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

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.