Microstructural Changes of Membrane Electrode Assemblies during PEFC Durability Testing at High Humidity Conditions

Abstract

Morphological changes occurring in membrane electrode assemblies ͑MEAs͒ were monitored using transmission electron microscopy ͑TEM͒ during the course of life testing of H 2 /air polymer electrolyte fuel cells ͑PEFCs͒. In the fresh catalyst layers, anode Pt particles were found to have smaller particle sizes, better dispersion, and less agglomeration on the carbon-support surfaces than did the cathode Pt 3 Cr alloy particles. The operation-induced agglomeration of catalyst particles was evaluated for both the anode and cathode after defined life testing periods. Agglomeration of catalyst particles occurred primarily during the first 500 h of testing, which was confirmed by both TEM analysis and electrocatalytic surface area measurement. After 500 h, degradation of the recast Nafion ionomer network within the catalyst layers likely contributes more significantly to MEA performance degradation. Migration of metal catalyst particles toward the interface between the catalyst layer and membrane was observed at both electrodes. The Pt anode catalyst was less stable than the Pt 3 Cr cathode catalyst under high current density and high humidity conditions, which was confirmed by the higher extent of migration observed for the pure Pt than for the Pt 3 Cr. Some Pt particles ͑from both electrodes͒ were found to migrate into the membrane during the testing period.

An impedance model for analysis of EIS of polymer electrolyte fuel cells under hydrogen peroxide formation in the cathode

“…The process responsible for the inductive characteristics at low frequencies has been attributed to hydrogen crossover through the PEM [5] to form hydrogen peroxide at the cathode. Hydrogen crossover from anode to cathode through conventional graphite for bipolar plates due to their porous nature could also be a responsible process for hydrogen peroxide formation and inductive loops in EIS measurements.…”

“…Roy et al [4] developed impedance models to account for the reaction mechanisms that may be responsible for the inductive response at low frequencies; the models propose the formation of hydrogen peroxide and platinum dissolution during the oxygen reduction reaction (ORR). It has been reported [5] that crossover of hydrogen to the cathode facilitates the reaction of oxygen and hydrogen at the cathode, generating hydroxyl and hydroperoxyl radicals which react further to produce hydrogen peroxide at the cathode. The hypothesis that hydrogen peroxide may be formed at the cathode of a fuel cell is supported by the results of Inaba et al [6].…”

An Electrical Circuit for Performance Analysis of Polymer Electrolyte Fuel Cell Stacks Using Electrochemical Impedance Spectroscopy

“…At low frequencies inductive effects on the EIS results were apparent for the three current densities. The literature [5,6,7,8] When the kinetics of the ORR dominate the cell performance such as in the low current density range of the polarisation curve, the impedance spectrum mainly represents the charge transfer resistance during the ORR and its diameter decreases with increasing current density [14]. At a current density of 0.3125 A/cm 2 there is an increase in the driving force for the interfacial oxygen reduction process.…”

The correlation of electrochemical and fuel cell results for alcohol oxidation in acidic and alkaline media

“…small clusters merge forming larger ones (Figure 24), furthermore, the smaller the particles the more rapid the agglomeration proceeds [131]. Three different mechanisms for nanoparticle agglomeration on a carbon support have been proposed [132]: 1) the Ostwald ripening process where small particles dissolve in the ionomer phase and redeposit on the surface of larger particles, 2) with random cluster-cluster collisions that is followed by liquidlike coalescence of these particles or 3) the growth can occur at the atomic scale by minimization of the clusters' Gibbs free energy.…”