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Issue 37, 2010
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Damage to fuel cell membranes. Reaction of HO˙ with an oligomer of poly(sodium styrene sulfonate) and subsequent reaction with O2

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Abstract

An understanding of the reactivity of oligomeric compounds that model fuel cell membrane materials under oxidative-stress conditions that mimic the fuel cell operating environment can identify material weaknesses and yield valuable insights into how a polymer might be modified to improve oxidative stability. The reaction of HO˙ radicals with a polymer electrolyte fuel cell membrane represents an initiation step for irreversible membrane oxidation. By means of pulse radiolysis, we measured k = (9.5 ± 0.6) × 109 M−1 s−1 for the reaction of HO˙ with poly(sodium styrene sulfonate), PSSS, with an average molecular weight of 1100 Da (PSSS-1100) in aqueous solution at room temperature. In the initial reaction of HO˙ with the oligomer (90 ± 10)% react by addition to form hydroxycyclohexadienyl radicals, while the remaining abstract a hydrogen to yield benzyl radicals. The hydroxycyclohexadienyl radicals react reversibly with dioxygen to form the corresponding peroxyl radicals; the second-order rate constant for the forward reaction is kf = (3.0 ± 0.5) × 107 M−1 s−1, and for the back reaction, we derive an upper limit for the rate constant kr of (4.5 ± 0.9) × 103 s−1. These data place a lower bound on the equilibrium constant K of (7 ± 2) × 103 M−1 at 295 K, which allows us to calculate a lower limit of the Gibbs energy for the reaction, (−21.7 ± 0.8) kJ mol−1. At pH 1, the hydroxycyclohexadienyl radicals decay with an overall first-order rate constant k of (6 ± 1) × 103 s−1 to yield benzyl radicals. The second-order rate constant for reaction of dioxygen with benzyl radicals of PSSS-1100 is k = (2–5) × 108 M−1 s−1. We discuss hydrogen abstraction from PSSS-1100 in terms of the bond dissociation energy, and relate these to relevant electrode potentials. We propose a reaction mechanism for the decay of hydroxycyclohexadienyl radicals and subsequent reaction steps.

Graphical abstract: Damage to fuel cell membranes. Reaction of HO˙ with an oligomer of poly(sodium styrene sulfonate) and subsequent reaction with O2

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Publication details

The article was received on 31 Mar 2010, accepted on 09 Jun 2010 and first published on 05 Aug 2010


Article type: Paper
DOI: 10.1039/C0CP00082E
Phys. Chem. Chem. Phys., 2010,12, 11609-11616

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    Damage to fuel cell membranes. Reaction of HO˙ with an oligomer of poly(sodium styrene sulfonate) and subsequent reaction with O2

    S. M. Dockheer, L. Gubler, P. L. Bounds, A. S. Domazou, G. G. Scherer, A. Wokaun and W. H. Koppenol, Phys. Chem. Chem. Phys., 2010, 12, 11609
    DOI: 10.1039/C0CP00082E

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