Reaction analysis for deprotonation of the sulfonic group of perfluorosulfonic acid molecules at low hydration levels

A reaction analysis for deprotonation of the sulfonic group in a model molecule of perfluorosulfonic acid (PFSA) at low hydration levels was performed. PFSA is usually adopted as a polymer electrolyte membrane in polymer electrolyte fuel cells. In hydration level three, the deprotonation reaction certainly occurs. The deprotonated state produced is more stable than the predeprotonated state by 3.72 kcal/mol. In addition, its activation energy is very low. Although quantitative discussion of this activation energy is difficult considering the computational error, it can be said qualitatively that H⁺ is abstracted smoothly from the sulfonic group because of a low activation energy. From the results of bond-order analysis, the produced H ₃O⁺ is strongly bound by the SO₃^- group. Thus, diffusivity of H₃O⁺ would be low. In hydration level four or more, we found a possibility that the diffusivity of H₃O⁺ increases because the hydrogen-bond strength between H₃O⁺ and SO₃^- is lower or SO ₃^- cannot bind H₃O⁺ directly by forming an Eigen cation.