Oxidative and Photochemical Stability of Ionomers for Fuel-Cell Membranes

Autor: Olga Delmer, Svetlin Mitov, Jochen Kerres, Emil Roduner
Rok vydání: 2006
Předmět:
Zdroj: Helvetica Chimica Acta. 89:2354-2370
ISSN: 1522-2675
0018-019X
DOI: 10.1002/hlca.200690219
Popis: To predict hydroxyl-radical-initiated degradation of new proton-conducting polymer membranes based on sulfonated polyetherketones (PEK) and polysulfones (PSU), three nonfluorinated aromatics are chosen as model compounds for EPR experiments, aiming at the identification of products of HO.-radical reactions with these monomers. Photolysis of H2O2 was chosen as the source of HO. radicals. To distinguish HO.-radical attack from direct photolysis of the monomers, experiments were carried out in the presence and absence of H2O2. A detailed investigation of the pH dependence was performed for 4,4′-sulfonylbis[phenol] (SBP), bisphenol A (= 4,4′-isopropylidenebis[phenol]; BPA), and [1,1′-biphenyl]-4,4′-diol (BPD). At pH ≥ pKA of HO. and H2O2, reactions between the model compounds and O2.− or 1O2 are the most probable ways to the phenoxy and ‘semiquinone’ radicals observed in this pH range in our EPR spectra. A large number of new radicals give evidence of multiple hydroxylation of the aromatic rings. Investigations at low pH are particularly relevant for understanding degradation in polymer-electrolyte fuel cells (PEFCs). However, the chemistry depends strongly on pH, a fact that is highly significant in view of possible pH inhomogeneities in fuel cells at high currents. It is shown that also direct photolysis of the monomers leads to ‘semiquinone’-type radicals. For SBP and BPA, this involves cleavage of a CC bond.
Databáze: OpenAIRE
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