Radical Attack and Damage Mitigation in Hydrocarbon-Based Ionomers

Abstract

In an operating polymer electrolyte fuel cell (PEFC) radical intermediates are formed as a result of the interaction of H2 and O2 on the surface of the Pt-based electrocatalyst. Of the various radical intermediates, •OH is of particular concern owing to its high oxidative strength, E°(•OH,H+/H2O) = 2.72 V) [1]. In PFSA ionomers, •OH reacts relatively slowly with the polymer and has a lifetime on the order of microseconds. This leaves sufficient time for additives, such as Ce(III), to scavenge the radicals and thus mitigate radical induced damage of the ionomer. In hydrocarbon-based ionomers containing aromatic units, •OH reacts within nanoseconds with polymer constituents [2]. Radical scavenging can therefore not be effective.In the development of alternative antioxidant strategies for hydrocarbon-based ionomers, it is crucial to elucidate polymer degradation mechanisms triggered by radical attack. In addition to studying the initial reactions of primary radicals, i.e. •OH, it is equally important to characterize the nature of the formed polymer intermediates, their lifetime, stability and reactivity. Sufficiently long-lived intermediates can be repaired by suitable additives, thereby restoring the original ionomer [3]. In the fuel cell, the thus protected membranes show a much lower rate of degradation in an accelerated stress test (AST) at open circuit voltage (OCV) [4].In this contribution, we provide an overview of methods to study reaction of radicals with small-molecule model compounds and ionomer constituents [5, 6]. The interaction of ionizing radiation (MeV electrons or photons) with water leads to the formation of radicals, such as •OH, at known rates. Water radiolysis can therefore be used to study the kinetics of radical attack and follow-up reactions as a function of pH, electron density of the aromatic ring, and presence of additives. Examples relevant to the acidic conditions in the proton exchange membrane (see Figure, Panel a) as well as the alkaline conditions in an anion exchange membrane (AEM) will be given. Moreover, the use of suitable antioxidants to mitigate degradation through the repair of intermediates will be discussed. Fuel cell tests with hydrocarbon-based membranes containing polymer-bound antioxidants will be shown (see Figure, Panel b) and prospects for long-term stability of non-fluorinated ionomers assessed.Figure caption: a) Analysis of the effect of Ce(III) ions on the extent of degradation of 1 mM of 4-cumenesulfonate (4CS) and 4-(tert-butyl)phenylsulfonate (BPS) at pH 0 (1 M H2SO4) in 1 mM H2O2 upon exposure to a radiolytic dose provided by a 60Co-source of 800 Gy (corresponding to a cumulative amount of •OH of 0.22 mM). ‘Excess degradation’ indicates the difference to the extent of degradation in the absence of Ce(III) and H2O2. Data for BPS from [5]. b) Ohmic resistance of single cells (average of several experiments) with partially fluorinated membrane containing a tethered crown ether with and without Ce(III) metal center during an accelerated stress test. Ion exchange capacity at the end of test was measured to represent remaining membrane state of health. Conditions: open circuit voltage (OCV), H2/O2, 80 °C, 2.5 bara, and 100% R.H. [4].