Chemical durability studies of ionomers and model compounds for fuel cell applications.

摘要

In this dissertation, a systematic investigation of the chemical durability study of perfluorinated sulfonic acid (PFSA) ionomers for polymer electrolyte membrane fuel cell (PEMFC) was conducted. Low molecular weight model compounds with various structural characteristics were employed as analogs to different moieties that are present in the ionomers. Model compounds and ionomers were degraded by hydroxyl radicals, which are the attacking species present in a running fuel cell. The hydroxyl radicals were created by Fenton's reagents, ferrous ion and hydrogen peroxide, or by direct UV photolysis of hydrogen peroxide. Fluoride release was measured and considered to be the measurement of chemical degradation of model compounds and ionomers. Degradation products from model compound systems and ionomer systems were identified by liquid chromatography-mass spectrometry (LC-MS) and 19F nuclear magnetic resonance (NMR) experiments. Certain intermediate radical species that are present during the degradation were trapped by using stable radical solutions of 4-hydroxy-2,2,6,6-tetramethyl-piperidinooxy (4-hydroxy-TEMPO), followed by identification using LC-MS.; The results from model compound systems revealed that: carboxylic acid groups are extremely labile toward the reaction with hydroxyl radicals, fluoroethers moieties can be cleaved by hydroxyl radicals, and the mechanistic steps involved in the ether cleavage reaction are proposed. The results from ionomer systems, i.e. commercial NafionRTM ionomer and 3M ionomers, showed good agreement with the conclusions reached from model compound systems. After degradation, low molecular weight fragments formed through the side chain cleavage from the fluoroether branching points of ionomers were identified by LC-MS. The proton conductivity of various ionomers also decreased after degradation.