Oxidative and Photochemical Stability of Ionomers for Fuel-Cell Membranes

✍ Scribed by Svetlin Mitov; Olga Delmer; Jochen Kerres; Emil Roduner

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: German
• Weight: 254 KB
• Volume: 89
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.200690219

✦ Synopsis

Abstract

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 H~2~O~2~ 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 H~2~O~2~. 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 ≥ p__K__~A~ of HO^.^ and H~2~O~2~, reactions between the model compounds and O~2~^.−^ or ^1^O~2~ 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 CC bond.