Platinum (Pt) nanoparticles is used as cathode catalyst in polymer electrolyte fuel cell (PEFC) to enhance oxygen reduction reaction, however high cost of Pt is preventing popularization of PEFC. Therefore, Pt-transition metal (M) alloy is expected to be used as cathode catalyst instead of Pt in order to lower the amount of Pt usage in PEFC. Both Pt and M dissolution under PEFC operating condition is one of the biggest problem, because their dissolutions lead to performance loss of PEFC. In this study, we selected Cu as additive elements, and the dissolutions and the consequent surface morphological changes of Pt-Cu nanoparticles were investigated by inductively coupled plasma mass spectroscopy (ICP-MS), and field emission-scanning electron microscopy (FE-SEM).

Plating bath was prepared by mixing 6 mM K₂PtCl₄, 6 mM CuSO₄·5H₂O, and 0.5 M Na₂SO₄, and pH of the bath was adjusted to 2.0 by adding H₂SO₄. Pt-Cu nanoparticles were pulse-electroplated on glassy carbon substrate in this bath. Electroplated nanoparticles were fully dissolved by aqua regia, and then tested aqua regia was analyzed with ICP-MS for determining chemical composition of the nanoparticles. Chemical composition was also investigated by using FE-SEM-energy dispersive X-ray spectroscopy, and was determined to be Pt-35 at. % Cu (Pt-35Cu). Pt-35Cu was subjected to potential cycling in 0.5 M H₂SO₄ at 298 K. A total of 5,000 cycles of potential cycling were applied between 0.05 V and 1.4 V vs. standard hydrogen electrode at 100 mV s^-1. Potential cycling was interrupted at the 50th, 200th, and 500th cycles, and every 500 cycles thereafter, and the test solutions were sampled for quantitative ICP-MS analysis of both Pt and Cu ions dissolved from Pt-35Cu. Furthermore, surface morphologies before and after potential cycling were observed by FE-SEM at identical location.

Before potential cycling, electroplated Pt-35Cu was composed of numerous Pt-Cu nuclei (less than 3 nm in diameter) that agglomerated to form larger secondary particles (30–50 nm). The surfaces of Pt-35Cu were rapidly flattened with increasing cycle number. After 1,000 cycles of potential cycling, the fine structure of primary nuclei completely vanished and reconstructed to form flattened surface. During this potential cycling, both Pt and Cu dissolved from Pt-35Cu, and the amount of dissolved Cu ions was much larger than the amount of dissolved Pt ions. Especially during the initial potential cycling, Cu was selectively dissolved from Pt-35Cu, and Pt-enriched layer was consequently formed on its surface. Cu dissolution was thereafter suppressed by Pt-enriched layer formation. Even the large Cu dissolution in the initial stage doesn’t affect to surface morphology change. Accordingly, surface morphology change during potential cycling is not likely due to Cu dissolution, but is likely due to Pt surface diffusion and/or Pt dissolution and re-deposition.

The authors acknowledge Ookayama Materials Analysis Division, Tokyo Institute of Technology, for assistance with the FE-SEM, and ICP-MS analysis.