Theoretical Studies of Pendant Effects on the Properties of Sulfonated Hydrocarbon Polymer Electrolyte Membranes

Six model compounds of hydrocarbon polymer electrolyte membrane (PEM) with different neighboring pendants have been investigated using density functional theory (DFT). The effect of the neighboring pendant on the proton dissociation properties of the PEMs and on the chemical stability of the key adjacent bond containing a sulfonic group was evaluated. Results of the proton dissociation properties of the six model compounds indicate that the introduction of a strong electron-withdrawing group, such as CF, CF₂, or CN, on the neighboring pendant of the acid group can improve the proton dissociation properties of PEMs. The calculated p<i>K</i>_a values confirm the relative acid strength of the six model compounds, whose properties are, to some extent, related to the proton conductivity. Our results demonstrate that a model compound containing a strong electron-withdrawing group in the neighboring pendant has stronger acid strength. DFT calculations on the C–S bond degradation reactions caused by OH or H radicals show that a −CF₂CF₂– group in the neighboring pendant improves the stability of the C–S bond against attack from a radical, while introduction of a −CH₂CH₂CH₂– or a CN group has little influence on the stability of the C–S bond