Study on Dissolution Behavior of Pt-M Binary Alloys Under Potential Cycling By Channel Flow Multi Electrodes

Abstract

Introduction Pt-M alloys catalysts have attracted attention over the years and expected to be used in proton exchange membrane fuel cells (PEMFCs) cathode catalysts due to their high oxygen reduction activity than pure Pt and reduction of Pt usage in PEMFCs [1]. However, Pt-M catalysts is easily degraded by both Pt and M dissolution from Pt-M alloys during on-off cycle and load cycle, which leads the performance loss of PEMFCs [2]. Thus, to understand the nature of dissolution mechanism of both Pt and M from Pt–M alloy catalysts is very significant to advance durability of PEMFCs under its operating conditions. In this study, we investigated that the dissolution mechanism of Pt-50 at% Fe alloy (Pt50-Fe50) and Pt50-Cu50 under potential cycling with a channel flow multi electrodes (CFME), and elucidate the effect of additive element to the dissolution behavior of Pt50-M50. Experimental Pt50-Fe50 and Pt50-Cu50 were subjected to potential cycling tests at 298 K using Ar-purged 0.5 M H2SO4 solution with a CFME. A double junction KCl-saturated Ag / Ag-Cl electrode was used as reference electrode, and Au coil was used as the counter electrode. Flow rate of the electrolyte was 10 cm s-1 in order to establish laminar flow condition. Potential cycling tests were employed between 0.05 V and 1.4 V vs. SHE at 20 mV s-1. Before potential cycling, Pt50-M50 was set at 0.45 V vs. SHE in order to remove Pt oxide layer formed on Pt–M alloy surface in ambient air and to measure baseline currents of collector electrodes (CEs). Reactions and potentials for detecting the dissolved Fe2+, Fe3+, and Cu2+ on Au-CEs are as follows, Fe2+ → Fe3+ + e- (E c = 1.0 V vs. SHE) (1) Fe3+ + e-→ Fe2+ (E c = 0 V vs. SHE) (2) Cu2+ + 2e-→ Cu (E c = 0 V vs. SHE) (3) Dissolved Fe2+, Fe3+, and Cu2+ from Pt50-M50 are monitored by the current change of CEs. Results and discussion Figure 1 shows M detection current with a CFME during potential cycling, and red and blue line show the results of Pt50-Fe50 and Pt50-Cu50, respectively. As shown in inset, Fe dissolution from Pt50-Fe50 starts from around 0.3 V in anodic scan. Then Fe dissolution is clearly enhanced between 0.6 and 1.4 V in both anodic and cathodic scan, and finally terminate at 0.3 V in cathodic scan. Wang et al reported that Pt dissolution occur above approximately 0.6 V [3], accordingly major Fe dissolution is enhanced by Pt dissolution. On the contrary, minor Fe dissolution between 0.3 and 0.6 V in both anodic and cathodic scan is contributed by Pt surface diffusion, because Pt atoms are easily diffuse on Pt-M alloy surface in this potential range due to no adsorbed species on its surface [4]. Cu dissolution from Pt50-Cu50 only appears above 0.6 V, whereas Fe dissolution from Pt50-Fe50 starts from 0.3 V. Moreover, the amount of Cu dissolved from Pt50-Cu50 is smaller than that from Pt50-Fe50, which indicate that Cu is more durable elements than Fe. This is explained by the difference of standard electrode potentials between Cu and Fe. Pt-enriched layer formed on Pt50-Cu50 is denser and flatter than that on Pt50-Fe50, because the amount of dissolved Cu is small due to more noble standard electrode potential than Fe. Flat Pt-enriched layer inhibits Pt surface diffusion between 0.3 and 0.6 V, and Pt dissolution above 0.6 V. Reference [1] V. R. Stamenkovic, B. S. Mun, M. Arenz, K. J. J. Mayrhofer, C. A.Lucas, G. Wang, P. N. Ross, and N. M. Markovic: Nat. Mater. 6 (2007) 241–247. [2] H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner: Appl. Catal. B: Environ. 56 (2005) 9–35. [3] Z. Wang, E. Tada, and A. Nishikata: J. Electrochem. Soc. 161 (4) (2014) F380–F385 [4] Q. Xu, E. Kreidler, and T. He: Electrochim. Acta 55 (2010) 7551–7557. Figure 1