Corrosion Resistance of Pt-Co Alloy Under Potential Cycling in Sulfuric Acid Solution

Pt-Co nano-particle is a promising material for the cathode catalyst of polymer electrolyte fuel cells (PEFCs) due to its high catalytic activity, but a decrease in the activity during a long-term operation of PEFC is a serious concern because the dissolution of Pt and Co is inevitable in the cathode operating condition.¹ In particular, Pt dissolution is a main origin of the particle growth of Pt catalyst in the electrochemical Ostwald ripening process,²and the dissolution of Pt-skin layer formed on Pt-Co surface is also an important factor for the stability of Pt-Co catalysts. In this study, the corrosion behavior of Pt-Co alloys in the simulated PEFC cathode condition was investigated to improve the stability of Pt-Co catalyst.

              Two types of Pt-Co alloys (ordered and disordered) were prepared by arc melting. The molar ratio of the alloy was 1:1. The ordered and disordered structures were formed by the heat treatments at 800 and 1200 °C in vacuum, respectively. The specimen was polished with diamond paste down to 0.25 mm prior to the heat treatment. Pt-skin layer on the Pt-Co surface was formed by dealloying in 0.5 M H₂SO₄ solution at 298 K open to air for 1 hour. 100 cycles of cyclic voltammetry (CV) was conducted to examine Pt and Co dissolution. The potential range of CV was from 0.6 to 1.4 V, and the sweep rate was 10 mVs^-1. 0.5 M H₂SO₄ solution at 298 K open to air was used as the electrolyte. After 100 cycles of CV, the solutions were analyzed by ICP-MS for the determination of the amount of dissolved Pt and Co. The electrochemical property before and after the dissolution test was evaluated by CV in the potential rage of 0.05–1.4 V in 0.5 M H₂SO₄solution at 298 K deaerated by Ar. The cross section of the specimen surface region was observed by STEM and analyzed by EELS.

              Figure 1 shows the amount of dissolved Pt and Co in the dealloying process. The ordered Pt-Co alloy exhibited a larger dissolution rate than the disordered one. STEM/EELS analysis of the cross-section of the Pt-Co surface indicated the thickness of Pt-skin layer of both alloys was less than 1 nm against the prediction that Pt was concentrated on the surface of the ordered Pt-Co alloy compared with that of disordered one from the ICP result. STEM observations also revealed the surface roughning of the ordered Pt-Co alloy was caused by the selective dissolution of Co, and the in-depth dissolution of Co for the ordered Pt-Co alloy explained why a large amount of Co dissolved as shown in Fig. 1.

              CVs of the ordered and disordered Pt-Co alloy were shown in Fig. 2. The electrochemical property of the ordered and disordered Pt-Co alloy was similar as that of Pt, this is evidence that a Pt-skin layer was formed on the surface of both Pt-Co alloy electrodes. Figure 3 the shows amount of dissolved Pt and Co in the dissolution test. Here, the electrochemically active surface area was used as the unit area to correct the influence of the roughness formed on the ordered Pt-Co alloy. The corrosion resistance of the Pt-skin layer on the Pt-Co surface was higher than bulk Pt, as shown in Fig. 3. This result explains the stability of Pt-Co cathode catalyst is higher than that of Pt catalyst in the PEFC operation.³ In addition, the ordered Pt-Co alloy has higher corrosion resistance than the disordered one. The suppression of Pt oxide formation for the Pt-Co surafce indicated in Fig. 2 is thought to have inhibited the Pt-skin layer dissolution because PtO₂ formaiton triggers the cathodic dissolution of Pt.⁴

1. S. Chen, H. A. Gasteiger, K. Hayakawa, T. Tada, and Y. Shao-Horn, J. Electrochem. Soc., 157(1), A82 (2010).

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

3. M. F. Mathias, R. Makharia, H. A. Gasteiger, J. J. Conley, T. J. Fuller, C. J. Gittleman, S. S. Kocha, D. P. Miller, C. K. Mittelsteadt, T. Xie, S. G. Yan, and P. T. Yu, Electrochem. Soc. Interface, 14(3), 24 (2005).

4. Z. Wang, E. Tada, and A. Nishikata, J. Electrochem. Soc., 161(4), F380 (2014).