DFT Study on Side Chain Detachment of Perfluorosulfonic Acid Ionomers by Radical-Assisted Nucleophilic Attack of Water
2022
Yamaguchi, Makoto
Chemical degradation of perfluorosulfonic acid (PFSA) ionomer membranes is still an unresolved critical issue in the development of polymer electrolyte fuel cells (PEFCs). Its reaction mechanism has not been settled yet even after experimental and computational studies for decades. In this study, the role of hydroxyl radical to side chain degradation is revisited and a new reaction mechanism is proposed. Its nucleophilic attack on the carbon atom in the ether group induces detachment of the side chain. The reactions are supposed to be feasible as their reaction barrier heights are only slightly higher than those of tertiary fluorine atom abstraction reactions by hydrogen radical in absence of the solvation effect. When a single water molecule is present with hydroxyl or hydroperoxyl radical it is the oxygen in the water that undergoes the nucleophilic attack on the carbon in the ether groups and the calculated barrier heights are lowered to 70∼100 kJ mol⁻¹ as the radical assists the reaction as a hydrogen acceptor. Possible reaction pathways of the products are also calculated to explain the occurrence of the experimentally observed degradation products of Nafion after chemical durability tests.
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