Effect of Potential on Dissolution of Pt-Fe Alloys Studied By Channel Flow Multi Electrode

Introduction

Core/shell structure Pt-M alloys catalysts are anticipated to be applied in PEMFCs cathode catalyst layer owing to their high catalytic activity than pure Pt [1]. However, dissolution of Pt and M is a significant problem, which causes the performance loss of PEMFCs [2], when the potential loads such as on-off cycle and load cycle were applied to the cathode catalysts. Therefore, understanding the detailed dissolution mechanism of Pt and M from Pt–M alloy catalysts is very important to improve durability of PEMFCs under its operating conditions. In this study, we investigated that the dissolution behavior of Pt-50 at% Fe alloy (Pt₅₀-Fe₅₀) under potential cycling with channel flow multi electrodes (CFME), and clarify the effect of potential range to the dissolution behavior of Pt₅₀-Fe₅₀.

Experimental

Pt₅₀-Fe₅₀ (1 mm x 5 mm) used as working electrode (WE), and two Au plates (1 mm x 2 mm) were used as collector electrodes (CEs). These electrodes were embedded into CFME cell by epoxy resin, and kept in parallel. The gap distance between WE and CEs was 100 μm, and the gap between two CEs was 1 mm.

Potential cycling tests 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 coil was used as the counter electrode. In order to satisfy laminar flow condition, flow rate of the test solution was 10 cm s^-1. In potential cycling tests, lower potential limit set at 0.05 V vs. SHE, and upper potential limit (E_upper) varied from 0.6 to 1.4 V vs. SHE, and scan rate was 20 mV s^-1. Before the potential cycling tests, the W.E. was kept at 0.45 V vs. SHE in order to eliminate Pt-oxide layer formed on alloy surface in ambient air. A reaction and potential for detecting the dissolved Fe²⁺ on Au-CE is as follows,

Fe²⁺→Fe³⁺+e^-  (E_c = 1.0 V vs. SHE)           (1)

Dissolved Fe²⁺ from Pt₅₀-Fe₅₀ are monitored by the current of the CE.

Results and discussion

 Fig. 1 shows Fe²⁺detection current (i_Fe(II)) change in anodic scan during potential cycling, and black, red, and blue lines represent results of E_upper = 1.4 V, 1.0 V, and 0.6 V, respectively. In case of E_upper = 1.4 V, Fe²⁺ detection peak was observed between 0.3 – 0.7 V. Similarly, Fe²⁺ detection peak was appeared in same region when E_upper set at 1.0 V, however, the peak height was much smaller compared with the peak observed when E_upper =1.4 V. From these results, Fe dissolution as Fe²⁺ was not suppressed during potential cycling when E_upper was 1.0 and 1.4 V. On the contrary, i_Fe(II) kept constant value between 0.05 – 0.6 V when E_upper set at 0.6 V, and it means that Fe dissolution as Fe²⁺ was suppressed during potential cycling. The similar trend was observed in the cathodic scan.

 The dissolution behavior of Pt₅₀-Fe₅₀ strongly depended on E_upper of potential cycling. This may be attributed to the structure of Pt-enriched layer formed on alloy surface. In case of E_upper = 0.6 V, potential cycling doesn’t induce Pt dissolution [3], thus dense Pt-enriched layer formed on alloy surface and Fe dissolution is suppressed by Pt-enriched layer. While, in case of higher E_upper like 1.0 and 1.4 V, Pt-enriched layer seems to be less effective.