Cerium Migration in Polymer Electrolyte Membranes

Abstract

Cerium enhances polymer electrolyte membrane (PEM) fuel cell durability by scavenging damaging radical species which are generated during operation. However, during cell fabrication, conditioning, and discharge, cerium migrates between the PEM and catalyst layers (CLs), which can reduce cell performance [1]. In this work, cerium migration within the cell and washout from it were quantified in membrane electrode assemblies (MEAs) which were fabricated using PEMs containing cerium concentrations of ~6.0µg/cm2. MEAs were subjected to accelerated stress tests (ASTs) at open circuit voltage (OCV) with cell temperatures of 90°C in hydrogen/air. ASTs were performed at three different humidity conditions: 100% RH, 30% RH, and wet/dry cycling ASTs, where the cell was cyclically exposed to humidified and dry reactant gasses for 30/45s, respectively [2]. After ASTs, through-thickness and in-plane cerium migration profiles were characterized in MEAs using X-ray fluorescence. In-plane cerium migration from the gasketed, inactive PEM border region into the active area was observed and correlated to the active area water content during the AST. Cerium concentration in the active area increased from ~6.0 to 15.3µg/cm2 after 2,000h of 100% RH operation. After 456h of 30% RH operation, active area water content was less than at 100% RH, and cerium concentration increased to 9.84µg/cm2. After 823h of humidity cycling, cerium concentration remained unchanged. Through-thickness cerium migration from the PEM into the CLs is enhanced by low humidity operation. 30% RH operation resulted in uniform migration of cerium into both the anode and cathode CLs. PEM cerium was reduced from ~6.0 to 3.7µg/cm2, while anode and cathode CL concentrations were uniformly increased from 0.0 to 2.3 and 3.5µg/cm2, respectively. Conversely, after 100% RH operation, more cerium remained in the active area of the PEM. In-plane concentrations were maintained at ~6.0µg/cm2 near the inlet, however, concentration increased linearly to ~15µg/cm2 near the outlet. These concentration gradients were attributed to the condensation and subsequent flow of liquid water from cell inlet to outlet. Subjecting the MEA to humidity cycling resulted in both significant through-thickness cerium migration out of the PEM, as well as concentration gradients from PEM inlet to outlet. Under these conditions, average PEM concentration was reduced to 0.98µg/cm2. PEM cerium was depleted from the inlet and its concentration was only 1.5µg/cm2 near the outlet. Average anode and cathode CL cerium concentrations increased to 1.9 and 3.1µg/cm2, respectively. Cerium depletion which results from such conditions could reduce the amount of available radical scavengers, leaving the PEM more susceptible to chemical attacks. Effluent cell water samples were collected from the anode and cathode during ASTs and analyzed to measure fluoride and cerium concentrations using ion chromatography and inductively coupled plasma mass spectrometry. Fluoride and cerium emissions are correlated, which suggests that ionomer degradation products serve as counter-ions for emission from the cell. The most aggressive test conditions, however, only reduced the total cerium inventory in the MEA by < 0.5%. Fluoride emission rates (FERs) are also correlated to the average PEM cerium content after ASTs, which indicates a relationship between membrane degradation and cerium migration (Figure 1). We propose that during ASTs, cerium migration from the PEM into the CLs is driven by both concentration gradients between the PEM and CL, which arise due to cell humidification, and from degradation, itself, which is proportional to AST aggressiveness (wet/dry cycling > 30% RH >> 100% RH). We also postulate that cerium interacts with carbon catalyst support particles in the CLs which prevents its ionic equilibration with PEM ionomer. Ex situ experiments demonstrate the ability of carbon black to stabilize quantities of cerium identical to those measured in the CLs after ASTs. In addition to water content and degradation, potential gradients and proton flux strongly influence cerium migration from the PEM into the CLs. Window cell experiments will be discussed, which determine the relative influence of these mechanisms and their effects in operating cells. Further understanding of these mechanisms will enable cerium stabilization within the active area of the PEM, in order to mitigate performance losses and further enhance cell durability. Acknowledgements This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos.