Selective Dissolution of Palladium and Consequent Enrichment of Platinum in Pt-Pd Alloys

Abstract

Introduction Polymer electrolyte fuel cells (PEFCs) are expected to be widespread as a clean energy source, and platinum (Pt) alloys are used as a cathode catalyst for low-temperature operation. Pt-palladium (Pd) alloy catalysts show higher oxygen reduction reaction activity than Pt catalysts.1 Furthermore, Pd exhibits a higher corrosion resistance than less noble metals such as cobalt, nickel, and cupper under PEFCs operating conditions. Thus, the durability of Pt-Pd alloy catalysts is expected to be greater than that of the other alloys. In this study, dissolution behavior of Pt-Pd alloys under potential cycling simulating PEFCs operating conditions was investigated. Experimental Single-phase and polycrystalline Pt-50at.% Pd (Pt-50Pd) and Pt-75Pd were prepared by arc-melting. A total of 1,000 cycles of potential cycling test (CV) was conducted with the potential range of 0.05-1.4 V vs. SHE, simulating start-up/shut-down cycles. The test solution (0.5 M H2SO4) was sampled and replaced every 100 cycles, and the amount of dissolved Pd and Pt from the alloys was quantified using an inductively coupled plasma-mass spectrometer (ICP-MS). After the CV, the outermost surface of the alloys was analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Results and Discussion The CV of Pt-50Pd consists of hydrogen (H) adsorption/desorption peaks, oxide formation/reduction peaks. These features are similar to the CV features of pure Pt, indicating that the Pt-enriched layer formed on the alloy surface after the 1,000th cycles of CV. On the other hand, the peak positions of the oxide formation/reduction of Pt-75Pd are in good agreement with those of pure Pt. However, H absorption peak was observed in the CV of Pt-75Pd. Thus, Pd atoms were partially exposed to the surface after the CV.The amount of dissolved Pt from Pt-50Pd and Pt-75Pd was almost similar that from pure Pt. In contrast, the amount of dissolved Pd from Pt-50Pd and Pt-75Pd was about 80-90% smaller than that from pure Pd. When the amount of dissolved Pt was compared with that of Pd of Pt-50Pd and Pt-75Pd during the last 100 cycles during CV, the ratio of dissolved Pt to Pd was 1:1.7 and 1:4.6, respectively. Hence, Pd selectively dissolved from Pt-Pd alloy, which confirmed that the Pt-enriched layer formation on the surfaces. The outermost surface analysis by TOF-SIMS also supported the Pt-enrichment. The dissolution rate of Cu and Pd from Pt-75Cu and Pt-75Pd during 100 cycles of CV were 3300 and 3.6 μg cm- 2, respectively, indicating that the selective dissolution rate of Pd is extremely small when the Pd concentration in the alloy is above parting limit.2 Based on these experimental results, the structures of Pt-enriched layer formed on Pt-50Pd and Pt-75Pd as a result of the selective dissolution of Pd might be different. References J. Zhang, M.B. Vukmirovic, Y. Xu, M. Mavrikakis, R.R Adzic, Angew. Chem. Int. Ed., 44(14) 2132 (2005)M. Artymowicz, J. Erlebacher, R.C. Newman, Philos. Mag., 89 1663 (2009).