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Dealloying; Dissolution; Oxygen Reduction Reaction; Pt-Cu; Scanning Flow Cell
Abstract:
Within the search for new catalysts for the oxygen reduction reaction in polymer electrolyte membrane fuel cells, alloys of Pt with other transition metals are of great interest due to their increased specific and especially mass activity. However, the drawback of these catalysts is their reduced stability due to the dissolution of the less-noble metal from the alloy. To resolve the potential dependence of these dissolution processes, we investigate a material library of Pt-Cu thin-film alloys with compositions ranging from 0 at% Cu up to 60 at% Cu. Utilizing our combinatorial scanning flow cell technique coupled to a mass spectrometer several aspects of dissolution are revealed. The onset of relevant Cu dissolution was found to be around 0.9 V-RHE independent of the composition. Although this is well below the onset potential of the Pt dissolution (1.15 V-RHE), the two dissolution processes are clearly correlated, especially when the surface is already depleted of Cu. In contrast to Pt, however, Cu preferentially dissolves during anodic polarization rather than during the cathodic sweep. Additionally, at Cu compositions above the parting limit between 50 and 57 at% Cu a breakdown of passivity and massive Cu dissolution leads to porosity formation. The critical potential for the alloy with 57 at% Cu was detected around 1.3 V-RHE, which is above the stability potential of Pt. While the absolute activity during the porosity formation increases due to the formation of more active sites in the pores, the specific activity decays to a value close to pure Pt. (C) 2014 The Authors. Published by Elsevier Ltd.
