Membrane degradation mechanisms in polymer electrolyte membrane fuel cells

Date of Completion

January 2006

Keywords

Engineering, Chemical|Energy

Degree

Ph.D.

Abstract

Membrane degradation and failure is one of the most important factors limiting the lifetime of polymer electrolyte membrane fuel cells (PEMFCs). Increasing the membrane life by developing degradation mitigation strategies in the cell or developing a new membrane with improved life requires a detailed understanding of the membrane degradation mechanism during operation in a PEMFC. ^ An in-situ and nondestructive technique, which relies on the measurement of the membrane degradation rate in a fuel cell, was used to study the chemical/electrochemical mode of membrane degradation. Nafion® membrane was used for the degradation study and fluoride emission rate (FER) as measured from the fuel cell effluent water analysis was used as a quantitative indicator of the membrane degradation rate. The degradation mechanism was studied by a detailed investigation of the effect of reactants, catalyst properties (location, potential, catalyst type, interaction with O2 and H2O), cell current, membrane thickness, Nafion® counterion, and direction of water movement on the membrane degradation rate. ^ Based on the experimental findings it is shown that commonly known membrane degradation mechanisms involving formation of active oxygen species from H 2O2 decomposition or the direct formation of active oxygen species in the oxygen reduction reaction are not the dominating membrane degradation mechanisms in PEMFCs. It is proposed that molecular H2 and O 2 react on the surface of Pt catalyst to form the membrane degrading species. Depending upon the catalyst location the source of H2 or O2 or both is from the reactant crossover through the membrane. The reaction mechanism is chemical in nature and depends upon the catalyst surface properties and the relative concentrations of H2 and O 2 at the catalyst. The membrane degradation rate also depends on the residence time of the species in the membrane and the reaction volume i.e. the membrane thickness. Thus, the membrane degradation may not be limited only to the polymer surface in contact with Pt catalyst. The sulfonic acid groups in the Nafion® side chain are key to the mechanism by which the radical species attack the polymer.^

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