DFT Studies of Perfluorosulfonic Acid Ionomer Degradation in Fuel Cells

Degradation reactions of perfluorosulfonic acid ionomers via H or OH radicals were investigated employing B3LYP/6-311++G (2d,2p) and omega 397XD/6-311++G (2d,2p). All important degradation reaction mechanisms of a model compound for perfluorosulfonic acid ionomers were analyzed, including optimized structures of related transition states, intermediates, and products. The ionomer backbones were more damaged by H radicals than by OH radicals. H radicals induced both main and side chain scission, while OH radicals mainly induced side chain scission. The C-S bond of the ionomers was the weakest site against both H and OH radical attacks. The ether group was also weak, suffering bond cleavage after H radical attacks on the backbone. H radicals preferred tertiary to secondary carbons, whereas there was no significant selectivity for OH radicals. In addition, carboxylic acids were primary intermediates during ionomer degradation, which was a key factor for ionomer stability. The results suggested various ideas for improving the stability of perfluorosulfonic acid ionomers in fuel cells.