Chemical degradation mechanism of model compound, CF₃(CF ₂)₃O(CF₂)₂OCF₂SO ₃H, of PFSA polymer by attack of hydroxyl radical in PEMFCs

To enhance the durability of perfluorosulfonic acid (PFSA) polymer for proton-exchange membrane fuel cells (PEMFCs), we theoretically analyzed the degradation mechanism of PFSA by the attack of a hydroxyl (OH) radical. We used CF₃(CF₂)₃O(CF₂)₂OCF ₂SO₃H as a model compound representing the PFSA side chain because the experimental result suggested that the ether group in the PFSA side chain is vulnerable to the OH radical attack. We performed density functional theory calculation to discuss the degradation reaction mechanism of the ether group in the model compound of the side chain and OH radical. Under high humidity condition, we clearly demonstrated the degradation mechanism and reactivity of C-O bond cleavage in the ether group by the OH radical. This result shows reasonable agreement with the experimental one. However, the OH radical prefers the reaction of the sulfonic acid group to the ether group under the low humidity condition. We found the different reactivity of the OH radical under the low and high humidity conditions. To improve the durability of PFSA, we proposed four directions: (i) enhancement of deprotonation, (ii) protection of ether group by steric hindrance, (iii) enhancement of C-O bond strength, and (iv) substitution of the ether group by other chemical groups. The latter two directions have been theoretically explored more in detail.