. Initial cyclic voltammograms show less Cu dissolution from Pt@PdCu5/C with 49% Pt compared to samples with less Pt, consistent with estimates of Pt coverage. The methanol tolerance of the Pt@PdCu5/C series decreases with increasing Pt content as indicated by the negatively shifting CO oxidation potential. For Pt@PdCu5/C - 14% Pt, the positively shifted CO oxidation potential indicates the presence of surface Pd leading to increased methanol tolerance compared to Pt/C. Cu dissolution from Pt@PdCu5/C in the membrane-electrode assembly greatly affects the single cell performance. Acid washing of the Pt@PdCus/C with 27% Pt cathode is required to obtain a stable direct methanol fuel cell polarization curve, and at methanol concentrations >0.5 M, the activity at 0.4 V becomes less than that with Pt/C.