Dissolution and the Consequent Surface Morphology of Pt-Cu Binary Alloys Under Potential Cycling

Abstract

Polymer electrolyte fuel cell (PEFC) is regarded as clean energy conversion system, however its high cost prohibits the progress of PEFC. Thus, a reduction in the amount of Pt used as cathode catalyst is required. In order to overcome this problem, Pt transition-metal (Pt-M) alloy catalysts have been developed instead of Pt catalysts, however a deterioration of Pt-M alloy catalysts is concerned because both Pt and M are dissolved from Pt-M alloy catalysts under PEFC operating conditions. In this study, potential cycling tests and inductively coupled plasma mass spectroscopy (ICP-MS), field emission-scanning electron microscopy (FE-SEM), and field emission auger electron spectroscopy (FE-AES) were combined, and dissolution behavior of Pt-Cu alloys has been investigated under potential cycling by using these techniques. Pt50-Cu50 (Pt-50 at% Cu) and Pt25-Cu75 were fabricated by arc-melting under an Ar atmosphere. These alloys were subjected to potential cycling tests at 298 K in 0.5 M H2SO4 solution with a two-compartment stationary cell. A double junction silver / silver-chloride electrode was used as reference electrode, and Au coil was used as the counter electrode. A total of 1,000 cycles of potential cycling tests were employed between 0.05 V and 1.4 V vs. SHE at 100 mV s-1. Tests was interrupted every 100 cycles, and the test solution was sampled for quantitative ICP-MS analysis of both Pt and Cu ions dissolved in it. In addition, surface structure and depth profile after potential cycling tests were evaluated by FE-SEM and FE-AES, respectively. FE-AES depth profiles before and after 100 cycle potential cycling confirmed that Pt-enriched layer formed on Pt25-Cu75 surface after potential cycling. Moreover, the amount of dissolved Pt and Cu ions during 1st sequential order of 100-cycles-potential cycling were 1 monolayer (ML) and 2370 ML, respectively. Hence, Pt-enriched layer formed by the large amount of Cu selectively dissolved from Pt25-Cu75. FE-SEM image of Pt25-Cu75 after 100 cycle potential cycling showed numerous pits (d pit = 2 – 5 nm) formed on its surface, and the large amount of Cu dissolution occurs from 2nd to 10th sequential order of 100-cycles-potential cycling. Thus, selective dissolution of Cu seems not to be suppressed in the latter cycles because Cu dissolution continuously occurs through the pits. On the other hand, FE-SEM image of Pt50-Cu50 after 100 cycle potential cycling was extremely flat. Accordingly, dense Pt-enriched layer formed on its surface, and it suppressed further Cu dissolution from the alloy. The authors acknowledge Ookayama Materials Analysis Division, Tokyo Institute of Technology, for assistance with the ICP-MS analysis.