Dissolution of Platinum Single Crystal Surfaces under Potential Cycling in Sulfuric Acid Solution

Pt dissolution is one of the most important topics in Polymer Electrolyte Fuel Cell (PEFC) field because of the origin of the Pt nanoparticle growth, which results in a cell performance loss of PEFC.¹ Recent reports showed a submonolayer of Pt dissolution occurs under potential cycling conditions simulating the PEFC operation in acidic environments.²Surface structural controls of Pt nanoparticles against Pt dissolution under potential cycling are a key point to improve the corrosion resistance of Pt surface. We investigated the effect of crystal orientations on the dissolution of Pt surface using Pt(100), Pt(110), and Pt(111) single crystal electrode.

   Commercial Pt(100), Pt(110) and Pt(111) single crystal electrodes were used as the specimen. The specimens were polished down to 0.25 mm diamond paste, annealed with an oxygen-hydrogen burner for 2 h, and cooled in a stream of Ar gas, prior to the electrochemical measurements. Cyclic voltammetry (CV) at single crystal surfaces was conducted for 100 cycles to examine Pt dissolution. The potential range of CV was from 0.5 to 1.0, 1.2, and 1.4 V. 0.5 M H₂SO₄solution at 298 K open to air was used as the electrolyte. After 100 cycles of CV, the solutions were analyzed with ICP-MS for the quantification of amount of dissolved Pt. The surface morphology of Pt single crystal electrodes was observed by electrochemical STM measurements.

   Figure 1 shows STM images of Pt(100), Pt(110), and Pt(111) single crystal surfaces after the anealing, and the cross section height profiles along the white arrows. The surfaces of Pt(100), Pt(110), and Pt(111) single crystal electrodes were composed of atomic height steps and smooth terraces. The saw-like step structures were observed at Pt(100) and Pt(110) surface. In addition, Pt(110) surface was relatively roughened.

   CVs at Pt(100), Pt(110), Pt(111) single crystal, and polycrystalline Pt surfaces were shown in Fig. 2. The voltammograms of single crystal electrodes exhibited characteristic hydrogen adsorption/desorption peaks between 0.05 and 0.4 V. These peaks were not sharp because of a exposure in air atmosphere during the setup of electrochemical measurements but similar to those reported in the literatures.³This demonstrates well-controlled surface structures of single crystal electrodes were obtained.

   Figure 3 shows the amount of Pt ions dissolved from single crystal surfaces under 100 cycles of CV in 0.5 M H₂SO₄solution. The horizontal axis means the upper potential limit of CV. The dissolved amount at Pt(100) and Pt(110) surfaces was almost the same, and increased with rising the upper potential limit. Pt(111) surface hardly dissolved at potentials below 1.2 V, and the amount of dissolution at this surface in the potential range of 0.5–1.4 V was about a half of that at Pt(100) and Pt(110) surfaces. This result indicates Pt(111) surface has the highest corrosion resistance among the three single crystal electrodes examined.

1.  Y. Shao-Horn, W. C. Sheng, S. Chen, P. J. Ferreira, E. F. Holby, and D. Morgan, Top Catal, 46, 285 (2007).

2.  Y. Sugawara, T. Okayasu, A. P. Yadav, A. Nishikata, and T. Tsuru, J. Electrochem. Soc., 159(11), F779 (2012).

3.  N. M. Marković, and P. N. Ross, Surf. Sci. Rep., 45, 117(2002).

Caption list:

Figure 1  STM images of Pt(100), Pt(110), and Pt(111) single crystal surfaces after the anealing treatment.

Figure 2  CVs at Pt(100), Pt(110), Pt(111) single crystal, and polycrystalline Pt surfaces at 10 mVs^-1 in 0.5 M H₂SO₄solution.

Figure 3  Amount of Pt ions dissolved from single crystal surfaces as a function of the upper potential limit of CVs. The CVs were carried out for 100 cycles at 10 mVs^-1 in 0.5 M H₂SO₄ solution.