Abstract
Polymer electrolyte fuel cell (PEFC) membranes are subject to radical-induced degradation. Antioxidant strategies for hydrocarbon-based ionomers containing aromatic units can focus on intermediates that are formed upon attack by hydroxyl radicals (HO·). Among the different intermediates, the cation radical P·+ is the most promising target for repair, for example by cerium(III). For the “repair” reaction of Ce(III) with radicals of a poly(α-methylstyrene sulfonate) oligomer we determined an activation energy of (9 ± 2) kJ mol−1 and a rate constant of 1.6 · 108 M−1 s−1 at 80 °C by pulse-radiolysis. For the reduction of Ce(IV) by hydrogen peroxide the activation energy was determined by stopped-flow as (30 ± 1) kJ mol−1 with a rate constant of 4.8 · 106 M−1 s−1 at 80 °C. These parameters are fed into a kinetics model to estimate the efficacy of the cerium (III)/(IV) redox couple as a catalytic repair agent in hydrocarbon-based fuel cell membranes. While cerium can mitigate polymer degradation, repair efficacy depends on the polymer degradation pathway and the nature and lifetime of the intermediates.
Highlights
Reaching the aforementioned New Energy and Industrial Technology Development Organization (NEDO) targets may require a drastic technological step in the form of next-generation fuel cell materials and cell engineering
Hydrocarbon ionomers react much faster with HO· radicals than perfluroalkylsulfonic acid (PFSA) ionomers: almost all of the hydroxyl radicals formed will react with the aromatic units of the polymer
Kinetics measurements.—When a deaerated (Ar saturated) solution of 0.1 mM PAMSS, 10 mM K2S2O8 and 1 mM H2SO4 is pulse-irradiated with a dose of 30 Gy, we expect the formation of faoprpmroxPi·m+,atreelayct6ioμnM(7)S,Ow4h·−ic. hTchaensebien turn oxidize observed by the polymer to a corresponding absorption build-up at 560 nm.[36]
Summary
Reaching the aforementioned NEDO targets may require a drastic technological step in the form of next-generation fuel cell materials and cell engineering. Ce(IV) reacts with H2O2, which is present in the fuel cell, and regenerates Ce(III).[30] The known sources of H2O2 in the PEFC are two-electron reduction of oxygen to hydrogen peroxide and reaction of O2 crossed-over from the cathode with hydrogen adsorbed on the Pt catalyst.[25] Hydrocarbon ionomers react much faster with HO· radicals than PFSA ionomers: almost all of the hydroxyl radicals formed will react with the aromatic units of the polymer. The estimated lifetime of HO· is three orders of magnitude lower than in a PFSA ionomer, i.e. around 1 ns, which prevents effective scavenging by cerium-ions at practical concentrations.[28] Under these circumstances, and in analogy to biology, antioxidant action cannot be based on damage prevention but must focus instead on repair and inhibition of damage propagation.[31] This may be possible if intermediates formed upon radical attack are sufficiently long-lived.[23]
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