A detailed thermochemical and kinetic analysis of PFSA ionomers and the reactive oxygen species formed in an operating fuel cell is reported. The analysis reveals that hydroxyl radical is the only oxygen species capable of abstracting a hydrogen atom from carboxylic acid intermediates, thereby propagating the PFSA main chain unzipping process. Two chemically specific, main chain scission mechanisms are proposed. The first involves formation of sulfonyl radicals under dry conditions and the second invokes the action of hydrogen atoms formed from hydrogen gas and hydroxyl radical. The thermochemical analysis provides a strong basis from which to rationalize the results from a broad range of in-situ and ex-situ fuel cell degradation studies.

Cerium and manganese ions are very effective reversible scavengers of •OH in an operating PFSA-based PEM fuel cell. The use of these ions in very small quantities can reduce the fluoride release rate by up to three orders of magnitude relative to an unmitigated sample and thereby afford extremely durable membranes. A chemically rational mechanism that accounts for the remarkable effectiveness of these chemical mitigants is presented.

We present experiments in an in situ fuel cell (FC) inserted in the resonator of the ESR spectrometer that offered the ability to observe separately processes at anode and cathode sides and to monitor the formation of HO and HOO radicals, H and D atoms, and radical fragments derived from the Nafion membrane. The presence of the radicals was determined by spin-trapping electron spin resonance (ESR) with 5,5-dimethylpyrroline N-oxide (DMPO) as a spin trap. The in situ FC was operated at 300 K with a membrane-electrode assembly (MEA) based on Nafion 117 and Pt as catalyst, at closed and open circuit voltage conditions, CCV and OCV, respectively. Experiments with H(2) or D(2) at the anode and O(2) at the cathode were performed. The DMPO/OH adduct was detected only at the cathode for CCV operation, suggesting generation of hydroxyl radicals from H(2)O(2) formed electrochemically via the two-electron reduction of oxygen. The DMPO/OOH adduct, detected in this study for the first time in a FC, appeared at the cathode and anode for OCV operation, and at the cathode after CCV FC operation of >or=2 h. These results were interpreted in terms of electrochemical generation of HOO at the cathode (HO + H(2)O(2) --> H(2)O + HOO) and its chemical generation at the anode from hydrogen atoms and crossover oxygen (H + O(2) --> HOO). DMPO/H and DMPO/D adducts were detected at the anode and cathode sides, for CCV and OCV operation; H and D are aggressive radicals capable of abstracting fluorine from the tertiary carbon in the polymer membrane chain and of leading to chain fragmentation. Carbon-centered radical (CCR) adducts were detected at the cathode after CCV FC operation; weak CCR signals were also detected at the anode. CCRs can originate only from the Nafion membranes, and their presence indicates membrane fragmentation. Taken together, this study has demonstrated that FC operation involves processes such as gas crossover, reactions at the catalyst surface, and possible attack of the membrane by reactive H or D that do not occur in ex situ experiments in the laboratory, thus implying different mechanistic pathways in the two types of experiments.

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