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

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 k_f = (3.0 ± 0.5) × 10⁷ M⁻¹ s⁻¹, and for the back reaction, we derive an upper limit for the rate constant k_r 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.