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DFT Study on the Chemical Degradation Mechanism of Perfluorobis(sulfonyl)imide Sulfonic Acid Ionomer Membranes

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journal contribution
posted on 19.01.2021, 07:15 by Makoto Yamaguchi
The development of proton-conducting membranes with high conductivity and durability is still one of the key issues for the wide application of proton-conducting membrane fuel cells (PEMFCs). Perfluorobis­(sulfonyl)­imide sulfonic acid (PFIA) ionomer membranes having multiple acid sites on each side chain can have lower equivalent weight and show higher proton conductivity than currently used perfluorosulfonic acid (PFSA) ionomer membranes without losing mechanical properties of the membranes. However, durability tests of membrane electrode assemblies (MEAs) with the PFIA membranes show greater loss of open-circuit voltage than the MEAs with the PFSA membranes, and sulfonamides are detected as degradation products. In this paper, reactions of hydrogen radical attack on imide nitrogen and following nitrogen sulfur bond cleavage are proposed to explain the formation of sulfonamides. Their calculated barrier heights are lower than those of the tertiary fluorine atom abstraction as the initial step of the PFSA membrane degradation by the hydrogen atom and consistent with the faster degradation of the PFIA membrane. These results suggest that the role of hydrogen radical to initiate the chemical degradation of perfluorinated ionomers should be further investigated both experimentally and theoretically.