Selective Dissolution Behavior of Fe from Pt-Fe Binary Alloy in Sulfuric Acid by CFDE

Abstract

Introduction Proton exchange membrane fuel cells (PEMFCs) is developed as a clean energy conversion device to overcome recent environmental problems. Core-shell structured Pt-M alloys are expected to be used in PEMFCs cathode due to their high catalytic activity than pure Pt1). However, many researchers reported performance loss of catalysts after PEMFCs long-run operation2), and selective dissolution of less noble metal (M) is one of the major issue of it3). Selective dissolution is inevitable for Pt-M alloy catalysts, thus the perception about dissolution rate of M is significantly important for catalyst application to PEMFCs. Hence, in this study, we investigated the dissolution behavior of Fe by in-situ channel flow double electrode (CFDE) system. Experimental Pt-50 at% Fe alloy (Pt50-Fe50) was made by arc-melting under Ar atmosphere. The specimen was vacuum-encapsulated in a silica glass tube at 2.0 x 10-5 torr, and homogenized at 1473 K for 1 week, and cooled in furnace. After heat treatment, Pt50-Fe50was cut into 1 mm x 5 mm, and used as working electrode (W.E.). Two Au plates whose geometric area are 1 mm x 2 mm was used as collector electrodes (C.E.). These electrodes are embedded into epoxy resin, and kept in parallel. The gap between W.E. and C.E. is 100 mm, and between two C.E. is 1 mm.Potentiostatic polarization measurements were employed at 298 K using Ar-purged 0.5 M H2SO4 solution. A double junction KCl-saturated Ag / Ag-Cl electrode was used as reference electrode and Au wire was used as the counter electrode. Flow rate of the test solution was 10 cm s-1. In potentiostatic polarization, W.E. (E w) was polarized at 1.0, 1.2, and 1.4 V vs. SHE. Before the polarization, the W.E. was kept at 0.45 V vs. SHE in order to eliminate Pt-oxide layer on alloy surface. A reaction and potential for detecting the dissolved Fe2+ and Fe3+on Au C.E. are as follows,Fe2+→Fe3++e- (E c= 1.0 V vs. SHE) (1)Fe3++e-→Fe2+ (E c= 0 V vs. SHE) (2)Dissolved Fe2+ and Fe3+ from Pt50-Fe50are monitored by the current of the C.E. Results and discussion Fig. 1 shows the changes of the W.E. current i w and C.E. current i c at E c = 1.4 and 0 V when the W.E was polarized at E w = 1.4 V. The i w immediately increased by changing W.E. potential from 0.45 V to 1.4 V, and then gradually deceased to a constant value. The i c at E c = 1.0 V for detecting the dissolution of Fe2+ does not respond to the i w change on W.E. On the other hand, the i c at E c = 0 V for Fe3+ shows similar trend to the i w change. As can be seen in Fig. 1(c), the selective dissolution of Fe3+ occurs just after the polarization and afterwards approaches the base current (approximately 200 nA ). This means that the surface of the Pt50-Fe50is quickly covered with a Pt-enriched layer. Selective dissolution mechanism of Fe from Pt-Fe alloy will be further discussed by using these CFDE results in the session. Reference [1] T. Toda, H. Igarashi, H. Uchida, and M. Watanabe, J. Electrochem. Soc. 146(1999) 3750.[2] H.A. Gasteiger, S.S. Kocha, B. Sompalli, and F.T.Wagner, Appl. Catal. B. Environ. 56(2005) 9.[3] S. Mukerjee, and S. Srinivasan, Handbook of Fuel Cells, vol2 (2003) 502.