A DFT study of bond dissociation trends of perfluorosulfonic acid membrane

The bond dissociation energy of a perfluorosulfonic acid (PFSA) molecule was investigated by density functional theory (DFT) to provide some dissociation trends of the PFSA molecule and the basic information for the design of more durable PFSA membranes. As the preliminary analysis, the chemical bond strengths of the PFSA molecule were analyzed exhaustively, and the vulnerable bond was identified by comparison of the results. The same calculations were performed for an ionized PFSA molecule to determine the influence of the ionization state on the bond strength. The C-S bond was the weakest among the side chain backbone in the neutral molecule; however, the C-S bond became stronger when ionized while it weakened the C-O bond connecting the side chain with the main chain. Analysis of the C-F bonds in the side chain showed that the dissociation energy decreases in the order of primary, secondary, and tertiary bonds, as also reported in the literature. Analysis of the main chain showed that the secondary C-F bonds neighboring the tertiary C-F bond connecting the side and main chains were the weakest. Ionization of the PFSA molecule weakens the average dissociation energy of C-F bonds. As the realistic analysis, the same calculations were performed considering the solvation effects to analyze the effects on the dissociation trend. Moreover, the possibility for improving the durability of PFSA membranes was investigated by partial substitution of H atoms or CH₃ groups for F atoms.