Cerium Migration during PEM Fuel Cell Assembly and Operation

Abstract

Cerium ions enhance the chemical stability and lifetime of polymer electrolyte membrane (PEM) fuel cell components by rapidly and reversibly scavenging degrading radical species which are generated during operation [1]. However, during cell fabrication and discharge, these ions readily migrate between the membrane and catalyst layers (CLs) of the membrane electrode assembly (MEA) [2]. Complete washout of cerium from the MEA has also been observed [3]. It is necessary to understand the mechanisms and magnitude of cerium migration during fuel cell operation, since cerium ions are ineffective outside of the catalyzed area of the MEA. These results can be applied to improve cerium stability in the active area of the MEA and localize it to areas of high radical generation, which can further extend the lifetime of PEM fuel cells. Table 1: AST conditions and flow fields Test Time (h) RH (%) Flow field I 500 30 50 cm2 single serpentine II 2,000 100 25 cm2 tri-serpentine III 2,000 100 25 cm2 single serpentine Membrane chemical stability accelerated stress tests (ASTs) [4] were performed on cerium-containing MEAs at 90°C in single-cell hardware (Fuel Cell Technologies), compressed with 8 x bolts at 50 in-lb of torque, using the conditions and hardware shown in Table 1. Nafion XL (DuPont) membranes were used, which contain a nominal cerium loading of 6 μg Ce/cm2. Carbon-supported platinum electrodes (TKK, 48% Pt, 0.1-0.2 mg Pt/cm2loading) and Sigracet 25BC GDLs (SGL) were also used. X-ray fluorescence (XRF) was performed on MEA components before and after the ASTs in order to measure in-plane cerium content in the membrane and CLs. After 500 hours of OCV operation at 30% RH, cerium moved uniformly from the membrane of MEA I into the CLs (not shown). Here, migration is attributed cell component hot pressing and interactions with the CLs [2]. Membrane cerium was reduced to 3.7± 0.68 μg Ce/cm2, while anode and cathode CL concentrations were increased to 2.3 ± 0.06 and 3.4 ± 0.22 μg Ce/cm2, respectively. At 100% RH, cerium also remained in the active area, however, membrane concentration increased from inlet to outlet (Figure 1a). This gradient may arise due to the increased presence and flow of liquid water during 100% RH operation. Only trace amounts of cerium remained in the CLs, except near the outlet, which suggests that under humidified conditions, the effects of CL interactions on cerium migration are reduced. After 2,000 hours of operation at 100% RH, flow field compression was observed to have implications on cerium migration out of the active area. The cerium profile of MEA III (Figure 1b) shows that it migrated from areas of high compression in the active area (shown in red) into low compression regions outside of the active area. In contrast, compression was higher around the active area of MEA II (Figure 1a), which prevents cerium from leaching from it. However, concentration was non-uniform, as discussed above. These preliminary results indicate that cerium migration and leaching out of the active area are affected by membrane water content and cell clamping pressure. It is believed that other factors such as electrical potential and temperature influence migration, as well. The authors wish to acknowledge the financial support of the Fuel Cell Technologies Program and the Technology Development Manager Nancy Garland.